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

Abstract

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.

Three Important Points that Relate to Improving the Hot Workability of Ledeburitic Tool Steels

“…It also influences the volume fraction of the eutectic colonies. [22][23][24][25][26][27][28] The positive effects of the higher cooling rate are limited to the regions situated nearer the surface of the ingot (Figure 3(a)), but in the central regions of the ingot, typical regions exhibit greater and lesser concentrations of eutectic colonies (Figure 4(a)). The microstructure in Figure 5(a) was obtained with a laboratory simulation [20] of the solidification process carried out on the Gleeble 1500D thermomechanical simulator, employing a relatively low cooling rate of 0.16 K/s.…”

Microstructures and high temperature properties of spray formed niobium‐containing M3 high speed steel

“…At natural cooling rates, the niobium rich MC carbides precipitated straight from melt will grow up to a larger scale to the end of solidification , which obviously affecting the toughness to a high degree. It is reported that high cooling rate can increase the heterogeneous nucleation and depress the growth of primary NbC carbides .…”

Formation Behavior of M₂C and M₆C Eutectic Carbides in M42 High-Speed Steel

“…3) As well recognized, the cutting performance of high-speed steels is intrinsically related to the characteristics of the carbide precipitates, such as size, amount, morphology, type and spatial distribution. 4) Large carbides not only deteriorate the hot workability of high-speed steels, 5) but also promote the nucleation of microcracks during service. 4,6) Similar to many other high-speed steels, two main types of eutectic carbides are formed during the solidification of M42 steel, i.e., M 2 C carbide and M 6 C carbide, and distributed in the interdendritic regions in the form of a continuous network.…”

“…4) Large carbides not only deteriorate the hot workability of high-speed steels, 5) but also promote the nucleation of microcracks during service. 4,6) Similar to many other high-speed steels, two main types of eutectic carbides are formed during the solidification of M42 steel, i.e., M 2 C carbide and M 6 C carbide, and distributed in the interdendritic regions in the form of a continuous network. [7][8][9][10] As-cast MC carbide is occasionally observed in M42 steel.…”