Refining the Carbide Size in AISI M50 High-Speed Steel Through Tailored Compositional Modifications

Area of Science:

• Metallurgical Engineering
• Materials Science

Background:

• M50 steel is a conventional material for high-temperature bearings.
• Existing M50 steel has limitations in hardness and carbide structure at elevated temperatures.

Purpose of the Study:

• To redesign the M50 steel composition for improved high-temperature performance.
• To refine carbide structures and enhance overall hardness.

Main Methods:

• Thermodynamic calculations were used to design a new steel composition.
• Laboratory development and heat treatment (quenching at 1070 °C, triple tempering at 540 °C) were performed.
• Hardness testing and microstructural analysis (carbide identification and size measurement) were conducted.

Main Results:

• The redesigned steel achieved a hardness of 66 HRC, a 7.57% increase over M50 steel (61 HRC).
• Significant hardness of 60 HRC was maintained at 400 °C.
• New Fe and Cr-rich M23C6 carbides were formed, contributing to hardness.
• Carbide refinement was achieved, with average primary carbide size reduced by 27.0% (4.6 ± 0.6 μm vs. 6.3 ± 0.1 μm).
• Detrimental V-rich MC carbides were eliminated.

Conclusions:

• The redesigned steel offers superior hardness and refined microstructure compared to conventional M50 steel.
• The new composition and heat treatment process are effective for high-temperature bearing applications.
• The formation of specific carbides (M23C6) and reduction in size of others are key to improved properties.