11Cr-5.5Co-2.5W马氏体耐热钢蠕变性能与长时持久强度预测

Creep properties and long-term rupture strength prediction of 11Cr-5.5Co-2.5W martensitic creep-resistant steel

耐热钢在高温环境下的长时蠕变持久强度评估和微观组织稳定性研究一直是业界关注的焦点。为此,以一种未添加氮的11Cr-5.5Co-2.5W(质量分数,%)马氏体耐热钢为研究对象,开展了650、675、700、725℃不同应力条件下的短时蠕变断裂试验,通过OM、SEM、EDS、XRD、TEM等多种表征技术,对试验钢微观组织和析出相进行分析;基于新蠕变模型结合Monkman-Grant关系式,对该耐热钢在600、650℃下服役10~5 h的持久强度进行评估,并将预测结果与同条件下的P92钢进行了比较。结果显示,该耐热钢经过正火(1100℃保温1 h)和回火热处理(780℃保温1 h)后,得到了全马氏体组织,析出相主要为M_(23)C_(6)、MX相,在蠕变试验后钢中还观察到了Laves相,未发现Z相等其他相生成。基于短时蠕变试验结果(断裂时间短于5000 h),通过新蠕变模型结合Monkman-Grant关系式预测得到,该钢在600、650℃下服役10~5 h的持久强度依次为108、52 MPa;蠕变试样微观组织和析出相定量分析结果显示,在测试应力比σ/σ_(TS)(应力/拉伸强度)区间(0.229~0.493)内,马氏体板条和边界处析出相尺寸的粗化速率随应力比项的降低无明显变化,表明利用新蠕变模型研究马氏体耐热钢的蠕变行为与微观结构演变之间的关系是可行的。此外,与P92钢相比,虽然在相同条件下该耐热钢展示了更高的高温抗拉强度,但其10~5 h蠕变持久强度的预测结果略低,这一差异主要与钢在2个过程中的强化机制和微观结构演变不同相关。

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 heatresistant 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 10~5 h of service at 600 and650℃were assessed,and compared with that of P92 steel under identical conditions.The results revealed that following normalizing at 1100℃for 1 h and tempering at 780℃for 1 h,the steel exhibited a fully tempered martensitic structure with primary precipitates of M_(23)C_(6) 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 5000 h),the 10~5 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 ratioσ/σ_(TS)(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,demonstrating 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 10~5 h creep rupture strengths were slightly lower.The difference can be attributed to the distinct strengthening mechanisms and microstructural evolutions during creep for the two steel grades.