In the present paper, the effect of different polishing methods (mechanical and electrochemical) on passive layer chemistry and the corrosion behavior of stainless steels is investigated. It was found that CrNiMo austenites have a substantially better corrosion behavior than CrMnN ones. The nickel is enriched underneath the passive layer, while manganese tends to be enriched in the passive layer. It was also noted that immersion of manganese into an electrolyte preferentially causes its dissolution. It was found that high amounts of chromium (27.4%), molybdenum (3.3%), nickel (29.4%), with the addition of manganese (2.8%) after mechanical grinding, generates a better corrosion resistance than after electrochemical polishing. This is most likely because of the introduction of phosphates and sulfates into its structure, which is known for steels with a high amount of manganese. For highly alloyed CrNiMo steels, which do not contain a high amount of manganese, the addition of phosphates and/or sulphates via the electropolishing process results in a decrease in pitting corrosion resistance, which is also observed for high manganese steels. Electropolished samples show detrimental corrosion properties when compared to mechanically polished samples. This is attributed to substantial amounts of sulfate and phosphate from the electropolishing electrolyte present in the surface of the passive layer.
Roller bearings in aircraft turbines are commonly made of AISI M50 steel, because enhanced heat stability as well as highest reliability is required for this application. With a chemical composition of approximately 0.8 C, 4.0 Cr, 4.5 Mo, 1.0 V (all in wt.%) and using a specific vacuum melting and remelting technology for highest cleanliness M50 provides the best properties for such application. Despite some studies on this steel, there is still a lack of information on microstructural evolution during rolling contact fatigue (RCF). Hence, in order to get a better understanding of the microstructural evolution and as a consequence of the crack initiation, in the framework of this study a comprehensive set of experimental techniques were combined. For the RCF loading a so called ball-on-rod test was used. Microstructural alterations were analysed by various methods including optical light microscopy, confocal microscopy, scanning electron microscopy (SEM) with cross-sectional cutting by focussed ion beam (FIB), transmission electron microscopy (TEM) and microhardness measurements. Testing methods showed the build-up of a so-called white etching area (WEA) in a certain region below the raceway and the formation of butterfly-wings (BW) with micro-cracks at local microstructural inhomogeneity. Furthermore, all tested samples showed BW's which initiated on carbides, often with micro-cracks near the boundary to the matrix.
Purpose: Applications for highly corrosive environments and cyclic loading are often made out of austenitic stainless steels. Corrosion fatigue and crack propagation behaviour has been studied to determine failure processes and damage mechanisms. Approach: CrNiMo stabilized austenitic stainless steel and CrMnN austenitic stainless steel in solution annealed and cold worked condition are compared. S/N curves and crack propagation rate curves are recorded in 43 wt% CaCl2solution at 120 °C, which resembles most severe potential service conditions. For comparison these experiments are also performed in inert glycerine. Additionally, the electrochemical behaviour of these materials has been studied. Findings: The CrMnN steels have excellent mechanical properties but are very susceptible to stress corrosion cracking in the test solution. The fatigue limit as well as the threshold for long crack growth are significantly reduced in corrosive environment. Moreover these steels exhibit a remarkable increase in the propagation rate, which is extremely pronounced in the near threshold region. This effect is enhanced by cold working. CrNiMo steels also show a reduction in the fatigue limit, but it is less pronounced compared to CrMnN steels. The threshold is significantly reduced in corrosive environment, but propagation rate is lower in corrosive environment compared to inert glycerine. Possible explanations of this surprising behaviour are discussed.
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