Effects of Solution Treatment and Cold Rolling on Microstructure and Mechanical Properties of Stainless Steel for Surgical Implants

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1 Materials Transactions, Vol. 49, No. 6 (8) pp to 1427 #8 The Japan Institute of Metals Effects of Solution Treatment and Cold Rolling on Microstructure and Mechanical Properties of Stainless Steel for Surgical Implants Yoshimitsu Okazaki Institute of Mechanical Systems Engineering, National Institute of Advanced Industrial Science and Technology, Tsukuba , Japan To obtain basic data for developing manufacturing processes for stainless steels for implant application, we determined how solution treatment and cold rolling affect the microstructure and mechanical properties of stainless steels. The grain size of the austenitic () phase in solution-treated ISO 5832 stainless steel tended to increase with solution-treatment temperature. The effects of solution-treatment temperature on the.2% proof strength (.2%PS ) and ultimate tensile strength ( UTS ) of this stainless steel were almost negligible, while total elongation (T. E.) tended to increase slightly with solution-treatment temperature. The mechanical properties of ISO 5832 stainless steel solution-treated at 175 C for 3 min were as follows:.2%ps, 33 3 MPa; UTS, 67 2 MPa; T. E., 65 2%; and reduction of area (R. A.), 71 2%. In the TEM image of % cold-rolled ISO 5832 stainless steel, many dense dislocations, which were produced by cold rolling, were observed. The.2%Ps and UTS of the cold-rolled stainless steel increased as the draft increased, whereas the T. E. decreased linearly with an increase in the draft. The.2%PS, UTS, T. E., and R. A. of % cold-rolled ISO 5832 stainless steel were MPa, 89 2 MPa, 22 2%, and 68 4%, respectively. In the microstructural observation of solution-treated high-n stainless steel, and Mn 2 SiO 4 were observed. The mechanical properties of this stainless steel were as follows:.2%ps, MPa; UTS, 83 4 MPa; T. E., 37 2%; and R. A., 48 7%. [doi:1.23/matertrans.mra7299] (Received November 29, 7; Accepted February 27, 8; Published May 25, 8) Keywords: microstructure, mechanical property, stainless steel, solution treatment, cold rolling 1. Introduction Stainless steel is increasingly being used as metallic material for orthopedic and vascular implants, e.g., bone plates, compression hip screws, intramedullary fixations, orthopedic wires, bone screws, artificial femoral heads, artificial hip stems, and coronary stents. 1 14) It has also received much attention as material for stents and stent grafts owing to its very good elasticity, high mechanical strength, excellent ductility and good cold workability. It has recently been reported that fretting corrosion is caused by the micromotion between bone cement and cemented artificial hip stems made of Ti alloy which has much lower elasticity than the high-strength stainless steel. 15) Therefore, cemented artificial hip stems made of stainless steel tend to replace those made of Ti alloy. The chemical requirements and mechanical properties of wrought stainless steel is specified in ISO , and those of wrought high-nitrogen (high-n) stainless steel for medical devices and surgical implants is specified in ISO In this study, to obtain basic data for developing such manufacturing processes, we determined the effects of solution treatment and cold rolling on microstructure and mechanical properties of stainless steel. The microstructure was analyzed by optical microscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD) analysis. Tensile test was also carried out at room temperature to examine the effects of above treatments on mechanical properties of stainless steel. 2. Materials and Methods 2.1 Alloy specimens and solution treatment Two kinds of stainless steel were prepared by vacuuminduction melting and mechanical alloying: ISO 5832 stainless steel specified in ISO and high-n stainless steel specified in ISO for surgical implants. An ISO 5832 stainless steel ingot (5 kg) was soaked at 1 C for 5 h and forged into billets. The billets were maintained at 1 C for 1 h and then hot-rolled. Each of the hot-rolled billet was solution-treated at 1 C, 15 C, and 11 C for 3 min and then quenched in water (solution-treated). A part of these solution-treated billets were cold-rolled to prepare 1%, 15% and %-reduced ISO The 1%, 15% and % indicate the change in cross section. High-N stainless steel specified in ISO was prepared in powder form by mechanical alloying in nitrogen gas atmosphere. The powder (3 kg) was hot-pressed at 15 C into rods. The rod specimens were solution-treated at 115 C for 1 h, and then quenched in water (solution-treated high-n). Through these processes, three types of stainless steel, namely, solution-treated ISO 5832, % cold-rolled ISO 5832, and solution-treated high-n stainless steels, were prepared for microstructural observation and mechanical test. The chemical compositions of the stainless steels are shown in Table Microscopic observation and XRD analysis The microstructures of solution-treated and cold-rolled ISO 5832 stainless steels, and solution-treated high-n stainless steel were analyzed by optical microscopy, TEM, and XRD analysis. Specimens for microscopic observation were cut from the sample alloys, covered with epoxy resin, and then polished with waterproof emery paper (1 to 24 grit) under running water. The sample surfaces were finished by buff cleaning using a high-quality SiO 2 (OP-S) suspension and etched before the observation. Then the microstructures were observed with Nikon Upright Microscope. TEM was performed using disc specimens of 3 mm diameter. The disc specimens were prepared by electrolytic polishing with 95%

2 1424 Y. Okazaki Table 1 Chemical compositions (mass%) of stainless steel specimens. Mo Ni Cu C N Mn Si P S Nb O Fe ISO <: Bal. High-N Bal. 1 C 15 C 5 µ m 5 µm c) 175 C d) 11 C 5 µ m 5 µ m Fig. 1 Effect of solution treatment temperature on microstructure of stainless steel. methanol + 5% perchloric acid solution. The microstructures were analyzed using a TEM system (Hitachi, Ltd. H- 8, at kv) equipped with an EDX system (Horiba, Ltd. EMAX-2). Precipitates were analyzed by electron beam diffraction and EDX analysis. The sample surfaces for XRD analysis were finished by buff cleaning. RINT15, manufactured by Rigaku Corp., was used for the measurement. The XRD patterns were obtained using an X-ray source of CuK at a tube voltage of 4 kv, a tube current of.2 A, and a scanning rate (2) of 1 /min. The lattice constant of precipitates was accurately measured. Silicon powder (NIST, 64C), for use in XRD analysis, was thinly spread on the buff-cleaned sample surface as flat as possible, according to the testing manual. Scanning rate (2) and sampling step were set to 1 /min and.1, respectively. To ensure the accurate calculation of lattice constant, the tested peak values were corrected by comparison with the standard Si peak values. 2.3 Tensile test To investigate the effects of solution treatment and cold rolling on mechanical properties of stainless steel, tensile test was also carried out at room temperature. The test was performed in accordance with the ISO 6892 standard. Each specimen of 1 mm diameter and 55 mm gauge length was pulled at a crosshead speed of 1 mm/min until a 1% strain was achieved. The crosshead speed was then changed to 5 mm/min until the specimen fractured. The means of.2% proof strength (.2%PS ), ultimate tensile strength ( UTS ), total elongation (T. E.), and reduction of area (R. A.), and standard deviations were calculated using three tested specimens. 3. Experimental Results and Discussion 3.1 Microstructures of solution-treated ISO 5832 stainless steel specimens Figures 1(, (, and (c) show optical micrographs of the ISO 5832 stainless steel specimens solution-treated at 1 C, 15 C, 175 C, and 11 C, respectively, for 3 min, and then quenched in water. The microstructures consisted of austenitic () phase. The grain size of the phase tended to increase slightly with solution-treatment temperature. Figures 2( and ( show TEM images of the stainless steel solution-treated at 175 C for 3 min. Many dislocations were observed. From the diffraction pattern shown in Fig. 2(c), the matrix was found to exhibit only the fcc () phase. The lattice parameter of the (fcc) phase was a ¼ :365 :1, which was obtained by XRD analysis. 3.2 Mechanical properties of solution-treated and coldrolled ISO 5832 stainless steel specimens Figure 3 shows the effects of solution-treatment temperature on the.2%ps, UTS, and T. E. of ISO 5832 stainless steel specimens that were solution-treated at 1 C, 15 C, 175 C, and 11 C for 3 min, and then quenched in water. The effects of solution-treatment temperature on the.2%ps and UTS were almost negligible; however, the T. E. tended to increase slightly with solution-treatment temper-

3 Effects of Solution Treatment and Cold Rolling on Microstructure and Mechanical Properties of Stainless Steel for Surgical Implants 1425 Table 2 Mechanical properties of stainless steels solution-treated at 75 C for 3 min and % cold-rolled..2%ps /MPa UTS /MPa T.E. (%) R.A. (%) ISO 5832 Solution-Treated % Cold-Rolled High-N Solution-Treated c) 1 µ m 25 nm σ.2%ps /MPa, σ UTS /MPa 1 σ UTS σ.2%ps Fig. 2 TEM images of stainless steel solution-treated at 175 C for 3 min. 6 σ.2%ps /MPa, σ UTS /MPa 8 σ UTS 6 4 σ.2%ps T. E. (%) Draft (%) Fig. 4 Effects of cold rolling on.2% proof strength (.2%PS ), ultimate tensile strength ( UTS ) and total elongation (T. E.). T. E. (%) T.E Solution Treatment Temperature, T / C Fig. 3 Effects of solution treatment temperature on.2% proof strength (.2%PS ), ultimate tensile strength ( UTS ), and total elongation (T. E.). ature. Table 2 shows the mechanical properties of ISO 5832 stainless steel solution-treated at 175 C for 3 min:.2%ps, UTS, T. E., and R. A. were 33 3 MPa, 67 2 MPa, 65 2%, and 71 2%, respectively. Figure 4 shows the effects of cold rolling on the mechanical properties of ISO 5832 stainless steel..2%ps and UTS increased linearly with the draft, whereas T. E. decreased linearly with an increase in draft. At % draft,.2%ps and UTS increased from 3 MPa to 75 MPa and from 68 MPa to 9 MPa, respectively, at T. E % or higher. The mechanical properties of % cold-rolled ISO 5832 stainless steel are shown in Table 2:.2%PS, UTS, T. E., and R. A. were MPa, 89 2 MPa, 22 2%, and 68 4%, respectively. These measures met the ISO standard of cold-worked stainless steel (.2%PS 69 MPa, UTS 86 MPa, T. E. 12%). Figure 5 shows a TEM image of % cold-rolled ISO 5832 stainless steel. Many dislocations, which were produced by cold rolling, were observed. As shown in the diffraction pattern in Fig. 5(, the matrix indicated only the (fcc) phase. 3.3 Microstructure of solution-treated high-n stainless steel specimens Figure 6 shows the optical micrographs of high-n stainless steel solution-treated at 115 C for 1 h followed by quenching in water. Precipitates were observed at grain boundaries

4 1426 Y. Okazaki Intensity (CPS) θ (deg) Fig. 7 X-ray diffraction pattern of high-n stainless steel solution-treated at 115 C for 1 h. 25 nm Fig. 5 TEM image of % cold-rolled stainless steel after solution treatment. 2 µ m 1µ m 5 µ m Fig. 8 TEM image of high-n stainless steel solution-treated at 115 C for 1 h. Fig. 6 Optical micrographs of high-n stainless steel solution-treated at 115 C for 1 h. and in phase grains. To identify these precipitates, XRD analysis was conducted. The XRD pattern is shown in Fig. 7. Along with peaks of the phase, small peaks of the ferrite phase (bcc) and many peaks of the phase were observed. The lattice parameters of the (bcc) phase was a ¼ :2877 :5 nm. The lattice parameters of the were a ¼ :4286 :1 and c ¼ :7382 :2 nm; those of Mn 2 SiO 4 were a ¼ :6243 :1, b ¼ 1:629 :1, and c ¼ :4897 :1 nm. Figure 8 shows a TEM image of solutiontreated high-n stainless steel. Precipitates were observed along grain boundary and in grain. A TEM image of the inclusions is shown in Fig. 9(. A plate precipitate and a spherical precipitate are referred to as 1 and 2 in Fig. 9. According to the diffraction patterns by EDX analysis, the plate precipitate was found to be with high peaks of and Nb (Fig. 9À), while the spherical precipitate was found to be Mn 2 SiO 4 with high peaks of Mn, Si and O (Fig. 9`). Figure 1 shows the XRD pattern of the extracted residue of high-n stainless steel. Only the peaks of Counts O Si C Mn 25 nm Counts kev Mn Nb C Nb Mn Fe Ni V Fe Fe Ni kev Fig. 9 TEM of inclusion precipitated in high-n stainless steel solutiontreated at 115 C for 1 h. À Diffraction and EDX patterns of plate inclusion (at 1); ` diffraction and EDX patterns of spherical inclusion (at 2).

5 Effects of Solution Treatment and Cold Rolling on Microstructure and Mechanical Properties of Stainless Steel for Surgical Implants 1427 Intensity (CPS) Fig. 1 steel. Ni Equivalent and Mn 2 SiO 4 were observed in the analysis. In Fig. 11, Ni and equivalents of high-n stainless steel are plotted by the square ( ) in the Schaeffler diagram. The square is located in the area where ferrite phase exists, which accounts for the result shown in Fig Mechanical properties of solution-treated high-n stainless steel specimens The mechanical properties of solution-treated high-n stainless steel are shown in Table 2:.2%PS, UTS, T. E., and R. A. were MPa, 83 4 MPa, 37 2%, and 48 7%, respectively. These values met the cold-worked high-n stainless steel (.2%PS 5, UTS 75, T.E. 16%) requirements specified in the ISO standard. 4. Conclusions 2θ (deg) To obtain basic data for developing manufacturing processes for stainless steels for implant application, we examined how solution treatment and cold rolling affect the microstructure of ISO 5832 and high-n stainless steels. Microstructural observations were carried out by optical microscopy, TEM, and XRD analysis. Tensile test, which was for examining these effects on mechanical properties, was also conducted at room temperature. The microstructure of solution-treated ISO 5832 stainless steel consisted of X-ray diffraction pattern of extracted residue of high-n stainless F+M A Austenite F Ferrite M Martensite 4 8 A+M Martensite 12 Austenite A+M+F M+F Equivalent 28 1% % Ferrite Ferrite 32 4% 8% 1% Fig. 11 Ni and equivalents of high-n stainless steel shown in Schaeffler diagram. 5% 36 4 austenitic () phase. Many dislocations were observed in grains and along grain boundaries. The grain size of the austenitic phase tended to increase slightly with solutiontreatment temperature. The effects of solution-treatment temperature on the.2%ps and UTS of the solution-treated ISO 5832 were almost negligible; however, T. E. tended to increase slightly with solution-treatment temperature. The mechanical properties of ISO 5832 solution-treated at 175 C for 3 min were as follows:.2%ps, 33 3 MPa; UTS, 67 2 MPa; T. E., 65 2%; and R. A., 71 2%. In the TEM image of % cold-rolled ISO 5832 stainless steel, many dislocations, which were produced by cold rolling, were observed. The.2%Ps and UTS of this stainless steel increased with an increase in the draft, whereas the T. E. decreased linearly as the draft increased..2%ps, UTS, T. E., and R. A. were MPa, 89 2 MPa, 22 2%, and 68 4%, respectively. In the microstructural observations of solution-treated high-n stainless steel, and Mn 2 SiO 4 were observed. The mechanical properties of this stainless steel were as follows:.2%ps, MPa; UTS, 83 4 MPa; T. E., 37 2%; and R. A., 48 7%. REFERENCES 1) D. Kuroda, T. Hanawa, T. Hibaru, S. Kuroda and M. Kobayashi: Mater. Trans. 45 (4) ) K. T. Oh, C. J. Hwang and K. N. Kim: Mater. Trans. 43 (2) ) D. Kuroda, T. Hanawa, T. Hibaru, S. Kuroda, M. Kobayashi and T. Kobayashi: Mater. Trans. 44 (3) ) D. Kuroda, T. Hanawa, T. Hibaru, S. Kuroda and M. Kobayashi: Mater. Trans. 44 (3) ) D. Kuroda, T. Hanawa, T. Hibaru, S. Kuroda and M. Kobayashi: Mater. Trans. 44 (3) ) L. Eschbach, G. Bigolin, W. Hirsiger and B. Gasser: Stainless Steels for Medical and Surgical Applications, ASTM STP 1438, eds. G. L. Winters and M. J. Nutt (ASTM, 3) pp ) N. Perot, J. Y. Moraux, J. P. Dichtel and B. Boucher: Stainless Steels for Medical and Surgical Applications, ASTM STP 1438, eds. G. L. Winters and M. J. Nutt (ASTM, 3) pp ) M. Windler and R. Steger: Stainless Steels for Medical and Surgical Applications, ASTM STP 1438, eds. G. L. Winters and M. J. Nutt (ASTM, 3) pp ) J. A. Disegi and L. D. Zardiackas: Stainless Steels for Medical and Surgical Applications, ASTM STP 1438, eds. G. L. Winters and M. J. Nutt (ASTM, 3) pp ) M. Windler, R. Steger and G. L. Winters: Stainless Steels for Medical and Surgical Applications, ASTM STP 1438, eds. G. L. Winters and M. J. Nutt (ASTM, 3) pp ) I. Tikhovski, H. Brauer, M. Mölders, M. Wiemann, D. Bingmann and A. Fischer: Stainless Steels for Medical and Surgical Applications, ASTM STP 1438, eds. G. L. Winters and M. J. Nutt (ASTM, 3) pp ) M. Zhu and M. Windler: Stainless Steels for Medical and Surgical Applications, ASTM STP 1438, eds. G. L. Winters and M. J. Nutt (ASTM, 3) pp ) B. Berli, R. Elke and E. W. Morscher: Stainless Steels for Medical and Surgical Applications, ASTM STP 1438, eds. G. L. Winters and M. J. Nutt (ASTM, 3) pp ) J. Z. Dennis, C. H. aig, H. R. Jr. Radisch, E. J. Jr. Pannek, P. C. Turner, A. G. Hicks, M. Jenusaitis, N. A. Gokcen, C. M. Friend and M. R. Edwards: Stainless Steels for Medical and Surgical Applications, ASTM STP 1438, eds. G. L. Winters and M. J. Nutt (ASTM, 3) pp ) S. R. Thomas, D. Shukla and P. D. Latham: J. Bone Joint Surg. 86B (4)