Aging Precipitation And Hardening Behavior Of Cast Duplex Stainless Steel

Cast duplex stainless steel has good mechanical properties and corrosion resistance,and is widely used in the petrochemical,marine engineering and mineral/mineral processing industries.Due to its long-term service in a harsh environment where wear and corrosion coexist,the service life of the material is often affected by its low hardness and poor wear resistance.Therefore,It is very important and necessary to study the influence of compositions and heat treatment process on the structure,hardness,corrosion resistance and wear resistance for cast duplex stainless steel.It's helpful to provide a theoretical basis for material and process selection in different service conditons by exploring the relationship between composition,process,structure and performances.In this paper,thermodynamic equilibrium phase diagrams and CCT curves of CD-4MCu steel were simulated by using JMat Pro software,and the types,contents and compositional changes of precipitated phases at different temperatures were analysed.The microstructure,phase content and composition of specimens after different heat treatment processes are characterized by means of optical microscopy,scanning electron microscopy and EDS,X-ray diffraction analysis,and the hardness of the material in different states is tested to analyse the hardening behaviour of the material.The kinetic equations are used to calculate the kinetic parameters of the precipitated phases and to analyse the precipitation behaviour and kinetic changes.Changes in corrosion and wear resistance of the materials under different states are investigated by means of polarisation curves,electrochemical impedance spectroscopy and wear tests.The main conclusions are as follows:1)It was simulated by using JMat Pro software the existence of two phase microstructures of?-phase and?phase in the diagrams in the temperature range of 1000?to 1300?.With the increase of temperature,the amont of?-phase gradually increases,and the ratio of?-phase to?phase is 60:40 at 1100°C.At 850°C,the brittle phase?was precipited in the shortest time,with a maximum percentage of 47%.2)With the rising of solid solution temperature,the?-phase content in CD-4MCu steel increases.It is related to the increase of Cr element content in the?phase and the decrease of Ni element which would control the transformation of?phase.During medium temperature aging from 440°C to 520°C,there is no obvious change in the structure,only some elements of Cr and Mo are segregated in the?phase.During high temperature aging from temperature of 650?to 850?,the?phase and?₂ phase will precipitate within the?phase and at the boundary of two phase.?phase precipitates according to the???+?₂ mechanism,and the precipitation amount rises first and then decreases with the increasing of temperature.3)Both bulk hardness and microhardness of ferrite phase increase with the temperature of solution heat temperature,but austenite phase hardness changes very little.It's the main contribution for body hardness that increasing of ferritic phase contents and enhanced solid solution strengthening of the ferritic phase.The highest ageing hardness is up to 350HBW at 460°C,with the best ageing strengthening effect.After high temperature aging,the hardness decreases and is lower than the solid solution state due to over aged.The substrate hardness of aged state at 750?is equivalent to that of solid solution state4)The average ageing thermal activation energy in the range of 440°C to 520°C is225.9 k J/mol,with high ageing thermal activation energy,and the ageing precipitation is influenced by the diffusion and polarization of Cr elements in the?-phase.Aging in the range of 650°C to 850°C,with changes in the magnitude of n and K values related to?phase precipitation time and desolvation rate.As the aging temperature and time increase,the time index n decreases,the reaction constant K increases and the?phase precipitation rate increases,with the maximum precipitation rate at 750°C.5)Electrochemical analysis shows that specimen of solid solution state at 1100°C pocess the highest corrosion resistance.Corrosion resistance decreased after aging at460°C which is related to the biased aggregation of Cr and Mo elements in the?phase to produce a potential difference and reduce the stability of the passivation film.The worst state of corrosion resistance aging at 750°C is caused by the precipitation of Cr-rich and Mo-rich?phases and the depletion of Cr and Mo elements in the surrounding matrix,which reduces the stability of the passivation film and leads to a reduction in corrosion resistance.6)The analysis of wear resistance shows that samples of cast state and the solid solution state are basically equivalent in terms of the weight loss.The smallest weight loss happens at aging state of 460°C which meaning the best wear resistance.The worst wear resistance at 750°C.The wear resistance matches the pattern of hardness variation.