Calorimetric Investigation of Thermal Stability of 304H Cu (Fe-17.7Cr-9.3Ni-2.95Cu-0.91Mn-0.58Nb-0.24Si-0.1C-0.12N-Wt Pct) Austenitic Stainless Steel

The sequence of phase instabilities that take place in a Fe-17.7Cr-9.3Ni-0.58Nb-2.95Cu-0.12N (wt pct) austenitic stainless steel (304H Cu grade) as a function of temperature has been investigated using dynamic calorimetry. The results obtained from this investigation are supplemented by Thermocalc-based equilibrium and Scheil-Gulliver nonequilibrium solidification simulation. The following phase transformation sequence is found upon slow cooling from liquid: L -> L + gamma -> L + gamma + MX -> gamma + MX + delta -> gamma + MX + M23C6 -> gamma + MX + M23C6 + Cu. Under slow cooling, the solidication follows austenite + ferrite (AF) mode, which is in accordance with Thermocalc prediction and Scheil-Gulliver simulation. However, higher cooling rates result in skeletal delta-ferrite formation, due to increased segregation tendency of Nb and Cr to segregate to interdendritic liquid. The solidification mode is found to depend on combined Nb + Cu content. Experimental estimates of enthalpy change associated with melting and secondary phase precipitation are also obtained. In addition a semi-quantitative study on the dissolution kinetics of M23C6 type carbides has also been investigated. The standard solution treatment at 1413 K (1140 degrees C) is found to be adequate to dissolve both Cu and M23C6 into gamma-austenite; but the complete dissolution of MX type carbonitrides occurs near the melting region. (C) ASM International (ASM) and The Minerals, Metals & Materials Society (TMS) 2016