Effect of composition and high energy rate forging on the onset of precipitation in an iron-base superalloy
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INTRODUCTION
A-286 is an Fe-base superalloy widely used for intermediate elevated temperature service. It is nominally a 25 wt pet Ni-15 wt pct Cr austenitic alloy containing Ti and A1 additions and other minor alloying additives. It is strengthened by an aging treatment which precipitates the ordered fcc Y' phase, Ni3 (Ti, A1), but variations in the minor alloying additives can significantly influence the precipitation sequence. Much work has been done on the precipitation sequence in commercial A-286 and a host of similar experimental compositions. ~-9 From these studies, it is known that a variety of minor secondary phases can form during aging, depending upon specific composition and aging time and temperature. These include the topological close packed (TCP) phases such as ~r, /~, X, and Laves; the 77-(Ni3Ti), G-(Nil3TisSi6), and Y-(titanium carbosulfide) phases; and the interstitial carbides Cr23C6 and TiC. Good reviews on second phase formation in Fe-base superalloys including A-286 are listed in References 10 to 12. The specific compositional modification investigated in this study was developed to reduce the susceptibility of A-286 to heat-affected-zone cracking during welding ~3'~4by removing Si and Mn and lowering the C and B contents while increasing the Ni content. The alloy was subsequently found to have better tensile ductility in hydrogen than other austenitic steels, 9 and it was granted a patent as a hydrogen compatible alloy, ts The need for the work described herein arose from early observations of variation in both the precipitation sequence and kinetics resulting from (i) heat-to-heat variations in composition, (ii) modifications to the standard A-286 heat treatment--993 K(720 ~ for 16 hours, and (iii) varying parameters in forging studies. Hence, one objective of this study was to establish the precipitation sequence over the entire aging temperature range up to the solution treatment temperature in a single heat of the alloy. It is known that prior deformation can alter the precipitation sequence of heat treatable alloys. Limited observations have been made concerning effects of prior cold T. J. HEADLEY and M. M. KARNOWSKY, Members of Technical Staff, and W. R. SORENSON, Engineering and Science Assistant, are with Sandia National Laboratories, Albuquerque, NM 87185. Manuscript submitted June 3, 1981. METALLURGICAL TRANSACTIONS A
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work on subsequent aging reactions in Fe-base superalloys 6'n'12 but less is known of the effect of prior hot deformation, in particular, thermomechanical treatments (TMT). One TMT with increasing application for precipitation hardening stainless steels is High Energy Rate Forging or HERFing. ~6 This treatment retains a dislocation substructure after forging and increases strength during aging by combining strain hardening and precipitation hardening effects. Other investigators ~7 have shown that strength increases of up to 50 pct can be achieved by HERFing and aging the modified alloy studied here. Since the effect of HERFing on precipitation sequence was
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