Cr Supermartensitic Stainless Steel Research Articles

Investigation on microstructure and mechanical properties of boron-modified 13Cr5Ni2Mo by powder-pack boriding

ABSTRACT The main objective of the present paper is to investigate the effect of pack boriding treatment at 950°C/4 h on surface 13 Cr supermartensitic stainless steel. Scanning electron microscopy and X-ray diffractometry were used to characterise the grown boride layers microstructures. In addition, we performed 3D profilometry, microhardness measurements, nanoindentation tests with a Berkovich diamond tip, Daimler-Benz Rockwell-C adhesion, and ball-on-disk wear tests in order to evaluate the mechanical properties. After boriding, the phases were found to be iron borides (FeB/Fe2B), chromium borides (CrB), nickel borides (Ni2B), and molybdenum borides (Mo2B). The results showed that the use of the powder-pack boriding process on 13Cr SMSS at 950°C/4 h conducted in the presence of dual phases (FeB/Fe2B) improved the mechanical properties compared to untreated material.

Structure related corrosion behavior of DLC films in high Cl− environment

Abstract The diamond-like carbon (DLC) films were successfully fabricated on the surface of 13Cr super martensitic stainless steel by plasma enhanced chemical vapour deposition, and the microstructure changed with the variation of pulse voltages of the high pulse power supply. The microstructure of the DLC films was characterized by atomic force microscope, and the corrosion behavior of the films in a high Cl− environment was analyzed by open circuit potential, polarization curve and electrochemical impedance spectroscopy. It is found that the substrate corrosion occurred first for the DLC films with open pores, followed by a substrate surface passivation before the final corrosion failure. The DLC film with closed pores can effectively prevent Cl− from attacking the substrate before the corrosion pits formed at the local defects.

Fatigue strength evaluation of a 13%Cr supermartensitic stainless steel by the thermographic method

The fatigue limits of a 13Cr super-martensitic stainless steel from a hot rolled seamless tube were determined for three heat treatment conditions, including the as received quenched and tempered (QT) condition, quenched (Q), and quenched and double tempered (Q-DT). These heat treatments were applied to produce different microstructures and different tensile mechanical properties. The microstructures were characterized by microscopy and magnetic methods. The fatigue strength was accessed through the utilization of the thermographic technique. The results show that fatigue limits measured were between 38% and 44% of the ultimate strength. The as quenched condition gave the higher fatigue limit (444 MPa). However, the material has the lower dutility at this condition. The results aid to decide the best heat treatment condition and give support to design in applications where dynamic loadings are expected

GTAW of 12% Supermartensitic Stainless Steel Using 625 Nickel Alloy as Filler Metal

Supermartensitic steels represent a new sub-class of martensitic stainless steels with higher toughness and corrosion resistance than conventional grades. These properties were improved by the drastic reduction of carbon content, and the addition of nickel and molybdenum. These chemical composition changes also increased the weldability of these steels, since the soft and ductile martensite formed is not susceptible to cold cracking during welding. However, post weld heat treatment may be necessary for uses in sour, aggressive and embrittlement environments. This work investigates the mechanical properties of a weld produced by gas tungsten arc welding (GTAW) of a pipe of 12%Cr supermartensitic stainless steel with 625 nickel alloy as filler metal. The response of the weld metal to post weld heat treatments at 650oC was also evaluated.

DLC (H) coated 13Cr SMSS by high-pulsed power CVD technique

ABSTRACT Hydrogenated diamond-like carbon (DLC: H) films were successfully fabricated on the surface of 13Cr super martensitic stainless steel by high-pulsed power chemical vapour deposition. The mechanical and tribological properties of the films related to the substrate bias voltages were investigated. The results showed that the surface hardness and tribological properties of the substrate after DLC film deposition were improved. The DLC films became dense, the grain size and roughness reduced, and the deposition rates increased as the bias voltage increased. In addition, the dense DLC films directly led to high nanohardness and elastic modulus. The H content decreased with an increase in the bias voltage, which may be the primary reason for nanohardness and modulus enhancement. Due to the relatively high H/E* and H 2/E*3 ratios of films corresponded to high elastic strain and plastic deformation resistances reduced the wear loss, and the localized graphitization on the surface provided solid lubrication.

Reheat treatment: effects in supermartensitic stainless stell

Reheat treatment: effects in supermartensitic stainless stell

In situ observation of high temperature microstructure and reverted austenite evolution in 13Cr supermartensitic stainless steel

In situ observation of high-temperature microstructure in 13Cr supermartensitic stainless steel was investigated by the high temperature laser scanning confocal microscopy (LSCM), and the effects of tempering temperature and tempering cooling rate on the morphology and contents of reverted austenite were characterized via transmission electron microscopy (TEM), x-ray diffraction (XRD) and electron backscattered diffraction (EBSD). The results indicate that only α phase appeared in the XRD diffraction pattern after quenching at 1050 °C, there was no retained austenite in the quenched microstructure. The martensite transformation starting temperature (Ms) was around 177.6 °C according to the results of LSCM. The amount of reverted austenite increased first and then decreased with the tempering temperature increasing, and it reached a maximum value when the sample tempered at 620 °C. With the cooling rate increasing, the amount of reverted austenite showed a sharp decrease after tempering at 620 °C. The reverted austenite precipitated on martensite laths, blocks and prior austenite grain boundaries, while the misorientation angles ranged from 0° to 60° in the tempered martensite structure.

Temperature Dependent Phase Transformation Kinetics of Reverted Austenite during Tempering in 13Cr Supermartensitic Stainless Steel

The formation and growth kinetics of the reverted austenite during tempering in 13Cr supermartensitic stainless steel were investigated by a combination X-ray diffraction (XRD), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) in a scanning electron microscope (SEM). The reverted austenite precipitated at the martensite blocks, sub-blocks, laths and grain boundaries. The growth kinetics was established by Johnson-Mehl-Avrami (JAM) kinetics equation according to the volume fraction of the equilibrium reverted austenite at room temperature. The Avrami exponent value is 0.5, and the activation energy was estimated to be 369 kJ/mol, the kinetic model indicates that the mechanism of reverted austenite is diffusion-controlled and the growth of reverted austenite closely relies on the diffusion of the nickel (Ni) element. The experimental measured orientations of the reverted austenite are in good agreement with the theoretical ones, implying that the reverted austenite has the same orientation with the surrounding martensite, which meets the Kurdjumov–Sachs (K-S) orientation relationship. The orientation relationships minimize the strain energy of the phase transformation by reducing the crystallographic mismatch between phases.

Microstructure Evolution and Orientation Relationship of Reverted Austenite in 13Cr Supermartensitic Stainless Steel During the Tempering Process.

The transformation mechanism of reverted austenite and the amount of reverted austenite during the tempering process in supermartensitic stainless steel have been investigated by X-ray diffraction (XRD), electron backscattered diffraction (EBSD), and a high-temperature laser scanning confocal microscope (HTLSCM). The results indicate that the microstructure mainly consists of tempered martensite and reverted austenite. The reverted austenite nucleates uniformly at the sub-block boundary and prior grain austenite boundary. The amount of reverted austenite strongly relies on the tempering time, showing a positive correlation in the supermartensitic stainless steel. The crystallographic orientation relationship between reverted austenite and martensite meets the Kurdjumov-Sachs(K-S) relationship and the deviation angle is mainly concentrated at about 2 degrees. The mechanism of reverted austenite transformed from martensite is a diffusion mechanism. The growth kinetics of the reverted austenite are dominated by diffusion of the Ni element and there is no shear deformation of the martensite matrix in the in situ observation. It can be deduced that the reverted austenite is formed by nickel diffusion during tempering at 620 °C for different tempering times.

Effect of nitrogen on microstructure and corrosion resistance of Cr15 super martensitic stainless steel

ABSTRACTTo investigate the influence of nitrogen on structure and corrosion resistance of Cr15 super martensitic stainless steels (SMSS), two types (N-free and N-0.12%) of specimens were quenched at 1050°C and tempered at different temperatures, and then, optical microscope, transmission electron microscopy, X-ray diffraction, potentiodynamic polarisation, immersion experiments and Kelvin Probe Force Microscope were used to characterize its microstructures and corrosion properties. The experimental results show that the microstructure in the N-free Cr15 super martensitic stainless steel is a biphasic tissue with alternating martensite and austenite distribution while quenched at 1050°C and tempered between 600 and 700°C. The nitrogen addition increases the content of austenite, and changes the austenite morphology significantly into the coarse block and strip distribution. What’s more, micro-galvanic corrosion is formed between austenite and martensite, which deteriorates the corrosion resistance of the SMSS.

Sliding wear of cobalt-based alloys used in rolling seamless tubes

Cobalt alloys are used in several industrial applications due to their good mechanical properties at high temperatures. During manufacturing of 13% Cr super martensitic stainless steel seamless tubes by Mannesmann process, the hot rolling guides made of cast Co30Cr19Fe alloy are currently used, but present severe wear, limiting their service life. Seeking an alternative, three cobalt-based coatings (Stellite 1, 6 and 12) were selected. These coatings were evaluated and compared with the alloy currently used by sliding wear tests performed at room temperature and at 500°C in a tribometer PLINT TE67 with pin-on-disc configuration, without lubrication and varying the normal load. The pin was made of 13% Cr super martensitic stainless steel. The discs were manufactured with cast Co30Cr19Fe alloy and some of them were coated by laser cladding process with Stellite alloys, that is, a total of four materials. The worn volume of the wear surface was analyzed by 3D profilometry. The micromechanisms were observed with a stereoscopic and scanning electron microscope. To assess the elemental composition of the wear track, analyses were performed using energy dispersive spectroscopy (EDS). The results indicate that the samples coated with Stellite presented better performance than the cast Co30Cr19Fe alloy. Among the coatings, the Stellite 1 showed the best wear resistance, both at room temperature and at 500°C. The Co30Cr19Fe, Stellite 6 and 12 alloys, presented predominantly plastic removal mechanisms. Whereas Stellite 1 presented micro-cutting and oxidative wear.

Influence of Heat Treatments on the Impact Toughness of a Ti-stabilized 12%Cr Supermartensitic Stainless Steel

The supermartensitic stainless steels (SMSS) are a relatively new class of corrosion resistant alloys developed to obtain a better combination of weldability, strength, toughness and corrosion resistance than conventional martensitic stainless steels. The final properties of SMSS are strongly influenced by quenching and tempering heat treatments. In this work, different routes of heat treatments were tested in a Ti-stabilized 12%Cr supermartensitic stainless steel with the objective to improve mechanical properties, specially the low temperature (-46oC) toughness. Double and triple quenching were tested and compared to single quenching heat treatments. Two tempering temperatures (500oC and 650oC) were tested. The results obtained with instrumented Charpy impact tests showed that a triple quenching treatment was able to increase the density of fine TiC particles and improve the mechanical properties of specimens heat treated by quenching and tempering at 650oC.

A study of the function mechanism of a corrosion inhibitor package for martensitic stainless steel tubulars

Purpose The purpose of this investigation was to study the function mechanisms of a corrosion inhibitor package used for martensitic stainless steel tubulars in acid solution at high temperatures. Design/methodology/approach The inhibition performance was evaluated by means of an acid corrosion test at high temperature and high pressure, and the functional mechanism of the inhibitor package at different temperatures was investigated using polarization curve and electrochemical impedance spectroscopy measurements. Findings The results showed that the corrosion inhibitor package chosen for high-temperature and high-pressure gas well applications was very suitable for use with 13Cr super martensitic stainless steel. At lower temperatures, the function mechanism of the corrosion inhibitor package was characterized as a type of negative catalytic effect. As the temperature was increased, the effect of the intensifier in the package became more significant and the function mechanism changed to be the geometric covering effect type. Originality/value This study has the important practical value for guiding the oil field to conduct reasonable screening and using the acidizing corrosion inhibitor for martensite stainless steel tubulars.

Effect of 0.2 and 0.5% Ti on the microstructure and mechanical properties of 13Cr supermartensitic stainless steel

The effect that a 0, 0.2, and 0.5 wt.% titanium content has on the microstructure and mechanical properties of 13Cr supermartensitic stainless steel was investigated using an optical microscope, transmission electron microscope, and X-ray diffraction. The resultant microstructures of the three steels were tempered martensite with a reversed austenite dispersed throughout the matrix. Additionally, the formation of Cr-rich carbides was suppressed by stable Ti(C, N), which improved the strength without severely decreasing in the Ti-microalloyed steel toughness. Nano-precipitation of Ni3Ti was found for the 0.5 wt.% Ti steel during tempering, which significantly increased the strength, but decreased the toughness. The reversed austenite volume fraction also significantly influenced the mechanical properties.

Modeling Localized Corrosion of Corrosion-Resistant Alloys in Oil and Gas Production Environments: Part I. Repassivation Potential

A model has been developed for predicting the localized corrosion repassivation potential (Erp) for alloys in environments containing chloride ions and hydrogen sulfide. The model has been combined with Erp measurements for a 13-Cr supermartensitic stainless steel (UNS S41425) at various concentrations of Cl− and H2S. The model accounts for competitive adsorption at the interface between the metal and the occluded site environment, the effect of adsorbed species on anodic dissolution, and the formation of solid phases in the process of repassivation. The effect of H2S is complex, as it may give rise to a strong enhancement of anodic dissolution in the occluded environment and may lead to the formation of solid metal sulfide phases, which compete with the formation of metal oxides. H2S can substantially reduce the repassivation potential, thus indicating a strongly enhanced tendency for localized corrosion and stress corrosion cracking. However, exceptions exist at lower H2S and Cl− concentrations, at whic...

On the measurement of austenite in supermartensitic stainless steel by X-ray diffraction

Sections of a 13Cr supermartensitic stainless steel were investigated to determine the optimum sample preparation for measurement of the austenite content by X-ray diffraction. The surface of several samples was mechanically ground or polished using media of grit sizes in the range 1–120μm. The strained surface layer was afterwards removed stepwise by electropolishing, and the austenite content measured at each step. It was found that any level of mechanical grinding or polishing results in a reduction of the measured austenite fraction relative to the true bulk value, and that coarser grinding media impart greater damage and greater reduction in the measured austenite content. The results thus highlight the importance of the electropolishing step in preparation of such samples, but suggest that the American Society for Testing and Materials standard E975-03 substantially overestimates the amount of material which needs to be removed to recover the true “bulk” content.

Influence of tempering on microstructure and mechanical properties of Ti alloyed 13%Cr supermartensitic stainless steel

Tempering is the final heat treatment applied to martensitic steels. In conventional martensitic stainless steels or in the newly developed supermartensitic grades, the mechanical properties are strongly influenced by tempering. In the present work, several tempering treatments were performed in a Ti alloyed supermartensitic stainless steel. Tensile mechanical properties, hardness and impact toughness were measured and correlated to microstructural features. Microstructural analysis was conducted in a field emission gun scanning electron microscope and also by magnetic measurements. The ductility parameters continuously increased with tempering temperature, while yield and ultimate strength decreased. Secondary hardening and temper embrittlement were observed by hardness and low temperature impact tests respectively, while tensile tests were not sensitive to these phenomena.

Influence of heat treatments on microstructure and pitting corrosion resistance of 15%Cr supermartensitic stainless steel

Supermartensitic is a new class of stainless steels in development and consolidation as commercial products. Significant changes on chemical composition of conventional martensitic stainless steels, such as the reduction of the carbon content to <0·03 wt-%, and the addition of Ni and Mo, marked the development of this new group of alloys. New grades containing higher amounts of Cr and small additions of Ti and/or Nb were developed recently. As a result, supermartensitic steels offer an interesting combination of high strength, toughness, weldability and corrosion resistance. In this work, the pitting corrosion resistance of a 15Cr supermartensitic steel with Ni, Mo and Cu additions was studied. It is well known that the mechanical properties and corrosion resistance of martensitic steels are adjusted by the final tempering treatment. Several single tempering treatments in the 300–650°C range and double tempering treatments were performed in order to obtain different microstructures. The pitting corrosion resistance was investigated by electrochemical test in 3·5%NaCl solution. It was found that the pitting potential slightly decreased with the increase in temperature and time of tempering. The analysis of pits formed during the corrosion tests showed that ferrite islands are more corrosion resistant than the martensitic matrix due to the higher Cr and Mo contents.

Effect of Cl&lt;sup&gt;-&lt;/sup&gt; Content, pH Value and CO&lt;sub&gt;2&lt;/sub&gt; on Electrochemical Corrosion Features of Cr15 Super Martensitic Stainless Steel

The corrosion behavior of a Cr15 super martensitic stainless steel (Cr15 SMSS) was investigated in NaCl solutions by means of potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Effects of Cl-content, pH value and saturated CO2on corrosion were sdudied. It was found that the parameters of maintaining passivity current density (ip), the critical pitting potential (Eb) and charge transfer resistance (Rt) of Cr15 SMSS varied widely under different conditions. The corrosion resistance of Cr15 SMSS decrease with increasing Cl-concentration and lowering pH value. BothipandEbincrease for Cr15 SMSS in CO2-saturated NaCl solution, which verified that the CO2in NaCl solution can result in lower pitting sensitivity and higher uniform corrosion rate to Cr15 SMSS.

Role of Nb in 13Cr super-martensitic stainless steel

The effect of Nb microalloying on structure and physical properties of quenched and tempered 13%Cr martensitic stainless steel was investigated. Excellent strength and adequate toughness properties were obtained by 0.10 wt% Nb addition to low interstitial (N 0.01wt%, C &lt; 0.02wt%) steel. The effect of Nb in 13%Cr steels with high N content was also studied in a commercial martensitic stainless steel sample containing higher levels of N and also alloyed with V. The microstructure, precipitate morphology and dispersion and volume fraction of reverse austenite were characterized. The strength properties obtained in the steel with 0.10%Nb are significantly higher than those of the V-containing steel. The study shows that whereas amount, size and dispersion of precipitates of microalloying elements contribute to enhanced strength, the optimum volume fraction of reverse austenite formed contributes to enhanced ductility and toughness properties. More importantly, high Nb additions to low N -13%Cr-1%Mo steel are found to improve significantly resistance against pitting corrosion significantly.