Microstructural Degradation and Creep Fracture Behavior of Conventionally and Thermomechanically Treated 9% Chromium Heat Resistant Steel

Abstract

The microstructural degradation and the creep fracture behavior of conventionally and thermomechanically treated Grade 91 steel were investigated after performing Small Punch Creep Tests. A remarkable reduction in creep ductility was observed for the samples thermomechanically treated in comparison to those conventionally treated under the tested conditions of load (200 N) and temperature (700 ºC). A change in the fracture mechanism from a ductile transgranular fracture to a brittle intergranular fracture was observed when changing from the conventionally treated to the thermomechanically treated processing condition, leading to this drop in creep ductility. The change in the fracture mechanism was associated to the localized concentration of creep deformation, close to coarse M23C6 carbides, at the vicinity of prior austenite grain boundaries (PAGB) in the thermomechanically treated samples. The preferential recovery experienced at the vicinity of PAGB produced the loss of the lath structure and the coarsening of the M23C6 precipitates. The electron microscopy images provided suggest that the creep cavities nucleate in these weak recovered areas, associated to the presence of coarse M23C6. After the coalescence of the cavities the propagation of the cracks was facilitated by the large prior austenite grain size produced during the austenitization which favors the propagation of the cracks along grain boundaries triggering the intergranular brittle fracture. This fracture mechanism limits the potential use of the proposed thermomechanical processing routes.

High-Chromium (9-12Cr) Steels: Creep Enhancement by Conventional Thermomechanical Treatments

“…The number density and average precipitate size of MX carbonitrides after the TMT are similar to the oxide particles in ODS steels. These latter steels possess high creep strength due to the high number density of oxides distributed in the [39].…”

“…The conventional treatments (AR) and TMT considered in this work were carried out on 10 mm in length and 5 mm in diameter cylindrical samples using a DIL 805A/D plastodilatometer (TA instruments) as described elsewhere [39,40]. Due to the limited amount of material available after the TMT is carried out in the plastodilatometer, the creep properties were investigated by means of the small punch creep test (SPCT) performed at 700°C as it has been previously reported [41,42].…”

“…The prior austenite grain boundary in this image has been highlighted with a dash line as a guide to the eye; (b) cavities associated with coarse M 23 C 6 precipitates have nucleated at a prior austenite grain boundary in sample G91-TMT 900_40. Cavities have been pinpointed with arrows and (c) EDS analysis of the particle marked with a red arrow in image (a), close to a cavity[39].…”

Study on Strengthening Mechanism of 9Cr-1.5Mo-1Co and 9Cr-3W-3Co Heat Resistant Steels

“…To achieve this goal, it is necessary to the increase the temperature and pressure of the main steam engine, which puts forward higher requirements for materials [ 3 , 4 , 5 , 6 , 7 ]. Because of its high creep strength, good temperature oxidation resistance, excellent thermal conductivity, low thermal expansion coefficient, and low sensitivity to thermal fatigue performance, 9–12% Cr martensitic heat-resistant steel has become the heat-resistant material in thermal power plants [ 8 , 9 , 10 ].…”

Example of using complex system approach methodology by degradation of selected technical object

“…• comparison of theoretical factors with real factors -determination of factors that can cause limit states and degradation processes of TO, • evaluation of TO degradation -TO analysis using microscopy methods, destructive and NDT tests, • processing results, determining the causes of TO degradation, • conclusions to prevent the formation of degradation processes of TO, resp. reduction of TO degradation [8][9][10][11][12][13][14].…”