Microstructure Evolution During Hot Deformation of REX734 Austenitic Stainless Steel
- PDF / 6,512,646 Bytes
- 10 Pages / 593.972 x 792 pts Page_size
- 82 Downloads / 301 Views
INTRODUCTION
AUSTENITIC stainless steels are widely used to manufacture orthopedic implants.[1,2] In the early 1980s, REX734 nitrogen- and niobium-bearing stainless steel was introduced for the fabrication of femoral prosthesis for a total hip replacement; this alloy is still one of the most common materials used for the fabrication of femoral stems.[3,4] REX 734 stainless steel for femoral stems is marketed under several brand names by different manufacturers and is standardized by both ISO (5832-9) and ASTM (F1586) standards. The addition of nitrogen in REX734 stabilizes austenite, improves resistance to crevice and pitting corrosion, as well as strength during both monotonic and cyclic loading. Compared to another austenitic stainless steel grade 316L commonly used in implants, REX 734 provides superior corrosion resistance and mechanical properties.[1,2] Failures of femoral stems made of REX 734 stainless steel are quite rare.[3–6] In some of the documented cases, femoral stems failed after as little as 6 months.[3] The
MYKOLA KULAKOV, JIANGLIN HUANG, MICHAIL NTOVAS, and SHANMUKHA MOTURU are with the Advanced Forming Research Centre, University of Strathclyde, 85 Inchinnan Drive, Renfrew PA4 9LJ, United Kingdom. Contact e-mail: [email protected] Manuscript submitted March 1, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
root cause of these failures is typically associated with the loss of proximal support, improper design, sizing or positioning; once these precursors for failure are met, femoral stems fail through corrosion-initiated fatigue.[3,5,7] A larger grain size and lower hardness are also often noted on the surface compared to the stem interior; the heterogeneity of grain structure was suggested as one of the factors contributing to fatigue failures.[3,5] Coarse and brittle second phase particles act as preferential sites for the initiation of fatigue cracks during cyclic loading.[8] Fatigue cracks can also initiate at R3 annealing twin boundaries in austenitic stainless steels.[9,10] The above observations of microstructure deficiencies that potentially contribute to femoral stem failures suggest the need to carefully control microstructure evolution during REX 734 hot forming. Several studies reported various aspects of the hot deformation behavior for a REX 734 alloy. The material behavior after solution annealing at 1523 K (1250 C) for 600 seconds was studied through hot torsion testing at strain rates 0.01 to 10 s1 and temperatures 1173 K to 1473 K (900 C to 1200 C).[11,12] According to the processing map, deformation temperature should be above about 1323 (1050 C) to avoid flow instabilities; in this temperature range, microstructure evolution is dominated by dynamic recrystallization. Hot torsion simulations of a multipass deformation process after solutionizing heat treatment at 1523 K (1250 C) for 300 seconds during continuous cooling showed that the highest non-recrystallization temperature of about
1398 K (1125 C) was achieved with an interpass time of 30 seconds.[13] The changes i
Data Loading...
