Alloy white Iron castings are used in the power, mining, cements, construction and other industries for applications where maximum abrasive wear resistance required with toughness being of lesser importance. Excellent abrasion resistance is obtained from these alloys because their microstructures contain 20%-40% of very hard carbides embedded in matrices which consist of martensite, bainite, austenite, secondary carbides. In National and International standards it is used to specify a minimum of around 550 Brinnel hardness and the alloy are divided into two broad groups; the well known Ni-Hard or Nickel-Chromium type and the High Chromium grades. The Chromium iron contains 12-25% Cr.

For a given iron the abrasion resistance is usually improved as the carbon content is raised owing to the greater volume of eutectic carbides which form on solidification, but at the same time the toughness and resistance to premature breakage is decreased. Additionally, the matrice structure can affect both wear and spalling resistance so that care is needed in the selection of the best iron for a given application.

An important consideration in the selection of abrasion-resistant irons and steels is their resistance to fracture. The toughness of white irons is as difficult to quantify as is resistance to abrasion. The usual methods for measuring toughness in ductile materials -like tensile elongation or notched or unnotched impact resistance -do not apply to the brittle white irons. So results are often meaningless and can be more representative of testing conditions and operator skill than of the material properties.

Generally, a decrease of the carbon (resp.carbide) content and rounded or discontinuous carbides improve the toughness.

White irons with a matrix of tempered martensite have the best overall fracture resistance. White irons containing higher amounts of austenite can appear quite tough in single impact or bending conditions, but can fail due to spalling or fracture under repeated loading. The reason is a strain-induced martensitic transformation resulting in high internal stresses. Irons containing bainite, where the bainite is an extremely fine grained mixture of ferrite and high carbon austenite, behave like irons with an austenitic matrix. Pearlitic white irons have a comparatively low toughness.

Again, it should be emphasized that hardness bears no direct relationship to toughness but is a useful indicator of the likely structure.

The practical meaning of toughness for an abrasion-resistant white iron is survival without breaking in a particular application. The condition resulting in fracture can be single or repeated impact, or static or dynamic bending stresses. Here the inherent low ductility of the iron is often over hidden by the presence of internal stresses due to shape and heat treatment of the casting and any casting defects. So the integrity of the castings and the care and control used in heat treatment are decisive factors in service performance.

High Chrome white cast iron is used extensively in the production of components for pumping equipment subjected to intense abrasive-corrosive wear. In comparison with other cast irons of this group, the above cast iron has high processing properties but its mechanical properties and wear resistance, especially in thick castings does not ensure the required service durability of equipment. This is caused by special role played by structure formation in the formation of the properties of high-chrome white cast iron. The reduction of the cooling rate of liquid metal increases the size of primary phases (austenite or chrome carbide) and this leads to the development of shrinkage porosity and reduction of the strength and machinability of castings.

Since the pump components are subjected to the total effect of the abrasive medium, corrosion, and cavitations which disrupts the surface, the service durability of these components depends on the initial structure of cast iron and on changes of the structure during service. Specifically, the white cast irons with a fine-grain structure have higher wear resistance, with other conditions being equal. Inoculation is an efficient method of refining the cast structure, eliminating transcrystalline solidification, and improving the properties of high-chrome white cast iron. Inoculation does not require additional investment and can be carried out in any of the existing casting shops.

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