Scale Formation & Prevention

Scale is the result of a series of chemical reactions that occur when a metal workpiece is exposed to elevated temperatures in the presence of oxygen. It forms as a thin layer of oxidized metal on the surface of the treated workpiece. This layer is commonly referred to as scale, and it is often brittle and non-adherent.

Scale on the surface of a piece of S7 tool steel that has been heat treated.

The process of scale formation during heat treatment begins with the oxidation reaction. When a metal workpiece is subjected to heat in an oxygen-rich environment, the high temperatures promote an oxidation reaction. This reaction involves the metal’s outermost layer reacting with oxygen from the surrounding atmosphere, leading to the creation of metal oxides.

The metal oxides produced as a result of the oxidation reaction are what constitute the scale layer. The composition of the scale depends on the specific alloy being treated, as it is a product of the reaction between the alloy and oxygen. The scale layer typically appears as a thin and sometimes flaky coating on the surface of the workpiece.

In certain heat treatment processes, particularly those where preserving the surface integrity of the workpiece is crucial, protective atmospheres may be used. These atmospheres, which can include gases like nitrogen or hydrogen, are employed to minimize the contact between the metal and oxygen, thus reducing the extent of scale formation. The presence of scale is a significant consideration in heat treatment, as it can impact the final material properties, and it is often addressed through subsequent mechanical or chemical removal processes to achieve the desired surface quality and finish, particularly in industries like aerospace and automotive manufacturing.