Soft spots and incomplete hardening

Root causes — most common first

1. 1Vapour blanketing in the quench — a steam or gas film trapped against a surface, or between nested parts, prevents contact with liquid at the critical moment.
2. 2Basket loading: parts nested, stacked or packed too densely for quenchant to reach every surface. The same recipe is often fine at a lower load.
3. 3Scale, drawing lubricant, oil or rust acting as an insulating film during the quench.
4. 4Quench oil out of condition — too hot, contaminated with water, or degraded, so its cooling curve no longer suits the grade.
5. 5Insufficient hardenability for the section: a plain-carbon or low-boron grade at the top of its size range cannot through-harden, which looks like soft spots but is metallurgy, not process.
6. 6Uneven furnace temperature or a short soak, leaving part of the load not fully austenitised before the quench.

Process controls that prevent it

• Fix loading first — a defined weight and layer depth per basket, no nesting, and fixtures for parts that trap gas.
• Control agitation so flow reaches the whole load, and maintain quench oil temperature, water content and condition on a schedule.
• Clean parts before hardening: degrease and remove scale and drawing lubricant so quenchant wets bare steel.
• Select the grade against the section — use a boron or alloy grade where a plain-carbon steel cannot make hardness through the diameter.
• Survey the furnace and count soak time from when the load reaches temperature, not from charging.
• Sample hardness from several positions in the load, including the worst position found in a load survey, instead of one convenient piece.