Effect of heat treatment on the chromium-depleted zones of a high carbon martensitic stainless steel

Heat treatments are commonly applied in the martensitic stainless steels to achieve high wear resistance and hardness; however, they can lead to a decrease in corrosion resistance due to the precipitation of chromium-rich carbides. The present work investigates the effect of austenitizing and tempering parameters on the microstructure, hardness, and passivation and reactivation currents of AISI 440C. A double-loop electrochemical potentiodynamic reactivation test is performed to evaluate the dissolution and precipitation of chromium-rich carbides. The volume fraction of phases after heat treatment is estimated by X-ray diffraction. Results show that an increase in the austenitizing temperature leads to an increase in the carbide dissolution, and also in the amount of retained austenite. As a consequence, low passivation and reactivation currents are obtained. When austenite is the main phase, the increase of austenitizing time contributed to an increase in the degree of sensitization due to austenite grain boundary carbon enrichment. However, the presence of austenite after tempering kept part of the carbon in a solid solution resulting in low passivation and reactivation currents.

Automated summary

Increasing the austenitizing temperature results in higher carbide dissolution and retained austenite, leading to lower passivation and reactivation currents. When austenite is the main phase after tempering, increasing the austenitizing time contributes to higher sensitization due to austenite grain boundary carbon enrichment. However, the presence of austenite after tempering retains some of the carbon in a solid solution, resulting in lower passivation and reactivation currents.