Tempering After Hardening: Why It Matters and How to Control Final Hardness

Hardening can form martensite and significantly increase the hardness and wear resistance of steel. A freshly quenched component, however, may also contain high residual stress, limited toughness, retained austenite, and an increased risk of cracking or distortion. For most hardened steel components, tempering is not an optional correction. It is an integral part of the heat-treatment route used to achieve the specified final properties. Tempering reheats hardened steel below the critical transformation range, holds it for a defined period, and cools it under controlled conditions. A suitable cycle relieves stress, adjusts toughness, stabilizes the microstructure, and delivers the properties required by the drawing or material specification. There is no universal tempering temperature. SKD11, SKD61, S45C, and SCM440 respond differently even when processed at the same nominal temperature. Steel grade, starting condition, austenitizing temperature, quench method, section size, number of tempering cycles, and service requirements must be considered together.

Multiple tempering is commonly considered for tool steels and certain high-alloy steels. After the first temper, some retained austenite may transform into fresh martensite during cooling. A subsequent temper can temper this newly formed martensite and improve microstructural stability. “Double temper” is not a universal rule. The number of cycles, temperature of each cycle, and cooling conditions must follow the steel producer's recommendation or the customer's specification. If sub-zero treatment is required, its sequence relative to tempering must also be defined before production.

Temper embrittlement is a loss of toughness that can occur under particular combinations of steel chemistry, tempering range, holding time, and cooling rate. It is inaccurate to apply one prohibited temperature range to every steel. For impact-loaded components, large sections, or alloy grades known to be sensitive, the heat-treatment provider should assess the risk using grade-specific technical data. The RFQ should describe the critical service condition and mechanical-property requirement rather than specify only one HRC value.

Acceptance hardness should normally be measured after the final temper and after the component has cooled and stabilized according to the procedure. A reading taken immediately after hardening is useful for process monitoring, but it does not represent the final delivered condition. The result also depends on method, scale, material thickness, surface curvature, surface finish, and test location. Oxide, decarburization, or a rough surface can bias the reading. If surface and core hardness must be distinguished, the drawing or inspection requirement should identify the location, depth, and test method. A practical inspection plan normally includes:

• Verification of steel grade and incoming condition.
• Hardness testing after the final heat-treatment operation.
• Defined scale, test locations, and number of readings.
• Distortion or dimensional inspection before finish grinding.
• Heat-treatment certification and test results when contractually required.

Most heat-treatment distortion develops during heating and quenching, but tempering continues to relieve stress and may cause further dimensional change. The magnitude depends on material, geometry, section uniformity, support method, furnace loading, and machining history. Tight-tolerance components should retain a planned allowance for grinding or finish machining after heat treatment. One allowance must not be applied to every part. Thin, long, deeply slotted, or asymmetric components require individual assessment because their distortion risk is higher.

For accurate process selection and quotation, please provide:

• Material grade, governing standard, and supply condition.
• Required hardness range, scale, and test location.
• Mandatory heat-treatment procedure or specification, if applicable.
• Requirements for mechanical properties, microstructure, surface hardness, or core hardness.
• Dimensional tolerances and GD&T applicable after heat treatment.
• Surfaces to be ground, designed allowance, and required roughness.
• Requirements for heat-treatment certificates, hardness reports, and lot traceability.
• Prototype quantity, batch quantity, and critical service conditions.