Wis5 weldability n.ppt 1 11

1. Faisal YusofCopyright © 2003 TWI Ltd TE Welding Inspection Weldability Course Reference WIS 5 2. Faisal YusofCopyright © 2003 TWI Ltd TE Plain Carbon Steels Steels are classified into groups as follows 1. Low Carbon Steel 0.01 – 0.3% Carbon 2. Medium Carbon Steel 0.3 – 0.6% Carbon 3. High Carbon Steel 0.6 – 1.4% Carbon Plain carbon steels contain only iron & carbon as main alloying elements, traces of Mn, Si, Al, S & P may also be present Classification of SteelClassification of Steel 3. Faisal YusofCopyright © 2003 TWI Ltd TE IRON CARBON DIAGRAM 4. Faisal YusofCopyright © 2003 TWI Ltd TE TTT DIAGRAM 5. Faisal YusofCopyright © 2003 TWI Ltd TE Diagram showing the Relationship between Carbon Content, Mechanical Properties, Microstructure and Uses of Plain Carbon Steels in the Normalised Condition 6. Faisal YusofCopyright © 2003 TWI Ltd TE An Alloy steel is one that contains more than Iron & Carbon as a main alloying elements Alloy steels are divided into 2 groups 1. Low Alloy Steels < 7% extra alloying elements 2. High Alloy Steels > 7% extra alloying elements Classification of SteelClassification of Steel 7. Faisal YusofCopyright © 2003 TWI Ltd TE (a) substitutional (b) interstitial Solid solution 8. Faisal YusofCopyright © 2003 TWI Ltd TE Carbon: Major element in steels, influences strength,toughness and ductility Manganese: Secondary only to carbon for strength toughness and ductility, secondary deoxidiser and also acts as a desulphuriser. Silicon: Primary deoxidiser Molybdenum: Effects hardenability, and has high creep strength at high temperatures. Steels containing molybdenum are less susceptible to temper brittleness than other alloy steels. Chromium: Widely used in stainless steels for corrosion resistance, increases hardness and strength but reduces ductility. Nickel: Used in stainless steels, high resistance to corrosion from acids, increases strength and toughness Steel Weld MetallurgySteel Weld Metallurgy 9. Faisal YusofCopyright © 2003 TWI Ltd TE  Aluminium:Deoxidiser,grain refinement  Sulphur: Machineability  Tungsten: High temperature strength  Titanium: Elimination of carbide precipitation  Vanadium: Fine grain – Toughness  Copper: Corrosion resistance and strength Steel Weld Metallurgy 10. Faisal YusofCopyright © 2003 TWI Ltd TE Increased strength: C, Si, Cu, Mn, Mo (also Nb and V; their exact effect depends on other factors also such as the rolling temperature and time, amount of carbon and nitrogen present, etc.) Hardening capacity: C, Mn, Mo, Cr, Ni, Cu Toughness: Ni, grain refinement (achieved via the presence of Nb, V, Al, Ti) Elevated Temperature Properties: Cr, Mo, V Atmospheric corrosion Resistance: Cu, Ni 11. Faisal YusofCopyright © 2003 TWI Ltd TE Steel Weld MetallurgySteel Weld Metallurgy The type and number of elements present in the material The temperature reached during welding and or PWHT. The cooling rate after welding and or PWHT The grain structure of steel will influence its weldability, mechanical properties and in-service performance. The grain structure present in a material is influenced by: 12. Faisal YusofCopyright © 2003 TWI Ltd TE Cooling RateCooling Rate The cooling rate of the weld zone depends on the following factors: •Weld heat : Also call arc energy, is the amount of electrical energy that is supplied to the welding arc over a given weld length ( an inch or mm) •Thickness of material •Preheating 13. Faisal YusofCopyright © 2003 TWI Ltd TE Heat Affected ZoneHeat Affected Zone The parent material undergoes microstructure changes due to the influence of the welding process. This area, which lies between the fusion boundary and the unaffected parent material, is called the heat affected zone (h.a.z.). 14. Faisal YusofCopyright © 2003 TWI Ltd TE Heat Affected ZoneHeat Affected Zone 15. Faisal YusofCopyright © 2003 TWI Ltd TE Heat Affected ZoneHeat Affected Zone 16. Faisal YusofCopyright © 2003 TWI Ltd TE Heat Affected ZoneHeat Affected Zone  Material composition  Cooling rate, fast cooling higher hardness  Arc energy, high arc energy wider HAZ  The HAZ can not be eliminated in a fusion weld The extent of changes will be dependent upon the following :- 17. Faisal YusofCopyright © 2003 TWI Ltd TE Arc energyArc energy Heat input = 1.6 kJ/mm Amps = 200 Volts = 32 Travel speed = 240 mm/min Arc energy= Amps x volts Travel speed mm/sec X 1000 Heat input = 200 X 32 X 60 240 X 1000 18. Faisal YusofCopyright © 2003 TWI Ltd TE High heat input - slow cooling  Low toughness  Reduction in strength Heat InputHeat Input Low heat input - fast cooling  Increased hardness  Hydrogen entrapment  Lack of fusion 19. Faisal YusofCopyright © 2003 TWI Ltd TE Carbon EquivalentCarbon Equivalent  The CE of steel primarily relates to its hardenability.  Higher the CE, lower the weldability  Higher the CE, higher the susceptibility to brittleness  The CE of a given material depends on its alloying elements  The CE is calculated using the following formula CE = C + Mn + Cr + Mo + V + Cu + Ni 6 5 15 Hardenability:The relative ability of a ferrous alloy to form martensite when quenched from high temperatures. 20. Faisal YusofCopyright © 2003 TWI Ltd TE WeldabilityWeldability  Weldability can be defined as the ability of a material to be welded by most of the common welding processes, and retain the properties for which it has been designed.  A steel which can be welded without any real dangerous consequences is said to possess Good Weldability.  A steel which can not be welded without any dangerous consequences occurring is said to possess Poor Weldability. Poor weldability normally generally results in the occurrence of some sort of cracking problem 21. Faisal YusofCopyright © 2003 TWI Ltd TE WeldabilityWeldability Weldability is a function of many inter-related factors but these may be summarised as: Composition of parent material  Joint design and size  Process and technique  Access 22. Faisal YusofCopyright © 2003 TWI Ltd TE WeldabilityWeldability It is very difficult to asses weldability in absolute terms therefore it is normally assessed in relative terms 23. Faisal YusofCopyright © 2003 TWI Ltd TE WeldabilityWeldability There are many factors which affect weldabilty e.g. material type, welding parameters amps, volts travel speed, heat input. 24. Faisal YusofCopyright © 2003 TWI Ltd TE WeldabilityWeldability Other factors affecting weldability are welding position and welding techniques. 25. Faisal YusofCopyright © 2003 TWI Ltd TE WeldabilityWeldability Basically speaking weldabilty is the ease with which a material or materials can be welded to give an acceptable joint 26. Faisal YusofCopyright © 2003 TWI Ltd TE CracksCracks 27. Faisal YusofCopyright © 2003 TWI Ltd TE Process CracksProcess Cracks Hydrogen induced cold cracking (HICC) Solidification cracking (Hot Tearing) Lamellar tearing Weld Decay 28. Faisal YusofCopyright © 2003 TWI Ltd TE CracksCracks When considering any type of crack mechanism, three elements must be present for it’s occurrence:  Stress: stress is always present in weldments, through local expansion and contraction.  Restraint: may be a local restriction, or through the plates being welded.  Susceptible microstructure: the structure is often made susceptible to cracking through welding, e.g high hardness 29. Faisal YusofCopyright © 2003 TWI Ltd TE HydrogenHydrogen CracksCracks 30. Faisal YusofCopyright © 2003 TWI Ltd TE Hydrogen CrackingHydrogen Cracking Hydrogen causes general embrittlment and in welds may lead directly to cracking, 31. Faisal YusofCopyright © 2003 TWI Ltd TE A combination of four factors is necessary to cause HAZ hydrogen cracking 32. Faisal YusofCopyright © 2003 TWI Ltd TE Hydrogen Cracking Characteristics  Also known as hydrogen induced cold cracking , delay cracking , underbead cracking and chevron.  Hydrogen is the major influence to this type of cracking.  Source of hydrogen may be from moisture or hydrocarbon such as grease , paint on the parent material, damp welding fluxes or from condensation of parent material  Hydrogen is absorbed by the weld pool from the arc atmosphere. 33. Faisal YusofCopyright © 2003 TWI Ltd TE • During cooling, much of this hydrogen escapes from the solidified bead by the diffusion but some also diffuses into the HAZ of the parent metal. • Type of cracking is intergranular along grain boundaries or transganular • Requires susceptible grain structure, stress and hydrogen and low temperature is reached. • Most likely in HAZ for Carbon Manganese steel and in weld metal for HSLA steel. 34. Faisal YusofCopyright © 2003 TWI Ltd TE Hydrogen induced weld metal cracking Hydrogen induced HAZ cracking Hydrogen CrackingHydrogen Cracking Micro Alloyed Steel Carbon Manganese Steel 35. Faisal YusofCopyright © 2003 TWI Ltd TE Hydrogen Cracking Factors responsible: Hydrogen cracking occurs when the conditions outlined in 1 – 4 occur simultaneously : 1.Susceptible grain structure – hardness value > 350 V.P.N That part of HAZ which experiences a high enough temperature for the parent steel to transform rapidly from ferrite to austenite and back again,produces microstructures which are usually harder and more susceptible to hydrogen embrittlement. 2.Hydrogen level - > 15 ml/100g This is inevitably present, derived from moisture in the fluxes used in welding and from other sources. 36. Faisal YusofCopyright © 2003 TWI Ltd TE Hydrogen Cracking 3.Temperature < 200o C for any steel and < 150o C for structural steel. The greatest risk of cracking occurs when temperatures near ambient are reached and cracking may thus take place several hours after welding has been completed ( normally after 72 hours ) 4.Stress > 50% yield strength of parent metal These arise inevitably from thermal contractions during cooling and may be supplemented by other stresses developed as a result of rigidity in the parts to be joined. 37. Faisal YusofCopyright © 2003 TWI Ltd TE  Pre heat, removes moisture from the joint preparations, and slows down the cooling rate  Ensure joint preparations are clean and free from contamination  The use of a low hydrogen welding process such as TIG or MIG/MAG  The use of Nickel and Austenitic filler metal  Ensure all welding is carried out under controlled environmental conditions  Ensure good fit-up as to reduced stress  The use of a PWHT with maintaining the pre- heat temperature  Avoid poor weld profiles  Use low hydrogen electrodes and baked as per manufacturer instructions Hydrogen CrackingHydrogen Cracking Precautions for controlling hydrogen cracking 38. Faisal YusofCopyright © 2003 TWI Ltd TE SolidificationSolidification CracksCracks 39. Faisal YusofCopyright © 2003 TWI Ltd TE Solidification Cracking Characteristics  Also known as hot cracking or center line cracking or crater cracking and liquation cracking Solidification cracking is intergranular type of cracking that is along the grain boundaries of the weld metal. It occurs during the terminal stages of solidification,when the stresses developed across the adjacent grains exceed the strength of the almost completely solidified weld metal. Impurities such as sulphur and phosphorous and carbon pick - up from parent metal increase the risk of cracking High joint restraint which produce high residual stress will increase the susceptibility to this type of cracking. 40. Faisal YusofCopyright © 2003 TWI Ltd TE • Occurs during weld solidification process from liquidus to solidus and at the last area to solidified. • Steels with high sulphur content (low ductility at elevated temperature ) whereby produce hot shortness to the weld metal • FeS form films at the grain boundaries whereby reduce the strength of the weld metal. • Addition of manganese will form MnS and forms globules instead of films( FeS) • Occur longitudinally down center of weld • Welding process that most susceptible to this type of cracking are SAW and MIG/MAG with spray transfer due to high dilution rate. Solidification Cracking 41. Faisal YusofCopyright © 2003 TWI Ltd TE Intergranular liquid film along the grain boundary Solidification CrackingSolidification Cracking 42. Faisal YusofCopyright © 2003 TWI Ltd TE Weld Centerline Solidification CrackingSolidification Cracking 43. Faisal YusofCopyright © 2003 TWI Ltd TE Solidification Cracking Factors responsible : Metallurgical factors a) Freezing temperature range –higher freezing range more susceptible to solidification cracking due to presence of FeS b) Primary solidification Phase – Less than 5% delta ferrite c) Surface tension – concave more susceptible than convex weld shape d) Grain structure of fusion zone – Coarse columnar grain more susceptible especially with high energy welding process. Mechanical factors a) Contraction stresses – Thicker material more susceptible. b) Degree of restraint – poor fit - up 44. Faisal YusofCopyright © 2003 TWI Ltd TE  Use low dilution welding process The use of high manganese and low carbon content fillers Maintain a low carbon content Minimise the amount of stress / restraint acting on the joint during welding The use of high quality parent materials, low levelsof impurities Use proper joint design, use Single J instead of single V Clean joint preparations, free from oil, paints and any other sulphur containing product. Joint design selection depth to width ratios Solidification CrackingSolidification Cracking Precautions for controlling solidification cracking 45. Faisal YusofCopyright © 2003 TWI Ltd TE LamellarLamellar TearingTearing 46. Faisal YusofCopyright © 2003 TWI Ltd TE 47. Faisal YusofCopyright © 2003 TWI Ltd TE Lamellar Tearing Characteristics  Lamellar tearing has a step like appearance due to the solid inclusions linking up under the influences of welding stresses Occurs at beneath of HAZ or near HAZ  It forms when the welding stresses act in the short transverse direction of the material (through thickness direction)  Low ductile materials containing high levels of impurities are very susceptible 48. Faisal YusofCopyright © 2003 TWI Ltd TE • Occur only in rolled direction of the parent material • Associated with restrained joints subjected to through thickness stresses on corners and tees • Presence of elongated stringers such of nonmetallic inclusion such as silicates and sulfides parallel to steels rolling plane will produce poor through thickness ductility of the plate. • Tearing will triggered by this such non metallic inclusion near the weld or it just outside HAZ during weld contraction. Lamellar Tearing 49. Faisal YusofCopyright © 2003 TWI Ltd TE Step like appearance Cross section Lamellar TearingLamellar Tearing 50. Faisal YusofCopyright © 2003 TWI Ltd TE Susceptible joint types Tee fillet weld Tee butt weld (double-bevel) Corner butt weld (single-bevel) Lamellar TearingLamellar Tearing 51. Faisal YusofCopyright © 2003 TWI Ltd TE Critical area Critical area Critical area Lamellar TearingLamellar Tearing 52. Faisal YusofCopyright © 2003 TWI Ltd TE Lamellar Tearing Precautions for controlling lamellar tearing  The use of high quality parent materials, low levels of impurities ( Z type material ) 53. Faisal YusofCopyright © 2003 TWI Ltd TE Friction Welded Caps Short Tensile Specimen Through Thickness Ductility Sample of Parent Material A test for a materials susceptibility to lamellar tearing Short Tensile TestsShort Tensile Tests The results are given as a STRA value Short Transverse Reduction in Area 54. Faisal YusofCopyright © 2003 TWI Ltd TE Lamellar Tearing Precautions for controlling lamellar tearing  The use of high quality parent materials, low levels of impurities ( Z type material )  Change joint design 55. Faisal YusofCopyright © 2003 TWI Ltd TE Lamellar TearingLamellar Tearing Modifying a Tee joint to avoid lamellar tearing Susceptible Susceptible Improved Non-susceptible Non-susceptible Gouge base metal and fill with weld metal before welding the joint Susceptible Less susceptible Prior buttering of the joint with a ductile layer of weld metal may avoid lamellar tearing 56. Faisal YusofCopyright © 2003 TWI Ltd TE Lamellar TearingLamellar Tearing Modifying a corner joint to avoid lamellar tearing Susceptible Non-Susceptible Prior welding both plates may be grooved to avoid lamellar tearing An open corner joint may be selected to avoid lamellar tearing 57. Faisal YusofCopyright © 2003 TWI Ltd TE Lamellar Tearing Precautions for controlling lamellar tearing  The use of high quality parent materials, low levels of impurities ( Z type material )  Change joint design  Minimise the amount of stress / restraint acting on the joint during welding  The use of buttering runs with low strength weld metal  Hydrogen precautions e.g use low hydrogen electrodes  Shift welding process such as Electro slag welding  Use forging or casting joint.  Place soft filler wire between the joint e.g T joint to reduce stresses during expansion and contraction of weld metal.  Pre heating helps on removal of Hydrogen on the plate. 58. Faisal YusofCopyright © 2003 TWI Ltd TE Weld DecayWeld Decay 59. Faisal YusofCopyright © 2003 TWI Ltd TE 60. Faisal YusofCopyright © 2003 TWI Ltd TE 61. Faisal YusofCopyright © 2003 TWI Ltd TE Weld Decay Characteristics  Weld decay may occurs in unstabilized austenitic stainless steels with carbon content above 0.1%  Also known as knife line attack or crack  Chromium carbide precipitation takes place at the critical range of 450o C-850o C (sensitising temperature )  At this temperature range carbon is absorbed by the chromium, which causes a local reduction in chromium content by promoting chromium carbides.  Loss of chromium content results in lowering the materials resistance to corrosion attack allowing rusting to occur 62. Faisal YusofCopyright © 2003 TWI Ltd TE Precautions for Weld DecayPrecautions for Weld Decay  The use of a low carbon grade stainless steel e.g. 304L, 316L, 316ELC with carbon content < 0.03%  The use of a stabilized grade stainless steel e.g. 321, 347, 348 recommended for severe corrosive conditions and high temperature operating conditions  Standard grades may require PWHT, this involves heating the material to a temperature over 1100o C and quench the material, this restores the chromium content at the grain boundary, a major disadvantage of this heat treatment is the high amount of distortion 63. Faisal YusofCopyright © 2003 TWI Ltd TE FatigueFatigue CracksCracks 64. Faisal YusofCopyright © 2003 TWI Ltd TE Fatigue TestingFatigue Testing 65. Faisal YusofCopyright © 2003 TWI Ltd TE Fatigue CracksFatigue Cracks  Fatigue cracks occur under cyclic stress conditions  Fracture normally occurs at a change in section, notch and weld defects i.e stress concentration area  All welded materials are susceptible to fatigue cracking  Fatigue cracking starts at a specific point referred to as a initiation point  The fracture surface is smooth in appearance sometimes displaying beach markings  The final mode of failure may be brittle or ductile or a combination of both 66. Faisal YusofCopyright © 2003 TWI Ltd TE Initiation points / weld defects Fatigue fracture surface smooth in appearance Secondary mode of failure ductile fracture rough fibrous appearance Fatigue CracksFatigue Cracks 67. Faisal YusofCopyright © 2003 TWI Ltd TE Fatigue CracksFatigue Cracks A fatigue failure on a small bore pipe work 68. Faisal YusofCopyright © 2003 TWI Ltd TE Precautions against Fatigue CracksPrecautions against Fatigue Cracks Toe grinding, profile grinding. The elimination of poor profiles The elimination of partial penetration welds and weld defects Operating conditions under the materials endurance limits The elimination of notch effects e.g. mechanical damage cap/root undercut The selection of the correct material for the service conditions of the component 69. Faisal YusofCopyright © 2003 TWI Ltd TE Any Questions?Any Questions? 70. Faisal YusofCopyright © 2003 TWI Ltd TE QU 1. Briefly discuss the four essential factors for hydrogen cracking to occur QuestionsQuestions QU 2. State four precautions to reduce the chance of hydrogen cracking QU 3. In which type of steel is weld decay is experienced and state how it can be prevented QU 4. State the precautions to reduce the chances of solidification cracking QU 5. State four the essential factors for lamellar tearing to occur