Effects Of Original Structures On Microstructures And Mechanical Properties Of Induction Heat-treated 40crni2mo Steel

At present, the main heat-treatment process of excavator ring rail is quenching and tempering, but the microstructure obtained by the heat-treatment process is prone to take place contact fatigue and form spalling of the product. Therefore, the service life of the product is deceased increasingly. According to above problems, in this paper austenitizing process of 40CrNi2Mo steel with different original structures is investigated; the effect of medium frequency quenching with different original structures on comprehensive mechanical properties is also analyzed. The theory basis to improve manufactured process of excavator ring rail is provided.The austenite nucleation and growth process of 40CrNi2Mo steel is observed by high temperature metallographic clearly, and the results show that when temperature increase to critical point Ac1, austenite begin to nucleate and grow; when the temperature is close to austenitizing temperature, the austenite grain continuously growth; when temperature is higher than austenitizing temperature, the grain size are almost unchanged. The analyses about final austenite grain size with three different original structures sorting shows that the grain size of annealing treatment is largest, and the quenching treatment is smallest, the grain size obtained by quenching and tempering is between them.The different morphology martensite is formed by the different pre-heat treatment and medium frequency quenching. The cryptocrystalline martensite structure, cryptocrystalline martensite and fine retained austenite, fine acicular martensite and retained austenite, coarse acicular martensite and coarse retained austenite are obtained by the quenching, normalizing, quenching and tempering, annealing treatment, respectively. The analysis of XRD results show that the grain size treated by the different pre-heat treatment process does not changes obviously, which can further explain the forming of fine grain martensite. The samples treated by medium frequency quenching will generate micro-stress, and fine grain size and micro-stress will result in the widening of diffraction peak. The further observation of TEM shows that samples used quenching as pre-heat treatment will form more fine body- centered-cubic martensite, and sample used normalizing ,quenching and tempering as pre-heat treatment will form fine body-centered-cubic martensite and dispersed, granular face-centered-cubic austenite, respectively. Sample use annealing as pre-heat treatment will form coarse body-centered-cubic martensite and coarse face-centered-cubic austenite.Comprehensively comparing the performance of 40CrNi2Mo steel treated by different pre-heat treatment and medium frequency quenching, we can determine that sample used quenching and tempering as pre-heat treatment following medium frequency quenching can present high strength and toughness, good wear resistance, even obtaining best comprehensive mechanical properties.