Welding faults Arc welding

Transcription

1 Welding faults Arc welding

2 Welding faults Arc welding faults Contents Cracks Heat Affected Zone (HAZ) hydrogen cracking Weld metal hydrogen cracking Solidification cracking Lamellar tearing Reheat cracking Imperfect shape Linear misalignment Excess weld metal Overlap Undercut Excess penetration bead Root concavity Cavities Porosity Restart porosity Crater pipes Lack of fusion/penetration Incomplete fusion Incomplete root penetration Inclusions Linear Inclusions Isolated inclusions Material degredation Incorrect phase balance Surface colour Loss of corrosion resistance Miscellaneous faults Stray arcing Copper pickup Spatter Surface pocking - gas flats References

3 Cracks Cracks may be detectable in severe cases by eye, or by surface NDT techniques, such as dye penetrant or magnetic particle inspection (ferritic materials) or eddy current testing. Buried cracks can only be found by volumetric inspection (radiography or ultrasonic inspection). Hydrogen cracks, in particular, may be tight, and difficult to detect by radiography if not ideally oriented.

4 Heat Affected Zone (HAZ) Hydrogen cracking Heat Affected Zone (HAZ) hydrogen cracking is a problem which may be experienced when welding ferritic and martensitic steels. It arises when hydrogen embrittles hardened HAZ microstructures. Potential sources of hydrogen include moisture, oil or grease, any of which may dissociate in a welding arc, and hydrogen containing weld process gases, which are only appropriate for austenitic stainless steels. Hydrogen cracking under the influence of residual stresses generally occurs when the plate cools to ambient temperature. It is most commonly experienced in thicker section materials. Steels will crack if the combination of stress, microstructure and hydrogen is too severe. Stress This is generally of yield magnitude, from thermal contraction, and although to some extent dependent on joint restraint, cannot be easily controlled. Stress concentrations resulting, for example, from poor fit up can increase the risk of cracking. Microstructure Hard microstructures are generally at greatest risk, and are produced by: rapid cooling due to: low heat input large heat sink (thick section) low preheat high carbon content and alloying additions Hydrogen The principal source of hydrogen is decomposition of: moisture in: fluxes poorly maintained gas supply system surrounding atmosphere grease and dirt on: wires weld preparations STRESS CRACK AT AMBIENT TEMP. MICROSTRUCTURE HYDROGEN Hydrogen embrittlement is greatest at around normal ambient temperatures, and cracking does not occur in most cases until the weld has cooled. Significant delays may be experienced beyond the initial cooling period before cracks initiate, particularly close to threshold cracking conditions.. Minimise hydrogen Use low hydrogen consumables/processes Ensure wire and preparations are clean and dry Apply correct preheat (encourages escape by diffusion)

5 Limit HAZ hardness Ensure high enough heat input Apply preheat Limit alloy content /carbon equivalent Ensure good fit-up The most powerful control is to reduce hydrogen. Both allowing hydrogen to escape and limiting hydrogen input are beneficial, as cracking will not occur while the joint is maintained above ambient temperature. Hydrogen diffusion is faster at higher temperatures. Hence, extending the time the temperature is over about 100 C, by applying preheat (and/or postheat), can encourage hydrogen escape and reduce the risk of cracking. Weld process gases rich in carbon dioxide, eg. Ferromaxx Plus and Ferromaxx 15, may help to limit absorption of hydrogen by the weld pool. Use of a long stick out and low current with tubular cored wires reduces hydrogen input. In carbon and carbon-manganese steels, a higher heat input will reduce the HAZ hardness. This is less beneficial in alloy steels. Applying preheat can also reduce HAZ hardness to some degree. Where there is the opportunity to select steels, attempt to minimise the carbon equivalent. Nomograms for determining safe combinations of hydrogen, heat input, steel composition and preheat for carbon and carbon manganese steels are available in EN , and in Welding Steels Without Hydrogen Cracking 2. Techniques for limiting the risks of cracking in more difficult (hardenable) steels are given in Welding Steels Without Hydrogen Cracking 2. It is important to ensure that controls are applied to tack welds and temporary attachment welds. Where possible, minimise joint restraint (eg. modify welding sequence). Where restraint is high, take extra care with other precautions. Weld metal Hydrogen cracking Weld metal hydrogen cracking is a problem which may be experienced when welding ferritic and martensitic steels. It arises when hydrogen embrittles weld metal microstructures. Potential sources of hydrogen include moisture, oil or grease, any or which may dissociate in a welding arc, and hydrogen containing weld process gases, which are only appropriate for austenitic steels. Hydrogen cracking under the influence of residual stresses generally occurs when the weld cools to ambient temperature. Steels will crack if the combination of stress, microstructure and hydrogen is too severe. See: Heat Affected Zone (HAZ) Hydrogen cracking for more information.

6 Minimise hydrogen Use low hydrogen consumables/processes Ensure wire and preparations are clean and dry Apply preheat (encourages escape by diffusion) Increase interpass time (encourages escape by diffusion) Limit weld metal strength Ensure good fit-up The most effective control is to reduce hydrogen. Both allowing hydrogen to escape and limiting hydrogen input are beneficial, as cracking does not occur until the joint cools to around ambient temperature, or a bit higher in alloy steels. In multi-pass welds hydrogen escape can be assisted by increasing interpass times. Heat input affects both weld metal hardness and hydrogen escape. Hydrogen escape is more difficult from large weld beads, and is hindered by high joint completion rates. In alloy steels the effect of heat input on weld metal hardness is generally marked, and outweighs the effect of heat input on hydrogen content. Thus the risk of weld metal cracking is greater at low heat inputs. In many carbon-manganese steel welds however, weld metal hardness is relatively insensitive to heat input, and hydrogen levels control the risk of cracking which often increases with increasing heat input. Limited guidance on safe procedures for multi-pass submerged arc welds is given in Welding Steels Without Hydrogen Cracking 1. Solidification cracking Weld beads contract as they solidify. This contraction may be increased by thermal or other strains imposed on the joint during cooling. When the supply of liquid metal to fill gaps which open up between developing crystals is insufficient, the weak solid-liquid-solid interface may rupture under the influence of contraction stress to produce a solidification crack. Weld pool composition Impurities which widen the solidification temperature range and encourage the formation of low melting point films should be minimised. Some elements can also limit the detrimental effects of others. Weld bead shape Welding speed Use welding conditions that encourage a depth to width ratio of ~ Aim for a slightly convex weld cap. Avoid wide root gaps. The use of high performance weld process gas mixtures, eg., may reduce the risk of solidification cracking by producing a broader penetration profile. Excessive welding speeds should be avoided, as these give rise to long weld pools, which may increase the risk of cracking.

7 The effect of composition on solidification cracking Ferritic steel Steels with high levels of carbon, sulphur, phosphorus and niobium tend to present a greater risk of solidification cracking, whereas additions of silicon and magnesium can help to reduce the risk of solidification cracking. For example, the risk of cracking in submerged arc welds is indicated by the UCS formula: UCS = 230C* + 190S + 75P + 45Nb -12.3Si - 5.4Mn - 1 where the composition is in wt%, and C* = the carbon content or 0.08, whichever is higher. It is the weld metal composition which should be used. The formula is valid for weld metals containing the following range of elements: C: Ni: <1 Ti: <0.02 S: Cr: <0.5 Al: <0.03 P: Mo: <0.4 B: <0.002 Si: V: <0.07 Pb: <0.01 Mn: Cu: <0.3 Co: <0.03 Nb: Note: Values of <10 indicate a high resistance to cracking, and >30 indicate a low resistance. Further information can be found in BS EN :2001 2, and Weldability of Ferritic Steels 3. Austenitic steel The key feature is the weld metal microstructure. Weld deposits containing a few per cent of the high temperature delta-ferrite phase are much more resistant to cracking than fully austenitic weld metal. The effect of composition on the microstructure is shown in the Schaeffler diagram (below). Generally, reducing sulphur and phosphorus is beneficial, as is increasing molybdenum and especially manganese. However, the effects of chemical composition are secondary to that of microstructure. Aluminium alloys Alloy-lean aluminium alloys are generally sensitive to solidification cracking. Many important heat treatable and low alloy 5XXX (Al-Mg) alloys can be prone to cracking. The problem may be overcome by using filler alloys which are over-alloyed with silicon, manganese or copper. The success of this approach depends on filler wire composition and dilution achieved. Guidelines for the selection of filler wires for British Standard alloys are given in the table over the page.

8 Guidelines for the selection of filler wires for British Standard alloys Parent metal combination Al-Si castings Al-Mg castings 3XXX 2XXX 1XXX XXX XXX NR (1) Al-Mg Al-Mg NR (2) Al-Mg 5556A Al-Mg Al-Mg Al-Mg 5556A 5XXX NR (1) Al-Mg Al-Mg NR (2) Al-Mg Al-Mg Al-Mg Al-Mg (3) Al-Mg (3) 5005 Al-Si Al-Mg Al-Si NR (2) Al-Si Al-Mg Al-Si Al-Mg (3) 6XXX Al-Si Al-Mg Al-Si NR (2) Al-Si Al-Mg Al-Si or Al-Mg 7020 NR (1) Al-Mg Al-Mg NR (2) Al-Mg 5556A 1XXX Al-Si Al-Mg Al-Si NR (2) Pure Al 2XXX NR (2) NR (2) NR (2) NR (2) 3XXX Al-Si Al-Mg 3103 Al-Mg castings NR (1) Al-Mg Al-Si castings Al-Si NR = not recommended. (1) The welding of alloys containing approximately 2% or more of Mg with Al-Si filler metal (and vice-versa) is not recommended because sufficient Mg2Si precipitate is formed at the fusion boundary to embrittle the joint. (2) 2XXX alloys covered by British Standards are not regarded as weldable alloys, but 4047A gives the best chance of success. (3) The corrosion behaviour of weld metal is likely to be better if its alloy content is close to that of the parent metal and not markedly higher. Thus for service in potentially corrosive environments it is preferable to weld 5154A with 5154A filler metal or 5454 filler metal. However, in some cases this may only be possible at the expense of weld soundness, so a compromise will be necessary. (4) For welding 1080A to itself, 1080A filler metal should be used.

9 Lamellar tearing Lamellar tears are cracks which form in the Heat Affected Zone (HAZ) of a weld when the strain imposed by the shrinkage of the weld exceeds the through thickness ductility of the parent material. Lamellar tearing only occurs in rolled materials, principally structural and pressure vessel steels Material controls Specify sufficient through thickness (short transverse) ductility Specify maximum sulphur content Use forgings or castings instead of rolled plate Design controls Avoid fusion boundaries parallel to the material rolling plane. Minimise weld volume Minimise joint restraint Welding Maintain low hydrogen conditions Use minimum allowable weld metal strength Butter surface, preferably with a low strength consumable Control bead sequence to minimise local strains The three principal controls for lamellar tearing are material quality, joint design, and welding technique. In most cases, sufficiently good material can be obtained to ensure that special joint design and/or welding techniques are not necessary. Other precautions may be required when modifying or repairing old structures or equipment. The judgement on how many precautions to apply will depend not only on material quality, but also on joint details (size and restraint), criticality of the component, and inspectability. Guidance on avoidance of lamellar tearing can be found in EN Material quality Steels with over 20 to 25% Short Transverse Reduction in Area (STRA) are essentially immune from lamellar tearing. Steels with 10 to 15% STRA or below should only be used for very lightly restrained joints.

10 Quoted STRA values are generally an average of three tests. Some attention should be given to the sampling location, as cleaner, less susceptible material will be found at the edges of plates rolled from either ingot cast or continuously cast slabs. Test data from such locations may lead to insufficiently conservative precautions being taken. In modern aluminium treated steels, STRA is largely dependant on the content of sulphide inclusions. STRA of less than 20% is unlikely with 0.005% sulphur or below. Higher STRA values for a given sulphur content may be expected for steels with a low rolling reduction (eg. continuously cast steels tend to have lower reductions than ingot cast) and for steels with sulphide shape control through calcium or rare earth metal additions. Non destructive (ultrasonic) techniques are not sufficiently quantitative to determine STRA values, but can identify particularly susceptible regions of plates, thus allowing best quality plate to be used in joints with a high risk of lamellar tearing. Joint design

11 Welding technique Since lamellar tearing is caused by local straining in the HAZ, any technique which can encourage strains to develop elsewhere instead, in particular in the weld metal, can reduce the risk of tearing. Deposition of a layer of weld metal on the susceptible surface ( buttering ) prior to filling the joint ensures that strains due to the weld passes in contact with the steel are low. Higher strains, developed as the more highly restrained joint is completed, are accommodated principally in the weld metal. Some benefit can also be gained from control of bead deposition sequence. Welding should be carried out starting at the susceptible plate and working towards the other side of the joint. This technique provides in situ buttering. Buttering layers using low strength consumables are even more beneficial. Balanced welding may produce more uniform strains under T-butt or T-fillet welds. Buttering In-situ buttering Balanced welding Re-heat cracking Reheat cracking is completely intergranular in nature. It occurs in welds that are given a Post Weld Heat Treatment (PWHT). Cracks normally form as the weldment is being heated, and occur because grain interiors are strengthened by carbide precipitation, forcing the relaxation of residual stress by creep deformation to occur at weaker grain boundaries. These then cavitate and crack. Reheat cracking is aggravated by carbide forming alloying elements, coarse austenite grains, grain boundary embrittling elements and stress concentrations.

12 Wherever possible avoid susceptible steels, eg. 5Cr 1Mo, 2.25Cr 1Mo, 0.5Mo B, 0.5Cr 0.5Mn 0.25V (in order of increasing risk) and high strength steels containing chromium, molybdenum and vanadium. Use steel with low levels of grain boundary embrittling elements, eg. antimony, arsenic, tin and phosphorous. Steels with a G or PSR of less than 0 are not susceptible. G = Cr + 3.3Mo + 8.1V -2 PSR = Cr + Cu + 2Mo +10V +7Nb +5Ti -2 Reduce stress concentration by grinding weld toes Reduce Heat Affected Zone (HAZ) austenite grain size by using a weld procedure to produce fine grained HAZ microstructures, eg. by two layer techniques and controlling the angle of attack.

13 Imperfect shape In general, imperfect shape can be identified by visual examination or measurement. Where access cannot be obtained, eg. the reverse side of a joint, radiography may reveal excess or missing material. Undercut can be confused with cracking when using magnetic particle inspection.

14 Linear misalignment s Incorrect assembly Base material dimensional variation - particularly common on pipes where ovality cannot be guaranteed Distortion during welding. Use of alignment jigs Size pipes before machining the weld preparation Use of strong backs and other clamping techniques Examples of clamping/alignment techniques flat bridge insert plate Bridges Flat bridges Cleats Glands Insert Bars Wedges block stiffener to be welded Tongue - groove clamps Dogs

15 Excess weld metal Deposition of too much weld metal above the parent metal surface. This only constitutes a fault if it exceeds the specification limit. Reduce current and/or increase voltage and/or increase travel speed. Reduce filler wire or electrode size. Pay close attention to final bead placement in multi-pass deposits. Avoid close butt joints if using filler metal. Increase joint gap. Ensure correct weld process gas selection. Overlap Weld metal flows over, without fusing into, the base material surface. Proper control of arc manipulation and torch/electrode angles. Ensure adequate heat input Increase travel speed. Pay close attention to bead placement. Ensure correct weld process gas selection. To determine the best gas for your application consult the weld gas selector:airproducts.co.uk/welding_selector

16 Undercut Base material is melted by the arc and undercut is formed when there is insufficient flow of weld metal to replace the original material. Undercut is often present as a shape discontinuity at the weld toe which only constitutes a defect if it exceeds the specification limits. Reduce travel speed.. Reduce welding voltage. Reduce welding current. Maintain correct electrode/torch angles. Ensure proper control of dwell at weld pool edges, when using arc oscillation or electrode weaving. Ensure correct weld process gas selection. Excess penetration bead Root penetration bead extends below the parent metal surface. This constitutes a defect if it exceeds specification limits. Modify edge preparation to provide additional weld metal support at the root: Reduce the gap. Increase root face. Ensure correct root gap is maintained. Reduce current. Increase travel speed.

17 Root concavity Root concavity is generally caused by shrinkage of the molten pool on the underside of the weld root, but it can be caused, or assisted, by the pressure of backing gas if present. Increase heat input. Reduce root gap. Reduce backing gas pressure. Reduce current for overhead welds.

18 Cavities Porosity is generally detected by radiography.

19 Porosity s Porosity can be present in the form of elongated wormholes or uniformly distributed spherical pores, which can be wholly contained in the weld, or can be surface breaking. Porosity can be caused by: Inadequate shielding. Contamination of weld preparation or consumables. Welding onto material which contains dissolved gases and/or reacts to form them. Porosity can occur in three ways. Firstly, as a result of chemical reactions within the molten pool, eg. if a molten pool of steel is inadequately deoxidised, oxides of iron may react with the carbon present to liberate carbon monoxide. Secondly, by rejection of gas from solution as the weld solidifies, as can occur in the welding of aluminium alloys when hydrogen originating from, eg. moisture, is absorbed in the pool and later evolves. Thirdly, by gas entrainment, eg. the trapping of weld process gas in the root of turbulent molten pools, or the trapping of gas evolved during the welding of the second side of a fillet in primed steel plate. Aluminium Alloys Porosity is readily formed in aluminium alloy welds. The most important factor is the rejection of absorbed hydrogen during solidification. Hydrogen can be introduced into the weld from: Moisture absorbed on the surfaces of the workpiece and/or filler wire. Surface treatment procedures. Moisture in the atmosphere or weld process gas. Internal hydrogen within the aluminium workpiece or filler wire.

20 These sources are controlled by: Specification of cleaning procedure and minimum time for completion of welding. Removal of organic, anodic and conversion coatings from regions to be welded. Working in a warm, enclosed environment, where possible. Using a reputable weld process gas supplier. Allowing sufficient purge times before commencement of welding. Taking care that all gas supply pipework and hoses are in good condition, regularly maintained and moisture free. Purchasing of parent material and filler wires from reputable suppliers. Guidance on quality levels in aluminium alloy welds is given in BS EN 30042: Ensure satisfactory cleanliness of the workpiece. Remove grease, oil, moisture, rust, paint and dirt before welding Ensure adequate (but not excessive) weld process gas coverage. Eliminate contamination of the welding consumables. Purchase materials from reputable suppliers. Use filler wire with sufficient deoxidants when welding steels Pay attention to welding parameters when welding double sided fillet welds and welding into sealed crevices Ensure weld process gas delivery system is contamination and leak free. Eliminating contamination of the welding consumables Ensure that electrodes and filler wires are clean. Follow proper electrode and flux drying and storage procedures, as recommended by the manufacturer. Eliminate water leaks if using water cooled torches. Ensure that gas delivery lines are in a good condition and are the recommended type for the application. Eliminate leaks from the gas delivery system. Check: Delivery lines Welding torch All connections After a period of in-activity the gas delivery system should be purged to remove residual moisture. Pay attention to welding parameters when welding double sided fillet welds and welding into sealed crevices Use appropriate grade of weld process gas for specific application. Ensure optimum gas flow rate is used. Flow must be sufficient: ensure all air is removed from weld zone excessive flow rates are not only wasteful, but may cause turbulence in the gas or weld pool. Eliminate draughts in welding area. Use properly maintained gas lens. Use correct torch nozzle size. Reduce welding travel speed. Reduce arc length. Reduce contact tip to work piece distance. Ensure that gas delivery lines are in a good condition and are the recommended type for the application. After a period of inactivity the gas delivery system should be purged to removed residual moisture. To determine the best gas for your application consult the welding gas selector: airproducts.co.uk/welding_selector

21 Restart porosity This is evident at the beginning of a weld run and is caused by inadequate shielding of the weld pool. For gas-shielded processes allow adequate pre-purge time before starting to weld. Use proper arc initiation technique (refer to diagram below). Ensure that the arc is started and the weld pool is formed before travelling forward. Initiation technique Correct electrode manipulation to avoid restart porosity Slag removed Arc started and then moved back over the previous weld end Weld start must subsequently be ground if not to be re-fused in a multipass weld Arc started on previous weld bead and moved over weld end

22 Crater pipes Crater pipes are caused by shrinkage of the weld pool on completion of a weld run, often assisted by some gas evolution (porosity). Gradually tail off the welding current towards the end of the weld run. For gas shielded processes ensure that a post purge of the weld pool is provided. For manual welding ensure that the electrode/torch is maintained in position while the weld pool solidifies.

23 Lack of fusion/penetration Lack of fusion/penetration is generally evident on radiographic inspection and is generally detectable using ultrasonic inspection.

24 Incomplete fusion Below left: Lack of side fusion Below middle: Lack of inter-run fusion Below right: Lack of root fusion. This occurs when the arc fails to impinge onto and/or melt the base material. It can be caused by incorrect alignment of the arc with the joint faces or by the weld pool moving ahead of the arc, which insulates the arc from the base material. High parent material thermal conductivity, eg. copper, can also cause this problem. This fault is often characterised as: Lack of side fusion Lack of inter-run fusion Lack of root fusion. Increase heat input Increase welding current. Ensure joint faces are free of excessive mill scale and slag from previous weld passes. Avoid too large a weld pool Ensure correct manipulation of the torch/electrode Modify joint design Ensure correct weld process gas selection. Reduce travel speed. Increase travel speed. Reduce arc oscillation or electrode weave width. Select correct weld process gas. Correct alignment with joint faces and/or seam. Use correct electrode/torch angle. Correct bead placement for multi-pass welds. Pay close attention to dwell times when using arc oscillation or electrode weaving. Ensure the equipment is in good working order. Increase preparation angle. Increase root gaps. Reduce root face. To determine the best gas for your application consult the welding gas selector: airproducts.co.uk/welding_selector

25 Copper Pure grades of copper and high-conductivity alloys, such as Cu-Be or Cu-Cr, may require preheating to obtain fusion in section thicknesses greater than about 3mm. The level of preheat required depends on the heat sink provided by the work pieces (usually interpreted in terms of section thicknesses), the welding process and the nature of the weld process gas. Weld process gases which contain helium (eg. ) produce hotter arcs and allow lower preheats to be effective, compared to argon and argon-rich gas mixtures. Nitrogen gas promotes the hottest arc conditions, but it is not widely used because of weld quality considerations. Level of pre-heat Preheat temperature for TIG welding of copper using various weld process gases. Preheat temperatures for MIG welding of copper using various weld process gases. Incomplete root penetration This occurs when the weld fails to penetrate completely and fill the root preparation. Increase heat input Modify root preparation Increase welding current. Reduce travel speed. Increase preparation angle. Increase root gaps. Reduce root face. Modify joint design to improve access to weld root. Use proper welding technique Avoid excessive contact tip to workpiece distance. Avoid excessively large electrode/torch angles (measured from normal to the workpiece). For double sided welds increase depth of backgouging and/or grinding. Ensure correct weld process gas selection.

26 Inclusions Inclusions are generally evident on radiographic inspection.

27 Linear inclusions s These may consist of continuous or intermittent slag lines parallel to the length of the weld and in the weld root zone. They are often associated with undercut or an irregular and excessively convex bead in the underlying weld layer. It can also be caused by incomplete slag removal from preceding weld passes. Avoid undercut (See also: undercut in the imperfect shape category). Select a consumable with good slag removal characteristics. Increase joint preparation angle. Modify welding parameters to avoid a convex profile Increase arc voltage. Reduce travel speed. Reduce welding current. Carry out adequate inter-run de-slagging. Carry out inter-run dressing of weld surface. Ensure correct weld process gas selection. Isolated inclusions s Pay close attention to the condition of both weld preparations and consumables. Normally caused by the presence of mill scale and/or rust on prepared surfaces, or electrodes with coverings that are cracked or damaged. May also be caused by isolated undercut in underlying passes of multi-pass weld. Tungsten inclusions can result from TIG welding. This is caused by poor technique, either when striking the arc or during welding. Simple touch starting (by striking the electrode along the material) is liable to result in tungsten inclusions. Start TIG welds by moving the electrode slowly upwards to initiate the arc or use high frequency initiation. Implement inter-run procedures as appropriate, ie. slag removal, weld dressing etc.

28 Material degradation Degradation of material, mechanical or corrosion properties cannot usually be detected by normal NDT techniques.

29 Incorrect phase balance s The composition and manufacture of wrought duplex and super-duplex stainless steels are controlled to give a 50/50 ferrite/austenite phase balance. This gives optimum mechanical and corrosion properties. The thermal cycle experienced during fusion welding can significantly alter the phase balance. The high temperature Heat Affected Zone (HAZ) transforms to ferrite, and the weld metal solidifies to ferrite: for both regions, cooling from peak temperature is too slow for complete reformation of austenite to occur. In addition, nitrogen may be lost from the weld pool, reducing the capacity of the weld metal to transform to austenite. Ferrite is promoted by increased Cr and Mo contents, while austenite is promoted by increased nickel and nitrogen levels. Guidance on the effects of composition can be obtained from the WRC or the Q-factor diagrams, shown below. Nitrogen is particularly significant in welding, since it diffuses rapidly and enhances the rate of austenite formation on cooling. Weld metal austenite content, % Nieq = Ni + 35 C + 20 N Cu Q factor Q = Cr + 1.5Mo + Mn Si 2Ni + 12C + 12N Creq = Cr +Mo Nb Control material composition Select base material and filler metal to balance ferrite formers and austenite formers. Select weld process gas(es) to balance nitrogen level lost. Control welding procedure Avoid excessively rapid cooling. Control heat input in multi-pass welds 1kJ/mm. Low heat input and rapid cooling suppress austenite formation and should be avoided. However, excessive arc energy must be avoided to minimise the formation of detrimental intermetallic phases in the weld area. Duplex steels become increasingly tolerant to low heat input during arc welding when materials are selected with nitrogen content towards the upper limit of commercial steel specifications. Beam processes must be regarded with caution. Primary austenite forms in the weld metal and HAZ as a weld run cools. Secondary austenite can develop on reheating in multi-pass welds. This tends to reduce corrosion resistance, especially in weld metal. Thus consistent arc energy should be used throughout the joint: low arc energy root runs and high arc energy second or third passes should be avoided.

30 Conventional TIG welding with pure argon shielding can lead to loss of nitrogen from the weld pool. This can be countered by use of nitrogen-containing gas mixtures: Duplex 1 gas (1.1% N2, 20% He, balance Ar) is applicable to duplex alloys, and Duplex 2 gas (2.25% N2, 20% He, balance Ar) to the superduplex grades. Nitrogen additions to the weld process gas above these levels must be regarded with caution, since they may lead to electrode degradation and process instability, spitting and sparking and to weld metal porosity in multi-pass welds. The high performance Duplex gas mixtures provide additional benefits when compared to argon, in particular: Reduced ozone emissions. Increased welding speed. Ozone emissions The effect of weld process gas on ozone (breathing zone) Autogenous TIG welding of duplex stainless steel. Weld process gas Surface colour of Titanium welds Ti metal Thin layers of oxide on the surface of titanium welds cause the generation of interference colours. The colour observed is related to the thickness of the oxide film. This surface oxidation may be associated with embrittlement of the underlying weld metal and Heat Affected Zone (HAZ). Interference colours The success of the gas shielding arrangements is judged, conventionally, by monitoring the surface colours on the completed weld and Heat Affected Zone (HAZ). Successful Hot shielding should result in uncoloured (bright, silvery) weld beads and HAZs. Some darkening/coloration at the edge of the gas shield is normally considered acceptable. As the thickness of the surface oxide increases, interference colours are produced giving a change in surface coloration. Acceptance criteria can give various degrees of relaxation from the "no colour" condition, but all colour criteria are, fundamentally, arbitrary. For example, disturbance of the gas shield around the arc can cause contamination without causing coloration, providing satisfactory shielding is maintained under the trailing shield. On the other hand, oxidation at low temperatures may be associated with the formation of a coloured surface but no serious underlying embrittlement.

31 All titanium surfaces which are heated above 500 C must be protected from contamination by the absorption of interstitial elements (O, N, H, C). Clean surfaces to remove grease, dirt, lubricants etc. before welding. Use inert gas-filled or vacuum chambers for welding when appropriate. For "open air" welding, additional shielding is usually necessary, compared to that employed for other alloys: Use trailing shields to protect the hot metal during cooling. Maintain gas coverage after the arc is extinguished. Backpurging with inert gas is essential in most cases. Surface colours The following colours are ranked with respect to increasing oxide thickness, ie. white - greatest oxide thickness: Silver Light straw Dark straw/magenta Dark blue Light blue Grey blue (mixed second order colours) White (flaky loose deposit) Loss of corrosion resistance of Stainless Steel weld metal by carbon pickup from weld process gas Stainless steel weld metals pick up carbon if a weld process gas mixture containing > 20% carbon dioxide is used for solid wire MAG welding. The carbon which is produced due to dissociation of the carbon dioxide in the arc can cause loss of corrosion resistance via the formation of chromium carbides as the joint cools. Chromium carbides form on grain boundaries on cooling from about 850 C to 550 C. The surrounding metal is depleted in chromium, and has corrosion resistance below that of the grain centres. This causes "sensitisation" to intergranular attack. At higher carbon levels, carbide formation is more rapid and more severe. Hence weld metal carbon pickup from incorrect weld process gas mixture is potentially detrimental. The occurrence of intergranular attack depends also on the environment, and carbon levels must be minimised for service in aggressive, highly oxidising media such as nitric acid. Use correct weld process gas mixtures with low carbon dioxide levels: Inomaxx Plus high performance mixture for spray, pulsed and dip transfer welding. Inomaxx 2 standard mixture for spray, pulsed and dip transfer welding. Note(s): The joint area must be clean of carbonaceous material prior to welding. Low contents of carbon dioxide in the gas can help wetting and avoid scattered porosity: these do not constitute a hazard. In weld deposits containing a few per cent of ferrite, the chromium carbides form on ferrite/austenite interfaces: the carbide density is reduced, and so is the degree of chromium depletion.

32 Miscellaneous faults

33 Stray arcing d by accidental contact of electrodes or welding torch with plate surface remote from weld. These usually result in small hard spots just beneath the surface which may contain cracks, and are thus to be avoided. Can also occur at the earth contact if the connection is poor. Adherence to proper electrode/torch manipulation. Ensure good earth connections. If present, arc spots should be ground out and checked by surface Non-Destructive Testing (NDT). Copper pickup s There are three principal causes of copper contamination in welds: Accidental gross contamination, eg. incorporation of part of the contact tip into the weld pool. Impingement of copper contact tips on the side wall of the joint and/or stray arcing back to the contact tips. Melting of copper backing bars. Liquid copper in contact with stressed solid steel will penetrate along the grain boundaries, thereby weakening them. Shrinkage of a weld with such grain boundary penetration results in cracking along the weakened boundaries. Copper may be in the form of metal in the weld pool, which has remained unmixed or has been rejected into interdendritic regions during weld solidification, or metal (eg. backing bar) in contact with weld or Heat Affected Zone (HAZ) surface. Liquid metal penetration can only take place above the melting point of copper (1085 o C), but the cracking may continue at lower temperatures as shrinkage strains develop.

34 Attention to the design and dimensions of the copper backing bar to avoid direct impingement of the arc. Avoid joint gaps when welding onto a copper backing bar. Ensure the welding torches are in good order Avoid arcing from the torch to the preparation Replace worn contact tips. Use correct tensions on wire feed rollers For narrow gap welding ensure the torch is correctly aligned with the preparation Ensure the traverse is in good working order Spatter Spatter is caused by arc instabilities during metal transfer which can cause molten metal droplets to be generated from the arc and weld pool. Optimise welding parameters. Ensure correct weld process gas is used (see below) Ensure welding equipment is in good working order If arc blow is suspected take steps to alleviate the problem Replace worn contact tips. Replace worn cable liners. Ensure proper tensions on wire feed rollers. Avoid bends in wire feed cable. Counter balance magnetic field. De-magnetise base material Change to AC welding, if appropriate. Change location of earth connection. Optimise welding parameters If the welding parameters are outside the optimum operating conditions, spatter will generally result. Arc length, electrode stick out and torch to workpiece distance all need to be set in the optimum range for spatter-free welds. Ensure correct weld process gas is used Use of incorrect weld process gases will increase the degree of spatter during MIG/MAG welding. To produce minimal spatter levels with all commonly welded materials, are recommended, as follows: Carbon steel - solid wire - Carbon steel - cored wire - Stainless steel - Aluminium -

35 Surface pocking / gas flats Surface pocking is a surface depression on the face of welds, caused by gas pockets between the slag and the solidifying weld bead. For sub arc welding, it occurs with inadequately dried flux or flux containing insufficient deoxidents. Wire drawing lubricants retained on the surface of rutile flux-cored wires may also cause this defect. Dry flux according to the manufacturer's recommendations. Check that the correct flux is used for the weld material. References 1. Bailey N, Coe FR, Gooch TG, Hart PHM, Jenkins N and Pargeter RJ, 2004: 'Welding Steels Without Hydrogen Cracking', 2nd edition (revised), Woodhead Publishing, Cambridge. 2. Welding - Recommendations for welding of metallic materials, Part 2: Arc welding of ferritic steels. BS EN :2001, Inc. Amendment 1: Bailey N. Weldability of ferritic steels Abington Publishing 1994.