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

Hot compression experiments conducted on a Gleeble-3500thermo-mechanical simulator and metallographic observation tests were employed to study the critical conditions of dynamic recrystallization（DRX）of 316 LN austenitic stainless steel.The true stress-true strain curves of 316 LN were obtained at deformation temperatures ranging from 900℃to 1 200℃and strain rates ranging from 0.001s-1 to 10s-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 316 LN was determined,and the quantitative relationship between the critical strain and the deformation parameters of 316 LN was elucidated.The results demonstrated that the onset of DRX corresponds to the constant normalized strain hardening rate（Γ）,namely,the critical strain hardening rateΓcfor316LN 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 re gression 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 mierostructural 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 microstrueture.

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 inclusiondeformation state obtained by finite element simulation is verified experimentally.