Abstract: Microstructure and properties of the welded joints of the grade-B cast steel and Q450NQR1 steel were studied by optical microscope,micro-hardness analyser,scanning electron microscope and three-point bending fatigue tester.The results show that the small dendritic ferrite and pearlite were found at the welding zone.Martensite was observed at the seam near heat affected zone(HAZ)of the grade-B steel.Hardness of the weld seam reached 240-295 HV,which was higher than that of the two sides of the base metal.Hardness variation of HAZ of the grade-B cast steel was the largest,the hardness could reached maximum of 370 HV.Compared with the welded state,weld toe mechanical polishing could make the joint fatigue strength increase by 12%,and the whole shot peening increase by 15%.Under the cyclic loading,the weld seam cracked most from the root of the weld joints.