The Effect of Hot Working on Structure and Strength of a Precipitation Strengthened Austenitic Stainless Steel
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INTRODUCTION
FORGING is used extensively to produce complex, high strength components from austenitic stainless steels such as 304L, 21-6-9, and JBK-75 (a modified A286). 1,2,3The need for high strength to weight ratios, for aerospace applications, results in stringent mechanical property and microstructural requirements. Most often the annealed strength levels of 304L and 21-6-9 and the solution treated plus aged strength of JBK-75 do not meet design requirements and, therefore, a significant strengthening contribution must be provided by thermo-mechanical treatment. Substructure and property development in any alloy depends uniquely on strain (e), strain rate (~), and temperature (T) history. Ultimately, the maximum strengthening that can be achieved during hot working is determined by the competition between work hardening and dynamic and static restoration processes. Thermo-mechanical history, in turn, is dependent upon processing variables (die and workpiece temperatures, die-workpiece friction, die and workpiece geometry, and die displacement rate) and material variables (strain rate sensitivity, strain hardening or softening behavior, temperature dependence of the flow stress, thermal conductivity, specific heat, and phase transformation kinetics). 4 Many of these variables may change during the forging operation leading to concomitant transients in e, k, and T. Although great strides have been made in developing an understanding of the production and utility of disM.C. MATAYA is a Senior Research Specialist with Rockwell International, Energy Systems Group, Golden, CO 80401. M.J. CARR, formerly with Rockwell International, Energy Systems Group, is a Member of Technical Staff at Sandia National Laboratories, Albuquerque, NM, 87185. G. KRAUSS is AMAX Foundation Professor with Colorado School of Mines, Golden, CO 80401. Manuscript submitted June 16, 1983. METALLURGICALTRANSACTIONS A
location substructures, 5 it is not surprising, considering the general complexity of thermo-mechanical history, that there is still a lack of general ability to predict the evolution of substructure as a function of e, k, and T for any given composition and starting structure. 6 Instead, process boundary conditions which guarantee a specific metallurgical state must be determined via systematic experiments which define the role of the process variables in the generation of substructure and properties. Such experimentation has been used to determine appropriate forging cycles for Ti alloys, 7-13 but little systematic work has been done to determine the effect of forging variables on substructure and property generation in austenitic steels, particularly in precipitation hardening alloys such as JBK-75. The development of macrostructure in JBK-75 has been shown to be extremely sensitive to variations in processing variables. 2'1~ In these studies, the substructure is very complex and highly dependent on deformation conditions. This suggests that the associated mechanical properties would also vary significantly, depending on for
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