Properties of 13KhMF steel after operation and degradation under the laboratory conditions
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PROPERTIES OF 13KHMF STEEL AFTER OPERATION AND DEGRADATION UNDER THE LABORATORY CONDITIONS I. R. Dzioba
UDC 621.181:669.018
The influence of cooling rate in the process of thermal treatment on the formation of the microstructure of 13KhMF heat-resistant steel is investigated. The relationships between the microstructure, strength characteristics, crack-resistance characteristics, and creep resistance of the material are determined. The transformations of the structure are modeled by the changes in the duration of tempering. The bainitemartensite structure is characterized by the best combination of properties and the largest period of transformation of the structure. The comparison of the results of laboratory and long-term in-service degradation and their influence on the structure, strength, and crack-resistance characteristics shows that they are similar. Keywords: heat-resistant steel, microstructure, strength, crack resistance, degradation.
Economically alloyed heat-resistant Cr–Mo–V steels are extensively used in the contemporary thermal power-generating industry for structures operating at high temperatures (500–550°) and under high stresses (30–75 MPa). As a result of long-term operation, their structure and the characteristics of strength, plasticity, creep resistance, and crack resistance undergo significant changes [1–10]. At the same time, despite the preservation of the mechanical properties on an admissible level even after operation for ~210 5 h , elements of the equipment sometimes fail [1–5]. This is why the assessment of the serviceability of the metal after long-tem operation is an urgent problem both for the Ukraine and for the European countries [1–6]. The procedures traditionally used for this purpose are based on the analysis of long-term strength at working temperatures. However, the process of operation also includes low-temperature periods (startups, shutdowns, and hydraulic tests). In some cases, the equipment fails even at low temperatures [1–3]. Thus, the procedure used for the evaluation of the serviceability of the metal must take into account the characteristics of the metal under the conditions of both high- and low-temperature modes of operation. For the last 20 years, numerous procedures were developed for the evaluation of the engineering state of structural elements. These are the ETM [11], BS-7910 [12], API-579 [13], R6 [14], and SINTAP [15] procedures. These procedures were generalized in the FITNET (Fitness for Service Procedure) document, basic for the countries of the European Community [16]. According to this document, the serviceability of metallic structures (including welded structures with cracks) under static and cyclic loads can be estimated in terms of the strength and crack resistance of the materials within fairly broad temperature ranges [16–19]. The most dangerous (and the most conservative) estimate is based on the use of the minimum values of these quantities but the possibility of using these values should be additionally justified. Producers of the pipe
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