Question:
We are looking for guidance to better understand the contribution of heat treating to stresses present in gear wheels so we can continue to make improvements in our products.

Answer:
Let’s continue our discussion of how variations in heat-treatment parameters can influence the development of stress in gear wheels. We now consider the influence of:

Prior to Heat Treatment
Many of the processes prior to heat treatment involve the removal of material. These can include grinding, broaching, turning and other machining operations. Because of the speed and feed-rates needed for these operations to be profitable, a large amount of residual stresses can be created in the part, which will be relieved during heat treatment, resulting in part distortion. To minimize the formation of residual stresses, it is often necessary to turn the part over several times during the machining operation.

Heat Treatment
Parts containing residual stresses prior to heat treatment will relieve those stresses during heat treatment. The relaxation of these stresses will cause distortion as the part finds a stress-free equilibrium. Heat-up rates in the furnace can also cause distortion, as thermal gradients are formed and the thinner sections reach temperature quicker. There will be differential thermal expansion, which can cause sizable thermal strains to be developed within the part. If these thermal strains are large enough, plastic deformation and distortion can occur. The use of a preheat stage to allow thicker sections to “catch-up” to the thinner section will reduce distortion. The same thing can occur if the furnace has nonuniform temperature within the work zone.

Racking of parts is extremely important in the heat-treating process. Proper racking minimizes part-to-part interactions as well as allowing heat to reach all parts. It further allows the quenchant to evenly extract the heat from parts in a uniform fashion. Properly racked parts minimize distortion during quenching and allow them to be evenly heated.

One area that is often overlooked in the control of distortion and residual stresses is the role that atmosphere plays. Most gears are carburized to achieve a hard-wearing surface. In non-wear critical areas, carburizing is not needed. These regions are plated or coated with a carburizing stop-off to prevent the diffusion of carbon into the steel.

As steel transforms from austenite to martensite, there is a volumetric expansion that increases as the carbon content increases. Typically, this amount is between 3-5% for carburized steel. This volume change will cause differential transformational strains, which may cause distortion. While these strains may not cause a distorted part to occur immediately after heat treat, these strains can appear immediately after any subsequent machining steps as the part tries to achieve a new static equilibrium. These residual stresses can also manifest themselves by shortened fatigue life.

Proper atmosphere control is important. Excessive soot can be carried into the quench oil, creating dirty parts and shortening the life of the quench oil. Proper atmosphere control can also reduce the amount of retained austenite, which can also cause residual stresses and distortion.

More to follow…