Abstract: Using MMS-200 thermal simulation testing machine,the change law of microstructure and hardness of NM400wear-resistant steel welded coarse grain area under different t_8/5conditions was studied,and the continuous cooling transition curve of the heat affected zone wsa obtained through the methods of metallographic structure observation and hardness testing.The results show that proeutectoid ferrite is precipitated in the high temperature transformation zone,and bainite and martensite transformation occurs in the middle and low temperature zone.When t_8/5 ≤ 10 s,the hardness value does not change much,and the structure is mainly lath martensite.When t_8/5>10 s,with the increase of t_8/5time,the hardness decreases significantly,the number of lath martensite gradually decreases,and the number of bainite increases.When t_8/5=60 s,the martensite structure disappears,and the structure in the interval of 60-150 s is mainly bainite.When t_8/5=150 s,ferrite is precipitated,and lath bainite gradually transforms into granular bainite.According to the SH-CCT curve,NM400wear-resistant steel should strictly control the line energy during welding,and by controling t_8/5 ≤ 10 s,the strength of the welded joint showld be ensured.