Alloy Design And Effect Of N/C Ratio On Creep And Solidification Behaviors Of Novel Nb-bearing Austenitic Heat-resistant Cast Steels

In order to comply with more stringent emissions and fuel economy regulations worldwide,the operation temperature of exhaust components is now reaching to as high as 1000 ℃.As a result,the incumbent materials are being pushed beyond their high-temperature strength and oxidation limitations.There is an urgent demand from automotive industries to develop novel and cost-effective alloys that are durable against these increased temperatures.Nb-bearing austenitic steels are potential candidates,owning to their improved oxidation and mechanical properties at 850 ℃.However,to date,limited researches have been conducted to investigate the mechanical property and solidification path of these alloys at temperatures higher than 1000 ℃,and the alloying effects of Mo,W,C and N on mechanical and solidification behaviors remains unclear.In order to establish and optimize the alloy design criterion for Nb-bearing austenitic cast steels and improve their mechanical properties,the current research was carried out in three steps:1)effects of macro（Mo,W）and micro（N）alloying elements on mechanical properties of austenitic cast steels at 1000 ℃ 2)alloy design and effects of N/C ratio on microstructures and creep behaviors of Nb-bearing austenitic cast steels and 3)effects of N/C ratio on solidification behaviors and paths of different phases in these alloys.The investigation on alloying effects of Mo,W and N is to clarify the phase fraction limitations for designing Nb-bearing austenitic cast alloys.Investigation results suggest that it is of limited effects to improve the creep properties of these alloys through Mo,W and high-N（>0.4%）additions.On the contrary,the excessive additions would deteriorate the creep resistance of these alloys.These alloying additions did not change the formation of Nb（C,N）,but significantly improved the precipitation of（Cr,Fe）₇C₃,（Cr,Fe）₂₃C₆,（Cr,Fe）₂（C,N）and x-phase.During creep deformation at 1000 ℃and 50 MPa,these Cr-rich phases grew and coalesced significantly and accelerated the nucleation and propagation of creep cracks.Therefore,the quantity of these Cr-rich phases should be strictly limited for the alloy design in the next step.The alloy design was carried out with concentrations on the alloying effects of C and N in Nb-bearing austenitic cast steels.The fractions of different phases(NbC/Nb（C,N）、（Cr,Fe）₂₃C₆、（Cr,Fe）₂（C,N）and δ-ferrite)were predicted and controlled using the CALPHAD（CALculation of PHAse Diagrams）methods,based on which the C and N contents were optimized.Four alloys with variant N/C ratios were designed accordingly and microstructural characterization results suggest that the measured quantities of various phases agreed favorably with the CALPHAD results.The morphology of NbC/Nb（C,N）was changed from the"Chinese-script" to a mixed flake-blocky morphology and then to the faceted blocky morphology as the N/C ratio increased and thus three microstructural models were established:script,flake-blocky and blocky.The script model alloys showed the best creep properties,owning to the "Chinese-script" NbC/Nb（C,N）that strengthened the grain boundary and interdendritic regions as well as the secondary nano-scale NbC/Nb（C,N）that pinned the dislocations in the austenitic matrix.（Cr,Fe）₂₃C₆ and δ-ferrite that accelerated the nucleation and propagation of creep cracks should be further limited.Investigations on the solidification behaviors of these three model alloys demonstrate that the solidification temperatures of NbC/Nb（C,N）and δ-ferrite were increased with increasing the N/C ratio,thereby altering the solidification paths as well as the solidified microstructures.The lamellar or "Chinese-script"NbC/Nb（C,N）was formed during the eutectic reaction at the end of solidification,while the peritectic reaction occurred as δ-ferrite solidified ahead of y-austenite.The solidification sequence of different phases（NbC/Nb（C,N）,8-ferrite andγ-austenite）predicted by CALPHAD methods agreed well with the directional solidification results.This indicates that it is feasible to predict the solidified structure of Nb-bearing austenitic cast steels using this CALPHAD method.Based on the above results and understandings,the relationship among composition,solidification path,microstructure and creep property was established,and thereby an alloy design method was developed based on the Integrated Computational Materials Engineering（ICME）methodology for austenitic heat-resistant cast steels.This method integrated multi-scale simulations and experiments into a holistic system of composition-processing-structure-property that can be used efficiently for developing advanced alloys and reduce development costs.