Effects of heat treatment on wear resistance and fracture toughness of duo-cast materials composed of high-chromium white cast iron and low-chromium steel

The objective of this study is to investigate effects of heat treatment on wear resistance and fracture toughness in duo-cast materials composed of a high-chromium white cast iron and a low-chromium steel as a wear-resistant part and a ductile part, respectively. Different size, volume fraction, and distribution of M₇C₃ carbides were employed in the wear-resistant part by changing the amount of chromium, and the volume fraction of martensite in the austenitic matrix was varied by the heat treatment. In the alloys containing a small amount of chromium, an interdendritic structure of eutectic M₇C₃ carbides was formed, and led to the improvement of wear resistance and fracture toughness. After the heat treatment, the selective wear of the matrix and the cracking or spalled-off carbides were considerably reduced since the hardness difference between carbides and matrix decreased by the increase in the matrix hardness, thereby leading to the improvement of the wear resistance. However, the fracture toughness of the heat-treated alloys was lower than that of the as-cast alloys because the matrix containing a considerable amount of martensite did not effectively prevent the crack propagation.     

Correlation of microstructure and fracture toughness in high-chromium white iron hardfacing alloys

A correlation is made of microstructure and fracture toughness in hypereutectic high-chromium white iron hardfacing alloys. In order to investigate the matrix effect of these alloys, in particular, four different matrices such as pearlite, austenite, and a mixture of pearlite and austenite were employed by changing the ratio of Mn/Si, while the total volume fraction of carbides was fixed. The hardfacing alloys were deposited twice on a mild steel plate by the self-shielding flux-cored arc-welding method. Fracture toughness was increased by increasing the volume fraction of austenite in the matrix, whereas hardness and abrasion resistance were nearly constant.In situ observation of the fracture process showed that cracks initiated at large primary carbides tended to be blocked at the austenitic matrix. This suggested that fracture toughness was controlled mainly by the amount of austenite in the matrix, thereby yielding the better toughness in the hardfacing alloy having the austenitic matrix. Considering both abrasion resistance and fracture toughness, therefore, the austenitic matrix was preferred for the high-chromium white iron hardfacing alloys.     

Raising powder steel wear resistance by electrocontact coating with white cast iron

A process has been developed for hardening powder steel by surface coating with white cast iron. The electrical and chemicothermal parameters have been determined theoretically. Formulas are derived that define the heating conditions and cooling rate. Structural features of the hardened layers have been examined. Electrocontact melting raises the resistance to wear in SP60 powder steel under conditions of dry friction and lubricated friction by a factor six by comparison with hardened high-speed steel.     

Correlation of microstructure with the wear resistance and fracture toughness of hardfacing alloys reinforced with complex carbides

A correlation was made of the microstructure, wear resistance, and fracture toughness of hardfacing alloys reinforced with complex carbides. The hardfacing alloys were deposited twice on a low-carbon steel substrate by a submerged arc welding (SAW) method. In order to investigate the effect of complex carbides, different fractions of complex carbide powders included inside hardfacing electrodes were employed. Microstructural analysis of the hardfaced layer showed that cuboidal carbides, in which a TiC carbide core was encircled by a WC carbide, and rod-type carbides, in which W and Ti were mixed, were homogeneously distributed in the bainitic matrix. In the surface layer hardfaced with FeWTiC powders, more complex carbides were formed, because of the efficient melting and solidification during hardfacing, than in the case of hardfacing with WTiC powders. As the volume fraction of complex carbides, particularly that of cuboidal carbides, increased, the hardness and wear resistance increased. In-situ observation of the fracture process showed that microcracks were initiated at complex carbides and that shear bands were formed between them, leading to ductile fracture. The hardness, wear resistance, and fracture toughness of the hardfacing alloys reinforced with complex carbides were improved in comparison with high-chromium white-iron hardfacing alloys, because of the homogeneous distribution of hard and fine complex carbides in the bainitic matrix.     

Optimizing fracture toughness and abrasion resistance in white cast irons

A series of twelve Cr-Mo white irons varying in carbide volume from 7 to 45 pct were tested for dynamic fracture toughness and wet sand abrasion resistance. Carbon content was varied from 1.4 to 3.9 pct. Two matrix microstructures were employed, and the compositions (copper and chromium content) were varied to assure constant matrix compositions. Chromium was varied from 11.6 to 25.7 pct. In addition, one composition of white iron was subjected to thirty different heat treatments to define the effect of matrix microstructure on dynamic fracture toughness and abrasion resistance. It was shown that for the abrasive wear system used, a carbide volume of about 30 pct represented an optimum quantity, above which abrasion resistance decreased. Martensitic irons provided consistently better abrasion resistance than austenitic irons. Dynamic fracture toughness decreased with carbide volume, as expected. Higher toughness values were obtained with predominantly austenitic matrix microstructures than with predominantly martensitic matrix microstructures. Considering both abrasion resistance and fracture toughness, it was shown that heat treated irons could provide an optimal combination of these properties. Formerly Visiting Research Metallurgist, Climax Molybdenum Co. Research Laboratory.     

Correlation of microstructure and fracture toughness in three high-speed steel rolls

The objective of this study is to clarify the fracture characteristics of high-speed steel (HSS) rolls in terms of microstructural factors such as matrix phase and primary carbide particles. Three HSS rolls with different chromium contents were fabricated by centrifugal casting, and the effect of the chromium addition was investigated through microstructural analysis, fracture-mechanism study, and toughness measurement. The hard and brittle primary carbides, as well as the eutectic carbides (ledeburites), were segregated in the intercellular regions and dominated overall properties. Observation of the fracture process revealed that these primary carbides cleaved first to form microcracks at low stress-intensity factor levels and that the microcracks then readily propagated along the intercellular networks. The addition of chromium to a certain level yielded microstructural modification, including the homogeneous distribution of primary carbides, thereby leading to enhancement of fracture toughness of the HSS rolls.     

Correlation of microstructure with hardness and wear resistance of VC/steel surface-alloyed materials fabricated by high-energy electron-beam irradiation

Correlation of microstructure with hardness and wear resistance of VC/carbon steel surface-alloyed materials fabricated by high-energy electron-beam irradiation was investigated. The mixtures of VC powders and flux (50 pct MgO-50 pct CaO or CaF₂) were deposited on a plain carbon steel substrate, and subsequently irradiated using a high-energy electron beam. The surface-alloyed layers of 1.2 to 3 mm in thickness were homogeneously formed without defects, and contained a large amount (about 10 vol pct) of VC precipitates in the bainitic or martensitic matrix. This microstructural modification including the formation of hard precipitates and hardened matrix in the surface-alloyed layers improved hardness and wear resistance. Particularly in the surface-alloyed material fabricated with the lower input energy density, the wear resistance was greatly enhanced over the steel substrate because of the increased size and volume fraction of VC particles, although the thickness of the surface-alloyed layer decreased. Microstructural modifications including melting, solidification, precipitation, and phase transformation of the surface-alloyed layer were also predicted from a thermal transfer modeling and a Fe-V-C ternary phase diagram. The predicted results were found consistent with those data from actual electron-beam irradiation and microstructural analysis.     

Thermodynamic aspects of the modification of low-chromium cast iron

Thermodynamic aspects of the modification of low-chromium (3–5% Cr) cast iron with Fe-Si-Mg alloy are considered. Such modification has an effect similar to that obtained on alloying: a phase transformation occurs in the structure of cast iron. Instead of a ledeburite eutectic with simple cementite Fe₃C and alloyed cementite (Fe, Cr)₃C, a eutectic with carbide (Cr, Fe)₇C₃ is formed. This carbide usually appears with at least 8–9% Cr in the cast iron. This effect may be attributed to the quasi-equilibrium state of the cast iron in treatment by a modifying agent. On the one hand, the melt is in a nonequilibrium state, because it contains many microzones with high Si and Mg content. On the other, the melt within the microzones may be regarded as in a quasi-equilibrium state. Within these microzones, on account of the high carbon activity, conditions are created for the formation of the carbide (Cr, Fe)₇Cr₃, as is confirmed by thermodynamic calculations. This effect permits marked improvement in the properties of cast iron, without significant expense.     

Correlation of the microstructure and fracture toughness of the heat-affected zones of an SA 508 steel

In this study, microstructures of a heat-affected zone (HAZ) of an SA 508 steel were identified by Mossbauer spectroscopy in conjunction with microscopic observations, and were correlated with fracture toughness. Specimens with the peak temperature raised to 1350 °C showed mostly martensite. With the peak temperature raised to 900 °C, the martensite fraction was reduced, while bainite or martensite islands were formed because of the slow cooling from the lower austenite region and the increase in the prior austenite grain size. As the martensite fraction present inside the HAZ increased, hardness and strength tended to increase, whereas fracture toughness decreased. The microstructures were not changed much from the base metal because of the minor tempering effect when it was raised to 650 °C or 700 °C. However, fracture toughness of the subcritical HAZ with the peak temperature raised to 650 °C to 700 °C was seriously reduced after postweld heat treatment (PWHT) because carbide particles were of primary importance in initiating voids. Thus, the most important microstructural factors affecting fracture toughness were the martensite fraction before PWHT and the carbide fraction after PWHT.     

The fracture toughness and toughening mechanisms of nickel-base wear materials

Nickel-base wear materials are typically used as weld hardfacing deposits, or as cast or hot isostatically pressed (HIP) inserts that provide the needed wear resistance to a base material with the desired mechanical properties. Most nickel-base wear materials contain high levels of chromium, silicon, carbon, and boron, which results in complex microstructures that are comprised of high volume fractions of silicide, carbide, and/or boride phases. The volume fraction of nickel-phase dendrite regions typically ranges from 40 to 70 pct, and these dendrite-phase particles are individually isolated by a matrix of silicide, carbide, and boride phases. The continuous matrix of brittle silicide, carbide, and boride phases results in a low damage tolerance for nickel-base wear materials, which is a concern in applications that involve high stresses, thermal transients, or shock loading. Fatigue crack growth (FCG) and fracture toughness (K _IC) testing in accordance with ASTM E399 methods has been used to quantify the damage tolerance of various nickel-base wear materials. Fractographic and microstructure examinations were used to define a generic toughening mechanism for nickel-base wear materials. The toughness of nickel-base wear materials is primarily controlled by the plastic deformation of the nickel-phase dendrites in the wake of a crack moving through the matrix of brittle silicide, carbide, and/or boride phases, i.e., crack bridging. Measured K _IC values are compared with calculated K _IC values based on the crack-bridging model. Microstructure examinations are used to define and confirm the important aspects of the crack-bridging model. This model can be used to predict the toughness values of nickel-base wear materials and direct processing methods to improve the K _IC values.     

Characterization and corrosion behavior of high-chromium white cast irons

Erosion-resistant high-chromium white cast irons (CWIs) are widely used in hydrotransport components, particularly in oil-sand operations. Due to the acceptance that corrosion processes can accentuate material degradation by erosion processes andvice versa, it is important to understand the corrosion resistance of these materials in the environments in which they are used. Three CWI alloys with different chemical compositions—chromium (26 to 40 wt pct) and carbon (2.5 to 4.3 wt pct)—were investigated in this study. Electrochemical DC potentiodynamic polarization and potentiostatic tests were carried out in a solution of 1000 ppm Cl⁻ at a pH of 8.5 (obtained by adding NaOH) that simulates a recycle cooling water. A detailed characterization of the microstructures was also performed. There are significant effects of microstructural features and alloy composition on the corrosion behavior of CWIs. Two key factors have been shown to determine the corrosion behavior: the primary carbide area fraction and the amount of chromium as well as other elements in the matrix. The corrosion resistance of the CWI alloys strongly depends on the ratio of chromium content in the M₇C₃ carbide to that in the matrix (Cr_M7C3/Cr_matrix).     

Evaluation of static and dynamic fracture toughness in ductile cast iron

Crack extension behavior and fracture toughness of ductile cast iron were examined by three-point bend tests, where various detection methods of crack initiation under static and dynamic loading conditions were adopted. Loading on the specimens was interrupted at various displacement points, and the final fracture surfaces of the specimen were observed via scanning electron microscopy (SEM). Crack-tip opening displacement (CTOD) obtained under the dynamic loading condition was smaller than that under the static loading condition in ferritic ductile cast iron, and CTOD additionally decreased with increasing pearlite content in the matrix. The relationship between J (ΔC) obtained by the compliance changing rate method and J(R) established by the intersection of the crack extension resistance curve and the theoretical blunting line varied with pearlite content. The average value of .J(ΔC) and J(R), that is J (mid), was proposed to define the fracture toughness of ductile cast iron; J (mid) was considered to be a reasonable measure for the fracture toughness of ductile cast iron, irrespective of loading condition and the pearlite content in the matrix.     

The influence of heat treatment on the high-stress abrasion resistance and fracture toughness of alloy white cast irons

The influence of a range of austenitizing and subcritical (tempering) heat treatments on the high-stress abrasion resistance and fracture toughness of four commercially significant grades of alloy white cast iron was investigated. Complementing an earlier study^[1] on the influence of a more limited range of heat treatments on the gouging abrasion performance of the same alloys, the results showed that the effect of austenitizing temperature on high-stress abrasion pin test weight loss differed for each alloy. With increasing austenitizing temperature, these results ranged from a substantial improvement in wear performance and retention of hardness through to vir-tually no change in wear performance and substantial falls in hardness. Fracture toughness, however, increased markedly in all alloys with increasing austenitizing temperature. Tempering treatments in the range 400 °C to 600 °C, following hardening at the austenitizing temperature used commonly in industrial practice for each alloy, produced significant changes in both hard-ness and wear performance, but negligible changes in fracture toughness. Most importantly, the data showed that selection of the correct temperature for subcritical heat treatment to reduce the retained austenite content for applications involving repeated impact loading is critical if abrasion resistance is not to suffer.     

Effects of alloying elements on microstructure,hardness, and fracture toughness of centrifugally cast high-speed steel rolls

A study was made of the effects of alloying elements on the microstructure, hardness, and fracture toughness of centrifugally cast high-speed steel (HSS) rolls. Particular emphasis was placed on the role of hard carbides located along solidification cell boundaries and the type of the tempered martensitic matrix. Microstructural observation, X-ray diffraction analysis, hardness and fracture-toughness measurements, and fractographic observations were conducted on the rolls. The constitution and morphology of carbides observed within the intercellular boundaries varied depending upon the predominant alloying elements that comprised them. These massive carbide formations strongly influenced the bulk material hardness and fracture toughness due to their high hardness. The effects of alloying elements were analyzed on the basis of the liquidus-surface diagram which and indicated that the proper contents of the carbon equivalent (CE), tungsten equivalent, and vanadium were 1.9 to 2.0, 10 to 11, and 5 to 6 pct, respectively. The roll material, containing a small amount of intercellular carbides and lath-type tempered martensitic matrix, had excellent fracture toughness, since carbides were well spaced. Therefore, it was suggested that the optimization of alloying elements was required to achieve the homogeneous distribution of carbides.     

Correlation of microstructure and fracture toughness in two 4340 steels

A correlation is made of plane strain fracture toughness and microstructure in two steels corresponding to AISI 4340 composition. The steels were deoxidized with aluminum and titanium-aluminum additions, respectively. In the case of the aluminum killed steel, austenitizing at temperatures above 950 °C led to large austenite grain sizes, whereas in the titanium steel grain sizes were maintained below about 70 μm even after austenitizing at temperatures approaching 1200 °C. This allowed a comparison of variations in plane strain fracture toughness with austenitizing temperature between microstructures that underwent large increases in grain size and those that did not. The results are interpreted using a simple fracture model which indicated that particle spacing is of primary importance in controlling toughness. The overall observed phenomenology, however, is not explainable using simple models that essentially require that either critical stresses or critical strains be achieved over distances scaling with microstructure. This finding suggests that more detailed crack tip models than presently exist are required if the full effects of heat treatment are to be understood and explained. Formerly Graduate Student at Brown University     

Influence of microstructure on fracture toughness of austempered ductile iron

An investigation was carried out to examine the influence of microstructure on the plane strain fracture toughness of austempered ductile iron. Austempered ductile iron (ADI) alloyed with nickel, copper, and molybdenum was austenitized and subsequently austempered over a range of temperatures to produce different microstructures. The microstructures were characterized through optical microscopy and X-ray diffraction. Plane strain fracture toughness of all these materials was determined and was correlated with the microstructure. The results of the present investigation indicate that the lower bainitic microstructure results in higher fracture toughness than upper bainitic microstructure. Both volume fraction of retained austenite and its carbon content influence the fracture toughness. The retained austenite content of 25 vol pct was found to provide the optimum fracture toughness. It was further concluded that the carbon content of the retained austenite should be as high as possible to improve fracture toughness.     

脉冲电流参数对过共晶高铬铸铁近液相线熔化后凝固组织的影响

对过共晶高铬铸铁近液相线溶化后的凝固过程进行脉冲电流处理,研究了脉冲电流参数（脉冲电压和频率）对凝固组织的影响.发现脉冲频率和电压的增大都有助于过共晶高铬铸铁凝固组织中碳化物的细化和粒化.脉冲频率的增大促进大量细小颗粒状初生碳化物的析出,离异共晶现象变得更加明显,层片状共晶碳化物减少.脉冲电压的增加促进初生碳化物细化和粒化,片状共晶碳化物变短.过高的电压也促进离异共晶,导致更多的初生碳化物的形成.     

Correlation of fracture toughness with the intergranular failure fraction in powder iron specimens

Powder Metallurgy and Metal Ceramics -     

Influence of matrix structure on the abrasion wear resistance and toughness of a hot isostatic pressed white iron matrix composite

The influence of the matrix structure on the mechanical properties of a hot isostatic pressed (hipped) white iron matrix composite containing 10 vol pct TiC is investigated. The matrix structure was systematically varied by heat treating at different austenitizing temperatures. Various subsequent treatments were also employed. It was found that an austenitizing treatment at higher temperatures increases the hardness, wear resistance, and impact toughness of the composite. Although after every different heat treatment procedure the matrix structure of the composite was predominantly martensitic, with very low contents of retained austenite, some other microstructural features affected the mechanical properties to a great extent. Abrasion resistance and hardness increased with the austenitizing temperature because of the higher carbon content in martensite in the structure of the composite. Optimum impact energy values were obtained with structures containing a low amount of M (M₇C₃+M₂₃C₆) carbides in combination with a decreased carbon content martensite. Structure austenitized at higher temperatures showed the best tempering response. A refrigerating treatment was proven beneficial after austenitizing the composite at the lower temperature. The greatest portion in the increased martensitic transformation in comparison to the unreinforced alloy, which was observed particularly after austenitizing the composite at higher temperatures,^[1] was confirmed to be mechanically induced. The tempering cycle might have caused some additional chemically induced transformation. The newly examined iron-based composite was found to have higher wear resistance than the most abrasion-resistant ferroalloy material (white cast iron).