The heat treatment of fasteners is a diverse and complex subject.

Figure 1. Hot bolts

The heat treatment of fasteners (Fig. 1) is a diverse and complex subject. In some instances, the challenge is bulk loading and high production volumes of small low-carbon or low-alloy fasteners. In other instances, the fastener sizes are larger and quantities smaller, but more advanced materials are involved. Here are some simple tips to help the heat treater do his job better.  

Tip #1: Know the Steelmaking Process
In selecting the type of steel, attention should be given to the deoxidation practice for the grades used for fastener manufacturing. A number of factors should be considered, including: heat-treated property requirements, heat-treat conditions, fastener size and steel availability.  

Silicon, for example, acts as a ferrite strengthener and, therefore, in the absence of aluminum produces steel with somewhat greater hardenability. Silicon-killed steel tends to have coarse (large) austenitic grains. For the same carbon grade and heat-treatment conditions with and without aluminum, complete transformation of the fastener core during heat treatment can take place in a larger section using a coarse-grain steel. Silicon-killed, fine-grain steel has both silicon added as the deoxidizer followed by the addition of aluminum for grain-size control.  

Austenitic grain size is not usually a factor for consideration in cold forming, but it has a significant effect in subsequent fastener heat treatment. Aluminum not only deoxidizes the steel but also refines the grain size (aluminum-killed steel). Aluminum also reacts with nitrogen in the steel to form aluminum-nitride particles that precipitate both at the grain boundaries and within the austenitic grains. As a result, the size of the grains is restricted even when the steel is reheated for carburizing or neutral hardening, hence the term fine grain.  

In the two types where silicon is added, the silicon content can have several ranges, with the most common being 0.15-0.30%. When aluminum is added to these steels for grain-size control, the aluminum content is generally in the 0.015-0.030% range. The aluminum content in fully aluminum-killed steels is generally 0.015-0.055%, which is somewhat higher on average since the aluminum must both deoxidize and control grain size at the same time.  

The disadvantage of silicon-killed steels is reduced ductility and tool life during cold heading because of its ferrite strengthening characteristic. Aluminum-killed steels are usually more formable and, therefore, provide somewhat improved tool life but reduced heat-treatment response during heading, particularly in larger fasteners. For this reason, the recommended maximum diameter for oil-quenched, aluminum-killed carbon grades is typically 4.8 mm (0.190 inch).