Titanium and Its Alloys

•Relatively new engineering materials that possess an extraordinary combination of properties

–Low density (4.5 g/cm3)

–High melting temperature (1668°C), high elastic modulus (107 GPa)

–Extremely strong: 1400 MPa tensile strength at room temperature, highly ductile and easily forged and machined

–Limitations

ØChemical reactivity with other materials and oxidation problem at elevated temperatures

ØCost

•Applications: airplane structures, space vehicles, and in the petroleum and chemical industries

Aluminum and Its Alloys

•Aluminum and aluminum alloys are the most widely used nonferrous metals.

•Aluminum alloys: strengthened by cold working and alloying (Cu, Mg, Si, Mn, and Zn)

–Nonheat-treatable: single phase, solid solution strengthening

–Heat treatable: precipitation hardening (MgZn2)

•Properties

–Low density (2.7 g/cm3), as compared to 7.9 g/cm3 for steel

–High electrical and thermal conductivity

–Resistant to corrosion in some common environments

–Easily formed and thin Al foil sheet may be rolled

–Al has an FCC crystal structure; its ductility is retained even at very low temperatures

–Limitation: low melting temperature (660°C)

High-Carbon Steels

•0.60 ~ 1.4 wt%C

•Used in a hardened and tempered condition

•Hardest, strongest, and yet least ductile; especially wear resistant and capable of holding a sharp cutting edge

•Containing Cr, V, W, and Mo; these alloying elements combine with carbon to form very hard and wear-resistant carbide compounds (e.g., Cr23C6, V4C3, and WC)

•Applications: cutting tools and dies for forming and shaping materials, knives, razors, hacksaw blades, springs, and high-strength wire

Ferrous Alloys — Steels

•The carbon content is normally less than 1.0 wt%.

•Plain carbon steels: containing only residual concentrations of impurities other than carbon and a little manganese

  About 90% of all steel made is carbon steel.

•Alloy steels: more alloying elements are intentionally added in specific concentrations.

•Stainless steels

Classification of Ferrous Alloys

•Based on carbon content

–Pure iron (< 0.008wt% C)

  From the phase diagram, it is 

composed almost exclusively of 

the ferrite phase at room 

temperature. 

–Steels (0.008 ~ 2.14wt% C

In most steels the microstructure 

consists of both a and Fe3C phases.

  Carbon concentrations in commercial steels rarely exceed 1.0 w

t–Cast irons (2.14 ~ 6.70wt% C)  Commercial cast irons normally contain less than 4.5wt% C   .

Metal Alloys

•Ferrous Alloys: Alloys containing 

more than 50wt.%Fe

–Classification of Steels

–Designation of Steels

•Nonferrous Alloys: Alloys 

containing less than 50wt.%Fe

–Aluminum

–Copper

–Magnesium

–Titanium

–Refractory metals

–Superalloys

–Noble metals

FAILURE ANALYSIS

MATERIALS  FAILURE  ANALYSIS

Our materials engineers and metallurgists provide independent expert opinions regarding weld or material failure analysis.   Forensic investigation of your product failure is accomplished by analyzing and testing materials and welds.  Root cause analysis of your weld or material failure  can be identified by our failure analysis experts.  Features of our product failure analysis capabilities include:

Weld failure analysis         Analysis conducted by registered      professional engineers         Failure mode identification         Broad failure analysis experience         Fracture evaluations         Root cause analysis determination         Photographic documentation

EXPERIENCE

Our staff has had broad industrial experience.   Examples of industries in which our staff has had welding, manufacturing or failure investigation experience:

• Aircraft / Aerospace	• Gas turbine engine components
• Weldments.	• Petrochemical / Pharmaceutical plants
• Offshore structures	• Industrial machinery
• Heat exchangers	• Food processing equipment
• Ships	• Oil and gas transmission pipelines

   

LITIGATION  INVESTIGATION  AND  EXPERT  WITNESS

Our services can assist defendant/plaintiff from the initial legal proceedings through trial.  We provide support in the following areas:

• On site accident investigation

• Product liability

• Technical advisors to counsel

• Conformance to industry specifications

• Evaluations conducted by registered professional engineers

• Failure mode and root cause identification

• Forensic Macro/Microphotographic documentation

• Industrial and consumer product failure analysis

 

If your company is experiencing these or other welding problems you can engage AMC to solve your welding or failure issues.  Hire our consultants to act as your weld failure analyst.   

 

DISTORTION

DISTORTION

Welding involves highly localized heating of the metal being joined together.  The temperature distribution in the weldment is therefore nonuniform.  Normally, the weld metal and the heat affected zone (HAZ) are at temperatures substantially above that of the unaffected base metal.  Upon cooling, the weld pool solidifies and shrinks, exerting stresses on the surrounding weld metal and HAZ. 

If the stresses produced from thermal expansion and contraction exceed the yield strength of the parent metal, localized plastic deformation of the metal occurs. Plastic deformation results in lasting change in the component dimensions and distorts the structure.  This causes distortion of weldments.

Several types of distortion are listed below:

         Longitudinal shrinkage

         Transverse shrinkage

         Angular distortion

         Bowing

         Buckling

         Twisting

 

 

Factors affecting distortion

If a component were uniformly heated and cooled distortion would be minimized.  However, welding locally heats a component and the adjacent cold metal restrains the heated material.  This generates stresses greater than yield stress causing permanent distortion of the component.  Some of the factors affecting the distortion are listed below:

         Amount of restraint

         Welding procedure

         Parent metal properties

         Weld joint design

         Part fit up

Restraint can be used to minimize distortion.  Components welded without any external restraint are free to move or distort in response to stresses from welding.  It is not unusual for many shops to clamp or restrain components to be welded in some manner to prevent movement and distortion.  This restraint does result in higher residual stresses in the components.

Welding procedure impacts the amount of distortion primarily due to the amount of the heat input produced.  The welder has little control on the heat input specified in a welding procedure.  This does not prevent the welder from trying to minimize distortion.  While the welder needs to provide adequate weld metal, the welder should not needlessly increase the total weld metal volume added to a weldment.   

Parent metal properties, which have an effect on distortion, are coefficient of thermal expansion and specific heat of the material.  The coefficient of thermal expansion of the metal affects the degree of thermal expansion and contraction and the associated stresses that result from the welding process.  This in turn determines the amount of distortion in a component. 

Weld joint design will effect the amount of distortion in a weldment.  Both butt and fillet joints may experience  distortion.  However, distortion is easier to minimize in butt joints.   

Part fit up should be consistent to fabricate foreseeable and uniform shrinkage.    Weld joints should be adequately and consistently tacked to minimize movement between the parts being joined by welding.

If your company is experiencing these or other welding problems you can engage AMC to reduce your welding distortion.  Hire our consultants to act as your welding specialist.   

 

 

دمای بين پاسی

دمای بين پاسی

 

تعريف:
دمای بين پاسی عبارتست از دمای قطعه در ناحيه جوشکاری درست قبل از اعمال پاس دوم و يا بين هر دو پاس متوالی. در عمل حداقل دمای بين پاسی اغلب برابر است با دمای پيشگرم قطعه٫ هرچند که طبق تعريف اين مورد الزامی نميباشد.

اهميت دمای بين پاسی:
اهميت دمای بين پاسی از نظر تاثير بر خواص مکانيکيو ميکروساختار قطعه٫ اگر بيشتر از اهميت دمای پيشگرم نباشد از آن کمتر هم نيست. بعنوان مثال استحکام تسليم و استحکام کششی فلز جوش تابعی از دمای بين پاسی ميباشند. مقادير بالای دمای بين پاسی باعث کاهش استحکام فلز جوش ميشود. علاوه بر اين دماهای بين پاسی بالا اغلب باعث بهبود خواص ضربه و تافنس جوش ميشود. هرچند که در صورت افزايش اين دما به بالاتر از ۲۶۰ درجه سانتيگراد اين اثر عکس خواهد شد.

حداکثر دمای بين پاسی:

هنگامی که دستيابی به خواص مکانيکی مشخصی در فلز جوش مد نظر باشد٫ کنترل حداکثر دمای بين پاسی اهميت ويژه ای ميابد. درصورتيکه طراح حداقل استحکام را برای قطعه ای که ممکن است در اثر شرايط جوشکاری به دماهای بين پاسی بالايی برسد٫ مشخص کرده باشد٫ بايد حداکثر دمای بين پاسی نيز تعيين گردد. در غير اينصورت ممکن است استحکام جوش بشدت کاهش يابد.

کنترل حداکثر دمای بين پاسی همچنين در جوشکاری فولادهای کونچ و تمپر شده (مانند A514 ) نيز اهميت خاصی دارد. بدليل اينکه عمليات حرارتی خاصی روی اين فولادها اجرا شده است٫ دمای بين پاسی بايد در محدوده مجاز کنترل شود تا به خواص مکانيکی مورد نظر در فلز جوش و HAZ دست يابيم. البته کنترل حداکثر دمای بين پاسی در همه موارد الزامی نيست.

در مورد فلزات حساس٫ حداقل دمای بين پاسی بايد به حد کافی باشد تا از ايجاد ترک جلوگيری نمايد٫ در حاليکه حداکثر دمای بين پاسی نيز جهت دستيابی به خواص مکانيکی مناسب بايد کنترل شود. برای رسيدن به يک تعادل بين ايندو٫ پارامترهای زير نيز بايد مد نظر قرار گيرد: زمان بين اعمال پاسها٫ ضخامت فلز پايه٫ دمای پيشگرم٫ شرايط محيطی٫ خصوصيات انتقال حرارت و حرارت ورودی حين جوشکاری.

برای مثال جوشهايی با سطح مقطع کوچکتر طبيعتا دمای بين پاسی را افزايش ميدهند. بدين صورت که با ادامه عمليات جوشکاری دمای قطعه بدليل انتقال حرارت کمتر٫ بطور مداوم افزايش ميابد. بعنوان يک قانون کلی اگر سطح مقطع جوش کمتر از ۱۳۰ سانتيمتر مربع باشد٫ دمای بين پاسی در اثر اعمال هر پاس ( درصورت ثابت بودن سرعت عمليات ) افزايش ميابد. در حاليکه اگر سطح مقطع بيشتر از ۲۶۰ سانتيمتر مربع باشد٫ دمای بين پاسی در صورت عدم وجود منبع حرارتی ديگری٫ در خلال جوشکاری کاهش ميابد.

اندازه گيری و کنترل دمای بين پاسی:

يک روش پذيرفته شده برای کنترل دمای بين پاسی استفاده از دو شمع حرارتی يکی با دمای ذوبی برابر با حداقل دمای بين پاسی يا دمای پيشگرم و ديگری با دمای ذوبی برابر با حداکثر دمای بين پاسی ميباشد. جوشکار ابتدا ناحيه اتصال را گرم ميکند تا زمانی که شمع حرارتی اول ذوب شده و رسيدن به دمای پيشگرم را تاييد کند. پس از اينکه قطعه به دمای پيشگرم رسيد پاس اول اجرا ميشود. درست قبل از اعمال پاس دوم ( و پاسهای بعدی) حداقل و حداکثر دمای بين پاسی توسط شمعهای حرارتی در محلهای مناسب کنترل ميشود. بدين صورت که شمع اولی (با دمای ذوب کمتر) بايد ذوب شود (نشاندهنده رسيدن به حداقل دمای بين پاسی) در حاليکه شمع دوم ( با دمای ذوب بيشتر) نبايد ذوب شود ( نشاندهنده عدم عبور دمای بين پاسی از حداکثر تعيين شده). اگر شمع حرارتی مربوط به دمای ذوب کمتر ذوب نشود بايد حرارت بيشتری به قطعه اعمال گردد و درصورتيکه شمع حرارتی مربوط به دمای بيشتر ذوب شود بايد قطعه در هوای محيط به آهستگی سرد شود تا حدی که ديگر شمع دمای بالاتر ذوب نشده ولی شمع اولی ذوب شود. در اين هنگام ميتوان پاس بعدی را اعمال کرد.

محل اندازه گيری دمای بين پاسی:

محل اندازه گيری دمای بين پاسی در استانداردها مشخص شده است. بعنوان مثال در AWS D 1.1 و AWS D 1.5 چنين آمده که دمای بين پاسی بايد در فاصله ای حداقل برابر با ضخامت قطعه ضخيمتر ( اما نه کمتر از ۳ اينچ يا ۷۵ ميليمتر) در تمامی جهات از نقطه جوشکاری٫ اندازه گيری شود. اين حالت برای اندازه گيری حداقل دمای بين پاسی قابل درک است. اما وقتی کنترل حداکثر دمای بين پاسی نيز ضروری باشد٫ دمای ناحيه مجاور جوش ممکن است بسيار بالاتر از حد مشخص شده باشد. در اين حالت بهتر است دما در فاصله يک اينچی از کناره گرده جوش ( Weld Toe ) اندازه گيری شود. در موارد ديگری نيز صنايع خاص دستورالعملهای مخصوص به خود را دارند. بعنوان مثال در صنايع کشتی سازی٫ دمای بين پاسی معمولا در فاصله يک اينچی از کناره گرده جوش و در ۳۰۰ ميليمتر اول از نقطه آغاز جوشکاری اندازه گيری ميشود. در اين حالت خاص پيشگرم از طرف مقابل محل اندازه گيری اعمال ميشود تا از پيشگرم شده کامل ضخامت قطعه اطمينان حاصل شود.

نظرات ديگری نيز در مورد محل اندازه گيری دمای بين پاسی وجود دارد که بيشتر تجربی هستند. در مجموع همان فصله يک اينچی از کناره گرده جوش روش مناسبی بنظر ميرسد.

Discontinuities

Welding Discontinuities

Some examples of welding discontinuities are shown below.  Evaluation of the discontinuity will determine if the discontinuity is a defect or an acceptable condition:

	Incomplete Fusion - A weld discontinuity in which fusion did not occur between weld metal and fusion faces or adjoining weld beads.
	Undercut - A groove melted into the base metal adjacent to the weld toe or weld root and left unfilled by weld metal. Overlap - The protrusion of weld metal beyond the weld toe or weld root.
	Underfill - A condition in which the weld face or root surface extends below the adjacent surface of the base metal. Incomplete Joint Penetration - A joint root condition in a groove weld in which weld metal does not extend through the joint thickness
Partial joint penetration groove welds are commonly specified in lowly loaded structures.  However, incomplete joint penetration when a full penetration joint is required, as depicted above, would be cause for rejection.  A fix for an incomplete penetration joint would be to back gouge and weld from the other side.   Another acceptable partial penetration joint is shown below.
	Partial penetration joint on the left without discontinuities is an acceptable condition where appropriate.  Appropriate engineering decisions need to be applied to determine what type of joint should be specified for a given application.

Engineering should be contacted to determine whether partial penetration of full penetration joints are appropriate for a particular situation.

Above are several different representations of weld Cracking

Below is a representation of a convex fillet weld without discontinuities.

 

If your company is experiencing these or other welding problems you can retain AMC to improve your weld processing.  Hire our consultants to act as your welding specialist.   

 

 

Defects/imperfections in welds - reheat cracking

Brittle fracture in CrMoV steel pressure vessel probably caused through poor toughness, high residual stresses and hydrogen cracking

The characteristic features and principal causes of reheat cracking are described. General guidelines on 'best practice' are given so that welders can minimise the risk of reheat cracking in welded fabrications.

Identification

Visual appearance

Reheat cracking may occur in low alloy steels containing alloying additions of chromium, vanadium and molybdenum when the welded component is being subjected to post weld heat treatment, such as stress relief heat treatment, or has been subjected to high temperature service (typically 350 to 550°C).

Cracking is almost exclusively found in the coarse grained regions of the heat affected zone (HAZ) beneath the weld, or cladding, and in the coarse grained regions within the weld metal. The cracks can often be seen visually, usually associated with areas of stress concentration such as the weld toe.

Cracking may be in the form of coarse macro-cracks or colonies of micro-cracks.

A macro-crack will appear as a 'rough' crack, often with branching, following the coarse grain region, (Fig. 1a). Cracking is always intergranular along the prior austenite grain boundaries (Fig. 1b). Macro-cracks in the weld metal can be oriented either longitudinal or transverse to the direction of welding. Cracks in the HAZ, however, are always parallel to the direction of welding.

 

	Fig.1a. Cracking associated with the coarse grained heat affected zone
	Fig.1b. Intergranular morphology of reheat cracks

Micro-cracking can also be found both in the HAZ and within the weld metal. Micro-cracks in multipass welds will be found associated with the grain coarsened regions which have not been refined by subsequent passes.

Causes

The principal cause is that when heat treating susceptible steels, the grain interior becomes strengthened by carbide precipitation forcing the relaxation of residual stresses by creep deformation at the grain boundaries.

The presence of impurities which segregate to the grain boundaries and promote temper embrittlement eg sulphur, arsenic, tin and phosphorus, will increase the susceptibility to reheat cracking.

The joint design can increase the risk of cracking. For example, joints likely to contain stress concentration, such as partial penetration welds, are more liable to initiate cracks.

The welding procedure also has an influence. Large weld beads are undesirable as they produce a coarse grained HAZ which is less likely to be refined by the subsequent pass and therefore will be more susceptible to reheat cracking.

Best practice in prevention

The risk of reheat cracking can be reduced through the choice of steel, specifying the maximum impurity level and by adopting a more tolerant welding procedure / technique.

Steel choice

If possible, avoid welding steels known to be susceptible to reheat cracking. For example, A 508 Class 2 is known to be particularly susceptible to reheat cracking whereas cracking associated with welding and cladding in A508 Class 3 is largely unknown. The two steels have similar mechanical properties but A508 Class 3 has a lower Cr content and a higher manganese content.

Similarly, in the higher strength, creep resistant steels, an approximate ranking of their crack susceptibility is as follows:

 

5 Cr 1Mo	lower risk
2.25Cr 1 Mo
0.5Mo B
0.5Cr 0.5Mo 0.25V	higher risk

Thus, in selecting a creep resistant, chromium molybdenum steel, 0.5Cr 0.5Mo 0.25V steel is known to be susceptible to reheat cracking but the 2.25Cr 1Mo which has a similar creep resistance, is significantly less susceptible.

Unfortunately, although some knowledge has been gained on the susceptibility of certain steels, the risk of cracking cannot be reliably predicted from the chemical composition. Various indices, including G1, P_SR and Rs, have been used to indicate the susceptibility of steel to reheat cracking. Steels which have a value of G of less than 2, P_SR less than zero or Rs less than 0.03, are less susceptible to reheat cracking

 

G1	=	10C + Cr + 3.3Mo + 8.1V - 2
PSR	=	Cr +Cu + 2Mo + 10V +7Nb + 5Ti - 2
Rs	=	0.12Cu +0.19S +0.10As + P +1.18Sn + 1.49Sb

Impurity level

Irrespective of the steel type, it is important to purchase steels specified to have low levels of trace elements (antimony, arsenic, tin and phosphorus). It is generally accepted that the total level of impurities in the steel should not exceed 0.01% to minimise the risk of temper embrittlement.

Welding procedure and technique

The welding procedure can be used to minimise the risk of reheat cracking by

• Producing the maximum refinement of the coarse grain HAZ
• Limiting the degree of austenite grain growth
• Eliminating stress concentrations

The procedure should aim to refine the coarse grained HAZ by subsequent passes. In butt welds, maximum refinement can be achieved by using a steep sided joint preparation with a low angle of attack to minimise penetration into the sidewall, (Fig 2a). In comparison, a larger angle V preparation produces a wider HAZ limiting the amount of refinement achieved by subsequent passes, (Fig 2b). Narrow joint preparations, however, are more difficult to weld due to the increased risk of lack of sidewall fusion.

	Fig.2a. Welding in the flat position - high degree of HAZ refinement
	Fig.2b. Welding in the horizontal/vertical position - low degree of HAZ refinement

Refinement of the HAZ can be promoted by first buttering the surface of the susceptible plate with a thin weld metal layer using a small diameter (3.2mm) electrode. The joint is then completed using a larger diameter (4 - 4.8mm) electrode which is intended to generate sufficient heat to refine any remaining coarse grained HAZ under the buttered layer.

The degree of austenite grain growth can be restricted by using a low heat input. However, precautionary measures may be necessary to avoid the risk of hydrogen assisted cracking and lack-of-fusion defects. For example, reducing the heat input will almost certainly require a higher preheat temperature to avoid hydrogen assisted cracking.

The joint design and welding technique adopted should ensure that the weld is free from localised stress concentrations which can arise from the presence of notches. Stress concentrations may be produced in the following situations:

• welding with a backing bar
• a partial penetration weld leaving a root imperfection
• internal weld imperfections such as lack of sidewall fusion
• the weld has a poor surface profile, especially sharp weld toes

The weld toes of the capping pass are particularly vulnerable as the coarse grained HAZ may not have been refined by subsequent passes. In susceptible steel, the last pass should never be deposited on the parent material but always on the weld metal so that it will refine the HAZ.

Grinding the weld toes with the preheat maintained has been successfully used to reduce the risk of cracking in 0.5Cr 0.5Mo 0.25V steels.

Copyright 2000, TWI Ltd

 

Defects - lamellar tearing

BP Forties platform lamellar tears were produced when attempting the repair of lack of root penetration in a brace weld

Lamellar tearing can occur beneath the weld especially in rolled steel plate which has poor through-thickness ductility. The characteristic features, principal causes and best practice in minimising the risk of lamellar tearing are described.

Identification

Visual appearance

The principal distinguishing feature of lamellar tearing is that it occurs in T-butt and fillet welds normally observed in the parent metal parallel to the weld fusion boundary and the plate surface , (Fig 1). The cracks can appear at the toe or root of the weld but are always associated with points of high stress concentration.

Fracture face

The surface of the fracture is fibrous and 'woody' with long parallel sections which are indicative of low parent metal ductility in the through-thickness direction, (Fig 2).

	Fig. 1. Lamellar tearing in T butt weld
	Fig. 2. Appearance of fracture face of lamellar tear

Metallography

As lamellar tearing is associated with a high concentration of elongated inclusions oriented parallel to the surface of the plate, tearing will be transgranular with a stepped appearance.

Causes

It is generally recognised that there are three conditions which must be satisfied for lamellar tearing to occur:

1. Transverse strain - the shrinkage strains on welding must act in the short direction of the plate ie through the plate thickness
2. Weld orientation - the fusion boundary will be roughly parallel to the plane of the inclusions
3. Material susceptibility - the plate must have poor ductility in the through-thickness direction

Thus, the risk of lamellar tearing will be greater if the stresses generated on welding act in the through-thickness direction. The risk will also increase the higher the level of weld metal hydrogen

Factors to be considered to reduce the risk of tearing

The choice of material, joint design, welding process, consumables, preheating and buttering can all help reduce the risk of tearing.

Material

Fig. 3. Relationship between the STRA and sulphur content for 12.5 to 50mm thick plate

Tearing is only encountered in rolled steel plate and not forgings and castings. There is no one grade of steel that is more prone to lamellar tearing but steels with a low Short Transverse Reduction in Area (STRA) will be susceptible. As a general rule, steels with STRA over 20% are essentially resistant to tearing whereas steels with below 10 to 15% STRA should only be used in lightly restrained joints (Fig. 3).

Steels with a higher strength have a greater risk especially when the thickness is greater than 25mm. Aluminium treated steels with low sulphur contents (<0.005%) will have a low risk.

Steel suppliers can provide plate which has been through-thickness tested with a guaranteed STRA value of over 20%.

Joint Design

Lamellar tearing occurs in joints producing high through-thickness strain, eg T joints or corner joints. In T or cruciform joints, full penetration butt welds will be particularly susceptible. The cruciform structures in which the susceptible plate cannot bend during welding will also greatly increase the risk of tearing.

In butt joints, as the stresses on welding do not act through the thickness of the plate, there is little risk of lamellar tearing.

As angular distortion can increase the strain in the weld root and or toe, tearing may also occur in thick section joints where the bending restraint is high.

Several examples of good practice in the design of welded joints are illustrated in Fig. 4.

• As tearing is more likely to occur in full penetration T butt joints, if possible, use two fillet welds, Fig. 4a.
• Double-sided welds are less susceptible than large single-sided welds and balanced welding to reduce the stresses will further reduce the risk of tearing especially in the root, Fig. 4b
• Large single-side fillet welds should be replaced with smaller double-sided fillet welds, Fig. 4c
• Redesigning the joint configuration so that the fusion boundary is more normal to the susceptible plate surface will be particularly effective in reducing the risk, Fig. 4d

Fig. 4 Recommended joint configurations to reduce the risk of lamellar tearing
Fig. 4a
Fig. 4b
Fig. 4c
Fig. 4d

Weld size

Lamellar tearing is more likely to occur in large welds typically when the leg length in fillet and T butt joints is greater than 20mm. As restraint will contribute to the problem, thinner section plate which is less susceptible to tearing, may still be at risk in high restraint situations.

Welding process

As the material and joint design are the primary causes of tearing, the choice of welding process has only a relatively small influence on the risk. However, higher heat input processes which generate lower stresses through the larger HAZ and deeper weld penetration can be beneficial.

As weld metal hydrogen will increase the risk of tearing, a low hydrogen process should be used when welding susceptible steels.

Consumable

Where possible, the choice of a lower strength consumable can often reduce the risk by accommodating more of the strain in the weld metal. A smaller diameter electrode which can be used to produce a smaller leg length, has been used to prevent tearing.

A low hydrogen consumable will reduce the risk by reducing the level of weld metal diffusible hydrogen. The consumables must be dried in accordance with the manufacturer's recommendations.

Preheating

Preheating will have a beneficial effect in reducing the level of weld metal diffusible hydrogen. However, it should be noted that in a restrained joint, excessive preheating could have a detrimental effect by increasing the level the level of restraint produced by the contraction across the weld on cooling.

Preheating should, therefore, be used to reduce the hydrogen level but it should be applied so that it will not increase the amount of contraction across the weld.

Buttering

Buttering the surface of the susceptible plate with a low strength weld metal has been widely employed. As shown for the example of a T butt weld (Fig. 5) the surface of the plate may be grooved so that the buttered layer will extend 15 to 25mm beyond each weld toe and be about 5 to 10mm thick.

Fig. 5. Buttering with low strength weld metal
	a) general deposit on the surface of the susceptible plate
	b) in-situ buttering

In-situ buttering ie where the low strength weld metal is deposited first on the susceptible plate before filling the joint, has also been successfully applied. However, before adopting this technique, design calculations should be carried out to ensure that the overall weld strength will be acceptable.

Acceptance standards

As lamellar tears are linear imperfections which have sharp edges, they are not permitted for welds meeting the quality levels B, C and D in accordance with the requirements of BS EN 25817 (ISO 5817).

Detection and remedial action

If surface-breaking, lamellar tears can be readily detected using visual examination, liquid penetrant or magnetic particle testing techniques. Internal cracks require ultrasonic examination techniques but there may be problems in distinguishing lamellar tears from inclusion bands. The orientation of the tears normally makes them almost impossible to detect by radiography.

Copyright 2000, TWI Ltd

 

Defects - hydrogen cracks in steels - identification

Preheating to avoid hydrogen cracking

Hydrogen cracking may also be called cold cracking or delayed cracking. The principal distinguishing feature of this type of crack is that it occurs in ferritic steels, most often immediately on welding or after a short time after welding.

In this issue, the characteristic features and principal causes of hydrogen cracks are described.

Identification

Visual appearance

Hydrogen cracks can be usually be distinguished due to the following characteristics:

• In C-Mn steels, the crack will normally originate in the heat affected zone (HAZ) but may extend into the weld metal (Fig 1).
• Cracks can also occur in the weld bead, normally transverse to the welding direction at an angle of 45° to the weld surface. They are essentially straight, follow a jagged path but may be non-branching.
• In low alloy steels, the cracks can be transverse to the weld, perpendicular to the weld surface, but are non-branching and essentially planar.

 

Fig. 1 Hydrogen cracks originating in the HAZ (note, the type of cracks shown would not be expected to form in the same weldment)

On breaking open the weld (prior to any heat treatment), the surface of the cracks will normally not be oxidised, even if they are surface breaking, indicating they were formed when the weld was at or near ambient temperature. A slight blue tinge may be seen from the effects of preheating or welding heat.

Metallography

Cracks which originate in the HAZ are usually associated with the coarse grain region, (Fig 2). The cracks can be intergranular, transgranular or a mixture. Intergranular cracks are more likely to occur in the harder HAZ structures formed in low alloy and high carbon steels. Transgranular cracking is more often found in C-Mn steel structures.

In fillet welds, cracks in the HAZ are usually associated with the weld root and parallel to the weld. In butt welds, the HAZ cracks are normally oriented parallel to the weld bead.

Fig. 2 Crack along the coarse grain structure in the HAZ

Causes

There are three factors which combine to cause cracking:

• hydrogen generated by the welding process
• a hard brittle structure which is susceptible to cracking
• residual tensile stresses acting on the welded joint

Cracking is caused by the diffusion of hydrogen to the highly stressed, hardened part of the weldment.

In C-Mn steels, because there is a greater risk of forming a brittle microstructure in the HAZ, most of the hydrogen cracks are to be found in the parent metal. With the correct choice of electrodes, the weld metal will have a lower carbon content than the parent metal and, hence, a lower carbon equivalent (CE). However, transverse weld metal cracks can occur especially when welding thick section components.

In low alloy steels, as the weld metal structure is more susceptible than the HAZ, cracking may be found in the weld bead.

The effects of specific factors on the risk of cracking are::

• weld metal hydrogen
• parent material composition
• parent material thickness
• stresses acting on the weld
• heat input

Weld metal hydrogen content

The principal source of hydrogen is the moisture contained in the flux ie the coating of MMA electrodes, the flux in cored wires and the flux used in submerged arc welding. The amount of hydrogen generated is determined mainly by the electrode type. Basic electrodes normally generate less hydrogen than rutile and cellulosic electrodes.

It is important to note that there can be other significant sources of hydrogen eg moisture from the atmosphere or from the material where processing or service history has left the steel with a significant level of hydrogen. Hydrogen may also be derived from the surface of the material or the consumable.

Sources of hydrogen will include:

• oil, grease and dirt
• rust
• paint and coatings
• cleaning fluids

Parent metal composition

This will have a major influence on hardenability and, with high cooling rates, the risk of forming a hard brittle structure in the HAZ. The hardenability of a material is usually expressed in terms of its carbon content or, when other elements are taken into account, its carbon equivalent (CE) value.

 

The higher the CE value, the greater the risk of hydrogen cracking. Generally, steels with a CE value of <0.4 are not susceptible to HAZ hydrogen cracking as long as low hydrogen welding consumables or processes are used.

Parent material thickness

Material thickness will influence the cooling rate and therefore the hardness level, microstructure produced in the HAZ and the level of hydrogen retained in the weld.

The 'combined thickness' of the joint, ie the sum of the thicknesses of material meeting at the joint line, will determine, together with the joint geometry, the cooling rate of the HAZ and its hardness. Consequently, as shown in Fig. 3, a fillet weld will have a greater risk than a butt weld in the same material thickness.

 

Fig.3 Combined thickness measurements for butt and fillet joints

Stresses acting on the weld

The stresses generated across the welded joint as it contracts will be greatly influenced by external restraint, material thickness, joint geometry and fit-up. Areas of stress concentration are more likely to initiate a crack at the toe and root of the weld.

Poor fit-up in fillet welds markedly increases the risk of cracking. The degree of restraint acting on a joint will generally increase as welding progresses due to the increase in stiffness of the fabrication.

Heat input

The heat input to the material from the welding process, together with the material thickness and preheat temperature, will determine the thermal cycle and the resulting microstructure and hardness of both the HAZ and weld metal.

A high heat input will reduce the hardness level.

Heat input per unit length is calculated by multiplying the arc energy by an arc efficiency factor according to the following formula:

 

V = arc voltage (V)
A = welding current (A)
S = welding speed (mm/min)
k = thermal efficiency factor

In calculating heat input, the arc efficiency must be taken into consideration. The arc efficiency factors given in BS EN 1011-1: 1998 for the principal arc welding processes, are:

 

Submerged arc (single wire)	1.0
MMA	0.8
MIG/MAG and flux cored wire	0.8
TIG and plasma	0.6

In MMA welding, heat input is normally controlled by means of the run-out length from each electrode which is proportional to the heat input. As the run-out length is the length of weld deposited from one electrode, it will depend upon the welding technique eg weave width /dwell.

Bill Lucas prepared this article with help from Gene Mathers and Dave Abson.

Copyright by TWI Ltd, 2000

 

Defects - solidification cracking

Weld repair on a cast iron exhaust manifold

A crack may be defined as a local discontinuity produced by a fracture which can arise from the stresses generated on cooling or acting on the structure. It is the most serious type of imperfection found in a weld and should be removed. Cracks not only reduce the strength of the weld through the reduction in the cross section thickness but also can readily propagate through stress concentration at the tip, especially under impact loading or during service at low temperature.

Identification

Visual appearance

Solidification cracks are normally readily distinguished from other types of cracks due to the following characteristic factors:

• they occur only in the weld metal
• they normally appear as straight lines along the centreline of the weld bead, as shown in Fig. 1, but may occasionally appear as transverse cracking depending on the solidification structure
• solidification cracks in the final crater may have a branching appearance
• as the cracks are 'open', they are easily visible with the naked eye

 

Fig. 1 Solidification crack along the centre line of the weld

On breaking open the weld, the crack surface in steel and nickel alloys may have a blue oxidised appearance, showing that they were formed while the weld metal was still hot.

Metallography

The cracks form at the solidification boundaries and are characteristically inter dendritic. The morphology reflects the weld solidification structure and there may be evidence of segregation associated with the solidification boundary.

Causes

The overriding cause of solidification cracking is that the weld bead in the final stage of solidification has insufficient strength to withstand the contraction stresses generated as the weld pool solidifies. Factors which increase the risk include:

• insufficient weld bead size or shape
• welding under high restraint
• material properties such as a high impurity content or a relatively large amount of shrinkage on solidification.

Joint design can have a significant influence on the level of residual stresses. Large gaps between component parts will increase the strain on the solidifying weld metal, especially if the depth of penetration is small. Therefore, weld beads with a small depth-to-width ratio, such as formed in bridging a large gap with a wide, thin bead, will be more susceptible to solidification cracking, as shown in Fig. 2. In this case, the centre of the weld which is the last part to solidify, is a narrow zone with negligible cracking resistance.

 

Fig. 2 Weld bead penetration too small

Segregation of impurities to the centre of the weld also encourages cracking. Concentration of impurities ahead of the solidifying front weld forms a liquid film of low freezing point which, on solidification, produces a weak zone. As solidification proceeds, the zone is likely to crack as the stresses through normal thermal contraction build up. An elliptically shaped weld pool is preferable to a tear drop shape. Welding with contaminants such as cutting oils on the surface of the parent metal will also increase the build up of impurities in the weld pool and the risk of cracking.

As the compositions of the plate and the filler determine the weld metal composition they will, therefore, have a substantial influence on the susceptibility of the material to cracking.

Steels

Cracking is associated with impurities, particularly sulphur and phosphorus, and is promoted by carbon whereas manganese and silicon can help to reduce the risk. To minimise the risk of cracking, fillers with low carbon and impurity levels and a relatively high manganese content are preferred. As a general rule, for carbon-manganese steels, the total sulphur and phosphorus content should be no greater than 0.06%.

Weld metal composition is dominated by the consumable and as the filler is normally cleaner than the metal being welded, cracking is less likely with low dilution processes such as MMA and MIG. Plate composition assumes greater importance in high dilution situations such as when welding the root in butt welds, using an autogenous welding technique like TIG, or a high dilution process such as submerged arc welding.

In submerged arc welds, as described in BS 5135 (Appendix F), the cracking risk may be assessed by calculating the Units of Crack Susceptibility (UCS) from the weld metal chemical composition (weight %):

UCS = 230C* + 190S + 75P + 45Nb - 12.3Si - 5.4Mn - 1
C* = carbon content or 0.08 whichever is higher

Although arbitrary units, a value of <10 indicates high cracking resistance whereas >30 indicates a low resistance. Within this range, the risk will be higher in a weld run with a high depth to width ratio, made at high welding speeds or where the fit-up is poor. For fillet welds, runs having a depth to width ratio of about one, UCS values of 20 and above will indicate a risk of cracking. For a butt weld, values of about 25 UCS are critical. If the depth to width ratio is decreased from 1 to 0.8, the allowable UCS is increased by about nine. However, very low depth to width ratios, such as obtained when penetration into the root is not achieved, also promote cracking.

Aluminium

The high thermal expansion (approximately twice that of steel) and substantial contraction on solidification (typically 5% more than in an equivalent steel weld) means that aluminium alloys are more prone to cracking. The risk can be reduced by using a crack resistant filler (usually from the 4xxx and 5xxx series alloys) but the disadvantage is that the resulting weld metal is likely to have non-matching properties such as a lower strength than the parent metal.

Austenitic Stainless Steel

A fully austenitic stainless steel weld is more prone to cracking than one containing between 5-10% of ferrite. The beneficial effect of ferrite has been attributed to its capacity to dissolve harmful impurities which would otherwise form low melting point segregates and consequently interdendritic cracks. Therefore the choice of filler material is important to suppress cracking so a type 308 filler is used to weld type 304 stainless steel.

Best practice in avoiding solidification cracking

Apart from the choice of material and filler, the principal techniques for minimising the risk of welding solidification cracking are:

• Control joint fit-up to reduce gaps.
• Before welding, clean off all contaminants from the material
• Ensure that the welding sequence will not lead to a build-up of thermally induced stresses.
• Select welding parameters and technique to produce a weld bead with an adequate depth to width ratio, or with sufficient throat thickness (fillet weld), to ensure the weld bead has sufficient resistance to the solidification stresses (recommend a depth to width ratio of at least 0.5:1).
• Avoid producing too large a depth to width ratio which will encourage segregation and excessive transverse strains in restrained joints. As a general rule, weld beads whose depth to weld ratio exceeds 2:1 will be prone to solidification cracking.
• Avoid high welding speeds (at high current levels) which increase the amount of segregation and the stress level across the weld bead.
• At the run stop, ensure adequate filling of the crater to avoid an unfavourable concave shape.

Acceptance standards

As solidification cracks are linear imperfections with sharp edges, they are not permitted for welds meeting the quality levels B, C and D in accordance with the requirements of BS EN 25817 (ISO 5817). Crater cracks are permitted for quality level D.

Detection and remedial action

Surface breaking solidification cracks can be readily detected using visual examination, liquid penetrant or magnetic particle testing techniques. Internal cracks require ultrasonic or radiographic examination techniques.

Most codes will specify that all cracks should be removed. A cracked component should be repaired by removing the cracks with a safety margin of approximately 5mm beyond the visible ends of the crack. The excavation is then re-welded using a filler which will not produce a crack sensitive deposit.

Copyright by TWI, 1999

 

Defects/imperfections in welds - slag inclusions

Prevention of slag inclusions by grinding between runs

The characteristic features and principal causes of slag imperfections are described.

Identification

Fig. 1. Radiograph of a butt weld showing two slag lines in the weld root

Slag is normally seen as elongated lines either continuous or discontinuous along the length of the weld. This is readily identified in a radiograph, Fig 1. Slag inclusions are usually associated with the flux processes, ie MMA, FCA and submerged arc, but they can also occur in MIG welding.

Causes

As slag is the residue of the flux coating, it is principally a deoxidation product from the reaction between the flux, air and surface oxide. The slag becomes trapped in the weld when two adjacent weld beads are deposited with inadequate overlap and a void is formed. When the next layer is deposited, the entrapped slag is not melted out. Slag may also become entrapped in cavities in multi-pass welds through excessive undercut in the weld toe or the uneven surface profile of the preceding weld runs, Fig 2.

As they both have an effect on the ease of slag removal, the risk of slag imperfections is influenced by

• Type of flux
• Welder technique

The type and configuration of the joint, welding position and access restrictions all have an influence on the risk of slag imperfections.

Fig. 2. The influence of welder technique on the risk of slag inclusions when welding with a basic MMA (7018) electrode
	a) Poor (convex) weld bead profile resulted in pockets of slag being trapped between the weld runs
	b) Smooth weld bead profile allows the slag to be readily removed between runs

Type of flux

One of the main functions of the flux coating in welding is to produce a slag which will flow freely over the surface of the weld pool to protect it from oxidation. As the slag affects the handling characteristics of the MMA electrode, its surface tension and freezing rate can be equally important properties. For welding in the flat and horizontal/vertical positions, a relatively viscous slag is preferred as it will produce a smooth weld bead profile, is less likely to be trapped and, on solidifying, is normally more easily removed. For vertical welding, the slag must be more fluid to flow out to the weld pool surface but have a higher surface tension to provide support to the weld pool and be fast freezing.

The composition of the flux coating also plays an important role in the risk of slag inclusions through its effect on the weld bead shape and the ease with which the slag can be removed. A weld pool with low oxygen content will have a high surface tension producing a convex weld bead with poor parent metal wetting. Thus, an oxidising flux, containing for example iron oxide, produces a low surface tension weld pool with a more concave weld bead profile, and promotes wetting into the parent metal. High silicate flux produces a glass-like slag, often self detaching. Fluxes with a lime content produce an adherent slag which is difficult to remove.

The ease of slag removal for the principal flux types are:

• Rutile or acid fluxes - large amounts of titanium oxide (rutile) with some silicates. The oxygen level of the weld pool is high enough to give flat or slightly convex weld bead. The fluidity of the slag is determined by the calcium fluoride content. Fluoride-free coatings designed for welding in the flat position produce smooth bead profiles and an easily removed slag. The more fluid fluoride slag designed for positional welding is less easily removed.
• Basic fluxes - the high proportion of calcium carbonate (limestone) and calcium fluoride (fluospar) in the flux reduces the oxygen content of the weld pool and therefore its surface tension. The slag is more fluid than that produced with the rutile coating. Fast freezing also assists welding in the vertical and overhead positions but the slag coating is more difficult to remove.

Consequently, the risk of slag inclusions is significantly greater with basic fluxes due to the inherent convex weld bead profile and the difficulty in removing the slag from the weld toes especially in multi-pass welds.

Welder technique

Welding technique has an important role to play in preventing slag inclusions. Electrode manipulation should ensure adequate shape and degree of overlap of the weld beads to avoid forming pockets which can trap the slag. Thus, the correct size of electrode for the joint preparation, the correct angle to the workpiece for good penetration and a smooth weld bead profile are all essential to prevent slag entrainment.

In multi-pass vertical welding, especially with basic electrodes, care must be taken to fuse out any remaining minor slag pockets and minimise undercut. When using a weave, a slight dwell at the extreme edges of the weave will assist sidewall fusion and produce a flatter weld bead profile.

Too high a current together with a high welding speed will also cause sidewall undercutting which makes slag removal difficult.

It is crucial to remove all slag before depositing the next run. This can be done between runs by grinding, light chipping or wire brushing. Cleaning tools must be identified for different materials eg steels or stainless steels, and segregated.

When welding with difficult electrodes, in narrow vee butt joints or when the slag is trapped through undercutting, it may be necessary to grind the surface of the weld between layers to ensure complete slag removal.

Best practice

The following techniques can be used to prevent slag inclusions:

• Use welding techniques to produce smooth weld beads and adequate inter-run fusion to avoid forming pockets to trap the slag
• Use the correct current and travel speed to avoid undercutting the sidewall which will make the slag difficult to remove
• Remove slag between runs paying particular attention to removing any slag trapped in crevices
• Use grinding when welding difficult butt joints otherwise wire brushing or light chipping may be sufficient to remove the slag.

Acceptance standards

Slag and flux inclusions are linear defects but because they do not have sharp edges compared with cracks, they may be permitted by specific standards and codes. The limits in steel are specified in BE EN 25817 (ISO 5817) for the three quality levels. Long slag imperfections are not permitted in both butt and fillet welds for Quality Level B (stringent) and C (moderate). For Quality Level D, butt welds can have imperfections providing their size is less than half the nominal weld thickness. Short slag related imperfections are permitted in all three quality levels with limits placed on their size relative to the butt weld thickness or nominal fillet weld throat thickness.

Article prepared by Bill Lucas with help from Gene Mathers and Colin Eileens.

Copyright © 2003, TWI Ltd

Link to related content, selected on the basis of keyword matches using the keyword set on the 'Advanced search' screen

Defects/imperfections in welds - porosity

The characteristic features and principal causes of porosity imperfections are described. Best practice guidelines are given so welders can minimise porosity risk during fabrication.

Identification

Porosity is the presence of cavities in the weld metal caused by the freezing in of gas released from the weld pool as it solidifies. The porosity can take several forms:

• distributed
• surface breaking pores
• wormhole
• crater pipes

Cause and prevention

Distributed porosity and surface pores

Distributed porosity (Fig. 1) is normally found as fine pores throughout the weld bead. Surface breaking pores (Fig. 2) usually indicate a large amount of distributed porosity

	Fig. 1. Uniformly distributed porosity
	Fig. 2. Surface breaking pores (T fillet weld in primed plate)

Cause
Porosity is caused by the absorption of nitrogen, oxygen and hydrogen in the molten weld pool which is then released on solidification to become trapped in the weld metal.

Nitrogen and oxygen absorption in the weld pool usually originates from poor gas shielding. As little as 1% air entrainment in the shielding gas will cause distributed porosity and greater than 1.5% results in gross surface breaking pores. Leaks in the gas line, too high a gas flow rate, draughts and excessive turbulence in the weld pool are frequent causes of porosity.

Hydrogen can originate from a number of sources including moisture from inadequately dried electrodes, fluxes or the workpiece surface. Grease and oil on the surface of the workpiece or filler wire are also common sources of hydrogen.

Surface coatings like primer paints and surface treatments such as zinc coatings, may generate copious amounts of fume during welding. The risk of trapping the evolved gas will be greater in T joints than butt joints especially when fillet welding on both sides (see Fig 2). Special mention should be made of the so-called weldable (low zinc) primers. It should not be necessary to remove the primers but if the primer thickness exceeds the manufacturer's recommendation, porosity is likely to result especially when using welding processes other than MMA.

Prevention

The gas source should be identified and removed as follows:

Air entrainment

- seal any air leak

- avoid weld pool turbulence

- use filler with adequate level of deoxidants

- reduce excessively high gas flow

- avoid draughts

Hydrogen

- dry the electrode and flux

- clean and degrease the workpiece surface

Surface coatings

- clean the joint edges immediately before welding

- check that the weldable primer is below the recommended maximum thickness

Wormholes

Elongated pores or wormholes

Characteristically, wormholes are elongated pores (Fig. 3) which produce a herring bone appearance on the radiograph.

Cause
Wormholes are indicative of a large amount of gas being formed which is then trapped in the solidifying weld metal. Excessive gas will be formed from gross surface contamination or very thick paint or primer coatings. Entrapment is more likely in crevices such as the gap beneath the vertical member of a horizontal-vertical, T joint which is fillet welded on both sides.

When welding T joints in primed plates it is essential that the coating thickness on the edge of the vertical member is not above the manufacturer's recommended maximum, typically 20µ, through over-spraying.

Prevention

Eliminating the gas and cavities prevents wormholes.

Gas generation

- clean the workpiece surfaces

- remove any coatings from the joint area

- check the primer thickness is below the manufacturer's maximum

Joint geometry

- avoid a joint geometry which creates a cavity

Crater pipe

A crater pipe forms during the final solidified weld pool and is often associated with some gas porosity.

Cause
This imperfection results from shrinkage on weld pool solidification. Consequently, conditions which exaggerate the liquid to solid volume change will promote its formation. Switching off the welding current will result in the rapid solidification of a large weld pool.

In TIG welding, autogenous techniques, or stopping the wire before switching off the welding current, will cause crater formation and the pipe imperfection.

Prevention

Crater pipe imperfection can be prevented by removing the stop or by welder technique.

Removal of stop

- use run-off tag in butt joints

- grind out the stop before continuing with the next electrode or depositing the subsequent weld run

Welder technique

- progressively reduce the welding current to reduce the weld pool size

- add filler (TIG) to compensate for the weld pool shrinkage

Porosity susceptibility of materials

Gases likely to cause porosity in the commonly used range of materials are listed in the Table.

Principal gases causing porosity and recommended cleaning methods

Material	Gas	Cleaning
C Mn steel	Hydrogen, Nitrogen and Oxygen	Grind to remove scale coatings
Stainless steel	Hydrogen	Degrease + wire brush + degrease
Aluminium and alloys	Hydrogen	Chemical clean + wire brush + degrease + scrape
Copper and alloys	Hydrogen, Nitrogen	Degrease + wire brush + degrease
Nickel and alloys	Nitrogen	Degrease + wire brush + degrease

Detection and remedial action

If the imperfections are surface breaking, they can be detected using a penetrant or magnetic particle inspection technique. For sub surface imperfections, detection is by radiography or ultrasonic inspection. Radiography is normally more effective in detecting and characterising porosity imperfections. However, detection of small pores is difficult especially in thick sections.

Remedial action normally needs removal by localised gouging or grinding but if the porosity is widespread, the entire weld should be removed. The joint should be re-prepared and re-welded as specified in the agreed procedure.

Copyright by TWI, 1999

 

آلومينيم

Aluminum Welding

Aluminum is the most difficult alloy to weld.  Aluminum oxide should be cleaned from the surface prior to welding.  Aluminum comes in heat treatable and nonheat treatable alloys.  Heat treatable aluminum alloys get their strength from a process called ageing.  Significant decrease in tensile strength can occurs when welding aluminum due to over aging.  For more information on aluminum welding processes, benefits of welding processes, welding discontinuities, or common welding problems please visit our homepage or any of the links to your left.  Take advantage of our aluminum welding experience in developing your welding processes.

Welding Aluminum Alloys

Aluminum Alloys can be divided into nine groups.

Designation	Major Alloying Element
1xxx	Unalloyed (pure) >99% Al
2xxx	Copper is the principal alloying element, though other elements (Magnesium) may be specified
3xxx	Manganese is the principal alloying element
4xxx	Silicon is the principal alloying element
5xxx	Magnesium is the principal alloying element
6xxx	Magnesium and Silicon are principal alloying elements
7xxx	Zinc is the principal alloying element, but other elements such as Copper, Magnesium, Chromium, and Zirconium may be specified
8xxx	Other elements  (including Tin and some Lithium compositions)
9xxx	Reserved for future use

 

Aluminum alloys are readily available in various product forms.  To establish a proper welding procedure it is necessary to know the material properties of the Aluminum alloy being welded.

 

 

Below are some of the factors affecting the welding of Aluminum.

        Aluminum Oxide Coating

        Thermal Conductivity

        Thermal Expansion Coefficient

        Melting Characteristics

 

Wrought Aluminum Alloys

1xxx Series.  These grades of aluminum are characterized by excellent corrosion resistance, high thermal and electrical conductivities, low mechanical properties, and excellent workability. Moderate increases in strength may be obtained by strain hardening. Iron and silicon are the major impurities.

2xxx Series.  These alloys require solution heat treatment to obtain optimum properties; in the solution heat-treated condition, mechanical properties are similar to, and sometimes exceed, those of low-carbon steel. In some instances, precipitation heat treatment (aging) is employed to further increase mechanical properties. This treatment increases yield strength, with attendant loss in elongation; its effect on tensile strength is not as great.

The alloys in the 2xxx series do not have as good corrosion resistance as most other aluminum alloys, and under certain conditions they may be subject to intergranular corrosion.  Alloys in the 2xxx series are good when some strength at moderate temperatures is desired.  These alloys have limited weldability, but some alloys in this series have superior machinability.

3xxx Series.  These alloys generally are non-heat treatable but have about 20% more strength than 1xxx series alloys. Because only a limited percentage of manganese (up to about 1.5%) can be effectively added to aluminum, manganese is used as a major element in only a few alloys.

4xxx Series. The major alloying element in 4xxx series alloys is silicon, which can be added in sufficient quantities (up to 12%) to cause substantial lowering of the melting range.  For this reason, aluminum-silicon alloys are used in welding wire and as brazing alloys for joining aluminum, where a lower melting range than that of the base metal is required.  The alloys containing appreciable amounts of silicon become dark gray to charcoal when anodic oxide finishes are applied and hence are in demand for architectural applications.

5xxx Series. The major alloying element is Magnesium and when it is used as a major alloying element or with manganese, the result is a moderate-to-high-strength work-hardenable alloy.  Magnesium is considerably more effective than manganese as a hardener, about 0.8% Mg being equal to 1.25% Mn, and it can be added in considerably higher quantities.  Alloys in this series possess relatively good welding characteristics and relatively good resistance to corrosion in marine atmospheres.  However, limitations should be placed on the amount of cold work and the operating temperatures permissible for the higher-magnesium alloys to avoid susceptibility to stress-corrosion cracking.

6xxx Series. Alloys in the 6xxx series contain silicon and magnesium approximately in the proportions required for formation of magnesium silicide (Mg2Si), thus making them heat treatable.  Although not as strong as most 2xxx and 7xxx alloys, 6xxx series alloys have relatively good formability, weldability, machinability, and relatively good corrosion resistance, with medium strength.  Alloys in this heat-treatable group are sometimes formed in the T4 temper (solution heat treated but not precipitation heat treated) and strengthened after forming to full T6 properties by precipitation heat treatment.

7xxx Series. Zinc, in amounts of 1 to 8% is the major alloying element in 7xxx series alloys, and when coupled with a smaller percentage of magnesium results in heat-treatable alloys of moderate to high strength. Usually other elements, such as copper and chromium, are also added in small quantities. Some 7xxx series alloys have been used in airframe structures, and other highly stressed parts.  Higher strength 7xxx alloys exhibit reduced resistance to stress corrosion cracking and are often utilized in an overaged temper to provide better combinations of strength, corrosion resistance, and fracture toughness.

Aluminum Welding Services

Visit our homepage for detailed information on arc welding processes, welding procedures, weld failure analysis, and expert witness testimony.  AMC can solve your companies aluminum welding procedure problems.  Hire our consultants to act as your welding specialist.   

 

جوشکاری آلياژ AL-6XN

جوشکاری آلياژ AL-6XN
 

آلياژ AL-6XN يک فلز سوپر آستنيتی با مقاومت خوردگی بسيار عالی و ساختار پايدار تا دمای 540C ميباشد. مزيت ويژه اين فولاد مقاومت عالی آن در برابر خوردگی شياري٫ حفره ای شدن٫ خوردگی ناشی از کلرايد و ترک خوردگی تنشی (SCC) ميباشد که ميتواند جايگزين بسيار مناسبی برای فولادهای زنگ نزن در محيطهای خورنده و حاوی کلرايد باشد. اين آلياژ ابتدا برای مصارف دريايی توليد شد ولی اکنون در صنايع متفاوتی از جمله صنايع غذايی٫ دارويی و شيميايی مورد مصرف پيدا کرده است.

اين آلياژ در دمای ۶۵۰ تا 980C فاز چی (Chi Phase) (ترکيب کرم-آهن-موايبدن) در راستاس مرزدانه ها تشکيل شده و نواحی اطراف را از موليبدن و کرم فقير ميسازد. اين موضوع باعث ايجاد خوردگی بين دانه ای ميگردد. برای کاهش اين اثر به ترکيب اين آلياژ نيتروژن افزوده ميگردد تا اين تغيير فاز را کاهش داده و مقاومت خوردگی را بهبود بخشد. اين موضوع هنگام جوشکاری از اهميت بالايی برخوردار ميگردد و لذا برای جوشکاری بايد نکات خاصی را رعايت نمود.

نکات جوشکاری :
در جوشكاري لوله در سايت از رينگهاي جوشكاري با عناصرآلياژي بيشتر از فلز پايه استفاده ميشود. برای ديگر جوشکاريها ميتوان از رينگ و يا سيم جوشهای خاص استفاده نمود. فلز جوش بايد دارای موليبدن بيشتری نسبت به فلز پايه باشد تا کاهش موليبدن فلز پايه هنگام سرد شدن را جبران نمايد. معمولا سيم جوش حاوی ۹٪ موليبدن (آلياژ ۶۲۵) مناسب است٫ اما ميتوان از سيم جوشهای ديگر نيز استفاده کرد (جدول زير).
 

Welding Process Designations			Consumables
			Filler Metal Alloy	Specifications		Classifications
AWS	Common	Form		AWS	ASME	AWS	UNS
GTAW	TIG	Bare Welding Rods and Wire	625 276 22	A5.14 A5.14 A5.14	SFA5.14 SFA5.14 SFA5.14	ERNiCrMo-3 ERNiCrMo-4 ERNiCrMo-10	N06625 N10276 N06022
GMAW	MIG	Bare Welding Rods and Wire	625 276 22	A5.14 A5.14 A5.14	SFA5.14 SFA5.14 SFA5.14	ERNiCrMo-3 ERNiCrMo-4 ERNiCrMo-10	N06625 N10276 N06022
SMW	Stick or Covered Electrodes	Coating Electrodes	112 276 22	A5.11 A5.11 A5.11	SFA5.11 SFA5.11 SFA5.11	ERNiCrMo-3 ERNiCrMo-4 ERNiCrMo-10	W86112 W80276 W86022

بايد از گاز خنثی بعنوان گاز تورچ و گاز محافظ استفاده کرد. هر دو گاز آرگون و هليوم قابل استفاده ميباشند اما استفاده از آرگون معمولتر است. ميتوان ۳ تا ۵٪ نيتروژن به گاز اضافه کرد تا جبران مقدار نيتروژن سوخته شده فلز پايه حين جوشکاری را جبران نمايد.
حرارت ورودی بايد تا جای ممکن کم باشد بطوريکه کمترين تاثير را بر منطقه جوش و HAZ گذاشته و تشکيل اکسيدهای رنگی اطراف جوش حداقل گردد. اکسيدهای تيره ايجاد شده روی سطح بايد با گرد اکسيد آلومينيوم و اسيد شويی برطرف شوند. درصورت عدم تميزکاری مناسب سطح و باقيماندن لايه های اکسيدی مقاومت به خوردگی کاهش ميابد.
فلز پايه نبايد پيشگرم گردد مگر در مواقعی که دمای قطعه کمتر از 10C باشد. درصورتيکه دمای قطعه زير نقطه شبنم باشد بايد آنرا به آرامی تا بالای نقطه شبنم گرم کرد و از نشست رطوبت روی سطح جلوگيری نمود.
جوشکاری بايد در ناحيه جوش استارت شود. درصورتيکه اين امر غير ممکن باشد بايد ناحيه قوس پس از انجام جوشکاری با سنگ زنی بطور کامل برداشته شود.

 

گازهاي محافظ در جوشكاري TIG:

گازهاي محافظ در جوشكاري TIG :

گازهاي محافظي كه در كپسولهل ذخيره ميشوند ميتوانند گاز خالص ( تك گاز)، مخلوطي از دوگاز ( مخلوطهاي دوتايي معروف)، يا مخلوطي از سه گاز ( مخلوطهاي سه تايي معروف) باشند.

براي جوشكاري تيگ معمولا گازهاي خنثي مانند آرگون يا هليوم يا مخلوط آن دو براي حفاظت بكار ميروند، كه اغلب در فرآيند تيگ از گازهاي مخلوط خنثي استفاده ميشود، در بعضي موارد هم از مخلوطي كه كمي گاز فعال دارد استفاده ميشود (مانند مخلوط آرگون اكسيژن و… ).   

هنگام جوشكاري با پروسه ميگ MIG گازهاي خنثي خالص در جوشكاري فولاد، قوس با مشخصات خوب فراهم نميكنند، در حاليكه گاز دي اكسيد كربنCO2  خالص كه گازي فعال است، قوسي با مشخصات خوب فراهم ميكند. همچنين در فرآيند ميگ MIG آرگون با مقدار كمي اكسيژن خصوصيات نفوذ را بهبود بخشيده و مهره جوش را كنترل ميكند ( ظاهر جوش خوبي ميدهد). و همچنين سوختگي كناره جوش، ناشي از عمل خيس شدگي را رفع ميكند.

مخلوط گازهاي آرگون و دي اكسيد كربن CO2)) مخلوط خوبي براي جوشكاري فولاد است. مخلوط سه تايي گازهاي آرگون، دي اكسيد كربن و اكسيژن يا مخلوطهاي سه تايي آرگون، دي اكسيد كربن وهليوم تركيبات ويژه أي هستند كه در فرآيندهاي تيگ و ميگ براي جوشكاريهاي خاص فلزاتي با فلزپايه پيچيده بكار ميروند.     

گاز آرگون:

آرگون گازي است بي رنگ، بي بو، بي مزه و بطور نسبي در مقايسه با گازهاي بي اثر ديگر فراوانتراست. گازآرگون گاز فرعي كه درهوا وجود دارد ( هر يك ميليون فوت مكعب هوا شامل 93 هزار فوت مكعب گاز آرگون است و همچنين گاز آرگون 1.4 برابراز هوا و 10 برابراز هليوم سنگينتر است).

يكي از روشهاي توليد گاز آرگون اين است كه ابتدا هوا را در زير فشار ودر دماي پايين به مايع تبديل ميكنند، سپس با بالا بردن (گرم كردن) دما مايع اجازه مي دهند تا مايع تبخير شود. آرگون در دماي 184 – درجه سانتيگراد ( 302 – درجه فارانهايت ) به مايع تبديل ميشود. درصد خلوص آرگون بايد تقريبا 99.99% درصد باشد. آرگون از هوا سنگين تر( چگالتر، چگالي KG/M3  1.784 كيلوگرم بر متر مكعب است و 23%از هوا سنگين تر است)، و براي همين آرگون براي حفاظت جوش در عمق شيار مناسب است و بايد در نظر داشته باشيم كه هنگاميكه ما جوش بالا سر مي دهيم نبايد از آرگون بعنوان گاز محافظ استفاده كنيم. 

آرگون در جوشكاري فلزات غير آهني ( مانند آلومينيم، منيزيم، برليم و مس) در فرآيندهاي ميگ و تيگ مانند يك محيط محافظ عمل ميكند. آرگون بخاطر اينكه ولتاژ يونيزاسيون پاييني دارد( ولتاژيونيزاسيون اوليه 15.45 ولت ) و به آساني و سريع يونيزه ميشود، اين امكان را فراهم مي سازد كه قوس به راحتي برقرار شده و پايدار بماند و بنابراين مناسب است براي كار با جريان AC ، و همچنين گاز آرگون  شروع قوس را در جريان AC آسانتر ميكند.

گاز آرگون يك ستون قوس جمع شده ومتمركز توليد ميكند و نسبت به گازهاي ديگر قابليت هدايت حرارتش كمتر است. بدليل اينكه گاز آرگون باعث تثبيت ( ثابت نگه داشتن قوس) ميشود، در بيشتر مخلوط گازهاي محافظ از آن استفاده ميشود.            

با اينكه گاز آرگون سمي نيست اما در مكانهايي كه جريان هوا وجود ندارد يا محدود است ( مثلا تانكر ها وجاهاي بسته) باعث خفگي ميشود. همچنين كارهاي تجربي روي مقاطع نازك آلياژهاي مقاوم به حرارت نشان داده است كه آرگون براي جوشكاريهاي دستي از هليوم بهتر است.

مخلوط آرگون با 1% يا 2% اكسيژن:

افزودن مقدار كمي اكسيژن به آرگون دماي قوس را بالا مي برد و اكسيژن مانند يك عامل خيس كننده در حوضچه مذاب عمل ميكند، همچنين اكسيژن سياليت مذاب را بيشتر كرده و قوس را تثبيت ميكند. اكسيژن سبب كاهش كشش سطحي ميشود و نفوذ و ذوب خوبي توليد ميكند.

در فرآيند تيگ افزايش خيلي كم اكسيژن ( كمتر از1% ) به تقويت قوس كمك ميكند. اكسيژني كه معمولا اضافه ميشود مقدارش 1% تا 2 %  يا  3% تا 5% است. اكسيژن باعث ميشود كه انتقال مذاب بصورت اسپري انجام شود.

مخلوط غني از قبيل آرگون و تا حدود 25% دي اكسيد كربن CO2 با افزايش اكسيژن، انتقال فلز را بصورت گلوله اي براي جوشكاري ورقه هاي نازك و فولاد ميسازد، مخلوط آرگون +1.2% اكسيژن بكار ميرود براي فولاد زنگ نزن ( استيل ) و مخلوط آرگون + 1% اكسيژن براي جوشكاري فولاد زنگ نزن (استيل) به روش پالس و اسپري بكار ميرود و همچنين مخلوط آرگون + 2% اكسيژن براي جوشكاري با روش گلوله أي بكار ميرود.

نكته قابل توجه در مورد اكسيژن اين است كه اكسيژن، از افزايش ضرر و زيانهاي ناشي از منگنز و سيليسيم جلوگيري ميكند.         

آرگون + هيدروژن:

با افزودن مقدار كمي هيدروژن به آرگون، ولتاژ و حرارت قوس افزايش مي يابد. مخلوطهاي آرگون كه شامل تقريبا 5% هيدروژن هستند براي جوشكاري نيكل و آلياژهاي نيكل و براي جوشكاري مقاطع بزرگ فولادهاي زنگ نزن آوستنيتي ( استيل ) بكار ميروند.

مخلوط آرگون با 25% هيدروژن براي جوشكاري فلزات ضخيم كه ضريب حرارتي بالايي دارند، ازقبيل مس بكار ميرود. اين مخروط يك مزيت در جوشكاري اتوماتيك با سرعت بالا، محسوب ميشود. افزايش هيدروژن نمي تواند براي جوشكاري فولادهاي كم آلياژي و ميان آلياژي و فولادهاي ساده كربني و سختي پذير بكار رود واين بخاطر خطر بروز نقص هيدروژن تردي و مشكلات ناشي از افزايش هيدروژن است. همچنين هيدروژن نبايد براي جوشكاري آلومينيم و منيزيم بكار رود.

آرگون + نيتروژن (ازت):

در بعضي كشورها از نيتروژن براي جوشكاري (ميگ) مس استفاده ميشود. كيفيت جوش حاصل به آن خوبي كه مي خواهيم نيست، افزودن 50% تا 75% آرگون به نيتروژن جوشي با كيفيت بالا توليد ميكند.

آرگون +دي اكسيد كربن CO2 :

مخلوط گازهاي آرگون با دي اكسيد كربن براي جوشكاري تيگ بكار نمي رود. اما اين تركيب براي فرآيند ميگ يكي از بهترين مخلوطها، مخلوط 75% آرگون و25% دي اكسيد كربن CO2 است، در حاليكه خارج از آمريكا مخلوط بهتر 80% آرگون و 20% دي اكسيد كربن است.

اين مخلوط در فولادهاي كم كربن، ميان كربن و داراي درصدي منگنز بصورت نامحدود بكارميرود، اين مخروط همچنين درجوشكاري فولادهاي با ضخامت كم (نازك ) نيز مناسب است . در ضمن جايكه عمق نفوذ و عرض جوش ضروري نيست و ظاهر جوش مهم است از اين تركيب استفاده ميشود.

اين تركيب همچنين باعث ميشود جرقه (پاشش) شديدا كاهش يابد. و در جوشكاري توپودري اين تركيب بطور موفق بكار ميرود.

آرگون + هليوم:   

در فرآيند تيگ براي جوشكاري فلزات غيرآهني ( مس، آلومينيم و…) زمانيكه نفوذ زياد وقوس آرام هر دو مورد نظر باشد، استفاده ميشود. افزايش 75% تا50% هليوم ولتاژ و حرارت قوس را بالا مي برد.

اين تركيب همچنين براي جوشكاري ضخامتهاي بالا در فلزات غيرآهني و براي جوشكاري بالاسر با درصد هليوم بيشتر مفيد است و باعث بهبود سرعت و كيفيت جوش در جوشكاري AC آلومينيم ميشود. مخلوط 25% آرگون + 75% هليوم براي فرآيند تيگ با سيم پركننده گرم بكار ميرود. همچنين مخلوط آرگون +هليوم براي جوشكاري فلزات غير آهني در فرآيند ميگ بكار ميرود.    

دي اكسيد كربن CO2 :

اين محصول فرعي بوسيله فرآيندهاي صنعتي از قبيل آمونياك ( تبديل به آهك در اجاق آهك ) از سوختن سوختها، ( نفت يا كك ) در اكسيژن هوا، يا از تخمير مداوم و تدريجي الكل ساخته ميشود. CO2 دي اكسيد كربن گازي است غير سمي، غير قابل اشتعال و سودمند براي كاهش مشكلات جرقه، همچنين گاز دي اكسيد كربن قبل از بسته بندي تميز، تصفيه و خشك ميشود و سپس در سيلندرهاي استيل كه محتوي تقريبا 35 كيلو گرم مايع دي اكسيد كربن هستند، ذخيره ميشود ويك نوع المنت گرم كننده الكتريكي مستقيما در راه خروج گاز دي اكسيد كربن قرار مي دهند. همچنين گاز دي اكسيد كربن تركيبي است از 27% كربن و 73% اكسيژن كه از پيوند دو اتم اكسيژن ويك اتم كربن بوجود آمده است.

گازدي اكسيد كربن در دما وفشار معمولي هوا، گازي بيرنگ، غير سمي و نميسوزد. همچنين CO2 كمي بوي زننده و اندكي هم ترش مزه است. آن در حدود 1.5 برابر سنگين تر از هوا است و در فضاي محدود مانند مخازن جاي هوا را مي گيرد و باعث خفگي جوشكار ميشود. در دماي بالا گاز دي اكسيد كربن به اكسيژن و كربن تجزيه ميشود. در جوشكاريهاي قوسي 20% تا 30% از اين گاز به اكسيژن و كربن تجزيه ميشود.

بايد توجه داشت كه گاز دي اكسيد كربن خالص از گازهاي محافظ ديگر ارزانتر است، و ميتواند مانند گاز محافظ براي جوشكاري فولادهاي تا 4% كربن و فولادهاي كم آلياژي بكار رود. در جوشكاري با گاز محافظ دي اكسيد كربن، دي اكسيد كربن بطور اختصاصي با اكسيژن تركيب ميشود. همانطور كه دي اكسيد كربن سطح قوس را ترك ميكند، آن دوباره به سرعت با اكسيژن تركيب ميشود.

خلوص دي اكسيد كربن ميتواند نسبت به فرآيند ساخت، تغييرات قابل توجهي داشته باشد. در دي اكسيد كربن نرخ قطرات نسبت به آرگون خالص كمتر است، ولتاژ قوس بالاست و مقدار اوليه ولتاژ براي انتقال اسپري نسبت به آرگون خيلي بالاتر است. نيروي انتقال قطرات كه در سراسر قوس منتقل ميشوند، نسبت به آرگون +اكسيژن كمتر است و بنابر اين قوس آرام نيست و كمي جرقه ( پاشش ) دارد وحالت قوس نيز نسبت به آرگون + اكسيژن خيلي بحراني است.

هنگام استفاده از دي اكسيد كربن در انتقال اسپري، يك نرخ بالا از رسوب فلز و خواص هيدروژني پايين بدست مي آيد.استفاده از دي اكسيد كربن روشي است كه بيشتر براي جوشكاريهاي تكراري پيشنهاد ميشود. همچنين اين روش در بعضي زمينه ها با فرآيند قوس دستي الكترود كه پودر آهن درآن بكاررفته رقابت ميكند. در اين روش فولادهاي تا ضخامت 75 م م  ميتواند با عملكرد كاملا اتوماتيك جوشكاري شود.

در قوس دي اكسيد كربن مقداري كربن بطور تصادفي بوجود مي آيد، همچنين در بعضي رسوبها به سبب وجود كاربيد كرم در طول مرز دانه ها و افزايش مقدار كربن در جوش، مقاومت به خوردگي كاهش مي يابد. در جوشكاري با گاز دي اكسيد كربن، نتيجه جوشهاي چند پاسه كاهش مقاومت به خوردگي است، اما با سيم پركننده تثبيت شده و انتقال گلوله أي در مقاطع نازكتر جوشهاي يك پاسه رضايتبخش و خيلي با صرفه ميتوان توليد كرد.

آرگون + دي اكسيد كربن20% يا 5%:

افزايش دي اكسيد كربن به آرگون براي جوشكاري فولاد عمل خيس كنندگي را بهبود مي بخشد، كشش سطح را كاهش ميدهد، و سياليت حوضچه مذاب را بيشتر ميكند. هر دو مخلوط بالا با روش اسپري و غوطه أي ميتوان با آنها جوشكاري كرد.

هليوم :

هليوم محصول فرعي از گاز خنثي صنعتي است. وزن آن 7/1 وزن هوا است ( هليوم داراي چگالي 0.178 كيلوگرم برمتر مكعب و ولتاژ24.58 ). هليوم گازي بيرنگ، بي بو، بي مزه و غير سمي و داراي ضريب هدايت حرارتي بالا مي باشد.