Effect of tempering temperature on the stress rupture properties of Grade 92 steel Lakshmiprasad Maddi a,n , A.R. Ballal a , D.R. Peshwe a , R.K. Paretkar a , K. Laha b , M.D. Mathew b a Department of Metallurgical and Materials Engineering, Visvesvaraya National Institute of Technology, Nagpur 440 010, India b Mechanical Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India article info Article history: Received 27 February 2015 Received in revised form 19 May 2015 Accepted 21 May 2015 Available online 27 May 2015 Keywords: P92 steel Normalizing Tempering Stress rupture Microstructural evolution abstract P92 steel is used in normalized and tempered condition for optimal creep properties. Effect of varying tempering temperatures in the range of 740–780 °C on the stress rupture properties has been in- vestigated in this study. High dislocation density and fine laths resulted in high rate of microstructural evolution in 740 °C tempering case, hence the steep slope of rupture curve was observed as compared to higher tempering temperatures. Quantification of lath width and precipitate size under Scanning elec- tron microscopy (SEM) and transmission electron microscopy (TEM) revealed increase in lath width and precipitate coarsening with tempering temperature and exposure time. Increase in lath width was more pronounced in 740 °C tempering case. The results were supported by the damage parameter (λ) and hardness measurements. Variation in fractographic features was associated more with rupture time, for a particular tempering temperature. Coarser precipitates were responsible for cavity initiation, inducing some brittle fracture at higher rupture times. & 2015 Elsevier B.V. All rights reserved. 1. Introduction Increasing efficiencies and reduction in CO 2 emissions are the prime factors governing choice of materials for fossil fired power plants. Petrochemical industries, thermal and nuclear power plants require materials which offer resistance to creep as well as high temperature oxidation. The last two decades witnessed de- velopment of various 9–12 wt% Cr steels aimed at providing long term service lives at temperatures of 600–650 °C in boilers, steam lines, and turbines of ultra supercritical power plants [1–6]. High dislocation density, complicated (fine) lath structure, precipitates of M 23 C 6 and MX are the requirements for high temperature strength. P92 is one of the candidate materials for piping in power plants and petrochemical industries. While the high degree of misorientation associated with lath martensite makes slip difficult, precipitates of M 23 C 6 (M¼ Fe, Cr, Mo) and MX (M¼ V, Nb and X ¼ C, N) arrest the boundaries [7,8], reducing the boundary migration. P92 steel is subjected to normalizing and tempering heat treatments to obtain the optimal microstructure of tempered martensite. During normalizing, carbides pinning the boundaries dissolve, leading to solid solution of C in austenite. Usually, higher normalizing temperatures are associated with better homogeneity in the distribution of C, resulting in superior properties [9]. The process results in the formation of lath martensite with high dis- location density in P92 steel. However, to introduce some degree of toughness, tempering treatment is followed. During tempering, dislocation density is reduced and the dissolved carbon is pre- cipitated out in the form of carbides, thereby, softening the ma- terial. In general, lower end tempering temperatures are chosen for strength based applications such as turbine blades, while high tempering temperatures are chosen for piping applications which require sufficient ductility [10]. Maile [11] reported the formation of M 2 X (M¼ Cr, V) at temperatures less than 700 °C. M 2 X being a brittle phase is detrimental to creep properties. It has also been reported that high end tempering temperatures of 835 °C led to marked fall in creep strength [12]. The range of tempering tem- perature given by steel manufacturers [13] is 730–810 °C. As per ASTM standard A213, only a minimum of 730 °C is specified. On this background, three tempering temperatures viz., 740, 760 and 780 °C are chosen following which creep properties are studied in the present work. Rupture data was generated for  5000 h to study the microstructural evolution with time. 2. Experimental work The material was procured from Mishra Dhatu Nigam Limited Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A http://dx.doi.org/10.1016/j.msea.2015.05.062 0921-5093/& 2015 Elsevier B.V. All rights reserved. n Corresponding author. E-mail address: prasadmlp@gmail.com (L. Maddi). Materials Science & Engineering A 639 (2015) 431–438