Abrasion Resistant White Cast Irons

Abrasion-resistant white cast irons are distinctly different from the usual gray or nodular cast irons.

In many respects, they are more similar to high-strength steels. In fact, iron foundries who have neglected this fundamental difference have often experienced much greater difficulties when starting the production of high-quality Abrasion-resistant iron castings than steel foundries.

The main difference from normal cast irons is that Abrasion-resistant cast irons solidify without graphite. This dramatically changes their feeding characteristics. The volume shrinkage at solidification of Abrasion-resistant cast irons is not compensated for by the volume expansion of the graphite precipitation as it is in gray or nodular irons.

Typically, a solidification shrinkage of about 4-5% will occur. This is almost identical to the shrinkage found in steel castings. The risering and feeding design of the mold is, therefore, similar to that of steel castings.

However, in contrast to steel, Abrasion-resistant irons have little or no ductility. Their solidification mode, their carbidic structure and their various transformations at relatively low temperatures make Abrasion-resistant irons prone to tearing and cracking in all stages of processing.

Furthermore, martensitic Abrasion-resistant irons of the Ni- Hard family and the High-Chromium irons are high-alloy materials. Such high-alloy materials require special process controls in both melting and casting.

From the viewpoint of foundry production practice, the Abrasion-resistant irons can be divided into the following five groups:

1. Fully White Pearlitic: Abrasion-resistant irons that are white throughout the whole section, with pearlitic matrices in the as-cast condition.

2. Chilled: Abrasion-resistant irons for castings that have a white iron working layer and a gray iron backing. These irons are usually produced by casting against a chill and have pearlitic matrices in the as-cast condition.

3. Ni-Hard 1 and Ni-Hard 2: Nickel-chromium abrasion resistant cast irons of the Ni-Hard 1 and 2 type, which are martensitic as cast.

4. Ni-Hard 4: High-alloyed Nickel-chromium abrasion resistant cast iron with an austenitic- martensitic matrix in the as-cast condition; often heat treated to obtain a more martensitic matrix.

5. High-Chromium: Cast irons with a wide variety of as-cast structures. These irons are usually heat treated to obtain maximum hardness or toughness, or resistance to repeated impact.