Microstructure and mechanical properties of high boron white cast iron with about 4 wt% chromium

The microstructure and mechanical properties of high boron white cast irons with about 4 wt% chromium before and after treating with rare earth magnesium alloy were studied in this article. The experimental results indicate that the cast irons comprise a dendritic matrix and interdendritic eutectic borides M₂B and M′_0.9Cr_1.1B_0.9 that distributed in the form of continuous network in as-cast condition. The matrix is made up of fine pearlite in the alloys with and without modification, but the grain size of the matrix is decreased greatly after modification. After water quenching at 1,303 K and tempering at 473 K, the matrix of the alloy mostly changes to lath-type martensite. For the alloy without modification the boride morphology remains almost unchanged after heat treatment. And a secondary precipitation of M₂₃(C,B)₆ compound appears in the central region of dentritic matrix grains. The morphology of the eutectic borides is changed to the form of isolated blocks after heat treatment and there is only little intragranular M₂₃(B,C)₆ particles in the matrix are found in the alloy modified with rare earth magnesium alloy. The modification by rare earth magnesium alloy can refine the primary austenite and the eutectic borides. Combined with a high austenitizing temperature the modification can improve the morphology of the borides which results in the improvement of toughness and tensile strength.     

Secondary carbide precipitation in an 18 wt%Cr-1 wt% Mo white iron

High chromium (18%) white irons solidify with a substantially austenitic matrix supersaturated with chromium and carbon. The austenite is destabilized by a hightemperature heat treatment which precipitates chromium-rich secondary carbides. In the as-cast condition the eutectic M₇Ca₃ carbides are surrounded by a thin layer of martensite and in some instances an adjacent thicker layer of lath martensite. The initial secondary carbide precipitation occurs on sub-grain boundaries during cooling of the as-cast alloy. After a short time (0.25 h) at the destabilization temperature of 1273 K, cuboidal M₂₃C₆ precipitates within the austenite matrix with the cube-cube orientation relationship. After the normal period of 4 h at 1273 K, there is a mixture of M₂₃C₆ and M₇C₃ secondary carbides and the austenite is sufficiently depleted in chromium and carbon to transform substantially to martensite on cooling to room temperature.     

Development of microstructure during heat treatment of high carbon Cr–Mo steel

Abstract

Stainless steels containing enhanced chromium and carbon contents are particularly attractive for applications requiring improved wear and corrosion resistance. The as cast microstructure of such steels is composed mainly of ferritic matrix along with a network of interdendritic primary carbides. It has been shown that heat treatment of these steels results in microstructures that contain more than one type of carbide. A selective dissolution technique has been employed to isolate carbides from the matrix. Scanning electron microscope and X-ray diffraction studies of the as cast steels have shown that the primary carbides are essentially of M₇C₃ type, whereas in heat treated specimens both M₇C₃ (primary) and M₂₃C₆ (secondary) type carbides have been observed. The relative amounts of these carbides are found to be dependent on the heat treatment temperature. In addition, nucleation of austenite occurs above 950°C and at ～1250°C the matrix transforms entirely to austenite, which is retained completely on quenching to room temperature.     

Microstructural characteristics of gas tungsten arc synthesised Fe-Cr-Si-C coating

Abstract

The gas tungsten arc (GTA) method was used to synthesise Fe-Cr-Si-C alloy coatings, and processing effects on the coating were investigated experimentally. Coatings were developed on an AISI type 1040 steel substrate. Four different regions were obtained in the surface coating; and in these regions either a hypoeutectic or a hypereutectic microstructure was found. The hypoeutectic microstructure consisted of primary dendrites of austenite (γ) phase and eutectic M₇C₃ (M=Cr,Fe) carbides. On the other hand, the hypereutectic microstructure consisted of M₇C₃ primary carbides and eutectic. A hypoeutectic or hypereutectic microstructure was determined by the combination of particularly carbon concentration, solidification rate, and extent of substrate melting. The higher hardness of the hypereutectic microstructure is attributed especially to the formation of M₇C₃ primary carbides. The lower hardness of the hypoeutectic microstructure is related to three effective parameters: first, the presence of γ phase in the primary dendrites; second, excessive dilution from the base material; and third, relatively low concentrations of chromium and carbon.     

烧结Cr15高铬铸铁组织与性能的研究

为研发耐磨性能优良、成本相对低廉的高铬铸铁,本文分别以亚共晶、过共晶的水雾化Cr15高铬铸铁粉末为原料,采用超固相线液相烧结工艺制备了烧结高铬铸铁(SHCCI),并对其显微组织、力学性能和冲击磨粒磨损工况下的耐磨性能进行对比研究。结果表明,烧结高铬铸铁主要由M7C3碳化物、马氏体和奥氏体组成;在亚共晶烧结高铬铸铁中,通过电解腐蚀萃取的M7C3碳化物三维形貌呈珊瑚状,沿晶界均匀分布,材料抗冲击耐磨性能优良;在过共晶烧结高铬铸铁中,优先形成的初生碳化物可能成为共晶碳化物的生长基底,形成核-壳结构的M7C3碳化物,沿晶界相互连接呈网状,严重割裂基体。亚共晶、过共晶烧结高铬铸铁的力学性能分别为:硬度HRC63.9、HRC64.3,冲击韧性7.92、3.04 J/cm^2,抗弯强度2112.65、1624.87 MPa。     

Carbide transformation in carburised zone of 25Cr35NiNb+MA alloy after high-temperature service

Microstructure evolution due to carburising of a 25Cr35NiNb+MA ethylene pyrolysis furnace tube was investigated after service for approximately 4 years. The microstructure of 25Cr35NiNb+MA alloy was examined by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) equipped with energy-dispersive X-ray spectroscopy and high-resolution transmission electron microscopy (TEM). The results revealed that M₂₃C₆ carbides of the 25Cr35NiNb+MA alloy had coarsened and transformed to M₇C₃ carbides with a heavily faulted structure during carburising. The content of M₇C₃ carbide increased closer to the inner wall. A large number of dislocations had been observed surrounding the carbides and some formed dislocation walls. The carbide transformation mechanism and the effect of the dislocation walls are discussed.     

The morphology and microtexture of M7C3 carbides in Fe-Cr-C and Fe-Cr-C-Si alloys of near eutectic composition

The microtexture of M₇C₃ carbides in undercooled 40 g samples of hyper- and hypo-eutectic Fe-Cr-C alloys was determined by electron back scatter diffraction. In the hyper-eutectic alloy the carbides were monocrystalline, while those in the hypo-eutectic alloy were polycrystalline. While in the former the preferred growth direction of the M₇C₃ carbides was [0001], in the hypo-eutectic alloy there was a relatively weak texture near [10¯11]. There was no evidence for the presence of growth twins in either the M₇C₃ carbide rods or in the branching mechanism in the joint between the carbide rods of the hypo-eutectic sample. The morphologies of the M₇C₃ carbides resulting from undercooling were used to explain the microstructure of hardfacing Fe-Cr-C weld deposits applied by the manual metal arc process. The effect of silicon additions on the morphology of M₇C₃ carbides in Fe-Cr-C-Si alloys is explained in terms of the effect of silicon on undercooling.     

Influence of carbon content on microstructure and tempering behaviour of 2 1/4 Cr 1 Mo steel

Transmission electron microscopic studies aimed at elucidating the effect of carbon level on the tempering behaviour of 2 1/4 Cr 1 Mo steels have been carried out. Specimens with two different carbon levels (0.06% and 0.11 %) were cooled in flowing argon gas (AC) from an austenitization temperature of 1323 K and tempered at 823, 923 and 1023 K for times ranging from 2 to 50 h. The tempering behaviour at these temperatures for the two carbon levels is found to differ in the nature of secondary hardening at lower temperatures, variation in the time to peak hardness and the saturation level of hardness at long tempering times. Based on a detailed study, using analytical electron microscopy, on the morphology, crystallography and microchemistry of secondary phases, the factors governing the observed variations in tempering behaviour are related to the difference in the dissolution rate of bainite, nucleation of acicular M₂C carbides and transformation rate of primary carbides into secondary alloy carbides. The carbides which promote softening were identified as M₇C₃, M₂₃C₆ and M₆C, whereas hardening is mainly imparted by M₂C.     

Effect of long-term service exposure on microstructure and mechanical properties of Alloy 617

The present work was carried out to investigate the effect of long-term service exposure on microstructure and mechanical properties of a gas turbine hot gas path component, made of Alloy 617. The results showed significant service-induced microstructural changes, such as excessive grain boundary Cr-rich M₂₃C₆ carbides formation and some oxidation features in the exposed material in compare with the solution-annealed material. Also it was found that the yield strength and hardness of the alloy have increased while the ductility of the alloy has decreased. In the similar test conditions, the stress-rupture life of the exposed alloy decreased considerably compared to the solution-annealed sample, which could be attributed to the microstructural degradation, especially formation of continuous M₂₃C₆ carbides on grain boundaries.     

Thermal-induced evolution of secondary phases in Cr–Mo–V low alloy steels

Four low alloy steels with different contents of molybdenum and vanadium were investigated. The steels were annealed at 773, 793, 853, 873, 933, 973, and 993 K for 500, 1000, 3000, and 10000 h. Techniques of transmission electron microscopy and thermodynamic calculations (ThermoCalc) were used to characterise influence of the steel bulk composition and the annealing conditions on evolution of carbides M₃C, M₂C, M₇C₃, M₂₃C₆, M₆C, and MC (M=metallic element). Changes in structure types and metal compositions of the carbides were characterised in detail. The work was done with the intention to obtain more information about the secondary phase evolution in low alloy steels used in energy industries.     

Microstructure evolution of HP40-Nb alloys during aging under air at 1000 °C

Two as-cast HP 40 alloys provided by different manufacturers were aged at 1000 °C under laboratory air. They had the same as-cast microstructure consisting of austenite dendrites delineated by a network of eutectic Nb-rich MC and Cr-rich M₇C₃ carbides. After aging for several months, they showed similar microstructures in the bulk materials, though M₇C₃ carbides have been replaced by M₂₃C₆ carbides. As expected, a sub-surface zone depleted in chromium has appeared where a tetragonal CrNbC could be identified in both materials. However, the composition of the transition zones between the surface and the bulk materials differed, mainly because one of the materials underwent significant nitrogen pick-up with associated precipitation of M₆(C,N) and M₂(C,N) phases. On the contrary, the other alloy did show only one intermediate zone with a mix of CrNbC, M₂₃C₆ and MC carbides. A full account of the microstructures observed in the aged materials is given.     

Creep resistance of Fe–Ni–Cr heat resistant alloys for reformer tube applications

Relations between microstructure, phase transformations and creep resistance of austenitic Fe–Ni–Cr alloys are investigated. As-cast alloys with different silicon contents and an ex-service tube are submitted to laboratory agings to trigger specific phase transformations, and subsequently creep-tested at 950°C under stresses of 24–48?MPa. As-cast microstructures contain interdendritic chromium-rich M₇C₃ carbides with niobium-rich MC carbides. After aging at 950°C, primary M₇C₃ carbides transform into chromium-rich M₂₃C₆ carbides, associated to a loss in creep strength. The G phase present in the ex-service alloy is reversed into MC carbides by a heat treatment at 1100°C, associated to a slight decrease in creep resistance. Besides, the addition of silicon is highly detrimental to creep strength. Results can be used for alloy design.     

Non-equilibrium synthesis of Fe-Cr-C-W alloy by laser cladding

Synthesis of Fe-Cr-C-W alloy using the laser cladding technique offered an opportunity to produce a novel wear-resistant material with fine and uniform microstructure. Use of preheating during laser cladding Fe-Cr-C-W provided crack-free clads. The preheating temperature was very critical to eliminate cracks in the clad. Different complex types of carbide were observed in this research. Overall laser process parameters such as power density or specific energy as well as preheating temperature affected the characteristics of the carbide precipitates in the matrix. The increase of solid solubility and high cooling rate resulted in good metallurgical characteristics. Mostly M₆C or M₂₃C₆ type carbides were observed. Usually diamond-shaped M₆C carbides showed good tribological characteristics. In general, increasing the power density brought an increase of average hardness, while decreasing the power density brought a decrease of wear scar width. The laser-clad Fe-Cr-C-W alloy showed better wear properties than laser-clad Fe-Cr-Mn-C and several times smaller scar width as compared to Stellite 6 hard-facing during line-contact wear testing.     

Fe-Cr-C hardfacing alloys for high-temperature applications

The microstructure and phase chemistry of a Fe-34Cr-4.5C wt% hardfacing alloy has been investigated using transmission electron microscopy and microanalytical techniques. The microstructure is found to consist of large primary M₇C₃. carbides in a eutectic mixture of austenite and more M₇C₃. The results indicate that the microstructure of the undiluted alloy becomes configurationally frozen at a temperature of about 1150° C during deposition by the manual metal arc welding technique. This allows the metastable austenite phase to contain a large chromium concentration ( 16 to 17 wt %), thus imparting good corrosion and oxidation resistance. Experimental data on the partitioning of chromium, manganese and silicon between the carbide phases are discussed in the context of the high-temperature stability of the alloy.     

Failure of Alloy 625 tube stub ends – Effect of primary nitrides

This paper reports a failure analysis of Alloy 625 stub ends of ammonia cracker tubes that failed during operation after 47,000 h of service operation. The failure occurred due to the formation of M₂₃C₆ carbides at grain boundaries, which made them very weak and brittle. However, the formation of M₂₃C₆ carbides at grain boundaries was surprising since they formed at temperatures around 550 °C, which is much below their expected temperature of formation and occurred in a period less than half the designed life. Precipitation of the grain boundary carbides has been attributed to the presence of primary nitride particles in the present alloy instead of primary carbides which are usually observed in these alloys. Formation of nitrides consumed Ti that binds C in the form of primary MC carbides in these alloys. This left free carbon in the alloy matrix for easy formation of M₂₃C₆ carbides which otherwise form due to degeneration of primary MC carbides.     

Effect of thermal exposure on the stability of carbides in Ni-Cr-W based superalloy

The microstructure evolutions of Ni-Cr-W based superalloy during thermal exposure have been investigated systematically. M₆C carbides in the alloy decompose into M₂₃C₆ carbides at temperatures from 650 to 1000 °C due to its high content of Cr. The M₆C carbides decompose dramatically from 800 to 900 °C. At temperatures up to 1000 °C, a few M₂₃C₆ carbides form on the surface of M₆C carbides. The decomposition behavior of primary M₆C can be explained by the following reaction: M₆C → M₂₃C₆ + Me (W, Ni, Cr, Mo). At temperatures below 900 °C, coarse lamellar M₂₃C₆ carbides precipitate at the grain boundaries. The carbide lamellae line almost perpendicular to the grain boundaries. While the temperature is above 1000 °C, discrete M₂₃C₆ carbides precipitate at the grain boundaries. Moreover, there are lots of small M₂₃C₆ particles precipitated around M₆C carbides from 650 to 1000 °C.     

A microtexture study of eutectic carbides in white cast irons using electron back-scatter diffraction

White cast irons have a microstructure composed of a ferrous matrix with hexagonal and/or orthorhombic carbides, M₇C₃ and M₃C, respectively. The US Bureau of Mines has implemented a study programme to improve microstructure and thereby wear resistance of these alloys. This work included electron back-scatter diffraction in a scanning electron microscope to identify the hexagonal and orthorhombic phases from their crystallography and subsequently establish their microtextures. Crystal structures of these carbides were identified and their microtextures were obtained from specific spatial locations within micrometre resolution. Such resolution allowed the identification of thin shells of M₃C (1–5 m thick) to be distinguished from cores of M₇C₃ (10–20 m thick). Microtexture results showed that M₇C₃ carbides without M₃C shells grew predominantly in the [0 0 0 1] direction, whereas those with M₃C shells were much less textured with the M₃C shell phase having a random microtexture.     

Role of preheating and specific energy input on the evolution of microstructure and wear properties of laser clad Fe-Cr-C-W alloys

Synthesis of Fe-Cr-C-W alloy (10 : 4 : 1 : 1, wt(%)) was carried out on AISI 1016 steel substrate using laser cladding technique which lead to the development of a suitable alternate for cobalt bearing wear resistant alloys. This study involved understanding of process variables like preheating temperature and specific energy input on the evolution of microstructures and their effect on wear resistance properties. The microstructure was examined with a scanning electron microscope and various types of complex carbides were identified using both energy dispersive x-ray and auger spectroscopy facilities. A combination of MC, M₇C₃ and M₆C types of carbides of certain proportions (formed at a preheating temperature of 484°C with specific energy input of 9.447 KJ/cm²) has been found to be most attractive for achieving an optimum combination of microhardness and steady state friction coefficient values. A similar advantage may be derived at a lower level of specific energy input of 8.995 KJ/cm² but with a higher preheating temperature of 694°C. However, increasing the specific energy input to 12.376 KJ/cm² can significantly soften the matrix.     

The influence of vanadium on fracture toughness and abrasion resistance in high chromium white cast irons

The influence of vanadium on wear resistance under low-stress conditions and on the dynamic fracture toughness of high chromium white cast iron was examined in both the ascast condition and after heat treatment at 500 °C. A vanadium content varying from 0.12 to 4.73% was added to a basic Fe-C-Cr alloy containing 2.9 or 19% Cr. By increasing the content of vanadium in the alloy, the structure became finer, i.e. the spacing between austenite dendrite arms and the size of massive M₇C₃ carbides was reduced. The distance between carbide particles was also reduced, while the volume fraction of eutectic M₇C₃ and V₆C₅ carbides increased. The morphology of eutectic colonies also changed. In addition, the amount of very fine M₂₃C₆ carbide particles precipitated in austenite and the degree of martensitic transformation depended on the content of vanadium in the alloy. Because this strong carbide-forming element changed the microstructure characteristics of high chromium white iron, it was expected to influence wear resistance and fracture toughness. By adding 1.19% vanadium, toughness was expected to improve by approximately 20% and wear resistance by 10%. The higher fracture toughness was attributed to strain-induced strengthening during fracture, and thereby an additional increment of energy, since very fine secondary carbide particles were present in a mainly austenitic matrix. An Fe-C-Cr-V alloy containing 3.28% V showed the highest abrasion resistance, 27% higher than a basic Fe-C-Cr alloy. A higher carbide phase volume fraction, a finer and more uniform structure, a smaller distance between M₇C₃ carbide particles and a change in the morphology of eutectic colonies were primarily responsible for improving wear resistance.     

Influence of secondary carbides precipitation and transformation on the secondary hardening of laser melted high chromium steel

The influence of secondary carbides precipitation and transformation on the secondary hardening of laser melted high chromium steels was analyzed by means of scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The microstructure of laser melted high chromium steel is composed of austenite with supersaturated carbon and alloy elements and granular interdendritic carbides of type M₂₃C₆. Secondary hardening of the laser melted layer begins at 450 °C after tempering, and the hardness reaches a peak of 672HV at 560 °C and then decreases gradually. After tempering at 560 °C, a large amount of lamellar martensite was formed in the laser melted layer with a small quantity of thin lamellar M₃C cementite due to the martensitic decomposition. The stripy carbides precipitating at the grain boundaries were determined to be complex hexagonal M₇C₃ carbides and face centered cubic M₂₃C₆ carbides. In addition, the granular M₂₃C₆ carbides and fine rod-like shaped M₇C₃ carbides coexisted within the dendrites. As a result, the combined effects of martensitic transformation, ultrafine carbide precipitations, and dislocation strengthening result in the secondary hardening of the laser melted layer when the samples were tempered at 560 °C.