high-Cr White Cast Iron Research Articles

Microstructure and wear behavior of spray-formed and conventionally cast rolls of 18Cr–2.5Mo–Fe alloy

Relationships among the carbide morphology, microstructures and the wear behavior of sand-cast and spray-formed high Cr white cast irons were studied. The eutectic carbides in the spray-formed material are of small blocky shaped, uniformly distributed M 3C and M 7C 3, while in the sand-cast materials, they are non-uniformly distributed, large needle shaped M 3C . Hardness is nearly the same for the as-spray-formed and as-sand-cast materials. After the heat treatment, the sand-cast material had a large increase in hardness, while the spray-formed material had only a slight increase, and the former shows a much higher hardness than the latter, primarily due to higher C and Mo contents of the former. The spray-formed material showed a better fracture-resistance than the sand-cast material. The wear mechanisms were mainly adhesive wear for both soft and hard counterparts for the spray-formed materials, and switched from mainly adhesive wear for the soft counterpart to mainly abrasive wear for the hard counterpart for the sand-cast materials.

Heat Treatment in High Cr White Cast Iron Nb Alloy

Wear resistance of high Cr white cast irons can be improved by means of heat treatment. This type of cast iron alloy may present a microstructure with retained austenite. The amount of retained austenite changes with the applied heat treatment, which will have an influence on wear properties. The purpose of this work was to study the influence of several parameters such as quenching and tempering temperatures and subzero treatment in the wear performance of the high Cr white cast iron Nb alloy. In this way, the performance was evaluated using pin-on-disc abrasion test. The worn surface was examined by scanning electron microscopy, and the main wear mechanisms were identified. The microstructural characterization was also performed with carbide identification. This Fe alloy has proven to be good for applications in mining and alcohol-sugar industries.

Effects of retained austenite on abrasion wear resistance and hardness of hypoeutectic high Cr white cast iron

A hypoeutectic white cast iron of 2.3%C—26%Cr—l%Ni—0.5%Mo was heat-treated with three different methods in order to obtain the specimens with different levels of retained austenite(Vr) in the matrix. Then, the effect of the Vr value on the abrasion wear resistance, and the macro- and micro-hardness of the iron was investigated to derive a mutual relationship among them. While the Vr value is 70.2% in the as-cast specimen, those of the heat-treated specimens range from 0.3 to 65.4%, depending upon the types of heat-treatment. In the scratching abrasion wear test, a linear relationship between wear loss and wearing time was obtained after the primary stage of unsteady wear in all the specimens. Therefore, the rate of wear(Rw: mg/s), equivalent to the slope at the portion of straight line, was adopted as an index of wear resistance. The Rw values range from 2.77×10−2 to 4.12×l0−2, the lowest Rw being obtained in the as-cast specimen with the highest Vr value and the highest Rw in the homogenized specimen with the lowest Vr value. The Rw value decreases with an increase in the Vr value probably due to a transformation of austenite into martensite during abrasion wear test. While the range of micro-hardness is from HV 358 to HV 756, the macro-hardness ranges from HV 529 to HV 785 due to the involvement of eutectic M7C3 carbides in measurements. The highest hardness value is obtained in the specimen heat-treated by fan-air hardening with austenitizing condition of 1323K for 7.2ks. On the other hand, the lowest hardness is obtained in the as-cast specimen due to the presence of large amount of austenite and of less martensite, and the absence of secondary carbides. Mutual relationships among Rw, Vr and hardness show a good correspondence.

Prediction of solidification path of cast irons using thermodynamic databases in a commercial FEM package

This paper describes an interface between a thermodynamic calculation of phase diagram parameters for multicomponent alloys and a micromodel for calculation of fraction solid and enthalpy in the preprocessing module of a commercial FEM software package. This calculation can be performed under LEVER (equilibrium), SCHEIL (complete diffusion in liquid and no diffusion in solid), and BACK-DIFFUSION (complete liquid mixing with solid diffusion for a fixed cooling rate) conditions. The calculated data can be used as thermophysical properties for a subsequent solidification modeling study of a casting. Simulation studies are performed on three high-Cr white cast irons using SCHEIL and LEVER models. A general agreement is found while comparing thermal parameters, carbide volume fractions, and types of phases that appear between simulated and published experimental data. This study also highlights the potential experimental error and limitation.

Effect of high temperature and sub-ambient treatments on the matrix structure and abrasion resistance of a high-chromium white iron

The addition of NbC to high chromium white cast irons (WCIs) has potential to improve component wear lives in mining and ore beneficiation environments, due to the combined presence of Cr-rich and niobium carbides. While the effect of chromium carbide volume fraction (Cr-CVF) on abrasion performance of plain-Cr WCIs is well understood, its effect on NbC-bearing WCIs is unknown. This work studies the effect of Cr-CVF on high-stress abrasion of this newer class of dual-reinforced high-Cr WCIs, using the ball mill abrasion test with quartzite and basalt. The alloys cover a wide range of Cr-CVF, from M7C3-free NbC-reinforced steel to hyper-eutectic WCI. Bulk hardness increased linearly with Cr-CVF, at a rate of 3.9 HV per 1% Cr-CVF. In basalt, abrasion resistance increased linearly with Cr-CVF, at an average life improvement rate of 24% per 1% Cr-CVF. This confirms that performance is maximized by increasing Cr-CVF even in the presence of substantial volumes of cubic carbides. In quartzite the trend is again linear up to 30 vol% M7C3 (3.7% life increase per 1% Cr-CVF), but the hyper-eutectic alloy shows poorer resistance than slightly hypo-eutectic alloy. SEM studies of the worn surfaces demonstrate that micro-fracture of the hard carbides was the controlling damage mechanism. Primary M7C3 was observed to suffer fragmentation in high-stress abrasion in quartzite, confirming the expectation from the quantitative trend.

Solidification structure and abrasion resistance of high chromium white irons

Superior abrasive wear resistance, combined with relatively low production costs, makes high Cr white cast irons (WCIs) particularly attractive for applications in the grinding, milling, and pumping apparatus used to process hard materials. Hypoeutectic, eutectic, and hypereutectic cast iron compositions, containing either 15 or 26 wt pct chromium, were studied with respect to the macrostructural transitions of the castings, solidification paths, and resulting microstructures when poured with varying superheats. Completely equiaxed macrostructures were produced in thick section castings with slightly hypereutectic compositions. High-stress abrasive wear tests were then performed on the various alloys to examine the influence of both macrostructure and microstructure on wear resistance. Results indicated that the alloys with a primarily austenitic matrix had a higher abrasion resistance than similar alloys with a pearlitic/bainitic matrix. Improvement in abrasion resistance was partially attributed to the ability of the austenite to transform to martensite at the wear surface during the abrasion process.

Effect of carbide orientation on abrasion of high Cr white cast iron

Wear resistant, high chromium white cast irons contain high volume fractions of M 7C 3 type carbides, which grow as rods and blades during solidification to produce a highly anisotropic microstructure. In this study, the effect of carbide orientation on the wear properties of these alloys is examined using both a high-stress pin-on-drum abrasion test and a single scratch test. It is found that when the carbides are oriented with their long axes perpendicular to the wear surface, abrasion resistance is considerably reduced. When the long axes of the carbides are parallel to the wear surface, the cast irons are more wear resistant, and the alloys show the least wear when the long axes of the carbides are parallel to the wear surface and oriented perpendicular to the wear direction. Scanning electron microscopy reveals that fracture of the carbides depends upon the orientation of the carbides with respect to the wear surface and the wear direction.

Abrasion resistance of the columnar zone in high Cr white cast irons

Abrasion resistance of the columnar zone in high Cr white cast irons

Abrasion resistance of the columnar zone in high Cr white cast irons

High chromium white cast irons are extremely abrasion resistant materials due primarily to their high carbide content. In the columnar zone of cast structures, eutectic carbides grow parallel to the direction of heat flow, forming a highly anisotropic morphology. In this US Bureau of Mines investigation, specimens taken from the columnar zone of these castings were tested utilizing a pin abrasion technique in order to understand the abrasive wear behavior of the eutectic carbides in two directions: parallel (longitudinal) and perpendicular (transverse) to the long axis of carbides. Also the effect of the matrix, as modified through a series of heat treatments, on wear rate was investigated. The longitudinal specimens showed 27–67% less volume loss than the transverse specimens depending upon heat treatment.

Fractal analysis of caride morphology in high-Cr white cast irons

Fractal analysis of caride morphology in high-Cr white cast irons