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WELDING METALLURGY OFCARBON STEEL
• EFFECT OF WELDING HEAT• Change in microstructure• Change in hardness
• Weld Bead Sectioning consists of:• Weld Bead Sectioning consists of:• Weld Metal (WM)• Heat Affected Zone (HAZ)• Base Metal (BM)
• Zone 1) WM – Metal melted & solidified to austenite and then to ferrite(course grain structure, pro-eutectoid ferrite in GB with bainite atcentre, lower hardness)
• Zone 2) HAZ - Metal heated above UCT but not melted (area of graingrowth, Austenite changes to Martensite or Troostite (fine ferrite +growth, Austenite changes to Martensite or Troostite (fine ferrite +cementite) in high rate of cooling, region of underbead cracking due tohydrogen solubility)
• Zone 3) HAZ- Metal heated between LCT and UCT (Grain refinement,hardness increases as compared to BM)
• Zone 4) BM – heated below LCT & cooled (no structural change)
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THE IMPORTANCE OF PREHEATING& INTERPASS TEMPERATURE
• To control rate of cooling
• To minimise the formation of hardening • To minimise the formation of hardening structure s.a. cementite
• To avoid hydrogen trapped
• To avoid cracks
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SPECIFICATION OF STEELS
• By Chemical Composition
• By Mechanical Characteristics• By Mechanical Characteristics
• By Standard Specification
• By an industrial accepted practice
• By end product
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PREFERRED ANALYSESCOMPOSITION FOR STEELS
ELEMENT PREFERRED HIGH*
• C 0.06 - 0.25% 0.35%• C 0.06 - 0.25% 0.35%
• Mn 0.35 – 0.80 1.40
• Si 0.10 or less 0.30
• S 0.035 or less 0.05
• P 0.030 or less 0.04• P 0.030 or less 0.04
*Additional care required
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EFFECT OF COMPOSITIONON WELDABILITY
• CARBON - main hardening element- increase strength- increase strength- decrease ductility, brittle- contributor to crack= LOW WELDABILITY>0.25% - HAZ becomes hard & brittle
• MANGANESE- <0.35% promote internal porosity& cracks at weld bead.& cracks at weld bead.
- >0.80% promote crack- For good weldability Mn:S ≥ 10:1- MnS less dangerous- FeS causes hot short cracks in WM
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• SULPHUR (SULFUR)– Increase Machinabilty
– Decrease transverse ductility
– Decrease Impact Toughness, brittle
– Promote hot short cracks– Promote hot short cracks
= LOW WELDABILITY
- Acceptance up to 0.035%
- >0.05% serious threat s.a. layers of FeS on fusion line
• SILICON• Acts as Deoxidiser (e.g. Si + MnO → SiO + Mn)• Acts as Deoxidiser (e.g. Si + MnO → SiO + Mn)
• Stimulate cracks on high C content
• For good weldability keep 0.10% or less
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• PHOSPHORUS– In welding P acts as impurity
– Decrease ductility & impact strength
– >0.04% causes brittleness, cracks– >0.04% causes brittleness, cracks
– Lower surface tension of molten WM
– LOW WELDABILITY
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WHERE CAN WE OBTAIN STANDARD SPECIFICATION ON STEEL?
• AISI (American Iron and Steel Institute)
• SAE (Society of Automotive Engineers)• SAE (Society of Automotive Engineers)
• ASTM (American Society for Testing and Materials)
• AISI/SAE → sheet, strip and bar product
• ASTM → Plate & Structural Shape• ASTM → Plate & Structural Shape
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SAMPLE OF AISIDESIGNATION OF STEEL
• E.g. AISI Grade 1035• 35 = 0.32 - 0.38%C• 35 = 0.32 - 0.38%C• 10xx = Non resulphurised• 11xx = Resulphurised• 12xx = Resulphurised, Rephosphurised• Exxxxx = Electric Furnaced Steel• Bxxxxx = Acid Bessemer Steel• No prefix = Basic Open Hearth, Basic Oxygen,
Electric Furnaced SteelElectric Furnaced Steel• xxLxx = Leaded Steel• xxBxx = Boron Treated Steels• xxHxx = Steel with narrow chemical hardenability ranges
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SPECIFICATION OF STEELBASED ON END PRODUCT
• Formable Steel
• Drawable Steel• Drawable Steel
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