Evaluation of the formability properties of nitrogen alloyed metastable austenitic stainless steels

Master Thesis


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University of Cape Town

This study examines the formability of an AISI 301 based metastable austenitic stainless steel, in which nitrogen partially substitutes nickel. In order to understand the formability of the experimental alloys, the tensile behaviour of the alloys is characterised. The tensile properties of metastable austenitic stainless steels are governed by austenite stability which is related to alloy composition and test temperature. At certain alloy compositions, transformation induced plasticity (TRIP) occurs. TRIP depends on the manner in which deformation induced martensite forms in the steels. Incipient necking is resisted if the martensite forms gradually and selectively, preventing propagation of micronecks and microcracks. Tensile tests performed from -5 to 100°C were used to study the effect of TRIP on the ductility of these alloys and optimum tensile properties were obtained at room temperature. In addition, the effect of copper on TRIP and subsequently formability were ascertained using copper alloyed stainless steels. Important stretch formability parameters were obtained from the tensile test which is an intrinsic formability test. TRIP results in improved formability of metastable austenitic stainless steels, and a simulative Engelhardt test was performed to ascertain the effect of TRIP on drawability of the test alloys. It was found that alloys with TRIP characteristics exhibited good drawability and in all cases the test alloys had better limiting drawing ratios than AISI 304 stainless steel. Delayed cracking occurred in alloys with more than 0.2 percent nitrogen content and a low austenite stability, probably as a result of embrittlement of the deformation induced martensite by nitrogen. A study of the cavitation erosion of the test alloys was initiated because it is known that TRIP enhances cavitation erosion resistance in stainless steels. Results indicate that the metastable test alloys demonstrate superior erosion resistance when compared with the stable experimental alloys. Cavitation induced martensite was found in metastable alloys using x-ray diffraction.

Bibliography: pages 92-95.