The process of through-hardening steels and then finish machining them is time consuming and expensive; to avoid this, prehardened or pretoughened steels have been developed,the effect the steel is heat-treated in a similar way to normal through-hardened steels but it is tempered back to a lower hardness.
For example, tempering at a temperature of 200 °C gives a hardness of approx* 50 Rc, while tempering at 600 °C gives a hardness of around 30 Rc. The big advantage is that while steel at 50 Rc cannot be milled or turned, material at 30 Rc can be. This means that all machining can be carried our without any need for further heat treatment. A commonly used version of pretoughened steel is A/Sf P20. The main reason for using pretoughened steel is that it does not have to undergo subsequent heat treatment hence avoiding the risk of distortion from a full hardening process.
Clearly, however, the through-hardened AISI H13 at 50 Rc is better suited to long-running tools than the pretoughened AISI PZ0 at 30 Rc .The lower the hardness of the steel,the quicker it will wear and suffer damage.P20 steel can, however, be subsequently treated with implantation to increase surface hardness by up to 15%. Pretoughened steels would normally be chosen where the risk of distortion on delicately forms is unacceptable- It would also be selected for medium-run jobs where the cost does not jusrify full hardening and finishing operations.
Carburising or Case Hardening
This process is a carry-over from the days when suitable alloy steels were unavailable. It consists of heating low-carbon steel (mild steel) in a carbon-rich gas, in a liquid (cyanide salt bath) or in a container filled with a form of carbon material. The carbon ingresses into the surface of the steel, where a hard structure is formed giving a component with a hard surface and a tough, more flexible interior. Modern hardening techniques have largely superseded carburising but there is a trend towards the use of this technique in recent years where the advantages of the hard skin and soft core are attractive.
Carburising may be used to advantage where any deflection of components is taking place such that through-hardened parts may break much earlier, A disadvantage of carburising is that it is quite prone to distort parts, so that subsequent machining is required to combat this effect, Carburised parts are not suitable for heavy direct loading applications where they will fail. For such loading conditions, the through-hardened alloy steels should be used. Carburiying is used frequently for surface hardening of cavities and puncht’s for very large mold tools where through-hardening would not be possible- This proccsy is also used fur hardening cavities that have been hohbed.