Study Of Strength-toughness Mechanisms And Wear Resistance Of Low-alloy Cast Steel On Mill Lines

Mill lines steel is developed by using alloying cast steel and optimizing heat treatment technique, based on the actual ball mill liner working conditions in this paper. By adding Mn, Si, Cr, Mo and other alloying elements, its sulphur and phosphorus contents were strict controlled. The medium-carbon-low-alloy cast steel was melted. The effect of different quenching temperature and tempering temperature on microstructure and mechanical properties of the medium-carbon-low-alloy cast steel were researched. The best heat treatment process was optimized. The HRD-150 electric rock well hardness tester and JB-300B impact tester as well as SHIMADZUAG-I250KV on electronic tensile testing machine were used to test the mechanical properties of materials. By means of JSM-5610LV SEM and JEM-2100 TEM as well as German Brux D-8 X diffractometer explored the microstructure and phase structure of materials, analyzed the wear morphology of materials, and systematically studied the strengthening and toughening mechanism of materials after the second tempering process. Finally, employing MMS-1G high velocity pin-disc friction wear test machine, we approached wear character of the steel.The results indicate that the medium-carbon-low-alloy cast steel whose alloy components is C0.42%, Mn1.35%, Si1.84%, Cr2.10% and Mo0.35% has good mechanical properties with HRC54,αk43J/cm~2 andσb1820MPa after oil quenching at 930℃×2h then tempering at 240℃×2h and tempering at 240℃×2h. It is ductile fracture with a large number of dimples in fracture surface.Through JEM-2100 TEM observation and German Brux D-8 X diffractometer analysis, the medium-carbon-low-alloy cast steel after the second tempering process of strengthening and toughening mechanism has been studied. The results indicate that: (1) after the second tempering the martensite sub-structure of the sample steel is mainly high-density dislocations and a small amount of twins, when compared with the once tempering of high-density dislocations and twins are more visible everywhere. The mainly because that the twins in part in response, disappeared after the second tempering, the number of twins were significantly reduced. (2) Through the high-resolution transmission electron microscopy observed the dislocation density of the sample steel under different magnifications: compared with the once tempering the dislocation density after the second tempering decreased significantly. After the second tempering the dislocation dynamic recovered and disappeared, tangle of high-density dislocation cells gradually restore, as a result, leading to large diameter of dislocation cells. (3) The analysis and calculation showed that: phase construction of the sample steel after second tempering was M+3.54%A'+11.6%M23C6; phase construction of the sample steel after second tempering was M+5.78%A'+6.4%M23C6; After the second tempering the more number and larger size of carbides mainly in granular dispersed in the lath martensite matrix and the high-dislocation density. (4) Compared with the once tempering, after the second tempering the content of the retained austenite from 5.78% to 3.54%, by using TEM the morphology of retained austenite was film shaped, distributed in the beam between the lath martensite by the way of lamellar phases. The result of these combined factors was the strengthening and toughening mechanism of the medium-carbon-low-alloy cast steel.The results of friction and wear experiments show that: after the second tempering the wear resistance of the medium-carbon-low-alloy cast steel increased over time by nearly 10%. The friction coefficient of the sample steel decrease with load and wear velocity increase; the wear volume increases rapidly with the load increase, increases first and then decreases with the wear velocity increase. Applied load is the major influencing factor of wear volume; however the wear velocity has little effect on wear volume. JSM-5610LV SEM was employed to analyze the wear morphology of the sample steel, the results show that: the abrasive wear plastic deformation and adhesion wear are presented mainly; the more enhanced precipitation phase and good plastic matrix toughness improve the wear resistance of the medium-carbon-low-alloy cast steel.