High Cr White Iron Research Articles

Effects of Chemical Composition and Solidification Rate on the Solidification Behavior of High-Cr White Irons

The effects of chemical composition and solidification rate on the solidification behavior of high-Cr white irons were investigated through directional solidification. Increasing the solidification rate in hypoeutectic alloys caused finer dendrite-arm spacing, as expected. The eutectic structure, which formed in the interdendritic region, was comprised of M7C3 and austenite; however, secondary dendrite arms of hypoeutectic alloys contained a few M7C3 particles that solidified prior to the eutectic structure. The transition from cellular to dendritic solidification occurred at a solidification rate between 50 µm/s and 100 µm/s in a near-eutectic alloy. In the near-eutectic alloy with cellular solidification, a directionally arrayed in-situ composite of M7C3/austenite formed within the cell. Speckle-like features appeared in the intercellular region due to M23C6 carbide precipitation during subsequent cooling after freezing. Like dendrite-arm spacing in hypoeutectic alloys, the inter-speckle spacing and the inter-fiber spacing became finer with an increasing solidification rate in the cellular solidification range.

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.

Effects of Primarily Solidified Dendrite and Thermal Treatments on the M23C6 Precipitation Behavior of High-Chromium White Iron

The precipitation behavior of M23C6 carbide during thermal treatment of high-Cr white iron with various fractions of primarily solidified dendrite was studied and reviewed. M23C6 precipitation in the primarily solidified dendrite occurred preferentially during conventional heat treatment, whereas it occurred scarcely in the eutectic austenite. The reaction between M7C3 and austenite caused the dissolution of M7C3 into austenite, followed by precipitation of M23C6 along the periphery of eutectic M7C3. Relatively low-temperature thermal treatment (modified heat treatment) led to precipitation of M23C6 particles in the eutectic austenite, which is presumed to be caused by solubility difference depending on temperature.

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.

Developing a M6C-Reinforced High-Cr White Iron for Abrasive Wear Application

Fast removal of soft phases (e.g., pearlite and ferrite) in the iron matrix limits the wear life of high-Cr white irons. To address this shortcoming, the authors successfully produced fine networks of M6C carbide in a high-Cr white iron through extensive thermodynamic calculations. Fishbone-like networks of M6C carbides were observed with an optical microscope. It was experimentally determined that such carbide networks protected the soft matrix and increased the overall hardness. Additionally, electron backscattered diffraction was conducted, which showed that the alloy contained phases of M7C3, M6C, ferrite, and retained austenite. Solidification sequence was determined by correlating the thermodynamic equilibrium calculation results with the size and distribution of each phase. A dry sand/rubber wheel apparatus following ASTM standard G65 Procedure A was utilized to assess the abrasive wear performance of the developed alloy. Results showed that the volume loss of the developed material was 25 pct less than that of conventional high-Cr white irons. Wear scars were investigated using a scanning electron microscope, and the improved wear resistance was attributed to the “buffer” effect and plastic deformation of the introduced M6C carbide networks.

Vanadium Additions to a High-Cr White Iron and its Effects on the Abrasive Wear Behavior.

AbstractFrom the present study, vanadium additions up to 6.4% were added to a 14%Cr-3%C white iron, and the effect on the microstructure, hardness and abrasive wear were analysed. The experimental irons were melted in an open induction furnace and cast into sand moulds to obtain bars of 18, 25, and 37 mm thickness. The alloys were characterized by optical and electronic microscopy, and X-ray diffraction. Bulk hardness was measured in the as-cast conditions and after a destabilization heat treatment at 900°C for 45 min. Abrasive wear resistance tests were undertaken for the different irons according to the ASTM G65 standard in both as-cast and heat-treated conditions under a load of 60 N for 1500 m. The results show that, vanadium additions caused a decrease in the carbon content in the alloy and that some carbon is also consumed by forming primary vanadium carbides; thus, decreasing the eutectic M7C3 carbide volume fraction (CVF) from 30% for the base iron to 20% for the iron with 6.4%V;but overall CVF content (M7C3 + VC) is constant at 30%. Wear behaviour was better for the heat-treated alloys and mainly for the 6.4%V iron. Such a behaviour is discussed in terms of the CVF, the amount of vanadium carbides, the amount of martensite/austenite in matrix and the amount of secondary carbides precipitated during the destabilization heat treatment.

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.

Kinetic Parameters of Secondary Carbide Precipitation in High-Cr White Iron Alloyed by Mn-Ni-Mo-V Complex

This study presents kinetics of precipitation of secondary carbides in 14.55%Cr-Mn-Ni-Mo-V white cast iron during the destabilization heat treatment. The as-cast iron was heat treated at temperatures in the range of 800-1100 °C with soaking up to 6 h. Investigation was carried out by optical and electron microscopy, dilatometric analysis, Ms temperature measurement, and bulk hardness evaluation. TTT-curve of precipitation process of secondary carbides (M7C3, M23C6, M3C2) has been constructed in this study. It was determined that the precipitation occurs at the maximum rate at 950 °C where the process is started after 10 s and completed within 160 min further. The precipitation leads to significant increase of Ms temperature and bulk hardness; large soaking times at destabilization temperatures cause coarsening of secondary carbides and decrease in particles number, followed by decrease in hardness. The results obtained are discussed in terms of solubility of carbon in the austenite and diffusion activation of Cr atoms. The precipitation was found to consist of two stages with activation energies of 196.5 kJ/g-mole at the first stage and 47.1 kJ/g-mole at the second stage.

Microstructural characteristics and corrosion behaviour of high-chromium cast iron alloys in sugar media

The work is aimed at using high-Cr White Irons in manufacturing equipment for sugar cane industry.

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.

Influence of reinforcement volume fraction and size on the microstructure and abrasion wear resistance of hot Isostatic pressed white iron matrix composites

The changes in the microstructure and wear resistance of a powder metallurgical high-Cr white iron after the incorporation of TiC particles were studied in the present work. Various reinforcement volume fractions and sizes were used in order to examine their influence on the three-body abrasion wear resistance. The experiments were carried out at three different austenitizing temperatures. The most important observation after a microstructural examination was the increased amount of martensite in the composites subjected to identical heat treatment procedures with the unreinforced alloy. The austenite-to-martensite transformation in the composites increased with the TiC volume fraction and with the austenitizing temperature. This indicates that the two parameters have a key role in the transformation mechanism, which seems to be mechanically induced. The increasing of martensitic transformation with the TiC content in the composites enhanced continuously the supporting ability of the iron alloy matrix to the TiC particles, which in turn increased the wear resistance of the composites. The abrasion wear resistance increased with the TiC volume fraction until the onset of spalling. However, in composites containing coarse reinforcements, spalling occurred earlier in the wear process. This decreased wear resistance significantly because spalled TiC particles contributed additionally to wear.

Columnar to equiaxed transition in high Cr white iron castings

The microstructure and properties of 17 wt.% Cr–2.9 wt.% C cast irons with up to 3.17 wt.% Nb additions, in both as-cast and heat treated state, have been studied. Also the influence of titanium and cerium on the structure and properties of 17 wt.% Cr–2.9 wt.% C-2 wt.% Nb alloys are examined. NbC carbides present in the structure of tested alloys, due to their characteristic morphology, show higher wear resistance and toughness than M7C3 carbides. Increasing amount of this type of carbides, caused by the increase of niobium in Fe–Cr–C–Nb alloys, contributes to the improvement of wear resistance and dynamic fracture toughness. The alloy containing approximately 3% Nb gives the best compromise between wear resistance and fracture toughness. This alloy shows about 30% greater dynamic fracture toughness and about 30% greater abrasion wear resistance than the basic Fe–Cr–C alloy. Titanium and cerium affect the crystallisation process of Fe–Cr–C–Nb alloys and the transformation of austenite during the cooling after solidification. The addition of 0.26% Ti results in a substantial change in the morphology and distribution of NbC carbides. The secondary carbides which precipitate in the matrix regions of the tested 17 wt.% Cr–2.9 wt.% C-2 wt.% Nb white iron containing titanium has an impact on the abrasion behaviour and fracture toughness. The alloy containing 0.28% Ti and 0.19% Ce has pearlite–austenitic matrix microstructure in as-cast condition. The pearlite, due to its high microhardness, improves the wear resistance under low-stress abrasion conditions, but drastically reduces the toughness of tested alloy.