Cr Heat Resistant Steel Research Articles

Microstructural evolution in 12% Cr heat-resistant steel during compression deformation at 650°C

Microstructural evolution in 12% Cr heat-resistant steel during compression deformation at 650°C

Two fatigue fracture modes at different temperatures of 10%Cr heat-resistant steel weldment filled with Ni-based alloy

9–12%Cr heat-resistant steels are widely used as ultra-supercritical thermal power generation components for their excellent high temperature performance and economic applicability. In order to overcome the problems of welding cracking and carbon migration of 9–12%Cr heat-resistant steels’ welded joints, Ni-based alloy welding materials are being more and more widely used. However, a very distinct interface will form between 9–12%Cr heat-resistant steels and Ni-based alloy welding materials due to their different chemical compositions and microstructures. In this work, the microstructure of the interfacial transition zone between a 10%Cr heat-resistant steel and Ni-based alloy welding material is investigated. The influence of temperatures on the fatigue behavior of the welded joint is studied. Two fatigue fracture modes at different temperatures are found, and the underlying mechanism of the different fracture modes are discussed in detail.

Atomic insights into the preferential dissolution of Laves phase of 9% Cr heat resistant steels in the strong alkaline solution

Two types of 9% Cr heat resistant steels were used to investigate the preferential dissolution mechanism of Laves phase in 8 mol/L NaOH solutions in an atomic scale by quasi-in-situ TEM analyses and first principles calculations. The dissolution of Laves phase is due to the unprotective oxide film. The dissolution rate of Laves phase was associated with the crystal orientation, and the stacking faults hinder the dissolution of Laves phase in a strong alkaline solution. Further, DFT calculations were used to illustrate the dissolution process of Laves phase, and the result is consistent with the TEM observation.

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.

Initiation of high-temperature fatigue cracks at the most tip of fractured grain boundaries

Two 2.2Cr heat resistant steel samples that had been fatigue tested respectively at 550 °C and 650 °C are analyzed on the atomic scale to reveal the different crack initiation mechanisms. Cracks formed along prior austenite grain boundaries (PAGBs) are selected and manufactured into foil specimens with the aid of focused ion beam (FIB). Hence the interior crack progressing routes are illustrated and the atomic structure particularly at the most tip of cracks are characterized by using the high resolution TEM. It is revealed that the cracks mainly progress within a spinel phase (Cr2MnO4) while not its interface with neighbouring martensitic matrix. Moreover, geometric phase analysis (GPA) and lattice misfit at the most tip of cracks further manifest a elastic dominated and a plastic dominated mechanism corresponding to the two specimens, respectively. Cracking of the 650 °C specimen is mainly attributes to the accumulation of plastic strain that caused by the viscoelastic transformation of PAGBs, which causes intriguing periodic grain boundary sliding (GBS) waves ahead of the crack tip.

Features of formation of austenite grains in 12 % Cr heat-resistant ferritic-martensitic steels

Ferritic-martensitic heat-resistant high-chromium steels (FMHS) with chromium content of 11 – 12 % are quenched to martensite from temperatures of 1050 – 1100 °С. Possible undesirable consequences of heating to such high temperatures are an increase in the size of austenite grains, increase in the amount of delta ferrite in the final structure, and a decrease in mechanical characteristics. In this work, the change of all these factors during heating of FHMS to quenching temperatures in the range of 950 – 1250 °С was studied. Ratios of the contents of martensite (its amount was identified with the proportion of austenite before quenching) and high-temperature delta ferrite on metallographic sections were analyzed. It was found that behavior of structure of the studied FHMS upon heating to temperatures of 1150 °С and above depends on the steels structural class. In steels whose structure at room temperature consists of martensite and delta ferrite, or in which delta ferrite begins to form at heating temperatures of 1200 °С and higher, size of austenite grain decreases with increasing temperature in the range of 1200 – 1250 °С, and the amount of delta ferrite – increases. Such structural transformations can be associated with features of the phase equilibrium diagrams of steels of this class. Such structural transformations can be associated with a change in the position and (or) inclination of boundaries of the high-temperature region of coexistence of austenite and delta-ferrite in the phase equilibrium diagrams of FHMS at a change in heating temperature in this range. Compression tests at 20 °С of 15Cr12Mn3SiMoW2VB steel samples after heat treatment with heating to temperatures for hardening 1000 – 1250 °С showed that formation of an additional amount of delta ferrite at temperatures above 1200 °С is a stronger factor than the refinement of austenite grains. This causes a decrease in yield strength of the samples quenched from these temperatures followed by high tempering.

Investigation on gradual degradation of mechanical property and microstructure in 9% Cr heat-resistant steels via interrupted creep test

Investigation on gradual degradation of mechanical property and microstructure in 9% Cr heat-resistant steels via interrupted creep test

Optimization of Temperature Field by Differentiating Number of Circumferential Control Zones in Local Post Weld Heat Treatment on 9%Cr Heat-Resistant Steel Pipe

Abstract This paper reports the optimal use of control zones to achieve required uniformity of temperature distribution in local postweld heat treatment of welds in 9% Cr heat-resistant large pipe system. In this research, local post weld heat treatment tests on temperature distribution with different control zones were carried out on 9%Cr steel pipes (OD 710 mm × 35 mm and OD 575 mm × 35 mm), which was further used for the development of the thermal analysis of the postweld heat treatment model via abaqus. The research results revealed (1) the effect of number of control zone on the uniformity of temperature distribution; (2) the effect of size of pipe diameter on the circumferential temperature and the through-thickness temperature gradients. This article discusses the possible reasons for the temperature difference at various positions of the pipe caused by the air flow inside the pipe with different control zones. Based on the obtained results, a practical method was designed for the selection the number of circumferential control zones on 9%Cr heat-resistant steel pipeline according to the required degree of temperature distribution uniformity. This paper contributes to the specific knowledge and the generic methodology.

9–12% Cr Heat-Resistant Martensitic Steels with Increased Boron and Decreased Nitrogen Contents

As a promising alloying approach, the modification of chemical composition by increasing the B content and decreasing the N content has been applied to improve the creep resistance of various 9–12% Cr heat-resistant martensitic steels. This paper presents an overview of the creep strength and related microstructural features of the 9% Cr and 10–12% Cr martensitic steels with high B and low N contents. The factors that determine the optimal B/N ratio in steels are considered. The creep properties are compared with those for similar steels with conventional B and N contents. The relationships between the stability of lath structure and precipitates of M23C6, Laves, and MX phases and the creep strength of steels are considered. Further perspectives of this modification of alloying by high boron and low nitrogen are outlined.

High-temperature creep life prediction of 9%Cr steel based on creep cavitation modelling

ABSTRACT In present work, the creep rupture life of 9% Cr heat-resistant steel (P91) was evaluated based on the creep cavitation modelling. At 873 K, a series of creep rupture tests were carried out under the stresses of 140–190 MPa to achieve several sets of ruptured specimens . The constrained and unconstrained models of cavity growth were introduced to investigate the creep behaviour of P91 steel. The obtained results showed that the rupture mechanism of these specimens was mainly attributed to the ductile rupture under the present creep conditions. Besides, reasonable extrapolations were calculated to predict the creep rupture life of P91 steel by combining different constitutive equations with the cavity growth model. It suggested that the model predictions with constraints were better than those with unconstrained conditions. In particular, the Sinh equation based constrained cavity model shows greater potential in predicting the long-term creep life of P91 steel.

Investigation on Gradual Degradation of Mechanical Property and Microstructure in 9% Cr Heat-Resistant Steels Via Interrupted Creep Test

Investigation on Gradual Degradation of Mechanical Property and Microstructure in 9% Cr Heat-Resistant Steels Via Interrupted Creep Test

Multi-axial creep-fatigue analysis of a cracked groove structure subjected to cyclic thermal loading

The aim of this study is to analyze the local change in stress–strain and damage behavior in an inlet groove structure of a power plant component under the effect of the crack geometrical constraint. A robust constitutive model is used to describe the inelastic behavior of advanced 9–12% Cr heat-resistant steels at high temperature. Steam temperature and loading profiles corresponding to the idealized startup, holding and shutdown sequence of the structure are assumed. Thirteen cycles of this sequence are chosen for the loading conditions. Transient heat transfer analysis is conducted to estimate the thermal field throughout the structure. Structural analysis is then performed using the material constitutive model. The results show that the crack geometrical constraint not only changes the stress–strain behavior, but also the damage distribution. Compared to the results without the crack effect, the damage locations demonstrate a major change. Specifically, the damage starts underneath the surface of the groove structure. This is attributed to the more complex multi-axial features in the regions close to the surface of the structure.

Microstructure and Mechanical Properties of Modern 11%Cr Heat-Resistant Steel Weld Joints.

In addition to good high-temperature creep resistance and adequate heat resistance, steels for the power industry must have, among other things, good weldability. Weldability of such steels is one of the criteria determining whether or not the material is suitable for applications in the power industry. Therefore, when materials such as martensitic steel Thor 115 (T115) are introduced into the modern power industry, the quality and properties of welded joints must be assessed. The paper presents the results of metallographic and mechanical investigations of T115 martensitic steel welded joints. The analysis was carried out on joints welded with two filler metals: WCrMo91 (No. 1) and EPRI P87 (No. 2). The scope of the investigations included: microstructural investigations carried out using optical, scanning and transmission electron microscopy and mechanical testing, i.e., Vickers microhardness and hardness measurement, static tensile test and impact test. The macro- and microstructural investigations revealed correct structure of the weld, without welding imperfections. The microstructural investigations of joint No. 1 revealed a typical structure of this type of joint, i.e., the martensitic structure with numerous precipitates, while in joint No. 2, the so-called Nernst’s layers and δ-ferrite patches were observed in the weld fusion zone as well as the heat affected zone (HAZ). The mechanical properties of the test joints met the requirements for the base material. A slight influence of the δ-ferrite patch on the strength properties of joint No. 2 was observed, and its negative effect on the impact energy of HAZ was visible.

Numerical and experimental investigations on the temperature field in local post weld heat treatment of 9 % Cr heat resistant steel welded pipes

Local post weld heat treatment (PWHT) is usually employed in the field fabrication of large-sized 9% Cr heat resistant steel welded components, such as power plant boilers. Temperature field in local PWHT plays critical roles in both achieving PWHT purposes and avoiding detrimental effects of local heating. The objective of this research is to quantify the effects of local PWHT parameters and pipe dimensions on the temperature field in local PWHT of 9% Cr heat resistant steel pipes and finding the optimal process window to engineer desired weld properties. Finite element (FE) models were developed to simulate the temperature field of local PWHT and carefully verified by eight experiments using four 9% Cr heat resistant steel pipes with typical dimensions employed in ultra-supercritical (USC) boilers. The results show that the through-thickness temperature gradient is severely aggravated with the increase of pipe diameter and wall thickness, which will lead to inadequate tempering in the weld metal close to the inside surface of the pipe. To enlarge heated band (HB) width is an efficient way to mitigate the through-thickness temperature gradient, and the axial temperature gradient can be effectively relieved by augmenting gradient control band (GCB) width. The through-thickness temperature gradient is the major factor in implementing successful local PWHT for 9% Cr heat resistant steel pipes. A much smaller process window is found in 9% Cr heat resistant steel pipes due to their higher PWHT temperatures and narrower range of permissible PWHT temperatures.

Microstructure and Mechanical Characteristics of Dissimilar TIG Welded 9% Cr Heat-Resistant Steels Joints

Dissimilar 9% Cr heat-resistant steels (G115 and CB2) with good creep properties for ultra-supercritical steam turbines were butt-joined by tungsten inert gas welding. The microstructure of welded metal (WM) was quenched martensite without carbide precipitates and lath packets existed inside prior austenite grains (PAGs), which leaded to higher hardness of WM. Partially melted zone at G115 side was composed of untempered martensite within equiaxed PAGs. The lowest hardness occurred in both G115 and CB2 steels which was attributed to tempered martensite with many M23C6 precipitates. The heat-affected zone consisted of three sub-grains and their microstructure was detailly analyzed in current work. As current increased from 130 to 150 A, both the tensile strength at room temperature and 650 ℃ increased while strength had no obvious change with further increasing current. The values of 673 MPa and 309 MPa corresponded to the tensile stress with 150 A at room temperature and 650 ℃, respectively. The fracture mode of joints at room temperature was cleavage and ductile failure at 130 and 150 A, respectively. The high-temperature fracture surface at 150 A was composed of deep and fine dimples.

A study into enhanced oxidation resistance and its mechanism in Cr1-xAlxN/CrN/Cr multilayer films deposited on 9–12 % Cr heat-resistant steel

Cr1-xAlxN/CrN/Cr multilayer films with a range of different Al contents were deposited on 9–12% Cr heat-resistant steel using direct current magnetron sputtering (DCMS). The chemical composition, roughness, cross-sectional structure, oxidation resistance and kinetic model of oxidation of the resulting films were investigated using various techniques. We discovered that Al atoms substituted for Cr atoms, while the crystal structure remained unchanged. Since Al doping decreases the lattice constants and hinders grain growth, the roughness of the film decreases, and the microstructure becomes denser, which is beneficial to the enhancement of oxidation resistance. According to the kinetic model, internal diffusion is the main restrictive step in this process. Increasing the Al content aids in the formation of an amorphous oxide surface layer that is abundant in Al. When the Al content is sufficiently high, the diffusion of oxygen into the 9–12% Cr steel substrates is prevented by the oxide surface layer of the film, which protects the substrates from further oxidation.

Effects of prior austenite grain boundaries on high-temperature fatigue properties of a 2.2Cr heat resistant steel

High-temperature fatigue mechanism of two 2.2Cr heat resistant steels that featured different prior austenite grain size were studied by using a strain-stress cooperative control strategy. Softening of grain boundaries led to a transition of deformation mechanism, during which the reaction of subboundaries within grains was replaced by the sliding of prior austenite grain boundaries (PAGBs) as the temperature increased from 550 °C to 650 °C. Moreover, increase of strain rate at 650 °C brought opposite effects on peak stress and fatigue life for the two matrixes, which was caused by the microscopic deformation anisotropy that induced by the viscoelasticity of PAGBs.

Evolution of strengthening factors during long-term aging at 650 °C in advanced 10% Cr heat-resistant steel

The effect of long-term aging for 1000…∼40,000 h at 650 °C on the tensile strength at ambient temperature was studied in a low-nitrogen and high-boron 10%Cr martensitic steel. In order to establish the reason for increasing the yield stress and ultimate tensile strength after 10,000 h of aging, the evolution of strengthening factors was analyzed. A decrease in the substructure and solid solution strengthening during long-term aging is compensated by an increase in the dispersion strengthening due to the precipitation of V-rich MX carbonitrides.

Investigation on solidification and phase transformation of heterogenous interface structure in dissimilar metal weld between high Cr heat-resistant steel and nickel-based alloy 617

The present study characterized heterogenous interface structure in dissimilar metal weld between a high Cr martensitic heat-resistant steel and nickel-based alloy weld metal (WM), and revealed its solidification and phase transformation processes during welding. The heterogenous interface structure consists of a martensitic peninsula structure and a partially mixed zone (PMZ) of austenitic stainless steel. The K–S relationship between peninsula structure and its surroundings indicates: 1. phase transformation of (δ-Fe) to (γ-Fe) of peninsula occurs only after previous austenitic PMZ has solidified; 2. the subsequent WM solidifies epitaxially on (γ-Fe) of peninsula. Due to that the liquidus temperature of PMZ is lower than that of base metal (BM), the peninsula structure can form in the early stage of solidification by quick freezing of liquid BM.

Improvement in Weldment of Dissimilar 9% CR Heat-Resistant Steels by Post-Weld Heat Treatment

The effect of the post-weld heat treatment on the microstructures and mechanical properties of the dissimilar joint of G115, a novel developed martensite heat resistant steel, and CB2 steel, currently used in an ultra-super-critical power unit, was investigated. The results indicate that the quenched martensite underwent decomposition and transformation, and the amount of dislocations were sharply decreased in the weld metal after post-weld heat treatment (PWHT). Many nano-scale M23C6 precipitates present in the weldment were distributed on the grain and grain boundary in a dispersed manner with PWHT. The average microhardness of the weldment decreased from about 400 HV to 265–290 HV after PWHT and only a slight decrease in the microhardness of CB2 steel was detected after PWHT at 760 °C. In contrast to the case of the as-received joint, the tensile strength of the joint was improved from 630 MPa to 694 MPa and the fracture location moved from the weld metal to the base metal after PWHT. The fracture surface consisted of a cleavage fracture mode without PWHT, whereas many dimples were observed on the fracture surface with PWHT.