The effects of nitrogen and nickel on the microstructure and mechanical properties of 16 wt.% chromium stainless steels

Master Thesis


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University of Cape Town

An investigation has been carried out on the effects of heat treatment on the microstructures and mechanical properties of a number of experimental 16 wt.% chromium dual-phase ferritic-martensitic stainless steels. A comparison was made between an alloy containing 2.5 wt.% nickel (low interstitial content [C + N = 0.03 wt%]), and three alloys possessing low nickel (0 - 1 wt.%) and high nitrogen contents (0.06 - 0.12 wt.%). Samples of AISI 304, 430 and 431 were included in the investigation for comparison with the experimental alloys. The microstructural response of the alloys to heat treatment was examined using light and scanning electron microscopy techniques. Tensile and Charpy V-notch impact tests were carried out on the alloys in their various heat treated conditions. Fracture surfaces, and deformation markings on the tensile gauge surface, were examined in the scanning electron microscope, while cross-sections of fracture surfaces were examined using light microscopy. Dilatometric traces were obtained for the experimental alloys in order to determine the effects of variations in composition on the inter-critical temperature range. The combination of good toughness and tensile strength that can be achieved in the low interstitial, nickel alloyed steel suggests that it could be a favourable alternative to both AISI 430 and 431 in many engineering applications. Toughness values superior to those of AISI 430 and 431 can be achieved in the high nitrogen stainless steels by tempering at 700°C, although the heat treatment results in a substantial loss in strength, and the low toughness exhibited by these alloys in the solution treated condition suggests that their weldability is no better than that of AISI 430 and 431. It is also shown that the formation of a lamellar ferrite/martensite compos.ite pha5e through intercritical annealing can provide attractive combinations of tensile strength, toughness and ductility in certain of the alloys. However, the ductility of alloys containing a lamellar composite phase is dependent on the o-ferrite content, and the toughness of the composite phase is adversely affected by a high nitrogen content. The yielding characteristics of ferritic-martensitic stainless steels are dependent on the hardness difference between the ferrite and martensite phases, and on the volume fraction of martensite. In addition, the morphology of the martensite phase exerts a strong influence on the ductility of dual phase steels. Microvoid initiation in the experimental alloys in the solution treated condition (1000°C/ lhour/air cool) occurs primarily by fracture within the martensite phase. In the 700°C tempered condition alloys having a high nitrogen content may be susceptible to intergranular fracture.

Bibliography: pages 125-130.