Study Of Stress-relief Cracking Mechanism And Chemical Composition Modification Of New Low Alloy Heat Resistant Steel T23

T23 steel is a creep strength enhanced ferritic heat-resistant steel.Due to its high thermal conductivity,low thermal expansion coefficient,good weldability,and excellent creep property,T23 steel is regarded as an ideal material for manufacturing heating components of ultra-supercritical boilers as water walls,superheaters and reheaters.However,many leakages due to stress-relief cracking（SRC）occurred in water wall panels and reheaters manufactured using T23 tubes,which threatened the operational safety of power plants.At present,the SRC mechanism in T23 steel is still not clear,and there is no effective method for preventing SRC formation.Therefore,it is urgent to investigate the mechanism and influence of chemical composition on SRC and then explore the chemical composition modification of T23 steel for improving SRC resistance.Firstly,SRC mechanism in coarse-grained heat-affected zone（CGHAZ）of T23 steel was investigated.The implant test and isothermal slow strain rate tensile test were carried out to evaluate the SRC susceptibility of commercial T23 steel.Scanning electron microscopy（SEM）,transmission electron microscopy（TEM）and energy dispersive spectroscopy（EDS）were used to analyze the fracture surface morphology,crack characteristic,precipitation,and depletion of alloy element near the grain boundary to clarify the SRC mechanism.It was concluded that M₂₃C₆ carbide precipitated on grain boundaries resulted in significant grain boundary weakening,which is the major reason for SRC occurrence.M₂₃C₆ and M₇C₃ carbides precipitated inside grains enhanced the strength of grain interior,playing a minor role of causing SRC.Very few MX carbides precipitated inside grains during short-term tensile test,which did not affect SRC directly.The intra-granular M₂₃C₆ carbide promoted the void nucleation and resulted in depletion of alloy element in the matrix near the grain boundary,which accelerated the grain boundary weakening.Secondly,the microstructure evolution during aging at 650°C for 0-240 h were characterized and the SRC susceptibility of the aged CGHAZ was evaluated.The influence of microstructure on SRC was analyzed,which contributed to further understand the SRC mechanism.The CGHAZ of T23 steel exhibited a mixture microstructure of martensite and bainite with high hardness in as-welded condition.While after aging at 650°C,microstructure recovered,recrystallization occurred,the dislocation density decreased and lath width increased.In the early stage of aging（before 24 h）,the precipitations inside grain were mainly M₃C,M₇C₃ and a small amount of M₂₃C₆;while the precipitation on grain boundaries was M₂₃C₆.When aged for 24⁴8 h,MX precipitated inside grains extensively.Along with the aging proceeding,carbide precipitated and transformed as follows:M₃C→M₃C+M₇C₃+M₂₃C₆→M₃C+M₇C₃+M₂₃C₆+MX→M₂₃C₆+MX+M₆C.In a consequence of microstructure transformation,the hardness of CGHAZ dropped and the SRC resistance rose.The rise of SRC resistance can be attributed to the drop of grain interior strength and alloy elements homogenization.When the hardness dropped to lower than 250 HB,the CGHAZ did not susceptible to SRC.The 250 HB is a critical hardness to estimate the SRC susceptibility of T23 steel CGHAZ.After confirming that M₂₃C₆ precipitation on grain boundary was the major factor of SRC occurrence,the influences of carbon,boron,and titanium on precipitation and SRC were studied.The segregation behavior of boron on grain boundary was analyzed using electron probe micro analyzer（EPMA）.The SRC susceptibility reduced while reducing the carbon content.The reason was that a less carbon brought about a lower strength of grain interior and a less precipitation on grain boundaries.The two aspects helped to decrease the strength difference between grain interior and grain boundary,which helped improve the SRC resistance.Boron was prone to segregated on grain boundaries in as-welded CGHAZ.The segregation of boron contributed to strengthen grain boundary directly and suppress M₂₃C₆ precipitation and growth on grain boundary.It was effective for keeping the strength of grain boundary and reducing the strength difference of grain interior and grain boundary.Therefore,the plastic deformation could form inside grains,which improved the whole ductility and reduced SRC susceptibility.Titanium had a much lower effect on SRC compared to carbon and boron.The titanium did not affect the M₂₃C₆ precipitation directly because the precipitation of M₂₃C₆proceded the MX during reheating in T23 CGHAZ.Finally,the relationship between carbon,boron content and SRC susceptibility were established.Carbon and boron affected the SRC markedly,while controlling the carbon and boron content to satisfy:[%B]>-1.4×[%C]²+0.35×[%C]-0.0115,the modified T23 steel with high SRC resistance was obtained.After analyzing the influence of various elements,the following criteria for predicting the SRC susceptibility of T23 steel was proposed:F_S=10C+0.47Cr+0.14Mo+0.038W-58.9B-1.45,when F_S≥0,susceptible to SRC;when F_S<0,not susceptible to SRC.This thesis enriched and extended weldability theory of low alloy steel and solved the bottleneck problem in the application of the new low alloy heat resistant steel T23.It has important theoretical significance and practical value for the material application and development of high-parameter thermal power units.