Materials and their behaviour during welding
Here the exam probes material grouping, hardenability, cracking mechanisms and the metallurgical reasons behind preheat, interpass and heat-treatment rules.
Core rules
- ISO/TR 15608 groups steels; group 1 covers unalloyed and low-alloy steels with Re ≤ 460 MPa, and the group defines the qualification range of a WPQR.
- IIW carbon equivalent: CEV = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15. Above about 0.40, combined with thickness and hydrogen, cold-cracking risk rises steeply; ISO/TR 17671-2 (formerly EN 1011-2) gives preheat methods.
- Hydrogen designations H5/H10/H15 follow ISO 3690; lower hydrogen permits lower preheat.
- Austenitic stainless: chromium-carbide sensitisation occurs roughly between 425 and 850 °C; control with L-grades, stabilised grades or heat input.
- Duplex: ferrite/austenite balance; excessive heat input promotes sigma phase and 475 °C embrittlement.
- Aluminium: oxide film and hydrogen porosity dominate; 5xxx and 6xxx need different filler choices.
Worked example
C 0.14, Mn 1.35, Cr 0.08, Mo 0.02, V 0.01, Ni 0.12, Cu 0.18. Mn/6 = 0.225; (Cr+Mo+V)/5 = 0.022; (Ni+Cu)/15 = 0.020; CEV = 0.407. At 30 mm thickness with H10 consumables this sits at the 0.40 threshold, so preheat around 100 °C plus interpass control is justified. Ignoring thickness in the same calculation would understate the risk.
Traps
Dividing Cr+Mo+V by 6; treating CEV and Pcm as interchangeable (Pcm serves very low-carbon steels); assuming stainless never needs preheat, when it may need it to dry the joint and control ferrite.