Martensitic stainless steels, first discovered by Harry Brearley, have advanced a long way since 1913. The addition of 12 % Cr to give sufficient corrosion resistance to the material also imparts very high hardenability, so large sections can easily “through-harden” up to 500 mm thick. With additions of various alloying additions, for example, molybdenum, the analysis can be tailored to suit certain applications.
The plastic moulding industry is a large consumer of these alloys. The key properties for the moulds are through-hardenability to prevent erosion, corrosion resistance to resist chlorides in the plastic and good thermal conductivity for cooling of the part to allow it to be ejected from the mould. A characteristic of martensitic steels is the formation of carbide stringers. These can be attacked by the plastic resulting in defects in the mould, which are then imparted to the moulded part. This problem has been addressed by developing low carbon grades with nitrogen acting as the main strengthening element. Different grades provide a range of hardness and ductility combinations to suit specific applications.
Quality is further improved by using ESR (Electroslag Remelting) which gives a uniform, clean structure which improves the surface of the mould and hence the quality of the moulded part. Typical plastic products are: spectacle lens mouldings, car rear lamp mouldings and plastic syringes. The same properties are required for extruder screws used in the production of food items like breakfast cereals. So now we know how rear lights are linked to cornflakes!