Research On Hot Deformation Behavior Of High Nitrogen Austenitic Stainless Steel For Non-magnetic Drill Collars

With the rapid development of China's industry,oil,natural gas and other resources have been excessively consumed,and their reserves gradually decrease,resulting in the increase of drilling depth and the complex of the drilling environment.High nitrogen austenitic stainless steel is widely used in the manufacture of deep oil and gas drilling and production equipment because of its excellent non-magnetic properties,corrosion resistance,mechanical properties and fatigue properties.However,the research and development of high nitrogen austenitic stainless steel started late in China,and there are still many manufacturing problems,among which the cracking during hot working is the key problem to limit its industrial production.Therefore,it is of great significance to clarify the hot deformation behavior and optimize the hot working process for promoting the localization of high nitrogen austenitic stainless steel.In this study,the single-pass hot compression test was carried out by Thermomaster-Z thermal simulator,and the hot deformation behavior of high nitrogen austenitic stainless steel P550 for non-magnetic drill collars was studied systematically.Based on the analysis of the flow stress curve,the optimal constitutive model for P550 was established.The deformation microstructure was analyzed by metallographic microscope and transmission electron microscope,and the effects of deformation temperature and strain rate on dynamic recrystallization were revealed.According to the theory of the dynamic material model,the hot working diagram was established.The deformation microstructures in the stable and unstable regions were analyzed by EBSD and TEM.The instability mechanism was clarified and the optimal hot working window was obtained.The main conclusions are obtained as follows:The flow stress curve of high nitrogen austenitic stainless steel P550 was a typical dynamic recrystallization curve,and the thermal deformation resistance decreased with the increase of deformation temperature or the decrease of strain rate.Meanwhile,the prediction accuracy of the original Johnson-Cook(JC)model,exponential JC model,coupled JC model and Arrhenius model was evaluated.It was found that the Arrhenius model had the highest prediction accuracy(R=0.9827,AARE=5.19%).So the Arrhenius model was selected as the final prediction model of P550,and its hot deformation activation energy(Q)was 850.54 kJ·mol-1.With the increase of deformation temperature,the degree of dynamic recrystallization increased.This was due to the fact that increasing temperature provided more sufficient energy for the hot deformation,which accelerated the annihilation of dislocation and the nucleation of dynamic recrystallization grains.With the increase of strain rate,the degree of dynamic recrystallization decreased first and then increased.This was due to the fact that the critical dislocation density of dynamic recrystallization was low and recrystallization was easy to take place at a low strain rate(0.05 s-1).Increasing strain rate(0.5 s-1)resulted in a decrease of deformation time and the increase of the critical dislocation density of dynamic recrystallization,which increased recrystallization difficulty.When the strain rate was further increased(5.0 s-1),the occurrence of adiabatic heating led to higher local deformation temperature,which promoted the dynamic recrystallization.Therefore,at high deformation rate,the deformation time was shortened,and its inhibition effect on dynamic recrystallization was less than of the promotion effect of adiabatic heating on dynamic recrystallization.Thus,the dynamic reciystallization was promoted.Otherwise,the dynamic recrystallization was inhibited.At low temperature and low strain rate(900?,0.05 s-1),low deformation temperature was the main cause of flow instability.Low deformation temperature caused insufficient for deformation energy,which suppressed the dislocation movement and the transition from low angle grain boundary to high angle grain boundary(mainly ?3 or ?9 grain boundary),which hindered the dynamic recrystallization.At medium temperature and high strain rate(1100?,5.0 s-1),high strain rate was the main cause of flow instability.High strain rate would cause adiabatic heating,leading to local concentrated deformation and flow instability.At high temperature and low strain rate(1200?,0.05 s-1),low strain rate was the main cause of flow instability.The low strain rate extended deformation time and increased the grain confusion degree,resulting in deformation instability.Based on the hot working map and microstructure analysis,the optimal hot working window of P550 was:1100?1200?,0.5?4.5 s-1.