The Effect of Nickel Content on the Mechanical Properties and Microstructure of a High Toughness Secondary Hardening Steel

Certain structural applications require materials to possess both high strength and high toughness, two properties which can often be inversely related with one increasing at the expense of the other. Currently, only two classes of steels exist that fulfill both of these application-critical criteria – maraging steels and some cobaltcontaining secondary hardening steels. Unfortunately, these alloys are expensive due to their high cobalt and nickel contents. It is thought that in these alloys nickel offsets the detrimental effects cobalt can have on the ductile-to-brittle transformation and that by removing cobalt, an inexpensive steel alternative with comparable mechanical properties to the currently available alloys could be developed. This research examines the effect of nickel additions to a base secondary hardening steel on mechanical properties and microstructure. The work was divided into separate studies in which five experimental alloys with differing nickel amounts were examined in order to determine the effect of nickel on the strength, ductile-to-brittle transition temperature (DBTT), and room temperature fracture toughness. In the first study, the effect of nickel on strength was investigated. Increasing nickel content generally increased strength, although the effect of nickel on yield strength and ultimate tensile strength was highly dependent on the quench rate from the austenitizing temperature. Results indicate that fine precipitates of VC and Mo2C contribute to the strength of these alloys. The second study looked at the effect of nickel on the DBTT. Test specimens were heat treated uniformly and then Charpy impact energies were determined for five test temperatures. The DBTT decreased approximately 200°C when nickel content was increased from 0 to 5 and 6 wt.%. Heat treatment did not have a significant effect on the DBTT. Results of testing for the nickel-free alloys suggest that rare-earth inclusions are slightly more effective in promoting quasi-cleavage than calcium or aluminum oxide inclusions. The DBTT was primarily controlled by nickel content, as the effects of prior austenite grain size and retained austenite content were negligible. In the third and last study, the effect of nickel on room temperature toughness was analyzed. Fracture toughness increased with increasing nickel content and tempering temperature. The experimental alloys with zero nickel additions had unexpectedly low toughness compared to previous work in heats with nearly identical composition. This was determined to be the result of the large inclusions in these steels which promote quasi-cleavage fracture in the absence of nickel. In addition, the difference in toughness between the two experimental base heats could be due to the low coefficient of thermal expansion of rare-earth inclusions, which could increase tensile circumferential stresses that promote quasi-cleavage. Examination of KIC fracture surfaces revealed no correlation between average area fraction of secondary voids and fracture toughness.