Study On Cracking Behavior Of Steels For Large Size Piston

As a new type of hydraulic engineering equipment,hydraulic breaker is widely used in the crushing and engineering construction of mine rock.The piston is the key component of the direct work of the hydraulic breaker impact system.In the work,the striking end face is prone to chipping and cracking,thus shortening the service life of the hydraulic breaker.For this reason,this article analyzes the causes of cracking failure of piston steel,and aims at the special service environment of hydraulic breaker pistons,studies the influence of heat treatment process on the microstructure and mechanical properties of steel for pistons,with a view to providing support for improving the level of China's hydraulic breaker hammer piston.The results show that the microstructure and mechanical properties of GCr15 bearing steel and 9CrV steel are uneven.The surface is martensite structure with high hardness and strength and low elongation.From the surface to the center,martensite gradually transitions to pearlite type structure,the hardness and strength gradually decrease,and the elongation increases.Segregation of the banded carbides in the steel for the piston increases the sensitivity of the piston cracking.The residual stress distribution along the section of the piston is the surface tensile stress,the core is subjected to compressive stress,and there is a stress mutation of 259 MPa at the position of 22 mm and 30 mm from the surface,which leads to the formation of cracks under the action of stress,and collapse cracking occurs under the repeated action of impact force in the subsequent work.By deep cooling the piston,the stress mutation at this location is reduced by 27.4%.The effects of heat treatment on the microstructure and mechanical properties of GCr15 bearing steel and 9CrV steel for piston were studied.The study found that the steel for piston is quenched at 840? and tempered at 300?,the microstructure is tempered martensite?undissolved carbides and a small amount of residual austenite.The quenching temperature is lowered to 800?,the amount of undissolved carbides is increased,and the structure is refined.The tempering temperature is rised to 400?,the microstructure changes from tempered martensite to tempered troostite,the dislocation density decreases,and the material softens.The combination of the two effects makes the hardness of the piston steel to decrease,and the impact absorption energy is increased improve.Quantitative analysis of the carbide distribution around the secondary crack near the impact fracture of the steel for the piston shows that as the quenching temperature decreases,the tempering temperature increases,and the number of carbides around the secondary crack increases,but the proportion of large-sized carbides is reduced,which reduces the damage caused by large-size carbides,the toughness of steel for pistons increases.