Skip to main content

Double Tempering Tool Steel and High-Alloy Components: Cycles and Rationale

Double tempering is the practice of running two full temper cycles, back-to-back with a cool to room temperature between them, after the austenitize-and-quench step on high-alloy tool steels and other grades that retain a significant fraction of austenite in the as-quenched microstructure. 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 first temper tempers the primary martensite and destabilizes retained austenite so that it transforms to fresh martensite on cool-down; the second temper then tempers that newly formed martensite, so the finished part contains no untempered martensite and no unstable retained austenite. This article explains the retained-austenite mechanism, the grades that require double (or triple) tempering, and the cycle parameters typically specified for common tool steel and secondary-hardening grades.

Why does high-alloy tool steel need to be tempered twice?​

When a high-alloy tool steel is quenched from its austenitizing temperature, the martensite start temperature (Ms) is depressed well below room temperature by the heavy alloying — chromium, molybdenum, vanadium, tungsten — that give these grades their hardenability and high-temperature strength. The result is that the martensite transformation does not run to completion during the quench: a significant fraction of austenite, typically 10–25% by volume, remains untransformed at room temperature. This retained austenite is soft (30–40 HRC versus 60+ HRC for fresh martensite), dimensionally unstable, and prone to transforming to fresh untempered martensite in service as the part sees thermal or mechanical cycling — which causes dimensional growth, cracking at the martensite-austenite interfaces, and loss of hardness uniformity. The first temper serves two functions: it tempers the primary as-quenched martensite, and it destabilizes the retained austenite by precipitating fine carbides within it, raising its Ms temperature so that it transforms to fresh martensite when the part cools back to room temperature after the temper. That freshly formed martensite is itself brittle and untempered. The second temper tempers this newly formed martensite, producing a fully tempered structure with minimal retained austenite. For some grades with very high retained austenite (M-series high-speed steels, heavily alloyed cold-work tool steels in large sections), a third temper is specified to handle martensite formed during cooling from the second temper (ASM Handbook, Vol. 4A, ASM International, 2013; Roberts, G., Krauss, G., Kennedy, R., Tool Steels, 5th ed., ASM International, 1998).

Which grades require double tempering as a standard practice?​

Double tempering is standard specification practice for essentially all high-alloy tool steels and for several secondary-hardening alloy steel grades. Cold-work tool steels: D2 (12% Cr), D3, D7, and the A-series (A2, A6) are routinely double-tempered because their high chromium content depresses Ms below room temperature and produces 15–25% retained austenite after oil or air quench. Hot-work tool steels: H13 (5% Cr-Mo-V) and H11 are double-tempered because their service requirements — hot-work dies, die-casting molds, forging dies — demand dimensional stability at elevated service temperatures, and any retained austenite would transform in service and cause tool distortion. High-speed steels: M-series (M2, M4, M42) and T-series (T1, T15) are always at minimum double-tempered, commonly triple-tempered, because their austenitizing temperatures are so high (2,050–2,250 °F) that retained austenite fractions of 20–30% are typical after quench. Secondary-hardening alloy steels: H13 when used at its secondary-hardening peak (tempering at 1,000–1,100 °F where alloy carbide precipitation produces a hardness peak above the as-quenched value) requires double tempering to fully develop that peak. Some PH stainless steels also specify double aging for the same reason. For standard through-hardening alloy steels (4140, 4340, 1045), single tempering is typically adequate because retained austenite fractions are generally below 5% and cause no service problems (ASM Handbook, Vol. 4A, ASM International, 2013; Heat Treater's Guide: Irons and Steels, 2nd ed., ASM International, 1995).

What is the typical double-temper cycle for D2 cold-work tool steel?​

D2 (AISI D2, ~1.5% C, 12% Cr, 1% Mo, 1% V) is austenitized at 1,825–1,875 °F (996–1,024 °C) for 30 minutes per inch of section thickness, quenched in oil or air depending on section size, and arrives at room temperature with 15–20% retained austenite and as-quenched hardness of roughly 62–64 HRC. The typical double-temper cycle is: first temper at 400–500 °F (204–260 °C) for 2 hours after the part reaches temperature throughout, cool to room temperature, then second temper at the same temperature or 25–50 °F lower, again 2 hours after temperature equalization, cool to room temperature in still air. Finished hardness is typically 58–62 HRC depending on the exact temper temperature — at 400 °F temper, approximately 61–62 HRC; at 500 °F, 60–61 HRC; at 950–1,000 °F (for secondary hardening), 60–62 HRC with better toughness than the low-temper condition but different metallurgical character. The retained austenite fraction after double temper is typically reduced to 3–8%. For D2 tooling used in high-impact service, some specifications call for a sub-zero treatment between the first and second temper — cooling to −100 °F or below — to force the retained austenite through its Ms before the second temper. This is distinct from standalone cryogenic treatment and serves specifically as a retained-austenite-conversion step (Roberts, Krauss, Kennedy, Tool Steels, 5th ed., ASM International, 1998; ASM Handbook, Vol. 4A, ASM International, 2013).

How does double tempering differ for H13 at secondary-hardening temperatures?​

H13 (AISI H13, ~0.4% C, 5% Cr, 1.5% Mo, 1% V, 1% Si) is a hot-work tool steel whose tempering behavior differs fundamentally from low-alloy steels because it exhibits secondary hardening. H13 is austenitized at 1,825–1,900 °F (996–1,038 °C), air or oil quenched (forced air for large sections, oil for thin), and retains 10–20% austenite at room temperature with as-quenched hardness of 52–56 HRC. Unlike low-alloy steels where hardness decreases monotonically as temper temperature rises, H13 shows a hardness peak at approximately 1,000–1,100 °F (538–593 °C) due to precipitation of fine coherent M2C and MC alloy carbides (molybdenum and vanadium carbides) that add strengthening beyond what the martensite alone provides. The typical production cycle is: first temper at 1,025–1,075 °F (552–579 °C) for 2 hours, cool to room temperature, second temper at the same temperature for 2 hours, cool, and optionally a third temper at 25 °F below the second temper for 2 hours. The finished hardness for die casting applications is typically 44–48 HRC; for forging dies 44–52 HRC; the exact target is set by the balance between wear resistance and thermal fatigue resistance required for the specific tool. The double (or triple) temper is critical for H13 because (a) retained austenite must be eliminated — it would transform in service at the 600–1,000 °F operating temperatures of die casting dies — and (b) the secondary-hardening precipitation process requires multiple cycles to reach peak hardness with a uniform carbide distribution. UTEC Industrial's car-bottom furnace runs these multi-cycle temper programs as a single programmable ramp-hold-cool-hold-cool sequence, with the chart record capturing each soak and return-to-ambient step for the documentation package (ASM Handbook, Vol. 4A, ASM International, 2013; NADCA #207 recommended practices for die steels).

How are double tempering and triple tempering specified on high-speed steels?​

High-speed steels (M-series like M2, M4, M42; T-series like T1, T15) present the most demanding retained-austenite conditions of any commonly heat-treated steel because their austenitizing temperatures are near the solidus — typically 2,150–2,250 °F (1,177–1,232 °C) for M2 and above 2,200 °F for T-series grades — to dissolve the massive alloy carbide structure and produce the supersaturated austenite that yields secondary-hardened martensite on tempering. As-quenched retained austenite is commonly 20–30% and the martensite transformation continues on cooling after each temper cycle. The standard specification is a triple temper at 1,025–1,050 °F (552–566 °C), each cycle 2 hours after reaching temperature, with cool to room temperature between each cycle. The first temper precipitates secondary carbides, destabilizes retained austenite, and produces fresh martensite on cool-down. The second temper tempers that martensite and may destabilize any remaining retained austenite, producing still more fresh martensite. The third temper tempers any final fresh martensite. Finished hardness for M2 is typically 62–66 HRC; for M42 and the cobalt-bearing grades, 66–70 HRC. Some specifications allow a double-temper if hardness verification on a representative test coupon confirms the required value and the retained austenite fraction — measured by X-ray diffraction — is below 3–5%. Skipping the third temper without that verification is a specification violation that leaves untempered martensite in the final part, reducing toughness and increasing susceptibility to grinding cracks during post-heat-treat sharpening (Roberts, Krauss, Kennedy, Tool Steels, 5th ed., ASM International, 1998; ASM Handbook, Vol. 4A, ASM International, 2013).

What happens if only a single temper is run on a grade that requires double tempering?​

Skipping the second temper on a grade that metallurgically requires it leaves two distinct problems in the finished part. First, the retained austenite that remains after a single temper is now in a destabilized state — its carbide precipitation has already begun, its Ms has been raised, and it will transform to fresh untempered martensite during any subsequent service exposure: thermal cycling, grinding, shock loading, even long-term storage in cold conditions. This fresh martensite is brittle and untempered, and the volumetric expansion that accompanies its formation causes dimensional growth that can take a precision tool out of tolerance. Second, any fresh martensite that did form during the cool-down from the single temper remains brittle and untempered; the part's hardness reading may meet the specification, but its toughness is compromised. Observable failure modes include: dimensional growth of precision dies in the first weeks of service, grinding cracks that appear after tempering when the part is ground to final dimension (grinding heat triggers retained austenite transformation at the surface, and the resulting fresh martensite cracks under the thermal gradient), chipping of cutting-tool edges in service from the untempered-martensite brittleness, and inconsistent hardness readings across a single tool as different regions have different retained-austenite fractions. For these reasons, the double temper is not optional on the grades that require it — the specification must be followed as written, and the documentation must show both cycles. For UTEC Industrial's HT documentation, both temper cycles appear in the chart record with the measured temperature, soak time, and cool-down for each (ASM Handbook, Vol. 4A, ASM International, 2013; Totten, G.E. (ed.), Steel Heat Treatment Handbook, 2nd ed., CRC Press, 2006).

When is sub-zero or cryogenic treatment specified between tempers instead of a second temper?​

Sub-zero treatment — cooling the part to −100 °F (−73 °C) or lower — and cryogenic treatment at liquid nitrogen temperatures (−320 °F / −196 °C) are sometimes specified to supplement or replace the retained-austenite-conversion function of a second temper. The rationale: cooling the part below its depressed Ms forces direct martensitic transformation of the retained austenite without relying on the destabilization-and-cool-down mechanism of the first temper. Sub-zero treatment is typically inserted after the quench and before the first temper, or between the first and second tempers on grades where the first temper alone cannot drop the Ms low enough to transform the retained austenite. For D2, A2, and some H13 applications, a sub-zero step after the quench reduces retained austenite from 15–20% to under 5% before any tempering, after which a standard single or double temper completes the cycle. Cryogenic treatment at −320 °F with extended hold (24 hours or longer) is a more aggressive version claimed to further refine the carbide distribution and improve wear resistance; the metallurgical evidence for the carbide-refinement mechanism is mixed, but the retained-austenite-conversion benefit is well established. Critically, sub-zero or cryogenic treatment does not eliminate the need for tempering — any martensite formed during the cold treatment is fresh and untempered, and must be tempered subsequently. UTEC does not offer sub-zero or cryogenic treatment as standalone services; when a specification calls for these steps, parts are typically sent to a specialty heat treater for the cryo portion and returned for the tempering cycle. Buyers whose drawings require cryogenic treatment should plan the process route accordingly (ASM Handbook, Vol. 4A, ASM International, 2013; Barron, R.F., "Cryogenic Treatment of Metals to Improve Wear Resistance," Cryogenics, Vol. 22, 1982).

How is a double-temper job documented and verified?​

A double-temper job requires documentation of both temper cycles plus hardness verification confirming the finished condition. The typical documentation package includes: the austenitizing cycle record (ramp, soak at temperature, quench time after furnace exit); the quench record (medium, bath temperature, agitation condition, time to immersion); the first temper cycle record (ramp, soak at temperature, cool-down to room temperature); the cool-down to room temperature between tempers with a return-to-ambient verification before the second cycle begins; the second temper cycle record (ramp, soak at temperature, cool-down); hardness test results with the method (Rockwell C typical for tool steel at 55+ HRC), number of tests, test locations, and specification limits; and part identification traceable to the job order, steel heat, and operator. For triple-temper jobs (M-series high-speed steels, some H13 die-casting die specifications), all three cycles are recorded. For specifications that require retained austenite verification, X-ray diffraction measurement of the finished part confirms the residual austenite fraction — this is a specialty metallurgical laboratory service, not a standard heat treater deliverable, and is typically specified only on aerospace or critical precision tooling. The programmable ramp-and-soak control on UTEC's car-bottom furnace runs the multi-cycle temper sequence as one integrated program so that the chart record shows each soak, each cool, and each re-ramp as a continuous timeline for the documentation package (ASTM E18: Standard Test Methods for Rockwell Hardness; ASM Handbook, Vol. 4A, ASM International, 2013).

Related Articles

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.
  • Roberts, G., Krauss, G., and Kennedy, R. (1998). Tool Steels (5th ed.). ASM International.
  • Totten, G.E. (ed.). (2006). Steel Heat Treatment Handbook (2nd ed.). CRC Press / Taylor & Francis.
  • Barron, R.F. (1982). Cryogenic Treatment of Metals to Improve Wear Resistance. Cryogenics, Vol. 22, No. 8, pp. 409–413.
  • NADCA #207: Recommended Practices for Die Casting Die Steels. North American Die Casting Association.
  • ASTM E18: Standard Test Methods for Rockwell Hardness of Metallic Materials. ASTM International.

Need In-House Heat Treating for Heavy Industrial Parts?​

UTEC Industrial operates a 6' × 10' × 17' car-bottom furnace (1,800 °F, 50-ton capacity), in-house induction hardening with per-part hardness verification, and automated vibratory stress relief at our Spokane, WA facility. Weldment stress relief, annealing, quench and temper, and induction hardening — all under one roof, with full documentation on every job.

Request a Quote →

Questions? Call (509) 922-1832 or email sales@utec.co