Question: We have a 4340 steel that is used for gear production by our customer. The production sequence is melting, forging, primary heat treatment (normalizing and/or annealing), machining, stress relieving, final heat treatment (surface hardening). To obtain the optimum balance of machinability and high hardness, which will be better for the primary heat treatment, normalizing or annealing?

Answer: Most gear blanks are normalized prior to machining of the gear teeth to minimize distortion during hardening or carburizing. A normalized gear blank is typically very machinable but harder than an annealed part.

A necessary first step in the heat-treatment process involves normalizing, a process designed to present a homogenous microstructure to the carburizing process. To reduce overall part distortion, it is highly undesirable to have this homogenization take place during the carburizing cycle of a finished gear component. A separate normalizing cycle removes the problems associated with pearlite and ferrite segregation. Ferritic areas do not transform to the same hardness and stress levels as pearlitic areas when homogenized during the carburizing cycle, resulting in more –not less – distortion.

Normalizing is a process that involves heating a part above the upper critical temperature and then typically air cooling outside the furnace to relieve residual stresses in a gear blank and to aid dimensional stability. Normalizing is often considered from both a thermal and microstructural standpoint. In the thermal sense, normalizing is austenitizing followed by cooling in still or slightly agitated air or nitrogen. In a microstructural sense, the areas of the microstructure that contain about 0.8% carbon are pearlitic, while the areas of low carbon are ferritic. A normalized part is very machinable but harder than an annealed part. It also relieves any residual stresses present from the steelmaking and forging processes that could cause later distortion during carburizing.

A good normalizing cycle consists of holding the rough material "at temperature" for two hours minimum or one hour per inch (25 mm) of section thickness. The temperature used should be between 25°F and 50°F (13-27°C) higher than the carburizing temperature.