Martensitic stainless steel (420B) microstructure show carbides precipitate at grain boundaries. Chromium, present in the matrix to guarantee corrosion resistance, is taken away to form chromium carbides. This causes a strong reduction in the corrosion resistance just near the carbides. This form of corrosion is called intergranular corrosion and the aspect is represented in the picture. Corrosion develops mainly along the grains boudaries, that is the chromium impoverished area, susceptible to the corrosion attack. The component in figure has failed also because the presence of tensile stresses.
Cylinder block is made of one piece of spheroidal cast iron. It broke due to the impact of a failed piston rod. It is evident observing the picture, there are no possibilities to repair the damaged cylinder block. Spheroidal cast iron can not be repaired by welding as the material can not be welded in a easy way. Mechanical repear with metal locking was not feasible due high geometrical complexity and to high deformation of the engine block. For this component a complete subtitution was necessary.
Achilles’heel of austenitic stainless steel is stress corrosion cracking. This steel family, born to sove any problem in corrosion is particularly susceptible to SCC in mildly aggressive chloride environments. In the image the typical aspect of a SCC crack. It propagates normal to the applied stress (in this case residual welding stress), but highly branched. In the studied case, propagation is transgranular.
Ferritic stainless steel resistance welding
The welding of ferritic stainless steel presents certain problems, at times underestimated. We must not only worry about the precipitation of carbides at the grain border, the embrittlement due to a constant 475°C, or the embrittlement due to a a permanent elevated temperature. The failure analysis conducted on ferritic stainless steel sheet metal demonstrates that welding within this particular class of steel products can potentially become disastrous. In this case, welding, with its rapid heating and cooling, provokes the precipitation of martensite at the border of the ferritic grain. This structure, very fragile and with low corrosion resistance, triggers a breakdown as evidenced in the images: decohesion of the crystal granules provokes the component failure.
Martensitic microstructure is present on the thread flank and no diffusion area on the base material are present.
Microhardness indentations confirm the observation: martensite hardness is around 750 HV, instead base material is around 350 HV.
Martensite (white layer) is present on many threads.
In order to check martensite presence during RCFA, many metallographic section have been taken (etch NITAL 2%).
After in order to confirm the observaton, microhardness tests have been made.
In tutti i casi non c’è evidenza di una zona di diffusione.
Stress corrosion cracking on a carbon steel.
Stress corrrosion cracking is a material failure that present a rapid crack propagation. SCC happens in a particular environment, for a particular material in presence of tension stresses. The image, taken during the tuna can steel failure analysis, represent the crack due to SCC. A further confirmation that not only austenitic stainless steel are subjected to stress corrosion cracking phenomenon.