Thermomechanical Treatments Applied to Ultra-High Strength Steels

摘要

The response of 9% nickel and 4% cobalt steels to thermomechanical treatments was evaluated with particular emphasis on fracture toughness. Two carbon contents, 0.25 and 0.45%, were studied. The thermomechanical treatments involved austenite deformation processes and strain-tempering processes with either martensite or bainite being the transformation product. The properties of undeformed structures were also measured for comparison purposes. Thermal and strain-tempering treatments applied to 9-4-45 develop two bands of precracked Charpy impact energy versus yield strength, one for martensites and one for bainites. In the yield strength range 200,000 to 340,000 psi the bainites possess substantially higher impact strengths than the martensites at equivalent strength levels. Austenite deformation processes do not improve the impact strength of 9-4-45 for treatments involving 50% deformation. Strain-tempering of the 9-4-25 steel develops yield strengths from 200,000 to 300,000 psi. At these strength levels this steel has higher impact energies than the 0.45% carbon steel. The level of both K sub IC and yield strength in the 9-4-xx steels may be increased by thermomechanical treatments, such that combinations of K sub IC and yield strength are obtained that are equivalent or superior to those developed in the 18 nickel maraging alloy. (Author)