Microstructure and mechanical properties of HR3C austenitic steel after service

Abstract

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

Effects of Long‐Term Ageing at High Temperatures on Oxide Scale Development and Evolution of Austenitic Steels Microstructure

“…The M 23 C 6 carbides are characterized by low stability which results in their growth and the formation of a continuous network during prolonged exposures at the grain boundaries affecting mechanical properties of the material (mainly toughness) and also increase susceptibility to intergranular corrosion . This negative effect was observed in HR3C steel by Golański et al and further by Zhu et al In HR3C steel after the long‐term test at 700 °C, at the grain boundaries and inside the grains, Cr‐ and Si‐rich precipitates were observed (Figure B). According to Ibañez et al and Lin et al, Si accelerates nucleation and growth of the σ phase in austenitic stainless steels such as HR3C.…”

“…Such type of steels are predominantly fabricated to withstand severe conditions in a power generation sector; the steels are predominantly used for super‐heaters (SH) and re‐heaters (RH) sections, where the highest temperature of a steam pressures is observed. Henceforth, microstructural evolution under a long temperature exposure, examination, and research into extended exposure times is an important factor . In addition, microstructural stability governs creep resistance, high‐temperature corrosion resistance, and oxidation resistance that are the most important indicators in overall performance of the structural materials for power generation system.…”

Analysis of Phase Transformation in Nickel-Base Alloy after Ageing

“…However, this effect disappears in case of formation of the so-called continuous network of precipitates by particles, which results in the increase in brittleness and contributes to a change in cracking mechanism from ductile to brittle, frequently intercrystalline [3]. The effect of negative impact of particles precipitated at the grain boundaries in austenitic matrix alloy is visible on the example of HR3C steel [3,13].…”

“…M 23 C 6 carbides ( Fig. 4a) are characterised by low stability at elevated temperature and a tendency to form the so-called continuous network of precipitates at the grain boundaries, which results in the decrease in mechanical properties, and also in the reduction in corrosion resistance [3][4][5]13]. According to [5,11,12], not only the amount of M 23 C 6 particles precipitated at the grain boundaries, but also their shape determine their impact on the properties.…”

Microstructure and mechanical properties of HR6W alloy dedicated for manufacturing of pressure elements in supercritical and ultrasupercritical power units

“…5) are also likely to result in significant reduction in ductility (not studied in this paper) and a change in cracking mechanism from ductile to brittle, and may cause an increase in sensibility. The negative impact of numerous precipitates on the performance of creep-resistant austenitic alloys is particularly visible in HR3C steel [4,20].…”