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High speed steels with improved sinterability and mechanical properties (CADOSTS)

A 3-phase region, austenite plus carbide(s) plus liquid, is essential to the successful supersolidus sintering of several standard high-speed steels: T1, M3/2, T42. Until now only determination of sintering curves could identify the sintering gate and the effect of compositional variations on sinterability. The major aims are to design alloys having the attributes of high speed steels by phase diagram computations, which can be:
sintered to full density;
with a wide sintering gate;
of different, possibly leaner, compositions to existing materials;
which sinter at temperatures possibly as low as 1150 C.
It is also proposed to develop richer alloys which cannot be processed by conventional means.

Water atomized powders subsequently processed to near full density include ASP60 type and carbon enriched T1 alloys with extended sintering windows (up to 50 C) and lower processing temperatures (down to 1270 C).
Microstructures and crystallisation paths of these and a number of model quaternary systems were determined; metallographic (scanning electron and optical) observations were supplemented by X-ray and differential thermal analysis studies.
Phase diagrams for carbon-iron-molybdenum, carbon-iron-molybdenum-chromium, carbon-iron-tungsten and carbon-iron-tungsten-chromium systems have been recalculated (CALPHAD) for 4% weight of chromium and varying contents of molybdenum and tungsten in the temperature range 800-1900 C. Differences between theoretical calculations and experimental observations relating to the liquidus temperature were frequently 10 C or less.

The newly developed alloys generally have rupture strengths, after heat treatment at hardness in the range of 860-950 HV30, between 1.4 and 1.8 GPa, comparable to conventional high speed steels. Single point turning trials are also encouraging; for a quinary easily sinterable alloy of iron, carbon, chromium, molybdenum and cobalt, the results on annealed 817 M40 steel workpiece were superior to those of M2 and T1.Similarly cutting properties of water atomised vacuum sintered high alloy high speed steels compare favourably with ASP HIPed counterpart.

Commercial exploitation of new alloys with increased sinterabiliy is being considered.

Contact

Andrew WRONSKI, (Professor of Engineering Materials)
Tél.: +44-274-384556
Fax: +44-274-305340
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