Some of these steels are considered to be secondary-hardening tool steels. The secondary hardening must not be confused with the tempering and decomposition of retained austenite that might be present, which would originate as a result of the incomplete transformation of austenite to fresh martensite.
The carbides (also known as the secondary-hardening elements) are formed as a result of interactions between the carbon content of the steel being treated and the carbide-forming elements of chromium, tungsten, molybdenum, vanadium, manganese, titanium, zirconium or niobium.
Carbides will not form as a result of the non-carbide forming elements being present in the steel analysis. These are silicon, nickel, cobalt, copper, aluminum and nitrogen.
The low tempering temperatures from 400°F and above will initiate the precipitation of the small, fine carbide particles and will begin to show an increase in the toughness of the steel. The tempering temperatures that will initiate the complete carbide precipitation are usually found around the 930°F mark and above, up to the A1 line of the Iron Carbon Equilibrium Diagram at approximately 1320°F. Above that temperature, the steel enters the mixed-phase area of ferrite plus austenite.
The strong carbide formers are tungsten, molybdenum, vanadium and chromium. It is necessary to dissolve these elements into the steel during the austenitize phase of the treatment (above the A3 line of the Iron Carbon Equilibrium diagram) so that they will be incorporated into the transformed martensite phase when cooled rapidly so that the complete secondary-hardening effect can be achieved.
The secondary-hardening carbide formation will occur as fine needles in the martensite. The degree to which the change will occur will be dependent on:
- The hardness required (after tempering)
- The amount of carbide-forming elements present in the steel analysis
- The tempering temperature selected
- The time at the tempering temperature
- The number of tempering procedures given to the steel (generally at least two to three tempers)