Microstructural stability of titanium-modified type 316 and type 321 stainless steel
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The present communication reports studies of the microstructural stability of thermal-mechanically pretreated samples of both a titanium-modified type 316 (316 TiM) and a type 321 steel using transmission electron and optical microscopy and X-ray diffraction of bulk extracted residues. The results are discussed and compared to related results in the literature. EXPERIMENTAL The compositions of the two steels investigated are given in Table I. Both steels were vacuum melted heats produced by Carpenter Technology Corp., San Diego, California. The 316 TiM material was received as one half in. diam rods which were further processed in such a way as to produce 0.125 in. diam rods which contained 0, 10, 20, or 40 pct reduction in area by swaging. Except for a final grain size control anneal at 1093°C, solution and intermediate annealing treatments during processing were carried out at 1204°C. The type 321 material was received as 0.25 in. outside diam by 0.014 in. wall thickness tubing. This tubing contained either 0 or 20 pct reduction in area. Except for the difference between swaging and tube drawing the processing of this material was similar to that of the 316 TiM samples. Aging treatments were performed either under an argon atmosphere or in evacuated Inconel alloy 600* capsules *Registered trademark of the International Nickel Co., Inc.
for times up to 4000 h at temperatures in the range 450°C to 950°C. Bulk extraction of the precipitates for X-ray diffraction analysis was carried out electrolytically in a 10 pct HCl in methanol electrolyte with a current density of 175 to 225 ma/cm 2 . The weight fraction of precipitate was determined by weighing the bulk samples before and after partial dissolution and also weighing the extracted and dried residue. The X-ray patterns were made using a graphite crystal diffracted beam VOLUME 6A, NOVEMBER 1975-2023
Table I. Chemical Analysis of Steels Composition, Wt Pct Type 316 TiM 321
0.057 0.06
Cr
Ni
Mo
Ti
Mn
Si
17.52 17.48
13.96 9.58
2.51 -
0.29 0.50
1.41 1.69
0.03 0.54
monochromator with the extracted powder mounted on a single crystal of silicon in order to improve peak to background ratios." RESULTS As -Pretreated Materials Bulk extractions carried out on the as-pretreated samples revealed that the extractable particle content of the type 321 material was about 0.4 wt pct while that of the 316 TiM was only 0.04 wt pct. In both steels X-ray diffraction revealed that these particles were mainly Ti 4 C2S2 with some TiC and TiN. In the 321 material the lattice parameters of the TiC and TiN were very near the values reported in the literature 1z for these phases [a o (TiC) = 4.33A, a o (TiN) = 4.24Á]. The lattice parameters for these phases in the 316 TiM alloy were ao (TiC) = 4.32A, ao (TiN) = 4.25A indicating an apparent tendency for some solution of nitrogen in the carbide to form Ti (C, N) and some solution of carbon in the nitride to form Ti (N, C). Based on the weights of the extracted precipitate, the analyses of Table 1, and estimates of the fraction of p
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