Inverse hardness distribution and its influence on mechanical properties

Studying the pattern of hardness distribution on round bars cross-section after quenching, Shimizu and Tamura have introduced the expression of "inverse hardening". Opposite to normal hardness distribution it shows lower hardness at the surface and higher hardness in the core. This phenomenon takes place dependent on: hardenability of the steel, cross-section size of the workpiece and on quenching condition. It is related to the delayed quenching with discontinuous change of cooling rate, and to the incubation period consumed before changing the cooling rate. Own experiments using cylindrical specimens of 50 mm Dia, made of the same heat of AISI-4140 stee1 have shown that polymer solutions (PAG) of high concentration may reproducible cause delayed quenching and yield inverse hardness distribution. This Controllable Delayed Quenching (CDQ) technology, influencing intentionally the dynamics of heat extraction, has a great potential to increase the depth of hardening, compared to conventi onal quenching practice. After tempering a workpiece having inverse hardness distribution to suitable tempering temperature, uniform microstructure of tempered martensite, giving the best impact toughness, can be obtained through the whole cross-section. Bending fatigue tests with adequate specimens have shown a significant increase of the fatigue life for tempered specimens with inverse hardness distribution after quenching, in comparison with specimens having normal hardness distribution.