13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Creep and fracture behaviors of an advanced heat resistant austenitic stainless steel for A-USC power plant Guocai Chai1,2* Patrik Kjellström1 and Magnus Boström1 1 Strategy Research, Sandvik Materials Technology, 811 81 Sandviken, Sweden 2 Engneering Materials, Linköping University, 581 83 Linköping, Sweden * Corresponding author: guocai.chai@sandvik.com Abstract Creep and fracture behaviors in a newly developed austenitic stainless steel grade UNS S31035 (Sandvik Sanicro® 25) for use in A-USC power plant have been investigated. This new grade shows very good resistances to steam oxidation and hot corrosion, and higher creep rupture strength than other austenitic stainless steels available today. This makes it an interesting alternative for super-heaters and reheaters in future high-efficient coal fired boilers. This paper will mainly focus on the study of the creep and fracture behaviors of the material at temperatures from 600°C to 750°C by using SEM and TEM. The creep and fracture mechanisms at different temperature and loading conditions have been identified. The interactions between dislocations and precipitates and their contribution to the creep rupture strength have been discussed. In this paper, free temperature model has been used to evaluate the long-term creep behavior of the grade. A creep rupture strength near 100MPa at 700°C for 100 000h has been predicted. Keywords Heat resistant austenitic stainless steel, Superheater, A-USC, Creep, Fracture 1. Introduction The demand for electric power is continuously increasing around the world. Meanwhile the consciousness of the environmental impact from human action is growing. Although combustion processes generate carbon dioxide, coal-fired thermal power generation is still one of the most important methods in the medium to long-term future to satisfy this demand, since coal is available at a competitive price and often is the single domestic energy source [1]. However, the biggest challenge facing coal-fired power plants is to improve their energy efficiency. This can be accomplished by increasing the maximum steam temperature and the steam pressure. Conventionally, the heat efficiency of coal-fired power plants has stayed at around 41% in the super critical (SC) condition with a temperature of 550°C and pressure of 24.1MPa. In order to attain a power generating efficiency of about 43%, ultra super critical (USC) conditions with a steam temperature at about 600°C should be reached. By increasing the temperature up to 700°C (A-USC condition) and pressure of above 300 bars, a power plant efficiency of higher than 50% can be reached and CO2 emission can be reduced by about 45% comparing with that of SC condition [2]. However, the steam data in practice will be limited by the material properties of the boiler tubes, especially tensile strength at elevated temperatures and creep strength combined with corrosion resistance. A new austenitic stainless steel grade, UNS S31035 (Sandvik Sanicro 25), has recently been developed for the purpose of A-USC [3] in collaboration with a number of different industrial partners within the Thermie-project in Europe, intended for use in super-heaters and reheaters in advanced ultra-supercritical boilers at temperatures up to 700°C. They have been test-installed in different boilers in Europe and have run for more than five years, and are still in very good conditions [4]. In this paper the creep and fracture behaviors of this new grade are discussed.
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