Surface Hardening

     The goal of surface hardening is to produce a layer of Tempered Martensite on the surface of a metal.  This is used to increase the wear resistance of a part while keeping a soft, tough and ductile core.  The core is Ferrite and Perlite micro structure.  Surface hardening is used on gear teeth, bearings, shafts, cams, cranks and many more products.  

Non-Composition Altered processes
  1. Flame Hardening
  2. Induction Hardening

1. Flame Surface Hardening
  • Start with a steel that has a carbon content of .3% or greater
  • Heat the surface of the steel to the Austentizing temperature
  • Follow the heating of the surface of the steel, quench the steel to form a Martensite case
  • To temper the Martensite case to a desired hardness re-heat with torch
  • With this form of surface hardening you have the least control in the depth of the Hardened layer

2. Induction Surface Hardening
  • Start with steel that has a carbon content of .3% or greater
  • Heat surface of steel using on induction coil to its Austenizing Temperature
  • Follow the heating of the surface of the steel, quench to form a Martensite case
  • To temper the Martensite case to a desired hardness re-heat part
  • With this form of surface hardening you have greater control of the depth of the hardened layer



Composition Altered Processes
  1. Carburizing
  2. Nitriding
  3. Carbonitriding


1. Carburizing
  • Produce a part from steel with less then .3% carbon.
  • Heat to austonite temperature in a carbon rich atmosphere.
    (Furnace with CO2 gas, Pack the part in Coke, Rich oxy-fuel torch)
  • The carbon will diffuse into the steel and raise the carbon level to greater than .3% on the outside. (depth of the hardened surface is controlled by temperature and time)
  • Quench to cause a case of martensite to form on the outside.
  • Temper to desired Hardness.

2. Nitriding
  • Make Part from steel with less then .3% carbon.
  • Heat the part up to approximately 1000 degrees F in a nitrogen rich atmosphere.
  • Nitrogen reacts and forms hard nitride compounds (Aluminum Nitride, TiN, etc) on the surface of the steel.
  • There is no quench required which helps to minimize distortion.
  • Max hardness is approximately 72 HRC which is better for wear resistance.
  • The surface that is hardened is very thin and this process should not be used where finish grinding is needed.


3. Carbonitriding
  • Make part from steel with less then .3% carbon.
  • Heat the part between 1400F - 1598F in an furnace that has an atmosphere has an hydrocarbon (propane or methane) mixed with Ammonia (NH3).
  • The part is then quenched in natural gas (oxygen free) atmosphere.
  • Tempering is needed to reduce brittleness