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.     

The effect of Si on the relationship between orientation and carbide morphology in high chromium white irons

Electron back-scatter diffraction (EBSD) has been shown to be the most appropriate technique to study the orientation and carbide morphology of small (<0.5 μm) regions of microstructure of high chromium white irons. The carbides in a slightly hypo-eutectic Fe–Cr–C alloy show a distinct texture close to [1 0 1 1] whereas those in a 1.3 wt% Si commercial white iron have a diffuse texture, with regions near to major crystal directions, i.e., [0 0 1 1], [1 2 1 0], [0 1 1 0], unpopulated. Using EBSD, it has been shown that the interconnectivity of the eutectic (Cr, Fe)₇C₃ carbide is less in a 1.3 wt% Si alloy compared with a low (0.1 wt%) Si alloy. This reduced interconnectivity is consistent with the increased fracture toughness in the as-cast condition. This revised version was published online in November 2006 with corrections to the Cover Date.     

The effect of heat treatment on the toughness, hardness and microstructure of low carbon white cast irons

The effect of destabilisation and subcritical heat treatment on the impact toughness, hardness, and the amount and mechanical stability of retained austenite in a low carbon white cast iron have been investigated. The experimental results show that the impact energy constantly increases when the destabilisation temperature is raised from 950°C to 1200°C. Although the hardness decreases, the heat-treated hardness is still greater than the as-cast state. After destabilisation treatment at 1130°C, tempering at 200 to 250°C for 3 hours leads to the highest impact toughness, and secondary hardening was observed when tempering over 400°C. The amount of retained austenite increased with the increase in the destabilisation temperature, and the treatment significantly improves the mechanical stability of the retained austenite compared with the as-cast state. Tempering below 400°C does not affect the amount of retained austenite and its mechanical stability. But the amount of retained austenite is dramatically reduced when tempered above 400°C. The relationship between the mechanical properties and the microstructure changes was discussed.     

Potentiostatic etching of duplex stainless steels and high chromium white cast irons

Abstract

The reasons for potentiostatic etching and the way in which it is carried out are explained. The technique was applied to some commercially important materials, and the information obtained is described. It was found that the chromium content of duplex stainless steels had a significant effect on the etching potential, but did not greatly influence the etching time. In high chromium white cast irons (HCWCIs), the composition of the various alloys influenced the etching potential very little, and it was possible to deduce a generally suitable etching range of 950–1050 mV (SCE) for the whole group. Manganous sulphate proved to be a superior etching agent, and very fine detail, e.g. different types of carbide, could be detected in HCWCIs after potentiostatic etching in this solution. Quantitative phase determinations were also possible for all materials investigated, provided that suitable etchants were used. Optimum etching times were from 10 to 15 s, regardless of the group of materials investigated.

MST/1665     

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.     

Improvement of toughness of high chromium white cast iron: duplex ferritic-austenitic matrix

It is attempted to enhance the impact toughness of industrially used high chromium white cast iron (WCI) without sacrificing wear resistance. The microstructure is engineered by cyclic annealing to obtain features such as duplex grain matrix, where austenite envelops ferrite grain, refined M₇C₃ carbide. The newly cast and heat-treated alloy shows remarkable impact toughness i.e. 13J with improved wear resistance. The fracture micro-mechanism is studied through extensive scanning electron microscopy and it is ascertained that enhanced impact toughness results from crack arrest at duplex grain boundaries. A few other toughness enhancing features are also discussed. The results are compared with standard ASTM grade Class-III high chromium WCI and are found to be encouraging.     

Structure,nucleation, growth and morphology of secondary carbides in high chromium and Cr-Ni white cast irons

Heat-treated high chromium and Cr-Ni white cast irons are widely used by the mining and mineral industries for impact and abrasion resistance. With certain heat treatments, Fe-Cr carbides are precipitated within the chromium- and carbon-rich austenitic matrix, thereby destabilizing the austenite which transforms substantially to martensite on subsequent cooling. The crystal structures of these carbides were determined indirectly by referring electron microprobe analyses of the austenitic matrix to the appropriate isothermal solid-state sections of the Fe-Cr-C phase diagram and directly by microprobe analyses of exposed secondary carbides. The nucleation, growth and morphology of these carbides were studied by a combination of selective removal of the austenitic matrix and subsequent scanning electron microscopy of the exposed carbides.     

The effect of spray forming on the microstructure and properties of a high chromium white cast iron

The effect of spray forming on the structure and properties of a 17% Cr, 2.5% C white cast iron is described and compared with conventionally cast material of the same composition. Spray forming resulted in a substantial reduction in microstructural scale (eutectic (Cr,Fe)₇C₃ fields of up to 500 m in conventionally cast material were replaced by discrete carbides of typically 2–8 m diameter in the spray cast deposit). Carbide size varied as a function of position in the spray deposit, being approximately twice the size at mid section compared with either surface or interface with the collector. Carbide size was not altered by the gas to metal ratio used to atomise the spray. Spray forming increased transverse rupture stress and work of fracture by 50% compared with the conventionally cast material. Forging of the spray formed material was possible at 950°C, without inducing carbide fracture or void formation in the matrix. Quenching into iced water from 300°C induced extensive macroscopic cracking in the conventionally cast material whereas 400°C was required in the spray cast material to induce similar damage. The relationship between processing, microstructure and mechanical properties is discussed.     

The role of secondary carbide precipitation on the fracture toughness of a reduced carbon white iron

The fracture toughness of a high chromium, reduced carbon white cast iron was measured using the K_Ic fracture toughness test. The toughness was found to increase with increasing heat treatment temperature for the temperature range of 1273–1423 K. Increases in the fracture toughness were due to crack deflection into the dendritic phase. Cracking in the dendrites was promoted by the presence of secondary carbides which formed during the high temperature heat treatment employed. The characteristic distance for brittle fracture as calculated by the Ritchie–Knott–Rice model correlated well with the centre to centre mean free path of the secondary carbides on the fracture plane.     

Breakup of eutectic carbide network of white cast irons at high temperatures

The fracture toughness of white cast irons is related to the morphology of eutectic carbides,being better when isolated than when network-like. In this paper observations on the breakup of eutectic cementite network during annealing treatment of white cast irons are reported using a high temperature microscope and scanning electron microscopy (SEM). Dissolution-induced breakup and capillarity-induced breakup are identified. The former occurs through growth of holes and pre-existing fissures as well as through fragmentation at narrow necks and narrow roots of branches. The latter is observed through growth of perturbations. Dissolution-induced breakup is closely associated with the morphology of the as-cast eutectic cementite. A combination of solidification processing and heat treatment thus produces a more positive breakup effect.     

Improving fracture toughness and hardness of Fe2B in high boron white cast iron by chromium addition

The effect of chromium containing 0%, 0.49%, 1.02%, 2.1%, 3.2% (in wt.%) on the morphology, fracture toughness and micro-hardness property of Fe₂B in high boron white cast iron was investigated. The results indicated that, with an increase of chromium addition, the morphology of Fe₂B becomes larger and changes from the block to rod shape, its micro-hardness increases and the fracture toughness increases first and then decreases. Compared with the fracture toughness (3.8 MPa m^1/2) of Fe₂B without chromium addition, the toughness at 2.1 wt.% chromium addition can be improved by above one time, achieving 7.8 Mpa m^1/2, and the result was also qualitatively testified by the micro-cracks in Fe₂B based on scanning electron microscope micrographs.     

The effects of heat treatment on the mechanical properties of multicomponent white cast irons

Multicomponent white cast irons contain many kinds of strong carbide-forming elements in order to obtain a very hard microstructure characterized by the presence of different carbides that are well dispersed in a martensitic matrix. The heat treatment of these products consists of high temperature austenization followed by quenching and two temperings, as required in order to increase their overall hardness and to completely eliminate residual austenite. The influence of tempering temperatures on the mechanical properties of these products, determined using tensile, hot compression and fracture toughness tests, was studied in this research work. Their corresponding failure micromechanisms were defined by means of the analysis of fracture surfaces.     

Hot workability of a high carbon high chromium tool steel

In this work, hot tension tests were conducted on as cast and wrought samples of a high carbon high chromium tool steel to study the hot workability under the rolling conditions. The flow curves illustrate the classical shape of dynamic recrystallization (DRX). It is observed that broken carbide nets in the wrought samples result in lower deformation activation energy 398 kJ/mol in comparison to the as cast samples 432 kJ/mol. Necking strains were calculated using the inflection point of the work-hardening (θ) vs. stress curves. Wrought samples show higher hot ductility and lower maximum stress than the as cast ones. It is shown that lower activation energy of deformation and lower stress concentration around the smaller carbides in the wrought samples is responsible for their higher hot workability.     

Thickness and moisture content effect in the fracture toughness of Scots Pine

Fracture toughness tests were performed on specially prepared specimens taken from a single tree. These were kiln dried, machined to various thicknesses and then sets of them conditioned to various moisture contents. It was found that about half the specimens were rejected because of warping and splitting for thicknesses greater than 10 mm, but below this the rejection rate decreased. Fracture toughness values showed thickness variations in the kiln dried state, but the toughness decreased and became constant when the moisture content was changed. An explanation is proposed in terms of residual stresses induced by the drying which increases the toughness and constraint stresses which cause a decrease. A characteristic length of the annual ring spacing seems appropriate in defining the various transitions.     

The effect of carbon on oxygen precipitation in high carbon CZ silicon crystals

To clarify the role of carbon impurities in the formation of oxygen precipitates, the behavior of CZ silicon wafers with varying carbon concentrations from 0.2 ppm to 2 ppm were studied under different heat treatment conditions. It is found that the rate of reduction in interstitial oxygen is not a function of carbon concentration as reported previously for both the cases of medium and low temperature annealing. Under medium temperature (1050°C) annealing no carbon reduction was detected, even though the reduction in interstitial oxygen can be very large, while with low temperature (750°C) annealing, oxygen reduction is always associated with the carbon reduction, but is not dependent on carbon concentration alone. A heterogenous precipitation model is presented to explain the observed phenomena.