456A Metallurgy & Heat Treatment 4

Question 1

What is 'weld metal solidification cracking' and which welding parameter most effectively controls it?

Accepted Answer

Cracking along weld centreline during solidification; controlled by maintaining a width-to-depth (W/D) ratio greater than 1 and reducing sulfur/phosphorus impurities

Answer

Centreline solidification cracking occurs when the last liquid (enriched in impurities like S and P) in the weld pool solidifies under tensile shrinkage stress. A weld bead that is deeper than it is wide (W/D < 1) concentrates this liquid at the centerline under maximum stress. Maintaining W/D > 1 distributes the liquid at the top surface where stresses are lower and prevents centreline crack formation.

Question 2

What is 'reheat cracking' (stress relaxation cracking) and in which steels does it occur?

Accepted Answer

Intergranular cracking in the coarse-grained HAZ during PWHT of Cr-Mo and high-strength alloy steels containing carbide-forming elements (V, Nb, Ti)

Answer

Reheat cracking occurs during PWHT or high-temperature service when the coarse-grained HAZ cannot accommodate relaxation of residual stresses by plastic deformation. Carbide-forming elements (V, Nb, Ti) that precipitate during PWHT strengthen the grain interiors, forcing relaxation to occur at the weaker grain boundaries, causing intergranular cracking. Cr-Mo steels (used in power generation and pressure vessels) are most susceptible.

Question 3

What is 'duplex stainless steel' and what special welding consideration applies to it?

Accepted Answer

A two-phase (austenite + ferrite) stainless steel that requires heat input and interpass temperature control to maintain the 50/50 phase balance and avoid sigma phase or excessive ferrite

Answer

Duplex stainless steels (e.g., 2205) contain approximately 50% austenite and 50% ferrite in the microstructure, providing high strength and excellent corrosion resistance. Welding must maintain the phase balance: insufficient heat input or fast cooling produces excessive ferrite (reducing toughness and corrosion resistance); excessive heat input or slow cooling produces sigma phase (brittle intermetallic compound). Heat input and interpass temperature must stay within WPS limits.

Question 4

What does a Brinell hardness number (HBW) of 300 indicate about a steel's condition relevant to welding?

Accepted Answer

The steel is in a significantly hardened condition (approximately 30 HRC / 1000 MPa UTS) that may require special welding procedures, preheat, and post-weld treatment

Answer

300 HBW corresponds to approximately 30 HRC and a UTS of about 1000 MPa, indicating a hardened condition. Such steel has limited ductility and is susceptible to hydrogen-induced cracking in the HAZ. Special welding procedures including high preheat temperatures, low-hydrogen processes (E7018 or better), controlled heat input, and possibly PWHT are required.

Question 5

What is the effect of nitrogen pickup in weld metal?

Accepted Answer

Causes porosity and embrittlement of the weld metal, reducing toughness; in austenitic stainless, it can be used as an austenite stabilizer if controlled

Answer

Nitrogen in excess is harmful: it forms N2 gas bubbles (porosity) if it exceeds solubility limits during solidification, and it can embrittle ferritic and martensitic steels by strain aging. In austenitic stainless steels, controlled nitrogen additions (as in 316N or 304N) stabilize austenite and increase strength. In welding, nitrogen contamination from poor shielding is a defect source.

Red Seal Welder 456A Practice Test

Red Seal Welder 456A Practice Test

456A Metallurgy & Heat Treatment 4