According to the metallographic structure, stainless steel is divided into five categories, namely ferritic stainless steel, martensitic stainless steel, austenitic stainless steel, austenitic-ferritic duplex stainless steel and precipitation hardened stainless steel.
1. Stainless steel metallographic test sample preparation. The preparation of the stainless steel metallographic specimen is basically the same as that of the general high alloy steel. Among them, the austenitic stainless steel matrix structure is soft, the toughness is high, and it is easy to work harden. It is difficult to prepare the sample, which is easy to produce mechanical slip and disturb the metal illusion of the metal layer and affect the normal metallographic analysis and inspection. Austenitic and martensitic steels become martensite if improperly prepared. Therefore, the preparation of the sample should be based on the principle of not causing the change of the structure. The sample should be carefully ground. When the grinding wheel is leveled, do not cause the high temperature of the stainless steel forging sample. When the sandpaper is polished, the force should not be too large, try to use new sandpaper to reduce the grinding time. In the mechanical polishing, the plush fabric and the diamond grinding material with large grinding capacity should be used. The polishing time should not be too long, and the pressure should not be too large. The ideal polishing method for stainless steel is electrolytic polishing, which results in high quality specimens and avoids artifacts.
2. Erosion of stainless steel metallographic specimens. Stainless steel has a high corrosion resistance, so the etchant showing its microstructure must be strongly aggressive in order for the tissue to be clearly displayed. Commonly used etchants are: 1 chlorinated high iron 5g + hydrochloric acid 50mL + water lOOmL; 2 hydrochloric acid 10mL + nitric acid 10mL + wine 100mL; 3 picric acid 48mL + hydrochloric acid 5mL + alcohol lOOmL
In addition, ferrite, austenite, carbide, ferrite, and the like may occur simultaneously in stainless steel forgings, which can be distinguished by chemical or electrolytic etching. In morphology, austenite has a twin structure, and the ferrite is often banded or dendritic; after being eroded with red blood potassium hydroxide solution, the ferrite is rosy and the austenite is bright.