Guia de Tratamento Térmico de Aço para Ferramentas M2

M2 tool steel is typically heat-treated by double preheating, austenitizing at 1190–1230°C, quenching in oil, air, or a salt bath, and double- or triple-tempering at about 538–595°C. A common process uses a first preheat at 540–650°C, a second preheat at 845–870°C, short austenitizing for about 2–5 minutes after temperature equalization, quenching to about 66–93°C, and immediate tempering. After proper hardening and tempering, M2 usually reaches about 60–65 HRC.

M2 is a molybdenum-tungsten high-speed tool steel used where high hardness, wear resistance, and red hardness are required. Its heat treatment window is narrow. Long soaking, overheating, inadequate atmosphere protection, delayed or insufficient tempering, can cause low hardness, grain coarsening, retained austenite, cracking, decarburization, or dimensional instability.

Aobo Steel supplies M2 / DIN 1.3343 / JIS SKH51 high-speed tool steel in an annealed condition for bulk orders. For chemical composition, equivalent grades, available sizes, tolerance, inspection requirements, and export supply details, please visit our M2 Tool Steel product page or contact us at [email protected].

M2 Tool Steel Heat Treatment Temperature Chart

The following chart presents the main heat-treatment data for M2 tool steel. The detailed process sequence is explained in the step-by-step section below.

Etapa do processo	Temperature / Condition	Controle de teclas
Recozimento	870–900°C / 1600–1650°F	Hold about 1 hour per inch of thickness. Furnace cool slowly to 650°C, then air cool. Typical maximum annealed hardness is about 241 HB.
Alívio do estresse	650–675°C / 1200–1245°F	Used after heavy machining. Hold 1–2 hours per inch of cross-section, then cool slowly.
First preheating	540–650°C / 1000–1200°F	Reduces initial thermal shock.
Second preheating	845–870°C / 1555–1600°F	Equalizes the tool before austenitizing.
Austenitização	1190–1230°C / 2175–2245°F	Lower range for toughness; higher range for hardness, wear resistance, and red hardness.
Soaking time	Usually 2–5 minutes after equalization	Avoid long soaking.
Resfriamento	Oil, air, or salt bath	Quench to about 66–93°C / 150–200°F.
As-quenched hardness	About 64–66 HRC	Temper immediately. Do not leave M2 in the as-quenched condition.
Optional sub-zero treatment	-100 to -195°C / -150 to -320°F	Used when dimensional stability is critical.
Têmpera	538–595°C / 1000–1105°F	Double tempering is required. Triple tempering is often used for demanding tools.
Typical final hardness	About 60–65 HRC	Depends on austenitizing temperature, quenching method, tempering temperature, and section size.

M2 should be heated in a vacuum furnace, in a controlled neutral atmosphere, or in a neutral salt bath whenever possible. At M2 hardening temperatures, surface decarburization can leave a soft outer layer, reducing wear resistance.

How to Heat Treat M2 Tool Steel Step by Step

M2 heat treatment should be controlled as a full process, not as separate heating and cooling operations. Each stage affects the next one. Poor preheating increases cracking risk. Incorrect austenitizing alters carbide solubility and the amount of retained austenite. Improper quenching affects hardness and distortion. Insufficient tempering leaves the structure unstable.

Step 1: Stress Relieving Before Hardening

Stress relieving is recommended when the M2 tool has been heavily machined before hardening. If a large amount of material has been removed, internal machining stress can cause movement or cracking during final heat treatment.

For stress relieving, heat the tool slowly to 650–675°C (1200–1245°F) and hold for about 1–2 hours per inch of cross-section. After holding, cool slowly to room temperature. This step does not harden the steel. Its purpose is to reduce internal stress before the high-temperature hardening cycle.

In production, rough machining should be completed before stress relieving, with sufficient allowance left for final grinding after hardening and tempering.

Step 2: Double Preheating for M2 Tool Steel

M2 should not be placed directly into the austenitizing temperature range. Because of its high alloy content and high hardening temperature, direct high-temperature heating can cause significant thermal shock and increase the risk of cracking.

Preheating Stage	Faixa de temperatura	Finalidade
First preheat	540–650°C / 1000–1200°F	Reduces the first temperature shock and begins uniform heating.
Second preheat	845–870°C / 1555–1600°F	Equalizes the tool before rapid heating to the austenitizing range.

The purpose of preheating is temperature equalization, not long soaking. Once the tool is uniformly heated, it should move into the austenitizing stage without unnecessary delay. Excessive holding during heating is unnecessary and may increase the risk of decarburization if the atmosphere is poorly protected.

Some references also list a general M2 preheating range of 730–845°C, or an elevated two-stage cycle at 843°C and 1010°C. For a clear technical guide, the 540–650°C plus 845–870°C sequence is easier for readers to follow and directly explains the staged heating process before hardening.

Step 3: Austenitizing Temperature and Soaking Time

M2 is normally austenitized at 1190–1230°C (2175–2245°F). This is the most sensitive part of the heat treatment cycle. The temperature must be high enough to dissolve sufficient alloy carbides, but not so high or so long that it causes grain coarsening, excess retained austenite, or overheating damage.

Austenitizing Target	Faixa de temperatura	Application Logic
Toughness priority	1175–1190°C / 2150–2175°F	Used when toughness and cracking resistance are more important than maximum hardness.
Standard M2 hardening range	1190–1230°C / 2175–2245°F	General range for M2 cutting tools and wear-resistant tooling.
Maximum hardness and wear resistance	Around 1230°C / 2245°F	Used when high hardness, wear resistance, and red hardness are required.
Salt bath adjustment	About 14°C / 25°F lower	Used when hardening from a salt bath.
High-carbon M2 adjustment	About 14°C / 25°F lower	Helps reduce overheating and retained austenite risk.

The soaking time at the austenitizing temperature should be short. After the tool reaches temperature, M2 is usually held for only about 2–5 minutes. Very large sections may require about 5–6 minutes, but M2 should not be soaked like low-alloy steels.

Over-soaking is one of the most common causes of M2 heat treatment failure. Excessive time at high temperature dissolves too much carbon and alloy into austenite, increases retained austenite, coarsens the grain structure, and reduces toughness. Underheating causes the opposite problem: insufficient carbide solution, lower hardness, and weaker secondary hardening during tempering.

For M2, the goal is not maximum furnace time. The goal is a controlled carbide solution within a narrow temperature-time window.

Step 4: Quenching Methods: Oil, Air, and Salt Bath

After austenitizing, M2 can be quenched in oil, air, or a hot salt bath. The best method depends on the tool size, the hardness target, the distortion tolerance, and the available heat-treatment equipment.

Método de resfriamento	Best Use	Main Limitation
Oil quenching	Higher hardness response	Higher distortion and cracking risk than air cooling.
têmpera por ar	Better dimensional stability	Hardness may be lower in larger or slower-cooling sections.
Hot salt bath quenching	Good temperature equalization and reduced thermal shock	Requires suitable salt bath equipment and strict control.

Oil quenching is often selected when maximum hardness response is required. The tool is cooled from the austenitizing temperature to about 66–93°C (150–200°F).

Air quenching is milder and can reduce the risk of distortion. It is more suitable for smaller sections or tools where dimensional stability is more important than the most aggressive cooling response.

Hot salt bath quenching provides better control of temperature equalization. The tool is held in the salt bath long enough for the section to equalize, then cooled further before tempering. This method can reduce thermal shock and distortion when properly controlled.

The most important rule is the same for all three methods: temper immediately after the tool cools to about 66–93°C. M2 should not be left fully cold in the as-quenched condition for an extended period. At this stage, the structure contains highly stressed martensite and retained austenite, so delayed tempering increases the risk of cracking and dimensional instability.

Step 5: Optional Sub-Zero Treatment

Sub-zero treatment is optional. It is mainly used when dimensional stability is critical, especially for intricate tools, precision components, or parts in which retained austenite must be further reduced.

A typical sub-zero range is -100 to -195°C (-150 to -320°F). After the part returns to room temperature, tempering should begin immediately. For intricate shapes, a brief, low-temperature stress-relief temper may be used before freezing to reduce the risk of cracking.

Sub-zero treatment should not replace proper austenitizing, quenching, and multiple tempering. It is only an additional stabilizing step when the tool design and service conditions require it.

Step 6: Double or Triple Tempering

Tempering must begin immediately after quenching, or immediately after the part returns to room temperature if a sub-zero cycle is used. M2 relies on secondary hardening, so tempering is not only for stress relief. It also controls the transformation of retained austenite, carbide precipitation, final hardness, and toughness.

The common tempering range for M2 is 538–595°C (1000–1105°F). Double tempering is the minimum practical requirement. Triple tempering is often used for cutting tools, precision tooling, and demanding service conditions.

Each tempering cycle should be followed by cooling to room temperature before the next temper begins. This cooling stage is important because retained austenite can transform into fresh martensite during cooling. The next temper then relieves the stress in that newly formed martensite.

A simple rule is: do not use single tempering for M2 when stable performance is required. One tempering cycle is usually not enough to stabilize the structure.

M2 Tempering Temperature and Hardness Chart

M2 has a strong secondary hardening response. Its hardness does not simply decrease as tempering temperature rises. In the high-tempering range, fine alloy carbides precipitate, and hardness can rise again.

The following chart shows typical hardness after double tempering when M2 is austenitized around 1230°C / 2250°F.

Temperatura de têmpera	Oil Quenched Hardness	Air Quenched Hardness
Como extinto	64.0–66.0 HRC	64.0–66.0 HRC
400°F / 204°C	63.0 HRC	63.0 HRC
500°F / 260°C	62.5 HRC	62.5 HRC
600°F / 316°C	62.5 HRC	62.5 HRC
700°F / 371°C	62.5 HRC	62.5 HRC
800°F / 427°C	63.5 HRC	63.5 HRC
900°F / 482°C	64.0 HRC	64.0 HRC
1000°F / 540°C	64.5–65.5 HRC	62.0 HRC
1025°F / 550°C	65.0 HRC	63.0 HRC
1050°F / 565°C	63.5–65.5 HRC	64.0 HRC
1100°F / 595°C	61.5–64.0 HRC	63.0 HRC
1150°F / 620°C	60.0–62.0 HRC	60.0 HRC
1200°F / 650°C	53.0–53.5 HRC	53.0 HRC

For many M2 cutting and tooling applications, tempering at around 540–565°C is common because it falls near the secondary hardening range. This range helps balance hardness, cutting performance, toughness, and stability.

M2 should not be undertempered. A low tempering temperature or insufficient tempering cycles can leave high internal stress and unstable retained austenite. Double tempering is the minimum practical requirement, and triple tempering is often used for more demanding tools.

Effect of Austenitizing Temperature on M2 Tempered Hardness

The austenitizing temperature affects the final hardness response. Higher hardening temperatures dissolve more carbon and alloying elements, thereby strengthening secondary hardening during tempering. However, they also increase the risk of retained austenite and overheating.

Temperatura de têmpera	Hardened at 1180°C	Hardened at 1200°C	Hardened at 1220°C	Hardened at 1240°C
Como extinto	66.0 HRC	64.0 HRC	65.0 HRC	64.0 HRC
200°C	63.0 HRC	61.5 HRC	62.5 HRC	61.5 HRC
300°C	62.7 HRC	61.5 HRC	62.5 HRC	61.5 HRC
400°C	63.0 HRC	62.0 HRC	62.5 HRC	62.0 HRC
500°C	63.5 HRC	64.0 HRC	64,5 HRC	64,5 HRC
525°C	64,5 HRC	65.0 HRC	65.5 HRC	66.0 HRC
550°C	64,5 HRC	65.5 HRC	66.0 HRC	66.5 HRC
575°C	64.0 HRC	63.5 HRC	64,5 HRC	66.0 HRC
600°C	62.0 HRC	62.5 HRC	62.5 HRC	63.0 HRC

This table shows why M2 should not be heat-treated only to chase the highest hardness number. A higher hardening temperature can produce stronger secondary hardening, but the safe processing margin becomes narrower. In production, the better target is stable hardness with acceptable toughness, controlled retained austenite, and reliable tool life.

M2 Heat Treatment Problems

Most M2 heat-treatment failures result from five areas: incorrect austenitizing, excessive soaking, poor atmosphere protection, improper quenching, and insufficient tempering.

Problem	Main Cause	Resultado
Low hardness	Underheating, insufficient carbide solution, poor quenching, or excess retained austenite	The tool cannot reach the required working hardness.
Grain coarsening	Overheating or over-soaking	Lower toughness and higher cracking risk.
Excess retained austenite	High austenitizing temperature, long soaking, or insufficient tempering	Dimensional change and unstable hardness.
Rachaduras por têmpera	Thermal shock, severe quench stress, or delayed tempering	Cracking during or after hardening.
Descarbonetação	Heating without atmosphere protection	Soft surface and poor wear resistance.
Under-tempering	Low tempering temperature or too few tempering cycles	Brittleness and unstable structure.

Overheating and over-soaking are serious because M2 is processed close to its high-temperature limit. Excessive temperature or time can coarsen the grain, increase retained austenite, reduce toughness, and lower practical tool reliability.

Underheating prevents enough alloy carbides from dissolving. The result is lower quenched hardness and weaker secondary hardening during tempering.

Retained austenite is expected in M2, but excessive retained austenite causes problems. It can transform later during service, leading to the formation of fresh martensite, dimensional growth, internal stresses, and an increased risk of cracking.

Decarburization is another major failure source. If M2 is heated without vacuum, neutral atmosphere, or salt bath protection, the surface can lose carbon and remain soft after hardening.

Tempering errors are especially costly. M2 requires multiple tempering cycles because the transformation of retained austenite and the tempering of fresh martensite cannot be completed properly in a single cycle. For stable performance, the tool should cool to room temperature between tempers.

Note: Aobo Steel supplies M2 / DIN 1.3343 / JIS SKH51 high-speed tool steel in an annealed condition. We do not provide final heat treatment, hardening, or tempering services. This article is prepared as technical support for our customers and is intended to help buyers better understand the principles of M2 heat treatment before arranging their own processing.

Actual heat treatment results may vary depending on furnace control, section size, tool design, quenching method, and the heat-treatment facility’s process control. For M2 tool steel supply, available sizes, inspection requirements, MTC, and export order details, please contact [email protected].

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