Flow stress and microstructural evolution during hot working of alloy 22cr-13ni-5mn-0.3n austenitic stainless steel
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INTRODUCTION
FORGING at elevated temperature is the primary method for producing complex-shaped components which must satisfy stringent physical and mechanical property requirements. For a particular alloy, final properties are dependent on microstructure, e.g., grain size, second-phase characteristics, and retained dislocation substructure. Evolution of microstructure depends on the extent and relative proportions of static and dynamic recrystallization and recovery during thermal mechanical treatment (TMT), governed in part by deformation temperature (/3, strain (e), strain rate (~), and cooling rate after deformation and in part by material characteristics such as stacking fault energy (SFE) and the size, distribution, and coherency of secondary phases. In general, metals and alloys with high SFE (e.g., A1, Zr, Ti, a-Fe, and ferritic steels alloyed with Cr and Si) tend to dynamically recover due to cross-slip. Metals with relatively low SFE (e.g., Cu, CuA1, CuZn, Ag, Ni-Fe, Co, Ni superalloys, tool steels, and some austenitic stainless steels, e.g., 304L), in which cross-slip is limited, tend to dynamically recrystallize during deformationY ,21 If dynamic recrystallization is suppressed by increasing ~ or initial grain size (do), or by decreasing T, static recrystallization immediately following deformation is likely to be the main restoration mechanism. Static recrystallization has been identified as the controlling mechanism after hot rolling of alloys 304 and 316,t31 after high-energy-rate forging of JBK-75E41 (a modified A-286) and 304L,[51 during multiplestep compression tests (simulating radial forging) of alloy
M.C. MATAYA, Associate Scientist, is with Safe Sites of Colorado, Golden, CO 80402-0464. C.A. PERKINS, Staff Environmental Engineer, is with Schuller International, Denver, CO 80217-5108. S.W. THOMPSON, Associate Professor/ISS Professor, and D.K. MATLOCK, Director, are with the Advanced Steel Processing and Products Research Center, Colorado School of Mines, Golden, CO 80401. Manuscript submitted February 27, 1995. METALLURGICALAND MATERIALSTRANSACTIONS A
718, I6] and during multiple-pass rolling of Nb-strengthened Si-Mn steel plate. I7] High strength-to-weight ratios in austenitic stainless steels for aerospace applications are typically achieved by relatively low final hot working temperatures in order to retain the dense dislocation substructure. [s.9,1~ Water quenching after forging helps to retain the worked microstructure. Additional strengthening through grain size refinement is insignificant compared to dislocation substructure strengthening; for example, the yield strength in alloy 304L is increased 200 pct by applying a strain of only 0.5 at 700 ~ but is increased only 13 pct by reducing the grain diameter from 0.85 to 0.038 mm.[5] In many austenitic stainless steels, precipitation of complex chromium-bearing carbides on grain boundaries tlq or transformation of delta ferrite to sigma phase,I~2.~3] which severely degrades corrosion resistance and fracture toughness, respectively, o
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