Quench Severity
Quench severity is the measure of how aggressively a quenching medium extracts heat from steel during hardening, quantified in Grossmann's framework as the H-value. Higher severity means faster cooling and greater chance of full hardening; lower severity risks incomplete hardening in cores and thick sections. Typical ranges run from about 0.1 for quiescent oil to 1.0 in⁻¹ for water.
On the shop floor, quench severity is selected by matching the steel's hardenability, the part's critical section size, and the required hardness at the deepest section that must transform. The practical workflow compares the alloy's Jominy curve against the expected cooling power of water, oil, polymer, or air. For CNC-machined parts, this matters because stock is often rough-machined before heat treat, then finish machined or ground after quench and temper to hit final tolerance. If the quench is too aggressive, distortion and residual stress can move bores, thin walls, and shafts out of spec. A severe quench also raises cracking risk in sharp corners, heavy section transitions, or high-carbon steels. Shops therefore choose the mildest medium that still produces required hardness and depth. In controlled specs, quench severity ties to measurable flow: impingement targets above 0.76 m/s for water and 1.07 m/s for oil achieve desired surface transformation.
What does the H-value actually represent?
It is a Grossmann-style quench severity parameter that ranks the heat-extracting power of a quenchant or quench system. A higher H-value means more aggressive cooling and a greater likelihood of full hardening in a given section size.
Why does quench severity matter to CNC tolerance control?
Quench-induced transformation strains and thermal gradients change part geometry after rough machining. Tolerance-critical features therefore require post-heat-treat finishing or generous stock allowance to account for distortion and residual stress.
How do shops decide between water, oil, polymer, and air?
They balance required hardness against cracking and distortion risk by checking hardenability data, section thickness, and the cooling rate needed at the critical cross-section. The mildest medium that still achieves the specified depth of hardening is typically selected.