A three-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 can identify the sintering gate and the effect of compositional variations on sinterability. The major objectives 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 degreesC.
It is also proposed to develop richer alloys which cannot be processed by conventional means.
Initial work concentrated on modifications to the standard T1 grade (principally through increases in the carbon content) and processing by water atomisation and vacuum sintering of two alloy compositions developed for the gas atomisation plus hot isostatic pressing route: Erasteel ASP60 and Hitachi HAP70. This work proved successful and accordingly it progressed to study a number of compositions with prospective increased sinterability based on CALPHAD calculations. The alloys were water atomised, annealed, pressed, vacuum sintered, heat treated and examined as to microstructure, mechanical and cutting properties. The novel compositions include a quaternary Fe-1.3C-3.5Cr-14.1Mo and a quinary Fe-1.4C-4.0Cr-14.0Mo-8.0Co alloys. Increased sinterability, windows extending to 40 C and lowering of the optimum full densification temperature to 1170 C, have been achieved. In correctly heat-treated materials coarser microstructures (grain sizes typically 10-15 um and carbide sizes up to 20 um) are observed than in conventional or HIPed HSSs. HV ranges from 810 to 960 and modulus of rupture up to 2.4 GPa. Initial single point turning point turning trials with annealed 817M40 steel workpiece material are encouraging, with the performance of some novel alloys being comparable or superior to standard butt-welded T1 and M2 tools at speeds up to 45 m/min.
In all alloys studied the scientific hypothesis at the basis of the CADOSTS project has been substantiated: supersolidus (liquid) phase sintering takes place successfully only in the three phase region of the phase diagram comprising liquid, austenite, carbide(s). This approach can now, with expectations of success, be extended to other multicomponent alloys systems to be developed with the objective of being powder metallurgy processing friendly.
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