HARDENING

The purposes of hardening are: (a) to harden the steel to resist wear, and (b) to enable it to cut other metal. The metal is heated 30–50°C above the upper critical temperature for hypoeutectoid steel and above the same amount above the lower critical temperature for hypereutectoid steel. It is left for soaking for considered time. Quenching of high carbon steel heated to 1,100–1,300°C is done in a current of air. Quenching 150–200°C per second in solution 3–10% caustic soda and 5–15% salt is more rapid than the quenching effect in water at 20°C and 32–42°C for oil quenching