A new approach for interpretation of strengthening mechanism of martensitic steel through characterization of local deformation behavior

Mechanical characterization in a sub-micron scale by nanoindentation technique is applied to Fe-C base martensitic steels to discuss the strengthening mechanisms. Matrix strengths of block structures were probed to reveal a dependence on a carbon content, tempering temperature and metallurgical process. A grain boundary effect was also evaluated to consider the strengthening factors on the macroscopic strength. It has been found that the grain boundary effect is significantly large on the strength of the Fe-C based martensite due to a fine effective grain of block structure. A locking parameter in the Hall-Petch relation is reduced at tempering temperature above 450 degrees C leading to a "hump" on the temper-softening curve because film-like carbides on grain boundaries are disappeared above 450 degrees C. A microstructure in a conventional quench-tempered process includes high-density carbides on a grain boundary corresponding to a higher locking parameter than that of an ausform-tempered martensite. The lower locking parameter in the ausform-tempered martensite reduces an stress concentration at an interface between the carbide and ferrite phase, which contribute to the improved delayed fracture property of the ausform-tempered martensite.