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Tempering Temperature and Hardness: Relationship by Steel Grade

Tempering is the step after quenching that transforms brittle as-quenched martensite into service-ready tempered martensite — and the tempering temperature is the single most important parameter determining the final hardness, strength, and toughness of a heat-treated steel part. UTEC Industrial provides in-house induction hardening, through-hardening, and quench-and-temper heat treating services for industrial components in the Pacific Northwest, with integrated CNC machining and reverse-engineering capability. The relationship between tempering temperature and resulting hardness is well-characterized for each steel grade: there is a predictable, monotonically decreasing hardness curve as tempering temperature increases. Understanding these curves — and the exceptions, embrittlement ranges, and grade-specific anomalies that modify them — is essential to specifying quench-and-temper correctly and to verifying that the specified temper was actually run.

What happens to steel during tempering — and why does hardness decrease?​

When steel is quenched to form martensite, the carbon that was dissolved in the austenite becomes trapped in the body-centered tetragonal (BCT) martensite lattice — it cannot diffuse out quickly enough during the rapid cooling. This trapped carbon is what makes martensite so hard: the lattice distortion from the carbon atoms creates a highly stressed crystal structure that resists dislocation motion (the mechanism of plastic deformation). Tempering reheats the steel to a sub-critical temperature to allow controlled, limited diffusion of this trapped carbon. The process occurs in stages as temperature increases: at 200–400 °F, fine epsilon carbides (Fe₂.₄C) precipitate from the most highly distorted regions — slight hardness reduction, relief of the most extreme lattice strain. At 400–600 °F, the epsilon carbides convert to cementite (Fe₃C), and retained austenite (if present) transforms to bainite — this stage includes the temper embrittlement susceptibility range for alloy steels. At 600–1,200 °F, the cementite coarsens by Ostwald ripening, the martensite lath structure spheroidizes, and the overall lattice strain is progressively relieved — producing a progressively softer, more ductile structure. Each stage increases ductility at the cost of some hardness. The final hardness after tempering at any given temperature is determined by the thermodynamic equilibrium between carbon in solution and carbide precipitation at that temperature, modified by the alloy content that controls diffusion kinetics. Higher tempering temperature → more carbon precipitated as coarse carbide → lower lattice strain → lower hardness (ASM Handbook, Vol. 4A, ASM International, 2013; Totten, Steel Heat Treatment Handbook, 2nd ed., CRC Press, 2006).

What does the tempering curve look like for AISI 4140?​

4140 is the most widely specified through-hardenable alloy steel in industrial manufacturing, and its tempering behavior is the benchmark for medium-alloy grades. As-quenched hardness for 4140 is typically 54–58 HRC (depending on carbon content and quench rate). Tempering produces the following approximate hardness values: 400 °F → 52–54 HRC; 500 °F → 50–52 HRC; 600 °F → 47–50 HRC; 700 °F → 44–47 HRC; 800 °F → 42–45 HRC; 900 °F → 38–42 HRC; 1,000 °F → 34–38 HRC; 1,050 °F → 32–35 HRC; 1,100 °F → 30–33 HRC; 1,150 °F → 28–32 HRC; 1,200 °F → 26–30 HRC. The curve is monotonically decreasing — higher temperature always produces lower hardness. Common industrial callouts for 4140 by application: crane wheels and mill rolls → 321–401 HB (32–43 HRC, tempered approximately 900–1,100 °F); precision shafting and gearing → 285–341 HB (29–36 HRC, tempered approximately 1,050–1,150 °F); wear plates and tooling → 50–54 HRC (tempered approximately 400–500 °F). The tempering curve for a specific heat of 4140 may vary by ±2 HRC from these values due to carbon content variation within the specification range (0.38–0.43% C) and minor alloy variation. Published detailed tempering curves appear in ASM Handbook Vol. 4A, the ASM Heat Treater's Guide, and in the technical data sheets of major steel producers (ASM Handbook, Vol. 4A, ASM International, 2013; Heat Treater's Guide, ASM International, 1995).

How does the tempering curve for 4340 differ from 4140?​

4340 (0.38–0.43% C, 1.65–2.00% Ni, 0.70–0.90% Cr, 0.20–0.30% Mo) has higher hardenability than 4140 and slightly different tempering behavior. As-quenched hardness for 4340 is typically 55–60 HRC — marginally higher than 4140 due to the additional alloy content. The tempering curve for 4340 runs approximately 2–5 HRC higher than 4140 at equivalent tempering temperatures, because the Ni-Cr-Mo alloy content retards carbide coarsening and maintains a finer, harder tempered structure at the same temperature. Representative 4340 tempering values: 600 °F → 52–55 HRC; 800 °F → 47–50 HRC; 1,000 °F → 40–44 HRC; 1,200 °F → 35–40 HRC. The higher alloy content of 4340 also increases susceptibility to temper embrittlement (discussed below) — when impact toughness is critical, 4340 is sometimes processed with a rapid cool from the tempering temperature to minimize segregation. For applications requiring higher hardness at a given temper temperature than 4140 can achieve — heavy-section shafting, large crane wheels, high-strength bolting — 4340 is the common upgrade path. The Charpy impact toughness of 4340 at any given hardness level is higher than 4140 at the same hardness, which is why 4340 is the standard specification for aerospace structural parts, large shafts in impact service, and components requiring both hardness and fracture toughness (ASM Handbook, Vol. 4A, ASM International, 2013; SAE J1397).

What is temper embrittlement and how is it avoided?​

Temper embrittlement is a phenomenon in alloy steels (particularly Cr-Mo, Ni-Cr, Ni-Cr-Mo grades including 4140, 4340, 8640) where tempering in the 450–570 °F range produces a marked decrease in impact toughness at room temperature — even though the hardness at that tempering temperature may appear appropriate on a Rockwell test. The mechanism is the diffusion of phosphorus, antimony, tin, and arsenic (trace impurities present in commercial steels at ppm levels) to the prior-austenite grain boundaries during the slow cooling from the embrittling temperature range or during prolonged hold in this range. The grain boundary film of segregated impurities provides a low-energy fracture path, dramatically reducing Charpy impact energy — from 40–80 ft·lb at tempering temperatures of 400 °F (below the embrittlement range) or 600–700 °F (above the embrittlement range) to as low as 5–15 ft·lb if tempered at 500 °F. The hardness values are similar — the embrittlement is not detectable by routine Rockwell or Brinell testing, only by impact testing. The practical rule is clear: for alloy steels in impact service, do not temper between 450 °F and 570 °F. Specify tempering temperatures below 400 °F (for maximum hardness applications) or above 600 °F (for toughness applications). Specify the tempering temperature explicitly on the drawing, not only the hardness range — a hardness callout of "36–40 HRC" on a 4140 part could legally be satisfied by tempering at 530 °F, which falls directly in the embrittlement range. If high hardness (50+ HRC) is required for a 4140 part in dynamic service, process at 375–400 °F and accept slightly increased cracking risk from the very low tempering temperature, rather than temper in the embrittlement zone (ASM Handbook, Vol. 4A, ASM International, 2013; Briant, C.L., and Banerji, S.K. (eds.). (1978). Embrittlement of Engineering Alloys, Academic Press).

How does tempering behavior differ for tool steels (D2, H13)?​

Tool steels differ from structural alloy steels in several important respects: they contain significantly more carbon and more alloying elements, which changes both the as-quenched hardness and the shape of the tempering curve. D2 cold-work tool steel (1.5% C, 12% Cr) quenches to 62–65 HRC and retains substantial retained austenite that must be converted by tempering. The first temper of D2 (typically at 375–400 °F) reduces hardness to 60–62 HRC and converts some retained austenite, but a significant fraction of retained austenite remains. A double temper — after the first temper, allow to cool to room temperature or below (cryo treatment in some applications), then perform a second temper at the same temperature — converts the retained austenite that transformed to fresh martensite during the cool-down from the first temper, and then tempers that fresh martensite. D2 double-tempered at 400 °F achieves approximately 60–62 HRC with reduced retained austenite and better dimensional stability. H13 hot-work tool steel (5% Cr, 1.5% Mo, 1.0% V) undergoes secondary hardening — at tempering temperatures around 1,000–1,050 °F, fine alloy carbides (vanadium carbides, molybdenum carbides) precipitate and cause hardness to actually increase from the as-quenched level before dropping again at higher tempering temperatures. This secondary hardening peak is critical to H13's hot strength: H13 tools are routinely tempered at 1,025–1,100 °F specifically to achieve hardness in the secondary hardening range (52–54 HRC) with superior thermal stability for hot die, extrusion, and pressure die casting service. The secondary hardening behavior must be accounted for in specification — H13 tempered at 900 °F may be slightly softer than the same steel tempered at 1,025 °F despite the lower temperature (ASM Handbook, Vol. 4A, ASM International, 2013; Heat Treater's Guide, ASM International, 1995).

What is double tempering and when is it required?​

Double tempering — performing two sequential tempering cycles, typically at the same temperature, with a cool to ambient (or below) between cycles — is required for steel grades that retain significant amounts of austenite after the first temper-and-cool cycle. The reason: retained austenite (austenite that did not transform to martensite during quenching because the Ms temperature is depressed by high alloy content) remains metastable. During the first tempering cycle, some retained austenite transforms to martensite as the part cools from temper temperature — this fresh, untempered martensite is hard and brittle and must be tempered in a second cycle. Without the second temper, the part contains a mixture of properly tempered martensite and fresh as-quenched martensite, producing non-uniform hardness and reduced toughness. Double tempering is standard for: high-carbon and high-alloy tool steels (D2, M2, A2, H13) where retained austenite content after quenching is typically 15–30%; highly alloyed stainless steels; and any steel where dimensional stability is critical (retained austenite transformation in service is a well-known source of dimensional change in precision components). For standard structural alloy steels (4140, 4340) with moderate carbon and alloy content, the retained austenite fraction is low (5–10%) and a single temper is typically sufficient — double tempering is specified only for the most dimensionally critical applications. The double-temper cycle adds furnace time but produces measurably more stable and uniform properties in high-retained-austenite grades (ASM Handbook, Vol. 4A, ASM International, 2013; Heat Treater's Guide, ASM International, 1995).

How should tempering temperature be specified on an engineering drawing?​

The most common and most problematic method of specifying tempering on a drawing is hardness range alone: "Heat treat to 32–36 HRC" or "285–341 HB." A hardness-only callout is ambiguous because multiple tempering temperatures can produce the same hardness in the same grade, and the wrong temperature (for example, the embrittlement range for alloy steels) may be used to achieve the number. The preferred specification method for any part in dynamic, impact, or fatigue service includes: (1) the steel grade (not just "alloy steel"); (2) the quench-and-temper process designation; (3) the hardness range at a specified location and test method; and (4) the minimum tempering temperature (to prevent embrittlement range treatment for alloy steels). Example: "4140 steel, quench-and-temper per [applicable procedure], 32–36 HRC at surface, min tempering temperature 950 °F." This gives the heat treater unambiguous direction and prevents the embrittlement range problem. For non-impact applications where only hardness matters, a hardness range with grade and process type is often adequate. For stress-sensitive applications, specify the tempering temperature be at least 50 °F below any subsequent stress relief or PWHT cycle — this preserves the tempered hardness. Drawing specification quality is one of the most impactful factors in whether a heat treatment produces the right outcome — a clear, complete specification reduces re-work, non-conformances, and the back-and-forth between shop and heat treater that delays jobs (ASM Handbook, Vol. 4A, ASM International, 2013; ASTM E18; Machinery's Handbook, 31st ed., Industrial Press, 2020).

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References​

  • ASM International. (2013). ASM Handbook, Volume 4A: Steel Heat Treating Fundamentals and Processes. ASM International.
  • ASM International. (1995). Heat Treater's Guide: Practices and Procedures for Irons and Steels (2nd ed.). ASM International.
  • Totten, G.E. (ed.). (2006). Steel Heat Treatment Handbook (2nd ed.). CRC Press / Taylor & Francis.
  • Briant, C.L., and Banerji, S.K. (eds.). (1978). Embrittlement of Engineering Alloys. Academic Press.
  • Machinery's Handbook (31st ed.). (2020). Industrial Press.
  • SAE J1397: Estimated Mechanical Properties and Machinability of Steel Bars. SAE International.
  • ASTM E18: Standard Test Methods for Rockwell Hardness of Metallic Materials. ASTM International.

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