Effect Of Heat Treatments On The Microstructure And Corrosion Resistance Of Cr13-type Plastic Mold Steel

To extend the working life of plastic injection molds, the 13 wt.% Cr-type martensitic stainless steels, which possess high hardness, better wear and corrosion resistance, have been widely used as ―corrosion resistant plastic mold steel（CRPMS）‖ in production of the lastic injection molds. In this thesis, the effects of heat treatment on the microstructure, electrochemical properties（mainly the pitting resistance）, hardness uniformities and surface roughness of typical corrosion resistant plastic mold steels（30Cr13 and Stavax ESR steel） are studied via advanced material analytical technique and electrochemical methods.Increasing the austenitizing temperature could enhance the pitting corrosion resistance of the martensitic stainless steel. This is due to a greater homogeneity in the distribution of Cr and a dramatic reduction of the amount of Cr-rich carbides. The electrochemical impedance spectrum（EIS） measurements confirmed that the decrease in M₂₃C₆ carbides content among the MSS matrix by increasing the austenitizing temperature allowed more protective and well-structural passive films to form, which enhanced the pitting corrosion resistance of martensitic stainless steel. In addition, the comparative studies of the two different experimental steels showed that the Ti N inclusions could effectively decrease the pitting resistance of the CRPMSs, and the promotion of the cleanness of CRPMSs also increased its pitting resistance.The tempering temperature could strongly affect the microstructure and the corrosion resistance of the 13 wt.% Cr-type CRPMSs. Corresponding to the tempering temperatures of 300 ℃, 500 ℃ and 650 ℃, the precipitated carbides in CRPMSs were M3 C, nano-sized Cr-rich M₂₃C₆, and stable Cr-rich M₂₃C₆ in micron or submicron-size, respectively. Potentiodynamic polarization, EIS measurements and surface observation after immersion in 3.5 wt.% NaCl solution indicated that the relatively low temperature（300 ℃） tempering could slightly decrease the pitting potential（Epit） of the CRPMSs when compared with the as-quenched steels. However, during higher temperature tempering, the precipitation of Cr-rich M₂₃C₆ resulted in the formation of the relative Cr-depleted zones which was close to the matrix/precipitates boundary. Among these Cr-depleted zones, formation of the densely protective passive films was supposed to be hindered, therefore, the corrosion resistance of CRPMSs would be dramatically deteriorated, especially when the large amount of nano-sized Cr-rich M₂₃C₆ carbides are dispersed in the steel matrix as evidenced in the CRPMSs that tempered at 500 ℃. The present results suggest that the tempering temperature for the 13% Cr-type plastic mold steels should be much lower or higher than 500 ℃ to avoid the massive precipitation of nano-sized Cr-riched M₂₃C₆ carbides.Quenching and partitioning（Q&P） treatments could effectively retain a significant amount of austenite in 30Cr13 steel. The thermodynamic condition of 0.3C-13 Cr MSS cannot fulfill the standard condition of CPE or CCE model, due to the fact that a portion of carbon in the partially transformed martensite was precipitated as carbides. In comparison with the conventional quenching and tempering treatments, the Q&P treatments improved the pitting resistance of the 30Cr13 steel, which could be attributed to the effects of hindering the formation of Cr-rich M₂₃C₆ carbides and maintaining sufficient amount of austenite in steel matrix via partitioning treatments. When the content of retained austensite is in the range of 8 vol.%-10 vol.%（350 ℃ and 450 ℃ partitioned）, the retained austenite distributed among the martensitic lath in the form of ―thick-film‖, and the pitting corrosion resistance of the Q&P treated CRPMSs was better than other Q-T and Q&P CRPMSs. When the austenite content of the Q&P treated steel was below 6 vol.%（300 ℃ and 500 ℃ partitioned） or exceeded 13 vol.%（400 ℃ partitioned）, the pitting corrosion resistance of CRPMSs were lower than the 350 ℃ and 450 ℃ partitioned CRPMSs, but slightly higher than the 350-450 ℃ tempered steels.