High-chromium Martensitic Steels Research Articles

Susceptibility of X17CrNi16-2 martensitic stainless steel to hydrogen embrittlement after conventional and deep cryogenic heat treatment

A low carbon/ high chromium martensitic stainless steel, X17CrNi16-2, was heat treated using two different hardening and tempering regimes (1050 °C/480 °C or 980 °C/600 °C) — promoting either a high strength or high toughness state, respectively − and further combined with deep cryogenic treatment (DCT) at −196 °C for 24 h. Over recent years DCT has been recognized as a promising technique to improve the properties of steel, predominantly with respect to its tensile strength, toughness and wear resistance. The influence of DCT on the hydrogen embrittlement resistance of martensitic stainless steel has not yet, however, been reported.A slow strain rate tensile test (SSRT) with simultaneous cathodic hydrogen charging was selected as the method to assess potential susceptibility to hydrogen embrittlement (HE). Relatively low-intensity hydrogen charging, utilizing a constant current density of 0.1 mA/cm2, in a non-corrosive, slightly alkaline buffer solution, led to a clear reduction in the ultimate tensile stress. This reduction, and therefore the HE susceptibility, was more pronounced in the steel with a higher strength (i.e. that subject to the 1050 °C/ 480 °C heat treatment condition). Furthermore, DCT did not appear to have any impact on the steel’s mechanical properties in the presence of hydrogen. Fractographic analysis showed clear evidence of HE in the hydrogen-charged specimens.This paper presents results of the SSRT tests and further fractography results, and discusses the impact of conventional and deep cryogenic treatment on HE susceptibility.

Microstructure and mechanical properties of 9Cr18Mo martensitic stainless steel fabricated by strengthening-toughening treatment

Microstructure and mechanical properties of strengthening-toughening treated (1020–1060 °C argon quenching + −73 °C isothermal quenching + 140–240 °C tempering) 9Cr18Mo martensitic stainless steel (9Cr18Mo steel for short) were investigated by using Thermo-Calc software, microstructure characterization and mechanical tests. It was found that the size range of carbides in the 9Cr18Mo steel was wide and approximately 0.2–15.0 μm. Most of micron-carbides (>1 μm) and submicron-carbides (<1 μm) were located at the grain boundary and in the grain interior of original austenite, respectively; and the 9Cr18Mo steel with higher volume fraction of submicron-carbides showed a better combination of strength and impact energy. It was detected that the carbides in the 9Cr18Mo steel mainly consisted of M23C6 and M7C3 with primary element of Cr, Fe, and secondary element of Mo. There were core-shell (M7C3-M23C6) structured carbides with two morphologies of coarse irregular (>10 μm) and micron near-spherical, as well as individual micron-M7C3, micron- and submicron-M23C6 with near-spherical morphology. Quasi-equiaxed grains with body-centered cubic (bcc) structure formed via recovery and recrystallization, {112}<111> bcc twins accompanied with hexagonal ω-Fe phase at the twin boundaries, retained lath- and blocky-austenite were observed in the tempered martensitic matrix of 9Cr18Mo steel. Moreover, the twins and retained austenite were prone to decomposition at higher tempering temperature. It was indicated that the yield strength of 9Cr18Mo steel was mainly correlated with subgrain block size of tempered martensite and dispersion parameter of carbides via the analysis of statistical data, and the subgrain block contributed more to the yield strength of 9Cr18Mo steel. The M7C3-M23C6 core-shell structured carbides and {112}<111> bcc twins associated with hexagonal ω-Fe phase found in this study were also helpful to understand the correlation between microstructure and mechanical properties of other high carbon chromium martensitic steels.

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.

Influence of homogenizing annealing on reducing the content of δ-ferrite in high-strength high-chromium martensitic steel grade 07Kh15N5D4B obtained by selective laser alloying

The work studies effects of temperature – time parameters of homogenizing annealing and related structural changes on the content of δ-ferrite in high-chromium martensitic steel grade 07Kh15N5D4B, obtained by selective laser alloying. It is shown that homogenizing annealing at a temperature of 1150°C with a holding time for 8 hours reduces the content of δ-ferrite, eliminates the cellular structure, and also increases the strength properties of the above-mentioned steel.

Effects of Helium Production and Displacement Damage on Microstructural Evolution and Mechanical Properties in Helium-Implanted Austenitic Stainless Steel and Ferritic/Martensitic Steel

The effects of helium concentration and displacement damage on microstructural evolution at low dpa and low helium concentration were mainly investigated in specimens of austenitic stainless steel 316FR or SUS304 and a high chromium martensitic steel (HCM12A). The 316FR and HCM12A specimens were implanted uniformly with helium at 823 K up to 30 appm-He or 50 appm-He by 50 MeV cyclotron accelerator using energy degraders. After the helium implantation, the microstructures were examined by a transmission electron microscopy and positron annihilation lifetime measurements. Irradiation hardening behaviors were analyzed using SUS304 and HCM12A steels at 823 K implanted with He ion up to 100 appm with different He/dpa ratios in the HIT ion irradiation experiments and the hardening behaviors were examined by nano indentation method. In the irradiation and annealing specimens, these mechanical properties and microstructures were examined to understand the effects of helium production, displacement damage and annealing on microstructural development, and kinetic Monte Carlo (kMC) simulations were also performed to understand the microstructural development, and the results were compared with the results of TEM observation and positron annihilation lifetime measurements. Important some differences in the microstructural developments such as cavity formation and growth between austenitic stainless steel and martensitic steel were observed in low dpa and low helium concentration conditions.

Creep behaviour of a high chromium martensitic steel under stress varying creep loading conditions: Primary creep regeneration (PCR)

Primary creep regeneration (PCR) is an important phenomenon observed for some alloys loaded at high-temperature and under stress varying loading conditions. For example, for a specimen deforming in the secondary creep regime, a stress reversal might result in an enhanced creep rate upon reloading due to full/partial re-activation of the primary creep stage (i.e. primary creep regeneration). The extent of PCR activation is expected to be dependent on different loading parameters (e.g. magnitude and duration of reverse loading) and the characteristics of the loaded alloy. In this study, a comprehensive testing program was designed and conducted to investigate the PCR behaviour of an advanced 10%Cr martensitic steel at 600 °C. The experimental observations characterized the influence of different loading parameters on the extent of PCR activation. It was found that the inelastic strain accumulation during reverse loading periods is the most important parameter governing the extent of PCR activation after stress transients. Furthermore, the gathered experimental data was analysed to understand the effect of different loading parameters on anelastic recovery behaviour of the steel. It was shown that a larger amount of strain recovers after unloading for tests with higher forward stress magnitudes and/or larger amounts of pre-accumulated inelastic strain.

Impact Toughness of Heat-Affected Zones of 11Cr Heat-Resistant Steels

Aiming at the requirements of structural steel in Gen-IV nuclear reactor, the high-chromium martensitic heat-resistant steels containing 10–12% chromium were developed. The toughness of heat-affected zones (HAZs) is one of the important factors for evaluating the weldability of steels. In this paper, the simulated HAZs were fabricated using tempered SIMP steels. The effects of microstructures on the impact toughness of materials were analyzed using Vickers hardness tester, optical microscope, transmission electron microscope. Experimental results demonstrated that the HAZs of weldment were poor in toughness, much lower than that of the base metal. However, after experiencing post-weld heat treatment, the toughness of the HAZs increased greatly. The toughness became better in terms of CG-HAZ, FG-HAZ and IC-HAZ for the two steels, regardless of as-welded or after PWHT. Compared with SIMP7 steel, chemical compositions, such as C, Si, Mn and Cr, were adjusted to a lower content; the toughness of base metal and simulated HAZs was better in the case of SIMP11. The conjunct roles of dislocation density and carbon contents retained in the martensite led to poor impact toughness of the as-welded HAZs, because dislocations and carbon atoms affected the inner stresses within lattices.

Frictional characteristics of impregnated graphite with different graphitization degree versus chromium stainless steel under varying PV values

An experimental study was performed by employing a seal pair of phenolic resin impregnated graphite and high chromium martensitic stainless steel 9Cr18. A purpose-built test rig was designed and fabricated to study the effect of face temperature on the surface morphology of a seal ring made of phenolic resin impregnated graphite with different graphitization degrees for varying PV values (pressure × velocity). As revealed by the results, the effect of rotational speed on heat generation between the two mating surfaces was more significant than that of fluid pressure. The effects of surface frictional temperature between the two faces were found different when the phenolic resin impregnated graphite ring with different graphitization degrees. The worn surfaces of the phenolic resin impregnated graphite rings were examined with the assistance of scanning electron microscopy and 3D measuring laser microscopy to investigate the wear mechanism for these different mating rings. Both adhesive and abrasive wear mechanisms were discovered under the working conditions. It was found out that the phenolic resin impregnated graphite ring with a high degree of graphitization was worn more severely at high PV values. Under the conditions of high speeds or high PV values, the phenolic resin impregnated graphite with fine and compact graphite matrix particles, medium degree of graphitization, high thermal conductivity and hardness is the top choice.

Effect of Delta Ferrite on Microstructure and Mechanical Properties of High-Chromium Martensitic Steel

The fusion zone of modified 9Cr-1Mo steel experiencing a peak temperature of 1450 °C and varying cooling rates between 1 and 40 °C/s from the peak temperature is simulated using Gleeble thermal simulator. Mechanical properties of the simulated samples are assessed by ball indentation and impression creep tests. The study evaluates the effect of cooling rate on volume fraction, morphology of retained delta ferrite in the simulated partially melted zone of an autogenous weld, and subsequently its effect on the mechanical properties of this zone. With varying cooling rate, volume fraction of delta ferrite does not change much; however, its morphology changes from polygonal island to grain boundary network in case of excessive slow cooling (1 °C/s). Morphological variation of delta ferrite affects the creep properties of this zone. Partially melted zone, which is cooled at 1 °C/s with delta ferrite present in the interdendritic regions, exhibits maximum creep strain and creep strain rate.

Energy-Based Water Droplet Impact Erosion Studies of Laser-Treated Austenitic and Martensitic Steels and their Applications

Abstract High-power diode laser (HPDL) surface treatments of high-manganese austenitic and high-chromium martensitic steel round specimens have been carried out, and their resistance to water droplet impact erosion (WDIE) in air was evaluated as per ASTM G73-10, Standard Test Method for Liquid Impingement Erosion Using Rotating Apparatus. This is because the water droplets always travel along with a fluid medium, either steam or air, and these are influenced by inertial, Coriolis, and centrifugal forces, which are missing in a vacuum. Because of these forces, the water droplets elongate and may break before hitting a target, resulting in WDIE damages entirely different from those in a vacuum. The WDIE damages on the test specimens were observed on the leading edge towards the suction side, where the boundary layer is attached, and the pressure gradients are high. These are similar to those occurring on the blades of an axial flow compressor of a gas turbine, a low-pressure steam turbine, and a helicopter rotor, because their leading edges are round. The WDIE testing is based on water droplet kinetic energy (KEd) and kinetic energy flux (KEfd). It has been proved experimentally that the product of KEd and KEfd can predict WDIE damages in a material, and this product can be used to compare the WDIE test results of different materials received from different laboratories. It is observed from the test results that the resistance to WDIE of HPDL-treated high-manganese austenitic steel (Hadfield’s steel) has reduced drastically, whereas that of high-chromium martensitic stainless steel has improved manifold. Fine microstructure and increased martensitic contents in high-chromium martensitic stainless steel after HPDL treatment are the main reasons for its improved performance, whereas the coarse microstructure having microcracks in HPDL-treated Hadfield’s steel is responsible for poor performance.

Effects of helium production, displacement damage on mechanical properties and surface acoustic wave in austenitic stainless steels and martensitic steel

The effects of helium production, displacement damage on mechanical properties and microstructures were investigated in some specimens such as austenitic stainless steels of SUS304 and SUS316 and a high chromium martensitic steel of HCM12A steels irradiated in a high fluence irradiation facility (HIT) of the University of Tokyo. The simultaneous dual ion (nickel plus helium ions) irradiations were adopted to simulate nuclear irradiation environments such as fast reactor and fusion reactor at HIT to 1 or 10 dpa and to 0, 1, 10, and 100 appm-He at 500, 550, and 600 °C. Thin foils for a transmission electron microscopy (TEM) were prepared with a focused ion beam (FIB) micro-sampling system. After the irradiation, the microstructures were observed by a transmission electron microscope and the irradiation hardening was measured by nano-indentation technique. In this study, the efficient of non-destructive measurement method of surface acoustic wave (SAW) excited by laser beam on the detection of radiation hardening behavior was also examined. Irradiation hardening was observed and the increment was tended to increase with dpa and He/dpa, which was especially effective in the condition of 100 appmHe/dpa even at 550 °C. Non-linear effect on amplitude of the excited SAW was also observed on the ion irradiated materials, and the propagation velocity of SAW was tended to be increased with irradiation dose. It was found that the behaviors of SAWs results were correlated with the changes of mechanical properties.

Effect of Normalizing on Microstructure and Properties of High Chromium Martensitic Heat Resistant Steels

Effect of Normalizing on Microstructure and Properties of High Chromium Martensitic Heat Resistant Steels

Investigation of Creep Processes and Microdamages in 10Kh9V2MFBR-Sh High-Chromium Steel

During the modernization and the new construction of power units at TPPs in Russia, high-chromium martensitic steels with higher heat-resistant properties than the traditional perlite steels are increasingly used as structural materials. High-chromium steels have a necessary regulatory support for their use in domestic power engineering. However, up to the present time, the issue of assessing the quality of these steels at the analysis of their state during long-term operation remains open. The article proposed is one of the first attempts to create a system of quality criteria for martensitic steels based on their microdamage parameters. Tests were carried out on the long-term strength and creep of samples from 10Kh9V2MFBR-Sh steel at high temperatures with the construction of creep curves in relative coordinates “deformation related to the deformation of fracture, current time related to time to failure.” For some samples, the tests were interrupted and the metal was subjected to metallographic studies consisting of the analysis of microdamage with reference to the accumulated creep strain. It has been shown experimentally that the deformation curve of high-chromium steel differs from the analogous curve of pearlitic steel by a longer and flat section of steady creep and by a sharper transition to the third accelerated creep stage, which has a very short time period (approximately 10% of the total durability). The tendency to the increase in the microdamage of the structure of steel as the accumulated creep strain increases with time was confirmed. The beginning of transition to the final creep phase is characterized by the formation of contours of future pore chains and by the appearance of individual large pores of up to 6 μm in size, the presence of which in the microstructure of the martensitic steel indicates a very significant accumulation of creep strain, and corresponds to the predestruction stage of metal. It is necessary to continue the research to obtain quantitative indicators on the accumulation of microdamage in high-chromium steel in a conjunction with the development of a metal resource under creep conditions.

Effect of Tungsten on the Temper Brittleness in Steels with 9% Cr

The impact toughness and the structure of high-chromium martensitic steels with different tungsten contents are studied after tempering at 300 – 800°C. It is shown that when the tungsten content is increased from 2 to 3%, the temperature range of the irreversible temper embrittlement is widened and the impact toughness is decreased by a factor of 4.

Sources of ultrahigh creep resistance of advanced martensitic steels

New approaches to alloying high-chromium martensitic steels have been discussed. The suggested strategy makes it possible to retain their long-term creep strength at temperatures up to 650°C and operation times up to 2 × 105 h by increasing the boron content and decreasing the nitrogen content.

Structural Changes in Steel 10Kh10K3V2MFBR Under Low-Cycle Fatigue

The characteristics of low-cycle fatigue of high-chromium martensitic steel 10Kh10K3V2MFBR are determined at room temperature and specified deformation amplitude varied within 0.25 – 1.0%. It is shown that the steel is susceptible to softening under low-cycle loads. The endurance of the steel is determined by the Baskvin – Manson – Coffin relation. Permanent softening of the steel until failure is caused by lowering of the dislocation density and coarsening of subgrains.

Analysis of temperature and strain rate dependencies of softening regime for tempered martensitic steel

This article aims to analyse the influence of temperature and strain rate on the mechanical behaviour of the high-chromium martensitic steel X20CrMoV12-1. The analysis is based on two series of high-temperature uniaxial tensile tests. In a first series, the tensile tests are conducted until rupture, and temperature as well as strain rate are varied systematically. The corresponding stress–strain curves show an extended softening stage. In order to examine softening, it is crucial to distinguish between microstructural changes and strain localisation due to necking. For this reason, a tensile test at low strain rate is performed several times, while the test is terminated at different strain levels in order to examine the onset of necking. Based on the test results and surface measurements of the deformed specimens, the strain level at which necking starts is determined, and possible interactions between softening and necking are discussed. The tensile tests have been conducted in order to calibrate a mechanical model which supplies reliable predictions on the material behaviour under different loading scenarios at elevated temperatures. For this reason, a framework based on microstructural processes is presented in the second part of the article. The model applies a binary mixture approach in conjunction with an iso-strain concept. Furthermore, backstress and softening variables are introduced to consider hardening and softening effects. This procedure results in a system of three differential equations describing the mechanical behaviour.

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.

The Role of Microstructure in Creep Strength of 9-12%Cr Steels

Tempered martensite lath structure (TMLS) plays a vital role in creep resistance of high chromium martensitic steels. Under creep conditions the TMLS could be stabilized by three agents: (i) a dispersion of boundary M23C6 carbides and Laves phase; (ii) a dispersion of M(C,N) carbonitrides, which are homogeneously distributed within ferritic matrix; (iii) substitutional alloying element within ferritic matrix. The boundary particles exert a large Zener drag force which effectively hinders migration of low-and high-angle boundaries. A dispersion of M(C,N) carbonitrides both within ferritic matrix and lath boundaries provides the pinning of mobile dislocations. This process is responsible for reliving long-range elastic stress field originated from lath boundaries. In addition, M(C,N) carbonitrides provide high threshold stress. Substitutional elements as W and Mo effectively slowing down diffusion in ferritic matrix retard climb of lattice dislocation that also prevents the aforementioned knitting reaction. The suppression of knitting reaction between lattice dislocation and low-angle boundaries prevents their transformation to subboundaries by concurrent operation of all three agent types. Depletion of W and Mo from solid solution leads to the occurrence of static recovery and precipitation of Laves phase at boundaries under long-term aging. This process is responsible for creep strength breakdown. The strain-induced formation of Z-phase at the expense of V-rich M(C,N) carbonitrides highly facilitates this process. However, slow strain-induced coarsening of M23C6 carbides and M(C,N) carbonitrides provides the suppression of the knitting reaction between mobile lattice dislocations and intrinsic dislocations of lath boundaries and replacement of TMLS by subgrain structure. Ostwald ripening of boundary M23C6 carbides and Laves phase leads to rapid creep rate increase with strain in tertiary creep and premature rupture owing to the formation of subgrain structure replaced TMLS and further subgrain growth.

Study of the structure and properties of metal of the major steam lines of a CCGT-420 unit made from high-chromium Х10CrMoVNb9-1 (P91) steel

The technology of manufacture of live steam lines and hot reheat lines at FINOW Rohrsysteme GmbH are discussed. These pipelines are designed for high-performance CCGT units and are made from high-chromium martensitic steel X10CrMoVNb9-1 (P91). The principles of certification and evaluation of conformance of thermal and mechanical equipment made from new construction materials with the TRCU 032-2013 technical regulation of the Customs Union are detailed. The requirements outlined in Russian and international regulatory documents regarding the manufacture of pipes and semifinished products for pipeline systems are compared. The characteristic features of high-chromium martensitic steel, which define the requirements for its heat treatment and welding, are outlined. The methodology and the results of a comprehensive analysis of metal of pipes, fittings, and weld joints of steam lines are presented. It is demonstrated that the short-term mechanical properties of metal (P91 steel) of pipes, bends, and weld joints meet the requirements of European standards and Russian technical specifications. The experimental data on long-term strength of metal of pipes from a live steam line virtually match the corresponding reference curve from the European standard, while certain experimental points for metal of bends of this steam line and metal of pipes and bends from a hot reheat line lie below the reference curve, but they definitely stay within the qualifying (20%) interval of the scatter band. The presence of a weakened layer in the heat-affected zone of weld joints of steel P91 is established. It is shown that the properties of this zone govern the short-term and long-term strength of weld joints in general. The results of synthesis and analysis of research data support the notion that the certification testing of steam lines and other equipment made from chromium steels should necessarily involve the determination of long-term strength parameters.