Effect of Long-Term Service on Microstructure and Mechanical Properties of Martensitic 9% Cr Steel

Abstract

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 M 23 C 6 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.

MICROSTRUCTURAL ASPECTS OF LONG-TERM AGEING OF MARTENSITIC 9% Cr STEEL WITH AND WITHOUT BORON ADDITION

“…The tendency to precipitate this unfavourable phase increases with the increase of chromium and niobium content in steel [16]. In 9% Cr steels phase Z is a coherent precipitate, with a size comparable to MX precipitates and occurs in the microstructure usually in the form of single particles [16,17], hence its influence on the properties of theses steels is relatively limited [18]. However, the inhibiting effect of MX precipitates on the dislocation movement was still observed in the tested steel.…”

Study on creep behavior and microstructure of a Fe–Cr–Al–Nb–W ferrtic heat-resistant steel based on C, N and Al element control

“…It has been reported [14][15][16] that the precipitation phases of high-Cr ferritic heat-resistant steel are mainly composed of M 23 C 6 , located at the boundaries or subgrain boundaries, MX grain interiors and Laves phases (type Fe 2 W), generated during aging. In the long-term service [15,16], the dislocation density decreased, the size of the martensite lath increased, M 2 N phases disappeared, Z-phase formed, Mo and W decreased in amount, while M 23 C 6 phases became coarser. In recent years, researchers [5] have attempted to qualitatively analyze the influence of microstructure degradation on creep strength.…”

Analysis of Phase Precipitation in Sanicro 25 Austenitic Steel after Ageing

“…These data are the basis for using this steel in supercritical and ultra-supercritical boilers. The creep strength determined for 100,000 h at 700 • C is 95 MPa, and it is comparable to that of the HR6W nickel-base alloy [1,8,9]. High functional properties of the Sanicro 25 steel were the basis for its implementation in supercritical and ultrasupercritical power units [1,9].…”