D. F. Susan, P. S. Duran, D. C. Robino, Sandia National Laboratories, Albuquerque, NM; J. R. Paules, Ellwood Materials Technologies, Ellwood City, PA
Large ingots of a modified HP9-4-20 steel (HP9-4-20M) were produced by air-melting, due to the unavailability of vacuum arc remelted (VAR) ingots in the large sizes required. Recent ingot casting and open-die forging experience have shown that very large diameter ingots of this alloy can suffer from cracking during forging or subsequent cooldown operations. The intergranular (IG) cracking found in the ingots and forgings was believed to be due to the very slow post solidification cooling rates and concomitant accumulation of internal residual stresses. An experimental study was undertaken to determine the conditions under which IG cracking would occur and to identify the microstructural mechanisms contributing to cracking. A Gleeble® thermomechanical simulator was used to austenitize, slowly cool, and mechanically test both air-melted and VAR HP9-4-20. Isothermal tensile tests of austenite were performed at temperatures ranging from 700°F(371°C) to 2250°F (1232°C) and “as-quenched” martensite was tested at 350°F(177°C) to 550°F(288°C). Strain rates of ~10-5 sec-1 were employed in order to simulate the slow strain rates expected from internal thermal stresses in a large ingot. For air-melted material, reduced ductility (“ductility-dip”) and IG fracture were observed in large-grain-size austenite in the 1200°F(649°C) to 1600°F(871°C) range, with the ductility decreasing as the strain rate was decreased. The results can be analyzed in terms of a possible “creep embrittlement” mechanism. Preliminary TEM characterization of grain boundary precipitates is reported and comparisons are made between VAR and airmelt material. ____________
*Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the US Dept. of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
Summary: A Gleeble® thermomechanical simulator was used to austenitize, slowly cool, and mechanically test both air-melted and VAR HP9-4-20. Strain rates of ~10-5 sec-1 were employed in order to simulate the slow strain rates expected from internal thermal stresses in a large ingot. For air-melted material, reduced ductility (“ductility-dip”) and IG fracture were observed in large-grain-size austenite in the 1200˚ F(649˚C) to 1600˚F(871˚C) range, with the ductility decreasing as the strain rate was decreased.
