Yazheng Liu

Correspondence between hydrogen enhancing dezincification layer-induced stress and susceptibility to SCC of brass

Dezincification layer formed during corrosion or stress corrosion cracking (SCC) of brass in the ammonia solution could induce an additive stress. The effect of hydrogen on the dezincification layer-induced stress and the susceptibility to SCC were studied. The dezincification layer-induced stress was measured using the deflection method and the flow stress differential method. The susceptibility to SCC was measured using a slow strain rate test. Results showed that both the dezincification layer-induced stress and the susceptibility to SCC increased with increasing hydrogen concentration, and the hydrogen concentration dependence of the susceptibility to SCC was in a good agreement with that of the dezincification layer-induced stress. Hydrogen in brass facilitates the selective dissolution of Zn in the ammonia solution, and then enhances the dezincification layer-induced stress.

Microstructure evolution, fracture and hardening mechanisms of quenched and tempered steel for large sized bearing rings at elevated quenching temperatures

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.

Prediction model of austenite growth and the role of MnS inclusions in non-quenched and tempered steel

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

Influence of high temperature pre-deformation on the dissolution rate of delta ferrites in martensitic heat-resistant steels

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

Analysis of the Formation of Surface Crack on Crankshaft After Die Forging

The causes are investigated for the formation of surface crack on a crankshaft based on numerical simulation, light microscopy and SEM analysis in this study. It is concluded that inclusion is not the reason for the crack initiation. Coarse microstructure and shrinkage cavities were found in the centre of billet, and then were observed to flow to the crack by 3D simulation analysis of forging process. In order to identify the simulation result, the size and distribution of prior austenitic grain and shrinkage porosities at the crack on the crankshaft were analyzed. Results show that the size of prior austenitic grain and shrinkage porosities and the volume fraction of shrinkage porosity are larger at the crack compared with that at other part of the crankshaft. The work shows that the coarse microstructure and shrinkage cavities flowing from the centre of the billet is the cause of the surface crack during the induction hardening process.

Surface decarburization behavior and its adverse effects of air-cooled forging steel C70S6 for fracture splitting connecting rod

Surface decarburization behavior and its adverse effects of air-cooled forging steel C70S6 for automobile engine fracture splitting connecting rod were investigated comprehensively by mechanical properties, microstructure and fracture morphology analysis. The results show that the surface decarburization in the outer surface of the fracture splitting at the big end bore and the micro-cracks in the decarburized layer are result in the uneven and spalling fracture surfaces of the waster connecting rod product. Besides, partial decarburization is produced between 900 °C and 1250 °C for heating 2 h, and decarburization sensitivity reach maximum at 1150 °C, but no complete decarburization forms for heating 2 h at 650-1250 °C. The decarburized depth follows a parabolic law with the increase of the heating time from 0.5 h to 12 h, and the decarburization sensitivity coefficient is 2.05×10-5 m·s-1/2 at 1200 °C. For the connecting rod manufacturing, surface decarburization must be under effective control during the hot forging process but not the control cooling process.