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A new family of patent pending high speed steel alloys has been developed. The compositions of the steels are related to the compositions of standard high speed steels (HSS) and die steels. However, the nominal carbon content of these alloys is approximately 0.20%. These are carburizable grades of HSS. The other major compositional differences between these alloys and conventional HSS are that the new alloys contain less than 1.50% chromium. Unlike alloy M50-Nil, these new steels do not have to be pre-oxidized prior to being carburized. The modified HSS alloys are carburized using standard processing identical to the procedures used with carburizable alloy steels such as 4320, 8620 or 8720. Once carburized, the alloys are then heat treated for maximum hardness using the time and temperature cycles employed for standard HSS. That is, the steels are preheated, austenitized, quenched, and double tempered.
To offset the effect of chromium on hardenability and secondary hardening, the molybdenum, tungsten, and vanadium contents of these alloys has been increased. Complete details of calculating the specific compositions of these types of alloys will be discussed.
Lab heats of modified alloys Ml, M2 and M50 achieve surface hardnesses of 65 HRC. Residual compressive stresses in excess of −400 MPa 0.50 mm below the surface are obtained in cylindrical test specimens. Additionally, low levels of retained austenite are observed in these steels.
The ease of machining and heat treating these alloys in conjunction with their high hardness and excellent hot hardness make the steels ideal materials to be used for bearings in normal, debris, or high temperature applications.
carburize, high speed steel, residual stress, retained austenite
Research Specialist, The Timken Company, Canton, Ohio