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:

Quenching
There are many sources of residual stress and distortion. In quenching, the primary source of distortion and residual stresses is differential temperatures from the center of the part to the surface or from different locations on the surface. By reducing the thermal gradients and differential temperatures, large reductions in residual stresses and distortion can be achieved. The largest factors that affect the creation of large thermal gradients in parts during quenching are temperature, agitation, the quenchant chosen and contamination of the quenchant.

Temperature
Increasing the oil temperature can reduce the distortion and residual stresses in a heat-treated component. As the temperature of oil is increased, the temperature gradients in the part are decreased. This is the basic principle of martempering. Using increased temperature can also reduce thermal gradients in cold oils, up to the recommended-use temperature of cold oil, typically 180-200ºF (80-95ºC). It has been reported that the speed of cold oil can be increased by increasing the temperature of the oil in the range of 160ºF (70ºC).

Agitation
Distortion occurs because of differential temperature gradients, whether from the center to the surface or from surface to surface. All three phases of cooling occur in the piece at different times, which means that some areas are cooled very slowly while other parts are cooled rapidly. This has the effect of creating thermal gradients on the surface of the part, which can cause distortion. The purpose of agitation is to minimize these surface gradients.

Quenching characteristics are influenced significantly by the degree of agitation. The degree of agitation reduces the stability of the vapor phase and increases the maximum rate of cooling. This also has the benefit of minimizing any vapor pockets that can occur and ensuring that there is a uniform heat transfer across the surface of a part.

Quenchant
There are many types of petroleum-based quenchants. For most gear heat-treating applications, the use of marquenching oils is used almost exclusively because of the benefits of reducing distortion. However, there are certain applications where cold oils are used, specifically in very large sections or where press quenching is used.

Contamination and Oxidation
The condition of the quench oil can also contribute to distortion of gears. Contamination of quenching oils with water must be avoided at all cost. As little as 0.05% of water in quenching oil influences quenching characteristics significantly and may cause soft spots, distortion or cracking. At concentrations of 0.5% or more, foaming during quenching is likely, and this can give rise to fires and explosions. Other contaminates, such as hydraulic oil and fire-resistant hydraulic fluids, can also alter the quenching characteristics, resulting in increased distortion and residual stresses.

The oxidation of quenching oil is measured by the Precipitation number or Total Acid number. As the oil oxidizes it forms organic acids. The formation of oxidized constituents decreases the stability of the vapor phase and increases the maximum cooling rate. This can increase the risk of distortion and cracking. The use of stable, high-quality quench oils will reduce the possibility of this occurring. The use of a proactive maintenance program of monthly or quarterly checks for contamination and oxidation will also prevent quenching irregularities.

More to follow ...