456A Metallurgy & Heat Treatment 3

Question 1

What is the purpose of 'quench and temper' heat treatment for structural steel plates (e.g., A514/T1 steel)?

Accepted Answer

Produces a fine martensitic microstructure with high strength and toughness not achievable by normalizing alone

Answer

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?

Accepted Answer

The amount of delta ferrite in the austenitic weld microstructure; 3–8 FN is typically targeted to prevent hot cracking

Answer

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?

Accepted Answer

The minimum cooling rate that produces a fully martensitic structure, bypassing pearlite and bainite transformations

Answer

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?

Accepted 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

Answer

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?

Accepted Answer

The percentage of base metal melted into the weld pool; higher dilution shifts weld metal composition toward the base metal composition

Answer

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.

Red Seal Welder 456A Practice Test

Red Seal Welder 456A Practice Test

456A Metallurgy & Heat Treatment 3