456A Metallurgy & Heat Treatment 3 — Questions and Answers
Question 1: What is the purpose of 'quench and temper' heat treatment for structural steel plates (e.g., A514/T1 steel)?
- Produces a fine martensitic microstructure with high strength and toughness not achievable by normalizing alone (Correct answer)
- Softens the steel to improve machinability before fabrication
- Relieves residual stresses from the rolling process
- Refines grain size by thermal cycling between transformation temperatures
Correct answer: Produces a fine martensitic microstructure with high strength and toughness not achievable by normalizing alone
Quench and temper (Q&T) heat treatment produces the high-strength, high-toughness microstructure of steels like A514 (T-1). The steel is austenitized, water or oil quenched to form martensite, then tempered to reduce brittleness while maintaining high strength. Q&T steels require special welding procedures to avoid softening the HAZ and must use low-hydrogen processes.
Question 2: What does 'ferrite number' (FN) indicate in stainless steel weld metal?
- The amount of delta ferrite in the austenitic weld microstructure; 3–8 FN is typically targeted to prevent hot cracking (Correct answer)
- The carbon content of the stainless steel weld deposit
- The number of weld passes required for full ferrite coverage
- The chromium-to-nickel ratio in the base metal
Correct answer: The amount of delta ferrite in the austenitic weld microstructure; 3–8 FN is typically targeted to prevent hot cracking
Delta ferrite in austenitic stainless steel weld metal prevents hot (solidification) cracking by interrupting the continuous austenite grain boundaries where low-melting films form. The target FN of 3–8 provides hot crack resistance without excessive ferrite, which can reduce corrosion resistance and toughness. FN is predicted by the Schaeffler or WRC-1992 diagrams from the weld metal composition.
Question 3: What is the 'critical cooling rate' for a steel?
- The minimum cooling rate that produces a fully martensitic structure, bypassing pearlite and bainite transformations (Correct answer)
- The cooling rate that causes maximum distortion in the weld joint
- The rate at which hydrogen escapes from the HAZ to prevent HIC
- The cooling rate required to prevent sensitization in stainless steel
Correct answer: The minimum cooling rate that produces a fully martensitic structure, bypassing pearlite and bainite transformations
The critical cooling rate is the minimum rate at which austenite must be cooled to suppress all diffusional transformations (pearlite, bainite) and produce a fully martensitic structure. Steels with higher hardenability (higher CE) have lower critical cooling rates and therefore form martensite more easily — including in the HAZ during normal welding cooling conditions.
Question 4: What is the Ac1 temperature in steel and why is it important for PWHT?
- The lower critical temperature (~727°C) below which austenite transforms to ferrite+carbide on cooling; PWHT must be performed below this temperature to avoid re-austenitizing the steel (Correct answer)
- The temperature at which martensite begins to form during quenching
- The temperature at which steel loses its magnetic properties
- The temperature above which all carbon dissolves in austenite (upper critical temperature)
Correct answer: The lower critical temperature (~727°C) below which austenite transforms to ferrite+carbide on cooling; PWHT must be performed below this temperature to avoid re-austenitizing the steel
Ac1 is the temperature at which austenite begins to form during heating (approximately 727°C for carbon steel). PWHT stress relief must be performed below Ac1 to avoid re-austenitizing portions of the HAZ, which would defeat the purpose of the treatment and could introduce new hard zones upon cooling. Typical stress relief temperatures (595–650°C) provide a margin below Ac1.
Question 5: What is the 'dilution' in weld metal and how does it affect weld metal properties?
- The percentage of base metal melted into the weld pool; higher dilution shifts weld metal composition toward the base metal composition (Correct answer)
- The amount of shielding gas dissolved in the weld metal
- The ratio of slag to weld metal produced during welding
- The reduction in strength caused by overheating of the weld metal
Correct answer: The percentage of base metal melted into the weld pool; higher dilution shifts weld metal composition toward the base metal composition
Dilution is the percentage of the weld metal composition contributed by the melted base metal (vs. the filler metal). High dilution means the weld chemistry is heavily influenced by the base metal. In cladding or overlay applications, multiple layers reduce dilution of the top layer. In dissimilar metal welding, dilution affects whether the weld metal composition produces the desired microstructure.
Question 6: What is the difference between 'yield strength' and 'ultimate tensile strength' (UTS) of a weld metal?
- Yield strength is the stress at which permanent deformation begins; UTS is the maximum stress the material can withstand before fracture (Correct answer)
- Yield strength is measured at room temperature; UTS is measured at elevated temperature
- UTS is the same as yield strength for weld metals due to work hardening
- Yield strength is measured in hardness units; UTS is measured in MPa
Correct answer: Yield strength is the stress at which permanent deformation begins; UTS is the maximum stress the material can withstand before fracture
Yield strength (0.2% proof stress in Canada, MPa) is the stress at which the material begins to deform plastically (permanently). UTS is the maximum engineering stress reached before the specimen fractures in a tensile test. Weld filler metals are classified by their minimum UTS (e.g., E7018 = 70 ksi/480 MPa UTS minimum). Structural design typically uses yield strength as the allowable stress basis.
456A Metallurgy & Heat Treatment 3
What is the purpose of 'quench and temper' heat treatment for structural steel plates (e.g., A514/T1 steel)?