Effect of Long-Term Service on Microstructure and Mechanical Properties of Martensitic 9% Cr Steel
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Effect of Long-Term Service on Microstructure and Mechanical Properties of Martensitic 9% Cr Steel Grzegorz Golan´ski, Anna Zielin´ska-Lipiec, Adam Zielin´ski, and Marek Sroka (Submitted August 16, 2016; in revised form December 14, 2016) The paper presents the results of research on the X10CrMoVNbN9-1 (T91) steel after long-term service. The material for testing was taken from a pipe section of a boiler superheater coil serviced for around 105,000 h at the temperature of 540 °C, at the pressure of 12.5 MPa. A quantitative analysis including the measurement of mean diameter of subgrains and precipitates as well as the density of dislocations of the examined steel was performed by means of TEM. The microscopic tests of T91 steel were complemented with the results of tests on mechanical properties which included also the short creep tests. After service, the investigated steel was characterized by a retained lath microstructure of tempered martensite with fine subgrain and quite large density of dislocations as well as numerous precipitates. In the microstructure, apart from the particles of M23C6 and MX (VX, NbC, V-wings), the precipitates of Laves phase and single particles of Z phase were revealed. It has been shown that the extent of degradation of the T91 steel microstructure was minor, which resulted from its low temperature of service. Performed tests of mechanical properties showed that these properties fulfilled the minimum requirements for this steel in the as-received condition. A favorable influence of fine precipitates of Laves phase on mechanical properties was observed. Moreover, an insignificant influence of single precipitates of Z phase on the creep resistance of the examined steel was stated. Keywords
creep and stress rupture, electron, mechanical microscopy, steel
1. Introduction The creep-resistant martensitic T/P91 steel was developed in the 1970s as a result of the modification and optimization of the chemical composition of P9 steel. It was a response to the needs of the conventional power industry aiming to obtain ‘‘pure’’ carbon technologies. T/P91 steel fills the gap between materials designed for work at elevated temperatures in the conditions of subcritical loads and the alloys that can transfer supercritical and ultra-supercritical states of load. T/P91 steel owes its high properties mostly to solution strengthening with Mo atoms and to precipitation strengthening with fine-dispersive precipitates of MX and M23C6 carbides. This steel is characterized by a number of significant qualities that have determined its already broad implementation in the power industry. T/P91 steel was introduced into the power industry in the 1990s, mostly as material for seamless hot-rolled pipes (Ref 1, 2). Many longterm studies were performed on this steel, such as but not limited to, the assessment of the stability of the microstructure and mechanical properties (Ref 3-5), low-cycle fatigue resis-
Grzegorz Golan´ski, C
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