TECHNICAL INFORMATION

COOLING RATE AND MICROSTRUCTURE Steel, the most widespread material for different mechanical tools, must perform the best resistance to tensile stress and wear. Those properties are directly connected with a high hardness but, on the other hand, a good toughness to cracks must be granted in order to make the mould resistant to pyrocracks and thermal fatigue. The highest hardness of a steel is linked to its microstructure and to obtain it, the steel must undergo a hardening process. The following tempering processes have two purposes: to reduce the hardness values obtained with a structural transformation of martensite and to complete the transformation of the left austenite, always present in the materials for the heat treatments due to their high alloy components like molybdenum and vanadium which tend to stabilize austenite slowing the martensitic transformation process. A complete transformation of residual austenite is of fundamental importance both for the deformations a tool undergoes after a particular heat treatment and for material tenacity which results seriously compromised when residual austenite values are consistent. For any steel tool, having particular shapes and dimensions, the possibility to obtain the needed distribution of structural components from the surface to the core depends substantially on the possibility to realize the necessary cooling rate in different points and choosing the extinguishing method. Obviously another parameter to be considered is the chemical composition of the steel, which determines the positioning of CCT curves. Generally the steels for the heat working, with high content of alloy elements, are virtually easily tempered (See fig.1), that is they can be tempered also with relatively low cooling rates. The difference between a tempering produced with a low cooling rate and one with a high cooling rate is the microstructure, which, as we can see, is the direct responsible of the behavior during the use with moulds. What we want to underline is that there is a precise connection between microstructure and cooling rate (hereafter c.r.), in the sense that a given microstructure may be obtained if only a given c.r. is performed, but this is not true for the connection hardness-c.r. In fact we can calculate the hardness we will obtain (if we know the obtained microstructure) but we cannot calculate the c.r. from the hardness and from the type of microstructure.

Fig.1: Curves CCT for steel UNI X37CrMoV 5 1 KU (1.2343) The request to obtain a certain hardness after the thermal treatment is at least incomplete (as we see after), especially for important tools as moulds for die casting. The single value of the hardness cannot be a parameter through which the life of a mould may be foreseen, as this value may be associated with a wide brand of different microstructures, having mechanical properties (i.e. toughness, elastic limit etc.) extremely different and those may influence the behavior of the component more than only hardness.