Prehardened 400 series stainless steel (DIN 1.2316) is generally used for calibration tools and mold inserts for the plastic processing industry. Today, plastic products are manufactured by mass production,

using thermoplastic and duroplastic bulk materials, which are processed by injection molding, blow molding and extrusion molding. The consumption of plastics has influenced the tool steel market, with a demand for more specialized type of tool steels1,5. Increasing ingot sizes can lead to microstructural inhomogenities—such as carbide segregation and the appearance of δ-ferrite—which have a negative influence on the properties of the typical prehardened stainless steel.

Experience has shown that such microstructural inhomogenities have a detrimental effect on properties such as the machinability, polishability and corrosion resistance. Furthermore, there also is a negative influence on the mechanical properties of the material. To prevent these negative effects, a new alloy (Bohler M303 Extra) has been created with a more homogeneous microstructure, and with better mechanical properties as well as better polishability and machinability.

Based on the chemical composition of DIN number 1.2316, Thermo-Calc calculations were carried out to clarify the occurrence of the ferrite phase. It has been found that at 16 weight percent of chromium, a delta ferrite phase occurs at forging temperatures up to 1200°C. This effect could be increased by chromium segregation. To reduce the effect of delta ferrite, nitrogen is one of the alloying elements that can be added6. This result was therefore used in the new alloying concept2,3,4.

The partial substitution of carbon by nitrogen reduces the amount of chromium rich M23C6-carbides2. As a consequence, the chromium content of the alloy can be reduced without lowering the chromium content of the matrix at typical hardening temperatures. This measure lowers the risk of carbide segregation.

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