Effect of phosphorus on stability of cold worked structure in 316Ti austenitic stainless steel

Abstract

The persistence of the effects of prior cold work through subsequent annealing treatment has been investigated by examining the changes in 0•2% yield strength and the microstructural evolution in phosphorus free and phosphorus containing type 316Ti stainless steels with various amounts of mechanical pretreatment. The results showed no specific influence of phosphorus on changes to a cold worked structure during short term treatment carried out in the range 800-1200°C where no second phase precipitation was observed. However, when second phase precipitation did occur the behaviour of the alloys examined was found to be slightly different. For long term annealing in the 600-800°C temperature range an abundant nucleation of fine carbides particles, particularly in the phosphorus containing steel, considerably improved the stability of the cold worked structure by decorating grain boundaries and by pinning the dislocations and, consequently, impeding their climb. This study has highlighted clearly the existence of an optimum annealing temperature for a given level of cold work and that there is an optimum amount of initial deformation for a given annealing temperature. The addition of phosphorus has been shown to result in an increase of strength in type 316Ti with perhaps an insignificant drop in ductility, although a full explanation of how the presence of phosphorus affects the recrystallisation process in a prestrained 316Ti type steel remains to be ascertained.MST/4314

A Calorimetric Study of Recovery and Recrystallization in Fe–14.4 Cr–15.4 Ni–2.4 Mo–2.36 Mn–0.25 Ti–0.04 C–0.05 P (mass %) Austenitic Stainless Steel

“…The average microhardness values are found to be 455 and 521 VHN, respectively for 70 and 85% coldwork conditions. These measured hardness values are in good agreement with the reported ones for coldworked 316 and 316‐Ti added austenitic steels . On the other hand, the 1323 K/15 min.…”

“…In some parts of microstructure, the presence of macro deformation bands (arrow #3) and numerous crisscrossing slip traces (arrow #4) are also noticed. In general the deformed microstructure appears to be inhomogeneous, as is already observed by others on severely deformed austenitic stainless steels …”

“…In general the deformed microstructure appears to be inhomogeneous, as is already observed by others on severely deformed austenitic stainless steels. [87][88][89][90][91] The average microhardness values are found to be 455 and 521 VHN, respectively for 70 and 85% coldwork conditions. These measured hardness values are in good agreement with the reported ones for coldworked 316 and 316-Ti added austenitic steels.…”

“…These measured hardness values are in good agreement with the reported ones for coldworked 316 and 316-Ti added austenitic steels. [26,90,91] On the other hand, the 1323 K/15 min. solution annealed microstructure possessed a hardness of about 145-150 VHN.…”

“…The occurrence of recrystallization is followed by two more thermal arrests of comparatively smaller magnitude which are centered around 1321 and 1374 K, respectively. It is probable that these smaller peaks are associated with secondary recrystallization phenomenon, which occurs following the dissolution of tiny TiC particles, which in this class of steels normally occurs in the temperature range of 1323-1373 K. [89,91] It may be added that the endothermic TiC dissolution event in itself is rather difficult to detect separately in a DSC experiment, as the volume fraction of the precipitate is rather small. However, the dissolution of TiC located on prior dislocations has an indirect effect, in that the grain boundaries whose mobility was earlier restricted due to the pinning effect of tiny TiC particles, have now been rendered free to undergo migration and contribute to secondary recrystallization.…”