Carbide-free Zone Research Articles

Evaluation of metal dusting resistance of thermal sprayed NiCrAlSiTaY and NiCrAlSiTaY/laser re-melted Al2O3 coatings on Inconel 625

The present study assessed the resistance against MD corrosion in a single-layer, oxide-forming NiCrAlSiTaY coating, and a double-layer NiCrAlSiTaY/laser-remelted (LRed) Al2O3 coating on an Inconel 625 (IN625) alloy. The NiCrAlSiTaY and Al2O3 coatings were applied onto the IN625 substrate using high velocity oxy-fuel spraying (HVOF) and air plasma spraying (APS) techniques, respectively. Coated and non-coated coupons were exposed, for 70 cycles, to a strong carburizing atmosphere of CO-50 % H2 in the temperature range of 550–750 °C to evaluate MD behavior. The results showed that the non-coated IN625 alloy contained a carbide free zone (CFZ), and secondary carbide precipitations of M23C6 and M6C and their subsequent in-situ oxidation were observed in the sub-surface layer. Such a development was indicative of the type III MD characteristic. The results also showed that the single-layer coating successfully protected the alloy, and only a part of the substrate underwent local oxidation. In addition, performing laser remelting (LR) on the Al2O3 coating significantly increased the resistance against the MD of the alloy through the removal of defects such as interconnected porosities, and via alumina phase transformation (γ → α). The lowest weight gain after 70 cycles of MD test was recorded for the double-layer coating coupons.

Microstructural characterization of HP50 stainless steel tube from ethylene plant radiant furnace after 4 years of operation

The microstructure of HP50 cast stainless steel tubes for radiant ethylene cracking furnace, which has been in service for four years at about 900 °C, was investigated. Optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy, and Rockwell hardness test were used to analyze the material properties. Extensive growth of the primary carbide network and secondary carbide precipitates caused by carburization were observed near the inner wall of the tubes. In contrast, the area near the outer wall showed minimum carbide growth due to decarburization. The quantitative analysis results showed that the fraction of carbide in the inner, mid, and outer areas were 33, 10, and 4% respectively. Hardness test data confirmed this observation and showed increasing hardness in the inner area and decreasing hardness in the mid and outer areas. Carbide-free zone and precipitation of silicon oxide were observed on both the inner and outer walls. Intergranular corrosion due to sensitization and severe carburization occurred on the inner wall.

Effect of liquid-solid volume ratios on the interfacial microstructure and mechanical properties of high chromium cast iron and low carbon steel bimetal

Bimetal casting made of high chromium cast iron (HCCI) and low carbon steel (LCS) was produced by liquid–solid casting technology. The influence of different liquid-solid volume ratios on microstructure and microhardness of the interface was studied. The interface microstructure and the elemental distribution from one side to the other side of the bimetal were analyzed using Energy Dispersive x-ray Spectroscopy (EDX). Wear behavior of the successfully produced bimetal was also investigated in order to emphasize the difference in the mechanical properties between the two bonded alloys. The results showed that a good metallurgical bonding area free of holes could be obtained with increasing the liquid to solid volume ratio to 10:1 and 12:1. By increasing this ratio, diffusion of the elements was accelerated resulting in expanded carbide-free zone and improved bonding of the bimetal. Accordingly, the microhardness decreased at the HCCI side and increased at the LCS part due to the elemental diffusion from the former to the later through the interface. Sliding wear test on both sides of the bimetal showed that the coefficient of friction was ∼0.79 for HCCI and 0.69 in case of LCS. The transition from the high wear resistant HCCI part to LCS was significantly observed on the worn surface of the interfacial area of the test sample. The wear mechanism involved in the two sides was delimitation and the size of the produced laminates was smaller in case of LCS than in HCCI.

Influence of the Base Element on the Thermal Properties of Non- Ferrous Chromium-Rich TaC-Containing Alloys Elaborated by Conventional Casting: Part 3: Surface and Cross-Sectional Metallographic Characterization of the Oxidized Alloys

The six alloys the thermal properties of which and the tendency to oxide spallation of which were studied in the first two parts of this work, were here characterized after oxidation for 70 hours at 1250°C. The external chromia scale, and also the CrTaO4 subsurface oxide, formed for all the alloys, almost independently of the Co and Ni proportions in the base element content. But, because of the formation of more CrTaO4 for the nickel-richest alloys probably due to the higher availability of Ta in the matrix and its easier diffusion towards the neighbourhood of the oxidation front, the adherence of chromia was weakened and spallation, suggested by the thermogravimetric curves in the second part of this work, is here really observed and the denuded part of alloys clearly seen. The degradation of the subsurface, which can be in a first time summarized by the development of a carbide-free zone and a {Cr, Ta}-depleted zone, depends on the Co and Ni proportions. The microstructure of the bulk is differently affected by long exposure at elevated temperature. The changes in carbide population characteristics are stronger for the nickel-based alloys than for the cobalt-based ones. Finally, the isothermal oxidation behaviour is best for the nickel-richest alloys but the oxide spallation behaviour and the potential mechanical properties are the best for the cobalt-richest alloys.

Formation of “carbide-free zones” resulting from the interplay of C redistribution and carbide precipitation during bainitic transformation

Advanced bainitic steels exploit their constitution of a phase mixture of bainitic ferrite and retained austenite. The desired microstructures are usually adjusted via isothermal or continuous cooling treatments below the bainite-start-temperature. In the present work, we analyze the microstructural evolution during bainite formation in detail and we report on a novel mechanism for the formation of “carbide-free zones” in the close proximity of austenite. Carbides are formed in the interior of bainitic ferrite, which is isolated from surrounding austenite, while the depletion of C in the vicinity of austenite suppresses their formation.

Effect of hot rolling on microstructure and properties of high-chromium cast iron hardfacing cladding plate

In this study, a high-chromium cast iron (HCCI) material hardfacing on a low-carbon steel (LCS) plate is fabricated into a symmetrical sandwich structure, which is subjected to multi-pass hot rolling on a rolling mill at a reduction of 15%, 25%, and 35%. The effects of different rolling reductions on the microstructure (interface and HCCI layer) and mechanical properties (bonding strength, hardness, and wear resistance) of the composite sheets are studied. Results show that the cracks in the hardfacing layer disappear. HCCI and LCS exhibit coordinated thermal deformation, and the shape of the composite plate after hot rolling is good without warpage. HCCI/LCS composite panels are successfully prepared. As the rolling reduction increases, the microstructure defects at the original bonding interface are eliminated. Moreover, a carbide-free zone and intergrowth are observed, and the bonding strength increases. Hot rolling results in the breakdown of a eutectic carbide chain network of hardfacing HCCI cast iron layer and the dispersion distribution of carbides. The precipitation of secondary carbides is observed in the HCCI layer after hot rolling at 35%, which results in increased hardness of HCCI.

Hot workability and corrosion behavior of EN31 grade steel

In the recent years across the globe, there is a remarkable shift in paradigm of manufacturing agricultural tools and accessories for farming in agricultural sectors. Most often, high carbon steels are usually used for making agricultural tools. In the present investigation, one of the used farming accessories was cut and accessed for compositional identification using spectrometer and based on its composition report, stock material (EN31 grade steel) was procured. Most of the agricultural tools especially ploughs are conventionally made using open die forging in rural areas to have intended properties. But morphological variation like size of the grain boundaries, carbide free zones, carbide rich zones and composition across the grain boundaries accompanied by forging, causes pitting corrosion. Hence the present investigation aims to study the effect of hot working at different temperatures while processing the selected EN31 grade steel on pitting potential and identifying the most suitable temperature for hot deformation and high corrosion resistance. All the processed steels exhibited both superior hardness and corrosion resistance compared to the base metal. Forged steel at 800°C has shown maximum corrosion resistance compared to steels forged at other temperatures and it may be attributed to the extent of dissolution of the carbides and uniform distribution of carbide particles in the parent matrix which act as insulator to the path of galvanic cell formation between cathodic of intermetallic particles and anodic cells i.e. parent microstructure. Even though the extent of deformation rate is found to be maximum while forging at 850°C, it has been restricted because of premature appearance of cracks, hence hot forging of base metal at 800°C is recommendable one.

Microstructure and creep resistance of a diffusionally aluminized Ni-base superalloy

An aluminide layer was formed on a wrought Ni-base superalloy by the diffusional aluminizing method, which involves a physical vapor deposition of Al followed by two-step heat treatment in vacuum. Microstructural analysis revealed the presence of an aluminide layer, inter-diffusion zone (IDZ), and affected substrate, all of which developed due to the inter-diffusion of deposited Al and elements in the matrix. In addition, a wide carbide free zone, in which grain boundaries were mostly denuded of carbides, was found below the IDZ. Depth profiling analysis using a glow discharge spectrometer confirmed the reduced carbon content in the carbide free zone. At 900 °C, the diffusionally aluminized specimens showed a decrease in creep-rupture life caused by the presence of the carbide free zone. Fracture surface and cross-section microstructure observation confirmed the detrimental effect of the carbide free zone on the creep resistance of the diffusionally aluminized Alloy 617.

Modeling carbide dissolution in alloy 602 CA during high temperature oxidation

The high-temperature oxidation resistance of the carbide strengthened nickel-base alloy 602 CA is provided by the formation of a protective outer Cr2O3 scale accompanied by internal precipitation of alumina and dissolution of Cr-rich carbides. The carbide free zone formation has been discussed in the literature to be a consequence of either chromium depletion or alloy decarburization.A new dissolution mechanism is proposed in this paper. Carbide dissolution in alloy 602 CA exposed to oxidizing atmospheres at 1100°C was studied. The alloy microstructure was characterized by means of SEM/EDX/EPMA and GDOES. The experimental observations were substantiated by CALPHAD based computations.

HIGH TEMPERATURE VACUUM CARBURIZATION BEHAVIORS AND PHASE EVOLUTION MECHANISMS OF Cr35Ni45Nb ALLOY UNDER SERVICE CONDITION

Carburization in Ni-Cr-Fe-based alloys is an important phenomenon, especially in ethylene cracking tubes which serve at high temperatures under highly carburizing environment. In this work, the Cr35Ni45 Nb tube subjected to service condition for 6 a was carburized by low-pressure vacuum carburizing(LPVC) at 1080 ℃.The carburization behaviors and corresponding mechanisms of phase evolution in the inner wall were comprehensively analyzed through SEM, XRD and EPMA. The results showed that oxidation behaviors of the tube at high temperature were consisted of external oxidation of Cr and internal oxidation of Si, resulting in formation of com-posite oxide scales. Depletion of Cr in the subsurface caused by surface Cr2O3 leaded to carbide dissolution and formation of carbide free zone and carburized zone. The critical concentration of Cr for carbide dissolution is about19.0%(mass fraction). By comparing carburization behaviors of specimens whose oxide scales were retained or removed, the carburization resistance of the composite oxide scales in carburizing environment was systematically investigated. The results showed that the composite oxide scales formed previously acted as an effective barrier to carbon infiltration. However, the outermost Cr2O3 scale tended to be carbonized to form carbide scale to spall from the surface in the strongly reducing environment with low oxygen partial pressure, while the Si O2 kept stable all along due to its excellent thermodynamic stability. However, a certain amount of carbon was still capable to penetrate the alloy interior through gaps of the Si O2 scale due to its discontinuity. Therefore, continuity, density and high-temperature stability of the oxide scales were crucial for the alloy to achieve excellent anti-carburizing performance. Once the oxide layers were removed or carbonized adequately, inconceivable internal carburization occured widely. Large amounts of secondary carbides precipitated again in the previous carbide free zone due to high carbon activity. Widespread precipitations of graphite called metal dusting in the range of about 0.5 mm in depth occurred after long exposure of specimens to the carburizing environment. The carbon activity gradually decreased with increasing distance from the surface. The primary carbides within the deeper carburized region were transformed from M23C6 to M7C3in situ, which were accompanied by precipitation of vermicular g phase in the primary carbides, phase transition from h to Nb C and decomposition of intragranular secondary carbides. Severe coalescing and coarsening of carbides and metal dusting caused the serious degradation of microstructure, formation of macrocracks and final thinning of the Cr35Ni45 Nb tube wall.

Effects of temperature and strain rate on microstructure and mechanical properties of high chromium cast iron/low carbon steel bimetal prepared by hot diffusion-compression bonding

The objective of this study is to develop a hot diffusion-compression bonding process for cladding low carbon steel (LCS) to high chromium cast iron (HCCI) in solid-state. The influence of temperature (950–1150°C) and strain rate (0.001–1s−1) on microstructure, hardness and bond strength of the HCCI/LCS bimetal were investigated. The interface microstructure reveals that the unbonded region can only be found for 950°C due to lack of diffusion, while the intergrowth between the constituent metals occurred at and above 1100°C. When bonding temperature increases to 1150°C, a carbide-free zone was observed near the interface on the HCCI layer, and the thickness of the zone decreases with an increase of bonding strain rate. These evolutions indicate that the bond quality was improved by raising temperature and reducing strain rate due to the increase of element diffusion. The hot compression process of the bonding treatment not only changes the carbide orientation of the HCCI, but also increases the volume fraction of Cr–carbide. Based on the microstructural examinations and mechanical tests, the optimum bonding temperature and bonding strain rate are determined to be 1150°C and 0.001s−1, respectively.

Determination of Diffusion Coefficients Using Thermogravimetric Measurements during High Temperature Oxidation

Thermogravimetry measurements associated to concentration profiles allow determining a diffusion coefficient at high temperature of the most oxidable one among the metallic elements belonging to the chemical composition of an alloy. In this work the employed method is described and applied to chromia-forming alloys essentially based on nickel but also to selected cobalt-based and iron-based alloys. More precisely DCr values were estimated for chromium diffusing through the carbide-free zones developed during high temperature oxidation. The effects of the base element, of the chromium carbides density and of the dendritic orientations on the chromium diffusion were evidenced.

High temperature oxidation behavior of chromium-rich alloys containing high carbides fractions. Part I: Nickel-base alloys

Six wear-resistant alloys based on nickel, containing 30 wt.% Cr and from 2.5 to 5.0 wt.% C, were elaborated by foundry and subjected to oxidation by air at 1000, 1100, and 1200 °C, for evaluating the oxidation behavior of hard bulk alloys. Their microstructures are rich in chromium carbides, eutectic, or pro-eutectic, and they also contain graphite for the highest carbon contents of interest. All the studied alloys obviously display a chromia-forming behavior, despite the initial low chromium content in the matrix. During oxidation, carbides disappear over an increasing distance from the oxidation front, with the consequence of the enrichment of the neighbor matrix in chromium. The minimal chromium content on the surface after oxidation decreases when the alloy is richer in carbon and increases with the oxidation temperature. The carbide-free zone tends to be deeper when the oxidation temperature increases, and also when the alloy's carbon content increases, in contrast with low-C Ni-30Cr alloys previously studied. The disappearance of carbides means a carbon loss as gaseous oxidized species. This probably disturbs the oxide scales growing on the external surface and may influence the oxidation behavior of the alloys. When present, graphite does not deteriorate dramatically the oxidation resistance of the alloys. The hardness of the alloys are lowered by the exposures to high temperature and by the presence of graphite.

Influence of Tantalum on the Rates of High Temperature Oxidation and Chromia Volatilization for Cast (Fe and/or Ni)-30Cr-0.4C Alloys

Four cast superalloys, Fe-base and (Fe,Ni)-base alloys, all containing 30%Cr and 0.4%C, were elaborated with addition of 3% and 6% of tantalum. Their oxidation behaviours were studied at 1000, 1100 and 1200°C during 50 hours. The oxidized surfaces of the samples were quantitatively characterized by measurements of thicknesses and surface fractions of internal oxides. The thermogravimetry files were treated according to the {m×(dm/dt) = Kp - Kv×m} equation, to obtain simultaneously the parabolic constant and the chromia volatilization constant. The internal tantalum oxides are more present in the Fe-base alloys and the carbide-free zones are less developed for the (Fe,Ni)-base alloys than for the others. The Fe-base alloys oxidize faster than the (Fe,Ni)-base and Ni-base alloys. The comparison with the corresponding Ta-free ternary alloys shows that the presence of Ta tends to accelerate the oxidation.

Experimental and thermodynamic study of nickel-based alloys containing chromium carbides, Part II: Study of the sub-surface characteristics of Ni–30 wt% Cr– [formula omitted]C alloys oxidized at high temperature using thermodynamic calculations

Six ternary Ni–30 wt% Cr– x C alloys ( x = 0.2 – 2.0 ) were oxidized at 1000, 1100 and 1200 ∘C. For several of them, the sub-surface was studied with accuracy by division into several successive parts for local measurements of carbide fractions by image analysis. Additionally thermodynamic calculations were performed for local analysis of the samples. By these means, the outer carbide-free zone and the inner zone enriched in carbides were characterized, with special attention to both local carbon contents and local solidus temperatures. Profiles of carbon contents (and thereafter of local solidus temperatures) across the zone affected by oxidation were obtained by thermodynamic calculations. The content of carbon still present in the carbide-free zone is necessarily very low. The carbon content in the zone enriched in carbides is higher than in the bulk. The loss of carbon leads to a higher refractoriness for the carbide-free zone, which could protect the bulk against fast oxidation if it becomes partially molten because of a sudden increase in temperature in service.

Effect of Cr Content on Supercritical Water Corrosion of High Cr Alloys

High Cr alloys were corrosion tested in supercritical water and the oxide scale was analyzed. Commercial grade two steel specimens; 9CrMoVNb steels, one 9CrMoVNbW steel, one 12Cr-MoVNbWCu steel and one 20Cr Fe-based O.D.S (Oxide Dispersion Strengthened) alloy specimen were investigated. Corrosion tests were conducted within non-deaerated pure supercritical water at 627, 550, and 500oC with 25 MPa. Corrosion rate was estimated by the weight change per unit surface area and the oxide layer was analyzed using a grazing incidence X.R.D (x-ray diffractometer), S.E.M (scanning electron microscope) and T.E.M (transmission electron microscope) equipped with an E.D.S (energy dispersive spectroscope). Corrosion rates of the 9Cr steel specimens were observed to follow the parabolic growth rate law, while those of the specimens with a 12 per cent or higher Cr content showed significantly lower rates. Oxide scale on the 9Cr steel specimen after a corrosion test in a supercritical water was found to consist of three distinctive layers. Through the cross-section T.E.M the outermost layer with about a 35 µm thickness after 200 hr at 627 oC was identified to be magnetite type Fe3O4, and about 25 µm thick intermediate layer was a Cr partitioned magnetite type (Fe,Cr)3O4. The outermost layer showed a coarse columnar structure, while the intermediate one revealed an agglomerate of tiny oxide particles (several tens nm in diameter). The innermost layer next to the matrix phase was found to be the internally oxidized zone. Oxygen atoms seemed to have attacked along the grain boundaries and the lath boundaries and formed oxide CrO3 along the boundaries. Also a Cr depleted zone, and consequently a carbide-free zone, was observed along the interface between the internal oxidation zone and the matrix phase.

The effect of selective oxidation of chromium on the creep strength of alloy 617

In order to investigate the effect on creep strength of the selective oxidation of chromium which causes the formation of a carbide-fi-ee subsurface zone, specimens of Ni22Cr12Co9Mo1Al (Alloy 617) were subjected to heat treatments to simulate a long-term service exposure of a thin-walled heat exchanger tube operating at high temperatures. In creep tests carried out at 900°C, specimens with extensive chromium-depleted and carbide-free subsurface zones exhibited higher creep strength than specimens thermally aged for the same durations, but having no carbide-free zone. The carbon released by dissolution of the carbides in the chromium-depleted zone migrated to the centre of the specimen, producing enhanced carbide precipitation. This led to an increase in the creep strength of the specimen core which offset the loss in creep strength of the subsurface zone.

Laser surface treatment and corrosion behaviour of martensitic stainless AISI 420 steel

Abstract The effect of laser surface treatment of AISI 420 steel was studied. The operation conditions of the laser treatment were optimized with regard to beam power and displacement rate in order to produce different surface microstructures. The corrosion behaviour was studied as a function of the microstructures developed in the material. Maximum hardness was observed in the non-melted and carbide-free zone which was transformed into martensite. The best corrosion behaviour was observed in completely melted areas; this behaviour is similar to that observed in the starting material which is quenched and homogeneously tempered. Heterogeneous zones exist after laser heat treatment which have to be considered for final applications.

Cyclic oxidation of Haynes 230 alloy

The cyclic oxidation of Haynes 230 alloy (Ni-Cr-W-Mo alloy) was investigated in air at three different temperatures, 871, 982 and 1093 °C. Studies indicated that during cyclic oxidation, protective scales formed which were predominantly Cr2O3, with Kirkendall voids formed both at the scale/alloy interface and grain boundaries. Intergranular oxides were observed at temperatures above 982 °C while internal oxide particles were found above 1093 °C. Both intergranular and internal oxides were identified as aluminium oxide. A 50 μm chromium-depleted zone developed after 70 h exposure at 1093 °C and was accompanied by disastrous scale spalling. The lowest chromium concentration within the depleted zone was 14 wt% which still provided a sufficient supply of chromium for development of a continuous Cr2O3 rich scale. Decarburization was observed at the higher temperature of 1093 °C, and a carbide-free zone developed. Also, it was found that Haynes 230 is subject to a sensitization process. At the lower exposure temperature of 871 °C, large amounts of chromium carbide formed preferentially at the grain boundaries. While at the surface region chromium carbide precipitation occurred at the twin boundaries.

Effect of carbon on the oxidation of Fe-5.5 Al-0.55 C alloy

An alloy with the chemical composition Fe-5.5 wt% Al-0.55 wt% C is employed to investigate the effect of carbon on the oxidation at 600, 800 and 1000° C in dry air. Kinetic curves were determined by thermogravimetry analyses. Optical metallography and electron probe microanalysis were used to examine the oxide scales formed on the alloy surfaces. The kinetic curves observed at 600, 800 and 1000° C had simple, three-stage and two-stage parabolic rate laws, respectively. No carbide-free layer could be observed in the alloy which was oxidized at 600° C. In contrast, a carbide-free zone was found in specimens of the alloy after oxidizing at 800 and 1000°C.