High Chromium White Iron Research Articles

Solidification structure versus hardness and impact toughness in high-chromium white cast irons

Cast macrostructure has long been considered an important metallurgical feature in determining the mechanical properties of high-chromium white irons. To explore this relationship, the US Bureau of Mines has looked at a series of high-chromium white irons cast at different superheat temperatures (75 and 250 °C) and then impact tested. All materials were cast into sand moulds, each of which produced three impact specimens (50 × 50 × 180 mm). The macrostructure of the alloys is shown to consist of columnar and equiaxed growth zones depending upon chemical composition (i.e. hypoeutectic, eutectic or hypereutectic) and superheat pouring temperature. An explanation of this solification behaviour is given by differential thermal analysis and standard solidification theories. Cast microstructure is illustrated by image analysis (carbide volume fraction and particle size) and optical metallography (carbide and matrix phase identification). Brinell hardness measurements were also taken along cut surfaces (perpendicular to the columnar and equiaxed zones) and across the top of the impact specimens (semi-chilled surface). Macrostructural and microstructural results are then related to impact toughness results. As would be expected, both macro- and micro-structure have strong effects upon the impact toughness of high-chromium white cast irons.

Secondary carbide precipitation in an 18 wt%Cr-1 wt% Mo white iron

High chromium (18%) white irons solidify with a substantially austenitic matrix supersaturated with chromium and carbon. The austenite is destabilized by a hightemperature heat treatment which precipitates chromium-rich secondary carbides. In the as-cast condition the eutectic M7Ca3 carbides are surrounded by a thin layer of martensite and in some instances an adjacent thicker layer of lath martensite. The initial secondary carbide precipitation occurs on sub-grain boundaries during cooling of the as-cast alloy. After a short time (0.25 h) at the destabilization temperature of 1273 K, cuboidal M23C6 precipitates within the austenite matrix with the cube-cube orientation relationship. After the normal period of 4 h at 1273 K, there is a mixture of M23C6 and M7C3 secondary carbides and the austenite is sufficiently depleted in chromium and carbon to transform substantially to martensite on cooling to room temperature.

Microstructure-property relationships in high chromium white iron alloys

High chromium white irons are ferrous based alloys containing 11–30 wt-% chromium and 1.8–3.6 wt-% carbon, with molybdenum, manganese, copper, and nickel sometimes added as additional alloying elements. The microstructure of these alloys typically consists of hard primary and/or eutectic carbides in a matrix of austenite or one of its transformation products. The presence of hard alloy carbides results in excellent abrasion resistance and, consequently, these alloys are commonly used for materials handling in the mining and minerals processing industries. Alloy content, solidification parameters, and thermal processing can dramatically alter the microstructure that is produced, and this in turn can influence the properties and hence performance of white iron alloys during service. This review outlines the development of the microstructure in high chromium white irons through solidification and thermal processing. The metallurgical effects of conventional processing techniques are discussed, and advances in aspects such as alloying and cryogenic treatments covered. The results of laboratory abrasion tests are summarised, and the effect of microstructure on the wear properties are discussed. The toughness and impact resistance of white cast irons, which are often thought to represent a limiting factor in their use, is reviewed, with particular regard given to the effects of microstructural constituents.

Microstructure-property relationships in high chromium white iron alloys

AbstractHigh chromium white irons are ferrous based alloys containing 11–30 wt-% chromium and 1.8–3.6 wt-% carbon, with molybdenum, manganese, copper, and nickel sometimes added as additional alloying elements. The microstructure of these alloys typically consists of hard primary and/or eutectic carbides in a matrix of austenite or one of its transformation products. The presence of hard alloy carbides results in excellent abrasion resistance and, consequently, these alloys are commonly used for materials handling in the mining and minerals processing industries. Alloy content, solidification parameters, and thermal processing can dramatically alter the microstructure that is produced, and this in turn can influence the properties and hence performance of white iron alloys during service. This review outlines the development of the microstructure in high chromium white irons through solidification and thermal processing. The metallurgical effects of conventional processing techniques are discussed, and ad...

The electro-discharge machining surfaces of high-chromium white irons

Electro-discharge machining (EDM) is widely used with hard and brittle materials, which are difficult to machine using conventional techniques. In the EDM process, spark discharges between the electrically conductive workpiece and a tool electrode increase the temperature so that localized melting occurs. Approximately 15% of this molten metal is removed by flushing, while the remainder resolidifies on the workpiece surface to produce a re-cast layer [1]. This re-cast layer has a rapidly quenched strucmre. A heat-affected layer may also be formed in the workpiece material due to the high temperatures involved in the process. The total depth of the recast layer and the heat affected regions may be between 2 and 130 ~m, and increases with the pulse energy used in the EDM process [2]. The extreme thermal and transformation stresses generated in the re-cast layer may also produce extensive surface cracks, which generally do not propagate deeper than the re-cast layer. The presence of these surface cracks and tensile residual stresses in the re-cast layer can deleteriously affect mechanical properties, such as fatigue resistance [3].

Influence of the metal/ceramic interface on the microstructure and mechanical properties of HIPed iron-based composites

Iron-based composites are an important class of wear- and corrosion-resistant materials for the chemical and process industries. The influence of the metal/ceramic interface on the microstructure and mechanical properties of iron-based composites with a high-chromium white iron matrix has been investigated. Three ceramic particle reinforcements (Al 2O 3, TiC, Cr 3C 2) with different chemical behaviour in contact with iron alloys were selected in order to reveal the influence of the resulting different interfaces on the microstructure and mechanical properties of the composites. It was found that the interface of the metal/ceramic system affects the amount of martensitic transformation in the iron-alloy matrix. Regarding the mechanical properties, wear resistance and hardness were not dependent on the nature of the interface; however, toughness deteriorated to a great extent with a weak interface.

Effect of rare earth elements on microstructure and properties of high chromium white iron

The influence of rare earth elements, such as cerium, lanthanum, and neodymium, on the microstructure and properties of high chromium white cast iron are examined. It has been demonstrated that rare earth elements can change the microstructural characteristics of white iron containing 18 wt-%Cr and that these elements improve its mechanical properties. The optimum content of cerium, lanthanum, and neodymium in high chromium white iron is 0·13–0·26 wt-%. The wear resistance of these alloys is improved by 10% in comparison with a basic alloy containing no rare earth elements, and there are no changes in fracture toughness.MST/1980

Effect of rare earth elements on microstructure and properties of high chromium white iron

Effect of rare earth elements on microstructure and properties of high chromium white iron

A substructure within the austenitic matrix of high chromium white irons

A substructure within the austenitic matrix of high chromium white irons

Microstructural aspects in application of high chromium white iron powders: M. Sittner (National Inst. for Materials Research, Praha, Czech Republic)

Microstructural aspects in application of high chromium white iron powders: M. Sittner (National Inst. for Materials Research, Praha, Czech Republic)

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.

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.

L'amélioration et la recherche de nouveaux matériaux résistant à l'abrasion

In grinding equipment, ferrous alloys with the highest carbon content have the best abrasion resistance, but, because of the severe stresses undergone in service, the material should have sufficient toughness to avoid failures.Unalloyed or low alloy iron-carbon alloys already have a low toughness in the martensitic condition with carbon contents starting from 0.4 %, and their toughness is very low in the case of hypereutectoid steels and white irons because of the morphology of the cementite that they contain. If alloy elements are added in sufficient quantities, carbides of a different type to cementite can be obtained, which have a higher hardness, thus enhancing the abrasion resistance. These carbides also have a morphology which is more favourable for the toughness. Alloys produced according to this principle, such as high chromium white irons, may be further improved.The possibilities for improving work-hardenable austenites have not yet been exhausted, not to mention the conventional austenitic steels with 12 and 6 % of manganese.

Tensile and creep behavior of graphites at temperatures above 3000°F : H. E. Martens, D. D. Button, D. B. Fischbach, and L. D. Jaffe. Jet Propulsion Laboratory, California Institute of Technology. JPL Progress Report 30-18, October 15, 1959. 18pp.

The impact-abrasion behavior of a series of low alloy white cast irons with different morphologies of eutectic carbide was investigated using a repetitive impact-abrasive wear tester. By means of a defined morphological parameter, the roundness area fraction SFi, the morphology of the eutectic carbide in each alloy was quantitatively assessed. It was found that the mean weight loss of the experimental alloys increased with increasing volume fraction of the network eutectic carbide. This is in general agreement with the results reported for high chromium white irons. Through one-time impact-abrasion, static indentation and pure repetitive impact tests in addition to the usual wear surface scanning electron microscope (SEM) examination, various kinds of wear damage of low alloy white cast irons in the repeated impact-abrasion testing were revealed. In otder to understand the wear damage, the concept of true impact stress on the wear surface was emphasized and used to analyze the mechanical interaction between the abrasive particles and the wear surface.