G115 Steel Research Articles

Evolutions on Microstructure and Impact Toughness of G115 Steel after Long-Term Aging at 700 °C

The microstructure and impact toughness evolution of G115 steel after long-term (ranging from 500 h to 10,000 h) aging at 700 °C were investigated in this study. The results showed that the microstructure of the G115 steel evolved from a finer-grained matrix with minor precipitates to a coarse-grained matrix with more precipitate with aging time, presenting a decrease in the local deformation degree in the matrix. The impact toughness of the steel decreased with aging time, presenting the largest decline at the initial aging times. The decrease in impact toughness was attributed to the coarsening of precipitates (M23C6 and Laves phase) in the steel matrix. The stable impact toughness during the whole aging process (from 500 h to 10,000 h) should be related to the comprehensive effects, including the precipitation of the Laves phase, the increase in high-angle grain boundaries, and the softening of the metal matrix.

Effect of aging on microstructure and mechanical properties of dissimilar steel welded joints between G115 and Sanicro25 steel

This paper studied the high-temperature stability of dissimilar steel welded joints between G115 and Sanicro25 steel. The joints were exposed to 700 °C for time up to 2000h. The microstructure evolution of base metal (BM) and fine-grain heat-affected zone (FGHAZ) were analyzed. Tensile, microhardness, and impact experiments were conducted on the joints, and the explanations for the changes in mechanical performance are derived from the microstructure. The results showed that the Laves phase were precipitated after aging for 100 h, which has two nucleation methods. Nucleation adjacent to M23C6 is the main way in this research. Laves phase mainly grows up to consume M23C6. The microstructure in BM and FGHAZ is obviously different after aging. After aging for 1000 h, mechanical properties such as tensile strength, Vickers hardness, impact energy, elongation and area reduction are reduced, except for the yield strength. The softening caused by the microstructure evolution of FGHAZ is the main reason for the joint fracture in FGHAZ.

Importance of cold rolling and tempering on the microstructure evolution, precipitation behavior and mechanical responses of 9Cr3Co3W1Cu ferritic/martensitic steel

The effects of thermomechanical treatment (cold rolling + tempering) on the microstructure evolution, precipitation behavior, and mechanical properties of the 9Cr3Co3W1Cu martensitic steel (G115) were investigated, by adjusting the cold rolling reduction (0, 20% and 40%), tempering temperature (750 °C and 800 °C) and time (30 min and 120 min). The results indicate that the G115 steel with 20% cold-rolling reduction rate followed by tempering at 800 °C for 30 min exhibits the optimum mechanical response (yield strength (σs) 899 MPa, elongation (δ) 26%), compared with that of 40% cold-rolled (σs 812 MPa, δ 17%) and undeformed steels (σs 670 MPa, δ 15%). The optimum yield strength in 20%-800 °C-30 min steel is attributed to the dislocation strengthening and precipitation strengthening, for the relatively low recrystallization rate and high density Cr23C6 and NbC precipitates, and the good ductility is attributed to the low recrystallization ratio and the uniform microstructure. As a comparison, in 40%-800 °C-30 min steel, a ferrite martensite dual phase structure formed. Due to the hardness difference between the two phases, there may be significant inharmony during the deformation process, leading to premature fracture.

Deformation behavior, microstructure evolution, and rupture mechanism of the novel G115 steel welded joint during creep

To promote the wide application of the G115 steel in ultra-supercritical (USC) power plants, creep deformation, microstructure evolution and rupture mechanism of the G115 welded joint were systematically investigated. The fabricated G115 welded joint exhibits superior creep strength to the P92 or MARBN steels. The deformation behaviors are analyzed by the Norton's power law, proving that the deformation mechanism is closely associated with the stress levels. During short-term creep under higher stress, the weld metal (WM) and heat-affected zone (HAZ) exhibit outstanding microstructure stability, and the rupture occurs within the base metal (BM) due to the lath fracture. While, during long-term creep under lower stress, the brittle Type IV cracking occurs within the inter-critical heat-affected zone (ICHAZ) or fine-grained heat-affected zone (FGHAZ) due to the formed micro-voids along boundaries, obviously deteriorating the creep strength. The weld strength reduction factors are calculated under different stresses, and prove that synergistic action of back stress and dislocation motion can enhance the creep resistance of the G115 welded joint. Additionally, the allowable service stress of G115 weldments should be designed according to the creep strength of the welded joints under expected performance life.

Microstructure characteristics in weld zone of the novel thick 08Cr9W3Co3VNbCuBN heat-resistant steel welded joint by fusion welding

ABSTRACTOne of the primary candidate materials used in 650°C ultra-supercritical (USC) thermal power plant units is a novel thick grade of heat-resistant steel, 08Cr9W3Co3VNbCuBN (G115), for which China has complete intellectual property rights. In this study, G115 steel with a 115 mm thick wall was successfully welded using gas tungsten arc welding (GTAW) + shielded metal arc welding (SWAM), which was a significant step toward achieving its application in large-diameter boiler tubes. The influence of the welding process on the microstructure and mechanical properties of the G115 welded joint after post-weld heat treatment (PWHT) at 770°C for 11 h was investigated. The findings demonstrate that the weld zone (WZ) after PWHT was composed of martensite and ferrite, and M23C6 was the precipitate. The average hardness of WZ was 273.1 HV, and the average impact toughness was above 39J, which was sufficient for actual production applications. It offered the necessary theoretical foundation for the practical development of thick-walled G115 heat-resistant alloy steel during welding.

Continuous Cooling Transformation Behavior in Welding Coarse-Grained Heat-Affected Zone of G115 Steel With the Different Content of Boron

Continuous Cooling Transformation Behavior in Welding Coarse-Grained Heat-Affected Zone of G115 Steel With the Different Content of Boron

Active element variation of G115 heat-resistant steel and its dependence on slag composition during electroslag remelting

The steel-slag reactions and the changes of boron, aluminum, and silicon contents of G115 heat-resistant steel for ultra-supercritical power plants during protective argon gas atmosphere electroslag remelting (ESR) were investigated. The pickup of boron and aluminum was inhibited with the increase in SiO2 content of the slag during protective argon gas atmosphere ESR through inhibiting steel-slag reactions. The contents of silicon and aluminum in steel decrease, and the boron content increases with the increase in the B2O3 content of slag in protective argon gas atmosphere ESR. The change of aluminum and boron content of the steel during ESR is driven by chemical reactions [Si]-(Al2O3) and [Si]-(B2O3). A kinetic model for describing and predicting alloying element changes of G115 heat-resistant steel and their dependence on slag composition during ESR is developed. The kinetic model indicates that the proportions of aluminum pickup, silicon loss, and the pickup or loss of boron at the tip of electrode during protective argon gas atmosphere ESR are 57.62–68.69 mass%, 90.31–98.19 mass%, and 94.48–98.99 mass%, respectively. The boron content of the ingot increases from 0.0059 mass% to 0.0203 mass% as the B2O3 content of the slag increases from 0.2 mass% to 1.0 mass%. Alternative slag for electroslag remelting of G115 steel with the target of boron content (ranging from 0.0080 mass% to 0.0150 mass%) and aluminum content (lower than 0.0100 mass%) is composed of 47.34 mass% CaF2, 20.46 mass% CaO, 9.00 mass% SiO2, 0.50 mass% B2O3, 22.00 mass% Al2O3, and 0.50 mass% MgO.

Microstructures and properties of a novel 115 mm thick 08Cr9W3Co3VNbCuBN heat-resistant steel tube joints welded by shielded metal arc welding

A novel heat-resistant steel tube, 08Cr9W3Co3VNbCuBN (G115), is the primary choice for crucial components of ultra-supercritical (USC) thermal power plants. In this paper, the 115 mm thick G115 steel tube was welded successfully by shielded metal arc welding (SMAW) and gas tungsten arc welding (GTAW), and the microstructures and mechanical characteristics of joint were studied. The results indicated that the microstructure of the weld zone (WZ) was mostly ferrite and martensite, and the content of ferrite at the weld root was 19.9% lower than that at the weld center. However, at the weld toot the grain size was reduced by about 20%, and the content of M23C6 was increased by 30.9%. Previous austenite grain boundaries (PAGBs) and martensite occurred in the heat-affected zone (HAZ) because to varying heat input, and a tiny quantity of fine Cu-rich phase precipitated from the root of HAZ. The hardness distribution of joint from the base metal (BM) to WZ demonstrated a low to high phenomena, with the average hardness of the BM being 235 HV and the average hardness of WZ being enhanced by 1.16 times. The fine secondary phases precipitated and diffused in the WZ, therefore reinforcing the martensite and improving its mechanical properties. The average impact absorption energy of the weld zone was 43.89 J, and the average impact energy of the HAZ was 54.57 J, both of which were sufficient to fulfill the actual standard (>31 J). The fracture morphology revealed a complex fracture mechanism with dimples and cleavages on the surface of joint, and the toughness was positive. The causes of the decrease in hardness and the rise in toughness might be the Cu-rich phase precipitated from the root of HAZ. Furthermore, the corrosion resistance of the welded joint (WZ: Icorr = 10.542 μA cm−2, HAZ: Icorr = 11.241 μA cm−2) was lower than that of the BM (Icorr = 9.4844 μA cm−2).

Improvement of Mechanical Properties of G115 Steel Welded Joint by Adjusting Welding Current

Improvement of Mechanical Properties of G115 Steel Welded Joint by Adjusting Welding Current

Development of weld filler material to match the advanced martensitic heat resistance steel G115 and tailoring the performance by tempering temperature

The lack of filler materials is the crucial issue that restricts the wide application of the advanced heat-resistance steel G115 in ultra-super critical (USC) power plants. In the present work, a new welding filler material with alloying system of 8.8Cr-2.7W-2.8Co was successfully developed to fabricate the G115 steel welded joint by the gas tungsten arc welding (GTAW), and post weld heat treatments (PWHT) involving different temperatures were proposed to tailor the mechanical properties. The filler material exhibited perfect compatibility with the base material (BM), and the manufactured welded joint displayed satisfactory appearance and defect-free weld section. After PWHT, full martensitic lath structure decorated with high density of dislocations and dispersed strengthening precipitates M23C6 and MX formed in the weld metal (WM), which was similar with that of the BM. Meanwhile, the welded joint was sensitive to the PWHT temperature, and an optimal combination of hardness (∼240 HV), tensile strength (∼679.9 MPa at 25 °C, and ∼344.6 MPa at 650 °C), elongation (∼13.1% at 25 °C, and ∼15.6% at 650 °C) and impact toughness (∼73.8 J/cm2) could be achieved after PWHT at 760 °C for 240 min. Comprehensively considering the obtained results, the designed filler material was feasible and reliable for matching the novel G115 steel, and satisfied the service demands of the G115 steel pipes in USC power plants.

Precipitation and Strengthening Behavior of M23C6 Carbides in Tempered G115 Steel

Precipitation and Strengthening Behavior of M23C6 Carbides in Tempered G115 Steel

The high-temperature creep mechanism of G115 steel joint under the vacuum electron beam welding

The welding joints of G115 steel were obtained from vacuum electron beam welding technology (VEBW), and their creep behavior was tested under the temperature at 630[Formula: see text]C, 650[Formula: see text]C and 670[Formula: see text]C with different stresses. According to the results, at the same temperature, the time of creep rupture decreases significantly with the test stress increasing, while the time of creep rupture declines significantly with the increased temperature at the same stress condition. The critical stress of creep for the welding joints at 630[Formula: see text]C, 650[Formula: see text]C and 670[Formula: see text]C is 115.28 MPa, 104.73 MPa and 96.98 MPa respectively. The stress index of welding joints between different temperature and stress conditions is about six, which indicates that the mechanism of creep deformation is controlled by the dislocation-climbing mechanism.

Microstructural evolution and the effect on strength of G115 steel after long-term creep at 923 K

The microstructural evolution and the effect on the strength of G115 steel after creep under 923 K and 130 MPa for up to 7288 h were systematically investigated using OM, SEM and TEM. The results show that the variation tendency of total calculated strengthening contribution is in a good agreement with the measured hardness of G115 steel after creep at 923 K. A new nucleation mechanism of Laves phase nucleated adjacent to Cu-rich phase in the grain interiors is found in G115 steel during creep at 923 K. Compared with Laves phase at grain boundaries and martensitic lath boundaries, the growth velocity of Laves phase nucleated adjacent to Cu-rich phase is lower. Laves phase and Cu-rich phase play a vital role in the strength of G115 steel during creep at 923 K in addition to M23C6 carbides. Not only Laves phase and Cu-rich phase in the grain interiors help to increase the strength by enhancing the dislocation strengthening and precipitation strengthening, but also Laves phase and Cu-rich phase affect the strength variation by influencing the precipitation strengthening obviously. In contrast, Laves phase at martensitic lath boundaries is easy to coarsen, which may be unfavorable to the strength of G115 steel due to the weakened martensitic lath strengthening. In order to increase the beneficial effect of Laves phase and Cu-rich phase, it is necessary to increase the volume fraction of Cu-rich phase and Laves phase in the grain interiors, and control the growth of Laves phase. Increasing the amount of Cu-rich phase can promote the precipitation of Laves phase with low growth rate by providing more nucleation sites for Laves phase in the grain interiors.

Strengthening and toughening optimizations of novel G115 martensitic steel: Utilizing secondary normalizing process

Novel G115 martensitic steel (G115 steel) with excellent creep properties, oxidation resistance and low manufacturing cost, has been widely used for ultra-super critical (USC) power plants. Microstructural evolutions were characterized using multiple methods, and their effects on mechanical properties of G115 steel after secondary normalizing process at different temperature (1000 °C, 1050 °C and 1100 °C) were also investigated. The results show that the average width of martensitic lath decreases with increasing secondary normalizing temperature, which is inversely proportional to the dislocation density. The volume fraction of M23C6 precipitates increases with increasing secondary normalizing temperature. The maximum yield strength of G115 steel is achieved after hot-rolling, which is mainly derived from martensitic laths strengthening and dislocations strengthening. Compared to single normalizing treatment, the yield strength, the elongation, and the impact toughness of G115 steel can be improved after secondary normalizing treatment.

A new strategy to improve the creep strength of a novel G115 steel by accelerating the precipitation of nano-sized MX and Cu-rich particles

A new thermo-mechanical treatment strategy is proposed to improve the creep resistance of a recently developed G115 steel. The goal of this strategy is to construct a favourable microstructure with superior thermal stability at high temperatures up to 650 °C. The improved creep strength can be ascribed to the reinforced interactions between dislocations and nano-sized MX and Cu-rich precipitates. Creep analysis suggests that these MX and Cu-rich particles exhibit more important contribution to the creep resistance than the M23C6 carbides. Following this strategy, the creep rupture life of the G115 steel can be significantly prolonged by more than two times.

Creep behavior, microstructure evolution and fracture mechanism of a novel martensite heat resistance steel G115 affected by prior cold deformation

In this paper, the influences of prior cold deformation on creep behavior, microstructure evolution, and fracture mechanism of the novel G115 steel were systematically investigated. It was found that the creep rupture life of the G115 steel could be significantly prolonged by moderate deformation reduction, which was a synergetic effect of deformation-induced dislocations and dispersed strengthening particles. The optimal deformation reduction was approximately 20%, at which the precipitation of Laves phases could be obviously promoted, and then restricted the coarsening of M23C6 carbides due to the competitive growth relationship. Besides, more nano-sized Cu-rich particles precipitated within lath interior, and obstructed the free dislocation motions. As a feedback, the dislocation recovery and the subgrain formation could be impeded, thus extending the creep rupture life. With the further increase of deformation reduction to 30% or higher, the coarsening of precipitates and the recovery of dislocations were intense, which could facilitate the formation of subgrains and then shorten the creep life. For the G115 steel with a deformation reduction within 30%, the creep damage was identified as microstructural degradation with limited contribution from necking. And, for the steel with a deformation reduction of 45%, the creep damage was cavity growth controlled by power law and diffusion creep.

Microstructural Evolution during Thermal Exposure of G115 Steel and Its Influence on Mechanical Properties

Microstructural evolution and mechanical property degradation of the newly developed G115 steel during thermal exposure are highly important for the safe power plant operation. In this paper, the microstructural stability and strength as well as impact toughness were investigated after aging at 650°C and 700°C up to 5000 h. It was found that when aged at 650°C in the G115 steel, the size of M23C6 carbides and the width of martensitic laths had little change even increasing aging time up to 5000 h. Both the room-temperature and high-temperature strengths remained stable after aging for 5000 h. In comparison, after aging at 700°C, high-density Laves phases precipitated accompanied by coarsening with increasing aging time. The majority of martensitic lath structure was replaced by sub-grains and the strength decreased linearly. Most importantly, the impact toughness of G115 steel aged at both 650°C and 700°C decreased sharply and embrittlement occurred rapidly in the initial aging period. The evidence provided by fracture morphologies proved that Laves phase was the culprit of decreasing the impact toughness.

Cold metal transfer plus pulse (CMT+P) welding of G115 steel: Mechanisms, microstructure, and mechanical properties

G115 steel is a prime candidate for key components in ultra-supercritical (USC) power plants. In this study, G115 welded joint of 15 mm thickness had been successfully fabricated using Cold metal transfer plus pulse (CMT + P) welding and the effect of heat input on microstructure and mechanical properties of G115 welded joint had been studied. The microstructure and mechanical properties were evaluated after PWHT (780 °C, 2 h). With the increase of heat input, the high temperature duration of welded joints increased, and the high temperature tempering effect increased, inducing more precipitates along grain boundaries and grains and thus enhancing the pinning effect of precipitates. Then the hardness and strength were improved 1.2 times and 1.1 times respectively as the heat input increased from 0.51 kJ/mm to 0.89 kJ/mm. However, as the heat input increased from 0.51 kJ/mm to 0.89 kJ/mm, the content of ferrite decreased, inducing the Charpy impact toughness was reduced by 50％.

Tailoring the tempered microstructure of a novel martensitic heat resistant steel G115 through prior cold deformation and its effect on mechanical properties

In this paper, the influences of prior cold deformation and tempering time on the precipitation behavior, substructure evolution, and mechanical properties of a novel G115 martensitic heat resistant steel were systematically investigated. It was found that the deformation reduction and tempering time had significant influences on the precipitation behavior of secondary strengthening particles, the recovery of dislocations, and the formation of subgrains, which played a crucial role in determining the tensile properties and micro-hardness. When the deformation reduction was lower than 20%, the increase of reduction could induce more dislocations or defects, which provided additional nucleation sites for M23C6 particles and obstructed the motion of dislocations and sub-boundaries, thus enhancing the tensile strength. However, when the deformation reduction was higher than 20%, the excessive reduction could accelerate the coarsening of M23C6 particles by providing more diffusion channels, thus weakening the pinning ability for boundaries and leading to substructure instability. For the G115 steel with a deformation reduction of 20%, an optimal combination of mechanical properties with an average hardness of 283.4 ± 8 HV, a tensile strength of 1102 ± 6 MPa, a yield strength of 783.6 ± 31 MPa, and a considerable ductility of 19 ± 2% could be achieved after tempering for 60 min.

Evolution of Microstructure in Welding Heat-Affected Zone of G115 Steel with the Different Content of Boron.

Welding thermal simulation was performed to investigate the effects of boron content (0, 60, and 130 ppm), welding peak temperature (Tp), and cooling time from 800 to 500 °C (t8/5) on the microstructure, carbide, subgrain, and microhardness of heat-affected zone (HAZ) in G115 steel. According to the experimental results, the microstructure of coarse-grained HAZ (CGHAZ), fine-grained HAZ (FGHAZ), inter-critical HAZ (ICHAZ), and sub-critically HAZ (SCHAZ) was martensite, martensite containing a small amount of undissolved carbide, martensite, and over-tempered martensite, tempered martensite, respectively. The presence of B element improved the thermal stability of M23C6 carbide, thereby resulting in a greater amount of undissolved carbides with a larger diameter in the materials with higher B content under the same Tp. Element B is effective in improving Ac1 and Ac3 for the material. Besides, compared with the material without and containing 60 ppm B, the Ac1 and Ac3 of the material containing 130 ppm B increased by 95 and 108 °C, 69 and 77 °C, respectively. Meanwhile, the FGHAZ area of the material containing 130 ppm B was significantly lower than the material without or containing 60 ppm B, indicating that element B can significantly reduce the formation range of FGHAZ. The alloy content in austenite of ICHAZ of materials without or containing 60 ppm B increased, compared with CGHAZ, its Ms and Mf declined by 50 and 7 °C, 46 and 7 °C, respectively. In contrast, the alloy content in austenite of the material with 130 ppm B content decreases, its Ms and Mf was 37 °C and 32 °C higher than CGHAZ, respectively. The microhardness of HAZ was ranked in descending order as CGHAZ, FGHAZ, ICHAZ, and SCHAZ. Differently, the microhardness of CGHAZ and FGHAZ showed an increasing trend with the rise of B content but exhibited a decreasing trend with the rise of t8/5.