Annealing, is a heat treatment that alters a metals property. It increase the materials ductility (how metals can be formed or shaped) and makes it more “workable”. It involves heating the material to above its critical temperature, maintaining a suitable temperature, and then cooling. Annealing can induce ductility, soften material, relieve internal stresses, refine the structure by making it homogeneous, and improve cold working properties.
The annealing process in band saw blades is done after Stellite tipping. Stellite alloy is a range of cobalt-chromium alloys designed for wear resistance. The operational time of a wood band saw blade can be increased considerably by tipping the teeth with wear-resistant alloys such as “stellite”. This is especially evident in sawing hard woods, but is today also used in the processing of softer woods like fir and pine. The stellite-tipped teeth give better surface finishes and considerably longer operational times. It is vital, however, that the grinding work is carried out correctly, preferably by using wet-grinding machines and suitable grinding wheels.
All stelliting processes, whether carried out manually with a welding burner or automatically using plasma or resistance welding, call for annealing of the tooth points after stellite-tipping. Due to the carbon contents of the saw blade a re-hardening of the tooth point will occur through the heat influence of the stelliting, developing untempered martensite. A martensite refers to any crystal structure that is formed by displacive transformation.
A part of the tooth point behind the stellite tip will become brittle and the risk of breakage will increase. Annealing will lower the high hardness of the steel in the heat affected zone and minimize the breakage risk.
Annealing can be carried out by different methods, using
• plasma burner
• high-frequency annealing unit
• gas flame
Whichever way is used to carry out the annealing, it is important that it is done in a duplicable and in a controlled way. This is necessary to be able to apply the same temperature and the same annealing, time after time to each tooth of the band saw blade.
In the cases of copper, steel, silver, and brass, this process is performed by heating the material (generally until it is glowing) for a while and then letting it cool to room temperature in still air. Copper, silver and brass can be cooled slowly in air, or quickly by quenching in water, unlike ferrous metals, such as steel, which must be cooled slowly in order to anneal. In this fashion, the metal is softened and prepared for further work—such as shaping, stamping, or forming.
Blade annealing differs by manufacturer, type of blade and band saw blade widths. Those differences in the blade steel metallurgy and the annealing requirements makes band saw blade welding complicated. Fortunately most welding machines have become more sophisticated and are often controlled by computer related technology that allows for precise input of data for any particular blade to allow for a finite process. The annealing process provides variable annealing time and voltage to achieve optimum consistent weld strength based on the metallurgy.