Martempering
Martempering, also called marquenching or interrupted quenching, is a steel heat-treatment process. An austenitized part is quenched into a hot oil, salt, or lead bath held just above the martensite start (Ms) temperature, held until the cross-section equalizes, then air-cooled to room temperature, typically followed by tempering. This reduces thermal stress and distortion compared with direct quenching.
In a CNC production environment, martempering is scheduled before final machining whenever a hardened steel blank must hold tight geometry. Consider a gear or long shaft with thin web sections: direct quenching slams the surface into martensite while the core is still hot, creating steep thermal gradients that pull flats, bores, and datum features out of tolerance. Martempering avoids that by dropping the part into a hot salt or oil bath held just above the Ms point. The surface and core equalize there, and only during the subsequent air cool does martensite form uniformly. On the shop floor this means less warp, fewer cracked corners, and less stock left for post-heat-treat grinding. It also helps maintain datums and hole positions, so machining can recover geometry with a lighter cleanup pass rather than re-establishing every reference surface. The process still requires a final temper cycle and final inspection.
- Bath temperature slides low: If the bath falls below the intended range, martensite can form before the core and surface equalize, bringing back steep thermal stress and risking checking, cracking, or bowing in long or asymmetric parts.
- Short holds in the bath: Holding too briefly leaves the core hotter than the surface, so later transformation creates internal tensile stress gradients that appear as twist, potato-chipping, or hole-to-hole drift after air cooling.
- Lean-alloy steel in a massive section: If the steel lacks hardenability or the section is too thick, incomplete hardening and mixed microstructures can occur, causing just as much distortion and failed hardness despite the interrupted quench.
What microstructural change is martempering trying to control?
It delays martensite formation until the cross-section reaches uniform temperature. Quenching stops just above the martensite start (Ms) temperature, so the part transforms during air cooling with a reduced thermal gradient, lowering residual stress and the risk of quench cracking.
Why not quench straight to room temperature?
Direct quenching pulls the surface below Ms while the core is still hot, causing martensite to form under severe thermal contraction stress. Martempering interrupts that pathway by equalizing temperature before transformation, producing more uniform dimensions and lower cracking risk.
When is martempering preferred over conventional quenching?
It is chosen for parts with critical tolerances, nonuniform thickness, or high quench-distortion risk, such as gears, shafts, tooling, and heavy sections, where post-heat-treat correction is expensive. It reduces corrective grinding or remachining but does not replace final tempering or inspection.