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Why Must D2 Material Be Tempered Immediately After Quenching?

D2 steel gains extremely high hardness after quenching yet has very poor toughness. Huge internal stress builds up inside quenched D2 material, and its microstructure stays highly unstable. Cracking will happen sooner or later without immediate tempering. This article explains why D2 tool steel must be tempered right after quenching, what problems delayed tempering will bring, and how to carry out correct tempering operations.

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What state is D2 material in after quenching?

After quenching, the microstructure of AISI D2 transforms from austenite to martensite. Therefore, quenched AISI D2 steel is hard, brittle, high‑stress and microstructurally unstable.

Extremely High Hardness of D2 Steel

D2 material can reach 60‑62 HRC after quenching.

Very Poor Toughness

The quenched martensite structure is extremely hard and brittle. The crystal lattice of quenched martensite suffers severe distortion with almost no plastic deformation capacity. Under load, this structure is easy to generate cracks and even surface spalling.

Huge Internal Stress

During quenching, volume expansion occurs when D2 material transforms from austenite to martensite. Since the surface and core of the workpiece cool at different rates, martensite transformation is asynchronous, which produces huge structural stress and thermal stress. These stresses distribute between martensite laths, around carbides and at grain boundaries. At this time, steel D2 is like a fully‑drawn bow that may snap at any moment.

Retained austenite remains

After quenching, 10‑20% austenite in D2 material cannot convert into martensite. Such retained austenite is an unstable phase. Without timely tempering, retained austenite will slowly turn into martensite during service, accompanied by volume expansion, causing dimensional change or even cracking.

Why must quenched D2 material be tempered immediately?

Tempering can relieve internal stress, decompose retained austenite, improve toughness and prevent cracking.

D2 Material Will Crack by Itself If Quenching Stress Is Not Removed

  • Extremely high internal stress remains inside quenched D2 material. Without timely tempering, these stresses concentrate on weak positions of material D2, such as carbide edges, sharp corners and cross‑section changes. D2 steel contains plenty of carbides which often show banded distribution, bringing higher brittleness at grain boundaries. Cracks will form once stress exceeds the material’s bearing limit.
  • Delayed tempering is one of the most common fatal mistakes in D2 material heat treatment. Even if you just set quenched D2 cold work tool steel aside to cool down to room temperature, it may crack spontaneously during cooling, handling or storage.

Tempering Decomposes Retained Austenite and stabilizes Dimensions

Part of retained austenite turns into fresh untempered martensite during the first tempering. For this reason, material D2 shall be tempered at least twice. The second tempering aims to remove stress from the newly‑formed martensite generated in the first tempering. After double tempering, retained austenite is mostly decomposed and internal stress is fully released, so dimensions become stable.

Tempering Relieves Stress and Improves Toughness

Essentially, tempering means reheating quenched D2 material to medium‑low temperature. Supersaturated carbon inside martensite precipitates to form carbides, meanwhile internal stress gets released. This process changes D2 mold steel from “extremely brittle” to “hard and tough”. Its hardness drops slightly while toughness rises greatly.

What are the specific consequences of delayed tempering?

Cracking

Quenched D2 metal carries extremely high internal stress. Delayed tempering gives time for stress concentration and propagation. Cracks may appear on the surface or form inside the material.

Dimensional Instability of D2 Material

For D2 tool steel without timely tempering, retained austenite is not fully decomposed. During service, such retained austenite slowly transforms into martensite with volume change, which leads to dimensional variation.

Cracking During Subsequent Processing

If quenched D2 material with high residual stress goes through grinding or wire‑cutting, combined thermal stress and residual stress will multiply the risk of cracking. Many steel D2 dies crack during finish machining due to untimely tempering.

Reduced Secondary Hardening Effect of D2 Tool Steel

One key advantage of AISI D2 is secondary hardening generated during high‑temperature tempering at 500‑560°C. Without prompt tempering after quenching, retained austenite cannot transform sufficiently, so the expected hardness and wear resistance from secondary hardening cannot be achieved.

How to perform proper tempering for D2 material

Timing (Immediately)

Temper D2 material right after air‑cooling to about 150°F (65°C). Do not wait for full cooling, and do not leave it until the next day.

Cycles (At Least 2 Times)

D2 material must be tempered at least twice. Some retained austenite changes into new martensite during the first tempering, yet only around 70% of internal stress gets removed. The second tempering removes stress from this new martensite, and over 90% stress can be eliminated after two tempering runs. Three tempering cycles are suggested for complex or high‑precision dies. Air‑cool to room temperature between two tempering operations.

Holding Time (No Less Than 2 Hours Each Time)

Each tempering holding time shall be no less than 2 hours. Add one hour for every 25 mm of material thickness. Cool down to room temperature in air after tempering before carrying out the next tempering.

Temperature (Choose Based on Target Hardness)

  • Low‑temperature tempering (180‑230°C): D2 material reaches 60‑62 HRC, fit for high‑wear and low‑impact working conditions.
  • High‑temperature tempering (500‑560°C): D2 die steel hardness ≥58 HRC, with lower internal stress and better dimensional stability. Recommended for high‑precision dies.
  • Secondary hardening temperature (482‑516°C / 900‑960°F): Highly recommended. Tempering within this range produces secondary hardening effect for D2 material and achieves the best balance between hardness and wear resistance.

FAQ:

Q1: How soon counts as "immediate" for tempering D2 material after quenching?

A: D2 material should be tempered right after air‑cooling to around 150°F (65°C). Do not wait until it fully cools to room temperature. Keep the time gap between quenching completion and entering the tempering furnace as short as possible, ideally within 30 minutes.

A: No. D2 tool steel must be tempered at least twice. Single tempering only removes about 70% of internal stress, while two tempering cycles eliminate over 90%. Three tempering runs are recommended for complex and precision dies. Each extra tempering cycle upgrades its dimensional stability.

A: It does not guarantee 100% cracking, yet the risk is extremely high. Cracking probability depends on part complexity, cross‑section size and quenching stress level.

A: Yes, but follow the correct sequence. The proper order is: quenching → cryogenic treatment → tempering. Perform cryogenic treatment (-70°C to -196°C) immediately after quenching to convert more retained austenite into martensite, then remove stress by tempering. Cryogenic treatment can lift D2 steel hardness by 4 units. Tempering must be carried out right after cryogenic treatment.

A: 

  1. Low‑temperature tempering (180‑230°C) for 60‑62 HRC, suitable for blanking dies, shears and cutting tools.
  2. High‑temperature tempering (500‑560°C) for stable dimensions and low internal stress, hardness ≥58 HRC, for precision dies and wire‑cut dies.
  3. Optimal secondary hardening effect, use 482‑516°C (900‑960°F). Tempering within this range delivers the best balance of hardness and wear resistance for AISI D2 steel.

Conclusion

D2 material must be tempered immediately after quenching for three key reasons:

  • Quenched D2 material has extremely high internal stress, and it will crack on its own if left untreated.
  • Immediate tempering relieves stress, decomposes retained austenite and improves toughness.
  • Delayed tempering is one of the most common fatal mistakes in D2 material heat treatment. Once cracking occurs, no repair can fix it.
There is only one thing you need to remember: temper D2 material while it is still hot right after quenching.

More D2 Material Resources

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