52100 Steel Heat Treatment Guide

AISI 52100 is a high-carbon, low-chromium hypereutectoid steel containing approximately 1.0% carbon and 1.5% chromium. It is widely used internationally for fully hardened bearings and high-wear applications.

The properties of 52100 steel are derived equally from its composition and from proper hardening heat treatment. While conventional tool steel heat treatment aims to minimize retained austenite to prevent deformation and cracking, 52100 steel heat treatment intentionally retains a portion of it. When 52100 is used in bearings, it generates high-frequency rolling contact fatigue. Residual austenite is specifically designed to accommodate this operating condition. Research indicates that 52100 bearings containing 5% to 15% RA typically exhibit longer rolling fatigue life than bearings with no RA content.

After receiving our annealed 52100 steel, our customers must perform their own heat-treatment steps, including stress-relief annealing and hardening. The hardening process follows standard tool steel procedures, encompassing preheating, austenitizing, quenching, and tempering. This document focuses on these steps.

Stress-relieving heat treatment

To ensure dimensional stability during the subsequent hardening stage of 52100 steel, stress-relief operations are essential, particularly after heavy machining or forming. Residual machining stresses in the workpiece are relieved during heat treatment, leading to significant dimensional changes.

Operating Procedure: Heat to 650°C to 700°C (1200°F to 1300°F). Maintain at this temperature for a sufficient duration to fully eliminate internal stresses. Subsequently, perform a slow furnace cooling to approximately 500°C, followed by final air cooling to room temperature.

Preheating

Preheating AISI 52100 steel prevents thermal shock and ensures a uniform temperature distribution between the surface and the core of the workpiece. This high-carbon steel has relatively low thermal conductivity. Rapid heating can create significant temperature differentials between the interior and exterior, increasing the risk of cracking in 52100 material. The preheating temperature range is 650°C to 705°C (1200°F to 1300°F).

Austenitizing

The purpose of austenitizing is to dissolve an appropriate amount of carbon and chromium into the austenite phase to ensure high hardness after quenching. Simultaneously, it preserves finely dispersed, finely distributed undissolved carbides to enhance the material’s wear resistance. 

The recommended austenitizing temperature for 52100 steel is typically 815°C to 845°C (1500°F to 1555°F). Hold for 10 to 30 minutes, depending on the workpiece’s cross-sectional dimensions. Overheating or excessive holding time must be avoided, as these practices generate excessive retained austenite and significantly reduce material toughness due to grain coarsening.

Quenching

Quenching is designed to rapidly transform the austenitic structure into martensite. AISI 52100 steel is oil-quenchable; water quenching is prohibited, as the intense cooling intensity of water quenching is highly prone to inducing stress cracks. The oil bath should be maintained between 40°C and 60°C (100°F to 140°F). This temperature range ensures stable cooling while minimizing deformation caused by thermal shock. 

Mechanical agitation during quenching is essential to ensure uniform cooling and prevent localized soft spots. When properly quenched, 52100 typically achieves a hardness of 63–65 HRC.

Tempering

When the 52100 workpiece cools during quenching to a residual temperature (50°C to 70°C), it should be promptly transferred to the tempering furnace for tempering. Tempering eliminates quenching stresses, reducing brittleness in 52100 steel and preventing spontaneous cracking. For bearing applications, standard tempering temperatures range from 180°C to 250°C to maintain hardness above 60 HRC.

Common Issues and Solutions in Heat Treatment of 52100 Steel

1. Decarburization

Decarburization primarily occurs when workpieces react with oxygen at high temperatures, leading to surface carbon loss and the formation of a so-called soft layer. To effectively prevent decarburization, it is recommended to use protective atmospheres, salt baths, or vacuum furnaces for heating during the 52100 hardening process. It is essential to maintain a strictly neutral-to-slightly reducing atmosphere within the furnace to prevent oxidation reactions.

2. Quenching Cracks

52100 quenching cracks typically originate from excessive cooling rates or thermal shock, occurring most frequently during water quenching or when heated beyond the recommended temperature. To address this issue, apply oil at 40-60°C as the quenching medium. Simultaneously, minimizing temperature differentials between the workpiece’s interior and exterior through thorough preheating, coupled with strict control of the austenitizing temperature within the recommended range, can significantly reduce the risk of cracking.

3. Dimensional Distortion

The primary causes of dimensional deformation in the 52100 workpiece include uneven cooling rates and the release of residual machining stresses during heating. To address this issue, it is recommended to incorporate a stress-relief treatment step after rough machining, performed at 650°C-700°C. Additionally, employing a step quenching method can achieve a uniform temperature distribution throughout the workpiece before the onset of martensitic transformation, thereby effectively controlling deformation.

4. Retained Austenite

Overheating during austenitization of 52100 steel lowers the martensite transformation start temperature (Ms), leading to excessive retained austenite after quenching. This structure is softer, leading to dimensional instability in the workpiece. The solution is to strictly adhere to the austenitization temperature range of 815-845°C. For special requirements, immediate deep cryogenic treatment (e.g., -70°C to -100°C) after quenching can be performed to promote complete transformation of retained austenite into martensite.

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