The heat treatment of stainless steel has two basic tasks to fulfil: to obtain the hardness for use and to achieve maximum oxidation resistance.
As both of these properties are important, particularly on the surface of the finished part, the heat treatment must be carried out in such a way that there is no change in the chemical composition of the surface.
Any lean chromium or decarburisation will lead to a reduction in the desired characteristics of the part and must be carried out in such a way that no distortion occurs in order to avoid subsequent correction work.
The techniques used to carry out these treatments are described below in order to obtain the required technical and economic characteristics in an exemplary manner.
Means of heat treatment
Heat treatment in an oxidising atmosphere, such as air, is practically no longer used because, particularly on stainless steel, strong layers of adhering scale are formed which must be removed by subsequent pickling.
In addition, decarburisation occurs in this atmosphere, which makes it necessary to grind the parts, not only to achieve the necessary surface characteristics, but also to remove the decarburised layer.
Most finished parts made of stainless steel, such as knives, surgical and dental instruments, ball bearing components, cylinders and needles, are often hardened by heating in a salt bath. As the required temperature is above 1000°C, only barium chloride salts can be used.
These salt baths have proven to prevent decarburisation, but lead to a loss of chromium on the surface of the part, which in turn leads to a reduction in corrosion resistance.
No matter how careful one is with the bath, it is impossible to avoid this loss of chromium. It is therefore necessary to machine the parts at least 0.008 mm after treatment to regain optimum corrosion resistance. In the case of parts with complex shapes, the cost of such an operation can be high.
Salt baths based on barium chlorate also have the disadvantage that these salts are difficult to dilute with water. Flushing therefore poses serious problems for parts with holes, threads or irregular shapes, and there is the additional problem of removing the flushing water and salt.
Protective gases such as dissociated ammonia (NH3), pure hydrogen (H2) and Formigas are by far the most economical and environmentally friendly means of heat treatment.
The only requirement is that these gases must be dry, i.e. have a dew point of -40 °C.
In this atmosphere, the chromium content is not reduced and therefore full corrosion resistance is maintained.
In addition, these gases do not react with carbon, thus avoiding any decarburisation and achieving maximum surface hardness.
Based on practical experience, the hardness values obtained in a gas atmosphere are significantly better than those obtained in a salt bath.
In the dissociated ammonia (NH3), the hydrogen content is 75% and the nitrogen content 25%. As hydrogen is a strong reducing gas, there is no possibility of oxidation of the parts in the furnace and they are completely white.
There are concerns that the 25% nitrogen content will produce nitrogenation, which has not been confirmed in practice.
Hardening
Annealed steel has a ferrite – carbide structure. The purpose of quenching is to transform this structure into an austenitic structure and to dilute the carbides. Maximum corrosion resistance and hardness can only be achieved if complete transformation and dilution of as much carbide as possible is achieved.
The hardening temperatures for different stainless steels range from 980 to 1050 ºC.
In most cases it has been shown that better results can be obtained at hardening temperatures of 1.040 to 1.080 ºC, achieving a more homogeneous structure and faster carbide dissolution without the possibility of forming a coarse-grained structure with all its drawbacks.
The holding time at the quenching temperature depends on the thickness of the part to be quenched. For parts of different thicknesses, the holding time should not be determined by the thinnest thickness, although this smallest thickness may be the most practical part of the part.
The length of holding time should be aimed at achieving complete transformation of the structure and extensive dissolution of the carbide, even in the thickest areas, if in these parts it is desired to achieve