Microstructures And Properties Of New Type High-Cr Cast Tunnel Boring Machine Cutter Steel

The full-face tunnel boring machine(TBM)has become an indispensable important construction machine for tunnel excavation because of its high excavation efficiency and excellent safety.At present,TBM has been widely used in infrastructure constructions such as railways,subways and urban rails.With the implementation of the "Thirteenth Five-Year Plan" and the "O ne Belt and One Road" strategy,the demand for TBM is increasing.The cutter is a key component of the tunnel boring machine's broken rock and is also a consumable part of the TBM.According to statistics,the cost of cutters consumed in tunnel construction can account for about 30% of the total construction cost.Therefore,to increase the cutter life and reduce cutter production costs can effectively reduce the total tunnel construction costs.At present,the TBM cutters manufactured by Wirth Company and Robbins Company have a good overall performance and market reputation,and have basically monopolized the market for TBM cutters.However,the production level of TBM cutter for China is relatively low,and the overall performance and service life of cutter cannot reach the international level.Most of the cutters still rely on import,which severely restricts the development of tunnelling.Therefore,the development of high-quality TBM cutter steel with independent intellectual property rights to improve the service life of the cutter and reduce the cutter production cost has a significant national economic significance.In this study,a new type of high-Cr cast tunnel boring machine cutter steel(HBC steel)was produced through alloy composition op timization and specific heat treatment process.Compared with the forged H13 steel,HBC steel has high strength,high hardness,high abrasive wear resistance,high thermal fatigue resistance,and high room temperature impact toughness.The main conclusions of this paper were as lollowed:(1)Through the research on the microstructure,mechanical properties,abrasive wear resistance and thermal fatigue resistance of the cast HBC steel,the optimum content of chemical composition of the HBC steel was determined.The optimal content of its mainchemical elements is as follows: C : 0.33 wt.%—0.40 wt.%,Cr: 9.5 wt.%—10.5 wt.%,Mo: 1.8 wt.%—2.1 wt.%,N i: 0.8 wt.%—0.9 wt.%,V: 0.8 wt.%—0.9 wt.%,Si: 0.6 wt.%—0.8 wt.%,Mn: 0.15 wt.%—0.25 wt.%.When the carbon content is 0.40 wt.%,HBC steel shows a good comprehensive performance,its hardness is about 58 HRC,the yield strength is about 1650 MPa,the tensile strength is about 2200 MPa,the elongation is about 7.4%,and the impact toughness is about 40 J/cm2.All performances have reached or exceeded the level of forged H13 steel,achieving the intended purpose of this study.(2)Determined the best heat treatment process for casting HBC steel: pretreatment+oil quenching + twice tempering.The pretreatment process includes 1230 oC homogenization,1080 oC normalizing and 750 oC annealing.After the homogenization treatment,the segregation of components in the as-cast microstructure of HBC steel is basically eliminated.The larger carbides are partially decomposed or incorporated into the matrix,and the size and shape of the carbides are significantly improved.The best quenching temperature for HBC steel is 1070 oC,and the best tempering temperature is 540—560°C.The tempering structure of HBC steel is mainly tempered lath martensite,and a large number of fine carbides with a size of only 30—60 nm are dispersed on the lath martensite.These nano-size carbides performed a good second-phase strengthening and ensured the strength of HBC steel.(3)With the increased of carbon content,the volume wear rate of HBC steel decreased first and then increased.When the carbon content is 0.40 wt.%,the volume wear rate reached the lowest,and the wear resistance of HBC steel increased about 45% compared with H13 steel.When the carbon content is less than 0.40 wt.%,the main abrasive wear mechanism of HBC steel is micro-cut wear and furrow wear.When the carbon content is more than 0.45 wt.%,besides the microscopic cutting wear and the furrow wear,the micro-fracture(peeling)wear is obviously increased,more small pits and brittle fractures appeared on the wear surface,and the wear resistance is reduced.(4)With the increased of carbon content,the thermal fatigue resistance of HBC steel shows a downward trend.The higher the carbon content,the higher the ther mal fatigue crack level.With the increased of the thermal fatigue cycles,the higher carbon content of HBC steel showed the more serious pitting phenomenon.More cracks are generated from pitting pits,especially,when the carbon content is more than 0.45 wt.%,the pitting phenomenon is more serious.During the thermal fatigue process,HBC steels with carbon contents of 0.33 wt.% and 0.40 wt.% exhibited high thermal stabilit y.After 10,000 thermal fatigue cycles,the hardness of the HBC steel only decreased by 1.5HRC,however,the strength of HBC steel not only decreased but increased slightly.In addition,during the thermal fatigue test,it was found that oxidation promoted the initiation and propagation of thermal fatigue cracks,so higher oxidation res istance is an important factor to ensure that HBC steel has good thermal fatigue resistance.