Baofeng Guo

Static Recrystallization Behavior of 316LN Austenitic Stainless Steel

The static recrystallization of 316LN austenitic stainless steel was studied by double-pass hot compression tests on a Gleeble-3500 thermomechanical simulator. The specimens were compressed at the deformation temperatures of 950, 1050, 1150 °C, strain rates of 0,01, 0,1, 1 s−1, strains of 0.1, 0.15, 0.2, and intervals of 1–100 s. The results show that the volume fraction of static recrystallization of 316LN increases with the increase of deformation temperature, strain rate, strain and interval, which indicates that static recrystallization occurs easily under the conditions of higher deformation temperature, higher strain rate and larger strain. Deformation temperature has significant influence on static recrystallization of 316LN. The volume fraction of static recrystallization could easily reach 100% at higher deformation temperatures. By microstructure analysis, it can be concluded that the larger the volume fraction of static recrystallization, the more obvious the grain refinement. The static recrystallization activation energy of 317882 J/mol and the exponent n of 0.46 were obtained. The static recrystallization kinetics was established. The predicted volume fraction of static recrystallization is in good agreement with the experimental results.

Study on hot deformation behaviour of 316LN austenitic stainless steel based on hot processing map

Abstract 316LN is a type of austenitic stainless steel whose grain refinement only depends on hot deformation. The true stress–strain curves of 316LN were obtained by means of hot compression experiments conducted at a temperature range of 900–1200°C and at a strain rate range of 0·001–10 s−1. The influence of deformation parameters on the microstructure of 316LN was analysed. Both the constitutive equation for 316LN and the model of grain size after dynamic recrystallisation were established, and the effect of different deformation conditions on the microstructure was analysed. The results show that the suitable working region is the one with a relatively higher deformation temperature and a lower strain rate, in which the dynamic recrystallisation is finely conducted. Moreover, the working region that should be avoided during hot deformation was indicated.

Prediction of Critical Conditions for Dynamic Recrystallization in 316LN Austenitic Steel

Hot compression experiments conducted on a Gleeble-3500 thermo-mechanical simulator and metallographic observation tests were employed to study the critical conditions of dynamic recrystallization (DRX) of 316LN austenitic stainless steel. The true stress-true strain curves of 316LN were obtained at deformation temperatures ranging from 900 °C to 1200 °C and strain rates ranging from 0.001 s–1 to 10 s–1. Based on the above tests, the critical conditions of DRX were determined and compared with those obtained from work-hardening theory and the Cingara-McQueen flow stress model. Furthermore, the microstructure was observed to validate the calculated results. The ratio of critical strain to peak strain (εc/εp) for 316LN was determined, and the quantitative relationship between the critical strain and the deformation parameters of 316LN was elucidated. The results demonstrated that the onset of DRX corresponds to the constant normalized strain hardening rate (Γ), namely, the critical strain hardening rate Γc for 316LN is equal to 0.65.

Flow Stress Behaviors and Microstructure Evolution of 300M High Strength Steel under Isothermal Compression

The compressive deformation behaviors of 300M high strength steel were investigated over a wide range of temperatures (850–1200 °C) and strain rates (0.001–10 s−1) on a Gleeble-3800 thermo-mechanical simulator. The measured flow stress was modified by the corrections of the friction and the temperature compensations, which nicely reflect negative effects of the friction and temperature on the flow stress. The corrected stress-strain curves were the dynamic recrystallization type on the conditions of higher deformation temperature and lower strain rate. Flow stress increases with the increase of strain rate at the same deformation temperature and strain. By contrast, flow stress decreases with the increase of temperature at the same strain rate and strain. Dependence of the peak stress on temperature and strain rate for 300M steel is described by means of the conventional hyperbolic sine equation. By regression analysis, the activation energy (Q) in the whole range of deformation temperature is determined to be 367.562 kJ/mol. The effects of the temperature and the strain rate on microstructural evolution are obvious. With the increase of the deformation temperature and the decrease of the strain rate, the original austenite grain sizes of 300M steel increase. At the same time, the corrected flow stress curves more accurately determine the evolution of the microstructure.

Formation Mechanism of Inclusion Defects in Large Forged Pieces

Nonmetallic inclusions mixed into large forged metal objects destroy the continuity in the metal and affect the quality of the forged product. Research on how inclusions affect the plastic deformation of a matrix shows the significance of the formation mechanism of inclusion defects. For upset forging, the nonlinear finite element model was shown to be appropriate for the ingot hot-forging process by comparing the results with experiments involving plastic and hard inclusions inserted into the forged piece. The high-temperature stress-strain curves of MnS plastic inclusions were obtained experimentally. The results show how, during upsetting, the morphology of MnS plastic inclusions varies from spherical to ellipsoidal, until finally becoming flat in shape. The larger the inclusion is, the larger the degree of deformation of the inclusion is, and large inclusions enhance the risk of the final product failing to pass inspection for inclusion flaws. Strain significantly concentrates in the matrix near a hard inclusion. When the hard inclusion reaches a certain size, conical fractures form on both sides of the inclusion. To pass inclusion-flaw inspection and close hole defects to the extent possible, the flat-anvil upsetting is recommended. Finally, the inclusion-deformation state obtained by finite element simulation is verified experimentally.

Influences of Ribs on the Residual Stress and Deformation of Long Stringer Aluminum Alloy Forgings During Quenching

To improve the mechanical properties of aluminum alloy forgings, solution treatment and quenching is necessary. However, it becomes difficult to control the residual stress and deformation after solution treatment and quenching which always results in obtaining a part with an undesirable size, especially for a long stringer forging with an existing rib. Therefore, this paper demonstrates a quenching experiment and residual stress measurements for a ribbed aluminum alloy forging; the calculated results are close to the actual convective heat transfer coefficients. In addition, the heat transfer coefficient is introduced into the quenching simulation of a long stringer forging consisting of rib-web forging and plate forging. The influence of ribs on the residual stress and deformation of the forging is compared and analyzed. The results show that the heat transfer coefficient on the web without a rib is highest and the heat transfer coefficient on the web below the rib is lowest. Compared with the plate forging, the deformation direction of the rib-web forging is opposite, and the deformation of the rib-web forging is obviously increased.

Hot-Deformation Behavior and Hot-Processing Maps of AISI 410 Martensitic Stainless Steel

Abstract The compressive deformation behaviors of 410 martensitic stainless steel were investigated on a Gleeble-1500 thermomechanical simulator, and the experimental stress–strain data were obtained. The measured flow stress was corrected for friction and temperature. A constitutive equation that accounts for the influence of strain was established, and the hot-processing maps at different strain were plotted. The microstructure evolution of the hot-deformation process was studied on the basis of microstructural observations at high temperatures. Phase-transformation experiments on 410 steel were conducted at high temperatures to elucidate the effects of temperature on the delta-ferrite content. The initial forging temperature and optimum process parameters were obtained on the basis of the processing map and the changes in the delta-ferrite content at high temperatures.