Austenite Layer Research Articles

Mechanism of increased strength and ductility of martensite-austenite composite steel with different thickness ratios

In this study, the mechanism of increased strength and ductility of a martensite-austenite composite steel with different thickness ratios was analysed. Results revealed that with increasing the thickness ratio between the martensite and austenite layers, the strength and uniform ductility of the composite steel initially increased and then decreased gradually. The best combination was obtained in a 2:1 specimen, with the strength and the uniform ductility being increased by 9% and 190% respectively, as compared with the monolithic martensitic steel. The slight diffusion of carbon atoms from the austenite layer into the martensite layer strengthened the steel, which offset the weakening effect of introducing the austenite layer. Moreover, the diffusion of heavy elements between the neighbouring layers, such as manganese and silicon, formed a transition layer with varying concentration which effectively coordinated the deformation behaviour of the neighbouring martensite and austenite layers.

Effect of warm-rolling on the strength and ductility of multilayered composite steel

Making composite steel is an effective method to improve the strength and ductility of steel, but it often follows the mixture rule. Further increasing the strength and ductility simultaneously of composite steel is challenging. In this study, warm-rolling was introduced into a multilayered martensite/austenite composite steel after the compositing process. Results revealed that some C atoms diffused from the martensite layer into the neighbouring austenite layer, resulting in a reduced hardness in the martensite layer of common composite steel. Introducing warm-rolling deformation not only hardened each layer effectively, but also clearly improved the ductility of the austenite layer. This facilitated the coordination deformability of the austenite layer and resulted in simultaneous improvements in strength and ductility for warm-rolled composite samples. Better mechanical properties (tensile strength of 1546 MPa and a uniform elongation of 14.9%) were obtained for a sample with a lower degree of reduction (10%). The mechanism leading to improved mechanical properties is analysed in detail in this paper.

Влияние нормальной силы на сглаживание и упрочнение поверхностного слоя стали 03Х16Н15М3Т1 при сухом алмазном выглаживании сферическим индентором

Introduction. Sliding burnishing minimizes roughness and hardens of the steel surface. Quality of the formed surface and strength characteristics of the surface layer are determined by the burnishing speed, force and feed. Due to the danger of the surface micro-destruction during burnishing, the problem of selecting the favorable value of the normal force at a given feed arises. The current investigation aims to study the effect of normal force during dry diamond burnishing with a spherical indenter on smoothing the surface microprofile and strain hardening of the 03Cr16Ni15Mo3Ti1 austenitic steel surface layer. Research methods. Profilometry, scanning electron microscopy (SEM), microdurometry are used. Results and discussion. As the result of dry burnishing of deformation-stable austenitic steel 03Cr16Ni15Mo3Ti1 with a spherical indenter with a 2 mm radius made of natural diamond at a sliding speed of 10 m/min and feed rate of 0.025 mm/rev, it is found that in the investigated variation range of the burnishing normal force (100...200 N) the value of the smoothing coefficient of the initial steel surface microprofile after finish turning is 79...90 %, the greatest smoothing with a decrease in the average roughness parameter Ra from 1.0 to 0.1 µm is achieved at a force of 150 N; during diamond burnishing the initial (after finish turning) surface is hardened by 15...43 % (up to 382...444 HV), as the burnishing force raises from 100 to 175 N, a non-monotonic increase of the average microhardness from 409 to 444 HV 0.05 takes place; burnishing with a load of 175 N forms a gradient-hardened layer with a thickness of 300...350 μm with the appearance of individual microfractures in the form of beadings and micro-cracks on the surface, the maximum hardening is caused by the formation of a highly dispersed surface layer of 30...40 μm thick with a structure of highly dispersed austenite and the corresponding activation of grain-boundary and dislocation strengthening mechanisms. The results can be used when selecting the diamond burnishing parameters of parts made of corrosion-resistant austenitic steels according to the criteria for obtaining low surface roughness without significant microfractures and effective strain hardening of the surface layer.

Effect of austenitic stainless steel weld clad layers on conduction heat transfer across the walls of pressure vessels

Cladding austenitic stainless steels are popular nowadays in pressure vessels to enhance surface qualities. In this work, austenitic stainless-steel clad layers deposited by flux cored arc welding process on structural steel plates used in boiler construction are investigated in the direction of heat energy conservation. This study is vital, as the low thermal conductivity of austenitic stainless-steel clad layer is capable of reducing heat transfer across walls of pressure vessels. Heat transfer studies were performed using theoretical approach, computational simulations and experimental approach. Results of these studies exhibit a heat conservation of 10.6% owing to the stainless-steel clad layers with optimum clad thickness. The results of theoretical and computational method are having good agreement with the experimental results. These dual-benefit findings of the present work could be supportive to impart corrosion resistance and energy conservation, thereby, in energy efficient design of the thermal equipment.

Plasma Carburizing of Laser Powder Bed Fusion Manufactured 316 L Steel for Enhancing the Surface Hardness

Austenitic stainless steels produced by laser powder bed fusion (L-PBF) are quite interesting materials owing to their specific microstructure consisting of dendrite walls built of dislocations pinned by many nano-oxides that involves significant strengthening without loss of ductility. In this work, different plasma treatments were performed to harden the surface of 316 L steel manufactured by L-PBF. The samples were characterized by X-ray diffraction (XRD), Raman spectroscopy (RS), light microscopy (LM) and micro-hardness tests. The experimental results show that all the plasma treatments enhance the hardness of the surface because a C-enriched layer of austenite (S-phase) forms with a thickness up to 25 μm. The plasma gas mixture, consisting of 2.5% (CH4) + 97.5% (H2), resulted in being the most effective and produced a surface hardness (547 ± 27 HV) more than double with respect to that of the untreated material. The treatment temperature was 475 °C, which represents a good compromise between the necessity to avoid the precipitation of M23C6 carbides and the compatibility of treatment time with the industrial practice. Moreover, it has been observed that a 2 μm-thick over-layer of amorphous C forms on the sample surface. The hardness of such over-layer, which depends on the specific treatment and is related to the degree of topological disorder, is generally greater than that of S-phase. The work demonstrates that plasma carburizing is quite effective in hardening the surface of 316 L steel manufactured by L-PBF and further improves its mechanical properties, which are basically superior to those of the same material prepared by conventional processes.

Quantification of Residual Stress Relief by Heat Treatments in Austenitic Cladded Layers.

The effect of the heat treatment on the residual stresses of welded cladded steel samples is analyzed in this study. The residual stresses across the plate’s square sections were determined using complementary methods; applying diffraction with neutron radiation and mechanically using the contour method. The analysis of the large coarse grain austenitic cladded layers, at the feasibility limits of diffraction methods, was only made possible by applying both methods. The samples are composed of steel plates, coated on one of the faces with stainless steel filler metals, this coating process, usually known as cladding, was carried out by submerged arc welding. After cladding, the samples were submitted to two different heat treatments with dissimilar parameters: one at a temperature of 620 °C maintained for 1 h and, the second at 540 °C, for ten hours. There was some difference in residual stresses measured by the two techniques along the surface of the coating in the as-welded state, although they are similar at the welding interface and in the heat-affected zone. The results also show that there is a residual stress relaxation for both heat-treated samples. The heat treatment carried out at a higher temperature showed sometimes more than 50% reduction in the initial residual stress values and has the advantage of being less time consuming, giving it an industrial advantage and making it more viable economically.

Kinetics of the UNS S32750 Super Duplex Stainless Steel Low-Temperature Plasma Nitriding

This work reports an experimental investigation on the thermodynamic and kinetic view-point of the nitrided layer growth behavior, carried out separately at ferrite and austenite phase grains, for the UNS S32750 super duplex stainless steel (SDSS). For this purpose, and motivated by the DSSs distinct behavior showing formation of two- or single-phase nitrided layers, which apparently depend on the steel substrate chemical composition, two series of treatment were studied here, one varying the nitriding temperature on the 300-450 °C range for 4 h time, and other one varying the time on the 2-8 h range at 350 °C. Microstructural characterization of studied layers by means of SEM and EDS analysis, XRD and microhardness measurements showed precipitation-free nitrogen-expanded austenite layer formation on both steel phases for all samples treated up to 400 °C. The nitriding kinetics study showed that the layer thickness on both steel phases is proportional to the square root of the treatment time and follows an Arrhenius law for the studied treatment temperatures. The nitrogen diffusion activation energy, separately determined from the layer growth on each steel phase, was 115±3.2 and 120±5.4 kJmol-1, for austenite and ferrite grains, respectively.

Excellent tension properties of stainless steel with a 316L/17-4PH/17-4PH laminated structure fabricated through laser additive manufacturing

Laminated metal structures (LMS), which are metals with site-specific properties, exhibit great potential for practical applications. In this study, a stainless steel with a laminated structure of alternating 316L/17-4PH/17-4 PH layer is fabricated using a laser additive manufacturing (AM) technique. Heterogeneous structures in the LMS are observed in two distinct areas, the austenite area (AU) and the martensite area (MA), based on optical microscopy and backscattered electron images. In the electron backscattered diffraction images, the AU area can be divided into austenite and mixture layers, while the MA area represents the martensite layer. Owing to the AM fabrication process, heterogeneity of both the micro-hardness and chemical composition exists among the three layers, which leads to different austenite stabilities in each layer. The tension test results reveal an excellent strength-ductility synergy with stronger ultimate strength and larger elongation than the pure 316L structure produced by AM with the same printing process parameters. During tension testing, the laminated structure causes strain partitioning, which evolves with the applied strain and delays the strain localization process. Owing to the change in austenite stability compared with pure AM 316L, the transformation-induced plasticity effect (TRIP) is triggered successively, and the laminate structure extends the TRIP effect to large plastic strains. As a result, the present study verifies the potential of using AM design for LMS stainless steel and may offer a framework for re-exploring LMS materials and components.

Technology of welding and repair works of hydropower plants units

In the course of long-term operation of hydroelectric units, they are subject to wear and tear, which negatively affects the power generating capacity of the hydroelectric power station. The existing repair technology provides the restoration of the geometric dimensions of the impeller chamber of the hydraulic unit using an austenitic cladding layer. However, this technology is characterized by the occurrence of cracks and delamination of the deposited layer during post-repair operation, which is negatively reflected in the production cycle of electricity. A new technology and welding consumables have been proposed for repair and restoration works of hydroelectric units at hydroelectric power plants, providing an increase in the quality level, high characteristics of hydroabrasive and cavitation resistance, and increasing of working period of hydroelectric power plants. Inspection of the body of the hydraulic unit was carried out, defects were identified and classified, the reasons for their occurrence were analyzed. The research results make possible to optimize the alloying system of the deposited metal, to determine the optimal welding mode, to reduce the amount of the martensite component and to develop new filler materials and the technology for welding and repair works of hydroelectric power plants.

Phase field modeling of crack nucleation and propagation in martensitic micro–structures

AbstractIn this paper, a combined phase‐field (PF) approach has been considered to model fracture behavior under martensitic transformation effects. The effect of temperature as another degree of freedom has been studied, which has not been taken into account in previous studies. The concept of thermodynamic driving force, the elastic energy, interfacial energy, heat transfer and the kinetics of phase field equations are introduced to obtain the fracture domain, the martensitic evaluation, the temperature field and displacements [1]. Due to the high amount of stress at the crack tip, the martensitic transformation can start and propagate through the austenitic structure. The internal stresses between martensitic and austenitic layers deviate the crack path, leading to changes in the martensitic formation [2]. In addition, temperature plays an important role in this phenomenon in two aspects; first of all a change in temperature can produce thermal stress; and secondly, it affects the conditions for martensitic transformation. An increase in temperature requires more energy for the phase transformation [3], and changes in martensitic formation induce a new driving force for crack growth [2,4].

Effects of Plasma-Chemical Composition on AISI 316L Surface Modification by Active Screen Nitrocarburizing Using Gaseous and Solid Carbon Precursors

Low-temperature plasma nitrocarburizing treatments are applied to improve the surface properties of austenitic stainless steels by forming an expanded austenite layer without impairing the excellent corrosion resistance of the steel. Here, low-temperature active screen plasma nitrocarburizing (ASPNC) was investigated in an industrial-scale cold-wall reactor to compare the effects of two active screen materials: (i) a steel active screen with the addition of methane as a gaseous carbon-containing precursor and (ii) an active screen made of carbon-fibre-reinforced carbon (CFC) as a solid carbon precursor. By using both active screen materials, ASPNC treatments at variable plasma conditions were conducted using AISI 316L. Moreover, insight into the plasma-chemical composition of the H2-N2 plasma for both active screen materials was gained by laser absorption spectroscopy (LAS) combined with optical emission spectroscopy (OES). It was found that, in the case of a CFC active screen in a biased condition, the thickness of the nitrogen-expanded austenite layer increased, while the thickness of the carbon-expanded austenite layer decreased compared to the non-biased condition, in which the nitrogen- and carbon-expanded austenite layers had comparable thicknesses. Furthermore, the crucial role of biasing the workload to produce a thick and homogeneous expanded austenite layer by using a steel active screen was validated.

The effect of thin surface layer of nitrogen-expanded austenite on bulk γ-α’ phase transformation in low-temperature deformation of 316L stainless steel

Deformation-induced γ-α’ phase transformation was studied in austenitic stainless 316L steel in low-temperature deformation regime (tension at 77 K). Before deformation, the surface of the steel specimens was saturated with nitrogen by ion-plasma nitriding at 673 K for 12 h. Thin 4-μm surface layer of nitrogen-expanded austenite (S-phase) does not influence mechanical properties but affects phase transformation in the bulk 1.5 mm-thick specimens. Surface nitriding effectively suppresses the inhomogeneity in deformation-induced martensite nucleation and formation of the α’-phase prior the macroscopic neck formation.

Controllable Martensite Transformation and Strain-Controlled Fatigue Behavior of a Gradient Nanostructured Austenite Stainless Steel.

Gradient nanostructured (GNS) surface layer with a controllable martensite fraction has been synthesized on 316L austenitic stainless steel by means of surface mechanical rolling treatment (SMRT) with temperature being controlled. The mean grain size is in the nanometer scale in the near-surface layer and increases gradually with depth. In addition, the volume fraction of martensite decreases from ~85% to 0 in the near-surface layer while the SMRT temperature increases from room temperature to 175 °C. Fatigue experiments showed that the strain-controlled fatigue properties of the GNS samples are significantly enhanced at total strain amplitudes ≥0.5%, especially in those with a dual-phase surface layer of austenite and pre-formed martensite. Analyses on fatigue mechanisms illustrated that the GNS surface layer enhances the strength-ductility synergy and suppresses the formation of surface fatigue defects during fatigue. In addition, the dual-phase structure promotes the formation of martensite and stacking faults, further enhancing fatigue properties at high strain amplitudes.

Investigation of the Heat-Affected Zone Properties During Cladding of Power Equipment with Austenitic Materials Using Control Mechanical Impacts on the Strip Electrode

During cladding an austenitic layer on low-carbon and medium-alloyed steels, the properties of the heat-affected zone, along with the resistance of the surface layer to resist corrosion, largely determine the performance and durability of the surfaced product. The work is devoted to the study of the dependence of the properties of the heat-affected zone during cladding of power equipment with austenitic materials on the parameters of the control mechanical impacts on the strip electrode and determination of their optimal range, which ensures high values of the mechanical properties of the deposited layer.

Computational analysis of austenite film thickness and C-redistribution in carbide-free bainite

In this work, a methodology for the computational analysis of some essential microstructural features of a bainitic microstructure is developed. The focus lies in the accurate prediction of the ferritic subunit size, the thickness of the residual austenite films, their corresponding C-enrichment and the accompanying stabilization of the residual austenite. Basis of the approach is the T 0-temperature concept in combination with the numerical simulation of C-diffusion profiles utilizing the cell diffusion module of the thermokinetic software package MatCalc. This methodology gives the opportunity to predict the C-distribution under consideration of consecutively forming subunits, which is necessary to estimate the C-content of austenite films. The simulations also take into account the effect of C trapping at the dislocations formed inside the ferritic platelets due to plastic deformation and its influence on the chemical potentials. Good agreement is achieved between measured and predicted retained austenite layer thickness and the C-enrichment of the layers accompanying the C redistribution process.

Enhancing the Corrosion Resistance of Austenitic Steel Using Active Screen Plasma Nitriding and Nitrocarburising.

AISI 316L steel was subjected to active screen plasma nitriding and nitrocarburising. The processes were carried out at 440 °C for 6 h. The nitriding process employed an atmosphere of nitrogen and hydrogen, while nitrocarburising was carried out in nitrogen, hydrogen and methane. The processes yielded structures consisting of nitrogen and nitro-carbon expanded austenite, respectively. Microhardness was measured via the Vickers method, surface roughness using an optical profilometer, microstructure by means of light microscopy, while a scanning electron microscope (SEM) served to determine surface topography. Phase composition, lattice parameter and lattice deformation tests were carried out using the X-ray diffraction (XRD) method. Corrosion resistance measurements were performed in a 0.5 M NaCl solution using the potentiodynamic method. The produced layers showed very high resistance to pitting corrosion, while the pitting potential reached 1.5 V, a value that has not yet been recorded in a chloride environment. After the passive layer was broken down, there was a clear deceleration of pitting in the nitrocarburised layer. It was found that in the case of nitro-carbon expanded austenite, pits are formed much slower compared to the nitrogen austenite layer.

Low-temperature carburized high-alloyed austenitic stainless steels in PEMFC cathodic environment

Austenitic stainless steels are promising materials for bipolar plates in polymer electrolyte membrane fuel cells (PEMFC). In this study, we explore the possibility to apply low-temperature carburized high-alloyed (high Cr, Ni, Mo) steels as bipolar plate materials. The electrochemical properties and the microstructural characteristics of the modified surfaces were analyzed by optical microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Auger electron spectroscopy. The carburizing treatment (Kolsterising®, Bodycote) induced the formation of a carbon-stabilized expanded austenite layer. The amount of carbon dissolved in the metal matrix was dependent on the alloy composition, in particular on the amount of Mo. Additionally, surface carbides were found at the surface of low-Ni containing steels. The interfacial contact resistance (ICR) measured in a simulated PEMFC was reduced after carburizing as a consequence of thinner oxide layer and change in oxide composition. Potentiodynamic tests revealed nobilitation of all the steels when carbon was present in solid solution. The occurrence of surface carbides, however, was highly detrimental to the corrosion properties. The ICR of carburized materials in aged condition was higher than the corresponding untreated alloys. The presence of localized corrosion in carburized materials could be responsible for this effect.

Comparative Analysis of Cavitation Resistance of Deposited/Sprayed Layers of Carbides, Stainless Steels and Metastable Austenite

The cavitation erosion resistance of deposited/sprayed layers prepared by thermal spraying and welding processes was evaluated using a new principle of ultrasonic vibratory testing [1]. A high velocity air fuel thermal spraying process (HVAF), gas tungsten arc welding (GTAW or TIG), and shielded metal arc welding (SMAW) were used to prepare the deposit deposited/sprayed layers. The WC-10Co4Cr powder was deposited on 20X13 stainless steel, while E308L-17 welding filler electrode, 06X19H9T welding solid wire, and Fe-Cr-Al-Ti cored wire (known as PPM-6) were deposited on AISI 316L stainless steel. For all materials, the curves of volume loss as a function of testing time were plotted. The obtained results showed that the deposited layers of PPM-6 metastable austenite steel prepared by TIG welding process was exhibited a higher resistance to cavitation erosion than the others. The maximum volume loss of PPM-6 steel was about 0.31 mm1, whereas the volume loss for the other materials was of 5-20 times high.

Evaluation of the sliding wear and corrosion performance of triode-plasma nitrided Fe-17Cr-20Mn-0.5N high-manganese and Fe-19Cr-35Ni-1.2Si high-nickel austenitic stainless steels

Low-temperature plasma nitriding has been widely studied and applied, in enhancing the wear performance of austenitic stainless steels (ASSs) without losing corrosion resistance. In this work the wear and corrosion behaviours of two specialty ASSs, i.e. Staballoy® AG17 (Fe-17Cr-20Mn-0.5N, in wt%) and RA330® (Fe-19Cr-35Ni-1.2Si, in wt%), were evaluated – and compared to AISI 304 – before and after low-temperature triode plasma nitriding (TPN) at 400 °C and 450 °C. A nitrogen interstitially-supersaturated expanded austenite layer (γN) was developed for all three ASSs after TPN treatment at 400 °C, which led to a) an approximately 4-fold increase in surface hardness, b) a reduction in specific wear rate of at least 2 orders of magnitude in unlubricated dry-sliding, and c) an improved resistance to pitting in 3.5 wt% NaCl aqueous solution. Large numbers of ‘linear defects’ (identified in TEM studies as strips of HCP-εN) were seen in the γN-AG17 layer, that could be correlated to comparatively higher surface hardness and better wear resistance. Several slip/shear bands were also seen in the γN-330 layer, where short-range Cr-segregation could occur, leading to localised corrosion. More importantly, after TPN treatment at 450 °C, alloys AISI 304 and AG17 presented a deterioration in corrosion performance, whereas good corrosion performance was maintained for alloy RA330. Redistribution of Si (in preference to Cr) was revealed in γN-330 after TPN treatment at 450 °C, whereby Si-alloying at a significantly higher level than in the other two alloys investigated appears (in addition to the high Ni content in alloy 330) to be beneficial in delaying CrN precipitation, and thus in maintaining the good corrosion performance of γN after nitriding at low-to-intermediate temperatures.

Diffusional model of expanded austenite layer growth on AISI 316L stainless steel during low-temperature hybrid thermochemical treatment

Diffusional model of expanded austenite layer growth on AISI 316L stainless steel during low-temperature hybrid thermochemical treatment