Austenite Boundaries Research Articles

Shielded metal arc welding of UNS S32750 steel: microstructure, mechanical properties and corrosion behaviour

In the present investigation, ER2594 (conventional) and ER2595 (consisting of additional Cu and W) electrodes were used to produce butt joints of 5.5 mm thick super duplex stainless steel (UNS S32750) sheets using shielded metal arc welding (SMAW) process. During SMAW process, the heat input was 0.81 kJ mm−1 for ER2594 electrode and 0.75 kJ mm−1 for ER2595 electrode. Optical microscope and scanning electron microscope were used to observe the microstructures of different zones, namely, fusion zone (FZ), heat affected zone (HAZ) and base metal (BM). Tensile test, impact test and micro-hardness measurements were carried out for both the weldments on samples cut across the weld. The microstructures of the FZ consisted of inter-granular austenite (IGA), grain boundary austenite (GBA) and Widmanstatten austenite (WA). There was minor change in the width of the HAZ of the weldments with the change in electrode. This is expected to be due to the minor change in the heat input during SMAW process. The weld zone (cap pass) of both the weldments predominantly showed WA and mode of solidification was found to be ferritic (F mode). The tensile strength of both the weldments was approximately same as that of base metal and both the samples fractured through the BM. Cr, Fe and Mo rich precipitates were found in the FZ (weld region), which are expected to be formed due to pick of gaseous content from the atmosphere during welding. The FZ of the weldments produced by ER2595 electrode showed better pitting corrosion resistance than those produced by ER2594 electrode.

Analysis of structure property relationship of super duplex stainless steel AISI 2507 weldments for severe corrosive environments

The present study centered on the effect of welding induced metallurgical changes on the mechanical properties and corrosion behavior of super duplex stainless steel (SDSS) weldments. The weld joints dealt with in the study has been fabricated through use of filler metal ER 2594 using gas tungsten arc welding process. Microstructural investigation in the weldment exhibited the evolution of austenite (gamma) phases in different morphologies such as grain boundary austenite, needle like elongated austenite (Widmanstätten structure) and intragranular austenite particles. Mechanical properties of the weldments were obtained and compared with the parent metal behavior by conducting Vicker’s micro hardness assessment, Charpy impact toughness investigation, tensile and three-point bend experiment. Cyclic polarization curves revealed higher corrosion resistance for the weldment than the parent metal in NaCl corrosion medium. Nickel enriched filler ER 2594 led to the nucleation of additional gamma phases in the fusion zone which are electrochemically more resistant than the delta ferrite phases. Moreover, accelerated stress corrosion cracking (SCC) test proved the SCC resistance of both parent metal and weld regions by protecting the surface without nucleation of any kind of fissures or cracks.

Reformation Behavior of Austenite in 2205 Duplex Stainless Steel with Rapid Heat Treatment

The reformation behavior of austenite in coarsened ferrite grains of 2205 duplex stainless steel is studied by means of rapid heat treatment process. With the prolongation of peak temperature holding time, ferrite grains coarsen significantly from 200 µm at 1 s to over 1000 µm at 10 min, and the reformation of grain boundary austenite is suppressed while fine equiaxed intragranular austenite is promoted during the subsequent isothermal treatment at 850 °C owing to the reduction of total ferrite grain boundary area. With the decreasing of isothermal temperature, the microstructure evolution of reformed austenite from widmanstatten austenite to acicular austenite and finally to equiaxed austenite is observed, due to the increasing of the nucleation site density. The orientation relationships (OR) between reformed austenite and ferrite vary at different isothermal temperatures. The N‐W OR gradually changes into the K‐S OR with the increasing of isothermal temperature to lower the coherent elastic strain energy induced in ferrite matrix.

Correlation between crystallographic and microstructural features and low hysteresis behavior in Ni50.0Mn35.25In14.75 melt-spun ribbons

In this work, crystallographic, microstructural and magnetocaloric investigations were performed on textured Ni50Mn35.25In14.75 melt-spun ribbons with low thermal (6 K) and magnetic-field induced hysteresis (−0.73 J kg−1 at 2 T) and moderate maximum magnetic entropy change ΔSMmax (11 J kg−1 K−1 at 5 T) at room temperature (302 K). The austenite in the ribbons crystallizes into a L21 structure, whereas martensite has a monoclinic incommensurate 6 M modulated structure as determined with the superspace theory. By means of electron backscatter diffraction technique, the morphological and crystallographic features of microstructure were systematically characterized. Ribbons possess a fine microstructure with an average grain size (initial austenite phase) of around 10 μm, whereas the 6 M martensite has a self-accommodated microstructure with 4 kinds of twin-related martensite variants. During inverse martensitic transformation, the austenite prefers to nucleate at the grain boundaries of initial austenite. By means of cofactor conditions and crystallographic orientation analyses, the good geometrical compatibility between austenite and martensite was confirmed. Based on the crystal structure and microstructure information obtained, the reason of the low thermal hysteresis was discussed.

Effect of Welding Process on Microstructure, Mechanical and Pitting Corrosion Behaviour of 2205 Duplex Stainless Steel Welds

An attempt has been made to weld 2205 Duplex stainless steel of 6mm thick plate using conventional gas tungsten arc welding (GTAW) and activated gas tungsten arc welding (A- GTAW) process using silica powder as activated flux. Present work is aimed at studying the effect of welding process on depth of penetration, width of weld zone of 2205 duplex stainless steel. It also aims to observe the microstructural changes and its effect on mechanical properties and pitting corrosion resistance of 2205 duplex stainless steel welds. Metallography is done to observe the microstructural changes of the welds using image analyzer attached to the optical microscopy. Hardness studies, tensile and ductility bend tests were evaluated for mechanical properties. Potentio-dynamic polarization studies were carried out using a basic GillAC electro-chemical system in 3.5% NaCl solution to observe the pitting corrosion behaviour. Results of the present investigation established that increased depth of penetration and reduction of weld width in a single pass by activated GTAW with the application of SiO2 flux was observed when compared with conventional GTAW process. It may be attributed to the arc constriction effect. Microstructure of the weld zones for both the welds is observed to be having combination of austenite and delta ferrite. Grain boundary austenite (GBA) with Widmanstatten-type austenite (WA) of plate-like feature was nucleated from the grain boundaries in the weld zone of A-GTAW process. Mechanical properties are relatively low in activated GTAW process and are attributed to changes in microstructural morphology of austenite. Improved pitting corrosion resistance was observed for the welds made with A-GTAW process.

Hydrogen Embrittlement Susceptibility and Safety Control of Reheated CGHAZ in X80 Welded Pipeline

Coarse-grained heat-affected zone (CGHAZ) exhibits the highest hydrogen embrittlement (HE) susceptibility, which changes under the influence of thermal cycle. In this study, slow strain rate tension (SSRT) tests were conducted to investigate the HE susceptibility of reheated CGHAZs and the critical hydrogen pressure for fracture failure. Results show that intercritically reheated CGHAZ (ICCGHAZ) possesses the lowest HE resistance. Analyses of HE index and fracture indicate that the critical hydrogen pressure is 3.5 MPa. Microstructure analysis reveals that HE susceptibility is associated with multiple factors, such as phase composition, grain coarsening, HAB density, and MA constituent. Blocky necklace-like MA constituent along prior austenite boundaries plays a predominant role in intensifying the HE susceptibility of ICCGHAZ.

Effect of Heat Input on Microstructure and Corrosion Behavior of Duplex Stainless Steel Shielded Metal Arc Welds

In the present work, UNS S32750 super duplex stainless steel sheets were welded by shielded metal arc welding process with E2595 electrode using two different heat inputs, 0.54 and 1.10 kJ/mm. Microstructural investigations (optical and scanning electron microscopy) showed very small differences in the heat affected zone for both the heat inputs. The weld metals showed presence of three different morphologies of austenite—Widmanstatten, intra-granular and grain boundary austenite along with ferrite. Ferrite content in the weld region was also nearly same and did not change significantly with the increase in heat input. Both the weldments showed similar mechanical properties (ultimate tensile strength, impact strength and hardness) and failed in a ductile manner. Electrochemical studies in 3.5% NaCl solution showed the degree of sensitization to less than 1% and nearly same pitting potential for both heat inputs. Since the effect of heat input on the weld behavior was negligible, low heat input may be preffered for welding UNS S32750 super duplex stainless steel.

Five-parameter crystallographic characteristics of the interfaces formed during ferrite to austenite transformation in a duplex stainless steel

The crystallography of interfaces in a duplex stainless steel having an equiaxed microstructure produced through the ferrite to austenite diffusive phase transformation has been studied. The five-parameter interface character distribution revealed a high anisotropy in habit planes for the austenite–ferrite and austenite–austenite interfaces for different lattice misorientations. The austenite and ferrite habit planes largely terminated on (1 1 1) and (1 1 0) planes, respectively, for the austenite–ferrite interfaces associated with Kurdjumov–Sachs (K–S) and Nishiyama–Wasserman (N–W) orientation relationships. This was mostly attributed to the crystallographic preference associated with the phase transformation. For the austenite–ferrite interfaces with orientation relationships which are neither K–S nor N–W, both austenite and ferrite habit planes had (1 1 1) orientations. Σ3 twin boundaries comprised the majority of austenite–austenite interfaces, mostly showing a pure twist character and terminating on (1 1 1) planes due to the minimum energy configuration. The second highest populated austenite–austenite boundary was Σ9, which tended to have grain boundary planes in the tilt zone due to the geometrical constraints. Furthermore, the intervariant crystallographic plane distribution associated with the K–S orientation relationship displayed a general tendency for the austenite habit planes to terminate with the (1 1 1) orientation, mainly due to the crystallographic preference associated with the phase transformation.

In-situ observation of microstructural evolution in reheated low carbon bainite weld metals with various Ni contents

Microstructural evolution in weld metals was in-situ observed through utilizing a laser scanning confocal microscope at two cooling rates. The specimens with various nickel contents were adopted for the observation. In the specimen with low fraction of Ni (≤2 wt. %), granular bainite microstructure (i. e. broad surface relief) transformation from intragranular nucleation site was in-situ observed, while, lath bainite microstructure originating from grain boundary of austenite was in-situ observed for specimens with high mass percentage of Ni (≥ 4 wt. %). With increasing nickel content, the transformation temperature dropped. The prior austenite grain size was initially depressed and subsequently coarsened dramatically with the addition of Ni. The microstructure difference was ascribed to various nucleation sites and growth direction in the matrix. On account of those observations, not only the chemical component, cooling rate and microstructure were systematically correlated, but also the microstructural evolution was definite.

Influence of heat input in electron beam process on microstructure and properties of duplex stainless steel welded interface

The influence of heat input in electron beam (EB) process on microstructure, mechanical properties, and pitting corrosion resistance of duplex stainless steel (DSS) welded interface was investigated. The rapid cooling in EB welding resulted in insufficient austenite formation. The austenite mainly consisted of grain boundary austenite and intragranular austenite, and there was abundant Cr2N precipitation in the ferrite. The Ni, Mo, and Si segregation indicated that the dendritic solidification was primarily ferrite in the weld. The weld exhibited higher hardness, lower toughness, and poorer pitting corrosion resistance than the base metal. The impact fractures of the welds were dominated by the transgranular cleavage failure of the ferrite. The ferrite was selectively attacked because of its lower pitting resistance equivalent number than that of austenite. The Cr2N precipitation accelerated the pitting corrosion. In summary, the optimised heat input slightly increased the austenite content, reduced the segregation degree and ferrite texture intensity, decreased the hardness, and improved the toughness and pitting corrosion resistance. However, the effects were limited. Furthermore, optimising the heat input could not suppress the Cr2N precipitation. Taking into full consideration the microstructure and properties, a heat input of 0.46 kJ/mm is recommended for the EB welding of DSS.

Effect of heat input and M-A constituent on microstructure evolution and mechanical properties of heat affected zone in low carbon steel

Microstructure evolution and impact toughness of simulated heat affected zone (HAZ) in low carbon steel have been investigated in this study. Thermal simulator was used to simulate microstructure evolution in HAZ with heat input of 10 – 100 kJ / cm welding thermal cycle. Results indicated that microstructure of HAZ mainly consisted of acicular ferrite (AF) inside grain and high volume fraction of grain boundaries ferrite (GBF) at prior austenite boundaries; the size of GBF and effective grain size increased with increasing heat input. Excellent impact toughness (more than 150 J at −40 °C) was obtained in HAZ with heat input less than 50 kJ / cm. When heat input was 100 kJ / cm, the impact toughness of HAZ decreased to 18 J because of the presence of large M-A constituent with lath-form in HAZ, assisting the micro-crack initiation and decreasing the crack initiation energy seriously. Effect of inclusions on acicular ferrite transformation in HAZ was also discussed.

Effects of austenitic and duplex electrodes on microstructure, mechanical properties, pitting, and galvanic corrosion resistance of ferritic and dual-phase stainless steel dissimilar joints

Abstract

Hot Ductility of 20Cr13 Steel at High Temperature

Hot ductility of 20Cr13 steel at high temperature was investigated through tensile test. The main phases of the steel in temperature range of 600-1400 °C were calculated with thermodynamic software. The fracture morphologies and microstructure were observed by scanning electron microscopy. The steel showed good hot ductility in temperature range of 1000-1200 °C, and the area reduction was 82 % or more. In temperature range of 800-950 °C, the M23C6 precipitated at the grain boundary of austenite, and it fractured in the tensile process. Thus the micro-crack or micro-hole formed at the grain boundary of austenite, and the area reduction was 52-68 %. The ferrite precipitated at 800 or 1250 °C, and the micro-voids formed at interface of ferrite and austenite because of the discordant deformation of two phases, which was harmful to hot ductility. Moreover, quasi-cleavage fracture happened at 800 °C because of the ferrite cleavage, which further decreased hot ductility of the steel.

Features of thermoelastic martensitic transformations, structure and properties in ternary B2-alloys based on NiMn – NiTi, NiMn – NiAl, NiMn – NiGa, Ni2MnGa – Ni3Ga quasi-binary systems

Structural phase transformations and properties of B2-alloys based on NiMn – NiTi, NiMn – NiAl, NiMn – NiGa, Ni2MnGa – Ni3Ga systems were studied. The effect of titanium, aluminum and gallium alloying on the structure, the phase composition, and the properties of quasi-binary alloys was studied over a wide temperature range. The influence of the alloy composition on the type of crystal structure in high-temperature austenite and martensite and the critical martensitic transformation temperatures was analyzed. A general phase diagrams of the structural and magnetic transformations in the alloys were plotted. It is shown, that alloing by third component reduces transformations temperatures. The temperature concentration boundaries of austenite and martensite phases were found. The predominant morphology of martensite is shown to be determined by the hierarchy of the packets of thin coherent lamellae of nano- and submicrocrystalline crystals with planar habit plane boundaries close to {011}B2. Martensite crystals are twinned along one of the 24 {011}<011¯>B2 “soft” twinning shear systems, which provides coherent accommodation of the martensitic transformation–induced elastic stresses. Investigations were carried out by methods of scanning and transmission electron microscopy, diffraction of x-rays and electrons, resistometry and magnetometry.

Studies on the structure–property relationships and corrosion behaviour of the activated flux TIG welding of UNS S32750

In the present study, three different fluxes namely NiO, MoO3 and SiO2 were employed to investigate the weldability of super-duplex stainless steel (UNS S32750). Bead on trial studies were conducted to determine the effect of welding current on the depth of penetration and depth to width (D/W) ratio. The studies demonstrated that complete penetration could be achieved in a single pass with the use of activated flux. Microstructure studies showed the formation of coarser grains of ferrite along with three different types of austenite which are Widmanstätten, intra-granular and grain boundary austenite in the fusion zones of all the weldments. Oxygen content and ferrite count analysis were also carried out at the fusion zones of the weldments. Tensile studies inferred that the tensile failures occurred at the weld zone for all the cases. The joint efficiencies of the activated flux tungsten inert gas (A-TIG) weldments employing NiO, MoO3 and SiO2 flux were reported to be 91.6%, 87.4% and 93.06%, respectively. The impact toughness of the A-TIG weldments was found to be lower than that of the base metal (82J). Potentiodynamic polarization studies were also performed to investigate the corrosion behaviour of the welds in 3.5% NaCl environment.

Morphology, crystallography, and crack paths of tempered lath martensite in a medium-carbon low-alloy steel

The tempered lath martensite and its crack propagation have significant influence on the ductility and toughness of the warm tempformed medium-carbon steel. The martensitic microstructures of these medium-carbon steels are transformed from twinned austenite and the orientation relationship of lath martensite (α′) with prior austenite (γ) is distinctive. In the present paper we investigate the microstructure and fracture mode of a quenched and tempered 0.4%C-2%Si-1%Cr-1%Mo steel using electron backscatter diffraction technique. The results showed that the orientation relationship between γ and α′ is Greninger-Troiano (G-T) relationship. A single γ grain was divided into 4 packets and each packet was subdivided into 3 blocks. The misorientation angles between adjacent blocks were ~54.3° or ~60.0° in a packet. Most γ grains were twins sharing a {111}γ plane. There were 7 packets in a twinned γ grain and the twin boundaries were in a special packet. Besides the common packet, there were three packets in each twin. Being different from the cleavage fracture along the {001} planes in conventional martensitic steels, both cleavage and intergranular cracks were present in our medium-carbon steel. The former was in the larger blocks and it propagated along the {001}, {011}, and {112} planes. The latter propagated along the block, packet, or prior austenite boundaries. The intergranular cracks were generally in the fine block region. These results suggested that the block size is the key factor in controlling the brittle fracture mode of lath martensitic steel.

Mechanism of hot-rolling crack formation in lean duplex stainless steel 2101

The thermoplasticity of duplex stainless steel 2205 (DSS2205) is better than that of lean duplex steel 2101 (LDX2101), which undergoes severe cracking during hot rolling. The microstructure, microhardness, phase ratio, and recrystallization dependence of the deformation compatibility of LDX2101 and DSS2205 were investigated using optical microscopy (OM), electron backscatter diffraction (EBSD), Thermo-Calc software, and transmission electron microscopy (TEM). The results showed that the phase-ratio transformations of LDX2101 and DSS2205 were almost equal under the condition of increasing solution temperature. Thus, the phase transformation was not the main cause for the hot plasticity difference of these two steels. The grain size of LDX2101 was substantially greater than that of DSS2205, and the microhardness difference of LDX2101 was larger than that of DSS2205. This difference hinders the transfer of strain from ferrite to austenite. In the rolling process, the ferrite grains of LDX2101 underwent continuous softening and were substantially refined. However, although little recrystallization occurred at the boundaries of austenite, serious deformation accumulated in the interior of austenite, leading to a substantial increase in hardness. The main cause of crack formation is the microhardness difference between ferrite and austenite.

Solute Segregation During Ferrite Growth: Solute/Interphase and Substitutional/Interstitial Interactions

The segregation of solutes to austenite/ferrite transformation interfaces during decarburization/denitriding of Fe-Mn-C, Fe-Mn-N, and Fe-Si-C ternary alloys was studied by using atom probe tomography. Manganese was found to segregate noticeably to the transformation interface in the presence of carbon, while no segregation could be detected in the presence of nitrogen. This result might indicate that manganese interacts little with the interface itself and that its interaction with the interstitial controls its segregation behavior. In the case of Fe-Si-C, the experiments were complicated by interface motion during quenching. Preliminary results suggest that silicon was depleted at the interface in contrast to the commonly observed segregation behavior of silicon at grain boundaries of ferrite and austenite. This observation could be explained by taking into account the repulsive interaction between silicon and carbon along with the intense segregation of carbon to the interface. This would lead to a net repulsive interaction of silicon with the interface even when considering the intrinsic tendency of silicon to segregate to the boundary in the absence of carbon. The results presented here emphasize the need to account for the interaction of all solutes present at the interface in ferrite growth models.

Microstructure and Tribological Properties of a Material with Cementite Eutectic Applied for Metallurgical Rolls

Three prototype metallurgical rolls were produced on the basis of G200CrNiMo4-3-3 material. The method applied for the microstructure forming was different for each roll: the roll marked WOT – as cast state (without a modification and heat treatment); the metallurgical roll marked WMT – during its casting the FeCaSi deoxidizing was applied and then modification by a complex inoculant and argoning; the metallurgical roll marked WNT – subjected to a heat treatment (incomplete normalizing).The mentioned above differences in the technology of making rolls caused changes in their microstructure.The cementite eutectic and pearlitic matrix occurred in each roll. The main differences in the microstructure of cast steel rolls concerned a morphology of precipitates of hypereutectoid cementite. In the WOT roll cementite was mainly in the Widmannstӓtten system. Precipitates of hypereutectoid cementite in the WMT roll occurred along grain boundaries of primary austenite. A large fraction of spheroidal hypereutectoid cementite, precipitated in the whole volume of the primary austenite grain, appeared in the WNT roll. The microstructure influenced the rolls hardness and was equal 260 ÷ 350 HBW.Tribological investigations indicated decreasing the abrasive wear resistance with increasing the hypereutectoid cementite fraction within the primary austenite grains.

Microstructural Characteristics of Plasma Nitrided Layer on Hot-Rolled 304 Stainless Steel with a Small Amount of α-Ferrite

The hot-rolled 304 stainless steel with γ-austenite and approximately 5 pct α-ferrite elongated along the rolling direction was plasma-nitrided at a low temperature of 693 K (420 °C). X-ray diffraction results revealed that the nitrided layer was mainly composed of the supersaturated solid solution of nitrogen in austenite (γN). Transmission electron microscopy (TEM) observations showed that the microstructure of the γN phase exhibited “fracture factor contrast” reflective of the occurrence of fine pre-precipitations in γN by the continuous precipitation. The occurrence of a diffuse scattering effect on the electron diffraction spots of γN indicated that the pre-precipitation took place in γN in the form of strongly bonded Cr-N clusters or pairs due to a strong attractive interaction of nitrogen with chromium. Scanning electron microscopy and TEM observations indicated that the discontinuous precipitation initiated from the γ/α interfaces and grew from the austenite boundaries into austenite grains to form a lamellar structure consisting of CrN and ferrite. The orientation relationship between CrN and ferrite corresponded to a Baker-Nutting relationship: (100)CrN//(100)α; [011]CrN//[001]α. A zigzag boundary line following the banded structure of alternating γ-austenite and elongated α-ferrite was presented between the nitrided layer and the substrate to form a continuous varying layer thickness, which resulted from the difference in diffusivities of nitrogen in α-ferrite and γ-austenite, along the γ/α interfaces and through the lattice. Microstructural features similar to the γN were also revealed in the ferrite of the nitrided layer by TEM. It was not excluded that a supersaturated solid solution of nitrogen in ferrite (αN) formed in the nitrided layer.