There are several benefits of neutral hardening, depending on the steel type:
- Heavy loaded parts can be given an optimal combination of high strength, toughness and, if applicable, temperature resistance
- Such parts can be made lighter and more stiff, due to higher strength
- Tools and dies get the required high wear and/or heat resistance while maintaining toughness
- Parts that need grinding to low roughness, acquire the required machinability
- For all these purposes, if the parts are made of martensitic stainless steels, the corrosion resistance is only available after the heat treatment
Tool steels: the desired properties of high hardness, wear resistance, heat resistance and machinability can only be given by hardening.
Martensitic stainless steels: these steels only get their maximum corrosion resistance by hardening.
For all steel types: during the shaping of the parts, (takes place before the heat treatment), the material is relatively soft and therefore easy to machine.
The hardening processes described here are typically neutral, which means that the chemical composition of the steel surface of the parts is not intended to be changed during the process.
Direct quench hardening is the most common practice for hardening of steel.
- The first step is to heat up in stages to the hardening temperature which is, depending on steel type, between 800 and 1220°C. At a temperature between 730 and 900°C (depending on steel type) a transformation of the microstructure into austenite takes place.
- The second step is to hold at this hardening, austenitising temperature to simultaneously fully equalise the temperature of the parts, and transform the microstructure into austenite. NB: this gives a reduction in the specific volume.
- The third step is quenching the part direct from the austenitising temperature in a cold medium. This kind of quench medium is usually water, liquid salt, oil or high pressure nitrogen, depending on the steel type and part dimensions. The quenching speed must be high enough to prevent the material from transforming back into the original soft structure.