456A Metallurgy & Heat Treatment 1 — Questions and Answers
Question 1: What is the carbon equivalent (CE) formula used in Canada (IIW formula) for assessing the weldability of carbon-manganese steels?
- CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15 (Correct answer)
- CE = C + Mn/4 + Si/4
- CE = C + (Mn+Si)/6 + (Cr+Ni)/10
- CE = C × (1 + Mn/6) × (1 + Cr/5)
Correct answer: CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15
The IIW (International Institute of Welding) carbon equivalent formula used in Canada is CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. A CE above approximately 0.40–0.45% indicates increased susceptibility to hydrogen-induced cracking, requiring preheat, low-hydrogen electrodes, and potentially PWHT. This formula is referenced in CSA W59 and used to determine preheat requirements.
Question 2: What microstructural phase forms in carbon steel when austenite is cooled extremely rapidly (quenched)?
- Martensite (Correct answer)
- Pearlite
- Bainite
- Ferrite
Correct answer: Martensite
Martensite forms when austenite is cooled faster than the critical rate (quenched), preventing carbon from diffusing out of the iron lattice. The carbon becomes trapped, distorting the crystal structure into a hard, brittle body-centred tetragonal (BCT) structure. Martensite in the HAZ of welded joints is a primary cause of hydrogen-induced cracking.
Question 3: What is the purpose of 'tempering' a hardened steel weld or HAZ?
- Reheating the hardened zone to a temperature below the lower critical temperature to reduce brittleness while maintaining hardness (Correct answer)
- Rapidly cooling the steel to lock in a hard martensitic structure
- Heating steel above the upper critical temperature to dissolve carbides
- Normalizing the steel by air cooling from above the critical temperature
Correct answer: Reheating the hardened zone to a temperature below the lower critical temperature to reduce brittleness while maintaining hardness
Tempering involves reheating hardened (martensitic) steel to a temperature below Ac1 (typically 150–650°C depending on the application). This allows some carbon to diffuse, relieving internal stresses, increasing toughness and ductility, and slightly reducing hardness. PWHT of carbon steels typically serves as a stress relief and temper.
Question 4: What is the 'heat-affected zone' (HAZ) in a weld and why is it often the weakest area of a welded joint?
- The area adjacent to the fusion line that experienced elevated temperatures during welding; it can contain coarse grains, martensite, or soft zones depending on the steel and cooling rate (Correct answer)
- The weld metal itself, which has lower strength than the base metal
- The zone where the weld reinforcement meets the base metal surface
- The area preheated before welding to prevent cold cracking
Correct answer: The area adjacent to the fusion line that experienced elevated temperatures during welding; it can contain coarse grains, martensite, or soft zones depending on the steel and cooling rate
The HAZ is the base metal zone that was heated by welding but did not melt. Temperatures in the HAZ range from near the melting point (at the fusion line) to just above ambient (at the outer edge). Depending on the steel's composition and the thermal cycle, the HAZ can contain coarse-grained austenite (reducing toughness), hard martensite (causing HIC susceptibility), or over-tempered soft zones in previously heat-treated steels.
Question 5: What is 'hot cracking' (solidification cracking) in welds and which elements in steel promote it?
- Cracking during weld metal solidification; promoted by high sulfur, phosphorus, and low melting point impurities that form liquid films at grain boundaries (Correct answer)
- Cracking after complete solidification due to residual stresses; promoted by high carbon
- Cracking during preheat application; promoted by high manganese
- Cracking during PWHT; promoted by high chromium
Correct answer: Cracking during weld metal solidification; promoted by high sulfur, phosphorus, and low melting point impurities that form liquid films at grain boundaries
Solidification cracking (hot cracking) occurs as the weld metal solidifies and shrinks. Low-melting-point liquid films (formed by sulfur, phosphorus, and other impurities) persist at grain boundaries after the surrounding metal has solidified, and weld shrinkage tears these films open. High S and P content, along with high restraint and convex bead profiles, increase susceptibility.
Question 6: What is 'lamellar tearing' and in what type of steel joint configuration does it occur?
- Subsurface cracking parallel to the plate surface caused by through-thickness tensile stresses acting on non-metallic inclusions; occurs in T-joints and corner joints with through-thickness loading (Correct answer)
- Surface cracking along the weld toe caused by fatigue loading
- Intergranular cracking in the HAZ of hardenable steels
- Root cracking in groove welds caused by hydrogen
Correct answer: Subsurface cracking parallel to the plate surface caused by through-thickness tensile stresses acting on non-metallic inclusions; occurs in T-joints and corner joints with through-thickness loading
Lamellar tearing occurs in rolled steel plate when through-thickness tensile stresses (from weld shrinkage) act on flattened non-metallic inclusions (typically sulfide stringers) parallel to the rolling plane. T-joints and corner joints where weld shrinkage pulls perpendicular to the plate surface are most susceptible. Low-sulfur steel (S < 0.005%) or Z-grade steel with good through-thickness ductility mitigates the risk.
456A Metallurgy & Heat Treatment 1
What is the carbon equivalent (CE) formula used in Canada (IIW formula) for assessing the weldability of carbon-manganese steels?