Methods for Controlling Residual Austenite in Heat Treatment

After quenching, parts invariably retain some residual austenite, to a greater or lesser extent. Excessive residual austenite is detrimental to the service life and hardness of the parts, potentially leading to soft spots and dimensional instability. However, a moderate amount of residual austenite can actually enhance the fatigue strength of the parts. By carefully controlling the level of residual austenite, we can effectively manage product quality and service life, thereby achieving the desired outcomes.

After quenching, parts invariably retain some residual austenite—more or less. Excessive residual austenite is detrimental to the service life and hardness of the parts, potentially leading to soft spots and dimensional instability. However, a moderate amount of residual austenite can actually enhance the fatigue strength of the parts. By carefully controlling the level of residual austenite, we can effectively manage product quality and service life, thereby achieving the desired performance.

1. The Influence of Residual Austenite on Various Parts

( 1 Rolling bearings are required to have excellent wear resistance, high rolling contact fatigue strength, and good dimensional accuracy stability. At typical stress levels, the presence of residual austenite has little impact on fatigue life. In actual production, bearing steels generally do not undergo cryogenic treatment after quenching.

( 2 ) Gears generally do not require cold treatment. Residual austenite is beneficial for enhancing their fatigue life.

( 3 For tool steels, retained austenite can enhance impact resistance. However, in cutting tools, retained austenite reduces hardness, thereby degrading cutting performance. For tools such as drills and taps that primarily experience torsional stresses, a moderate amount of retained austenite is beneficial. In die steels used for pressure working—especially in punches—having an appropriate amount of retained austenite is advantageous. Compared to martensite, retained austenite acts like a sponge: it can absorb and cushion impacts, improve toughness, increase surface contact fatigue strength, and extend the service life of punches.

( 4 ) For measuring tools, residual austenite is detrimental to ensuring dimensional accuracy and must be eliminated as much as possible through cold treatment.

2. Analysis of Factors Influencing Residual Austenite

As the alloying elements increase, the carbon content rises, the holding time during quenching is prolonged, the cooling rate slows down, and the quenching temperature is raised—all these factors contribute to an increase in retained austenite. When quenching is interrupted and held isothermally, the final transformation amount of martensite decreases, while the amount of retained austenite increases; this phenomenon is known as the thermal stabilization of austenite. The carbon content at the eutectoid point... 0.8 Left and right, residual austenite is present in... 25% Here, the residual stress is compressive. After carburizing, the surface of the part has a high carbon content, and after quenching, the amount of retained austenite increases.

The main factors determining the retained austenite content are:

( 1 ) The influence of alloying elements in raw materials: Mn 、 You 、 Cr Alloying elements increase the amount of retained austenite after quenching.

( 2 ) The increase in the carbon content of the raw materials leads to an increase in retained austenite.

( 3 ) Heat treatment In terms of processing, increasing the austenitizing temperature, raising the quenching temperature, elevating the quenching termination temperature, reducing the quenching cooling rate, and allowing intermediate pauses during quenching will all lead to an increase in retained austenite. Given a fixed part material, appropriately lowering the quenching temperature and adopting additional cold treatment (such as extended quenching) are effective measures for reducing retained austenite. After quenching, cold treatment, and tempering, the retained austenite in the parts is consistently reduced. ≤10%，GCr15 Bearing steel is generally in 5% Left and right.

3. Measures to reduce residual austenite

Generally, after quenching during heat treatment, a martensitic transformation occurs, inevitably leaving some residual austenite behind. To eliminate or control the residual austenite, the following methods are commonly employed:

( 1 ) Add cold treatment. Cold treatment is essentially an extension of quenching; its core principle is to lower the cooling termination temperature, thereby further transforming residual austenite into martensite. This is... GCr15 It is widely used in plunger pairs and represents the most effective method for promoting the transformation of residual austenite. Typically, the amount of residual austenite is controlled to... 10% Within.

( 2 ) Replace martensitic quenching with bainitic quenching—that is, raise the quenching termination temperature, typically to... Ms. Near the point of isothermal transformation, the reaction produces a near-equilibrium microstructure consisting of acicular lower bainite formed by ferrite and cementite. Since no martensitic transformation occurs, the amount of retained austenite is reduced.

( 3 ) Adjustment of heat treatment process parameters: ① During carburizing of low-carbon steel, control the carbon potential, regulate the surface carbon content, manage the levels of nitrogen-carbon compounds and carbides, and thereby control the retained austenite. ② Reducing the austenitizing quenching temperature and immediately performing tempering after quenching can also help reduce the content of retained austenite. ③ Increasing the tempering temperature can cause the residual austenite in steel to transform into martensite or decompose, thereby reducing the amount of residual austenite. Below... 200℃ Tempering—residual austenite in the steel does not decompose. After... 200 ~ 300℃ Tempering causes the residual austenite in steel to begin decomposing into lower bainite. Above... 300℃ Tempering causes the complete decomposition of residual austenite in steel. In high-speed steel... 560℃ During tempering cooling, a portion of the retained austenite undergoes martensitic transformation, increasing hardness and reducing the amount of retained austenite.

( 4 During carbon-nitrogen co-diffusion, ammonia gas and carbon-nitrogen compounds lead to an increase in retained austenite. Carburizing is employed. + Quenching process replaces Carbon-nitrogen co-diffusion Quenching, after being subjected to cold treatment, can make the... 500 Under a magnifying glass with twice the magnification, it cannot be observed with the naked eye; the residual austenite is essentially less than. 10% or 5%。

4. Production and Application

In actual production, it is applied to... CB18、CPN2.2-0401 Control of residual austenite in the stem and roller bushings after case-hardening. By controlling the atmosphere, the amount of ammonia gas introduced is... 40 ~ 80 L/h, Adjust to 20L/h ; Propane is controlled at 200L/h Control the reduction of surface nitrogen-carbon compounds and carbides. Lower the quenching temperature: by 850℃ Adjust to 820 ~ 830℃ , add cold treatment; the tempering temperature is by 180℃ Increase to 200℃ A series of process parameter adjustment measures were implemented, significantly improving the results and ensuring that residual austenite is kept below. 10% , meeting the technical requirements.