Performance of Low Temperature Tempering on Austenitic Precipitation Hardening Stainless Steel 0Cr17Ni5Mo3N (AM350)
Austenitic precipitation hardening stainless steel 0Cr17Ni5Mo3N (AM350) is a kind of duplex stainless steel. Also known as Alloy alpha high strength martensitic stainless steel, it has good formability and superior oxidation resistance, accompanied with high strength and great fatigue resistance. AM350 is commonly used in aircraft structures, railroad cars, pipelines and valves, and ship structural components. In order to investigate the performance of low temperature tempering on AM350, low temperature tempering is applied by induction heating method, then the microstructure, mechanical property and corrosion resistant property are studied.
The result of microstructure observation shows that, grain sizes of the specimen tempered from 930℃ to 550 ℃，and from 550℃ to 370℃ are between ASTM 6-7 and ASTM 5-6 classes respectively. After tempering for 6min, the ferrite amount of the specimen tempered from 930℃ to 550 ℃，and from 550℃ to 370℃ are 15.92% and 18.67% respectively, indicating that continuous tempering of AM350 is beneficial to the refinement of the ferrite grains, which is beneficial to the improvement of the properties of the specimen.
In order to study the mechanical properties, tensile tests were performed at the temperature of room temperature -50℃ and -150℃. The results show that the yield strength, tensile strength and elongation of 0Cr17Ni5Mo3N specimens tempered from 930℃ to550 ℃ and from 550℃ to 370℃ are enhanced under different field temperatures, compare with the specimen tempered at 930℃. In particular, the yield strength, tensile strength and elongation of this specimen tempered from 930℃ to550 ℃ and from 550℃ to 370℃ are significantly improved at the temperature of -150℃. The improved strength at low temperature is credited to the presence of a relatively high commonality of PHF issues, which is due to increased grain boundary and number of dislocations present in the steel. The increased dislocations and grain boundaries provide dislocation pinning points which enhance the ductility while preserving the strength of the steel.
At the same time, to study the corrosion resistant property of AM350, a series of electrochemical tests are conducted on AM350 specimens and the results show that, As the tempering temperature decreased, the corrosion rate of the specimens also decreases. After tempering for 6min, the corrosion rates of specimens tempered from 930℃ to 550℃ and from 550℃ to 370℃ are 0.403mm/a and 0.375mm/a respectively, which means that the corrosion resistance of the specimens is improved. This is due to the high level of PHF in the specimen, which decreased the effect of the oxygen “creep rate”.
To sum up, the low temperature tempering of austenitic precipitation hardening stainless steel 0Cr17Ni5Mo3N (AM350) is beneficial to the improvement of microstructure, mechanical property and corrosion resistant property. This can be used to enhance the performance of AM350 at low temperatures, further expanding its application.
