A STUDY OF THE EFFECTS OF VARYING SOLUTION TREATMENT ON THE TOUGHNESS OF A STAINLESS MARAGING ALLO

The effects of varying solution treatment on the mechanical properties and microstructure of Custom 455*--a martensitic precipitation hardening stainless steel--were evaluated. This steel is normally solution treated at 1500 F followed by aging in the range of 850 F to 1050 F. Severe embrittlement is experienced if the alloy is solution treated at a higher temperature (1700 F). This work shows that use of a 1250 F + 1500 F solution treatment as compared to the standard 1500 F treatment results in superior toughness and notch ductility if the aging temperature is 850 F or 900 F.;Austenite reversion measurements show that austenite at the lath boundaries is required for toughness at the lower aging tem- peratures. The presence of (TURN)3% reverted austenite is required for notch ductility. Aging at higher temperatures overages the structure and causes austenite reversion to occur following a 1500 F solution treatment.;The causes for the change in austenite reversion behavior are related to two factors (a) the amount of coarse precipitate, identified as Fe(,2)Ti, that is present in the as solution treated condition and (b) the lath size of the structure. The amount of Fe(,2)Ti present in the as solution treated structure affects the amount of aging phase precipitated. The aging precipitate was shown to be coherent ordered h.c.p. Ni(,3)Ti with lattice parameters a = 4.05 (ANGSTROM) and c = 2.48 (ANGSTROM) (c/a = 0.61). The h.c.p. precipitate/b.c.c. matrix orientation relationship is (0001)(,h.c.p.) // (111)(,b.c.c.) and 2110 (,h.c.p.) // 110 (,b.c.c.).;The aging precipitate maintains its coherency even after aging at 1050 F. The aging precipitate has the morphology of rods and precipitates in rows along the directions of the b.c.c. matrix and thus severely inhibits slip. This precipitate makes the alloy prone to lath boundary failure. Consequently, the ductility of the alloy can be improved by the presence of austenite at the lath boundaries.;This work shows that solution treatment at 1700 F results in a structure essentially devoid of Fe(,2)Ti. The absence of Fe(,2)Ti, the presence of excessive Ni(,3)Ti and the large lath size of this structure all contribute to reduced toughness and notch ductility by reducing austenite reversion. However, precipitation of the Fe(,2)Ti in such a structure by use of a 1250 F + 1500 F solution treatment restores the loss in toughness related to the presence or absence of precipitates.;The reduction in austenite reversion related to lath size is not restored. (Abstract shortened with permission of author.).;*Registered Trademark Carpenter Technology Corporation.