Paper,honoured with the Sir Charles Hatchett Award by the Institute of Metals,London: Developments in high speed tool steels

This paper describes a research programme at the Austrian School of Mines (Montanuniversität) at Leoben, carried out since 1981 in cooperation with the Max-Planck-Institute for metals research in Stuttgart, on the fundamentals of alloy design for high speed tool steels. Among the results, the development of niobium-alloyed grades has an important place. Controlled solidification studies with a gradient technique have clarified the influence of various alloying elements on the as-cast microstructure of ledeburitic tool steels. A procedure for accurate quantitative metallography in SEM, combined with EDX and STEM-EDX analysis of the chemical compositions of the carbide and matrix phases, has led to a quantitative model for the performance of high speed steels in metal cutting tools, in which the contributions of carbides and of the matrix are combined using empirically determined weight factors. An important role is played by the saturation of the matrix with vanadium and other carbide formers which are essential for secondary hardening. This saturation is related to the way in which these carbide formers are present in the annealed structure; this in turn is influenced decisively by the solidification path (via M₆C or M₂C) of the alloy. On the basis of these concepts, low alloyed, niobium-containing economy grades have been developed whose performance is comparable to that of commercial high speed steels, and perspectives for the development of economic super high speed steels with niobium as an alloying element are indicated.