How to use the quench hardness calculator
For plain and low-alloy steels the as-quenched hardness is governed almost entirely by carbon content, not by the alloying elements. Alloying controls hardenability, which is how deep the hardness reaches; carbon controls how hard fully martensitic steel can get. The widely cited metallurgical approximation for that ceiling is HRC ≈ 60√(%C) + 20, and this calculator applies it first, then adjusts for the martensite percentage actually formed and for the tempering step.
Cooling rate decides how much martensite you get. A thin section in agitated brine or water can reach 99% or more, a typical water quench about 95%, and oil quenching or the core of a heavy section commonly lands in the 80 to 90% band. The 50% martensite point used in hardenability testing sits at roughly 75% of the maximum hardness, which is why Jominy end-quench curves fall away so quickly once the section gets thick.
The tempering drops are taken from the standard tempering curve for AISI 4140 class steel at 400, 600, 800, 1,000 and 1,200 °F. Actual results vary with grade, section size, quenchant agitation and soak time, so use these numbers for process planning and confirm the final specification with hardness testing on real coupons. Tensile strength is an approximate hardness conversion in the style of ASTM A370.
Frequently asked questions
No. Martensite hardness effectively saturates near 0.6% carbon, and above that retained austenite increases so hardness flattens or even drops. This calculator caps the result at 65 HRC.
Hardenability of the specific grade, section thickness, quenchant agitation and tempering soak time all shift the result. For a firm answer, run a Jominy end-quench test and measure real coupons.