Materials and Their Behaviour During Welding
This module tests your ability to predict what the thermal cycle does to the parent metal and weld metal, and to specify the measures that prevent cracking and loss of properties.
The main damage mechanisms
- Hydrogen-assisted cold cracking needs four things together: diffusible hydrogen, a susceptible (hard) microstructure, tensile restraint and low temperature. Remove any one and cracking is prevented.
- Solidification (hot) cracking occurs at the centreline or in the mushy zone with high sulphur/phosphorus, deep narrow beads and high restraint.
- Lamellar tearing occurs in thick restrained T-joints where through-thickness ductility is poor in the parent plate.
- Reheat cracking affects coarse-grained HAZ in thick Cr-Mo and 2.25Cr steels during post-weld heat treatment.
HAZ control
Hardness in the HAZ is governed by the cooling time t8/5, which falls as heat input falls and as plate thickness rises. Preheat and heat input are chosen to keep HAZ hardness below the agreed limit (commonly 350 HV10 for carbon steels) using EN 1011-2.
Worked example: carbon equivalent
For C 0.15, Mn 1.20, Cr 0.10, Mo 0.02, V 0.01, Ni 0.10, Cu 0.10: CEV = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15 = 0.15 + 0.20 + 0.026 + 0.013 = 0.39. Below 0.40, the steel is normally weldable without preheat for thin sections, but a 40 mm joint still needs preheat because thickness drives hydrogen escape and restraint. Do not apply this formula to austenitic stainless steels.
Traps
- Believing preheat alone makes a high-hydrogen process acceptable - use low-hydrogen consumables and bake them.
- Quoting CEV for stainless or for high-carbon steels where it loses meaning.
- Ignoring that austenitic stainless is sensitised by chromium carbide precipitation around 450–850 °C, so heat input and interpass temperature matter.