To investigate the formation of internal cracks in GCr15 bearing steels during the soft reduction process in rectangular bloom continuous casting, fully coupled thermomechanical finite element models were developed using the commercial software MSC.MARC, and microstructures and fractographs were also observed. With the finite element models, the contours of temperature, equivalent plastic strain, and equivalent von Mises stress were simulated. It is observed that the fracture surfaces of internal cracks are covered by cleavage or quasi-cleavage facets. The region of internal cracks in the intergranular brittle fracture mode is in the mushy zone between the zero ductility temperature (ZDT) and the zero strength temperature (ZST). The simulated equivalent plastic strain in the crack region is 2.34%-2.45%, which is larger than the critical strain (0.4%-1.5%), and the equivalent von Mises stress is 1.84-5.05 MPa, which is within the range of critical stress (3.9-7.2 MPa), thus resulting in the occurrence of internal cracks. Reducing the soft reduction amount from 3 to 2 mm can lower the stress under the critical value.
To explicate the thermodynamics of the chromite ore lime-free roasting process, the thermodynamics of reactions involved in this process was calculated and the phrases of sinter with different roasting times were studied. The thermodynamics calculation shows that all the standard Gibbs free energy changes of the reactions to form Na 2 CrO 4 , Na 2 O·Fe 2 O 3 , Na 2 O·Al 2 O 3 and Na 2 O·SiO 2 via chromite ore and Na 2 CO 3 are negative, and the standard Gibbs free energy changes of the reactions between MgO, Fe 2 O 3 and SiO 2 released from chromite spinel to form MgO·Fe 2 O 3 and MgO·SiO 2 are also negative at the oxidative roasting temperatures (1 173− 1 473 K). The phrase analysis of the sinter in lime-free roasting process shows that Na 2 O·Fe 2 O 3 , Na 2 O·Al 2 O 3 and Na 2 O·SiO 2 can be formed in the first 20 min, but they decrease in contents and finally disappear with the increase of roasting time. The final phase compositions of the sinter are Na 2 CrO 4 , MgO·Fe 2 O 3 , MgO·SiO 2 and MgO. The results indicate that Na 2 CrO 4 can be formed easily via the reaction of Na 2 CO 3 with chromite ore. Na 2 O·Fe 2 O 3 , Na 2 O·Al 2 O 3 and Na 2 O·SiO 2 can be formed as intermediate compounds in the roasting process and they can further react with chromite ore to form Na 2 CrO 4 . MgO released from chromite ore may react with iron oxides and silicon oxide to form stable compounds of MgO·Fe 2 O 3 and MgO·SiO 2 , respectively.
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