The influence of substructure on the elevated and room temperature strength of a 26 Cr-1 Mo ferritic stainless steel
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I.
INTRODUCTION
SUBGRAINS dominate the substructural features observed in many crystalline solids after deformation at intermediate temperatures (0.35 to 0.65 T,,)) '2 The present investigation on a ferritic stainless steel attempts to achieve a better understanding of the effect of subgrains on strength. Toward this end, the strength and substructure of a 26 Cr-1 Mo ferritic stainless steel was studied after large torsional deformation at a variety of temperatures, strain rates, and strain levels. The subgrain size is perhaps the most studied characteristic of a substructure developed during warm working.'-4 Young and Sherby 3 have had good success in relating the steady state subgrain size to the modulus compensated flow stress for ferrous alloys. They have demonstrated that there is a notable lack of variation of subgrain size with temperature, stacking fault energy, mode of deformation, and alloy content. In fact, a simple relationship exists between subgrain size and flow stress as follows:
where or is the stress at a fixed plastic strain, oro is a "friction stress" that is a function of factors such as the solute content and stacking fault energy, K is a material constant that depends on the nature of the subgrain boundaries, and m is a material constant generally equal to unity. While subgrain size is of primary importance in determining the high and low temperature flow stress, other substructural characteristics may also have an effect. Examples of substructure characteristics of secondary importance include misorientation angle, subgrain shape, texturing, and dislocation density within subgrains. This investigation will consider some of these more subtle features that are developed during warm working and will discuss their influence on the flow stress during warm working and on the room temperature yield strength.
II.
E X P E R I M E N T A L PROCEDURE
The chemical composition of the electron beam melted ferritic stainless steel used in this investigation is as follows: h =A [1] 25.5 Cr, 1.17 Mo, 0.1 Ni, 0.22 Si, 0.001 C, 0.01 Mn, b 0.0076 N, 0.0038 O, 0.01 A1, 0.012 Cr, balance Fe. The material was received in the form of 16 mm (0.62 inch) hot In this equation, b is the Burgers vector and E is the dynamrolled plates. A large grain size was present in this material, ic average polycrystalline Young's modulus; A is a constant which sometimes produced fracture at relatively low strains equal to 4 • h and or are the steady state subgrain interduring torsion testing. In order to 'refine the grain size and cept size and flow stress, respectively. avoid this difficulty, the following procedure was used. AsThe low temperature flow stress has also been found to be received plates were vacuum annealed at 1000 ~ (1830 ~ a function of subgrain size. A relationship that has been for one hour, water quenched, rolled 50 pet at room temproposed both theoretically s and experimentally 3'6-m is perature by a series of 10 pct passes, and then recrystallized or = or0 + Kh-m; [2] by heating in vacuum to 890 ~ (1635 ~ for
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