Sintering of High Speed Steels and Tool Steels

Powder metallurgy processing offers several advantages to costly and highly alloyed tool steel materials. These advantages include uniform and finer microstructure, improved grindability, improved cutting performance, and capabilities of high-speed steels and tool steel alloys that cannot be made by conventional ingot metallurgy.

The use of conventional press-and-sinter technology offers the additional advantage of net shape or near-net shape capability. Cutting tools, bearings, and wear parts are being produced commercially by fully dense sintering. Wear parts also are produced by conventional P/M techniques to densities of 80 to 90%.

Current commercial fully dense sintering uses high green strength and compressible water-atomized tool steel powders that are compacted in rigid dies using uniaxial pressing or in flexible rubber molds using cold isostatic pressing to make green tools and parts. These parts are then sintered in a microprocessor-controlled vacuum furnace near the solidus temperature of the alloy to virtually full density (at least 98% and frequently 99+% of theoretical).

The flexibility of this process allows production of pressed and sintered tool and die parts in various forms. Generally, parts are pressed to 70 to 85% of theoretical density before sintering to full density. Pressing to lower green densities tends to require longer times at temperature to obtain full density and results in coarser microstructures. Pressing at higher pressures, which is required for increased green densities, results in increased tool wear and breakage. Parts with high green density also may require extra care in sintering. The surface can sinter rapidly to high density and entrap gases from the center of the part.

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