Effect Of Heat Treatment On Microstructure And Mechanical Properties Of Weathering Bolt Steel

In this paper;weathering and high-strength bolt steel NHL10,designed by ourselves,is used as test steel.The different quenching and tempering structures and mechanical properties of the test steels were obtained through different heat treatment processes.Meanwhile,the structure of Packet,Block,Lath and precipitate phase of martensite were characterized by optical microscope,filed emission type election scanning microscope and transmission electron microscope,and statistical analysis is made for the number and shape of each structure.The mechanical properties of tempered steel were tested by room temperature tensile test,impact test and Rockwell hardness test;The weathering resistance and delayed fracture resistance of NHL 10 steel were tested by cyclic infiltration accelerated test and constant load notched tensile test.The results show that,with the increase of quenching temperature,the size of original austenite grain and martensite Packet/Block structure of test steel grew obviously,but the size of Lath structure did not change obviously,and the number of precipitation phase decreased.The size of original austenite grain was linearly related to the Packet and Bock structure size of quenched martensite.With the rise of tempering temperature,the micro structure of Packet and Block grew slightly,the number of precipitates at the grain boundary increased significantly,and the spheroidization,size increased,and the tensile strength decreased,and the relationship between the Packet structure size and yield strength of tempered martensite conforms to the classical Hall-Petch.In addition,the ratio of large angle grain boundaries of martensite increased,which hindered the expansion of cracks and led to the improvement of impact properties.Based on the above research,the optimized quenching and tempering process of the test steel is:quenching at 875 ?,tempering at +500?.According to the above results,the trial production of NHL10 steel is carried out.The mechanical properties,weathering resistance and delayed fracture resistance of the products are all met the technical requirements.