9Cr-2W Steel Research Articles

Impact of trace silicon on irradiation hardening and embrittlement of RAFM steel subjected to neutron irradiation

The content of silicon (Si) element in reduced activation ferritic/martensitic (RAFM) steels varies significantly among different candidates of fusion reactor structural materials. Si is not intentionally added to the RAFM steel but rather remains as a residue during the deoxidation process in steel fabrication. To control and reduce the Si content, removing processes and special treatments are required. The additional refining processes significantly increase the cost of steel fabrication. Currently, the role of residual or trace Si in neutron irradiation of steel remains unclear. In the presented study, two 9Cr-2 W RAFM steels with Si contents of 0.1 wt.% and 0.01 wt.% were fabricated and subjected to a neutron irradiation at 563 K with a neutron fluence of 4.8 × 1023m−2. The main results show that 0.1 wt.% Si can contribute to the suppression of irradiation hardening and embrittlement, as evidenced by the reduction in irradiation-increased hardness and yield strength, while maintaining preferable elongation. Microscopic analysis suggests that an appropriate content of Si helps refine the grains in RAFM steel, increases the ductile fracture region of the fracture surface, and effectively reduces the density of dislocation loops induced by neutron irradiation. The findings evidence that retaining a certain amount of trace Si should be an informed and rational choice in the design of RAFM steel.

Si promotes nano-clusters and α phase separation in early-stage neutron irradiated low activation ferritic/martensitic steel

Two types of 9Cr-2W steels with one by an addition of 0.1 wt. % Si were neutron irradiated at 290°C to 0.1 dpa, and studied by positron annihilation spectroscopy. The addition of Si brought about the formation of Si-containing nanoclusters and an α phase separation under irradiation. Isochronal annealing indicates that the nano-clusters can survive and be promoted up to 450°C and recovered at 500°C. Our results suggest that even a trace Si can significantly influence the microstructural evolution under irradiation. The nano-clusters/precipitates are irradiation enhanced by the generation of interstitial clusters in the cascade process.

Effect of Co addition on microstructure and mechanical properties of new generation 3Cr-3W and 5Cr-3W steels

In this study, the effect of Cr and Co on microstructure and mechanical properties of low Cr-W steels was investigated. For this goal, 3% Cr-3%W and 5% Cr-3%W steels, this may be an alternative to new generation low Cr steels, containing 0%, 0.5%, 1.5%, 3%, 4.5% Co, produced by casting and hot rolling. The samples were annealed at 1100 °C for 1 h, followed by quenching in air and tempering for 2 h at 710 °C. Ferrite, bainite, and martensite phases were determined in various volume fractions according to the chemical composition. Carbides precipitated within the grain and grain boundaries during tempering. Co bearing caused an increase of A1, A3, and Curie temperatures. Co slightly increased hardness and strength in both 3 Cr and 5 Cr alloys. However, the hardness and strength of 5 Cr alloys with all Co ratios were almost same that of 3 Cr alloys. The Charpy-V impact energies of 5 Cr steel are lower than 3 Cr steel with the same Co amount. The addition of Co up to 1.5% in steels with 3 Cr did not make any major changes in the notch toughness. However, the addition of 3% Co to the alloy significantly reduced impact energy. In 5 Cr steel, notch toughness generally decreased with the increase of Co amount.

Influence of Heat Treatment on the Workability of Modified 9Cr-2W Steel with Higher B Content

In this study, the effect of heat treatment on the fracture behavior of alloy B steel with boron (B) contents as high as 130 ppm was investigated. The Alloy B are derived from Gr.92 steel with outstanding creep characteristics. The amounts of minor alloying elements such as B, N, Nb, Ta, and C were optimized to achieve better mechanical properties at high temperatures. Hence, workability of the alloy B and Gr.92 were compared. An increase in the B content affected the phase transformation temperature and texture of the steel. The development of the {111}<uvw> components in γ-fibers depended on the austenite fraction of the steel after the phase transformation. An increase in the B content of the steel increased its α-to-γ phase transformation temperature, thus preventing the occurrence of sufficient transformation under the normalizing condition. Cracks occurred at the point of the elastic-to-plastic deformation transition in the normal direction during the rolling process, thereby resulting in failure. Therefore, it is necessary to avoid intermediate heat treatment conditions, in which γ-fibers do not fully develop, i.e., to avoid an imperfect normalization.

Radiation-hardening and nano-cluster formation in neutron-irradiated 9Cr[sbnd]2W low activation steels with different Si contents

Two kinds of 9Cr2W steels with and without the addition of 0.1 wt% Si were irradiated with neutrons up to a fluence of 4.8 × 1023 n/m2 at 563 K. Precipitates, dislocation loops, radiation-enhanced nano-clusters and radiation-hardening were studied using advanced electron microscopy and micro-hardness test. Si reduced the number density of dislocation loops, and therefore the extent of radiation-hardening decreased. It also favored the formation of silicide-like nano-clusters. In this study, the role of Si in neutron irradiated 9Cr2W steels is discussed, and then extended to nano-clusters and irradiation-hardening.

Evaluation of Workability on the Microstructure and Mechanical Property of Modified 9Cr-2W Steel for Fuel Cladding by Cold Drawing Process and Intermediate Heat Treatment Condition

In this study, we evaluated the cold drawing workability of two kinds of modified 9Cr-2W steel containing different contents of boron and nitrogen depending on the temperature and time of normalizing and tempering treatments. Using ring compression tests at room temperature, the effect of intermediate heat treatment condition on workability was investigated. It was found that the prior austenite grain size can be changed by the austenite transformation and that the grain size increases with increasing temperature during normalizing heat treatment. Alloy B and Alloy N showed different patterns after normalizing heat treatment. Alloy N had higher stress than Alloy B, and the reduction in alloy N increased while the reduction in alloy B decreased. Alloy B showed a larger number of initially formed cracks and a larger average crack length than Alloy N. Crack length and number increased proportionally in Alloy B as the stress increased. Alloy B had lower crack resistance than Alloy N due to boron segregation.

Influence of B and N on the microstructural characteristics and high-temperature strength of 9Cr-2W steel during an aging treatment

The evolution of the microstructure and strength of sodium-cooled fast reactor (SFR) fuel cladding steel with different B and N contents based on 9Cr-2W steel after aged at 650°C is experimentally investigated in this study. The strength and hardness of a cladding tube fabricated containing relatively more B (0.013wt% B, 0.02wt% N) remained mostly stable with an increase in the aging time. This demonstrated the cause of the suppression of the growth of M23C6; the depletion of carbon in the matrix was prevented when B diffused to the C position in M23C6, thereby stabilizing the microstructures. A cladding tube fabricated containing more N (0.004wt% B, 0.0767wt% N) formed fine MX precipitates which were twice as fine as the cladding tube containing more B in the as-received state. Thus, the initial strength and hardness in the sample with more N are greater than when more B is used. Although the initial strength and hardness with more N are higher those with more B, the strength and hardness after 7000h of aging were low due to the rapid growth of the M23C6 in the main precipitates.

Role of Tungsten in the Tempered Martensite Embrittlement of a Modified 9 Pct Cr Steel

The effect of tempering on the mechanical properties and fracture behavior of two 3 pct Co-modified 9 pct Cr steels with 2 and 3 wt pct W was examined. Both steels were ductile in tension tests and tough under impact tests in high-temperature tempered conditions. At T ≤ 923 K (650 °C), the addition of 1 wt pct W led to low toughness and pronounced embrittlement. The 9Cr2W steel was tough after low-temperature tempering up to 723 K (450 °C). At 798 K (525 °C), the decomposition of retained austenite induced the formation of discontinuous and continuous films of M23C6 carbides along boundaries in the 9Cr2W and the 9Cr3W steels, respectively, which led to tempered martensite embrittlement (TME). In the 9Cr2W steel, the discontinuous boundary films played a role of crack initiation sites, and the absorption energy was 24 J cm−2. In the 9Cr3W steel, continuous films provided a fracture path along the boundaries of prior austenite grains (PAG) and interlath boundaries in addition that caused the drop of impact energy to 6 J cm−2. Tempering at 1023 K (750 °C) completely eliminated TME by spheroidization and the growth of M23C6 carbides, and both steels exhibited high values of adsorbed energy of ≥230 J cm−2. The addition of 1 wt pct W extended the temperature domain of TME up to 923 K (650 °C) through the formation of W segregations at boundaries that hindered the spheroidization of M23C6 carbides.

Effect of alloying on interfacial energy of precipitation/matrix in high-chromium martensitic steels

The effect of cobalt, tungsten, and boron on interfacial energy of precipitate/ferritic matrix in the 9% Cr martensitic steels on the base of creep tests at 650 °C under different applied stresses ranging from 80 to 220 MPa was investigated. An interfacial energy of M23C6 carbides, the Laves phase particles, and MX carbonitrides was estimated by comparison of theoretical curves obtained by Prisma software for the model steels for the exposure time of 2 × 104 h with experimental data measured by TEM in the gage sections of crept specimens. Addition of 3 wt% Co to Co-free 9Cr2W steel led to about 1.7 times increase in the interfacial energy of M23C6 carbides and MX carbonitrides, whereas Co did not effect on the interfacial energy of the Laves phase. Increasing W from 1.5 to 3 wt% in the Co-containing steels led to increase in the interfacial energy of the Laves phase up to 0.78 J m−2 under long-term exposure, whereas it did not effect on the interfacial energy of M23C6 carbides and MX carbonitrides. In the steel with increased B up to 0.012 wt% and decreased N to 0.007 wt%, a strong decrease in the interfacial energy of M23C6 carbides to 0.12 J m−2 occurred. Change in the interfacial energy of the precipitates was analyzed in comparison with coarsening rate constant.

Comparison between effect of B and N on the microstructure of modified 9Cr-2W steel during aging and creep

This paper reports on microstructural degradation by aging and creep, specifically with different B and N content in modified 9Cr-2W steels. This study used TEM to examine the growth and generation of precipitates across chemical composition changes resulting from aging and creep. The coarsening of precipitates, mainly M23C6, was evaluated during aging and creep, and the role of B and N on microstructural change during aging and creep was investigated.

The Irradiation Performance and Microstructural Evolution in 9Cr-2W Steel Under Ion Irradiation

Grade 92 steel (9Cr-2W) is a ferritic-martensitic steel with good mechanical and thermal properties. It is being considered for structural applications in Generation IV reactors. Still, the irradiation performance of this alloy needs more investigation as a result of the limited available data. The irradiation performance investigation of Grade 92 steel would contribute to the understanding of engineering aspects including feasibility of application, economy, and maintenance. In this study, Grade 92 steel was irradiated by iron ion beam to 10, 50, and 100 dpa at 30 and 500 °C. In general, the samples exhibited good radiation damage resistance at these testing parameters. The radiation-induced hardening was higher at 30 °C with higher dislocation density; however, the dislocation density was less pronounced at higher temperature. Moreover, the irradiated samples at 30 °C had defect clusters and their density increased at higher doses. On the other hand, dislocation loops were found in the irradiated sample at 50 dpa and 500 °C. Further, the irradiated samples did not show any bubble or void.

Evaluation of the effect of B and N on the microstructure of 9Cr–2W steel during an aging treatment for SFR fuel cladding tubes

In this study, the microstructure of sodium-cooled fast reactor (SFR) fuel cladding steel with different B and N contents after aging is compared. The addition of nitrogen produces a large quantity of MX precipitates with sizes of 0.1 μm or smaller during the initial thermal treatment process and this contributes to help such precipitates maintain stability without being excessively affected by aging. B is primarily distributed in the grain boundary precipitates and grain interior precipitates in the initial stage. The B distribution is believed to move to the Cr precipitates after 7000 h and to contribute to suppressing the growth of M23C6.

Formation of intermetallic compound at interface between rare earth elements and ferritic-martensitic steel by fuel cladding chemical interaction

The intermetallic compounds formation at interface between rare earth elements and clad material were investigated to demonstrate the effects of rare earth elements on fuel-cladding chemical interaction (FCCI) behavior. Mischmetal (70Ce-30La) and Nd were prepared as rare earth elements. Diffusion couple testing was performed on the rare earth elements and cladding (9Cr2W steel) near the operation temperature of (sodium-cooled fast reactor) SFR fuel. The performance of a diffusion barrier consisting of Zr and V metallic foil against the rare earth elements was also evaluated. Our results showed that Ce and Nd in the rare earth elements and Fe in the clad material interdiffused and reacted to form intermetallic species according to the parabolic rate law, describing the migration of the rare earth element. The diffusion of Fe limited the reaction progress such that the entire process was governed by the cubic rate law. Rare earth materials could be used as a surrogate for high burnup metallic fuels, and the performance of the barrier material was demonstrated to be effective.

Analysis of the Creep Behavior of P92 Steel Welded Joint

Different regions of heat-affected zone (HAZ) were simulated by heat treatment to investigate the mechanisms of the Type IV fracture of P92 (9Cr-2W) steel weldments. Creep deformation of simulated HAZ specimens with uniform microstructures was investigated and compared with those of the base metal (BM) and the weld metal (WM) specimens. The results show that the creep strain rate of the fine-grained HAZ (FGHAZ) is much higher than that of the BM, WM, the coarse-grained HAZ (CGHAZ), and the inter-critical HAZ (ICHAZ). According to the metallurgical investigation of stress-rupture, the FGHAZ and the ICHAZ have the most severely cavitated zones. During creep process, carbides become coarser, and form on grain boundaries again, leading to the deterioration of creep property and the decline of creep strength. In addition, the crack grows along the FGHAZ adjacent to the BM in the creep crack growth test (CCG) of HAZ.

Determination of creep and damage properties for P92 at 675 °C

In order to predict the service life of components that operate at high temperatures, such as steam carrying pipes in conventional power plants, the material creep behaviour needs to be determined. There are little creep data available on grade P92 (9Cr2W) steel (a potential successor to P91) as it is a relatively new material; therefore a testing programme has been undertaken. This paper presents the results of uniaxial and notched bar creep tests on P92 parent material (PM) and P92 weld metal (WM) at 675 °C. The PM had higher failure times and lower minimum creep strain rates for tests in the same stress range (80–100 MPa) as the WM, but the PM and WM values tend to converge at high stress, with a significant difference between the failure times as applied stress decreases. The notch strengthening effect was found to decrease as the applied stress decreased. Processing of the test data including the calculation of the minimum creep strain rates has been performed to determine the material constants required for Norton’s steady state creep and both the Kachanov and the Liu and Murakami creep damage models. Material constant sets for creep of P92 PM and WM at 675 °C, including a parameter to describe the effect of a multiaxial stress state, have been obtained that give a good fit to the test data. Validation was achieved using finite element analysis.

Reaction between a rare earth element and 9Cr-2W steel

We analyzed the reaction between a rare earth element and ferritic-martensitic stainless steel in order to evaluate the role of the rare earth element in the fuel cladding chemical interaction (FCCI). A diffusion couple test between Misch metal (70Ce-30La) and 9Cr-2W (Gr.92) steel at both 660 °C and 800 °C was conducted, and a microstructural analysis was carried out. The result showed that Fe in the Gr.92 material and Ce in the Misch metal mainly diffused and reacted to form intermetallic compounds, namely Fe2Ce and Fe17Ce phases, above 660 °C, whereas La hardly diffused or reacted in the Gr.92 material. In the specimen tested at 660 °C, the Fe diffused outside of the clad interface so that Cr-rich precipitated beneath the clad interface. In the specimen tested at 800 °C, a local reaction caused by the eutectic transformation of the (Fe,Cr)17Ce phase resulted in a local penetration across the clad.

Study on the cold working process for FM steel cladding tubes

A cold working process for FM steel cladding tubes of the sodium-cooled fast reactor was studied. Hot-rolled 9Cr–2W steel plates were heat-treated to dissolve the M 23C 6 and V-rich MX precipitates into the matrix, followed by several cold rollings and intermediate/final heat treatments with the purpose of nucleating the dissolved precipitates on the dislocations formed through the cold rolling. As a result, it was possible to obtain the FM steels with very fine and uniform precipitates which were more promising for superior tensile properties.

Fabrication Process for a High Strength 9Cr-2W Steel Sheet

This study deals with the fabrication of high strength ferritic/martensitic steels by a control of both the carbon concentration and the fabrication process parameters. The 9Cr-2W steels containing a carbon concentration of 0.05, 0.07 and 0.11 wt% were normalized at 1050oC for 1 h, followed by a tempering at 550 and 750oC for 2 h, respectively. The results of the tensile tests at room temperature indicated that the tensile strengths were increased with an increase of the carbon concentration from 0.05 wt% to 0.07 wt%, but no more increase was observed when the carbon concentration was increased further up to 0.11%. After a cold rolling from a 4 mm to a 1 mm thickness without/with an intermediate heat treatment and a final heat treatment, the results of the tensile tests exhibited that superior tensile properties were obtained when the fabrication processes were composed of a tempering at 550oC, and a cold rolling with several intermediate heat treatments. These results could be attributed to the finely distributed precipitates in the partially recrystallized matrix. The optimized carbon concentration and the controlled fabrication process parameters are thus suggested to fabricate a high strength 9Cr-2W steel sheet.

Effects of an intermediate heat treatment during a cold rolling on the tensile strength of a 9Cr–2W steel

The effects of an intermediate heat treatment during a cold rolling on the tensile strength of a 9Cr–2W steel were evaluated. Before a cold rolling, the steel was normalized at 1050 °C and tempered at 550 °C in order to avoid the formation of M 23C 6 and V-rich MX precipitates in the martensitic structure. A 75% cold rolling and a heat treatment at 750 °C for 30 min induced the formation of large M 23C 6 carbides in a fully recrystallized structure. However, three cold rollings with an intermediate heat treatment at 750 °C for 10 min after each cold rolling led to the formation of fine and uniform M 23C 6 carbides in a partially recrystallized structure, providing an enhanced tensile strength at 650 °C. It is thus concluded that an intermediate heat treatment during a cold rolling could be an effective procedure for fabricating a high strength 9Cr–2W steel at high temperatures.

Local creep evaluation of P92 steel weldment by small punch creep test

The aim of this research is to evaluate the local creep damage of various microstructures such as coarse grained heat affected zone (CGHAZ), fine grained heat affected zone (FGHAZ), intercritical heat affected zone (ICHAZ) known as Type IV region, weld metal, and base metal of P92(9Cr-2W) steel weldment by small punch (SP) creep test at 600°C. Also, to determine the weakest local part of the weldment, the effect of microstructures on creep rupture behavior in the weldment of P92 steel was studied. From the experimental results, the FGHAZ and Type IV region turned out to be very weak local parts of P92 weldment. Especially, the TYPE IV region showed the greatest displacement rate and had the shortest rupture life with many creep cavities widely spreaded in most of the region.