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JIS SKD11 - Complete Heat Treatment Process Guide

The main goal of heat treatment of JIS SKD11 is to create a fine and even martensite structure with carbides spread evenly through the steel by carefully controlling the three steps of heating, holding, and cooling.
This finally gives high hardness, high wear resistance, and good dimensional stability at the same time.

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SKD11 Tool Steel
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Annealing Of JIS SKD11

The purpose of annealing SKD11 tool steel is to soften the steel, remove internal stress, make the structure even, and prepare it for later machining and quenching.

The annealing temperature for SKD11 material is usually controlled at 800-850℃. Some sources also recommend 830-880℃ or the narrower range of 840-860℃. The holding time is usually 1-3 hours, depending on the effective thickness of the workpiece. After annealing, the steel must cool slowly inside the furnace at a rate of 10-20℃ per hour until it reaches about 600℃. It can then be taken out of the furnace and cooled in air. The hardness after annealing should be kept at ≤255 HB. Some high-quality materials can reach ≤248 HB or 207-255 HB.

Quenching Of JIS SKD11

Quenching is the key step that gives JIS SKD11 high hardness. The main point is precise control of preheating, austenitizing, and cooling. Each step directly affects the final hardness and structure.

H3: Preheating Of JIS SKD11

SKD11 steel must be preheated in stages before quenching. Two-stage preheating is normally used: first heat to 550-650℃ and hold for some time, then heat to 800-850℃ and hold. Some sources recommend a simpler method: preheat directly to 700-750℃. The purpose of preheating is to reduce the temperature difference between the inside and outside of the workpiece and prevent thermal-stress cracks caused by heating too fast.

H3: Austenitizing Of SKD11 Steel

Austenitizing is the key heating stage that decides the final hardness and performance. The austenitizing temperature range for SKD11 tool steel is 1000-1050℃. 

  • If low-temperature tempering is used, the austenitizing temperature can be controlled at 975-1025℃. With low-temperature tempering, the hardness can reach above 61 HRC.
  • If high-temperature tempering (secondary hardening) is used, the recommended austenitizing temperature is raised to 1000-1050℃. With high-temperature tempering, the hardness can reach above 58 HRC, and high-temperature hardness is improved.

The holding time at the austenitizing temperature must be calculated from the effective thickness of the workpiece. It is usually at least 30 minutes for every 25mm of thickness.

H3: Quench Cooling Of SKD11 Tool Steel

SKD11 tool steel is an air-hardening steel. The cooling method mainly depends on the thickness of the workpiece:

  • For most workpieces of normal size, cooling in still air is recommended. This is the main way to keep distortion as low as possible.
  • For thicker workpieces (6 inches/150mm or more), air cooling may not give enough hardening through the full thickness. Oil cooling is recommended, and the quenching temperature can be lowered to 980-1030℃.

Overall, the normal quenching temperature range for SKD11 tool steel is 980-1040℃.

Tempering Of JIS SKD11

Tempering must be done immediately after quenching to remove quenching stress, stabilize the structure, prevent cracking, and adjust the final hardness. 

Low-Temperature Tempering (Main Choice)

The temperature range is 150-200℃, and the hardness after tempering can reach ≥61 HRC. This route keeps the highest hardness and is suitable for cold-work dies that need very high wear resistance.

High-Temperature Tempering (Secondary Hardening)

The temperature range is 480-540℃, and the hardness after tempering is ≥58 HRC. This route can improve high-temperature hardness and is suitable for dies that work at higher temperatures or need nitriding later.

Whichever tempering route is chosen, JIS SKD11 must be tempered at least twice. The recommended holding time for each tempering cycle is at least 60 minutes for every 25mm of thickness.

Quick Reference For Heat Treatment Process Parameters

Annealing Parameters For SKD11 Material

  • Annealing temperature: 800-850℃ (or 830-880℃)
  • Holding time: 1-3 hours
  • Cooling method: slow cooling in the furnace at 10-20℃/h to 600℃, then air cooling
  • Hardness after annealing: ≤255 HB

Quenching Parameters

  • Preheating temperature: 550-650℃ → 800-850℃ (two-stage preheating)
  • Austenitizing temperature: 1000-1050℃ (975-1025℃ for the low-temperature tempering route; 1000-1050℃ for the high-temperature tempering route)
  • Holding time: at least 30 minutes for every 25mm of thickness
  • Cooling method: air cooling for thin parts; oil cooling for thick parts (≥150mm)

Tempering Parameters

  • Low-temperature tempering: 150-200℃, hardness ≥61 HRC
  • High-temperature tempering: 480-540℃, hardness ≥58 HRC
  • Number of tempering cycles: at least two
  • Holding time: at least 60 minutes for every 25mm of thickness

Protection And Cryogenic Treatment For JIS SKD11

Protection Of SKD11 Material Against Decarburization And Oxidation

A vacuum furnace or a protective-atmosphere furnace is recommended for heat treatment. A salt bath furnace can also be used, but the deoxidation condition of the salt bath must be controlled precisely.

Cryogenic Treatment Of SKD11 Steel

For precision dies that need very high dimensional stability, cryogenic treatment at -70℃ to -80℃ can be done after quenching. This changes more retained austenite into martensite and can raise hardness by 1-3 HRC. Cryogenic treatment should be done after quenching and before tempering.

Key Process Points

Temper Brittleness Range

SKD11 steel has a risk of temper brittleness in the 500-600℃ range. This range should be avoided during high-temperature tempering.

Tempering Without Delay

Tempering should be done immediately after quenching. The delay should not be more than 2-4 hours, or quenching stress may cause cracks.

H3: Even Cooling

During quench cooling, every part of the workpiece should cool evenly. This prevents distortion caused by different local cooling rates.

Heating Rate

Heating should not be too fast. The recommended rate is ≤200℃ per hour. Thick workpieces should be heated more slowly to reduce thermal stress.

FAQ of JIS SKD11

Q1: Why must JIS SKD11 use step pre‑heating before quenching instead of direct heating?

A: SKD11 steel is high‑alloy steel with poor thermal conductivity. If heated directly from room temperature to over 1000°C, huge temperature difference between inside and outside will generate large thermal stress and cause cracking. Step pre‑heating has two stages (550‑650°C → 800‑850°C). It lets inner and outer temperature of workpiece rise gradually to lower thermal stress, and prepares microstructure for final austenitization. This step cannot be skipped especially for thick parts.

A: It depends on tempering route. For low‑temperature tempering (150‑200°C), set austenitization temperature at 975‑1025°C to get hardness ≥61 HRC. For high‑temperature tempering (480‑540°C), raise temperature to 1000‑1050°C. It makes carbides dissolve fully and produces secondary hardening effect during high‑temperature tempering. The two routes serve different purposes. Temperature choice directly affects final performance.

A: It mainly depends on workpiece thickness. Regular‑size parts (<150 mm) can get full hardening under still‑air cooling with minimum distortion. Large‑section parts with thickness ≥150 mm cannot get enough hardenability by air cooling and show low core hardness. Oil quenching is required, and quenching temperature can drop properly to 980‑1030°C. Oil quenching brings faster cooling yet higher distortion risk.

A: The first tempering removes quenching stress and stabilizes martensite, but some retained austenite still exists. The second tempering re‑tempers new martensite formed in the first tempering and further stabilizes microstructure. Single tempering cannot remove residual stress completely. It may lead to dimension change or even cracking in later service.

A: Cryogenic treatment turns retained austenite (about 10‑20%) after quenching into martensite. It must be done before austenite gets stabilized by tempering. If tempering comes first, retained austenite will be locked by thermal stabilization. Later cryogenic treatment will lose most effect. Hardness gain drops from 1‑3 HRC to less than 0.5 HRC. Correct sequence: quenching → cryogenic treatment (-70°C to -80°C) → tempering.

A: Temper brittleness occurs in this temperature range and impact toughness drops obviously. Two avoidance methods: first, skip this range, use low‑temperature tempering at 150‑200°C or high‑temperature tempering above 620°C. Second, if tempering in this range is necessary (for later nitriding), shorten holding time and apply fast cooling (oil cooling or water cooling) after tempering to reduce brittleness tendency.

Conclusion

Heat treatment of JIS SKD11 requires control of annealing, preheating, austenitizing, quench cooling, and double tempering in order. The tempering and cryogenic treatment plans should be chosen based on the die size and performance needs. Keyspark Steel provides high-quality steel and custom services. We can match the material solution to the workpiece specifications and working conditions. Contact us for custom advice.

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