Effect Of Oxygen, Nitrogen Contents On Structure And Properties Of Martensitic Stainless Steel With High Yield-tensile Ratio

With several decades' development of stainless steel materials for hydraulic turbine runner, their comprehensive properties have been greatly improved. But water turbine unit is currently being developed towards large-scale, which requires higher properties of stainless steel materials to meet the need of this new trend. The high yield-tensile ratio martensitic stainless steel SRIF-2 is a kind of new materials for hydraulic turbine runner, which was produced based on traditional casting stainless steel ZG06Cr13Ni4Mo, by special heat treatment processing, the strength and the hardness of SRIF-2 were increased but its toughness and plasticity were not decreased, and the anti-fatigue, cavitation corrosion resistance were also obtained. But, the impact properties of the SRIF-2 welding joint showed high strength and low toughness compared with the base metal, which was very unfavourable to the engineering application. Therefore, it is necessary to found a method to enhance the toughness of the SRIF-2 welding joint. It is known that the purity of the welding joint, which is affected by the purity of the base metal, is an important factor to influence the impact property. Accordingly, it is very significant to research the effect of Oxygen, Nitrogen contents on the structure and properties of the SRIF-2.In the present work, the effect of Oxygen, Nitrogen contents on the structure and properties of the SRIF-2 was first studied. The influential regularities of Oxygen, Nitrogen contents on structure and properties were achieved. And the effect of Oxygen, Nitrogen contents on the structure and properties of welding heat affected zone that was simulated by high temperature furnace was also investigated. The main conclusion and creative work were given in the following: The microstructure of the SRIF-2 was composed of lath martensite andδferrite, the amount of theδferrite was increased with the increasing of the Oxygen, Nitrogen contents. But the formedδferrite could not be eliminate during the heat treatment.The effect of Oxygen, Nitrogen contents on the strength of SRIF-2 was unobvious but that on the impact toughness was very significant. With the increasing of Oxygen, Nitrogen contents, impact toughness decreased evidently. Moreover, the effect of the Nitrogen content on the impact toughness was more dramatic by the analyse of Multivariate Linear Regression Equation.Welding heat affected zone CCT Diagram of SRIF-2 was made out. It was found that the structure of SRIF-2 at room temperature was basically martensitic structure. The initial temperature of Martensitic transformation and the hardness of SRIF-2 test material were not affected by the cooling rate.Oxygen,Nitrogen contents had significant effect on impact toughness of heat affected zone of SRIF-2. With the increasing of Oxygen, Nitrogen contents, the impact toughness of the heat affected zone simulated and tempered was decreased significantly. The impact toughness of heat affected zone was lower than that before welding. The impact toughness of SRIF-2 was improved by 480℃tempering after welded, which was close to base metal. The influential regularities of various Oxygen, Nitrogen contents on impact toughness of the three states of SRIF-2 were similar. We can conclude that decreasing Oxygen, Nitrogen contents in base metal is the precondition of improving impact toughness of heat affected zone.