Research On Hot Workability And Optimization Of Hot-working Parameters Of High Alloy Materials

High-quality special steels and alloys are basic materials for national key construction and high-end equipment manufacturing.Owing to the high quantity of alloy elements,these materials are typically characterized by high deformation resistance,narrow processing window and limited workability,which lead to difficulty in shaping and microstructure control during hot working.Therefore,in order to obtain the desired microstructure as well as to avoid the occurrence of defects,how to control and optimize hot working parameters is becoming a critical problem that remains to be solved in practical operations and a hot research topic of hot workability.In this thesis,the hot deformation behavior,microstructure evolution,development of constitutive model,flow softening and the underlying mechanisms,control and optimization of hot-working parameters of three high alloy materials,i.e.Ti-15-3 titanium alloy,Fe-23Mn-2Al-0.2C high-Mn steel and 253MA austenitic stainless steel,have been investigated.The innovative achievements and main research results are drawn as follows.(1)A systematic study was carried out on hot workability of Ti-15-3 titanium alloy,Fe-23Mn-2Al-0.2C high-Mn steel and 253MA austenitic stainless steel from both mechanical and microstructural points of view.Abnormal dynamic recrystallization(DRX)at high strain rates in austenitic steels was reported.Microstructural evolution maps of the investigated materials were developed.At the initial stage of straining,flow curves of Ti-15-3 titanium alloy showed discontinuous yielding.Magnitude of discontinuous yield drop increased with decreasing temperature and increasing strain rate in the strain rate range of 0.001-1s-1,while decreased dramatically at 10s-1.Dynamic recovery(DRV)was found to be the dominant restoration mechanism in the temperature range of 850-1150? and strain rate range of 0.001-1s-1,with limited or without any dynamically recrystallized gains in the microstructure.Flow curves of both Fe-23Mn-2Al-0.2C high-Mn steel and 253MA austenitic stainless steel exhibited a broad stress peak at lower strain rates,i.e.0.01 and 0.1s-1.At high strain rate of 20s-1,flow curves of the two investigated austenitic steels showed significant softening at strain higher than 0.4.It was also found that DRX became sluggish with increasing strain rate and was,however,more active at high strain rates.In the higher strain-rate regime,DRX was more active in Fe-23Mn-2Al-0.2C high-Mn steel,while in the low strain-rate regime DRX was more active in 253MA austenitic stainless steel.(2)The effects of deformation heating and friction on flow stress,which have largely been ignored in the literature,were discussed.An easy-to-use method for correcting deformation heating was proposed based on theoretical analysis.The as-measured flow stress data was corrected for both friction and deformation heating.It was found that it was still necessary to correct the eff'ect of friction on flow stress,even though MoS2 lubricant provided good lubrication during hot compression.An easy-to-use method for correcting defamation heating was proposed on the basis of theoretical analysis.As compared to the widely-used method,this approach can be utilized by merely using the as-measured data,without requiring determination of any parameter.Under the identical deformation condition,deformation heating had a more significant influence than friction on the flow stress.It was also found that,whether or not the correction sequence had a significant effect on flow stress depended largely on the level of friction.Good lubrication reduced the effect of correction sequence on flow stress.(3)An approach by dividing the stress regime into "high stress region" and "low stress region" was proposed to evaluate the key parameter in Sellars-Tegart-Garofalo(STG)model,thereby avoiding a misconception concerning the development of constitutive model.An important parameter,i.e.strain,was introduced into the constitutive model through the material parameters in STG model.High-accuracy constitutive models of the investigated materials were developed.An approach was suggested to divide the stress regime into "high flow stress region"and "low flow stress region".Accordingly,the values of n' and ? can be determined by using the corresponding equations.STG model was initially proposed to describe the dependence of steady-state flow stress on strain rate and temperature,and hence did not incorporate strain,which is an important parameter during hot deformation.In the present study,strain was introduced into constitutive models through the material parameters(??n?Q and In A)in STG model.A 4th-order(Ti-15-3 titanium alloy)or 5th-order(Fe-23Mn-2Al-0.2C high-Mn steel and 253MA austenitic stainless steel)polynomial was found to represent the influence of strain on material parameters with very good correlation and generalization,allowing for establishment of constitutive models with high accuracy.(4)A study on the flow softening in titanium alloy with higher stacking fault energy(SFE)and austenitic steels with lower SFE at high strain rates was carried out.The underlying softening mechanisms were clarified on the basis of microstructural characterization and model.The optimum hot-working windows of the investigated materials were identified.Flow softening of Ti-15-3 titanium alloy at higher strain rates was attributed to DRV,deformation heating and flow localization.DRV was an important restoration mechanism at high strain rate,with the activation energy for hot deformation being slightly higher than the activation energy for self-diffusion in pure ? titanium.Deformation heating was another important mechanism which resulted in thermal softening in the material,especially at lower temperatures and higher strain rates.Flow instabilities in the form of shear bands and flow localization occurred in the temperature range of 850-1000? and strain rate range of 1-10s-1.The optimum processing window was identified as to be 1050-1150? and 1-10s-1.The mechanisms responsible for flow softening in the two austenitic steels deformed at higher strain rates were DRX,deformation heating and localized deformation bands.The occurrence of DRX was confirmed by Electron Backscattered Diffraction(EBSD)analysis and the double-differentiation method.Deformation heating was also found to be an important mechanism accounting for significant decrease in the flow stress.Flow instabilities were observed in the form of localized deformation bands in the temperature range of 900-950? and strain rate range of 1-20s-1.The two austenitic steels are suitable to be hot-worked at 1050-1150? and 1-20s-1.(5)The relationships between two types of criteria based on dynamic materials model(DMM)were established on the basis of theoretical analysis and experimental verification.The applicability of various criteria was verified by means of microstructural observation,thus providing support for selection of criterion.It was found that the criteria based on the same theory(i.e.Gegel's criterion and Alexander-Malas's criterion based on Lyapunov functions or Kumar-Prasad's criterion,Murty-Rao's criterion and Babu's criterion based on Ziegler's flow theory)yielded a similar prediction of the unstable region.The predictions of criteria based on different theories were almost opposite.Based on microstructural observation,it was shown that Gegel's criterion and Alexander-Malas's criterion gave a poor prediction of the occurrence of flow instability,which could not be used in the present study.Kumar-Prasad's criterion,Murty-Rao's criterion and Babu's criterion reasonably predicted the onset of flow instability in Ti-15-3 titanium alloy,but overestimated the unstable flow in two austenitic steels,especially at higher temperatures and higher strain rates.Murty-Rao's criterion gave a more accurate prediction of the occurrence of flow instability.