Phase transformation-induced grain refinement in rapidly solidified ultra-high-carbon steels
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RODUCTION
SEVERAL processing routes are available for the manufacture of precision parts, but they often involve an undesirably slow or complex series of steps. A method of near-netshape forming that is increasingly being explored involves fewer processing steps comprised of producing particulate material with a desired fine-scale microstructure and hot pressing the particle aggregate into the final geometry in a single step. Alloys that exhibit superplastic behavior are ideal candidates for this method of processing. In principle, high-performance structural components having complex shapes such as gears, wear rings, piston rings, shaft seals, compressor hubs, and guided missile aft-closures can be fabricated economically. One class of alloys in which superplasticity has been demonstrated is ultra-high-carbon steels (UHCS).[1] When these steels are processed to produce subm spheroidized carbide within a ferrite matrix of a few microns grain size, high room-temperature strength and ductility and excellent high-temperature formability via superplastic deformation are likely.[1] Conventionally processed UHCS are brittle at room temperature due to the presence of proeutectoid carbide along the grain boundaries. The thermomechanical processing techniques developed by Sherby and co-workers[2,3,4] involve breaking up this carbide network and producing a fine-scale microstructure. Thermomechanically processed UHCS exhibit engineering strains in excess of 20 pct at room temperature at tensile stress levels above 800 MPa.[5] Unfortunately, the extensive hot and warm working necessary to produce the fine microstructures in UHCS is expensive and places limits on product geometry. However, if fine-scale microstructures could be produced in powdered UHCS, then hot pressing, involving superplastic flow, should K.P. COOPER, Metallurgist, and H.N. JONES III, Materials Engineer, are with the Naval Research Laboratory, Materials Science and Technology Division, Washington, DC 20375. Contact e-mail: [email protected] Manuscript submitted December 3, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
yield parts of near-net shape with desirable properties at competitive costs. This approach was attempted by several researchers[6,7,8] using gas atomized powders. They all reported superplastic behavior but at low consolidation rates. Efforts to refine the microstructures through thermal treatments alone were met with limited success.[9] In all cases, the initial microstructures of the material were not sufficiently fine scale to permit direct superplastic forming. Results from these studies made it clear that if rapid consolidation of UHCS is to be achieved, then the ferrite and carbide grain sizes must not exceed 1 to 2 m. For the grains to be able to slide past each other and to rotate with respect to one another to achieve superplastic deformation, they need to be sufficiently fine scale. Our studies attempted to overcome some of the limitations mentioned previously by employing a much higher cooling rate process, chill-block
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