Supermartensitic Stainless Steel Research Articles

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.

Wear Resistance of Boron-Modified Supermartensitic Stainless Steel Coatings Produced by High-Velocity Oxygen Fuel Process

The wear resistance of boron-modified supermartensitic stainless steel coatings produced by high-velocity oxygen fuel (HVOF) was investigated through pin-on-disk measurements. It was shown that addition of boron leads to the formation of an interconnected and rigid boride network delimiting the grain boundaries of the martensitic matrix. A very refined structure was formed as result of the high cooling rates imposed to the molten alloy during the HVOF process. The specific wear rates of the HVOF coatings were about tenfold lower than the boron-free supermartensitic stainless steel, lying in the order of 10−5 mm3/N m. The refined boride skeleton along the HVOF coating was found to be effective to reduce the materials’ removal from the exposed softer martensitic matrix. While the supermartensitic stainless steel master alloy and the mild steel substrate displayed severe adhesive wear, the HVOF coatings exhibited mild delamination wear at low sliding velocities (10 and 20 cm/s) and abrasive wear at the highest tested velocity (40 cm/s). The studied boron-modified supermartensitic HVOF coatings are an interesting approach to protect the surface of inexpensive steel substrates against wear.

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.

Double-step inter-critical tempering of a supermartensitic stainless steel: Evolution of hardness, microstructure and elemental partitioning

The maximum allowed hardness for low carbon martensitic stainless steel components used in the oil and gas industry is 247 HV. Inter-critical tempering is an effective method for hardness control due to the production of stable reverted austenite. By conducting multiple-step tempering cycles, the austenite reversion kinetics can be accelerated and its thermal stability upon cooling can be greatly increased. In this work, supermartensitic stainless steel samples were subjected to two-step inter-critical tempering cycles. First, all samples were heat-treated at 625 °C for 2.5 h to minimize hardness through the maximization of stable austenite. Then, a second stage tempering with temperatures between 560 and 720 °C for 2.5 h was studied. The amount of stable reverted austenite at room temperature increased for second stage temperatures below 625 °C. Between 625 and 670 °C, the amount of reverted austenite notably increased at high temperature but it had limited thermal stability upon cooling. Above 670 °C, all newly reverted austenite was completely unstable during the cooling stage and partial dissolution of the stable austenite obtained after the first tempering cycle also occurred. Interestingly, hardness was mostly insensitive to the stabilization of additional austenite or to the newly formed fresh martensite.

A Corrosion Inhibition Study of a Novel Nonionic Gemini Surfactant Derived from Palm Oil for Supermartensitic Stainless Steel in Sour Solution

In the present investigation a novel non-ionic gemini surfactant, namely bis(2-((2-palmitoamidoetil) amino) etil) 1H-imidazol-4,5-dicarboxilate, was synthesized from the fatty acids contained in the palm oil and evaluated as corrosion inhibitor for UNS S41425 type supermartensitic stainless steel in a sour solution. The obtained results from electrochemical techniques showed that, the inhibition efficiency increased with increase of the inhibitor concentration. The maximum inhibition efficiency was reached 99% at 100 ppm. The potentiodynamic polarization measurements indicated that the gemini surfactant acts as anodic type inhibitor. The adsorption of the inhibitor on the UNS S41425 surface in the sour solution was found to obey Langmuir’s adsorption isotherm.

Fundamentals of isothermal austenite reversion in a Ti-stabilized 12Cr – 6 Ni – 2 Mo super martensitic stainless steel: Thermodynamics versus experimental assessments

This work addresses the fundamentals of inter-critical austenite reversion in a Ti-stabilized 12Cr-6Ni-2Mo (at.%) supermartensitic stainless steel, combining thermodynamic and experimental assessments. The calculation of the temperature and composition at which ferrite and austenite phases have the same free energy, i.e. T0 and C0(T), respectively, is discussed as a methodology to understand the austenite reversion and stabilization mechanisms. An ultra-fast heating rate of 500 °C s−1 provided isothermal austenite nucleation and growth from a fully solubilized martensite, allowing direct comparison with the compositional tie-lines and the transformation paths described by the free energy calculations. Isothermal transformation temperatures below and above T0 (625 °C) were used. Below T0, massive reversion was suppressed since it would imply a free energy increase. The opposite occurred above T0, since the critical Ni concentration for austenite reversion was lower than for the solubilized case. Transmission electron microscopy and atom probe tomography evidenced that, in all cases, lath growth occurred by local equilibrium partitioning of Ni, along with co-segregation of ferrite-stabilizing elements (Cr and Mo) at the advancing interface. The complex interaction between Cr, Ni and Mo on the energy gain upon nucleation of austenite revealed that Cr segregation can be beneficial while the adverse effect of Mo can be quickly outbalanced by Ni. The most stable reverted laths were obtained for transformation temperatures at least 15 °C below T0 with average austenite/martensite Ni partitioning factors higher than 2.0.

Effect of carbon partition in the reverted austenite of supermartensitic stainless steel

In this study, the reverted austenite transformation from martensite and the distribution of carbon element during tempering process in supermartensitic stainless steel have been investigated by x-ray diffraction analysis with whole pattern fitting refinement and electron backscattered diffraction (EBSD). The results revealed that the microstructure consists mainly of tempered martensite and reverted austenite. The martensite matrix has high density dislocation and the misorientation angles range of 0°–60° in the stucture, while the reverted austenites nucleate and precipitate at sub-block boundaries and prior grain austenite boudndaries. With the tempering temperature increasing, the amount of reverted austenite increases first and then decreases, which is strongly depended on the tempering temperature. The carbon element dissolved in the reverted austenite has an opposite tendency due to the difference of carbon element partitioning. At the same time, the hardness results have the same trend. It manifested that reverted austenite contents have an effect on the mechanical properties. The amount of carbon enrichment in the reverted austenite is considered to be one of the main factors.

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.

Temper embrittlement of supermartensitic stainless steel and non-destructive inspection by magnetic Barkhausen noise

Supermartensitic is a new class of stainless steels used as seamless pipes and forged parts in the oil and gas off shore production. The final mechanical properties of the steel are adjusted by quenching and tempering heat treatments. In this work, a Ti alloyed supermartensitic stainless steel was quenched and tempered at 500 °C, 570 °C, 620 °C and 640 °C. Specimens tempered at 500 °C showed very low toughness and brittle fracture in low temperature Charpy tests. This temper embrittlement effect was detected by magnetic Barkhausen noise (MBN) inspection. Specimens tempered at 620 °C and 640 °C were cooled in water or in furnace after tempering, and those slowly cooled had lower toughness and higher austenite volume fraction. In this case, MBN could not detect the embrittlement effect, probably due to the high austenite (paramagnetic phase) content.

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.

3D processing maps and a modified constitutive model of 16Cr-5Ni-Mo supermartensitic stainless steel

The hot deformation behavior of 16Cr-5Ni-Mo supermartensitic stainless steel was investigated using a Gleeble-1500D thermo-mechanical simulator in the temperature range of 850 to 1150 °C and strain rate range of 0.01 to 10 s−1. The hot deformation apparent activation energy of the steel was obtained as 424.173 kJ/mol. Arrhenius type constitutive equation was established according to flow stress data to describe the flow behavior. Meanwhile, the 3D maps of power dissipation and processing based on dynamic materials mode (DMM) were developed for the typical strains of 0.2, 0.4, 0.6 and 0.8, respectively. The results show that with the high deformation temperature (1000-1150°C) and low strain rate (0.01—0.1 s−1), it is dominated by the dynamic recrystallization (DRX) for the softening mechanism. And the critical strains for the onset of DRX at different conditions are obtained. The optimized processing parameters for hot working of 16Cr-5Ni-Mo supermartensitic stainless steel are in the temperature range of 980—1150°C and strain rate range of 0.02—0.36 s−1 at strains of 0.6 and 0.8.

Fundamentals of Isothermal Austenite Reversion in a Ti-Stabilized 12cr - 6 Ni - 2 Mo Super Martensitic Stainless Steels: Thermodynamics Versus Experimental Assessments

This work addresses the fundamentals of inter-critical austenite reversion in a 12Cr-6Ni-2Mo supermartensitic stainless steel, combining thermodynamic and experimental assessments. The calculation of the temperature and composition at which ferrite and austenite phases have the same free energy, i.e. T0 and C0, is discussed as a methodology to understand the austenite reversion and stabilization mechanisms. An ultra-fast heating rate of 500 °C.s-1 provided isothermal nucleation and growth from a fully solubilized martensite, allowing direct comparison with the compositional tie-lines and the transformation paths described by the free energy calculations. Isothermal transformation temperatures below and above T0 (625 °C) were used. Below T0, massive reversion was suppressed since it would imply a free energy increase. The opposite occurred above T0, since the critical Ni concentration for austenite reversion (C0-Ni) was lower than the solubilized case. Nevertheless, transmission electron microscopy and Atom Probe Tomography evidenced that, in all cases, lath growth occurred by local equilibrium partitioning of Ni, along with co-segregation of ferrite-stabilizing elements (Cr and Mo) at the advancing interface. The complex interaction between Cr, Ni and Mo on the energy gain upon nucleation of austenite revealed that Cr segregation can be beneficial while the adverse effect of Mo can be quickly outbalanced by Ni. The concentration of Ni along the reverted laths was rather inhomogeneous due to isothermal coalescence and lack of redistribution. However, the most stable laths were obtained for transformations at least 15 °C below T0, with average austenite/martensite Ni partitioning factors higher than 2.0.

Synergistic effect of Cu and Ni on the formation of reversed austenite in super martensitic stainless steel

ABSTRACTThe interplay of Cu and Ni on the formation of reversed austenite in super martensitic stainless steel (SMSS) was discussed in this study. Ni concentration in reversed austenite increases from 9.93 to 11.96 wt-% as Cu content increases from 1.5 to 3 wt-%, while Cu concentration increases from 1.43 to 1.89 wt-% as Ni increases from 5 to 6.5 wt-%. The enrichment of Ni and Cu in reversed austenite is much higher than that in martensite matrix. Reversed austenite formed in the dispersion area of ε-Cu inherits the twinning structure of ε-Cu and the crystal orientation relationship with the matrix during tempering. So Cu helps increase the enrichment of Ni, which in turn promotes Cu enrichment and induces the precipitation of ε-Cu due to the reduction of interfacial energy for reversed austenite formation. Our work reveals the synergism of Cu and Ni on the formation of reversed austenite.

Thermal Treatment Effect on Tribological and Corrosion Performances of 13Cr5Ni2Mo Super-Martensitic Stainless Steel

Wear behavior of 13Cr5Ni2Mo supermartensitic stainless steel (SMSS) were investigated in air and in argillaceous paste at room temperature. Prior to wear testing, the samples were subjected to two treatments, namely; quenching followed by double tempering. A pin-on- disc test rig was used to conduct the wear test with a tribo-pair consisting of supermartensitic stainless steel sliding against itself. It was observed that the microstructure of the thermally treated samples mainly consisted of tempered martensite and carbides. Wear results showed that oxidative and abrasive wear dominated the wear process of the treated samples in both dry conditions and in argillaceous paste. Additionally corrosion tests were performed in 5.0% NaCl solution via potentiodynamic polarization tests. It was found that the passive film formation provides a good corrosion resistance to the samples.

Surface mechanics and wear resistance of supermartensitic stainless steel nitrided by plasma immersion ion implantation

In materials employed in the marine oil extraction, a nitrided layer produced by plasma immersion ion implantation (PI3) can improve the surface resistance against abrasion and still work as a strengthened substructure for deposition of hard coatings. PI3-modified supermartensitic stainless steel (SMSS) surfaces consisted of an iron nitride-rich case followed by N-solid solution; thicknesses of both regions grew as the treatment temperature increased from 300 to 400 °C. The amount of nitrides scaled up with ion fluence/temperature. The thickest layer (25 μm) with the highest hardness profile (13 GPa) was produced at 400 °C, as a result of mixed iron nitrides precipitated in the matrix. Elastic recoveries after normal and tangential loadings were improved as well. Moreover, specific wear rates reduced up to two orders of magnitude and the coefficient of friction was 60% lower when compared with that of the untreated surface. Wear features varied from adhesive to abrasive as iron nitrides became the prevailing structure on the modified surfaces, from 300 °C to 400 °C. Even so, all the nitrided layers disclosed an overall ductile character under deformation. Nanomechanical analyses indicated that nitrides-containing top layers originated in the implantation and diffusion-based PI3 method were uniformly hardened in depth, in contrast with those from the conventional plasma nitriding.

Effect of titanium nitride (TiN) on the corrosion behavior of a supermartensitic stainless steel

This study evaluates the effect of high N, Ni, and Ti contents on the microstructure and corrosion properties of a supermartensitic stainless steel when immersed in a 3.5% NaCl solution. Tempering procedure at several temperatures resulted in a martensitic microstructure composed of grains ranging from 60 to 70 μm, TiN microparticles, and austenite. The sample treated at 610 °C showed better corrosion resistance due to the lower current density of the anodic polarization curve and less negative pitting potential, which was thoroughly influenced by the content of austenite (6.2 vol%). Anodic curves presented current fluctuation attributed to the presence of passive film instability, due to micro‐galvanic corrosion between TiN particles and matrix. Impedance analysis on the sample tempered at 610 °C confirmed that 6.2 vol% of austenite assists the protective film by making it thicker and more protective in chloride media.

Meta-equilibrium transition microstructure for maximum austenite stability and minimum hardness in a Ti-stabilized supermartensitic stainless steel

The maximization of stable reverted austenite at room temperature through inter-critical tempering is a widely used method to reduce hardness in supermartensitic stainless steels. Nevertheless, partial martensitic transformation might occur due to insufficient compositional stabilization. In this work, we conducted a time-resolved triple-step inter-critical tempering, specially designed to obtain maximum austenite stability and minimum hardness through the progressive suppression of the martensitic transformation. The mechanism behind the progressive increase in stable reverted austenite was the generation of a meta-equilibrium state, which imposed a limit in both high temperature austenite reversion and room temperature austenite stabilization. Such limit corresponded to the high temperature volume fraction of austenite, obtained right before cooling from the first cycle. This effect was associated to the Ni-rich fresh martensite laths acting as local Ni compositional pockets, providing site-specific austenite reversion; and to the suppression of any additional nucleation at the Ni-poor matrix as the T0 temperature for austenite reversion was strongly increased. The softening mechanism was mainly controlled by the carbon arrest effect by the precipitation of Ti (C, N), which was completed after the first tempering cycle. Nevertheless, maximizing reverted austenite and suppressing fresh martensite at room temperature did not result in additional hardness reductions.

Performance of nitrogen ion-implanted supermartensitic stainless steel in chlorine- and hydrogen-rich environments

Modified supermartensitic stainless steel surfaces were investigated as protective means against deterioration in Cl−- and H+-rich media. Nitrogen plasma immersion ion implantation at the 300–400 °C range produced top nitride-rich layers (with mainly γ′-Fe4N and ε-Fe2-3N, but also with α′N, according to the treatment temperature) followed by underneath expanded martensite cases. The 400 °C nitrided sample presented the best performance in potentiodynamic polarization tests with NaCl electrolyte, featured by 4.3 times increase in the corrosion potential and the absence of pits, attributed to the thickest and continuous ε-phase containing nitride-rich layer. The hydrogen embrittlement was assessed through cathodic hydrogenation tests. Both reference and 400 °C nitrided surfaces disclosed the phenomenon of intensified plastic flow under normal and tangential loadings. A decrease in hardness, elastic modulus and scratch resistance featured a ductile-to-brittle transition on the nitrided surface, possibly due to improved hydrogen trapping by nitride species with subsequent effects in plasticity. In summary, while the nitride layer played an advantageous role in protecting SMSS from chlorine attack, it was susceptible against the hydrogen corrosion.

Mechanical and Microstructural Characterization of Diffusion Bonded Super Martensitic Stainless Steel Joints

Super Martensitic Stainless Steels (SMSS) are low carbon high chromium steels. They possess high strength and resistant to corrosion for fluids like CO2 and H2S. Diffusion bonding is a welding technique capable of joining similar or dissimilar metals wherein the solid materials intersperse themselves with time. In this present study, the SMSS specimens were bonded at temperatures ranging from 800-10500C in the non-inert atmosphere and their microstructure and mechanical properties were analyzed.

Pitting Corrosion of Supermartensitic Stainless Steel in Chloride Solutions Containing Thiosulfate or H2S

Deepwater oil and gas extraction environment have high salinity, low oxygen content, and the presence of carbon dioxide and hydrogen sulfide (H2S). Handling of H2S-containing gaseous mixtures should be carried out in laboratories with special installations, owing to the high toxicity of this gas. An alternative is the use of non-toxic substances that can generate H2S in the medium, such as sodium thiosulfate. The objective of this work is to study the viability of substituting H2S by thiosulfate for the evaluation of the susceptibility to pitting corrosion in sour environments of the supermartensitic stainless steel (SMSS). The electrochemical behavior of SMSS in NACE TM0177-2016 solution C solution containing different concentrations of H2S or thiosulfate ions at 10−3 mol/L was performed by using cyclic potentiodynamic polarization (CPP). The condition of shorter OCP time (300 s) prevents the formation of a passive layer and allows the generation of H2S due to the reaction of the solution with the metal surface. The CPP curves of SMSS in the media containing 3% H2S/Ar (2.96 kPa) and 10−3 M of thiosulfate ions (OCP 300 s) were similar, the pitting potential was about − 322.5 ± 8.7 and − 338.8 ± 11.4 mVAg/AgCl, respectively, and the pit morphology showed several small pits on the steel surface in both conditions, suggesting the viability of the use of thiosulfate-containing brines to simulate H2S solutions.