An Experimental Study to Understand the Localized Corrosion and Environment-Assisted Cracking Behavior of AISI 420-Martensitic Stainless Steel

Sammanfattning: Motivation and aim: Currently steel molds are designed with cooling channels to reduce the solidification time of molten plastic within the mold to improve the productivity. As water is generally used as the cooling medium, corrosion and environment-assisted cracking (EAC) leading towards the dysfunction of mold, can increase the production downtime. This was observed in some cases. Hence the primary aim of this thesis is to study the corrosion and EAC behavior of a martensitic stainless steel (MSS) in Cl containing environment to further the current understanding thereby to optimize the existing alloy/s and to design and develop new steel grades.Methods: The MSS had been austenitised at 1020°C, and subsequently quenched in nitrogen gas at fast (3°C/s), and slow quenching rates (0.6°C/s). Then tempering was done at 250°C, and 500°C, respectively, twice for two hours. Microstructure was predicted and characterized using Thermocalc simulation, dilatometry, light optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, atomic force microscopy (AFM). Localized corrosion behavior was characterized using standard salt spray test, electrochemical experiments, scanning Kelvin probe force microscopy, in-situ AFM. Stress relaxation associated with 250°C, and 500°C tempering was characterized by a new method for both fast (FQ) and slow quenched (SQ) conditions. Based on the %stress relaxation, initial loading levels were altered and the corresponding environment-assisted cracking behavior was investigated at two different loading levels. Results: Samples tempered at 250ºC exhibited higher corrosion resistance than samples tempered at 500ºC in both FQ and SQ conditions. FQ samples exhibited higher corrosion resistance with an ability to passivate than SQ samples when tempered at 250ºC. However, when tempered at 500°C, the corrosion resistance was poor for both FQ and SQ samples. These observed differences clearly indicate the strong influence of microstructure on the corrosion behavior of the material. There are preferential active sites in the microstructure, which dictate the sequence of corrosion events. Secondary Cr-rich carbides formed during 500ºC tempering apparently deteriorate the corrosion resistance in spite of their smaller sizes as compared to undissolved Cr-rich carbides.  Stress relaxation increased with increasing tempering temperature. In the FQ condition, 250°C temper exhibited superior EAC resistance than 500°C temper in both loading scenarios, indicating the dominant role of corrosion resistance in delaying the failure. Whereas in SQ condition, 500°C temper exhibited superior EAC resistance than 250°C temper in both loading scenarios, indicating the dominant role of applied stress in delaying the failure. The pitting susceptibility increased with increasing applied stress on both FQ and SQ conditions. The fractographic features suggest that the mechanism of failure was mixed mode involving both active path dissolution and hydrogen embrittlement, which could have been operative during the failure in varying magnitude in respective scenarios. Conclusions: Based on this research work, it can be concluded that, in order to have a longer service life, both the localized corrosion behavior and the residual stresses are to be considered while recommending tempering temperature to mold makers.