Correlation of hardness with creep rupture strength, allowable stress and service/remaining life of Grade 91 heat-resistant steel
PENG Zhi-fang1, LIU Sheng1, YANG Hua-chun2, YANG Chao3, WANG Jia-qing4
1. School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, China; 2. Materials Research Institute, Dongfang Boiler Group Co., Ltd., Dongfang Electric Corporation, Zigong 643000, Sichuan, China; 3. Testing Center, Jiangsu Frontier Electric Technology Co., Ltd., Nanjing 211102, China; 4. East China Electric Power Test and Research Institute, China Datang Group Science and Technology Research Institute Co., Ltd., Hefei 230088, China
Abstract：The creep rupture strength data of Grade 91 steel specimens with various hardness values were used to study the correlation of hardness with creep rupture strengths and maximum allowable stresses of the steel. The results show that creep rupture tests at a certain temperature and stress applied to the specimens with a series of hardness values receive the results in both overestimation and underestimation of rupture properties, leading to some unrealistic effect on 105h creep rupture strengths determined in this way. An approach was thus proposed to determine the lower limits of hardness satisfying the maximum allowable stresses at given temperatures. It was found with this method that a hardness level of ≥ 201(205) HBW of any of the Type 1- and Type 2-Grade 91 components running at a temperature of ≤ 575(600)℃ can satisfy the requirement of the maximum allowable stresses at the corresponding temperature specified by ASME BPVC 2019, and a hardness value of ≥ 204HBW is effective for the grade 91 components with a wall-thickness of ≥ 75 mm running at a temperature of ≤ 575℃ to satisfy the requirement of the maximum allowable stresses at the corresponding temperature specified by ASME BPVC 2017. Therefore, the most recently modified specification brings, to some extent, about difficulty in continuously practicing the application of the lower limit of hardness values (190-250HBW) specified by ASME BPVC 2017-2019 because it is not satisfied with the requirement on the maximum allowable stress at some given temperatures. Thus, there is a need to raise the lower limit of hardness values to settle this issue in the future. In addition, the optimization of the function fitting the creep-rupture data currently used in the estimation calculation of service/remaining lives was studied, showing that the tendency of overestimation of rupture properties can be reduced by replacing the current power function with the logarithm one. Quite good fitness of the practical data with the logarithm function curves is contributed to separating the whole data group with a series of hardness values into the higher and lower hardness level groups in calculation. On this basis, the relationship of thickness, hardness and service life of the components with variable dimensions and hardness values can be obtained by integrating the technical parameters of the safety assessment, which is able to reflect the applicability, reliability and intuitiveness of this combination. The above results can be used as reference for both the revisions of the relevant technical standards and the practical applications of industry.
彭志方, 刘省, 杨华春, 杨超, 王家庆. Grade 91耐热钢的硬度与持久强度、许用应力和运行/剩余寿命的相关性研究[J]. 材料工程, 2021, 49(9): 109-118.
PENG Zhi-fang, LIU Sheng, YANG Hua-chun, YANG Chao, WANG Jia-qing. Correlation of hardness with creep rupture strength, allowable stress and service/remaining life of Grade 91 heat-resistant steel. Journal of Materials Engineering, 2021, 49(9): 109-118.
 BRETT S J, ALLEN D J, PACEY J. Failure of a modified 9Cr header endplate[C]// Proceedings of Conference on Case Histories in Failure Investigation. Milan: [s.n.], 1999: 873-884.  KIMURA K, KUSHIMA H, ABE F. Improvement of creep life prediction of high Cr ferritic creep resistant steels by region partitioning method of stress vs time to rupture diagram[J]. Journal of the Society of Materials Science Japan, 2003, 52(1): 57-62.  ARMAKI H G, MARUYAMA K, YOSHIZAWA M, et al. Prevention of the overestimation of long-term creep rupture life by multiregion analysis in strength enhanced high Cr ferritic steels[J]. Materials Science and Engineering: A, 2008, 490(1/2): 66-71.  CHEN R P, ARMAKI H G, YOSHIMI K, et al. Premature creep rupture and overestimation of rupture life in modified 9Cr-1Mo steel[J]. Tetsu-To-Hagane, 2010, 96(9): 564-571.  WILSHIRE B, SCHARNING P J. A new methodology for analysis of creep and creep fracture data for 9-12% chromium steels[J]. International Materials Reviews, 2008, 53(2): 91-104.  BENDICK W, CIPOLLA L, GABREL J, et al. New ECCC assessment of creep rupture strength for steel grade X10CrMoVNb9-1 (Grade 91)[J]. International Journal of Pressure Vessels and Piping, 2010, 87(6): 304-309.  KIMURA K, YAGUCHI M. Re-evaluation of long-term creep strength of base metal of ASME grade 91 type steel[C]//ASME 2016 Pressure Vessels and Piping Conference.Vancouver, British Columbia: ASME, 2016, 50435: V06BT06A014.  KIMURA K, TAKAHASHI Y. Evaluation of long-term creep strength of ASME Grades 91, 92, and 122 type steels[C]//ASME 2012 Pressure Vessels and Piping Conference. Toronto, Ontario: ASME, 2012, 55058: 309-316.  彭志方, 党莹樱, 彭芳芳. 9%-12%Cr铁素体耐热钢持久性能评估方法的研究[J]. 金属学报, 2010, 46(4): 435-443. PENG Z F, DANG Y Y, PENG F F. Study on creep-rupture property assessment method for 9%-12%Cr ferritic heat-resistant steels[J]. Acta Metallurgica Sinica, 2010, 46(4): 435-443.  PENG Z F, DANG Y Y, PENG F F. On creep-rupture property assessment for 9-12% Cr ferritic heat-resistant steels[C]//Advances in Materials Technology for Fossil Power Plants: Proceedings from the 6th International Conference.Materials Park, Ohio:ASM International, 2011: 705-714.  PARKER J, SIEFERT J, SHINGLEDECKER J. The benefits of improved control of composition of creep-strength-enhanced ferritic steel Grade 91[R]. Palo Alto, CA: EPRI, 2014.  PARKER J, SIEFERT J, SHINGLEDECKER J. An informed perspective on the use of hardness testing in an integrated approach to the life management of Grade 91 steel components[R]. Palo Alto, CA: EPRI, 2016.  MASUYAMA F. Hardness model for creep-life assessment of high-strength martensitic steels[J]. Materials Science and Engineering:A, 2009, 510: 154-157.  LIU S, YANG C, PENG Z F, et al. An approach to 570 ℃/105 h creep rupture strength prediction and safety assessment of Grade 91 components with reduced hardness after service exposures at 530-610 ℃[J]. International Journal of Pressure Vessels and Piping, 2020, 182: 104073.  刘成, 彭志方, 彭芳芳, 等. P92 钢 625 ℃持久实验过程中试件特征部位相参量的变化[J]. 材料工程, 2020, 48(3): 98-104. LIU C, PENG Z F, PENG F F, et al. Phase parameter changes of specific positions of P92 steel specimens during creep rupture test at 625 ℃[J]. Journal of Materials Engineering, 2020, 48(3): 98-104.  ALLEN D J. A hardness normalized model of creep rupture for P91 steel[C]//Creep and Fracture in High Temperature Components-Design and Life Assessment Issues. Lancaster, PA: DEStech Publications, 2009: 659-668.  MASUYAMA F, SHINGLEDECKER J P. Recent status of ASME code on creep strength enhanced ferritic steels[J]. Procedia Engineering, 2013, 55: 314-325.  NIMS creep data sheet C43AJ [DB/OL]. (2014-03-31)[2020-10-03]. http://smds.nims.go.jp/MSDS/pdf/sheet/C43AJ.pdf.  LARSON F R, MILLER J. A time-temperature relationship for rupture and creep stresses[J]. Transactions of the ASME, 1952, 74: 765-775.  MASUYAMA F. Creep rupture life and design factors for high-strength ferritic steels[J]. International Journal of Pressure Vessels and Piping, 2007, 84(1/2): 53-61.