| In order to achieve the China’s strategic goal of becoming a powerful manufacture country,the traditional manufacturing industry has been continuously upgraded.As an important supporting equipment for modern industry,centrifugal fan is widely used in energy recovery,aerospace and other fields.As the core component of the centrifugal fan,the fan impeller directly determines the service life of the centrifugal fan.G520 martensitic stainless steel can be widely used in the manufacture of fan impellers due to its excellent strength,toughness and economical applicability.However,the research of its heat treatment process and development matching metal powder flux cored wire are two significant reasons for limiting its application.Therefore,it is important to study the relationship between the heat treatment system of G520 martensitic stainless steel and its microstructure and properties,and the relationship between matching metal powder flux cored wire and welded joint.The thesis studied the heat treatment process for G520 martensitic stainless steel are solid solution plus overaging and thermal refining according to the high yield strength requirement of 1000MPa for steel for fan impeller.The effect of heat treatment on microstructure and properties are clarified by studying the effects of heat treatment on its microstructure and properties.The research shows that the structure of G520 martensitic stainless steel after heat-treatment is composed of acicular ferrite,a small amount of reversed austenite and second phase precipitate such as NbC,Cr23C6,AlNi and Mn3Si.As the tempering tem perature increases during the quenching and tempering process,the structure of reversed austenite is appearing,which is helpful to improve the plasticity and toughness of the steel;its tensile strength and hardness are continuously decrease,while the plasticity and impact energy are gradually increased.In the present work,G520 martensitic stainless steel matching welding materials are designed by controlling the content of alloy element,based on the strengthening and toughening mechanism and weld metallurgy characteristics of G520 martensitic stainless steel during heat treatment.The Cr and La in the metal powder flux cored wire are designed respectively.The influence mechanism of the alloy system matching the welding material on the microstructure and mechanical properties of the G520 martensitic stainless steel welded joint is investigated.The results show that the weld microstructure consists of martensite and austenite plus a small amount of δ-ferrite.With increase of Cr content,the ferrite in the weld microstructure transforms from interdendritic to lath and lace,and its tensile strength,plasticity,toughness and hardness increased first and then decreased;while the increase of La element made the grain of the weld structure refine,but when the content exceeds 0.5wt.%,the grain appears coarse,but the phase composition do not change,and the plasticity and toughness increase and then decrease,while the tensile strength and the hardness do not change too much.The heat affected zone(HAZ)structure is composed of coarse martensite and second phase precipitate plus reversed austenite structure,and its crystal grains are relatively coarse,which cause softening of HAZ.Obviously,the change of Cr content in the G520 martensitic stainless steel welding matching material is the key to control the evolution of the microstructure in the weld,and the addition of a small amount of rare earth La is beneficial to refine the microstructure of weld.The paper established the relationship between the heat treatment system of G520 martensitic stainless steel and its microstructure and properties,and the corelation between matching metal powder flux-cored wire and welded joint.It provides reliable theoretical guidance for the application of G520 martensitic stainless steel in the field of fan impeller manufacturing,and further broadens the application field of G520 martensitic stainless steel. |
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