Water hardening
Water hardening is quenching steel in water. Water cools very sharply and creates maximum hardness – but also maximum stress. Only unalloyed or low-alloy steels can handle it, and even then only with simple shapes.
Physical-Chemical Mechanism of Action
Water pulls heat away faster than anything else in the hardening room. The carbon has no time to escape, and you get the hardest you can get out of this steel. The catch is in the same process: a vapor blanket forms around the part, and it doesn't collapse uniformly everywhere. Where it lasts longer, the steel cools slower, and a soft spot remains there.
Metallurgical Relevance & Welding Risks
Sharp cooling means a steep gradient between surface and core, and that means maximum residual stress. Plus, martensite takes up more space than the previous structure. So the cracking risk is well above that of , and so is . This is only acceptable for unalloyed or low-alloy grades with low , and even then only for simple, uniform cross-sections.
Shop Practice & Action Instructions
Quench quickly and move the part to break the vapor blanket. Use hand-warm water, not ice-cold. Parts with holes, grooves, sharp edges, or big cross-section jumps don't go in water. They'll crack at the notch right away. After quenching, temper and don't let the part cool down completely before that.