Preparation Of Nanobainitic Hot Die Steel And Microstructure And Mechanical Properties After Tempering

Nanobainite structure is potentially used in bridges,ships,Bearing steel,rail and Vehicle armor plate because of its excellent comprehensive mechanical properties.Both toughness and thermal stability of bainite are higher than that of martensite,so it will be advantageous that the nanobainite is used in hot die steel.In this dissertation,nanobainitic microstructure is guaranteed to be obtained with low-temperature austempering by adding Si into H13 steel to obtain Si-rich H13 steel(Si-H13).The effects of conditioning heat treatment,austempering and multiple tempering on microstructure and mechanical properties was studied..After Si-H13 steel was quenched by 1050?C and 1150?C for 10 min+720?C tempering for 1 h,tempered troostite and spherical undissolved carbide were obtained.Then the final heat treatment is carried out,the preheated sample is heated to 1030?C for10 min and then quickly placed in 350?C and 360?C(20 and 30?C above Ms point)for austempering,then perform once tempering of 560?C×1h,twice temperings of 560?C×1h+580?C×1h and three times tempering treatments of 560?C×1h+580?C×1h+600?C×1h.Rockwell hardness,tensile properties and U-notch impact energy were tested on isothermally quenched and tempered samples,and the microstructure and fracture morphology of the heat-treated samples were analyzed with scanning electron microscope,transmission electron microscope and X-ray diffractometer.The results show that the M_s point of the original and pretreated sample of Si-H13steel after austenitizing at 1030?C for 10 min were both 330?C.After quenching and tempering pretreatment,Si-H13 steel was isothermally quenched to obtain a nano-bainite structure composed of bainite ferrite lath and thin film austenite.The thickness of bainite ferrite lath was 95?112nm.Under the same pretreatment conditions,the higher the austempering temperature,the less the amount of bainite is generated,the size of bainite ferrite lath become wider,due to the transformation from massive residual austenite to martensite during the cooling process after the end of austempering,the residual austenite content decrease,and the impact energy,tensile strength and elongation decrease.Under the same austempering conditions,compared with the 1050?C pretreated sample,there are more massive M/A structures in the 1150?C pretreated sample,with lower impact energy and elongation,and higher hardness.Under multiple tempering conditions,the samples of 1050?C and 1150?C pretreatment+austempering showed secondary hardening after once tempering,the hardness reached the highest value,and the impact energy reached the lowest value.The hardness of twice tempering and three times tempering showed a downward trend,while the toughness increased.The yield strength of1050?C and 1150?C pretreatment+austempering samples was low.After twice tempering treatments,the yield strength increased by 600 MPa,and the tensile strength and elongation decreased to a lesser extent.At 350?C austempering+two tempering conditions,we observed fine carbides were precipitated in the bainite ferrite lath boundary in the 1150?C pretreated sample,but not found precipitation of carbides in the 1050?C pretreated sample.After Si-H13 steel is subjected to 1050?C quenching+720?C tempering pretreatment,1030?C austenitizing+350?C austempering,it has excellent comprehensive mechanical properties,its hardness is 52.8HRC,tensile strength is 1996MPa,The elongation is 11%and the impact energy is 29J.The best tempering process is 560?C×1 h+580?C×1 h twice tempering.The hardness is 50.2HRC,the tensile strength is1950MPa,the elongation is 8.9%and the impact energy is 18J.