STRUCTURAL EVOLUTION DURING THERMAL TREATMENTS AND THE RESULTANT MECHANICAL BEHAVIOR OF HIGH STRENGTH LOW ALLOY STEELS
Draper, Matthew Charles
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HY steels were designed as a solid solution strengthened grade for both high yield strength and high impact toughness in structural applications for Naval vessels. These alloys are susceptible to both hydrogen and temper embrittlement which necessitates high expense manufacturing processes to preclude these conditions. Successful implementation of lower cost and higher reliability treatments requires an improved understanding of the structural evolution and corresponding changes in mechanical behavior for the alloy. This research combines mechanical and microstructural characterization methods along with thermodynamic and kinetic models to build a comprehensive understanding of the effects of thermal treatments on the structure-property relationship of the alloy system. The embrittlement rate was studied between 315°C and 565°C at varied logarithmic time intervals up to 40,000 minutes. The embrittlement recovery rate was studied between 593°C and 704°C at logarithmic time intervals up to 10,000 minutes. Finally, hydrogen aging was studied between 315°C and 565°C at varied thermodynamically equivalent time intervals. A variety of test methods were employed for characterization including: traditional metallographic techniques, mechanical testing, computational modeling, and a novel image analysis technique for carbide analysis. Metallographic along with computational work supports a conclusion that temper embrittlement and subsequent recovery cannot be solely explained by the segregation of phosphorus and other embrittling elements to grain boundaries. Rather it is shown for the first time that alloy carbides play a key role in embrittlement for this system. The evolution of these carbides serves both to create initiation sites for cleavage fracture and deplete the matrix of Mo, which is a P scavenger. Recovery from embrittlement is thus proposed to be related to both the removal of P from the boundary and the dissolution of carbides. From these results a series of kinetic models have been developed for the nucleation, dissolution, and coarsening of alloy carbides. Models developed for the mitigation of monatomic hydrogen show a novel treatment for hydrogen aging via performing the aging within the embrittlement range with follow on treatments designed to recover from embrittlement. This new treatment has the potential to reduce hydrogen aging times by up to 90% in industrial manufacture.