Welding Theory, Metallurgy, and Heat Input
This domain tests welding theory, metallurgy and heat input: how arc energy affects penetration, cooling rate, microstructure and cracking risk. Expect calculation items plus questions on the heat-affected zone (HAZ) and preheat.
Heat input
For arc welding, heat input is usually:
HI = (V × A × 60) / travel speed in J/mm, with travel speed in mm/min. For heat input per unit length, current and voltage are measured at the arc. If arc efficiency is requested, multiply by efficiency before comparing processes.
Metallurgy
- Fast cooling in the HAZ can form hard martensite in carbon and low-alloy steels.
- Hydrogen from moisture, grease or electrodes can cause delayed hydrogen cracking.
- Preheat slows cooling and helps hydrogen escape; it is not a substitute for dry consumables.
- Carbon equivalent estimates hardenability; higher CE usually needs more preheat.
Worked example
A process runs at 24 V and 180 A with a travel speed of 200 mm/min. HI = (24 × 180 × 60) / 200 = 1,296 J/mm. If travel speed doubles to 400 mm/min, HI halves to 648 J/mm at the same volts and amps. The reasoning: travel speed is in the denominator, so faster travel reduces heat input per length. If the answer choices use cm/min or in/min, convert before calculating.
Traps
Do not use current alone as heat input. Do not mix mm/min with cm/min. Do not assume faster travel always improves quality; it can reduce fusion. Do not confuse preheat with post-weld heat treatment.