Flapper valve, martensitic, bending fatigue, corrosion resistance, Zapp SF
In order to increase the energy efficiency of refrigerators, compressors, incorporating valves with new geometries have been developed over the past few years. In addition, higher stresses are increasingly placed upon the valve itself during operation. Hence, higher demands are set on the steel strip used to produce such valves. Today, compressor valves for refrigerators are mainly manufactured from hardened and tempered martensitic stainless steels. The main design criterion for the material selection is the endurance limit of the material under bending. However, impact loads and corrosive atmospheres often act additionally on the valve. To fulfil the increased demands for such new valves, the microstructure of the most commonly used stainless steel grade is refined. Through this method a new grade Zapp Super-Fatigue is developed, whereby the yield strength, ultimate tensile strength and bending fatigue are increased. In addition, the corrosion resistance of the material is also significantly increased. All the tests are performed with industrial scale processes and material. The properties are determined with samples taken from various production lots. Tensile properties as well as bending fatigue tests are performed on a strip with thickness typical for valves used for refrigerators. Current density potential measurements are conducted to confirm and explain the corrosion test results. Finally, the microstructure is characterized using a scanning electron microscopy with the electron back scatter diffraction method. For the first time, the microstructures of the most commonly used stainless steel and the improved grade Zapp Super-Fatigue are compared. It is demonstrated how the fatigue limit and the corrosion resistance of a martensitic stainless steel can be enhanced by tailoring the microstructure.