Tempering
Quick Definition
Tempering is a heat treatment applied to quenched steel by reheating to a temperature below the critical point and slow cooling, to reduce brittleness and relieve stresses while retaining acceptable hardness.
Tempering explained
Freshly quenched martensite is hard but contains enormous internal stresses and is too brittle for most engineering applications — it can shatter under moderate impact. Tempering transforms this as-quenched martensite into tempered martensite: carbon atoms and alloying elements partially redistribute, stresses relax, and toughness increases, all at the cost of some hardness reduction. This hardness-toughness trade-off is controlled precisely by choosing tempering temperature.
As tempering temperature increases from room temperature to below the critical point (~Ac1): at 100–200°C, transformation stresses and some carbon precipitation occur with minimal hardness change; at 200–350°C, retained austenite transforms and carbon segregates to dislocations; at 350–450°C (the "tempered martensite embrittlement" zone — often avoided); at 500–650°C, carbides coarsen significantly, hardness drops substantially, and toughness and ductility increase dramatically.
Tempering temperature selection is driven by the application. Cutting tools and cold-work dies: 150–200°C (maximum hardness retention, 60–64 HRC). Structural components requiring toughness: 550–650°C (35–45 HRC). Spring steels: 380–430°C (optimal spring elasticity). Specifications in drawings or material standards define the required hardness range, which the metallurgist converts to a tempering temperature from the alloy's specific tempering curve.
Synonyms & Related Terms
Real-World Example
Bearing rings made from 100Cr6 (SAE 52100) bearing steel are through-hardened to 63–65 HRC by quenching. They are then tempered at 160°C for 2 hours. Hardness drops to 60–62 HRC — still extremely hard for rolling contact fatigue resistance — but dimensional stability is improved and brittleness is reduced to acceptable levels. The bearing manufacturer's specification defines both hardness and the tempering temperature range to ensure consistency across millions of parts.
Why It Matters
Tempering is the essential second step after quenching that makes hardened steel usable. Without tempering, as-quenched parts would crack during machining, break during assembly, or shatter in service under the first impact load. The precise control of tempering temperature is what allows engineers to optimise the hardness-toughness balance for each specific application — a cutting tool needs maximum hardness; a hammer head needs maximum toughness; a gear needs an intermediate balance.
Industry Applications
Related Standards
Frequently Asked Questions
Does tempering always reduce hardness?
Yes — tempering always causes some hardness reduction compared to the as-quenched state. The extent depends on temperature: low tempering (150–200°C) reduces hardness by only 2–4 HRC. High tempering (600°C+) can reduce hardness from 60 HRC to 30–35 HRC. The trade-off is always hardness lost in exchange for toughness gained.
What is double tempering and why is it used?
Some high-alloy tool steels (HSS, H13 hot work steels) contain retained austenite after quenching that transforms to fresh martensite during the first temper. This new martensite is untempered and brittle, so a second temper cycle is required to temper it. Double or triple tempering is standard procedure for high-speed steels and complex die steels.
What is tempered martensite embrittlement (TME)?
TME is a loss of toughness that occurs when certain alloy steels are tempered in the 250–400°C range. Phosphorus and other impurity elements segregate to grain boundaries during this tempering range, dramatically reducing impact toughness. The standard solution is to avoid this temperature range — temper below 250°C or above 400°C.
How is tempering temperature chosen in practice?
The steel supplier provides a tempering curve (hardness vs tempering temperature). The engineer determines the required hardness from the application (e.g. 56–58 HRC for a gear), reads the corresponding temperature from the curve, and specifies a tempering temperature tolerance (e.g. 180 ± 10°C). Time at temperature is typically 1 hour per 25 mm of cross-section.
Can stainless steel be tempered?
Only martensitic stainless steels (410, 420, 440C series) can be quenched and tempered — they respond similarly to alloy carbon steels. Austenitic stainless (304, 316) and ferritic stainless cannot be hardened by quenching at all, so tempering is not applicable. Precipitation-hardening stainless (17-4 PH) uses an "ageing" treatment rather than tempering.
See Also
Related Glossary Terms
Quenching
Quenching is the rapid cooling of a metal from a high temperature by immersion in a cooling medium such as water, oil, or air, to lock in a hardened microstructure.
Annealing
Annealing is a heat treatment that softens a metal: you heat it, hold it, and cool it slowly. It restores ductility and relieves stress. Hardening does the opposite, using a fast quench to make steel harder and stronger.
Hardness
Hardness is a material's resistance to permanent indentation. It is measured by pressing a hard indenter into the surface under a set load, and reported on a scale such as Rockwell (HRC), Brinell (HB) or Vickers (HV). Each scale has its own number, so a hardness value must always name its scale.
Tensile Strength
Tensile strength, or ultimate tensile strength (UTS), is the highest stress a material can carry in tension: the maximum force in a tensile test divided by the original cross-section. Ductile metals reach it before they neck and break.