Martensitic Heat-resistant Steel Research Articles

Effect of post weld heat treatment on microstructural and mechanical properties of martensitic heat-resistant steel weldments

Effect of post weld heat treatment on microstructural and mechanical properties of martensitic heat-resistant steel weldments

A mechanism-based elasto-viscoplastic constitutive model for the creep deformation of martensitic heat-resistant steels

A mechanism-based elasto-viscoplastic constitutive model for the creep deformation of martensitic heat-resistant steels

Effects of radial bending stress on hot corrosion behaviour of dissimilar martensitic heat-resistant steel weldments in ultra-supercritical unit

The influences of the applied radial bending stresses on the corrosion behaviour of dissimilar weldments of two types of martensitic heat-resistant steel (F92/Co3W2) are investigated with NaCl–Na2SO4 deposited salts at 620 °C and 24 h employing a four-point bending jig. The applied external stress exhibits no obvious influence on corrosion products, which mainly consist of Fe2O3, Fe3O4, and FeCr2O4. However, the stress tends to aggravate the corrosion degree, and the weldments with stress addition yield more mass gain and greater corrosion scale thickness. Such phenomenon is more serious in the parent F92, implying F92 is more sensitive to the corrosion environment with stress. The corrosion scale of weldments without stress shows typical double-layer structure: outer Fe–O layer and inner Fe–Cr–O layer, while the weldments with bending stresses show interesting multi-layers (10 or more layers) with Fe-oxide and Fe, Cr-oxide alternately appearing. The mechanism for hot corrosion behaviour and the influence of hot corrosion on the local mechanical properties of different welding zones with and without stresses are further discussed.

Improving the Oxidation Resistance of G115 Martensitic Heat-Resistant Steel by Surface Treatment with Shot Peening

G115 steel is a novel martensitic heat-resistant steel, primarily utilized in the main steam pipelines and collectors of ultra-supercritical thermal power units. However, the oxidation resistance of martensitic steels in the high-temperature steam environment is usually suboptimal, significantly affecting the efficiency of power plants. In this paper, shot peening (SP) is employed as a surface treatment method for G115 steel, and the oxidation kinetics, oxide layer thickness, and microstructure of shot-peened G115 samples are compared with those of G115 steel. The results indicate that in the 650 °C steam environment, the oxidation kinetics of the shot-peened samples follow the parabolic law and that the oxidation weight gain is significantly smaller than that of the non-shot-peened samples. The higher the SP intensity, the smaller the oxidation weight gain and the better the oxidation resistance. This can be attributed to the fragmentation of the grains in the surface layer caused by external stress during SP, which creates a multitude of grain boundaries that can provide rapid diffusion pathways for corrosion-resistant Cr atoms, resulting in the accelerated outward diffusion of Cr atoms from the substrate. Simultaneously, a continuous and dense FeCr2O4 protective layer is produced at the interface between the SP layer and the substrate, obstructing the inward diffusion of oxygen and enhancing the oxidation resistance of G115 steel.

Improving creep strength of the fine-grained heat-affected zone of novel 9Cr martensitic heat-resistant steel via modified thermo-mechanical treatment

Improving creep strength of the fine-grained heat-affected zone of novel 9Cr martensitic heat-resistant steel via modified thermo-mechanical treatment

Distribution of Boron in 9.5Cr–1.5MoCoVNbNB Martensitic Heat-Resistant Steel Studied by Secondary Ion Mass Spectroscopy and atom Probe Tomography

Distribution of Boron in 9.5Cr–1.5MoCoVNbNB Martensitic Heat-Resistant Steel Studied by Secondary Ion Mass Spectroscopy and atom Probe Tomography

Spatiotemporal variations of creep damage in dissimilar welded joints

The influence of high-temperature creep on local microstructural, physical and mechanical performance for different regions of dissimilar welded joints of two kinds of martensitic heat-resistant steels F92 and Co3W2 is studied, under 873 K and 150 to 200 MPa. The rupture location is located in F92 for all testing specimens under various creep conditions and the fracture morphology exhibits a transgranular plastic fracture. Analysis on creep rupture data yields a 100,000 h creep rupture stress of 93.5 MPa, and a creep damage tolerance of 4.93. Creep damage micromechanisms like external cross section loss, microcavities’ nucleation and growth and microstructural deterioration are observed, and creep damages are more complicated in the crept dissimilar welds compared to single material because they are not only location dependent but also time dependent. Nanoindentation technique was implemented to acquire local nanohardness, reduced modulus, plasticity index, strain rate sensitivity and activation volume of different welding microzones with various degrees of creep damage. Co3W2 side displays better local physical and mechanical properties after creep compared to uncrept state due to the positive impact of creep, while the properties of F92 side improve for short creep and are decreased after longer creep, which are consistent with their microstructural evolution during creep.

Electrochemical Characterization Method of Laves Phase in 9Cr Martensitic Heat-Resistant Steel and Creep Life Prediction

The Laves phase formed during the long-term creep of 9% Cr heat-resistant steels plays an important role in their high-temperature mechanical properties. Detecting and quantifying the Laves phase is one of the main problems in predicting the creep residual life. This study detects and quantifies the Laves phase in various 9% Cr heat-resistant steel samples by anodic polarization. Both T92/P92 samples after creep and 9Cr ferritic martensitic samples after thermal ageing precipitate Laves phase particles, and the content of the Laves phase increases with the service time. Comparing the results obtained by the electrochemical method with those obtained by the traditional SEM observation method, it can be found that there is a similar two-stage linear relationship between the two results in all materials, which is related to the diameter change of Laves phase particles during service. According to the correlation between the Laves phase content and creep time, an extrapolation method for the prediction of the residual creep life is proposed. The results show that the creep life of P92 can be predicted by using the dissolved charge density of the Laves phase with high prediction accuracy. This method has the potential to evaluate the service states of materials and predict the long-term creep life in the industrial field.

Ductile ultrastrong China low activation martensitic steel with lamellar grain structure

The low high-temperature mechanical properties of reduced activated ferrite martensitic (RAFM) heat-resistant steels limit the maximum operating temperature of nuclear fusion reactors. However, it is difficult to improve the strength of the material without reducing its plasticity. In this study we prepared a China low activation martensitic (CLAM) steel with a layered structure through hot rolling followed by low-temperature tempering, which successfully improved its strength and ductility. In addition, we elucidate the ductility mechanisms in such lamellar structural materials. When the strength of the grain boundaries exceeds that of the grains, cracks will continuously extend along the layered grain boundaries, eventually resulting in a delamination fracture mechanism. When the grain strength exceeds that of the grain boundaries, cracks will continuously form through the relative sliding of the grain boundaries, and a large number of small cracks will initiate and propagate longitudinally along the layered boundaries. Both forms of crack propagation cause longitudinal expansion along the layered grain boundaries. The resulting longitudinal extension of cracks markedly diminishes local stress concentration to promote plastic deformation, thereby significantly increasing the ductility of the material. Employing the combination of this delamination ductility mechanism with interface strengthening and precipitation strengthening, the strength of the CLAM steel for nuclear applications is shown to be enhanced by ∼30 % without loss of elongation. The current delamination ductility strategy provides a unique pathway to develop materials with ultrahigh strength and superior ductility at economical material costs.

A robust life prediction model for a range of materials under creep-fatigue interaction loading conditions

This paper develops a robust creep-fatigue interaction (CFI) life prediction model which is superior to the existing methods. Specifically, the newly proposed creep damage incorporates the effect of creep strain, which introduces the critical creep strain energy density rate and creep strain evolution term. The predictions are carried out for various materials, including martensitic heat-resistant steel, austenitic stainless steel, nickel-based superalloy, and different loading conditions, including conventional CFI and hybrid-controlled CFI loadings. The results show that all of the tested data falls within ± 2.5 error band, demonstrating the broad applicability of the proposed model. Based on the proposed model, a continuous CFI failure envelope independent of material, temperature, and CFI loading type is proposed, with almost all the data points lying outside the envelope.

Influence of Different Weld Heat Input on Microstructure and Properties of New Martensitic Heat-resistant Steel Joints by Automatic Welding

In this article, the influence of different automatic weld heat input about the microstructure and properties of the joint of new martensitic heat resistant steel of P92 is studied. Based on this, the key factors to obtain excellent quality of P92 joint are proposed. The results show that the weld is tempered martensite microstructure, the grain of Heat-affected zone (HAZ) is finer with the weld heat input lower. The width of the slats and the crystalline size of the coarse-grained zone (CGHAZ) in the weld metal microstructure is gradually larger with the heat input higher. Meanwhile, as the welding heat input increase, the hardness of weld bead in the welded joint of P92 steel has a gradual downward trend, the impact toughness has a significant decrease, and the impact fracture morphology changes from ductile fracture to brittle fracture. Therefore, when the automatic welding technology is used to repair P92, so as to obtain better performance of welded joints, appropriate gas protection flow and welding parameters should be ensured according to welding thickness, and heat input should be strictly controlled.

Mechanism of mitigating quenching cracking for B-containing 9Cr martensitic heat resistant steel by austempering process

B-containing 9Cr martensitic steels are attractive materials for specific industrial applications due to their excellent high-temperature strength and ductility. However, quenching large forgings with water or oil often results in cracking. The correlation between quenching crack and microstructure was established in this work to evaluate the selection of the quench cooling methods, which make it possible to mitigate the quench cracking by tailoring microstructural evolution. Kinetics dynamic analysis, microstructural characterization, and phase-field simulation were used to clarify the mechanism of mitigating quenching cracking. It is demonstrated that austempering produces a mixed microstructure consisting of lathy martensite, ultra-fine martensite, and a small amount of C-riched retained austensite. This mixed microstructure translates into refining the effective grain size and reducing the internal stress, reducing cracking propagation by reducing phase transformation stress and enhancing the coordinated deformation. Thus, the quenching cracking can be inhibition by tailoring the fraction of ultra-fine martensite and retained austensite in B-containing 9Cr steels.

Ab-initio calculations on the interaction between OH− and Fe2Mo Laves phase

In the process of heat treatment at high temperature, A2B type intermetallic compound Fe2Mo can precipitate out in the martensitic heat-resistant steel. When the martensitic heat-resistant steel is used in high concentration alkali solution of 7 mol/L, the Fe2Mo Laves phase can dissolve. Meantime, the dissolution behaviors of different shapes of Fe2Mo Laves phase are also distinct. However, there is still no effective micro-mechanism to explain the dissolution behaviors of Fe2Mo Laves phase. Thus, in this work, the adsorption behavior of OH− on Fe2Mo(001), (010), (110) and (112) surfaces was simulated by ab initio calculations. And corresponding electronic effects were comparatively analyzed, the results of which show that the outmost surface of Fe2Mo changed significantly before and after adsorption, demonstrating that the Fe2Mo surface can be influenced by OH−. The results of adsorption energy, Mulliken population, density of states and electronic local function indicate that the Fe2Mo(001) surface has a greater adsorption trend, and the OH−/Fe2Mo(001)-b configuration has the largest adsorption energy. Electrons transfer from OH− to the Fe2Mo surface, and the adsorption type is chemisorption. The results provide a theoretical basis for studying the selective dissolution of Fe2Mo Laves phase in strong alkali solution.

Influence of multiple short-time normalizations on mechanical properties and microstructure of 9Cr-2.3W-3.0Co heat-resistant steels

In this study, the mechanical properties and microstructure of the 9Cr-2.3W-3.0Co martensitic heat-resistant steels were investigated after continuous transformation of martensite-austenite, multiple short-time normalizing and tempering at 780 °C for 180 min. The results showed that with increasing number of normalization cycles, the tensile strength of martensitic heat-resistant steel did not change significantly and remained at approximately 700 MPa, while the toughness increased from 182.8 ± 5 J after the first normalization to 369.7 ± 6 J after the fourth normalization. The size and particle spacing of the M23C6 particles in the matrix gradually increased, and the number density decreased with increasing number of normalizations. This was mainly due to the dissolution of fine M23C6 particles in the matrix caused by multiple normalizations. The main reason for the influence of the multiple normalizations on the toughness was grain refinement, which increased the proportion of high-angle grain boundaries, thereby increasing the difficulty of crack propagation. The reason for the stability in strength after multiple normalizations was attributed to the positive effect of grain boundary strengthening being counteracted by the negative effect of dispersion strengthening. Performing multiple normalizations is an effective method for improving the toughness of 9Cr-2.3W-3.0Co martensitic heat-resistant steel.

Research on liquid nitrogen cryogenic milling of 11Cr-3Co-3W martensitic heat-resistant steel

PurposeWhen processing 11Cr-3Co-3W martensitic heat-resistant steel, the traditional pouring cooling method often appears large cutting force, high cutting temperature, serious tool wear and poor surface quality. This paper aims to use new cooling methods for processing this problem.Design/methodology/approachDifferent performance indicators such as cutting force, tool wear and surface quality were measured and analysed under different continuous milling times. The relationship between liquid nitrogen flow and cutting force and surface roughness was analysed and measured.FindingsThe results show that with the increase of liquid nitrogen flow, the cutting force decreases, especially the Fx component, which decreases by 10%. When the liquid nitrogen flow reaches 8 L/min, the effect of increasing the liquid nitrogen flow on reducing the cutting force becomes smaller. The cutting force reduced by up to 15%, and the tool life increased up to 20% using liquid nitrogen cryogenic cooling than in cutting liquids cooling. When minimal quantities of lubricant (MQL) was added, the cutting force was reduced by 23%, and the tool life increased by 25%. When the cutting speed increases from 100 m/min to 250 m/min, the cutting force with cutting liquid cooling does not change significantly while the cutting force with liquid nitrogen cooling decreases with the cutting speed increasing. It shows that liquid nitrogen cooling is more suitable for high-speed machining. After the cutting length reaches 66 m, the surface roughness of the workpiece using liquid nitrogen cooling method larger than that of the cutting liquid cooling method. When MQL is added into liquid nitrogen, the lubrication performance is improved, and the surface roughness of the workpiece is reduced about 8%.Originality/valueMany studies had focused on the improvement of tool life and surface quality by different cooling methods, or on the injection process and chip mechanism. However, there are few relevant studies on the variation of cooling and lubrication properties with the change of cutting length in liquid nitrogen cryogenic processing. In this research, different performance indicators such as cutting force, tool wear and surface quality were measured and analysed under different continuous milling times. The relationship between liquid nitrogen flow and cutting force and surface roughness was analysed and measured.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-03-2023-0053/

Microstructure and creep properties of graded transition joints between nickel-based alloys and martensitic heat-resistant steels

Interfacial failure can significantly reduce creep lifetime of dissimilar metal welds (DMWs), bringing great economic losses and safety issues to power plants. The fundamental reason for interfacial failure is steep gradient of chemical compositions between different metals. Many researchers have made great efforts to develop graded transition joints (GTJs) aiming to eliminate the steep gradient, but the results of creep tests were rarely seen. The goal of the present work was to determine whether GTJs could effectively prolong creep life compared with conventional DMWs, and further to clarify the reason. In our study, a GTJ between creep-resistant steels and nickel-based alloys was fabricated by a dual-wire TIG method. It was shown that the gradients of chemical compositions and hardness were significantly reduced over a longer distance of ∼17 mm than that of ∼100 μm in conventional DMWs, and microstructures varied from full martensite to mixed martensite + austenite, and finally to full austenite. The formation of dual-phase region composed of martensite and austenite was due to the segregation behavior of Cr, Ni, and Mo to inter-dendritic regions during solidification. Creep tests showed that creep life of the GTJ was much shorter than conventional DMWs, and two different failure modes were found, one of which failed in COST E HAZ (creep life 790 h), and the other in the nickel-rich martensite zone of transition region (creep life 435 h). The failure in COST E HAZ was the combined results of creep, plastic deformation and oxidation, and the failure in nickel-rich martensite zone was due to localized austenization induced shrinkage during creep tests. Nickel-rich martensite was not observed after post weld heat treatment (PWHT) due to the sluggish kinetics of nickel atoms diffusion, while creep tests provided enough time for nickel atoms to diffuse into austenite, which transformed into martensite with nickel enriched after creep tests. Based on the results, it is concluded that the GTJ might not be a feasible solution to extend creep life of DMWs at least in the current situation, due to the inevitable occurrence of nickel-rich martensite.

Promoting the dissolution of delta ferrites by cyclic heat treatment in martensitic heat-resistant steel

Promoting the dissolution of delta ferrites by cyclic heat treatment in martensitic heat-resistant steel

Effect of tempering temperature on plasma-welded joint of martensitic heat-resistant steel

The effect of tempering temperature on microstructure and mechanical properties of plasma welded martensitic heat-resistant steel P92 was investigated by tempering heat treatment process (tempering temperature range was 550–750°C, temperature interval was 100°C). The results show that with the increase of tempering temperature, the microstructure of welded joint coarsens, the tensile strength and hardness decrease, and the toughness is improved.In the tempering process, M23C6 and Laves phases are produced in the welded joint. A certain amount of M23C6 precipitates can strengthen the mechanical properties of the welded joint. Excessive M23C6 carbides lead to the softening of martensite and the decrease of hardness; the formation of Laves phase can inhibit the excessive precipitation of M23C6, but the excessive precipitation of the Laves phase will lead to coarsening at the grain boundary and the formation of voids at the grain boundary. When the welded joint temperature is 650°C, the comprehensive mechanical properties of the welded joint are the best.

Study on Intelligent Classification of Aging Heat-Resistant Materials.

High chromium martensitic heat-resistant steel is considered as a candidate material for pressure components of the next generation of incinerators of the subcritical level or above in China due to its excellent high-temperature and corrosion resistance, but in the long-term service, aging will significantly affect the service safety of materials. So, accurate identification of its aging state is important to enhance the safety of a power plant. In this paper, an automatic aging grading model of high chromium martensite heat-resistant steel based on the depth residual network is proposed according to different scales of metallographic data. A multiscale data set is constructed by image reduction to verify the accuracy of the model in identifying microstructure images of high chromium martensitic heat-resistant steel with different scales. The experimental results show that the model using multiscale data sets performs well, and then, through feature pyramid network model training, the accuracy rate is further improved, and a relatively good prediction accuracy model is obtained. The validity of the deep learning method for the classification of damage and aging of P91 steel with different scales is verified.

Application of post-weld treatment (PWHT) in improving properties of martensitic heat-resistant steel thick plate by cold metal transfer plus pulse (CMT + P) welding

Application of post-weld treatment (PWHT) in improving properties of martensitic heat-resistant steel thick plate by cold metal transfer plus pulse (CMT + P) welding