Use of aqueous polymer quenchants for hardening of carbonitrided parts

Water, 10% aqueous polymer solutions, and conventional quench oil were evaluated to harden carbonitrided parts. The resulting as-quenched and quenched and tempered microstructures were compared. Typically, the gas carbonitriding process produced (carbonitrides) – martensite-retained austenite in the case and martensite-retained austenite or bainite-retained austenite in the core. The effect of the amount of retained austenite and carbides (carbonitrides) on hardness, abrasive wear resistance, and impact strength showed that the best results were obtained when a 10% aqueous solution of a polyalkylene-glycol quenchant was used. However, most surprisingly, the impact strength for the aqueous polymer quenched specimens was dramatically greater than either oil or water quenched specimens. A rationale for this important observation is presented, which suggests that thermal stresses may be more important than transformational stresses as it relates to these quenching conditions and the test pieces quenched.