Study On Metal Structure And Impact Toughness Of Weld Metal Of Heat Resistant Steel Metal Powder Core Welding Wire

With the development of science and technology,the demand for electricity is increasing,and thermal power generation is the main f orce of power generation in modern society.Because of the excellent properties of chromium-molybdenum(Cr-Mo)heat-resistant steel,i.e.low thermal expansion coefficient,high thermal conductivity,good welding performance and high temperature performance,it can meet t he demand of thermal power generation.In practice,both sound elev ated-temperature performance and optimized toughness are necessary f or Cr-Mo heat-resistant steel,so it is very meaningful to study the r elationship between microstructure and mechanical properties of weld metals.The microstructure and properties of the weld metal are not only affected by the alloying elements in the weld but also by the post-weld heat treatment.In this paper,investigation was conducted on the effect of postweld heat treatment on the impact toughness of low-chromium heat-re sistant steel welded joints.The effect of B element on the microstru cture and properties of low chromium heat-resistant steel weld metal is studied by metal powder cored wires.In this paper,investigation was conducted on the effect of post-weld heat treatment on the impa ct toughness of low-chromium heat-resistant steel welded joints.The effect of B element on the microstructure and properties of low chro mium heat-resistant steel weld metal is studied by metal powder core d wires.The research content of this subject is composed of two parts:(1)The effect of post-weld heat treatment on the microstructure and impact properties of low-chromium heat-resistant steel weld metals.T hree different post-weld heat treatments were applied to the heat-resi stant steel weld metal,which were 690? for lh,690? for 8h,and 730? for 2h.The weld metals were obtained through multi layers an d multi passes welding process.A number of impact samples were cu t out from the welded joint and the impact test was conducted at var ious temperatures,i.e.-20?,-10?,-5?,0?,20?,30?,60?,70?,80?.(2)The effect of B content on the microstructure and i mpact properties of heat-resistant steel weld metal.Here the content of B element was modified on the basis of designing the composition of the metal powder cored wire for heat-resistant steel,more specifi cally,the content of B element is 0.0028%,0.0054%and 0.0079%,r espectively.The impact experiments were carried out at eight tempera tures of-40?,-20?,-10?,0?,20?,30?,60?,and 80?,respec tively.The microstructures and fracture surface of the weld metals w ere analyzed by means of optical microscopy and scanning electron microscopy.It is shown in the first part of the research results that post-weld heat treatment will improve the impact toughness of heat-r esistant steel welded joints,and the highest impact toughness at 730?for 2h.There are two possible reasons:on one hand,although post-w eld heat treatment has trivial effect on ferrite grain size,the subgrai n size is reduced,and the subgrain size gradually decreases with the increase of the post-weld heat treatment temperature and time.The impact toughness of the heat-resistant steel welded joint is affected b y the fine-grain,which caused increasing grain boundaries to inhibit crack propagation.On the other hand,M23C6 and MX carbide precipi tated in the grain in the grain boundary after post-weld heat treatme nt.These carbides can not only improve the high temperature perfor mance of the heat-resistant steel welded joint,but also inhibit the gr owth of the sub-grain to ensure sound toughness of the heat-resistant steel welded joints.It is revealed in the results of the second part that the addition of B element can improve the impact toughness of the heat-resistant steel welded joint,and the toughness of the heat-resistant steel welded joint is the highest when the B element is added to 0.0079%,because the B addition will refine the austenite,which caused increasing grain boundaries to inhibit crack propagation.When the impact test temperature is 80?,the increase of boron element makes the increase of inclusion size and density in the metal structure of heat-resistant steel weld be the main reason for the lowest impact toughness of weld metal at 80? when the boron content is 0.0079%.The type of rare earth element added to the weld metal has not changed,but the segregation of the rare earth element in the structure at the grain boundary hinders the growth of the grain,and the austenite grain size in the weld metal structure decreases.The increase in the grain boundaries of the austenite hinders the expansion of the crack and improves the toughness of the welded joint of the heat-resistant steel.When the impact test temperature is 80?,the addition of rare earth elements makes the increase of inclusion size and density in the heat-resistant steel weld metal structure the main reason for the low impact toughness at 80?.