Microstructure and Mechanical Properties of M50 Steel by Combining Cold Rolling with Austempering

Dongsheng Qian,Yuangeng He,Yun Chen,Feng Wang,Xiaohui Lu

doi:10.3390/met10030381

Dongsheng Qian, Yuangeng He + Show 3 more

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https://doi.org/10.3390/met10030381

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Abstract

The microstructure and mechanical properties of M50 steel subjected to combining cold rolling (CR) with austempering are investigated. The microstructure is characterized using X-ray diffraction and scanning and transmission electron microscopy. The mechanical properties are measured using the uniaxial tensile and Charpy impact tests. It is observed that an excellent combination of ultimate tensile strength (2536 MPa) and impact toughness (128 J) was achieved by combining austempering with CR; of which the ultimate tensile strength increased by 10% compared with traditional martensite quenching-tempering (Q-T) specimen and the impact absorbed energy exhibited 2.3 times of that in Q-T specimen. The observation of the microstructure indicates that CR obviously refines the thickness of bainite sheaves, which is favorable for the formation of ultrafine equiaxed ferrite during tempering.

Highlights

M50 steel is widely used in the manufacture of high-end bearing steel for aeroengine shafts in the aerospace industry [1,2,3]
The martensite start (Ms) temperature of experimental steel without and with CR can be obtained at the cooling rate of 10 °C/s, which indicates a notable decrease from 184 °C to 174 °C after CR
The results show that the average thickness decreased from 142 nm to 86 nm compared with that in the specimens without CR, indicating that CR can significantly refine the bainite sheaves

Summary

Introduction

M50 steel is widely used in the manufacture of high-end bearing steel for aeroengine shafts in the aerospace industry [1,2,3]. Compared with the most common GCr15 low alloy steel, M50 steel grade is a high alloy steel that relies on tempering to achieve secondary strengthening [4] It is routinely treated by a martensitic quenched and tempered heat treatment to obtain the microstructure consisting of dominant tempered martensite, a small amount of retained austenite (RA) and carbides [5]. These microstructures lead M50 steel to achieve excellent strength, hardness, and dimensional stability. Compared with lower strength steels, the impact toughness of M50 steel is poor, which results in short service life and low reliability of bearings. How to improve the impact toughness of M50 steel is a major issue that needs further attention

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