1972 Volume 58 Issue 14 Pages 1961-1968
This study deals with the gas atmosphere heat treatment of high speed steel.
The hardening of the high speed steel is made at the temperature of 1200°C or above, during which the gas of the proper nature forming the atmosphere is worked into, and the hardening and weak carburization are affected simultaneously, and thus the hardness of the steel surface is brought upward, which is the ob-jective.
In order to improve the machinability and durability, efforts have been exerted in America and European countries of late to increase the carbon contents of various kinds of standardized steels, and the effectiveness of the efforts is being recognized. However, this way of thinking, viewed from the points of the equilibrium diagram of steel and of the technical practice, may have narrow limits. This study aims at the rise of the hardness by means of letting the gas atmosphere react and weak carburization take place at the time of hardening.
First of all, selection of the proper gas atmosphere was made going through preliminary experiments of extensive coverage. As a result, itis established that the carburization mechanism at a high temperature of 1200°C and above, entirely unlike the reaction at or around 930°C that is usually employed functions the other way, and the following result was obtained:
(i) The CnHm family gas or its mixture that is usually adopted to form the gas atmos phere does not meet the requirement.
(ii) It is confirmed that the gas atmosphere of weak carburization trend formed with the CO+N2 family is suitable for the purpose.
(iii) As had been expected, the surface hardness of the high speed steel of HrC 66-67.5 was obtained.