456A Metallurgy & Heat Treatment 4 — Questions and Answers
Question 1: What is 'weld metal solidification cracking' and which welding parameter most effectively controls it?
- Cracking along weld centreline during solidification; controlled by maintaining a width-to-depth (W/D) ratio greater than 1 and reducing sulfur/phosphorus impurities (Correct answer)
- Cracking in the HAZ after complete cooling; controlled by preheat temperature
- Surface cracking from weld metal shrinkage; controlled by travel speed
- Cracking at the weld toes from residual stress; controlled by post-weld grinding
Correct 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
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?
- Intergranular cracking in the coarse-grained HAZ during PWHT of Cr-Mo and high-strength alloy steels containing carbide-forming elements (V, Nb, Ti) (Correct answer)
- Cracking during the second weld pass over a previously deposited bead
- Surface cracking during post-weld water cooling
- Cracking in weld metal during stress relief of austenitic stainless steel
Correct 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)
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?
- 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 (Correct answer)
- A stainless steel with dual corrosion resistance grades for different services
- Stainless steel that requires two-pass welding to achieve adequate strength
- A stainless steel with two separate chromium and nickel content ranges
Correct 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
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?
- 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 (Correct answer)
- The steel is in a normalized, soft condition with good weldability
- The hardness indicates excellent weld toughness at low temperatures
- 300 HBW is the standard hardness for all structural steels
Correct 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
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?
- Causes porosity and embrittlement of the weld metal, reducing toughness; in austenitic stainless, it can be used as an austenite stabilizer if controlled (Correct answer)
- Increases yield strength without affecting toughness
- Improves corrosion resistance of carbon steel welds
- Has no significant effect below 500 ppm
Correct answer: Causes porosity and embrittlement of the weld metal, reducing toughness; in austenitic stainless, it can be used as an austenite stabilizer if controlled
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.
Question 6: What is the significance of the 'S curve' (TTT diagram - Time-Temperature-Transformation) for welding engineers?
- Shows the time required at each temperature for austenite to transform to pearlite or bainite, helping predict whether martensite will form at a given cooling rate (Correct answer)
- Maps the relationship between weld current, voltage, and travel speed for a given electrode
- Shows the carbon solubility limit in austenite at different temperatures
- Graphs the ductile-to-brittle transition temperature for different steel grades
Correct answer: Shows the time required at each temperature for austenite to transform to pearlite or bainite, helping predict whether martensite will form at a given cooling rate
The TTT (or continuous cooling transformation, CCT) diagram maps transformation start and finish lines for austenite as a function of temperature and time. By comparing actual weld cooling rates to the 'nose' of the S-curve, welding engineers can predict whether the HAZ will form pearlite (slow cooling), bainite (intermediate cooling), or martensite (fast cooling), guiding preheat and procedure requirements.
456A Metallurgy & Heat Treatment 4
What is 'weld metal solidification cracking' and which welding parameter most effectively controls it?