A study of the microstructural basis for the strength and toughness properties of as-quenched and quenched and tempered high copper HSLA-100 steel

Abstract

A program for the use of high copper HSLA-100 steel as a replacement for HY-130 in ship construction is in progress at the David Taylor Naval Research Center in Annapolis, Maryland. The characterization of the microstructure of this highly weldable high copper HSLA-100 steel is extremely important to this program. In the present work, optical, scanning electron and transmission electron microscopy were employed to analyze and characterize the microstructural basis for the mechanical properties off as-quenched and quenched and tempered high copper HSLA-100 steel in the form of 19.05 mm plate. The as-quenched and samples aged at 538 CC and 621°C were examined in great detail. In the as-quenched condition the microstructure consisted of small packets of highly dislocated lath martensite containing autotempered Fe 3C carbides with small amounts of retained austenite present as thin films (200-300 A in width) at lath boundaries. The basis for the high strength and toughness of this as-quenched sample is the small transformation product packet size and the highly dislocated structure. The best combination of strength and toughness occurred in the sample aged at 538°C. Contributing to these desirable characteristics was the recovery of the dislocation substructures, the precipitation of coherent BCC copper precipitates (12-20 A in size) further precipitation (FeM) 3 C carbides and the small transformation product packet size. In the overaged (621°C) condition, the toughness was increased and the strength decreased due to the absence of coherent BCC copper precipitates and the presence of FCC Ecopper incoherent precipitates. Further recovery of the dislocation substructures was also evident. All of the samples (lot GLB) studied met the MILSPEC for HY-130 and would be suitable replacements. As a result of this investigation it is suggested that the optimum aging temperature for the 19.05 mm as-quenched high copper HSLA-100 plate is around 550°C.