CA6NM Martensitic Stainless Steel Research Articles

Cavitation and strain-induced transformation: the austenite phase behavior in a soft martensitic and an austenitic stainless steel * *Preliminary results of this manuscript were presented at 3rd International Conference on Surface Engineering in Material Forming (in the 40 o SENAFOR), October 13-15th 2021, Porto Alegre-RS, Brazil.

In this work the behavior of the austenite phase at the surface of a low-carbon martensitic stainless steel and an austenitic stainless steel subjected to cavitation through ultrasonic vibration tests in liquid phase was studied. Emphasis was given to the behavior of two distinct types of austenite, in the case the reversed austenite of the as-tempered ASTM CA-6NM martensitic stainless steel, and the thermally-stable austenite of the solution-treated AISI 304 austenitic stainless steel. The evolution of phases fraction at the studied surfaces by XRD technique was characterized by two ways. Firstly, cavitation test intercalated with XRD measurements as a function of the test time was carried out. In the sequence, an indirect measurement technique comprising the use of Vickers indentation and controlled material removal by polishing, also intercalated with XRD characterization along the incubation period of both steels was used. This procedure was strong enough to determine the mechanism that precedes the erosive wear defining the incubation-acceleration stages transition and the cavitation-affected depth presenting γ(austenite)→α′(martensite) strain-induced transformation in the material microstructure, measured from the surface of the tested region into the interior of the substrate bulk of each studied steel. Such mechanism covers deformation of the steel matrix and strain-induced transformation of the austenite phase at the surfaces subjected to cavitation, strongly influencing the beginning of the significant mass loss process for both low-carbon stainless steels studied here.

A Novel Concept of Hybrid Treatment for High‐Hardenability Steels: Concomitant Hardening and Paraequilibrium Thermochemical Treatment to Produce Interstitially Hardened/Stabilized Austenite Surfaces

A novel concept of hybrid treatment for steels is presented. The goal is to produce a hard interstitially hardened/stabilized austenite layer, with an expected improved wear and corrosion resistance, in the surface of high‐hardenability steels. The novelty is in the fact that the thermochemical treatment is performed in the steel metastable austenite field, above the martensite start temperature, during the isothermal step of the martempering or austempering hardening treatment. As a proof of concept, CA6NM martensitic stainless steel samples are subjected to the novel hybrid treatment. The whole treatment is plasma assisted and the chosen thermochemical treatment is nitriding. In the presented case, the produced hard layer is constituted of N‐stabilized austenite. During the paraequilibrium thermochemical treatment step, the metastable austenite surface is converted into a hard interstitially hardened/N‐stabilized austenite. Once concluded the whole treatment, as the component is cooled down to the room temperature, surface does not undergo phase transformation, whereas the nonchemically altered substrate bulk undergoes metastable austenite to martensite phase transformation. So, by means of this case study, the feasibility and potentiality of this novel concept of hybrid treatment is proved.

Behavior of the reversed austenite in CA-6NM martensitic stainless steel under cavitation

As-hardened (fully martensitic) and as-tempered (with 14 ± 2% reversed austenite in a martensitic matrix) samples were subjected to cavitation testing in order to characterize the behavior of the reversed austenite and understand how it influences the CA-6NM steel cavitation behavior. Results showed similar nominal incubation periods of 3 h for both the studied conditions. For the first time, it is shown that the as-tempered CA-6NM steel incubation stage can be divided in two distinct parts: i) the IP I, period mainly related to γREV→α′ strain-induced transformation; and IP II, period mainly related to α′ deformation. It was concluded that the reversed austenite plays important role in the incubation stage of the CA-6NM steel, delaying the expected start of the cavitation erosion.

Through‐Thickness Residual Stresses, Microstructure, and Mechanical Properties of Electron Beam‐Welded CA6NM Martensitic Stainless Steel after Postweld Heat Treatment

In this study, the integrity of electron beam‐ (EB‐) welded CA6NM—a grade of 13% Cr‐4% Ni martensitic stainless steel—was assessed through the entire joint thickness of 90 mm after postweld heat treatment (PWHT). The joints were characterized by examining the microstructure, residual stresses, global mechanical properties (static tensile, Charpy impact, and bend), and local properties (yield strength and strain at fracture) in the metallurgically modified regions of the EB welds. The applied PWHT tempered the “fresh” martensite present in the microstructure after welding, which reduced sufficiently the hardness (<280 HV) and residual stresses (<100 MPa) to meet the requirements for hydroelectric turbine assemblies. Also, the properties of the EB joints after PWHT passed the minimum acceptance criteria specified in ASME sections VIII and IX. Specifically, measurement of the global tensile properties indicated that the tensile strengths of the EB welds in the transverse and longitudinal directions were on the same order as that of the base metal (BM). Evaluation of the local tensile properties using a digital image correlation (DIC) methodology showed higher local yield strengths in the fusion zone (FZ) and heat‐affected zone (HAZ) of 727 MPa and 740 MPa, respectively, relative to the BM value of 663 MPa. Also, the average impact energies for the FZ and HAZ were 63 J and 148 J, respectively, and attributed to the different failure mechanisms in the HAZ (dimples) versus the FZ (quasi‐cleavage consisting of facets and dimples). This study shows that the application of PWHT plays an important role in improving the weld quality and performance of EB‐welded CA6NM and provides the essential data for validating the design and manufacturing process for next‐generation hydroelectric turbine products.

Analysis of 410NiMo coating deposited by thermal spray in CA6NM martensitic stainless steel against erosion by cavitation

For most applications, martensitic stainless steels are subject to operating conditions where good mechanical properties and wear resistance are required. Soldering soft martensitic stainless steels features decreased tenacity of the welded joint which, along with the residual stresses, can shorten the life of component and accelerate the cavitation process. In order to increase the strength of the martensitic structures, thermal spraying is used to produce coatings, laid without resulting in thermal cycles that influence the structural integrity of the coated steels. This work was deposited by the electric arc and wire flame, the 410NiMo martensitic stainless steel, in wire and rod form, respectively, on the substrate of the martensitic stainless steel CA6NM. The samples were sent to Vickers microhardness tests and accelerated cavitation, according to ASTM G32-10. The results were obtained through optical microscopy, X-ray diffraction, SEM, and dispersive energy spectroscopy (DES); the samples featured high adhesion, low porosity, and cavitation erosion–resistant coatings. The thermal spray is suitable for the application of this type of coating.

Reprint of ‘‘Computational study of the particle size effect on a jet erosion wear device’’

Computational fluid dynamics (CFD) is a useful tool to predict the erosion behaviour over geometries exposed to conditions of severe erosion wear. This work shows how CFD can be used to virtually characterize the wear behaviour of materials used in several hydraulic components, including turbomachinery systems, in which it is important to consider the effect of particle size. In addition, this work develops a methodology for determination and validation of the constants involved in the well-known Tabakoff-Grant model for erosion prediction using the erosion wear obtained via erosion testing in a jet tribometer reported in the literature as a reference. From the experimental data, an optimization algorithm was performed to determine the optimal values of Tabakoff-Grant model constants for ASTM A743 grade CA6NM martensitic stainless steel. The simulated erosion rate agrees with the experimental data for the material analysed in jet erosion simulations with impact angles ranging from 15° to 90°. The change of the angle of maximum erosion rate for small particles, which has been reported in the literature via experimentation, was explained satisfactorily. The results showed that the erosion rate with smaller particles is affected by fluid flow, since small particles tend to follow the flow streamlines, while larger particles move according to the conditions imposed by the jet at its outlet. The effective impingement angle against the surface for small particles is lower than the impingement angle for large particles; therefore, the angle of maximum erosion rate, measured between the jet and the sample's surface, for small particles increases.

Computational study of the particle size effect on a jet erosion wear device

Computational fluid dynamics (CFD) is a useful tool to predict the erosion behaviour over geometries exposed to conditions of severe erosion wear. This work shows how CFD can be used to virtually characterize the wear behaviour of materials used in several hydraulic components, including turbomachinery systems, in which it is important to consider the effect of particle size. In addition, this work develops a methodology for determination and validation of the constants involved in the well-known Tabakoff-Grant model for erosion prediction using the erosion wear obtained via erosion testing in a jet tribometer reported in the literature as a reference. From the experimental data, an optimization algorithm was performed to determine the optimal values of Tabakoff-Grant model constants for ASTM A743 grade CA6NM martensitic stainless steel. The simulated erosion rate agrees with the experimental data for the material analysed in jet erosion simulations with impact angles ranging from 15° to 90°. The change of the angle of maximum erosion rate for small particles, which has been reported in the literature via experimentation, was explained satisfactorily. The results showed that the erosion rate with smaller particles is affected by fluid flow, since small particles tend to follow the flow streamlines, while larger particles move according to the conditions imposed by the jet at its outlet. The effective impingement angle against the surface for small particles is lower than the impingement angle for large particles; therefore, the angle of maximum erosion rate, measured between the jet and the sample's surface, for small particles increases.

Macro-defects characterization in cast CA-6NM martensitic stainless steel

Cast low carbon CA-6NM (13%Cr-4%Ni) martensitic stainless steel has been used for hydraulic turbine runners manufacturing since the 60's owing to its high strength, toughness, corrosion, and cavitation-erosion resistance. These castings contain several types of defects which may have detrimental effects on the performance of the produced parts. Several aspects of the casting defects in these steels have not been specifically studied in detail. It is common in industry to evaluate the integrity of cast components by nondestructive testing such as X-ray radiography before putting them in operation. However, tortuous and complex geometry of macro-shrinkage porosities makes them difficult to be characterized precisely only by 2D radiographic results. In the present study, the relation between radiography mapping and actual macro-defect morphologies has been studied by the “salami” cross sectioning method in order to document the real shape and morphology of defects. Furthermore, metallographic characterization of cast microstructure in CA-6NM stainless steels has also been performed using several techniques to document both chemical composition and microstructure around macro-defects. Briefly stated, the main purpose of the investigation described herein is to study several severity levels of macro-defects in cast martensitic stainless steels and investigate some characteristics of real defects which can impact on the fatigue properties of cast parts at microstructural level.

Hybrid Laser-Arc Welding of 10-mm-Thick Cast Martensitic Stainless Steel CA6NM: As-Welded Microstructure and Mechanical Properties

Cast CA6NM martensitic stainless steel plates, 10 mm in thickness, were welded using hybrid laser-arc welding. The effect of different welding speeds on the as-welded joint integrity was characterized in terms of the weld bead geometry, defects, microstructure, hardness, ultimate tensile strength, and impact energy. Significant defects such as porosity, root humping, underfill, and excessive penetration were observed at a low welding speed (0.5 m/min). However, the underfill depth and excessive penetration in the joints manufactured at welding speeds above 0.75 m/min met the specifications of ISO 12932. Characterization of the as-welded microstructure revealed untempered martensite and residual delta ferrite dispersed at prior-austenite grain boundaries in the fusion zone. In addition, four different heat-affected zones in the weldments were differentiated through hardness mapping and inference from the Fe-Cr-Ni ternary phase diagram. The tensile fracture occurred in the base metal for all the samples and fractographic analysis showed that the crack path is within the martensite matrix, along primary delta ferrite–martensite interfaces and within the primary delta ferrite. Additionally, Charpy impact testing demonstrated slightly higher fracture energy values and deeper dimples on the fracture surface of the welds manufactured at higher welding speeds due to grain refinement and/or lower porosity.

Improvement of the cavitation erosion resistance for low-temperature plasma nitrided Ca-6NM martensitic stainless steel

The present work reports increase of the cavitation erosion resistance for low-temperature plasma nitrided CA-6NM martensitic stainless steel, which was investigated by means of a 20kHz ultrasonic vibrator at peak-to-peak amplitude of 50μm. The study was emphasized for the three first cavitation stages, usually presented at the cumulative material mass loss–time curve, for surfaces treated in dc pulsed plasma at 350°C, in 5%N2+95%H2 gas mixture, for different nitriding times of 6, 12, and 24h. The changes on nitrided surfaces were evaluated by XRD and nanoindentation technique. The modification of the treated surface morphology as a function of the testing time in the incubation, acceleration, and maximum erosion rate stages was also verified by SEM analysis. The improvement of the cavitation erosion resistance for nitrided surfaces is believed to be due to the nitrogen-expanded austenite (γN-Fe) formation from a tempered martensite matrix.

Efeito da temperatura interpasse na microestrutura, tenacidade ao impacto e propagação de trinca por fadiga de uniões soldadas por GTAW do aço ASTM A743-CA6NM

Atualmente os aços inoxidáveis martensíticos tem sido utilizados para a fabricação de turbinas hidráulicas, devido principalmente a sua elevada tenacidade. Entretanto, estes aços apresentam algumas restrições com relação à regiões recuperadas por soldagem, principalmente em razão da formação de martensita não revenida, a qual gera redução na tenacidade. Considerando as aplicações de reparo de turbinas hidráulicas, há grande interesse em desenvolver procedimentos de soldagem que elevem a tenacidade ao impacto e evitem os tratamentos térmicos pós-soldagem (TTPS). O presente trabalho busca analisar a influência da temperatura de interpasse na microestrutura, tenacidade ao impacto e propagação de trincas por fadiga nas uniões soldadas multipasse do aço inoxidável martensitico CA6NM usando AWS410NiMo como metal de adição, e processo TIG (tungsten inert gas). Observou-se a influência da temperatura de interpasse na formação de ferrita d, com formação intergranular no campo bifásico δ+γ, na amostra com temperatura interpasse de 80ºC, enquanto que na amostra soldada a 150ºC a formação de ferrita d ocorreu principalmente no campo monofásico. A alteração na formação da ferrita d, com a menor temperatura, promoveu um aumento na tenacidade ao impacto e uma diminuição na velocidade de propagação de trinca, quando comparada com a amostra soldada com maior temperatura de soldagem. Os resultados obtidos indicam que o processo TIG apresenta-se como uma excelente alternativa para o reparo do aço CA6NM, observando-se também uma influência significativa da temperatura de interpasse.

Plasma nitriding using high H2 content gas mixtures for a cavitation erosion resistant steel

Plasma nitriding using high H2 content gas mixtures in CA-6NM martensitic stainless steel was studied in the present work. Nitriding was performed in H2+N2 gas mixtures for 5, 10 and 20% N2, in volume, at 773K (500°C), during 2h. Changes in the surface morphology and nitrided layer constitution were characterized by SEM, XRD, roughness analysis, and nanoindentation technique. Cavitation erosion behavior of the nitrided samples was also investigated by means of a 20kHz ultrasonic vibrator. The study was emphasized for the three first cavitation stages (incubation, acceleration, and maximum erosion rate stage) of the cumulative erosion–time curve. Results indicate that the gas mixture nitrogen content strongly influences the phases’ formation and its distribution on the nitrided layer. Better cavitation erosion resistance which was attributed to the finer and more homogeneous distribution of the nitrided layer phases was verified for samples treated at 5% N2. Otherwise, worse cavitation erosion behavior for samples nitrided at 20% N2 is supposed to be due to the formation of multiphase compound layer constituted by Fe4N+Fe2–3N+CrN, which can infer residual stress in treated surface.

Strong evidences of tempered martensite-to-nitrogen-expanded austenite transformation in CA-6NM steel

The present work reports strong evidences of the transformation from tempered martensite to nitrogen-expanded austenite in low-temperature plasma nitrided CA-6NM martensitic stainless steel, which are supported by XRD, Kakhovskii constitution diagram (SIMS profiles and WDS measurements) and microstructure analysis of the treated samples.

Cavitation erosion behaviour of niobium

Abstract Cavitation erosion behaviour of niobium was investigated by means of a 20 kHz ultrasonic vibrator at peak-to-peak amplitude of 50 μm, aiming to determine the niobium potential as a material for the manufacturing of hydraulic machine components. The study was emphasized for the three first cavitation stages of the cumulative erosion–time curve. The modification of the niobium surface morphology as a function of the testing time in the incubation, acceleration, and maximum erosion rate stages was verified by SEM analysis. Samples were prepared from 98.9% purity and 90% reduction cold-rolled niobium bar. The study was performed for niobium samples in both the cold-worked and annealed conditions. Samples of CA-6NM martensitic stainless steel, a typical material utilized for hydraulic turbines manufacturing, were also analysed for comparison purpose. Annealing treatment of niobium decreases its hardness and increases its ductility, leading to an increase of the incubation period when compared with the cold-worked niobium. Cavitation erosion failure mechanism in niobium occurs in a sequence of events comprising the work-hardening effect and the fracture of debris allied to the effect of fatigue and microcracks formation. Finally, annealed niobium presents similar incubation period but worse behaviour in the maximum erosion rate stage than CA-6NM steel.

Weldability of Low-Carbon ASTM A356 CA6NM Martensitic Stainless Steel Casting for Power Plants

Weldability, especially HAZ cold cracking, weld metal solidification cracking, and HAZ liquation cracking susceptibilities, of ASTM A356 CA6NM martensitic stainless steel casting was investigated and compared with that of 9-12% Cr ferritic steel castings. Irrespective of the Cr and Ni content in the castings, the HAZ maximum hardness increased with an increase of carbon content. CA6NM steel, which has the lowest carbon content, had the lowest HAZ hardness and showed no cold cracking in y-slit cracking tests. CA6NM steel, meanwhile, showed the largest weld metal solidification cracking susceptibility in varestraint tests because of its higher amount of impurity elements, phosphorus, and sulfur. All castings investigated had good high temperature ductility in hot ductility tests and showed little difference in liquation cracking susceptibility. (Received June 30, 2010)

Plasma nitriding of CA-6NM steel: effect of H2 + N2 gas mixtures in nitride layer formation for low N2 contents at 500 ºC

This work aims to characterize the phases, thickness, hardness and hardness profiles of the nitride layers formed on the CA-6NM martensitic stainless steel which was plasma nitrided in gas mixtures containing different nitrogen amounts. Nitriding was performed at 500 ºC temperature, and 532 Pa (4 Torr) pressure, for gas mixtures of 5% N2 + 95% H2, 10% N2 + 90% H2, and 20% N2 + 80% H2, and 2 hours nitriding time. A 6 hours nitriding time condition for gas mixture of 5% N2 + 95% H2 was also studied. Nitrided samples results were compared with non-nitrided condition. Thickness and microstructure of the nitrided layers were characterized by optical microscopy (OM), using Villela and Nital etchants, and the phases were identified by X-ray diffraction. Hardness profiles and hardness measured on surface steel were determined using Vickers hardness and nanoindentation tester, respectively. It was verified that nitrided layer produced in CA-6NM martensitc stainless steel is constituted of compound layer, being that formation of the diffusion zone was not observed for the studied conditions. The higher the nitrogen amounts in gas mixture the higher is the thickness of the nitrided layer and the probability to form different nitride phases, in the case γ'-Fe4N, ε-Fe2-3N and CrN phases. Intrinsic hardness of the nitrided layers produced in the CA-6NM stainless steel is about 12-14 GPa (~1200-1400 HV).