Materials and their behaviour during welding
This domain tests how base materials respond to the welding thermal cycle: softening, hardening, distortion, sensitisation and loss of toughness. The exam wants cause-and-effect reasoning from peak temperature and cooling rate.
The thermal cycle and HAZ
The heat-affected zone is characterised by peak temperature and cooling time from 800 to 500 °C (t8/5). Long t8/5, produced by high heat input, preheat or thick-section heat retention, coarsens grains and lowers toughness in some steels. Short t8/5, produced by low heat input or rapid extraction, favours hard martensite and cold cracking risk.
Heat input
For arc welding: HI = (60 × U × I) / (1000 × v) in kJ/mm, with v in mm/s. Travel speed is the most sensitive variable because it sits in the denominator.
Worked example
U = 26 V, I = 280 A, v = 6 mm/s. HI = (60 × 26 × 280) / (1000 × 6) = 436,800 / 6,000 = 0.73 kJ/mm. If the procedure quoted 360 mm/min, the correct conversion is 6 mm/s before substitution; otherwise the answer is wrong by a factor of 60. That 0.73 kJ/mm then feeds EN 1011-2 preheat assessment together with carbon equivalent and thickness.
Material-specific behaviour
- Aluminium: high thermal conductivity and no phase transformation; needs higher current, careful cleaning and often preheat on thick sections.
- Austenitic stainless: low conductivity, high expansion, distortion-prone; sensitisation occurs in the 450–850 °C range, so carbon content and t8/5 matter.
- Quenched and tempered steels: heat input limits protect the tempered microstructure.
Traps
Using mm/min directly in the formula; ignoring that preheat, plate thickness and joint type also change t8/5; assuming heat input alone defines toughness.