Crystallography and structural evolution during reverse transformation in an Fe–17Cr–0.5C tempered martensite

The mechanism and the crystallography of austenite and δ-ferrite formation from tempered martensite at temperatures of 900–1200°C have been studied by means of transmission electron microscopy in an Fe–17Cr–0.55C alloy. It was found that austenite nucleates within ferrite at low angle, high angle and twin-related lath boundaries as well as at high angle equiaxed grain boundaries in contact with M₂₃C₆ grain/lath boundary carbides. The austenite grains are in a cube–cube relationship with the M₂₃C₆ carbide particles and bear the Kurdjumov–Sachs orientation relationship with at least one of the adjacent ferrite grains. They are often in the Kurdjumov–Sachs relationship with both ferrite laths separated by a high angle boundary as far as the laths had formed from the same austenite. The {111}_A close packed plane of γ precipitate is parallel to the {110}_F plane most parallel to the grain boundary. The close packed planes of some austenite grains nucleating at the high angle lath boundaries are parallel to the close packed planes of both ferrite laths. These crystallographic features often result in a single variant of austenite orientation at a grain boundary. After nucleation, the austenite grains grow by the migration of both semicoherent and incoherent interfaces. These results demonstrate that a specific orientation relationship is preferred for the austenite nucleation, but is not necessary for the subsequent growth. The kinetics of austenite growth are controlled by chromium diffusion. The δ-ferrite particles precipitate at high temperatures as a non-equilibrium phase. No rational orientation relationship between δ-ferrite and retained austenite was found. The experimental results are discussed qualitatively in terms of the thermodynamic predictions using the software ThermoCalc, assuming local equilibrium at the moving interfaces.     

Crystallography and interface boundary structure of pearlite with M7C3 carbide lamellae

The crystallography of pearlite with M₇C₃ carbide lamellae and the atomic structure at the ferrite/carbide interface have been examined by means of transmission electron microscopy in an Fe–8.2Cr–0.96C alloy. Two orientation relationships with the corresponding habit planes were determined: OR-I and OR-II. Variants of these orientation relationships have been frequently observed. Microscopic steps at the ferrite/carbide interfaces accommodate the curvature of the habit planes. Each of these orientation relationships provides a small misorientation between the {110}_b close packed planes of ferrite and the coincident planes of M₇C₃ carbide, and the atomic planes are perfectly matched through the interface. The orientation relationship between the parent austenite and M₇C₃ carbide was also deduced assuming the ferrite/austenite orientation relationship so far obtained.     

Weldability of X100 linepipe

Abstract

Research has been carried out to identify weld metal compositions and microstructures capable of meeting high strength and toughness requirements for X100 seam welded linepipe. Single pass, multiwire submerged arc welds were made in experimental, high strength low alloy steel plates using consumables to give a wide range of weld metal alloying. Work has shown that the optimum strength and toughness are obtained in Mo–B–Ti alloyed weld metals with P _cm values between 0.218 and 0.250. Weld metal microstructures were almost fully acicular ferrite with an ultrafine grain size (1–2 µm). Dilatometric studies demonstrated that at typical weld cooling rates the optimised welds transformed at significantly lower temperatures than those reported for X65 plate deposits, which contain acicular ferrite in the form of idiomorphic primary ferrite and intragranular Widmanstätten ferrite. The maximum rate of transformation in the optimised welds occurred between 515 and 570°C, which indicates that the acicular ferrite in this case consisted of intragranular Widmanstätten ferrite and/or bainite. The ferrite would appear to have a fine plate morphology growing from large as well as small inclusions, but not very far before the onset of hard impingement, thereby ensuring an ultrafine grain size. Tensile strengths of 708–784 MPa were achieved with an 80 J Charpy impact transition temperature toughness between -68 and -115°C. More highly alloyed weld metals containing 2–3%Mn and 1.5%Si transformed at lower temperatures and showed increased strength, but there was a substantial loss of toughness, attributed to the relatively unimpeded growth of large ferrite plates from larger inclusions, and the replacement of ultrafine acicular ferrite between these plates by blocks of martensite–austenite. One pass per side, multiwire submerged arc welds manufactured to the optimum weld metal chemistry confirmed their applicability for thin section X100 linepipe.     

Fiber laser welding of WC-Co to carbon steel using Fe-Ni Invar as interlayer

A fiber laser procedure was developed for joining cemented carbides to carbon steels (3-mm-thick plates) in the butt joint configuration using fcc Fe-Ni invar as interlayer. The optimized welding parameters gave a required weld width and penetration. The as-welded microstructure was characterized as a function of welding speed through fiber laser welding. In fusion zone, non-equilibrium fcc (γFe, Ni) was the predominant phase with α-WC and some mixed carbides such asFe₃W₃C and Co₆W₆C. In heat affected zone (HAZ) of cemented carbide, the phases consisted of α-WC, (γFe, Ni) and mixed carbides. Bct–martensite was observed in HAZ near steel side. Which were confirmed by XRD, TEM, and nickel-iron phase diagram calculate using Thermo Cal Classic software. Thick and large sized η-phases were inhibited in the HAZ of cemented carbide and replaced by η-phases with carbon depletion. A higher energy input produced more non-equilibrium (γFe, Ni) and martensite. Small amounts of η-phases, non-equilibrium (γFe, Ni) besides the martensite. The thermal cycles associated with laser welding led to a rather small fusion zone, which usually inhibited η-phases formation due to the inhibition of normal grain growth and the decrease of iron/nickel diffusion across WC/Co interface. The results indicated that the maximum bend strength was 980.90MPa. Due to the formation of martensite/carbides and an excess of which led to embrittlement, welded-joint samples were broken at the HAZ or in the fusion zone instead of the base metal. The cracks exhibited the characteristics of hot crack ((γFe, Ni) in the fusion zone) and cold crack (α-WC at the HAZ of cemented carbide).     

Decomposition of martensite by discontinuous-like precipitation reaction in an Fe–17Cr–0.5C alloy

The mechanism of martensite decomposition and the kinetics of carbide precipitation have been studied in an Fe–17 wt% Cr–0.55 wt% C alloy. The morphology of carbide precipitates formed within the decomposed regions and the crystallography of their formation were examined by means of transmission electron microscopy after tempering at 735°C for various times. The martensite decomposition starts within less than 10 s, but it is not completed even after 10 min. The reaction initiates with the nucleation of fine cementite particles preferentially at the prior austenite grain boundaries and occasionally at the martensite lath boundaries. Cementite particles are related to the ferritic matrix with the Bagaryatsky orientation relationship. The decomposition of martensite proceeds heterogeneously by the migration of a reaction front. Various carbide morphologies were observed in the region close to the reaction front: rod-like, spherical or lamellae. The kinetics of martensite decomposition changes from carbon diffusion controlled to chromium diffusion controlled. After long time tempering, the alloy carbides, M₂₃C₆ and M₇C₃, precipitate at the reaction front. The M₂₃C₆ carbides are related with respect to the ferrite by the Kurdjumov–Sachs orientation relationship. Two specific orientation relationships were found between the M₇C₃ carbide and the ferrite, which are related to each other by a rotation of 30° about their common axis of [0001]_h//[110]_α. One of them has previously been reported. The specific features of discontinuous-like precipitation in martensite are discussed and are attributed to the presence of carbon and chromium atoms, which have different mobilities. The driving forces for diffusion of carbon and chromium were qualitatively determined with the software and database ThermoCalc by assuming local equilibrium at the moving interfaces.     

Solidification and microstructural characterisation of iron-chromium based hardfaced coatings deposited by SMAW and electric arc spraying

The microstructure and solidification of shielded metal arc welding (SMAW) hardfaced Fe-Cr-C deposits used in the sugar industry as well as electric arc-sprayed Fe-Cr-B coating have been determined using a combination of optical microscopy, image analysis, SEM and XRD. The aim of this study was to examine the morphology, microstructure and chemical composition of the coating. The weld microstructures consisted primarily of (Fe,Cr,Mn)₇C₃ carbides, austenite (γ) and ferrite (α), while the arc-sprayed coating was composed of two metallic phases, α (Fe,Cr) and Fe_1.1Cr_0.9B_0.9, which were intermingled with oxides of iron and chromium. The highest average hardness (850 kgf/mm²) occurred in weld coating A80, compared to the 730 kgf/mm² measured in the arc-sprayed coating. The results of the study also showed that different welding electrodes as well as weld procedure variation produced significant differences in the morphology of the carbides, structure of the deposit and microhardness. Although the microhardness of the welded deposits was higher than the arc-sprayed coating, the arc-sprayed coating exhibited a more consistent hardness value. Porosity and oxide inclusions were more evident in the arc-sprayed coating: 1% and 3% in the weld coatings S80 and A80, respectively, and 6.5% in the arc-sprayed coating. The implications of the result with respect to solidification and microstructure are discussed.     

Effects of high temperature and cryogenic treatment on the microstructure and abrasion resistance of a high chromium cast iron

Effects of deep cryogenic treatment on the microstructure, hardening and abrasion resistance behaviors of 16Cr1Mo1Cu cast iron subjected to destabilization treatment were investigated. The results show that the cryogenic treatment can effectively reduce the retained austenite after destabilization heat treatment, but cannot make retained austenite transform completely. Cryogenic treatment can markedly improve bulk hardness and abrasion resistance of the high chromium cast iron. In the course of destabilization treatment and then cryogenic treatment, the amount of precipitated secondary carbide, M₂₃C₆, was more than that in air cooling. The additional fine secondary carbide precipitated during the cryogenics treat after destabilization heat treatment comparing with air cooling, is the main reason for the increase of the bulk hardness and wear resistance.     

Microstructural investigations in binderless tungsten carbide with grain growth inhibitors

The microstructure of binderless tungsten carbide, with small additions of Cr₃C₂ or VC, was investigated using transmission electron microscopy associated with X-ray energy dispersive spectrometry. The distribution of Cr and V elements was determined. In the material sintered with Cr₃C₂, numerous (Cr,W)₂C grains were found, some of them displaying an epitaxy orientation relationship with the basal facet of adjacent WC grains, and Cr segregation was observed in all examined grain boundaries. In the carbide with VC additive, small V-rich carbides were found at triple junctions of WC grains. Unlike Cr, no V segregation was detected in grain boundaries. The grain growth inhibiting effect of Cr₃C₂ and VC is very likely different. For Cr₃C₂, it is supposed that both (Cr,W)₂C grains and Cr segregation reduce the mobility of grain boundaries. For VC, probably the grain boundary triple junctions are pinned by small V-rich carbide grains.     

Investigation on Microstructure and Impact Toughness of Different Zones in Duplex Stainless Steel Welding Joint

This paper investigated on microstructure and impact toughness of different zones in duplex stainless steel welding joint. High-temperature heat-affected zone (HTHAZ) contained coarse ferrite grains and secondary precipitates such as secondary austenite, Cr₂N, and sigma. Intergranular secondary austenite was prone to precipitation in low-temperature heat-affected zone (LTHAZ). Both in weld metal (WM) and in HTHAZ, the austenite consisted of different primary and secondary austenite. The ferrite grains in base metal (BM) presented typical rolling texture, while the austenite grains showed random orientation. Both in the HTHAZ and in the LTHAZ, the ferrite grains maintained same texture as the ferrite in the BM. The secondary austenite had higher Ni but lower Cr and Mo than the primary austenite. Furthermore, the WM exhibited the highest toughness because of sufficient ductile austenite and unapparent ferrite texture. The HTHAZ had the lowest toughness because of insufficient austenite formation in addition to brittle sigma and Cr₂N precipitation. The LTHAZ toughness was higher than the BM due to secondary austenite precipitation. In addition, the WM fracture was dominated by the dimple, while the cleavage was main fracture mode of the HTHAZ. Both BM and LTHAZ exhibited a mixed fracture mode of the dimple and quasi-cleavage.     

Effects of hot compression on carbide precipitation behavior of Ni—20Cr—18W—1Mo superalloy

The effect of hot compression on the grain boundary segregation and precipitation behavior of M₆C carbide in the Ni–20Cr–18W–1Mo superalloy was investigated by thermomechanical simulator, scanning electronic microscope (SEM) and X-ray diffraction (XRD). Results indicate that the amount of M₆C carbides obviously increases in the experimental alloy after hot compression. Composition analyses reveal that secondary M₆C carbides at grain boundaries are highly enriched in tungsten. Meanwhile, the secondary carbide size of compressive samples is 3–5 μm in 10% deformation degree, while the carbide size of undeformed specimens is less than 1 μm under aging treatment at 900 and 1000 °C. According to the thermodynamic calculation results, the Gibbs free energy of γ-matrix and carbides decreases with increase of the compression temperature, and the W-rich M₆C carbide is more stable than Cr-rich M₂₃C₆. Compared with the experimental results, it is found that compressive stress accelerates the W segregation rate in grain boundary region, and further rises the rapid growth of W-rich M₆C as compared with the undeformed one.     

Influence and effectivity of VC and Cr3C2 grain growth inhibitors on sintering of binderless tungsten carbide

For the production of hard, high temperature and abrasion resistant parts, like water-jet nozzles or pressing tools for forming glass lenses, binderless cemented carbide is used. In this work, the consolidation of tungsten carbide with additions of VC and Cr₃C₂ grain growth inhibitors is studied. Tungsten carbide powder dry or wet milled was consolidated by dry pressing, debindering and gas pressurized sintering and, alternatively, by spark plasma sintering. The effect of adding VC and Cr₃C₂ to binderless tungsten carbide on the grain growth was studied with contents being 0; 0.1; 0.3; 0.5; 0.7 and 1.0 wt.%. Samples with an ultrafine microstructure free of abnormal grain growth, a hardness of 25.5 GPa and a fracture toughness of 7.2 MPa·m^1/2 were archived by conventional sintering. Both carbides reduce grain growth, but with Cr₃C₂ a finer microstructure can be achieved at lower amounts. Compared to the same amount of Cr₃C₂, the addition of VC results in smaller grains but lower hardness and fracture toughness.     

Effects of secondary carbide precipitation and transformation on abrasion resistance of the 16Cr-1Mo-1Cu white iron

The relationship between secondary carbide precipitation and transformation of the 16Cr-1Mo-1Cu white iron and abrasion resistance were investigated. The results show that secondary carbide precipitation and transformation at holding stage play an important role in the hardness and abrasion resistance. After being held for a certain time at 853 K for subcritical treatment, the grainy secondary carbide, (Fe,Cr)₂₃C₆, precipitated first and then Fe₂MoC or MoC carbides precipitated in the alloy, both of which improve the bulk hardness and abrasion resistance of the alloy. The reasons for these improvements are the secondary carbide precipitates from the austenite and the retained austenite transforms into the martensite, both make the matrix strengthen. So the matrix has more effective support to the harder eutectic carbide against exterior abrasion. With expanding the holding time, the in situ transformation from the granular (Fe,Cr)₂₃C₆ carbide into laminar M₃C carbide causes the formation of the pearlitic matrix and an associated decrease of the alloy abrasion resistance.     

Veränderung der Werkstoffeigenschaften austenitischer CrNiFe-Legierungen durch Aufkohlung

Changes in material properties of austenitic CrNiFe-alloys by carburization In gravimetric experiments austenitic CrNiFe-alloys, especially Incoloy 800, have been carburized in CH₄-H₂ in the temperature range 800–1100°C. Upon carburization internal carbide formation occurs which is controlled by carbon diffusion. Samples in different states of carburization have been studied by metallographic, chemical and X-ray analysis. Upon carburization carbides of the types M₇C₃ and M₂₃C₆ (M = Cr, Fe, Ni) are formed in an austenitic Fe-Ni matrix. Through this process the volume of the material increases and changes of the heat conductivity and the magnetic properties occur. The oxidation resistance of carburized samples was tested at 1000°C in flowing air. Deeply penetrating internal oxidation occurs in weakly carburized samples with M₂₃C₆ precipitates at grain boundaries. Highly carburized samples oxidize under formation of a protective Cr₂O₃-layer, slower than virgin material. The internal carbide formation also changes the mechanical properties which have been tested by tensile and notch impact tests. With increasing carburization of the samples the tendency to brittle fracture increases.     

Biocompatibility of metal carbides on Fe-Al-Mn-based alloys

The biocompatibility and microstructural variation of Fe-Al-Mn (FAM)-based alloys have been investigated clearly. The ((Fe,Mn)₃AlC_x) carbide (κ′-carbide) and Fe_0.6Mn_5.4C₂ carbide (κ-carbide) as well as Cr₇C₃ (Cr-carbide) were formed on FAM alloy, following phase transformation by heat treatment and surface functionalization. These carbides have important role in forming nanostructure and oxidation layer. Moreover, it was found that the albumin adsorbed onto the surface increased with increasing nano-metal carbides. Therefore, heat treatment and surface functionalization such as electro-discharging and anodization not only generates a nanostructural precipitates, but also converts the alloy surface into a nanostructured oxide surface, then increasing the alloy biocompatibility.     

A microanalysis study of the bainite reaction at the bay in Fe-C-Mo

A microanalysis study of the eutectoid decomposition of austenite to ferrite and M₂C (bainite) at the bay in Fe-0.24C-4Mo is reported. The carbon remaining in the austenite showed little variation with position at any given reaction time, which ruled out carbon diffusion as a rate-limiting process. Fine-probe EDS at bainite-austenite growth fronts revealed Mo enrichment in the interface and up to 15 nm in the austenite. EDS of extracted M₂C carbides always revealed Mo enrichment, but with a non-equilibrium Fe/Mo ratio at early reaction times. It was concluded that alloy element partition between ferrite and alloy carbides at the reaction front is largely responsible for the slow kinetics in this and related alloys. A thermodynamic analysis showed that ferrite-carbide interfacial energy and non-equilibrium carbide compositions reduce the thermodynamic driving force for diffusive processes (Mo partition) by up to 20%, further slowing the kinetics.     

Influence of Step Annealing Temperature on the Microstructure and Pitting Corrosion Resistance of SDSS UNS S32760 Welds

In the present work, the influence of step annealing heat treatment on the microstructure and pitting corrosion resistance of super duplex stainless steel UNS S32760 welds have been investigated. The pitting corrosion resistance in chloride solution was evaluated by potentiostatic measurements. The results showed that step annealing treatments in the temperature ranging from 550 to 1000 °C resulted in a precipitation of sigma phase and Cr₂N along the ferrite/austenite and ferrite/ferrite boundaries. At this temperature range, the metastable pits mainly nucleated around the precipitates formed in the grain boundary and ferrite phase. Above 1050 °C, the microstructure contains only austenite and ferrite phases. At this condition, the critical pitting temperature of samples successfully arrived to the highest value obtained in this study.     

B元素对Fe-Cr-C系耐磨堆焊合金组织和耐磨性的影响

采用直径为1.6 mm的细径药芯焊丝,利用CO₂气体保护焊堆焊的方法制备了含有1.0%~3.0%C(质量分数),15%~20%Cr,0%~2.0%B的高铬堆焊合金.研究了B₄C含量对堆焊合金的硬度及耐磨性的影响.结果表明,堆焊合金的硬度从57.1 HRC增加到65.2 HRC,硬度提高14.2%;堆焊层合金的相对耐磨性从3.5倍提高到18.0倍.借助光学显微镜、扫描电镜和X射线衍射等微观分析方法,研究了堆焊合金的显微组织及碳化物分布形貌.结果表明,堆焊合金的显微组织主要由铁素体+奥氏体+(Fe,Cr)₇C₃组成,加入B₄C可显著改善堆焊合金层基体组织,使碳化物(Fe,Cr)₇C₃数量增加且呈弥散分布.     

Formation and morphology of M<Subscript>7</Subscript>C<Subscript>3</Subscript> in low Cr white iron alloyed with Mn and Cu

The presence of M₇C₃ carbide in white iron enhances its wear resistance because of high hardness. Scanning electron microscopy (SEM) revealed its morphology as a pencil-like hexagonal structure. On the basis of the SEM observations, elemental distribution studies, and differential thermal analysis (DTA) of some heat-treated hypoeutectic white irons alloyed with Cr, Mn, and Cu, it is concluded that M₇C₃ carbides form as a result of attainment of a favorable condition in the liquid phase present at the austenite grain boundaries. Segregation of phosphorus in the intercellular regions and formation of a copper-rich intermetallic is responsible for the formation of this liquid phase. Austenite was found to nucleate first, followed by the nucleation and growth of M₇C₃ carbide in its vicinity, because of rejection of C and Cr during formation of austenite. The rosette structure generally observed is formed from the joining of M₇C₃ carbides by precipitation of secondary carbides.     

Microstructure and properties of Cr3C2-modified nickel-based alloy coating deposited by plasma transferred arc process

The nickel-based alloy with 30 wt.% chromic carbide (Cr₃C₂) particles has been deposited on Q235-carbon steel (including 0.12 wt.% C) using plasma transferred arc (PTA) welding machine. The microstructure and properties of the deposited coatings were investigated using optical microscope, scanning electron microscope (SEM) equiped with X-ray energy spectrometer (EDS), X-ray diffraction (XRD), transmission electron microscopy (TEM), microhardness testes, and sliding wear test. It was found that the γ(Ni, Fe), M₇(C,B)₃, Ni₄B₃, and (Cr,Fe)₂B phases existed in the Cr₃C₂-free nickel-based alloy coating obtained by PTA process. The typical hypoeutectic structure and composition segregation in the solid solution could be found clearly. The addition of 30 wt.% Cr₃C₂ particles led to the existing of Cr₃C₂ phase and the microstructure changing from hypoeutectic structure into hypereutectic structure. The composition segregation in the solid solution could not be found clearly. The average microhardness of the Cr₃C₂-free nickel-based alloy coating increased by 450-500 HV after the addition of 30 wt.% Cr₃C₂ particles. The partial dissolution of Cr₃C₂ particles led to the enrichment of carbon and chromium in the melten pool, and hence caused the formation of more chromium-rich carbides after the solidification process. The undissolved Cr₃C₂ particles and the increasing of chromium-rich carbides was beneficial to enhance the hardness and wear resistance of the Cr₃C₂-modified nickel-based alloy coating deposited by PTA process.     

Thermal treatment effect on structural and mechanical properties of Cr–C coatings

In the present study, the effect of thermal treatment on the mechanical and structural properties of chromium carbide coatings with different thicknesses is evaluated. The coatings were deposited by cathodic magnetron sputtering on XC100 steel substrates. Samples were annealed in vacuum, at different temperatures ranging from 700 to 1000°C for 1?h, resulting in the formation of chromium carbides. X-ray diffraction (XRD), microanalysis X/energy-dispersive X-ray spectrometer (EDS), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy analysis were used to characterise the samples. Mechanical properties were evaluated by nano-indentation tests and the residual stress was calculated with the Stoney formula. The XRD analysis suggests the formation of the Cr₇C₃, Cr₂₃C₆ carbides at 900°C. For thin films, they transformed totally to ternary (Cr, Fe)₇C₃ carbides and their partial transformation has been observed in the case of thick films at 1000°C, without the formation of Cr₃C₂. The EDS and XPS showed the diffusion mechanism between the chromium film and the steel substrate for the Cr, Fe, C, O elements during the annealing treatment. The increase of chromium film thickness from 0.5 to 2.64?µm, contributed to the significant enhancement of mechanical properties such as hardness (H) (from 12 to 26.3?GPa) and Young's Modulus (E) (from 250 to 330?GPa), respectively.