Effect of matrix hardness on the creep properties of a 12CrMoVNb steel
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I. INTRODUCTION
II. EXPERIMENTAL PROCEDURE
IN fossil power plants, there is a trend to use larger steam turbines at higher temperatures in order to improve the thermal efficiency of the operation. At steam temperatures around 566 8C to 600 8C, conventional low-alloy steels such as 1CrMo or 2.25CrMo steels have poor oxidation and corrosion resistance and are in the process of being replaced by high-chromium ferritic steels with better properties. Since the 9 to 12Cr steels have better creep strength as well as higher thermal conductivity, numerous studies have been conducted on these materials.[1–6] Under a high temperature and pressure after a long period of time, material properties such as fracture toughness and creep rupture strength are greatly affected by microstructural degradation during service.[6,7,8] In the case of service lives over 105 hours, degradation of materials can be severe, and the creep-rupture and crack-growth properties of the same material at the initial and final stages of service life are very different. Therefore, it is difficult and potentially dangerous to predict failure times of these components using experimental data from tests lasting around 103 hours. In order to predict failure times reasonably accurately, it is necessary to understand the effects of material degradation on the creep properties. The material degradation is related to the microstructural evolution during creep, such as precipitate nucleation and coalescence, subgrain growth, reduction of dislocation density, etc., which can be indirectly measured by hardness testing. In the present work, a 12CrMoVNb steel was tempered to give three different matrix hardness (Rc) levels (Rc 20, Rc 25, and Rc 30), and the effects of the hardness on the creep properties are investigated through microstructural analysis.
The material for this work was manufactured by KHIC (Kyung-nam, Korea), and the as-received chemical composition is given in Table I. The material was cut to 30 3 30 mm2 square bars, austenitized at 1050 8C for 2 hours to yield the prior austenitic grain size of 50 mm,[9–12] and subsequently air cooled. To obtain the matrix hardness levels of Rc 20, Rc 25, and Rc 30, the specimens were tempered at 750 8C for 9 hours, at 750 8C for 2 hours, and at 700 8C for 2 hours, respectively, and then air cooled. The as-tempered microstructures were 100 pct tempered martensite without d-ferrite, which is known to have deleterious effects on impact properties.[13,14] Smooth bar specimens with a 36-mm gage length and a 7-mm diameter were prepared according to ASTM STP E139[15] and equilibrated for 10 hours at 650 8C before loading. The creep tests were conducted in air at 650 8C under constant loads, while displacements were measured by a linear variable differential transducer to the accuracy of 1 mm, and the temperature was kept constant within 62 8C. Fracture surfaces of crept specimens were examined using a scanning electron microscope (SEM), and microstructural studies were conducted using a transmission electron microscope (T
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