Influence of High Temperature Pre-Deformation on the Dissolution Rate of Delta Ferrites in Martensitic Heat-Resistant Steels

Junru Li,Jianjun Liu,Bo Jiang,Chaolei Zhang,Yazheng Liu 대한금속·재료학회 2017 METALS AND MATERIALS International Vol.23 No.2

The dissolution process of delta ferrites and the influence of high temperature pre-deformation on the dissolutionrate of delta ferrites in martensitic heat-resistant steel 10Cr12Ni3Mo2VN were studied by isothermalheating and thermal simulation experiments. The precipitation temperature of delta ferrites in experimentalsteel is about 1195 °C. M23C6-type carbides incline to precipitate and coarsen at the boundaries of delta ferritesbelow 930 °C, and can be rapidly dissolved by heating at 1180 °C. The percentage of delta ferrites graduallydecreases with heating time. And a Kolmogorov-Johnson-Mehl-Avrami equation was established to describethe dissolution process of delta ferrites at 1180 °C. High temperature pre-deformation can markedly increasethe dissolution rate of delta ferrites. Pre-deformation can largely increase the interface area between delta ferriteand matrix and thus increase the unit-time diffusing quantities of alloying elements between delta ferritesand matrix. In addition, high temperature pre-deformation leads to dynamic recrystallization and increases thenumber of internal grain boundaries in the delta ferrites. This can also greatly increase the diffusing rate ofalloying elements. In these cases, the dissolution of delta ferrites can be promoted.

Prediction Model of Austenite Growth and the Role of MnS Inclusions in Non- Quenched and Tempered Steel

Bo Jiang,Meng Wu,He Sun,Zhilin Wang,Zhigang Zhao,Yazheng Liu 대한금속·재료학회 2018 METALS AND MATERIALS International Vol.24 No.1

The austenite growth behavior of non-quenched and tempered steels (casted by continuous casting and moldingcasting processes) was studied. The austenite grain size of steel B casted by continuous casting process is smallerthan that of steel A casted by molding casting process at the same heating parameters. The abnormal austenitegrowth temperature of the steels A and B are 950 °C and 1000 °C, respectively. Based on the results, the models forthe austenite grain growth below and above the abnormal austenite growth temperature of the investigated steelswere established. The dispersedly distributed fine particles MnS in steel B is the key factor refining the austenitegrain by pinning the migration of austenite grain boundary. The elongated inclusions MnS are ineffective in preventingthe austenite grain growth at high heating temperature. For the non-quenched and tempered steel, the continuouscasting process should be adopted and the inclusion MnS should be elliptical, smaller in size anddistributed uniformly in order to refine the final microstructure and also improve the mechanical properties.

Microstructure Evolution, Fracture and Hardening Mechanisms of Quenched and Tempered Steel for Large Sized Bearing Rings at Elevated Quenching Temperatures

Bo Jiang,Zhen Mei,Leyu Zhou,Chaolei Zhang,Yazheng Liu 대한금속·재료학회 2016 METALS AND MATERIALS International Vol.22 No.4

Based on 42CrMo steel, a steel with a higher C and Ni content is developed for use in large sized bearing rings. The impact energy and hardness of the quenched and tempered steel increase with the quenching temperature, but then decrease when the temperature is above 925 °C. When the temperature is below 925 °C, some larger M23C6-type carbides (with average diameters of 255.6 μm) exist in the quenched and tempered microstructure. The number of carbides is reduced as the quenching temperature increases. At the same time, the fracture modes change from microvoid coalescence and quasi-cleavage to microvoid coalescence. The number of round quasi-cleavage fractures, which are formed around the carbides, decrease as the number of carbides decrease. The existence of larger M23C6-type carbides leads to round quasi-cleavage fractures and decrease the impact energy. The precipitation strengthening of M23C6-type carbides increases the hardness at a quenching temperature of 925 °C.

Effect of Transformation Temperature on the Ferrite–Bainite Microstructures, Mechanical Properties and the Deformation Behavior in a Hot-Rolled Dual Phase Steel

Bo Jiang,Xuewen Hu,Leyu Zhou,Haibo Wang,Yazheng Liu,Fugang Gou 대한금속·재료학회 2021 METALS AND MATERIALS International Vol.27 No.2

The ferrite–bainite microstructures and mechanical properties in a hot-rolled dual phase steel with different transformationtemperatures were investigated in this paper. The deformation behavior of bainite and ferrite was analyzed by using in situscanning electron microscope. The results showed that the ferrite and bainite can be refined and the volume fraction of bainitecan be slightly increased by 5% by decreasing the ferrite transformation temperature from 690 to 635 °C and increasing thebainite transformation temperature from 400 to 450 °C. The strengthening mechanism was dislocation difference in polygonaland acicular ferrite and then the grain size difference of ferrite. The hole expansion ratio increased from 52.9 to 83.7% withthe ferrite transformation temperature decreasing. The cracks or voids were smaller in size and also the number in steel withthe lower ferrite transformation temperature after expanding. The microstructure separation along the boundary betweenbainite and ferrite and the slip band in the ferrite appeared at the non-uniform deformation stage. However, fewer appearedin the microstructure of steel with lower ferrite transformation temperature. Cooperative deformation of ferrite and bainiteand finer microstructure in steel with lower ferrite transformation temperature were the mechanisms why the cracks couldnot easily nucleated and propagated, respectively. Thus this was also why there was a better hole expansion ratio.

Microstructural Characterization and Softening Mechanism of Ultra-Low Carbon Steel and the Control Strategy in Compact Strip Production Process

Bo Jiang,Xuewen Hu,Guoning He,Huan Peng,Haibo Wang,Yazheng Liu 대한금속·재료학회 2020 METALS AND MATERIALS International Vol.26 No.9

In this paper, the microstructures and properties of hot rolled ultra-low carbon steel sheet produced by different compact stripproduction (CSP) processes were investigated. The softening mechanism was also discussed and the control strategy wasproposed in order to obtain optimum properties. Result showed that the average ferrite grain sizes of austenite rolling sheetand multiphase rolling sheet were 31.0 μm and 74.6 μm, respectively. The sheet after austenite rolling had a slightly higheryield and tensile strength while had a 6.3% higher elongation than that of the sheet after multiphase rolling. The higher dislocationin the sheet after multiphase rolling increased the strength while decreased the elongation. The softening mechanismof the sheet after multiphase rolling was the coarsening of ferrite grain. The combined role of {001} and {111} orientationresulted in a slight increase of the r and ̄rvalue in the sheet after multiphase rolling. It was a wise choice to conduct rollingat the Ac1temperature in CSP process to increase the grain size and decrease the dislocation density. Then, the strength ofthe sheets could be further reduced and the elongation could also be improved.

Exploring the Heat Treatment Parameters, Microstructural Evolution, and Mechanical Properties of Ti–6Al–4V Alloy Fabricated by a Selective Laser Melting Process

Xing Gao,Ning Zhang,Yi Zhang,Mingrui Chen,Yan Ding,Bo Jiang,Yazheng Liu 대한금속·재료학회 2022 METALS AND MATERIALS International Vol.28 No.11

The effects of heat treatment parameters on the mechanical properties of a selective laser melting Ti–6Al–4V alloy werestudied using an L9 (34) type orthogonal test. Results showed that the solution temperature and aging temperature were themost influential factors relative to the mechanical properties. The optimum heat treatment was then obtained as 920 °C/2 h/water quenching + 550 °C/3 h/air cooling and the corresponding tensile strength was 1045.2 MPa and the elongation was13.6%. Compared to a solution temperature of 850 °C, more fine secondary α phase (αS) and higher geometrically necessarydislocations (GND) density were obtained when solution temperature was 920 °C, which promoted the increased strengthby increasing the interface strengthening effect of αS/β and the dislocation strengthening. However, the microstructure of thecontinuously coarsening αGB resulted in decreased elongation when the solution was at 920 °C. As the aging temperatureincreased from 550 to 650 °C, the GND density was reduced and the microstructures of primary α (αP), αS with variouscrystallographic orientations, lamellar α, and equiaxial αP were coarsened. Therefore, the elongation of the specimen agingat a higher aging temperature increased while the strength decreased.