Metallic parts surface failure in heat treatment

Heat treatment modifies the initial microstructure of a part's surface and/or core, to give it specific properties for use (improved wear resistance, fatigue life, corrosion resistance, etc.).

During heat treatment –, which involves heating the part in a gaseous atmosphere –, undesirable interactions between the surface of the part and the furnace atmosphere can occur, leading to surface degradation. Among the precautions to be taken, particular attention must be paid before treatment to the preparation of the surface of the part to be treated, which must be in a known and controlled state of contamination. The presence of surface residues can be the cause of thermal or physical barrier degradation and/or physico-chemical reactions between the part and the heated gaseous environment. Control of the treatment atmosphere (carbon potential adjustment, for example) also plays a major role in the physico-chemical reactions that can occur on the surface of parts and lead to their degradation. The combined effects of atmosphere and surface contamination prior to treatment have a direct impact on the quality of treated parts. Poor control of heat treatment parameters can therefore be the cause of degradation of treated parts, which may also reveal anomalies in the upstream manufacturing process.

The first section of this article is dedicated to the different sources of contamination that can be found on parts: their nature, origin and impact.

The second section describes the different heat treatment atmospheres and the impact of poorly controlled atmospheres on the quality of treated parts.

In addition to the above-mentioned elements, the various parts and equipment of heat treatment furnaces (belt, muffle, hearth...), which are also subjected to heated gaseous environments, can be the site of damage, and are discussed in this article.

Finally, we briefly discuss the various analysis techniques that can be used before and after heat treatment. The main rules of the trade for limiting damage to the parts to be treated or to furnace components are cited.

A table of acronyms and symbols is provided at the end of the article.