High Alloy White Cast Iron  ( Nihard & High Chromium)

  Wear resistant cast irons are white, carbide-solidified cast irons which contain a high level of hard particles in the form of iron carbide or chromium carbide. The carbides are held by a hard matrix. Generally the matrix is martensitic but there are also cases of an austenitic matrix as well, which only becomes more solid during the wear process associated with strain hardening.

   Since they are highly resistant to wear, white irons are particularly suitable for applications involving wear caused by minerals, for example in grinding tools, in reducing, mixing and conveying equipment and systems and in pumps.

  The wear properties of any particular type of cast iron depend on a range of factors. The main factors are the basic microstructure, the carbon content and the type and distribution of carbides. The carbides should be evenly distributed and not too small, otherwise it is possible that a carbide to become easily dislodged from the matrix by an abrasive particle brushing along it.

   The amount of carbide particles as hard material is dependent on the carbon content. The greater the carbon content, the greater the hardness if all other variables remain constant, and the lower the toughness. Hardenability can be increased with the elements nickel, copper, molybdenum and manganese.

   However, the amount of nickel that can be added is limited because excessive amounts will lead to excessive proportions of residual austenite and even lead to the formation of graphite, thereby drastically reducing the degree of hardness. As an alloy element, chromium offers graphite-free solidification and prevents the precipitation of graphite even during heat treatment. Like nickel, copper, molybdenum and manganese, chromium also improves hardenability.

   In some grades the desired microstructure can be achieved in the as-cast condition. Others require additional heat treatment to remove unwanted microstructure portions and achieve the desired properties.

  Nihard alloys is a family of cast iron alloyed with Ni and Cr and characterized with excellent abrasion resistance properties and usually recommended for parts used in crushing and grinding purposes. The Ni and Cr contents of the conventional Nihard types I and II are governed by the equation: %C + %Ni + %Cr + %Si = 4.2-5.0

Where Ni is added primarily to provide the necessary combination of toughness, strength and hardenability and Cr contributes mainly to the alloy’s hardness and corrosion erosion resistance.

  The increased Cr and Ni contents (up to 11.0% and 7.0% respectively) in the more recently developed type IV Nihard lead to increased toughness due to the breakdown of the continuous brittle carbide network into discontinuous Cr7C3 type. Retained austenite in these alloys may be partially transformed to martensite depending on chemical composition.

  The presence of such high contents of Cr, a very strong carbide stabilizer always excluded any possibility of graphite formation in Nihard alloys, and the completely white structure is responsible for the rather low machinability of these alloys.