Grzegorz Golański

Influence of long-term service on microstructure, mechanical properties, and service life of HCM12A steel

The investigated material was a pipe measuring φ38 × 6.3, made of HCM12A steel used for coils of boiler superheaters at supercritical and ultra-supercritical parameters of steam. The article presents the results of research on mechanical properties and microstructure of the examined pipe material in the as-received condition and after 100 000 h of service at the temperature of 585°C. Creep tests were carried out for the material serviced for 100 000 h to determine the available residual life. The influence of changes on mechanical properties and microstructure degradation on the loss of creep resistance was assessed compared with the as-received condition. Performed research has shown that the examined steel after 100 000 h of service fulfills the requirements set for this steel in the as-received condition within the range of strength properties at room and elevated temperature. The insignificant loss of life is confirmed by the results of creep test.

Microstructure and mechanical properties of the T23 steel after long-term ageing at elevated temperature

The paper presents the results of research on the microstructure and mechanical properties of T23 steel after long-term ageing up to 70 000 h at the temperature of 550 and 600 °C. It has been shown that the main mechanisms of degradation of the T23 steel microstructure were: recovery of the matrix, disintegration of the bainitic microstructure, growth of carbide diameter and precipitation of M6C carbides. These processes were more advanced in the steel aged at higher temperature. The changes in the microstructure resulted in a 10–15% decrease in mechanical properties (YS, TS) and hardness HV10, and a 30–40% decrease in the impact energy kV. A greater fall of impact strength occurred in the steel aged at the lower temperature. This was ascribed to the segregation of phosphorus to grain boundaries, which has a well-known adverse effect on ductility of low-alloy steels.

Estimation of long-term creep strength in austenitic power plant steels

This paper presents the characteristics of the performance of austenitic steel Super 304H and HR3C used for construction of boilers with supercritical and ultra-supercritical steam parameters. Characteristic microstructure images of the examined steels in the as received condition observed with the scanning electron microscope are shown. The method for strength assessment based on abridged creep tests carried out at a temperature higher than the design one is presented on the example of examined steels. It has been demonstrated that the obtained results do not deviate from the values of creep strength determined in long-term creep tests. The maximum difference is ± 20%, which is in compliance with the acceptable scatter band. The presented methodology can be used for verification of creep strength.

Properties, structure and creep resistance of austenitic steel Super 304H

Abstract This paper presents selected material characteristics of Super 304H austenitic steel used for elements of boilers with super- and ultra-critical steam parameters. In particular, abridged and long-term creep tests with and without elongation measurement during testing and investigations of microstructural changes due to long-term impact of temperature and stress were carried out. The practical aspect of the use of creep test and structure investigation results in forecasting the durability of materials operating under creep conditions was presented. The characteristics of steels with regard to creep tests and structure investigations developed in this paper are used for estimation of changes in functional properties of the material of elements operating under creep conditions.

Fatigue Life of GX12CrMoVNbN9 -1 Cast Steel in the Energy-Based Approach

The paper presents the research on the controlled strain and temperature on the stress strength of martensitic GX12CrMoVNbN9 – 1 in the energy-based approach. The fatigue tests were carried out for five levels of total strain amplitude ac and two temperatures (the room one and 600oC). Performed research has shown that in terms of energy the investigated cast steel is characterized by changes in cyclic properties. Moreover, it has been proved that the temperature of study significantly influences the per-unit value of strain energy. The influence is slight in the area of large strains and gets stronger as the level of cyclic strains decreases.

Microstructure and mechanical properties of HR3C austenitic steel after service

Purpose: The purpose of the investigations was to determine changes in the microstructure and mechanical properties of HR3C creep resisting austenitic steel after service. Design/methodology/approach: The investigations were performed on test specimens taken from a part of the steam superheater tube. The range of the investigations included: microstructural investigations light and SEM microscope; analysis of precipitates carbide isolates; investigations of mechanical properties hardness measurement, static tensile test, impact test. Findings: The precipitation processes at the grain boundaries lead to increase in intergranular corrosion of the HR3C steel resulting in loss of grains in the structure. The impact strength testing on test specimens with reduced width may result in overestimation of crack resistance of the material after service. Research limitations/implications: The comprehensive analysis of precipitation processes requires TEM examinations. Finding the correlation between the impact strength determined on standard vs. non-standard test specimens with reduced width. Practical implications: The obtained results of investigations are used in industrial practice for diagnosis of pressure parts of power boilers. Test procedures developed based on comprehensive materials testing conducted under laboratory conditions are used in upgrading and design of pressure parts of steam boilers. The results of investigations are also the element of database of the materials characteristics of steels and alloys as well as welded joints made of them working under creep conditions developed by the Institute for Ferrous Metallurgy. Originality/value: The results and analysis of the investigations of microstructure and mechanical properties of HR3C steel after service under actual boiler conditions are presented.

Low Cycle Fatigue of GX12CrMoVNbN9 -1 Cast Steel at 600°C Temperature

The paper treats of the low cycle fatigue (LCF) behaviour of GX12CrMoVNbN9-1 (GP91) cast steel after heat treatment. Fatigue tests at the temperature of 600oC within the scope of small amount of cycles to failure were carried out for five levels of controlled amplitude of total strain εac = 0.25, 0.30, 0.35, 0.50 and 0.60 %. The investigated cast steel within the scope of low cycle fatigue life reveals a three-stage course of changes in strength and strain. In the given scope of low cycle fatigue for GP91 cast steel, cyclic weakening was observed without the occurrence of stabilization period of its properties.

Mechanical Properties of G17CrMoV5 – 10 Cast Steel after Regenerative Heat Treatment

The paper presents results of research on the influence of regenerative heat treatment on structure and properties of G17CrMoV5 – 10 cast steel. Investigated material was taken out from a turbine frame serviced for over 250 000 hours (total service time) at the temperature of 535 oC. The cast steel after service revealed degraded bainitic-ferritic structure and was characterized by mechanical properties ranging below norm requirements. It has been proved that high tempering temperature in the case of cast steel with bainitic structure ensures optimum combination of mechanical properties and impact energy. It has also been shown that ferrite has a negative influence on impact energy of the cast steel with bainitic-ferritic structure.

Microstructural Aspects of Low Cycle Fatigue of Aged GX12CrMoVNbN9-1 Cast Steel

The paper presents the results of research on the changes in microstructure of GX12CrMoVNbN9-1 cast steel subject to aging at the temperature of 600°C and holding time of 8000 hours, followed by low-cycle fatigue. The characteristics of the microstructure of the examined cast steel after ageing and low-cycle fatigue was described using transmission electron microscopy (study of the dislocation microstructure) and morphology of precipitations. It has been shown that low cycle fatigue leads to the matrix softening as a result of the processes of recovery, polygonization and repolygonization. Moreover, the processes of precipitation of Laves phase and coagulation of M23C6 carbides were observed in the microstructure. Intensity of these processes depended not only on the temperature of fatigue tests, but also on the level of total strain amplitude εac.

Microstructural Investigation of the Ferritic GX12CrMoVNbN9-1 (GP91) Cast Steel

The paper presents research on the microstructure of high chromium martensitic GX12CrMoVNbN9-1 (GP91) cast steel. The steel was subject to investigation in the as-cast state, after hardening as well as after quenching and tempering. The microstructure and morphology of precipitates in GP91 cast steel was examined in terms of changes taking place during the technological process. It has been shown that the examined cast steel is characterized by a microstructure of lath martensite (and polygonized ferrite) with numerous precipitates, such as: M3C, M23C6 and MX (NbC, VX) of diverse morphology. Tempered martensite in the cast steel consists of former austenite grains separated by high angle grain boundaries. These grains are subdivided by high angle boundaries into blocks of martensite laths. The dislocation density of the tempered martensite is high (of about 1014 m-2).