Tempering hardness testing is a critical quality control procedure used to verify that quenched steel components have achieved the correct balance of hardness, toughness, and dimensional stability after the tempering process. This technical article explains the principles of tempering hardness determination, applicable international standards, sample requirements, testing methods, and typical industrial applications. Whether you are qualifying fasteners, tool steels, or automotive powertrain parts, understanding proper hardness measurement after tempering ensures reliable mechanical performance and compliance with specification.
Technical Overview
Tempering is a sub-critical heat treatment process in which a fully hardened (quenched) steel component is reheated to a temperature below the lower critical transformation point (Ac1), held for a specified time, and then cooled at a controlled rate. The primary purpose of tempering is to relieve residual quenching stresses, reduce brittleness of the as-quenched martensite, and adjust the hardness–toughness balance to suit the intended service conditions. Because hardness decreases predictably as tempering temperature and holding time increase, hardness measurement serves as the most practical and widely accepted indicator of whether the tempering cycle has been correctly executed.
Tempering hardness determination involves measuring the indentation hardness of a component or test coupon after the quench-and-temper (Q&T) cycle using standardized methods such as Rockwell, Vickers, or Brinell testing. The measured value is compared against drawing specifications, material standards, or established tempering curves to confirm conformity. For most hardened and tempered engineering steels, Rockwell C (HRC) is the preferred scale; for thin sections, case-hardened parts, or when a converted value of higher precision is required, Vickers (HV) testing is commonly applied. The relevant international standards include ISO 6508-1 and ASTM E18 for Rockwell hardness, ISO 6507-1 and ASTM E92 for Vickers hardness, and ISO 6506-1 and ASTM E10 for Brinell hardness, together with their corresponding national standards such as GB/T 230.1, GB/T 4340.1, and GB/T 231.1.
A key metallurgical phenomenon that must be considered during tempering hardness evaluation is secondary hardening, which occurs in steels containing strong carbide-forming elements such as chromium, molybdenum, vanadium, and tungsten. In these steels, hardness may initially decrease and then peak again at tempering temperatures around 500–600 °C due to the precipitation of fine alloy carbides. Accurate hardness determination therefore requires knowledge of the steel grade and its characteristic tempering response curve.
Testing Samples
Finished Quenched and Tempered Components
Direct measurement on finished parts such as shafts, gears, bolts, crankshafts, camshafts, and bearing rings is performed on accessible, functionally non-critical flat surfaces. The test location must be representative of the hardened zone, and any oxide scale, decarburized layer, or grinding burn present after heat treatment must be removed by light grinding or polishing before testing, since surface conditions can significantly distort the result.
Heat Treatment Witness Coupons
When the component geometry is unsuitable for direct indentation, witness coupons of the same steel grade are heat treated together with the production load and subsequently tested. Coupon dimensions typically follow the recommendations of the applicable hardness testing standard, ensuring sufficient thickness (generally at least ten times the indentation depth) and parallel support and testing faces.
Fastener and Threaded Product Samples
High-strength fasteners of property classes 8.8, 10.9, and 12.9 are tempered and then hardness tested in accordance with fastener standards such as ISO 898-1 or ASTM F3125. Testing is performed on the shank, head, or a transverse section, and the measured hardness must fall within the range specified for each property class after the mandatory tempering treatment.
Tool Steel and Die Block Samples
Cold-work tool steels, hot-work die steels, and high-speed steels are tempered, often with multiple tempering cycles, and hardness is verified on ground flats or on prepared cross-sections. For these materials, hardness confirmation after each tempering stage is essential to detect under-tempering, over-tempering, or retained austenite-related instability.
Case-Hardened and Nitrided Parts
For carburized or carbonitrided components that receive a subsequent low-temperature temper, hardness determination may include surface hardness together with a microhardness traverse across the case to establish the effective hardening depth, using Vickers or Knoop microindentation methods.
Testing Items
The scope of tempering hardness determination typically covers the following measurable parameters:
- Rockwell hardness after tempering, expressed as HRC, HRA, or HR30N depending on hardness level and section thickness, tested per ISO 6508-1, ASTM E18, or GB/T 230.1.
- Vickers hardness after tempering, expressed as HV10, HV30, HV50, or HV5 for small parts, tested per ISO 6507-1, ASTM E92, or GB/T 4340.1.
- Brinell hardness for medium-hardness tempered structural steels, expressed as HBW, tested per ISO 6506-1, ASTM E10, or GB/T 231.1.
- Hardness uniformity surveys across a component or a load of parts to evaluate the consistency of the quench-and-temper process.
- Surface hardness versus core hardness differentiation for case-hardened and induction-hardened parts that receive a tempering treatment.
- Hardness-depth profiles (effective case depth) determined by micro-Vickers traverse on a polished transverse section.
- Conversion of measured hardness values between scales where permitted, using conversion references such as ASTM E140, ISO 18265, or GB/T 1172, recognizing that conversions are approximate.
- Verification against specified acceptance bands in product standards, for example ISO 898-1 for fasteners, EN 10083 for quenched and tempered steels, EN 10025-6 for flat products, and GB/T 3077 for alloy structural steels.
Testing Methods
Rockwell Hardness Method
Rockwell testing is the most common method for tempered hardened steel because it is rapid, produces a small indentation, and reads hardness directly. The HRC scale applies a 150 kgf major load with a diamond cone indenter and is suitable for tempered hardness above approximately 20 HRC. The machine must be verified daily using certified reference blocks, and the test piece must be supported rigidly on a clean anvil with the test surface free of scale, oil, and decarburization. Ambient temperature is normally maintained between 10 °C and 35 °C as required by the standards.
Vickers Hardness Method
Vickers testing uses a diamond pyramid indenter with test forces commonly between 1 kgf and 100 kgf. It provides excellent repeatability for tempered tool steels and is preferred for thin sections, shallow cases, and when hardness must be measured at specific microstructural locations on a polished and etched cross-section. Indentation spacing rules must be observed: the distance between the centers of two adjacent indentations and from the specimen edge must be at least 2.5 to 3 times the diagonal length, and specimen thickness must be at least 1.5 times the diagonal length.
Brinell Hardness Method
Brinell testing with a tungsten carbide ball is applied to tempered steels of lower hardness, typically below about 450 HBW, such as quenched and tempered structural plates and large forgings. The large indentation averages local microstructural variations and is less sensitive to minor surface roughness, but it requires larger, thicker specimens and flat accessible areas.
Specimen Preparation Requirements
Reliable tempering hardness determination depends on correct preparation. The test surface must be prepared by removing any decarburized, carburized, or oxide layer introduced during heat treatment, normally by grinding a minimum depth specified by the testing procedure while avoiding overheating that could locally re-temper the surface. Both the test surface and the support surface should be flat, clean, and parallel. For microhardness work on cross-sections, specimens are sectioned without burning, mounted, ground, and polished to a finish suitable for accurate diagonal measurement.
Procedure and Data Evaluation
A minimum number of valid indentations, commonly at least three to five, is taken at representative locations, and the reported result is the mean value together with the range. Outliers caused by surface defects, inclusions, or edge effects are identified and excluded according to the governing standard. Results are then compared with the acceptance criteria on the engineering drawing or in the referenced material specification, and a pass/fail or measured-range report is issued, including test method, scale, force, location, temperature, and instrument calibration status.
Quality Assurance of the Measurement
Hardness testers must be periodically verified by indirect verification with certified reference blocks traceable to national metrology institutes, and direct verification of force, indenter geometry, and depth measurement must be performed at defined intervals in accordance with ISO 6508-2, ISO 6507-2, ISO 6506-2, and the corresponding ASTM verification practices. Testing is recommended to be conducted by trained personnel in accredited third-party laboratories or in in-house laboratories operating under an ISO/IEC 17025 compliant management system.
Applications
Tempering hardness determination is applied across virtually every sector in which hardened steel components are used:
- Fastener manufacturing, where quenched and tempered bolts, screws, and studs of classes 8.8 to 12.9 are hardness verified per ISO 898-1 and related ASTM specifications.
- Automotive powertrain components, including gears, crankshafts, camshafts, connecting rods, and transmission shafts, where tempering hardness confirms fatigue strength and wear resistance.
- Bearing production, where rings and rolling elements are tempered to stabilize dimension and hardness, and measured hardness confirms the heat treatment result.
- Tool and die industries, where cold-work, hot-work, and high-speed tool steels are tempered, frequently with triple tempering, and hardness is checked after each cycle.
- Spring manufacturing, where oil-tempered wire and quenched-and-tempered leaf and coil springs are hardness tested to verify the specified strength class.
- Structural steel plate and forged components for pressure equipment, bridges, and construction machinery, produced to quenched-and-tempered delivery conditions such as EN 10025-6 and ASTM A514, with hardness and tensile verification.
- Heat treatment job shops and metallurgical failure analysis, where tempering hardness measurements support process validation, root cause investigation of in-service brittleness or softening, and audit of supplier quality.
- Research and development, where tempering curves (hardness versus tempering temperature and time) are established for new steel grades or for optimizing existing heat treatment recipes.
FAQ
What is tempering hardness testing?
Tempering hardness testing is the measurement of the indentation hardness of a steel part after it has been quenched and subsequently tempered. Because tempering reduces hardness in a predictable way, the measured hardness value directly indicates whether the tempering temperature, holding time, and cooling were correct, and whether the part meets its specified mechanical requirements.
Why does hardness decrease after tempering?
During tempering, the supersaturated carbon in the as-quenched martensite precipitates as transition carbides and cementite, and the distorted martensite lattice relaxes into tempered martensite. This loss of lattice strain and carbon supersaturation lowers hardness while simultaneously improving toughness and reducing residual stress. The degree of softening increases with tempering temperature and holding time, except in secondary-hardening alloy steels where fine alloy carbide precipitation can temporarily raise hardness.
Which hardness scale should be used for tempered steel?
For hardened and tempered engineering and tool steels, typically ranging from about 30 to 65 HRC, the Rockwell C scale is standard. For thin parts, shallow hardened cases, or when a metallurgically precise value is needed, Vickers testing is preferred. Brinell testing is reserved for softer tempered structural steels, generally below 450 HBW, where the larger ball indentation averages over a greater area.
How is the surface prepared before testing?
The test surface must have any oxide scale, decarburized layer, or coating removed by careful grinding or polishing, because a soft decarburized skin or a hard oxidized crust will produce misleading readings. Care must be taken not to overheat the surface during grinding, since local frictional heating can re-temper or even re-harden the surface and falsify the result. The opposite support surface must also be flat and free of scale so the specimen sits rigidly on the anvil.
How many measurements are required?
Most standards and internal procedures require a minimum of three to five valid indentations at representative locations, with the average reported as the result. Indentations must be spaced at least 2.5 to 3 times the indentation diameter or diagonal apart and kept away from edges, and values influenced by visible defects may be discarded according to the rules of the applied standard.
Can hardness values be converted between HRC, HV, and HBW?
Approximate conversions are possible using standardized conversion tables such as ASTM E140, ISO 18265, or GB/T 1172, but conversions are valid only for the specific material class on which the tables were based. For acceptance purposes, direct measurement in the specified scale is always preferable to a converted value.
What does an out-of-specification tempering hardness result indicate?
A hardness value above specification usually indicates insufficient tempering, too low a tempering temperature, too short a holding time, or a mixed microstructure; it is often accompanied by unacceptable brittleness. A value below specification suggests over-tempering, an excessively high temperature, incomplete hardening before tempering, or surface decarburization. In either case, metallographic examination and a re-test on a properly prepared surface are used to confirm the finding before disposition or re-heat treatment of the parts.