Carbide Network Research Articles

Bulging of inner cover of a batch annealing furnace in a cold rolling mill

Bulging of an inner cover made up of AISI 309S austenitic stainless steel of a batch annealing furnace (BAF) in a cold rolling mill has been investigated. Inner cover envelopes the cold rolled (CR) coils to be annealed and shields the coils from direct heating by the burners on the heating hood surrounding the inner cover. The investigation of the bulged out inner cover consists of visual inspection, chemical analysis, characterization of microstructures by optical and scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), and measurement of micro-hardness profile. Visual inspection reveals deposition of soot at the inner surface, microstructural examination and EDS analysis indicate the formation of carbide network and sigma phase in the matrix, and micro-hardness profile shows gradual increase in hardness values from the outer surface to the inner surface. Analyses of the results suggest that carburization attack of the hydrocarbon gas generated at the annealing temperature from the rolling oil adherent to the coils favors the formation of carbide network at the inner cover matrix. Formation of carbides associated with volume expansion and depletion of alloying elements from the matrix deteriorates the mechanical properties of the inner cover material making it prone to bulging.

Thermal expansion of chromium-rich iron-based or iron /nickel-based alloys reinforced by tantalum carbides

Six alloys reinforced by TaC carbides based on iron (ferritic) or both iron and nickel (austenitic) were studied in thermal expansion between 100 and 1200°C for two microstructural orientations. The heating, isothermal, and cooling parts of the dilatometry curves were characterized. The thermal expansion of the ferritic alloys is less important than the thermal expansion of the austenitic alloys. A compressive deformation of the matrix subjected to stresses applied by the carbides network was observed. The importance of this phenomenon seems to depend more on the matrix nature than on the microstructural orientation.

An Investigation on the Cracking of Air Tubes of Rotary Kilns in a Sponge Iron Plant

Cracking of an air tube used for supplying air in rotary kiln of a sponge iron plant is investigated in this paper. The material of the air tube is ASTM A297 HK 40, a member of the heat-resistant cast alloy family (H series) steels widely used for enhanced high-temperature properties. Microstructural degradation occurring at high temperature affects mechanical and corrosion-resistance properties of the component. The failure analysis consists of visual observation, chemical analysis, examination of microstructures using optical and scanning electron microscopes, and characterization with energy-dispersive spectroscopy (EDS). Microstructural characterization reveals (i) intergranular corrosion at the cross section of tube wall, (ii) formation of severe chromium carbide network at the grain boundary, (iii) formation of sigma phases along the grain boundary as well as within the matrix, and (iv) generation of crack at the grain boundary filled with oxides. Analyses of these results suggest that formation of carbide network and sigma phases at the elevated service temperature depletes the alloying elements from the surrounding matrix, enhancing intergranular corrosion, which leads to cracking of the air tube.

An investigation of carburization resistance performance of ethylene furnace tube alloys

Carburization tests and analysis were performed on some samples of radiant tube alloys of ethylene furnaces at three petrochemical plants. These tubes had undergone carburization in service with some losing their structural integrity. This study evaluates and compares the carburization resistance performances of some of these tubes in service and identifies the cause of their degradation. The polished surfaces of the test samples were visually observed. The depth of carburization zones for each test sample was measured. Scanning electron microscopy and energy dispersive x-ray spectroscopy were used to examine the carburization zones and the rest part of the specimens and to characterize the microstructure and elemental composition of the tubes’ material. Optical microscope (Image analyzer) was used to examine the etched surface of each of the test samples. Microhardness testing was performed to determine their mechanical strength. Visual inspection revealed the sagging in some of the tubes. Metallurgical assessment indicated that the selected furnace tubes showed relative greater depths of carburized zones when compared with other tube materials tested. Microstructure of the tubes showed coarse Cr-carbide precipitation and continuous carbide networks at austenite grain boundaries. Key words: Ethylene furnace, carburization, degradation, high temperature(s), microstructure, tubes.

Effect of thermal cycling treatment on the structure of cast high-speed steel R6M5-Sh

The possibility is demonstrated of improving the microstructure of cast high-speed steel R6M5-Sh by use of thermal cycling treatment (TCT) as an intermediate production operation between preliminary isothermal annealing and final hardening with triple tempering. As a result of TCT there is a reduction in the amount of ledeburite eutectic, size of the carbide network, carbide inhomogeneity, and a change in carbide size and composition. Rational treatment regimes are determined.

Effects of modification on microstructure and properties of ultrahigh carbon (1.9 wt.% C) steel

Abstract The effects of a modifier that contains Rare Earths (RE), low melting point alloy (Al–Bi–Sb) and Ca–Si alloy on an ultrahigh carbon steel containing 1.9 wt.% C were studied. Microstructure characterization was carried out with optical microscopy (OM) and scanning electronic microscopy (SEM) combined with energy-dispersive spectrometry (EDS). Upon modification, the continuous eutectic carbide network structure was broken up and changed to a partly isolated and finer blocky structure in the as-cast alloy. Differential scanning calorimetry (DSC) revealed that the eutectoid temperature increased and the eutectic temperature decreased for the modified alloy. Modification also improved the impact toughness of the tempered steel, with a significant increase from 6.5 to 12.6 J cm −2 , despite the hardness remaining around 66 HRC. Furthermore, in pure sliding under loads of 20, 60 and 100 N for 600 s against a zirconia ball, the modified alloy shows slightly higher friction coefficient at all loads than the non-modified one. In addition, the friction coefficient for the steel specimen decreased with load from 20 N to 100 N attributing to a reduction in metallic wear and the formation of a thicker oxide film on the surfaces.

Microstructural and mechanical characterization of an ultra-high-strength Fe86.7Cr4.4Mo0.6V1.1W2.5C4.7 alloy

This study describes the correlation between microstructure and mechanical properties of an ultra-high-strength Fe86.7Cr4.4Mo0.6V1.1W2.5C4.7 (at.%) alloy manufactured under high cooling rates and pure conditions. The applied preparation conditions promote the formation of non-equilibrium phases such as martensite, retained austenite and special carbides already in the as-cast state. The carbides form a 3-dimensional skeleton-like structure between the retained austenite and the martensite. This hard and finely ramified carbide network distributed throughout the entire ingot is a specific characteristic of this alloy and important for its excellent mechanical properties. The material exhibits extremely high engineering compression strength of almost 5500 MPa combined with a large compression strain of about 23% due to deformation-induced martensite formation. Furthermore, the alloy possesses a high hardness and tensile strength in the as-cast condition. This combination of mechanical properties leads to an outstanding engineering material for a variety of structural applications in the automotive and tool manufacturing industry.

Microstructure and Mechanical Properties of an Advanced Nickel-Based Superalloy in the as-HIP Form

This study evaluates the suitability of as-hot isostatically pressed (HIP) RR1000 for non-critical applications in aero-engine components. RR1000, an advanced powder nickel-based superalloy, was developed for disc rotor components in aero-engines. For these critical applications, the consolidated alloy powder particles are extruded to break down carbide and oxide networks, known as prior particle boundaries (PPBs), and to refine the structure into a fine grain size for isothermal forging. In this study, hot isostatically pressed compacts, made from two different powder particle size fractions have been assessed following heat treatments below and above the gamma prime solvus temperature. A microstructural evaluation shows a greater degree of PPB decoration occurs in the finer powder particle size fraction. Following a super-solvus heat treatment these PPBs pin grain boundaries of the fine powder particle size compacts, whilst the reduction of PPB decoration in coarse powder particle compacts allows significant grain growth. Tensile test results of as-HIP RR1000 show, good yield strengths, ultimate tensile strengths and ductility, which are comparable with extruded and isothermal forged RR1000 disc material. Dwell crack propagation tests show that finer powder particle size compacts, which have received a sub-solvus heat treatment, give the highest crack growth rates; whilst the remaining material conditions show markedly improved crack growth resistance. In conclusion, as-HIP RR1000 demonstrates clear potential for use in non-critical applications, employing either powder particle size fraction used in this study subject to the appropriate solution heat treatment.

Effect of Calcium Modification on the Microstructure and Properties of High Speed Steel

The present work has investigated the influence of calcium on the microstructure and properties of AISI M2 high speed steels. The results show that the as-cast structure consists of the iron matrix and networks of M2C eutectic carbides, which are greatly refined in the ingot modified by calcium. Meanwhile, the morphology of M2C evolves from the plate-like shape into the fibrous one. Compared with the plate-like M2C, the fibrous M2C is less stable at high temperatures, which promotes the spheroidization and refinement of carbides. Therefore, M2 steels modified by calcium obtain higher hardness and red-hardness after heat treatment than those without the addition of calcium.

Study on the Solidification Microstructure of Aluminum Alloyed High Speed Steel

The present work has investigated the effect of aluminum on the solidification microstructure of M2 high speed steel. The results show that the as-cast structure is refined and the networks of M2C eutectic carbides are distributed more homogeneously with the addition of aluminum. Meanwhile, the morphology of M2C changes from the fibrous shape to the plate-like one. Despite of the same crystal structure, the microstructure differs greatly between the two carbides. Compared with the fibrous M2C, micro-twining is formed in the plate-like M2C, resulting in the orientation changes between different plates. It is expected that the solid/liquid interface structure of M2C may evolve from the non-faceted into the faceted due to the addition of aluminum.

The Effect of Prior Tempering on Sub-Zero Treatment to Reduce Retained Austenite

Abstract The consensus view is that a high carbon case gives gears the best overall properties, provided that there is no carbide network and the retained austenite has been reduced below 20 % by sub-zero treatment. This view is effectively enshrined in the SAE AMS 2759/7A standard. The sub-zero treatment usually takes place immediately after the quench to avoid austenite stabilization. However, for some parts with complex geometries that might crack during the treatment, a short low temperature temper is carried out first. Little is known on how this temper affects the subsequent sub-zero treatment. Three carburising steels used extensively in the aerospace industry were carburised to produce high retained austenite levels in the case using two different but typical carburising cycles. The retained austenite was determined by X-ray diffraction before and after sub-zero treatment was carried out in accordance with the standard and compared with that obtained when an intermediate temper was used. This study shows that for three typical carburising steels carburised using typical cycles, the efficacy of the sub-zero treatment is probably slightly reduced after the temper but not enough to be industrially significant.

Abrasion wear behaviour of alloyed and chilled cast irons

Abstract High-chromium white cast irons are widely used in a variety of applications that require high wear resistance. This outstanding performance is due to the presence of large amounts of chromium carbides which exhibit high hardness. However, for wear-resistant applications exposed to impact, ductile cast iron is a candidate material due to its good wear behaviour, reduced brittleness and lower production costs. That is the case of the working rolls employed in clay mass rolling operations which operate under significant wear conditions and impacts (due to foreign bodies mixed in the ceramic mass). In the present study, the effect of the chemical composition on microstructure and abrasion wear of two high-chromium white cast irons (13% Cr and 19% Cr) and a chilled ductile cast iron with 0.6% Cr was investigated. The microstructural characteristics and hardness were evaluated and correlated with the wear data. Ball cratering tests, using abrasive slurry containing SiC particles, were carried out to rank the abrasive wear resistance of the alloys, in the as-cast condition, obtained by permanent mould casting. The alloy with the highest chromium content (and higher ratio Cr/C = 6.8) presents a larger volume fraction of chromium-richer carbides providing better wear resistance. Ductile iron exhibits good wear performance in the chilled region due to a fine abrasion-resistant carbide network.

Metastable structure of austenite base obtained by rapid solidification in a semi-solid state

Material processing in a semi-solid state with rapid solidification is an innovative technology, which enables us to produce complex-shaped semi products in one operation. Unconventional properties and microstructures can be obtained in this way. Material processing in a semi-solid state has been used for materials with lower melting temperatures, particularly for Al alloys. This paper concentrates on the development of new technologies for production of miniature thin-walled steel components with complicated shapes. Ledeburitic steel with 1.8% of carbon and 11% of chromium was chosen for this experimental study. This material was used to produce very small thin-walled semi products. From the initial structure consisting of primary and secondary carbides distributed in a ferrite matrix was obtained a microstructure with over 90% of metastable austenite after cooling from the semi-solid state. The main aim of this experimental program was to describe the effect of two different methods of heating to the semi-solid state. The first method used unique heating equipment, combining high frequency and resistance heating. The second method consisted of conventional heating in a furnace. The influence of the cooling rate on the development of the microstructure was investigated. If was found that both heating and cooling rates influence grain size and the size and the morphology of carbide network placed between the globular austenite grains. Structure analysis was performed with the help of light microscopy, laser scanning confocal microscopy and scanning electron microscopy. EDX analysis was applied to determine chemical profiles of phases and X-ray diffraction phase analysis was used to establish volume fraction of austenite in the final microstructure. Characterisation of individual structural components was further completed by hardness measurements.

Breakdown Behavior of Eutectic Carbide in High Speed Steel During Hot Compression

The evolution of eutectic carbide in as-cast M2 high speed steel was investigated with hot compression test and metallographic examination. Initial rodlike or irregular eutectic carbides were broken into smaller particles during hot deformation by thermomechanical disintegration, while diffusion-controlled phase transformation was not remarkable. Combining with numerical simulation, the relationship between breakdown ratio of carbide network and deformation parameters was obtained. Strain was the most important driving force to shatter eutectic carbides and disperse products. Furthermore, critical strain values were obtained, beyond which carbide network disappeared, and fractured carbides kept a stable profile and they were deformed with matrix coordinately. A higher temperature or lower strain rate resulted in a lower critical strain.

Effect of Heat Treatment on Microstructure and Mechanical Properties of NF6357A Cast Alloy for Wear Resistance Application

The solidification structure of the as-cast consists of the matrix structure that is predominantly austenite and precipitated chromium carbide along the grain boundary. Under these circumstances and where the level of impact is relatively modest, such alloys in as-cast condition will perform. However, at higher levels of impact energy, a point is reached where excessive stress are built up within the component and eventually the materials strength is exceeded and the outcome is complete failure in a characteristic stress fracture mode. If this is to be prevented, it is therefore imperative that the casting be subjected to appropriate heat treatment, to obtain a structure which consist of Cr7C3 carbide and martensite at a hardness range of 650-750HB. The microstructure of NF6357A cast chromium steel containing 2.59% C- 0.7%Si-0.91%Mn-18.54%Cr-0.019%P-0.01%S- balance–Fe after appropriate heat treatment such as quenching and tempering process have been characterised by means of optical microscope, micro hardness tester, optical emission spectrometer and charpy testing machine. The results show that oil quenched samples were found to retained microstructural consistency for casting thicker than 120mm section. For economic argument, air quenched castings of less than 120mm thickness is not only cheaper alternative, but it is also environment friendly. The fracture toughness was found to be fairly consistent between 2.4-2.6%C range. However, at higher carbon level, the fracture process is dominated by the presence of segregated carbide network which act as a weak link in the microstructure. This weak link encourages dislocation pile-up and impaired material toughness.

Electrochemical and Microscopy Study of Localized Corrosion on a Sensitized Stainless Steel AISI 304

Corrosion has been frequently recognized as failure's root cause for diverse engineering components like those associated to production lines, like in boiling water reactor piping systems (1). It is important that several electrochemical techniques help conjointly to elucidate some of the corrosion specificities, like localized corrosion of traditional materials. Pitting corrosion has been usually reported to affect stainless steels because it engages more frequent than not, second phase particles that precipitate out at the matrix and the grain boundary network, thereby consuming alloying elements that, for example, participate on sustaining passivity. Thermally-dependent fabrication operations such as welding bring about sensitization, particularly within the 450 to 700 ºC range. Exposure propitiates nucleation and growth of chromium carbide networks: this makes the boundaries and neighboring areas sensitive to depletion. Thus the precipitates are directly suspect of significant decrements in protective potential as the chromium has been chemically bound to carbon, thereby decreasing the surface passivating abilities. Therefore, the present work reports the electrochemical and microscopy results obtained after immersing the heat treated SS304 samples for diverse periods in a suitable electrolyte to studying their potentiodynamic polarization and EIS behaviors.

Transformation‐induced plasticity in rapidly solidified Fe88.9Cr4.3V2.2C4.6

AbstractIn the present work, the mechanical behaviour at different strain rates of a high strength tool steel with the composition Fe88.9Cr4.3V2.2C4.6 (at.%), manufactured under relatively high cooling rates and highly pure conditions, was studied. The applied preparation conditions promote a microstructure composed of martensite, retained austenite and a fine network of special carbides already in the as‐cast state. This special phase combination yields a structural material with superior mechanical properties under quasi‐static loading, exhibiting an extremely high engineering compression strength of almost 4000 MPa combined with a large compression strain of about 20% due to deformation‐induced martensite formation. However, the knowledge of the performance of this material under dynamic loading conditions is also very important for its possible application as a novel kind of tool steel. For this reason, compression tests in the strain rate range of 10−3–103 s−1 were performed and the transformation‐induced plasticity (TRIP effect) was investigated. The analysis of the transformation behaviour of the retained austenite reveals that the rate of martensite formation decreases with increasing strain rate. Furthermore, an increase of the offset yield stress and the compression strength with increasing strain rate together with a remarkable plasticity was detected.

A Study on the Microstructure of AISI M2 High Speed Steel Manufactured by Continuous Casting

Continuous casting has been widely applied in the production of steels and other metals. However, it has been rarely used in producing high speed steels, which are still manufactured by the conventional method of mould-casting. Thus, little is known about the microstructure of high speed steels made by the continuous casting technology. In the present work, AISI M2 steel is produced by horizontal continuous casting and the difference of solidification microstructure of ingots by different casting technologies has been examined. The results show that the networks of M2C eutectic carbides are greatly refined in the ingot by continuous casting compared to that by mould casting. Meanwhile, the morphology of M2C eutectic carbides changes from the plate-like type to the fibrous one, due to the increasing cooling rates. Compared with the plate-like M2C, the fibrous M2C in continuous casting ingots is less stable and decomposes faster at high temperatures, spheroidizing obviously after heating and refining dimensions of carbides.

Toughening of Fe-based laser-clad alloy coating

An investigation is reported on crack-free laser clad Fe-based alloy by use of biaxial powder feeding shielded with argon gas. The microstructure and phase structure of the coating were studied, and mechanical properties were analyzed through hardness, tension strength and wear resistance of the coating. Microstructure analysis showed that there was retained austenite with spherical particles distributed therein in the interdendritic and nearby grain boundary regions. The mechanical test results showed that net-like distributed retained austenite in the interdendritic region had certain toughening effect through blunting crack-tip. Under wear condition of high sliding speed and high loading, the wear resistance of the coating with net-like retained austenite was much higher than that of the coating with some discontinuous carbide network or carbide blocks. The results showed that toughening of laser clad Fe-based alloy with high hardness over 850 HV could be achieved by modifying interdendritic phases from net-like carbide to net-like austenite with spherical particles.

Influence of the morphologic evolution of the eutectic carbides at high temperature on the thermal expansion behavior of refractory cast alloys

Two alloys particularly rich in carbides, a nickel-based one reinforced by chromium carbides and a cobalt-based one strengthened by tantalum carbides were characterized by metallography and their hardness and thermal expansion behavior tested, after alloys had undergone a more or less long aging treatment at high temperature. Aging induces a progressive coarsening and/or fragmentation of these interdendritic carbides, with as first consequence a decrease in hardness. Also due to these morphology changes, aging the alloys on long time leads to a loss of effect of the carbides on the thermal expansion behavior of the alloys. The total thermal expansion, initially lowered by the rigid carbides network, is then free again. Indeed, the high temperature phenomenon of plastic or viscous-plastic deformation in compression of the matrix by the initial continuous carbides network has almost disappeared.