Annealing of Wire Products: Atmospheres

The purpose of a furnace atmosphere varies with the desired end result of the heat-treating process. In general, the atmospheres used in the heat-treating industry have one of two common purposes: •To protect the material being processed from chemical reactions that could occur on their surfaces (e.g. oxidation), that is, to be passive (or chemically inert) to the metal surface. •To allow the surface of the material being processed to change (e.g. adding carbon, or nitrogen, or both), that is, to be reactive (or chemically active) to the metal surface. When annealing wire, rod and similar products, we most often want the atmosphere to protect the product in question rather than be reactive with it. Air atmospheres or those using products of combustion from direct-fired burners are not capable of preventing steel parts, for example, from oxidation or decarburization. It is for this reason that various protective atmospheres (generated, pure, or mixed) are used (Table 1). The type of gas or gas mixtures used depends on the metal being treated, the treatment temperature, part contamination limits and the surface requirements of the product being annealed. Atmosphere control is often an important consideration as well. Purging is another critical first step in the annealing process and should take place when the workload is cold (prior to heating). Nitrogen or lean (non-combustible) Exothermic gas are common choices and purging is considered complete when less than 1% oxygen is