Effect of intercritical quenching temperature on micro-structure and properties of Ni-Cr-Mo-V high strength hull steel
To improve the high yield ratio and narrow tempering range of high strength hull steel,the effect of intercritical quenching temperature on the microstructure and properties of Ni-Cr-Mo-V steel was investigated.The impact tensile properties and microstructure of the experimental steel were examined by changing the secondary quenching temperature,and characterized by MO,TEM,X-ray diffraction and other methods.The results show that in quenching temperature range,the properties of the material show two phenomena:when the quenching temperature between 640 to 700 ℃,the strength of steel increases with the increase of intercritical temperature,and the toughness decreases.In this quenching temperature range,the mechanical properties of the material are determined by the composition of F&M microstructure and the formation of reversed austenite.With the increase of the intercritical quenching temperature,the proportion of F in the dual phase structure decreases from 40％to 10％and the content of reversed austenite reduces from 11％to 1％.When the quenching temperature varies from 700 to 780 ℃,the change of strength and toughness of test material is not obvious.The main reason is that the intercritical quenching temperature exceeds the A3 temperature,the steel transforms into a single martensite structure after cooling,and the change of austenite grain size is not significant,specifically between 7.3～8.5 µm.The results show that the best match of strength and toughness can be achieved by intercritical quenching at 680 ℃.
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