Creep properties and long-term rupture strength prediction of 11Cr-5.5Co-2.5W martensitic creep-resistant steel
The evaluation of long-term creep rupture strength and the microstructural stability of creep-resistant steels at high temperature remains a critical concern in the industry.An 11Cr-5.5Co-2.5W martensitic heat-resistant steel(mass percent,%)without nitrogen addition was investigated.The short-term creep rupture tests were conducted under various stresses at temperatures of 650,675,700,and 725℃.Analyses of microstructure and precipitates of the steel were performed using OM,SEM,EDS,XRD,TEM and etc.Based on a new creep model and the Monkman-Grant relationship,the long-term creep rupture strengths of the steel at 105 h of service at 600 and 650℃were assessed,and compared with that of P92 steel under identical conditions.The results revealed that fol-lowing normalizing at 1 100℃for 1 h and tempering at 780℃for 1 h,the steel exhibited a fully tempered martens-itic structure with primary precipitates of M23C6 and MX.Laves phases were observed after creep tests,while no Z-phase or other phases were detected.Based on the short-term accelerated creep tests(creep rupture time less than 5 000 h),the 105 h creep rupture strength values at 600 and 650℃were predicted to be 108 and 52 MPa,respectively.Quantitative analysis of the microstructures and precipitates in the creep-ruptured specimens showed that within the test stress ratio σ/σTS(ratio of stress to tensile strength)range(0.229-0.493),the coarsening rate of martensite lath and precipitates sizes at the boundaries showed no significant change with the decrease of the stress ratio term,dem-onstrating the feasibility of investigating the creep behavior and microstructural evolution of martensitic heat-resistant steel by using the new creep model.Moreover,although this steel exhibited higher high-temperature tensile strengths compared to P92 steel under the same conditions,the predicted 105 h creep rupture strengths were slightly lower.The difference can be attributed to the distinct strengthening mechanisms and microstructural evolutions dur-ing creep for the two steel grades.
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