What is Heat Treating?
Heat treating cold-formed fasteners involves heating the parts to a specific temperature and then allowing them to cool before further work is undertaken. The speed at which the metal is cooled is just as important as the heating process itself. If the fasteners are cooled too slowly, the benefits of the heating process are lost.
Once heated to the desired temperature, the fasteners are quenched in water, oil, or even air, which rapidly brings down the temperature and encourages the formation of martensite (a dense and hard crystalline structure) and, in turn, changes the properties of the fasteners.
What Properties are Changed?
The heating, quenching, and cooling processes cause changes in the structure of the fasteners. The changes affect several aspects of each part, including:
- Toughness, and
- Wear resistance
The specific effect on each fastener depends on the type of steel that has been used, and the size and shape of each piece. The internal, or core, of a larger fastener will take longer to cool than that of a smaller, thinner fastener. It is crucial that these differences are taken into consideration prior to beginning the process. If two pieces of differing sizes are put through the same process for the same amount of time, the properties of the two will be significantly different at the end. The larger of the two will lack the strength of the smaller one (as the core will cool much slower), and be less affected by the quenching process. The end product will also differ depending on the quenching medium that is used.
At AKKO, three different types of heat treatment are utilized, with the type of treatment used depending on the type of fastener being treated:
- Neutral Hardening - mainly used on machine screws
- Case Hardening - mainly used on tapping screws such as sheet metal screws
- Induction hardening - Used on M6 or greater thread forming screws
Not every fastener undergoes heat treatment, but instead undergoes a process known as stress relief. This is used after cold heading to achieve the strength requirement stipulated by the customer. It is an important part for these parts as cold heading can raise the Rockwell hardness of the parts, leaving them unsuitable for individual clients.
Why is this Important?
The heat treating process ensures that each fastener is suited to its purpose. Without the treatment, the fasteners would be weaker and more likely to shear or snap under pressure. Where fasteners are being used to secure moving parts, they are constantly under changing pressure and stress. Any faults within the fastener could cause it to distort or warp, causing damage to other parts.
Even when fasteners are used between non-moving parts, they are under considerable stress – often holding together parts that are essential for supporting other elements. A fault in a fastener here would cause a failure that could be catastrophic. The heat treating process helps to avoid these issues by minimizing or removing internal stress within the piece that can be caused during the cold-forming process.
The heat treating process needs to be carried out by a certified professional in the right environment. The addition of heat treating to the overall processes also increases the cost of manufacturing. However, the costs of the process are far outweighed by the benefits. Parts that have been through the heat treating process add to the reliability and longevity of the completed product. In turn, a higher quality product adds to the positive reputation of both the fastener supplier and the company producing the finished product.