Presence Of Austenite Research Articles

Effect of charging temperature of continuous casting slab on the surface cracks of HSLA steel thick plate

In order to investigate the formation mechanism of the surface cracks on high-strength low-alloy (HSLA) steel thick plates, this paper simulated the temperature history of different hot charging processes, and deduced the microstructural transformation behavior and recrystallization austenitization process under different charging temperatures. This paper proposed four hot charging modes and ultimately elucidated the effect of charging temperature on the surface cracks of HSLA steel thick plates. Experimental results showed significant differences in the microstructure and recrystallized austenite grains under different charging temperatures. Analysis revealed that the recrystallized austenite grains were determined by the microstructure. The presence of austenite in the microstructure significantly increased the size of recrystallized austenite grains, thereby disrupting the uniformity of grains. Based on the relationship between charging temperature, microstructure, and recrystallized austenite grain state, the hot charging process of HSLA steel was divided into four temperature zones: high-temperature single-phase zone, high-temperature two-phase zone, medium-temperature three-phase zone, and low-temperature two-phase stable zone. The occurrence of the surface cracks in HSLA steel was caused by the recrystallized austenite mixcrystal structure generated during charging in the medium-temperature three-phase zone. The uneven recrystallized austenite grains caused by undercooled austenite were the direct cause of the surface cracks, while high charging temperature was the root cause. When the charging temperature entered the medium-temperature three-phase temperature zone, recrystallized austenite grains would exhibit mixcrystal structure. These mixcrystal structure deteriorated the surface quality, promoting the formation, deformation, and propagation of surface cracks. To reduce the occurrence rate of surface cracks, for steel grades sensitive to surface quality, the charging temperature should be controlled in the low-temperature two-phase stable zone.

Electrolyte-plasma surface hardening of hollow steel applicator needles for point injection of liquid mineral fertilizers

<abstract> <p>This paper presents the results of research on the effect of electrolyte-plasma hardening on tribological and mechanical properties of hollow needles of 12Kh18N10Т steel applicators for liquid fertilizer application. For the application of liquid fertilizers for processing and testing, the hollow needles of the applicator are made of 12Kh18N10Т steel of cylindrical shape with a diameter of 20 mm. To ensure uniformity of the hardening process, the part was rotated clockwise during the entire procedural cycle. To reveal the influence of the sample rotation speed on the uniform surface hardening, an experiment was conducted for three applicators with rotation speeds of 4, 6, and 8 rev/min. As a result of electrolyte-plasma surface hardening (EPSH), the phase composition of the specimen's surface is characterized by the presence of austenite (γ-Fe) and ferrite (α-Fe). It is revealed that the maximum value of microhardness after EPSH is 2 times higher than the initial value. According to the results of the performed works, the contract on application of tests in field conditions and revealing of perspectivity of needle applicators for liquid fertilizers application was concluded.</p> </abstract>

Microstructure and Wear Resistance in Medium and High C Steels Treated by Quenching and Partitioning

The quenching and partitioning (Q&P) steels have shown to be promising candidates to be applied in fields where wear resistance is required. In this study, a medium and a high C steel are heat treated by Q&P and the resulting microstructure, hardness, and wear resistance are characterized. The mechanical stability of the austenite phase under wear test conditions is investigated. It is found that the stability of austenite is very high in the high C steel and decreases in the medium C steel. Additionally, the hardness and wear behavior of the Q&P‐treated steels are compared with the results obtained for quenching and tempering (Q&T) treated samples, showing that, although the hardness of Q&P steels is quite lower, the obtained wear rates are similar. It means that in the studied Q&P steels, although the austenite transformation into martensite does not occur considerably, the presence of austenite might play a key role in the wear resistance.

Austenite Formation in the Oxidized Layer of Ultra-High-Strength 13Ni15Co10Mo Maraging Steel

Maraging steels are precipitation hardening alloys that can achieve an ultra-high yield strength (~3 GPa), however associated with low toughness. During exposure to high temperatures, an oxidation process occurs on the surface of these steels, generally, the oxides formed are hematite and/or magnetite. The aim of this study was to investigate oxidation on a maraging 13Ni15Co10Mo at annealing temperature of 900 °C. The bulk microstructure was investigated by several complementary techniques and the oxidized surface was characterized by Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS) and X-ray Diffraction (XRD). The results showed that the bulk microstructure, at annealed condition, consists of a lath martensite with a hardness of round 400 HV. The most external and oxidized surface contains the oxides hematite, magnetite and kamiokite. Finally, the presence of austenite was detected in the first 2 μm below the surface. Chemical microanalysis indicated that the austenite is stable at room temperature in this region due a composition gradient that makes this region rich in nickel and cobalt. The composition gradient is due atom diffusion during oxides formation. Austenite near to the surface is very convenient as it could avoid crack initiation and propagation, improving toughness.

Additive manufacturing of corrosion-resistant maraging steel M789 by directed energy deposition

Conventional maraging steels feature a combination of high strength and toughness, but they often suffer from low corrosion resistance. Hence, maraging steel M789 was developed to alleviate this issue while maintaining its strength. Most studies of maraging steels processed by additive manufacturing (AM) focus on utilizing laser powder bed fusion (LPBF). However, the research in laser-directed energy deposition (L-DED) fabrication of corrosion-resistant maraging steels is limited. The different cooling rates experienced by materials during L-DED and LPBF processing give rise to differing microstructures and mechanical properties. In this study, the L-DED process was adopted to manufacture nearly-fully dense M789 parts, which were subsequently subjected to direct aging and solutionizing + aging heat treatments. Electron backscatter diffraction (EBSD) analysis reveals a martensitic structure in both as-fabricated and heat-treated samples with the presence of austenite in the as-fabricated and directly aged samples. Scanning transmission electron microscopy (STEM) and transmission Kikuchi diffraction (TKD) reveal the presence of Ti- and Al-rich precipitates within the martensites after the solution and aging treatment, suggesting that Orowan looping around precipitates, grain boundary strengthening, and solid solution strengthening are responsible for the high yield strength of L-DED M789. Besides, the as-fabricated alloy shows higher pitting potential than solutioned and aged sample. This work serves as a guidance for the fabrication of corrosion-resistant maraging steels by L-DED and accelerate the implementation of maraging steels for marine and offshore applications.

An influence of nickel with heat treatment on the microstructure and fracture toughness of austempered ductile iron

ABSTRACT In this study, unalloyed and nickel alloyed permanent moulded Austempered Ductile Iron (PMADI) samples are utilised to evaluate and compare properties such as tensile hardness and fracture toughness. All PMADI samples were austempered at 350°C for 150 minutes to improve mechanical properties. When compared to unalloyed PMADI samples, PMADI samples alloyed with 1.5% nickel showed better hardness, tensile, and fracture toughness characteristics. In addition to the microstructure examination on samples to determine the presence of feathery dispersion of Bainite, an X-Ray Diffraction Analysis was done to confirm the presence of austenite.

Analysis of the influence of hot rolled plate steel treatment using temper and quench-temper method on vickers hardness number enhancement

This paper wants to know the effect of bending radius on the distribution of hardness, grain distribution and microstructure on the surface area of tensile stress and compressive stress after bending, quenching and tempering. Material testing helps determine and analyze material quality. The research was conducted on the bending of Hot Rolled Plate Steel material with a radius of 50 mm, 55 mm, 60 mm, 65 mm and 70 mm with a measurement distance of 1 mm, 2 mm and 3 mm, the highest value was obtained at a radius of 55 mm with a measurement distance of 1 mm. After getting the quench-temper treatment with a holding time of 30 minutes, the value of 498 HV was obtained at a radius of 70 mm with a measurement distance of 2 mm. Hardness test was performed using the austenite temperature of 900 °С, microstructure test results obtained finer grains in the compression area r=2.173 µm and in the tensile area r=2.34 µm. This observation aims to determine the microstructure of the material undergoing a heat treatment process at a temperature of 900 °С with a holding time of 30 minutes using water cooling media. The results of the observation of the microstructure of the test specimens before the quench-temper process showed that the structure of ferrite was more abundant than perlite, but after the quench-tempering process the results showed that there was more perlite than ferrite due to the presence of austenite. The treatment on the transformation of the Ar3 line causes the hardness to change the shape of the martensite microstructure into steel while the thickness of the carburizing layer increases with the increase in the carbonization temperature on the surface of the quenched specimen, resulting in the formation of martensite and residual austenite causing the coating to become hard.

Effect of heat treatment on the chromium‐depleted zones of a high carbon martensitic stainless steel

AbstractHeat treatments are commonly applied in the martensitic stainless steels to achieve high wear resistance and hardness; however, they can lead to a decrease in corrosion resistance due to the precipitation of chromium‐rich carbides. The present work investigates the effect of austenitizing and tempering parameters on the microstructure, hardness, and passivation and reactivation currents of AISI 440C. A double‐loop electrochemical potentiodynamic reactivation test is performed to evaluate the dissolution and precipitation of chromium‐rich carbides. The volume fraction of phases after heat treatment is estimated by X‐ray diffraction. Results show that an increase in the austenitizing temperature leads to an increase in the carbide dissolution, and also in the amount of retained austenite. As a consequence, low passivation and reactivation currents are obtained. When austenite is the main phase, the increase of austenitizing time contributed to an increase in the degree of sensitization due to austenite grain boundary carbon enrichment. However, the presence of austenite after tempering kept part of the carbon in a solid solution resulting in low passivation and reactivation currents.

Occurrence of liquid-metal-embrittlement in a fully ferritic microstructure

Abstracts Despite numerous studies making an effort to attain a thorough understanding of the liquid-metal-embrittlement (LME) phenomenon, the metallurgical facet of this catastrophic event remains unclear in iron/zinc (Fe/Zn) systems. While it has been frequently reported that the presence of austenite is an essential prerequisite for LME formation, the present study showed that fully ferritic structure is prone to LME phenomenon and has a high susceptibility to LME-cracking which makes it a novel observation adding to a pool of knowledge regarding LME occurrence. The elemental distribution analysis near the LME crack-tip indicated that liquid Zn was not present which confirmed solid-state grain boundary diffusion was a plausible description of LME-cracking. The occurrence of grain dropout as well as a Zn-containing crack in grain boundary without any branches with other cracks showed that grain boundary sensitization has assisted LME-cracking.

Macrosegregation Characteristics of Ferrite and Austenite Stabilizer Elements in Large Size High Strength Steel Ingot

In the present work, the segregation degrees of ferrite and austenite stabilizer alloying elements were analyzed for a high strength steel. For this, samples were taken from the surface and center of the hot-top and the upper section of a 40 MT ingot. The results showed that the positive segregation ratios for all the investigated elements were higher in the ingot center than in the surface with higher values for austenite stabilizer elements. The increase of austenite alloying element stabilizers was accompanied by the change in the primary solidification mode of the austenite phase. The obtained results are in good agreement with the observed presence of austenite, revealed by X-ray diffraction analysis, stabilized by the austenite alloying elements.

Grain boundary engineering to overcome temper embrittlement in martensitic steel

Grain boundary engineering (GBE) approach has been introduced in martensitic steel through quenching and partitioning (Q&P) heat treatment to overcome temper embrittlement. This heat treatment increases fraction of low energy coincidence site lattice (CSL) boundary from 0.12 in Q&P specimen to 0.22 in Q&P-tempered specimen through transformation of retained austenite to martensite. Presence of austenite in martensitic matrix produces different interfaces. Low energy austenite / austenite and austenite / martensite interfaces reduces segregation of tramp elements during tempering leading to increased toughness compared to normalized and tempered specimen having high energy martensite / martensite interface. Increased toughness is accompanied by a change in fracture morphology from intergranular fracture to typical dimple fracture.

High-temperature flow behaviour of grain-oriented and non-grain-oriented electrical steel

ABSTRACTPlane strain compression tests were performed on grain-oriented (GO) and non-grain-oriented steel specimens of identical silicon content. Though Arrhenius type constitutive equations showed excellent predictability of the experimental flow stresses, there were clear differences in the flow curves of the respective grades. More specifically, GO steel had higher flow stress and work hardening and showed clear presence of dynamically recrystallized grains. All these were attributed to the higher (0.06 versus 0.003 wt-%) carbon content and corresponding presence of austenite, at elevated temperatures, in the GO steel specimens.

Microstructural evolution of white and brown etching layers in pearlitic rail steels

The formation of White (WEL) and Brown Etching Layers (BEL) on rail raceways during service causes the initiation of microcracks which finally leads to failure. Detailed characterization of the WEL and the BEL in a pearlitic rail steel is carried out from micrometer to atomic scale to understand their microstructural evolution. A microstructural gradient is observed along the rail depth including martensite, austenite and partially dissolved parent cementite in the WEL and tempered martensite, ultrafine/nanocrystalline martensite/austenite, carbon saturated ferrite and partially dissolved parent cementite in the BEL. Plastic deformation in combination with a temperature rise during wheel-rail contact was found to be responsible for the initial formation and further microstructural evolution of these layers. The presence of austenite in the WEL/BEL proves experimentally that temperatures rise into the austenite range during wheel-rail contact. This is in agreement with finite element modelling results. Each wheel-rail contact must be considered as an individual short but intense deformation and heat treatment cycle that cumulatively forms the final microstructure, as shown by diffusion length calculations of C and Mn. The presence of secondary carbides in the BEL indicates that the temperature in the BEL during individual loading cycles reaches levels where martensite tempering occurs. Partially fragmented primary cementite laths, enriched in Mn, depleted in Si, and surrounded by a C-gradient and dislocations were found in the BEL. The initial step in the formation of BEL and WEL is the defect- and diffusion-assisted decomposition of the original microstructure.

TiN formation in steels depending of titanium and nitrogen

In today’s steel automotive industry, press hardened steels meet requirements of a lower CO2 consumption and of a better safety of drivers. These steels contain titanium nitrides which can downgrade the in-use properties. From this laboratory study, with a wide range of N (20–120 ppm) combined with high Ti variations (20–110.10−3 wt.%), clear conclusions can be highlighted. Titanium content is the key parameter to drive inclusions content, more than N, as there is a high segregation of Ti in the last liquid, due to a low partition coefficient combined with a low back-diffusion coefficient. Due to this supersaturation in Ti, TiC can be formed. It does not mean that N has no effect. It has to be noticed that the density of inclusions coarser than two microns is highly dependent of the product [Ti] × [N]. Carbon is a very important factor too to take into account as the surpersaturation in Ti in presence of austenite is higher than in presence of delta-ferrite. This gives a few guidelines in the design of inclusions controlled new grades.

Effect of Aging Heat Treatment H950 and H1000 on Mechanical and Pitting Corrosion Properties of UNS S46500 Stainless Steel

The effect of aging temperature on mechanical and pitting corrosion properties of UNS S46500 was investigated. Tensile and Hardness tests were carried out and the microstructure was analyzed by optical microscopy, scanning electron microscopy and X-ray diffraction; Thermo-Calc simulations helped in the phase identification. Pitting corrosion properties were investigated in 0.6M NaCl electrolyte with sulfate additions by Potentiodynamic Polarization (PP). Hardness, tensile and yield strength of the UNS S46500 steel after lower aging temperature, 510oC (H950), are higher than the ones found in the 538oC (H1000) aged steel.This result is explained by microstructure, X-ray diffraction and Thermo-Calc analysis, which indicated the presence of austenite, chi phase and probably Ni3Ti precipitates finely and uniform distributed throughout the martensite matrix. Pitting corrosion resistance is equivalent in both aging temperatures. The sulfate inhibitor effect on UNS S46500 steel was enhanced for 538oC condition when the electrolyte reaches 1Cl-:1SO4 2- ratio, which is explained by Ni sulfate adsorption and the amount of interfaces in the microstructure resulting in smaller amount of adsorption sites, such as coarsened Ni3Ti precipitates, smaller fraction of chi phase and recovery of dislocations in martensite structure.

The Effect of Low Temperature Aging and the Evolution of R-Phase in Ni-Rich NiTi

This study investigates the thermal and mechanical properties that arise from aging Ni-rich Ni–Ti (Nitinol) at temperatures below 250 °C, well below those commonly used to fabricate medical devices. We demonstrate that the Ni50.8–Ti49.2 composition decomposes at temperatures as low as room temperature and discuss the unusual changes in thermal and mechanical behaviors compared to common aging treatments, such as separation of Martensite and R-phase transformations. Using such aging treatments, superelasticity can be achieved without the presence of austenite at body temperature (Af > 45 °C, well above the body temperature). Furthermore, the influence of R-phase on mechanical response and the disparity between thermal and mechanical behavior is discussed in detail.

On the hardness of high carbon ferrous martensite

Due to the presence of retained austenite in martensitic steels, especially steels with high carbon concentrations, it is difficult to estimate the hardness of martensite independent of the hardness of the coexisting austenite. In the present work, the hardness of ferrous martensite with carbon concentrations in the range 0.23-1.46 mass-% was estimated by the regression analysis of hardnesses for hardened martensitic-austenitic steels containing various martensite fractions. For a given carbon concentration, the hardness of martensitic-austenitic steels was found to increase exponentially with an increase in the fraction of the martensitic constituent. The hardness of the martensitic constituent was subsequently estimated by the exponential extrapolation of the hardness of phase mixtures to 100 vol.% martensite. For martensite containing 1.46 mass-% carbon, the hardness was estimated to be 1791 HV. This estimate of martensite hardness is significantly higher than the experimental hardness of 822 HV for a phase mixture of 68 vol.% martensite and 32 vol.% austenite. The hardness obtained by exponential extrapolation is also much higher than the hardness of 1104 HV based on the rule of mixtures. The underestimated hardness of high carbon martensite in the presence of austenite is due to the non-linear dependence of hardness on the martensite fraction. The latter is also a common observation in composite materials with a soft matrix and hard reinforcing particles.

Brittle fracture analysis of Dissimilar Metal Welds between low-alloy steel and stainless steel at low temperatures

Dissimilar Metal Welds (DMW) between low-alloy steel (A533 steel) and austenitic 316L stainless steel are widely used within the French nuclear power plants where they connect the main components to the primary circuit pipes. In these DMW, the welding process and the post-weld heat-treatment generate a hard thin layer of carburized martensite and austenite in the vicinity of the fusion line (FL), resulting in a heterogeneous microstructure with high mechanical properties gradient, hence potentially, a higher sensitivity to brittle fracture in the hard layer. The present study aims to evaluate the brittle behavior of the DMW at low temperatures and more particularly of the interface between the carburized martensite and austenite (MA interface). Fracture toughness tests were carried out at temperatures between - 120°C and -50°C, with a precrack tip located on the ferritic side near the FL (i.e., between the ferritic steel and the stainless steel buttering). The analysis of the fracture behavior of the DMW was based on a SEM examination of CT (Compact Tension) specimens fracture surfaces. Coupled with a 2D plane strain numerical simulation of the tests, the results show that the presence of austenite in the fatigue precrack front and subsequent ductile tearing towards FL produced higher solicitations on the hard layer and caused intergranular fracture on the MA interface, resulting in lower toughness values for the specimen. To model the brittle behavior of the MA interface, a stress based criterion was used. Tensile tests on axisymmetric specimen, which were machined to initiate intergranular fracture in the MA interface and tested at -170°C, were used to define a threshold stress (σth), below which brittle fracture cannot occur. Then, a criterion to evaluate the brittle fracture risk of the DMW was proposed and applied to the CT specimen.

Effect of root pass filler metal on microstructure and mechanical properties in the multi-pass welding of duplex stainless steels

This paper is focused on the estimation of the effect of root pass chemical composition, in multi-pass GTA Weldments, on microstructure and mechanical properties of duplex stainless steel welds. We used two different filler metals, the super duplex ER 2594 and duplex ER 2209. Microstructures of different passes of welded joints are investigated using optical microscope and scanning electron microscope. The relationship between mechanical properties, corrosion resistance, and microstructure of welded joints is evaluated. It is found that the tensile and toughness properties of the first weldment, employing the combination of ER 2594 in the root pass and ER 2209 in the remaining, are better than that of the second weldment employing ER 2209 all passes, due to the root pass grains refinement and its alloy elements content as chromium Cr and nitrogen N. The microstructure indicates the presence of austenite in different forms on the weld zone of ER 2209, same in the case of ER 2594, but with higher content and finer grains size, in particular Widmanstatten austenite WA. Potentiodynamic polarization tests of the first weld metal evaluated in 3.5% NaCl solution at room temperature have been demonstrated a corrosion resistance higher than that of the second weld metal. This work addressed the improvement of the corrosion resistance using appropriate filler metal without getting any structural heterogeneity and detrimental changes in the mechanical properties.

Heat input effect on the microstructural transformation and mechanical properties in GTAW welds of a 409L ferritic stainless steel

Welds without filler metal and welds using a conventional austenitic stainless steel filler metal (ER308L) were performed to join a ferritic stainless steel with Gas Tungsten Arc Welding process (GTAW). Welding parameters were adjusted to obtain three different heat input values. Microstructure reveals the presence of coarse ferritic matrix and martensite laths in the Heat Affected Zone (HAZ). Dilution between filler and base metal was correlated with the presence of austenite, martensite and ferrite in the weld metal. Weld thermal cycles were measured to correlate the microstructural transformation in the HAZ. Microhardness measurements (maps and profiles) allow to identify the different zones of the welded joints (weld metal, HAZ, and base metal). Comparing the base metal with the weld metal and the HAZ, a hardness increment (~172 HV0.5 to ~350 HV0.5 and ~310 HV0.5, respectively) was observed, which has been attributed to the martensite formation. Tensile strength of the welded joints without filler metal increased moderately with respect to base metal. In contrast, ductility was approximately 25% higher than base metal, which provided a toughness improvement of the welded joints.