What is the hardening process?
Hardening is a kind of heat treatment process that can improve the plain-carbon steel properties in hardness and abrasive resistance. This process starting with steel is heated to the austenitizing temperature, held in furnace to acquire homogeneous austenitic structure and then quenched at such a rate that martensitic structure is produced or critical cooling rate. In order to obtain fully martensitic structure in steel, the related factors are recommended as following ;
1. Carbon content : The higher percentages of carbon in steel, the more occasion that martensitc structure is produced. Moreover,the alloying elements such as Nickel, Chromium and Molybdenum will also increase hardenability by decreasing the critical cooling rate.
2. Cooling rate : Cooling rate that required to provide martensite should not be lower than the critical cooling rate and also depends on severals factors as follwing ;
– The surface always cools faster than the center of the part. In addition, as the size of the part increases, the cooling rate at any location is slower. Consequently, the smaller part has more possibility to produce fully martensitic structure than the bigger one in the same condition.
– The different quenching media provide different cooling rate. For example, water and brine (water plus various percentages of Sodium choride or Calcium choride) provide a faster cooling rate than oil. In addition, agitation of the quenching media is one method which also increases the cooling rate
Reference: 1.Donald R. Askeland.,The Science and Engineering of Materials, 3rd editionn,PWS Publishing company (1994)