Effect of magnesium on inclusions and austenite grain in a GCr15 bearing steel
YU Zheng1, REN Ying1, REN Qiang2, ZHANG Lifeng3
1. School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China; 2. College of Mechanical Engineering, Yanshan University, Qinhuangdao 066004, Hebei, China; 3. School of Mechanical and Material Engineering, North China University of Technology, Beijing 100144, China
Abstract：In order to study the modification effect of inclusions in the bearing steel by magnesium and the influence of inclusions in the bearing steel on austenite grain size of different magnesium content conditions,the inclusion and austenite grain size in the bearing steel with different magnesium content were analyzed by laboratory experiments and thermodynamic calculations. The composition,size,morphology,and composition of inclusions in bearing steel samples with the mass percent of total magnesium of 0.000 2%,0.000 4%,0.001 2%,0.002 5%,and 0.003 8% were analyzed. The modification sequence of inclusions in the magnesium-treated bearing steel was Ca-Al-O,Al2O3→Ca-Al-Mg-O→MgAl2O4→MgO. With the increase of the mass percent of total magnesium from 0.000 2% to 0.002 5%,the number density of 1-2 μm inclusions in the steel increased from 6.04 piece/mm2 to 22.99 piece/mm2,when the mass percent of total magnesium was 0.003 8%,the number density of inclusions in the steel decreased slightly (17.59 piece/mm2),and the average size of inclusions decreased from 3.03 μm(the mass percent of total magnesium was 0.000 2%) to 2.02 μm (the mass percent of total magnesium was 0.003 8%). When the austenitizing time increased from 15 min to 120 min or when the austenitizing temperature increased from 900 ℃ to 1 050 ℃,the austenite grain size of a bearing steel showed an increasing trend. Thermodynamic calculation results showed that temperature rise led to the dissolution of carbides,the decrease of austenite grain boundary nailing,and the growth of austenite grains. The results of Zener nailing force calculation showed that when the size of inclusions was smaller and the volume percent of inclusions was larger,the inclusion had stronger nailing effect on the grain boundary,the migration rate of the grain boundary was smaller,and the austenite grain size was smaller. So the austenite grains of the samples with the mass percent of total magnesium of 0.001 2% and 0.002 5% showed a slow growth trend during the holding or heating process. When the holding temperature was 1 000 ℃ and the holding time was 30 min,the size of austenite grains with magnesium content of 0.001 2% was the smallest (26.4 μm),when the mass percent of total magnesium was 0.002 5%,the grain size was 28.4 μm,and the austenite grain size of the other bearing steels with magnesium content was greater than 35 μm. Based on the Arrhenius austenite grain growth model,the growth process of austenite grain in the magnesium-treated bearing steel was predicted,which was consistent with the growth process of austenite grain.
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2023, Vol. 58 
