Cr White Iron Research Articles

Oxidation behavior of high Cr white iron composites reinforced with TiC and nonstoichiometric TiCx in air at 600 °C

High Cr white irons (HCWIs) are often used as high-temperature components exposed to the temperatures exceeding 500 °C. Improving oxidation resistance of HCWI components is necessary for engineering applications. In the present study, oxidation behaviors of high Cr white iron (HCWI) and its composites reinforced with 1–2 μm nonstoichiometric TiCx resulting from the decomposition of Ti3AlC2 (in situ TiCx/HCWI), and 2–4 μm irregular-shape of TiC (TiC/HCWI) at 600 °C for 96 h in air were investigated. Results showed that the two composites exhibited better oxidation resistance than the pure HCWI material. The oxidation test revealed a weight loss for the unreinforced HCWI but a weight gain for the composites. Among these materials, the in situ TiCx/HCWI composite has the best oxidation resistance due to its dense and fine microstructure. Oxidative thermodynamics and oxidation mechanism were discussed. This work suggests that in situ formed TiCx is an effective reinforcement to improve the oxidation resistance of HCWIs.

Effect of Solidification Variables on the Tensile Property of 2.8 wt% C–26 wt% Cr White Iron

The present study aimed to investigate the increasing solidification rate during directional solidification developed from in situ composites of M7C3/austenite eutectic, to in situ composites with a small portion of dendrites, and to partial in situ composites with equiaxed structures. M7C3 fibre aligned along the solidified direction in the in situ composites; however, its orientation and shape became irregular among the dendrites. In situ composite structure has higher tensile strength than partial in situ composite or equiaxed material. Crack initiation in the fibre occurred because the fibre could not accommodate the deformation of the matrix under tensile stress. The tensile fracture was caused by both crack initiations in the M7C3 fibre or at the randomly oriented particles, and the crack propagation to matrix.

Effect of Dendrite Fraction on the M23C6 Precipitation Behavior and the Mechanical Properties of High Cr White Irons

High Cr white irons with various fractions of primary dendrite have been prepared through the modification of their chemical composition. Increasing C and Cr contents decreased the primary dendrite fraction. Eutectic solidification occurred with the phase fraction ratio of austenite: M7C3 = 2.76:1. The measured primary dendrite fractions were similar to the calculated results. ThermoCalc calculation successfully predicted fractions of M7C3, austenite, and M23C6. Conventional heat treatment at high temperature caused a destabilization of austenite, releasing it’s solute elements to form M23C6 carbide. Precipitation of M23C6 during destabilization preferentially occurred within primary (austenite) dendrite, however, the precipitation scarcely occurred within austenite in eutectic phase. Thus, M23C6 precipitation by destabilization was relatively easy in alloys with a high fraction of primary dendrite.

Comparison of high Cr white iron composites reinforced with directly added TiC and in situ formed TiCx

Two kinds of high Cr white iron (HCWI) composites reinforced with directly added TiC (TiC/HCWI) and in situ formed TiCx (TiCx/HCWI) have been successfully synthesized by powder metallurgy method. The directly added TiC grains are irregular in shape with a mean size of about 2−4 μm, but the in situ formed TiCx grains from the decomposition of Ti3AlC2 are spherical and about 1–2 μm in size. The microstructure, mechanical properties and wear resistance of the two composites were compared. The in situ TiCx/HXCWI composite possesses finer microstructures and superior mechanical and wear resistant properties than TiC/HCWI composite. The phase composition and microstructure were characterized with X-ray diffraction analysis, scanning electron microscopy and transmission electron microscopy.

Microstructure, Mechanical and Abrasive Wear Behavior of 8.0 wt pct Cr White Iron Subjected to Continuous and Cyclic Annealing Treatment

Continuous annealing treatment (austenitization for 4 hours followed by furnace cooling) and cyclic annealing treatment (four cycles of austenitization, each of 0.66 hours duration followed by forced air cooling) of 8.0 wt pct Cr white iron samples are undertaken at 1173 K, 1223 K, 1273 K, 1323 K, and 1373 K (900 °C, 950 °C, 1000 °C, 1050 °C, and 1100 °C) as steps of destabilizing the as-cast structure. Continuous annealing results in precipitation of secondary carbides on a matrix containing mainly pearlite, while cyclic annealing treatment causes similar precipitation of secondary carbides on a matrix containing martensite plus retained austenite. On continuous annealing, the hardness falls below the as-cast value (HV 556), while after cyclic annealing treatment there is about 70 pct increase in hardness, i.e., up to HV 960. Decrease in hardness with increasing annealing temperature is quite common after both heat treatments. The as-cast notched impact toughness (4.0 J) is nearly doubled by increasing to 7.0 J after both continuous and cyclic annealing treatment at 1173 K and 1223 K (900 °C and 950 °C). Cyclic annealing treatment gives rise to a maximum notched impact toughness of 10.0 J at 1373 K (1100 °C). Abrasive wear resistance after continuous annealing treatment degrades exhibiting wear loss greater than that of the as-cast alloy. In contrast, samples with cyclic annealing treatment show reasonably good wear resistance, thereby superseding the wear performance of Ni-Hard IV.

Effects of tungsten on the microstructure and on the abrasive wear behavior of a high-chromium white iron

In the present study, systematic additions of tungsten (up to 10.3wt%) and their effects on the microstructure, hardness, microhardness, and abrasive wear of 17%-Cr white iron was analyzed. Six high-chromium iron alloys with different tungsten additions were melted in an open induction furnace and cast into sand molds to obtain 50-mm×25-mm cross-sectional bars. The alloys were characterized by optical and electronic microscopy, energy dispersive spectroscopy, and X-ray diffraction. The bulk hardness and microhardness of the different phases of the microstructure (matrix and carbides) were measured in the as-cast conditions and after a destabilization heat treatment at 950°C for 45min. Abrasive wear resistance tests were undertaken for the different irons according to the ASTM G65 standard in both as-cast and heat-treated conditions. The results show that, when tungsten is added up to 4wt%, it partitions either to the matrix or to the M7C3 carbide, causing a moderate strengthening in both phases and contributing to an increase in the overall hardness of the alloys. When tungsten additions are higher than 4%, the presence of harder M2C and M6C carbides is prevalent in the microstructure and the bulk hardness of the alloys increased. The wear behavior was found to be consistent with the hardness values; an increase in the wear resistance occurred as the tungsten additions were increased. However, such an increase in the wear resistance is not considerable (just 13% higher for 10.3% tungsten addition to the iron). Tungsten was found not to have an important effect on the secondary carbide precipitation during heat treatment, and the wear behavior had the same trend as that in the as-cast alloys. The results are discussed in terms of the partition of tungsten to the carbide and matrix and on its tendency to form harder M6C-type carbides.

Boron effect on the precipitation of secondary carbides during destabilization of a high-chromium white iron

This work analyses the secondary carbides precipitation during the destabilization of a 17% Cr White iron containing 195 ppm boron. The experimental iron was characterised in the as-cast conditions to have comparable parameters with the heat-treated samples. Destabilisation heat treatments were undertaken at temperatures of 825, 900 and 975 °C for 25 min; each sample was air-cooled after this soaking time. Characterisation was undertaken by optical and electronic microscopy, image analysis and energy dispersive spectroscopy microanalysis, hardness and microhardness were also evaluated. It was found that the volume fraction of secondary carbides precipitated is always higher for the lowest destabilisation temperature (825 °C) due to the lower carbon solubility in austenite at low temperatures. A much higher precipitation for the irons containing boron than that for the iron without boron at any destabilisation temperature was also noticed. For the iron containing boron, a density of 23 carbide particles per square micron was measured when destabilised at 825 °C, and it decreased to about 10 particles per square micron when destabilised at 975 °C. In the case of the alloy without boron additions, about 10 carbides per square micron were counted when destabilised at 825 °C and about 5 when destabilised at 975 °C. Higher volumes of carbides precipitation implies higher values of bulk hardness and microhardness in the alloys. The results suggest that boron works as a nucleant for the precipitation of secondary carbides; this is discussed in terms of the limited solubility of boron in iron and the formation of boron-rich precipitates found in the iron in the as-cast conditions.

Abrasive wear of V–Nb–Ti alloyed high-chromium white irons

The effect of 2% V, 2% Nb plus 2% Ti added to a 17% Cr white iron was studied. The experimental alloys were made in an induction furnace by using high-purity raw materials. Alloying elements were added at the end of the ironmaking process before pouring. Thin bars of 25×15mm2 cross section and 250mm length were solidified into sand molds. Several bars of alloyed and unalloyed white irons were obtained for the present study. The as-cast microstructure consisted of eutectic M7C3 carbides in a matrix of austenite for the unalloyed iron, while for the alloyed iron the structure was composed of austenite, eutectic M7C3 carbides and MC primary carbides. The alloys were heat treated at 900°C to destabilize the austenitic matrix; a stronger mainly martensitic matrix reinforced with secondary carbides resulted from such treatment. The as-cast and heat treated alloys were tested under abrasive wear by using a rubber wheel testing machine and SiO2 as abrasive particles of 0.3mm diameter. The wear tests were undertaken by placing the rubbing surfaces against each other with loads of 25, 40 and 54N and pouring the abrasive SiO2 particles between the surfaces. The results indicate a considerable better wear resistance for the alloyed heat treated iron compared with those as-cast and unalloyed irons. The responsible for such better wear behavior is the strengthening of the matrix by the MC carbides plus the precipitation of secondary M7C3 carbides and the partial transformation of matrix from austenite to martensite after heat treatment. The strengthening of the matrix provides better support to the eutectic carbides against cracking which in turn prevents the surface destabilization.

Phase Transformations and Microstructural Observations During Subcritical Heat Treatments of a High-Chromium Cast Iron

In this study, Cr white iron of 18.23 wt.% was subjected to a series of subcritical heat treatments. At both temperatures of 350 and 450 °C, no precipitation of secondary carbides was observed, and the overall microstructure resembles to that of the as-cast condition. At 550 °C, hardness values increased slightly compared to the as-cast values. No evidence of secondary carbide formation was observed. At 650 and 750 °C, extensive-to-complete transformation to pearlite-ferrite structures has occurred. Some evidence of secondary carbide precipitation especially for prolonged treatment periods was not adequate to obstruct the hardness decrease due to the dominating effect of pearlitic-ferritic formation. At 850 °C, secondary carbide precipitation and martensite formation lead to high hardness values.

Effect of Destabilization Heat Treatments on the Microstructure of High-Chromium Cast Iron: A Microscopy Examination Approach

A 18.22 wt.% Cr white iron has been subjected to various destabilization heat treatments. Destabilization at 800 °C caused gradual precipitation of M23C6 secondary carbide particles with time leading to a gradual increase in the bulk hardness. At 900, 1000, and 1100 °C, an initial sharp increase in bulk hardness with time occurred, reaching a plateau that was followed by a slightly decreasing trend. The combination of martensite formed, stoichiometry, and morphology of the secondary carbides present (mostly M7C3) are responsible for the obtained values of hardness. At 1100 °C, severe dissolution of the secondary carbides and consequent stabilization of the austenitic phase took place. Maximum hardness values were obtained for destabilization at 1000 °C. The correlation between bulk hardness and microstructural features was elaborated.

Erosion–corrosion assessment of materials for use in the resources industry

The erosion–corrosion properties of a range of ferrous-based materials that are currently being used or have potential for use in the resources industry have been assessed using a slurry pot erosion–corrosion (SPEC) test rig that has the capability of establishing the separate components of erosion, corrosion and synergy. Testing was performed, at 30 °C, in an aqueous slurry containing 35 wt% AFS 50–70 silica sand and a 3.5 wt% NaCl solution. Erosive action was supplied through high-speed rotation of a rubber-lined impeller. Erosion–corrosion performance of materials evaluated was related to composition/microstructure and hardness. Test data correlated with available service experience. The results showed that the cast Cr white irons with (i) a structure that was essentially a duplex stainless steel containing a distribution of hard carbides and (ii) a near eutectic Cr white iron exhibited the highest erosion–corrosion resistance of the materials tested. The evaluation of the Cr white irons also highlighted the influence of Cr and C levels on the E–C properties of these materials. E–C assessment of selected carbon steels confirmed that the erosion-only rates and synergistic levels showed a general decline with increasing carbon content and hardness. As expected, a low C steel pipe product displayed very mediocre erosion–corrosion behaviour as a consequence of its very low intrinsic corrosion resistance and inferior wear properties. This reflected service experience, however, such products are still being used, due to the comparatively low initial cost. A TiC particle-reinforced AISI 316 stainless steel exhibited an almost 45% improvement in the E–C resistance, when compared with an AISI 316L stainless steel.

Sliding abrasion resistance assessment of metallic materials for elevated temperature mineral processing conditions

The multiplicity of harsh environments in mining, processing and transporting ore and related waste, cause severe wear, extremely high maintenance costs and lost production. Elevated temperature processing is one of the conditions that influence the performance of possible materials of construction. This takes the forms of reduced hardness and strength, deleterious changes in the structure and properties of materials during protracted exposure and increased oxidation and corrosion. Drag chain conveying of hot solids e.g. in smelting, typically results in three-body sliding abrasion and adhesive wear of connecting pins and hole surfaces in link assemblies and of moving paddles that impel the particulates in enclosed channels. Selected materials have been assessed for this type of service under reciprocating sliding abrasion contact conditions using an adapted Cameron-Plint TE77 wear rig at 20 °C and 350 °C. These include the current carburised low alloy steel, other steels, Cr white irons and Co-based alloys in bulk, overlay and surface treated forms. Examination of wear scars, using scanning electron microscopy, identified the main wear mechanisms affecting the highly resistant powder metallurgical (PM) tool steels and HVOF coating as micro-scratching and as indentation leading to micro-fracture. Materials with lowest resistance displayed evidence of significant material removal by micro-ploughing. The formation of oxide layers on some samples during testing appeared to be beneficial.

Effect of boron on the sliding wear of directionally solidified high-chromium white irons

The effect of small amounts of boron (less than 195 ppm) added to a 16%Cr white iron were studied. Bars of 13 mm diameter were directionally solidified into hot ceramic moulds and heat was extracted from the bottom by a water cooled copper plate. The microstructure showed aligned eutectic carbides in the direction of the heat flow with a finer structure for the iron with the higher boron amount. The alloys were then heat treated at 950 °C to destabilize the austenitic matrix; a stronger mainly martensitic matrix reinforced with secondary carbides resulted from such a treatment. The as-cast and -heat treated alloys were tested under dry sliding for wear behavior by using three different loads (50, 150 and 250 N) and a constant sliding speed of 0.7 m/s. Wear resistance resulted higher for: (i) the heat treated irons since the martensitic matrix reinforced with secondary carbides better supported the eutectic carbide phase against fracture, (ii) the boron added irons due to the finer structure, and (iii) the irons whose eutectic carbides were aligned parallel to the worn surface, when the carbides were aligned perpendicular to the contact surface, more carbide fracture was observed which contributed to destabilize the surface.

Kinetics of Secondary Carbides Precipitation in a High-Chromium White Iron

This work analyzes the rate of secondary carbides precipitation during the destabilization heat treatment of a 17% Cr white iron. The experimental iron was characterized in the as-cast conditions to have comparable parameters with the heat treated samples. Destabilization heat treatments were undertaken at temperatures of 900, 1000, and 1150 °C for between 5 min and 8 h; each sample was water quenched immediately after being taken out of the furnace. Characterization was carried out by optical and electron microscopy, image analysis, and energy dispersive spectroscopy (EDS) microanalysis; hardness and microhardness were also evaluated. It was found that most of the secondary carbides that precipitate (between 2–30% of the matrix volume) precipitated in less than 2 h for the lowest destabilization temperature (900 °C). The secondary carbides volume fraction was found to increase for lower destabilization temperatures and large soaking times. A very low carbide precipitation along with a stabilization of the austenite phase occurred for heat treatments at 1150 °C. The results are discussed in terms of the solubility of chromium and carbon in the austenite phase at the different treatment temperatures.

Formation and Morphology of M<SUB>7</SUB>C<SUB>3</SUB> in Low Cr White Iron Alloyed With Mn and Cu

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

The effect of boron on the stereological characteristics of the structural phases present in the structure of the 13% Cr white iron

In this paper, the results of the examination of the effect of boron, in the amounts of 0.26, 0.39 and 0.59 wt%, on the stereological characteristics of the structural phases present in the structure of 13%Cr–2.3%C white iron, are reviewed. The examined stereological characteristics were: volume fraction of phases, dendrite arm spacing-DAS and size of eutectic carbides.

High resolution observations of friction-induced oxide and its interaction with the worn surface

A detailed transmission electron microscopy study of oxide and oxygen-containing phase formation during the sliding wear of metals, composites and coatings is provided. A wide range of different materials types are reported in order to compare and contrast their oxidational wear behaviour: a low carbon stainless steel, a H21 tool steel containing 7%TiC particles, a 17%Cr white iron, an Al–Si/30%SiC composite, an Al–alloy (6092)–15%Ni 3Al composite and finally a 3rd generation TiAlN/CrN ‘superhard’ multilayer coating. For the ferrous alloys, nanoscale oxides and oxygen-containing phases were formed that exhibited excellent adhesion to the substrate. In all cases, an increase in oxide coverage of the surface was associated with a decrease in Lancaster wear coefficient. The oxide at the surface of the 316L and H21+7%TiC was found to deform with the substrate, forming a mechanically mixed layer that enhanced surface wear resistance. Evidence of oxidational wear is presented for the wear of the Al–Si–30%SiC composite, but this did not give a beneficial effect in wear, a result of the brittle nature of the oxide that resulted in detachment of fine (∼150nm) thick fragments. The worn surface of the Al–alloy (6092)–15%Ni 3Al and TiAlN/CrN coating was characterized by reaction with the counterface and subsequent oxidation, the product of which enhanced wear resistance. The observations are related to the classical theory of oxidational wear.

The effect of boron on the amount and type of carbides in chromium white irons

The effect of boron, in the amounts of 0.26, 0.39, and 0.59 wt.%, on the volume fraction and structure of carbides in Cr white irons was examined. It was demonstrated that the addition of boron can change the microstructural characteristics of white iron containing about 13 wt.% Cr. With increasing boron content, the volume fractions of M3C carbide increase, but the volume fracton of M7C3 carbide remains unchanged. The addition of boron tends to produce hard borides and/or borocarbides. It also prevents the formation of pearlite, which results in alloys possessing good wear resistance.

The Effect of Mo on the Structure of High Cr-Mo Alloy Iron

According to many former studies, the function of Mo in high Cr white iron is to increase its hardenability. In this paper, the results show that, as the precipitation of high Cr-Mo white iron takes place, the Mo precipitates in the form of χ phase from the austenitic matrix prior to other carbides, which results in a change of precipitate and precipitation.

Fracture toughness of high-chromium white irons: Influence of cast structure

The fracture toughness of two high-chromium white iron alloys in the as-cast condition has been investigated. Fracture toughness test pieces were extracted at various orientations relative to the columnar macrostructure. The toughness of a 27 Cr white iron alloy was very sensitive to orientation, and the toughness was much larger when the crack propagated in a direction perpendicular to the long dimension of the eutectic carbides. The toughness of the 15-3 Cr-Mo white iron was insensitive to the orientation. The different fracture behaviour of the two alloys was related to the anisotropy of the eutectic carbide structures in the as-cast material. The limitations in applying quantitative data on the eutectic carbide structure to models of fracture toughness in white iron alloys were illustrated.