Effect of gradient temperature rolling process on promoting crack healing in Q500 heavy plates

To ensure the quality of heavy plate products as determined by ultrasonic inspection, it is necessary to effectively control defects such as cracks and shrinkage cavities in heavy plates. Generally, some defects such as large size cracks exist due to insufficient deformation in the center of traditionally rolled plates. Compared with the traditional rolling process, gradient temperature rolling(GTR) process can effectively increase deformation inside heavy plates. In this study, the effect of GTR on crack healing was analyzed through a comparison experiment with the uniform temperature rolling(UTR). The results show that the GTR process could increase the plastic strain inside the heavy plate and effectively promote the healing process of the preset cracks. The degrees of crack healing at the center and quarter thickness position of the steel plate via GTR were greater than twice those of the plate via UTR. The GTR process can significantly reduce the internal defects of heavy plates and improve the defect detection level of heavy plate products. Also, The GTR process results in the formation of new crystal grains in the crack region, which is crucial to crack healing.

Effect of Rolling Process on Microstructure and Properties of 95CrMo Drill Steel

In order to improve the strength and toughness of 95CrMo steel and explore a short flow process, a direct rolling process was employed, and the effect of finishing rolling process on microstructure and mechanical properties was investigated. The results show that, with the decrease of finishing rolling temperature, inter-lamellar spacing in- creases, the strength as well as hardness has a general increase, and secondary cementite is distributed uniformly but represents a remarkable decrease in size. Based on Hall-Petch type equation, an effective relationship of yield strength and pearlite structure parameters was established. The correlation coefficients between the measured and calculated strength were more than 0.95, which indicated a high reliability of the relationship. By analyzing the individual strength contributions of pearlite structures, yield strength was found to have a more great dependence on pearlite inter-lamellar spacing than colony size.

Influence of Mo on Growth and Coarsening of Nanometer-sized Carbides in Low-alloy Ferritic Steels Containing Ti

By optical microscopy, transmission electron microscopy and energy dispersive spectroscopy, the changes with aging time in size, composition and distribution of nanometer-sized （Ti, Mo）C precipitated in Ti-Mo low-alloy ferritc steel have been studied in comparison with that of nanometer-sized TiC precipitated in Ti low-alloy ferritc steel. It was found that the growth rate of （Ti,Mo）C in Ti-Mo steel was less than that of TiC in Ti steel. Nanometer- sized carbides formed at 650 ~C （or 550 ~C） for 55 h were at transitional stage from growth to coarsening. When aging time reaches 55 h, the coarsening rates of nanometer-sized carbides in Ti and Ti-Mo steel tend to be the same and in- variable. The influence of Mo on growth and coarsening of nanometer-sized carbides tends to decrease with increasing aging time, and Mo contents in nanometer-sized carbides with the same size at different aging time were different.

Effect of tempering temperature on strain hardening exponent and flow stress curve of 1000MPa grade steel for construction machinery

To study the effect of tempering temperature on strain hardening exponent and flow stress curve,one kind of 1000 MPa grade low carbon bainitic steel for construction machinery was designed,and the standard uniaxial tensile tests were conducted at room temperature.A new flow stress model,which could predict the flow behavior of the tested steels at different tempering temperatures more efficiently,was established.The relationship between mobile dislocation density and strain hardening exponent was discussed based on the dislocation-stress relation.Arrhenius equation and an inverse proportional function were adopted to describe the mobile dislocation,and two mathematical models were established to describe the relationship between tempering temperature and strain hardening exponent.Nonlinear regression analysis was applied to the Arrhenius type model,hence,the activation energy was determined to be 37.6kJ/mol.Moreover,the square of correlation coefficient was 0.985,which indicated a high reliability between the fitted curve and experimental data.By comparison with the Arrhenius type curve,the general trend of the inverse proportional fitting curve was coincided with the experimental data points except of some fitting errors.Thus,the Arrhenius type model can be adopted to predict the strain hardening exponent at different tempering temperatures.

Effect of reduction pretreatment process on evolution of micro-porosity in 42CrMo billet

Reduction pretreatment(RP)process,which is implemented after the billet is completely solidified,is an alternative process to further improve billet quality.Finite element method and experiment were used to investigate the effect of the RP process on the internal quality of 42CrMo billet.An ultrasonic testing technology was used to study the influence of the RP process on the micro-porosities.Through a three-dimensional finite element model,the mechanism of the RP process on the porosity closure was studied.The experimental results show that the RP process eliminates the macro-porosities of the billet center.The number of micro-porosities in the billets is effectively reduced,and the increase in deformation contributes to the reduction in micro-porosities.Compared with isothermal rolling,the RP process leads to a larger equivalent strain and hydrostatic integration in the billet center because of the temperature gradient,and the effect of the temperature gradient is greater with the increase in deformation.The closure effect of micro-porosity is related not only to the stress state,but also to the distribution of micro-porosity.The increase in three-dimensional compressive stress is beneficial to the porosity closure.If the micro-porosities distributed densely,excessive stress could induce micro-porosity coalescence,which has the risk of inducing cracks.

Prediction for Flow Stress of 95CrMo Hollow Steel During Hot Compression

The compressive deformation behavior of 95CrMo hypereutectic steel was studied at temperatures ranging from 800 to 1050 ℃ and strain rates from 0.1 to 3 s-1 on a Gleeble-3500 thermo-simulation machine. The results showed that, with the decrease in deformation temperature and increase in strain rate, the fragmented retained austenite in finer and distributed more uniformly in the ferrite matrix as a result of the inhibited recovery. The recorded flow stress suggested that the stress level decreases with increasing temperature and decreasing strain rate. Based on the classical stress-dislocation relation, the constitutive equations of flow stress determined by work-hardening and softening mechanisms were estab- lished. A comparison between the experimental and calculated values confirmed the reliability of the model, and the predictability of the model was also quantified in terms of correlation coefficients and average absolute relative errors, which were found generally above 0.99 and below 2.50%, respectively. In the whole range of strain rate, the activation energy is 419.84 kJ/mol. By further identification based on Sch6ck＇s model and Kocks-Argon-Ashby model, the rate- controlling mechanism is found to be dislocation cross-slip.

A Flow Stress Model for High Strength Steels with Low Carbon Bainite Structure

Two kinds of steels （YP960 and YP690） with low carbon bainite structure were designed, and their flow stress and strain hardening exponents were studied. The results showed that, when Hollomon relation was applied to descrihe the flow stress, there were significanl errors between the experimental and calculated points in specimens tempered below 400 ℃, while a high precision was ohserved in samples tempered above 400℃. Whereas, the modijied Voce relation could effectively predici the flow stress as well as the strain hardening exponent at different tempe ring temperatures, which was verified by unbiased estimators such as maximum relative error （MRXE） and average ahsolute relative error （AARE）. Besides, the modified Voee relation was also applied to estimate the maximum uniform strain, and the correlation coefficients （R） between the experimental data and calculated maximum uniform strain were more than 0.91. The high correlation coefficients indicated that the modified Vote relation could effec lively predict the uniform deformation ability of high strength steels with low carbon bainite structure at different tempering temperatures.

Effect of high-temperature reduction pretreatment on internal quality of 42CrMo casting billet

Comparative analysis was conducted to illustrate the effect of high-temperature reduction pretreatment on the internal quality of 42CrMo casting billet.Analysis of macrostructure clarifies that reduction pretreatment could reduce the size of central cavities and equiaxed crystals and improve the segregation degree and porosity on statistics.Lath bainite matrix and granular structure on segregation bands mainly dominate the microstructure of original billet,while pro-eutectoid ferrite and its accompanying troostite are added due to the application of reduction pretreatment.Additionally,C,Mn,and Mo are suggested to be enriched in segregation bands where pores and inclusions tend to be generated.By clarifying the microstructure evolution during cooling process,physical compression and multiplication of prior austenite grain boundaries caused by recrystallization are regarded as the main reasons in improving the internal quality of 42CrMo billet.