High Cr Ferritic Heat-resistant Steel Research Articles

Development of the modified high Cr ferritic heat-resistant steel

High Cr ferritic heat-resistant steels have been the important structural materials for elevated temperature application in the advanced power plants. Owing to the severe difficulties encountered by energy shortage and environment pollution, it is imperative to improve the serving temperature up to 650° which would largely elevate the thermal efficiency of generating station. In this paper, the phase transformation behaviours, strengthening mechanism and the formation the precipitates of high Cr ferritic steels were summarised. Based on these, the modified high Cr ferritic steel for 650 °C condition was developed by our group. The tensile and creep tests indicate that the modified steel has the outstanding mechanical properties, suggesting it may be the candidate materials in advanced power plants for 650 °C grade. Furthermore, developments of the modified high Cr ferritic steels will be presented and discussed in views of microstructural control, alloy design, phase transformation behaviours and high-temperature performance.

Carbonitride Dissolution and Austenite Grain Growth in a High Cr Ferritic Heat-resistant Steel

Carbonitride Dissolution and Austenite Grain Growth in a High Cr Ferritic Heat-resistant Steel

Martensite–austenite transformation kinetics of high Cr ferritic heat-resistant steel

Abstract The isochronal transformation behavior from martensite (α′) to austenite (γ) of modified high Cr ferritic heat-resistant steel was analyzed using the kinetic information extracted from differential thermal analysis results. The thus obtained α′→γ transformation kinetic process has been described by a JMAK-like model including three overlapping processes: site saturation nucleation, diffusion-controlled growth, and impingement correction for random distribution of nuclei. It is found that both the diffusion of alloying components and the inhibition of carbide precipitates strongly affect the α′→γ transformation kinetic processes. The α′→γ transformation (at relatively low heating rates) is of diffusion-controlled nature, and the obstruction of carbides during the migration of the α′/γ interface leads to a decrease in the pre-exponential factor of diffusion coefficient.

Influence of austenitization temperature on phase transformation features of modified high Cr ferritic heat-resistant steel

The phase transformation features of high Cr ferritic heat-resistant steel have been investigated using Differential Thermal Analysis (DTA) in accordance with different austenitization temperatures as 950°C, 1050°C, 1150°C, 1200°C and predicted by Thermo-calc calculation. The results based on DTA curves on heating showed that the magnetic transition and the Ac1 temperature occurred at 744.9°C and 850.9°C, respectively. The measured Ac3 temperature was around 919.5°C, the obtained austenitization temperature range was about 69°C. Two new phenomena were found: firstly, the onset temperature of martensite transformation increases strongly and then not obviously with the increase of austenitization temperature; secondly, the δ-ferrite abnormally forms in the specimen austenitized at 950°C due to the existence of un-dissolved M23C6 particles.

Effect of groove configuration on Type-IV creep damages in Gr.122 steel welds

Creep strength of high Cr ferritic heat-resistant steel welds decreases more rapidly than base metals due to Type-IV creep damage formed in the heat-affected zone (HAZ) at high temperatures. This paper aims to elucidate the effect of groove configuration of welds on the formation and evolution of Type-IV creep damage. Creep tests using base metal, simulated HAZ and two-types of weld joint; gas tungsten arc and electron beam welds, of the ASME Gr.122 steel were conducted. Decreasing the groove angle and width of HAZ was effective in prolonging the creep life; however, it could not prevent Type-IV failure. Type-IV creep voids were mostly observed in the area about 1.0–3.0 mm below the specimen surfaces, whereas their detailed distributions were dependent on the groove configurations. Both the distribution of the multiaxial stress state and creep strain in HAZ can be considered to influence the formation of Type-IV creep damage. Damage mechanics analysis taking the multiaxial stress state into account can predict fairly well the distribution of Type-IV creep voids and failure times of the weld joints without regard to the groove configurations.

Martensite transformation in the modified high Cr ferritic heat-resistant steel during continuous cooling

Abstract

Effect of Austenitization Temperature on Phase Transition Features of High Cr Ferritic Heat-Resistant Steel

The phase transformation of high Cr ferritic heat-resistant steel has been investigated by using differential scanning calorimeter and predicted by Thermo-calc calculation. The steel specimens were hot rolled and followed by air cooling, and then heated from room temperature up to different austenitization temperature as 800°C, 900°C, 1000°C and 1100°C. The DSC curves during heating process showed that the magnetic transition temperature and the Ac1 temperature are 744.9°C and 850.9°C, respectively. The austenitization range was about 58°C. The onset and offset temperature of martensite transformation both increase with the increase of austenitization temperature. The experimental results and the Thermo-calc calculated results both displayed that M23C6 carbides precipitated at around 950°C, and δ-ferrite started to form at about 1020°C.

Effect of groove configuration on Type-IV creep damages in Gr.122 steel welds

Creep strength of high Cr ferritic heat-resistant steel welds decreases more rapidly than base metals due to Type-IV creep damage formed in the heat-affected zone (HAZ) at high temperatures. This paper aims to elucidate the effect of groove configuration of welds on the formation and evolution of Type-IV creep damage. Creep tests using base metal, simulated HAZ and two-types of weld joint; gas tungsten arc and electron beam welds, of the ASME Gr.122 steel were conducted. Decreasing the groove angle and width of HAZ was effective in prolonging the creep life; however, it could not prevent Type-IV failure. Type-IV creep voids were mostly observed in the area about 1.0–3.0 mm below the specimen surfaces, whereas their detailed distributions were dependent on the groove configurations. Both the distribution of the multiaxial stress state and creep strain in HAZ can be considered to influence the formation of Type-IV creep damage. Damage mechanics analysis taking the multiaxial stress state into account can predict fairly well the distribution of Type-IV creep voids and failure times of the weld joints without regard to the groove configurations.

Effects of PWHT Temperature on Mechanical Properties of High-Cr Ferritic Heat-Resistant Steel Weld Metals

Welded joints of ASME Gr. 91 steel are subject to post-weld heat treatment (PWHT) to obtain sufficient mechanical properties. However, when the PWHT temperature is higher than the lower critical transformation temperature (AC1) of both the base metal and the weld metal, the mechanical properties of the weld may be degraded. To avoid this, it is important to know the AC1 of the weld. The authors have investigated several methods of measuring the AC1 of Gr. 91 steel weld metals to select the most credible method and examined the effects of the PWHT temperature on the mechanical properties of the weld metal. In addition, the proper PWHT temperature was discussed in consideration of the measurements of AC1, test results of creep-rupture performance, and observation results of precipitates. In the case of a low Mn+Ni weld metal, the upper PWHT temperature limit should be lower than AC1. On the other hand, in the case of the high Mn+Ni weld metal, remarkable negative effects on the mechanical properties of the weld metal were not found, even though the PWHT temperature exceeds AC1 by about 30 °C.

Effect of Cooling Rate on Microstructure of Simulated CGHAZ for Modified High-Cr Ferritic Heat-Resistant Steel

The microstructure and carbide precipitate of simulated coarse grain heat affected zone(CGHAZ) in modified high Cr ferritic heat-resistant steel at different cooling rates have been investigated by means of thermal simulator, optical microscope, SEM and TEM . It was found that the microstructure of CGHAZ of testing steel was mainly lath martensite and δ-ferrite under the different welding thermal cycles. However, the prior austenite grain size reduced with increasing the cooling rate. Furthermore, with increasing the cooling rate, the amount of carbide precipitate inside laths of martensite increased, and the size and morphology of precipitates have changed from elongated and coarse to needlelike and fine.

The isochronal δ → γ transformation of high Cr ferritic heat-resistant steel during cooling

Thermal simulation technology was employed to investigate phase transformation in heat-affected zones (HAZ) of high Cr ferritic heat-resistant steel. The simulated continuous cooling transformation diagram was established based on the experimental results obtained from different cooling rates in the range of 0.02–60 °C/s. A theoretical model considering the site saturation nucleation at grain boundaries has been applied to calculate the austenite fraction as a function of cooling rate. It is found that both the austenite fraction and grain size decrease with the increase of cooling rates. The calculated results are mostly consistent with the experimental data.

Evaluation of long-term creep damage in high Cr ferritic steel welds

The creep strength of high Cr ferritic heat-resistant steel welds decreases due to the formation of Type IV creep damage in the heat affected zone (HAZ) during long-term service at high temperatures. In order to elucidate the processes of Type IV creep damage, creep rupture and creep interruption tests using ASME Gr.91 and Gr.122 steel welds were conducted. It was found that creep voids formed at an early stage (0.2 of life) and coalesced to form a macro crack at 0.8 of life for the Gr.91 steel weld. On the other hand, for the Gr.122 steel weld, a small number of Type IV creep voids formed at 0.5 of life, increased slightly until 0.9 of life and rapid crack growth occurred after that. Differences of creep damage behaviour between the Gr.91 and Gr.122 steel welds are discussed. The Type IV creep damage distributions obtained were compared with analytical results using the finite element method and damage mechanics.

Kinetics of isochronal austenization in modified high Cr ferritic heat-resistant steel

Employment of high Cr ferritic steels as a main structural material is considered as a way to achieve economical competitiveness of main steam pipe and nuclear reactors in power plants. Differential dilatometry and microstructure observation were employed to investigate the isochronal austenitic transformation of the modified high Cr ferritic steel. The kinetics of the isochronal austenitic transformation were described by a phase-transformation model involving site saturation (pre-existing nuclei), diffusion-controlled growth, and incorporating an impingement correction. The experimental results and kinetic analysis indicate that an increase of the heating rate promotes the diffusion-controlled austenitic transformation. The dissolving degree of precipitates during the austenization process affects the activation energy for diffusion and the undissolved precipitates lead to an increase of the onset temperature of the subsequent martensite transformation upon cooling.

Effect of Minor Compressive Deformation in Austenite on Martensitic Transformation in Modified 9-12%Cr Ferritic Steel

A modified high Cr ferritic heat-resistant steel was developed and subjected to the thermomechanical treatment. The effect of applied stress and temperature on martensitic transformation and microstructure was studied by using high-resolution differential dilatometer and optical microscope. Applied stress raises the amount of defection as dislocation and grain boundary defects, thus elevateMs. Applied stress also lead to the increase of transformation time due to the mechanical stabilization of austenite. Increase of applied stress or temperature would enhance these above effects.

Evaluation of long-term creep damage in high Cr ferritic steel welds

The creep strength of high Cr ferritic heat-resistant steel welds decreases due to the formation of Type IV creep damage in the heat affected zone (HAZ) during long-term service at high temperatures. In order to elucidate the processes of Type IV creep damage, creep rupture and creep interruption tests using ASME Gr.91 and Gr.122 steel welds were conducted. It was found that creep voids formed at an early stage (0.2 of life) and coalesced to form a macro crack at 0.8 of life for the Gr.91 steel weld. On the other hand, for the Gr.122 steel weld, a small number of Type IV creep voids formed at 0.5 of life, increased slightly until 0.9 of life and rapid crack growth occurred after that. Differences of creep damage behaviour between the Gr.91 and Gr.122 steel welds are discussed. The Type IV creep damage distributions obtained were compared with analytical results using the finite element method and damage mechanics.

Investigation of precipitation behavior in a weld deposit of 11Cr-2W ferritic steel

This article is aimed at investigating the difference in precipitation behavior in the fine-grained heat-affected zone (FGHAZ), coarse-grained heat-affected zone (CGHAZ), and base metal for the welded joint of high Cr ferritic heat-resistant steel (11Cr-0.4Mo-2W-1Cu-V-Nb, normalized 1323 K×1 h and tempered 1033 K×1 h). Simulated HAZ (SHAZ) specimens were used, whose thermal cycles were controlled to be the same as those in the actual welded joint with peak temperatures of 1523 and 1173 K to represent CGHAZ and FGHAZ, respectively. Based on scanning electron microscopy (SEM) observation, it looks that the precipitates in FGHAZ specimens (1173 K) were fewer and larger than those in CGHAZ (1523 K) specimens and base metal specimens. This phenomenon implied that the growth and coarsening of precipitates in FGHAZ may play a role in the deterioration of creep properties and type IV cracking, which was observed in previous creep tests. X-ray diffraction analysis for the electrolytic extraction showed that the types of precipitates are the same for the 1173 K specimens and base metal specimens, including M23C6, MX, Laves phase, and μ phase. Further, the elemental analysis of the extraction showed that the mass percentages of Cr, W, and Mo in the precipitates to specimen mass were higher in the FGHAZ specimen than those in the base metal specimen, especially during the period between 600 and 2464 hours. Finally, a two-dimensional (2-D) model was proposed to simulate the precipitation behavior of the Laves phase.