Since being questioned about a previous blog about annealed vs. unannealed wire I thought it might be a good idea to write a short blog about cold heading wire. To clarify, I am not an engineer or a metallurgist but this is how I understand it from my 35 years in the industry. Raw material comes from molten steel created from a combination of iron ore and/or scrap melted and poured into moulds. When it cools, the resultant slab of steel is called a billet. The billet composition is key to the quality of the wire we use to produce fasteners. Unwanted contaminants or impurities pose risk to the cold heading process. Cold heading quality billets are inspected for surface imperfections which can be ground out to reduce the risk of surface defects in the final rod or wire.
The billets are then rolled using a combination of heat and pressure in order to turn a 4000 lb slab of steel into a 4000 lb coil of rod. This is called green rod and is the most basic state of coiled raw material. It can then be annealed and coated to create annealed rod (once referred to as #2 rod). Or it can be drawn at high speed through a series of draw dies to reach it’s desired finished size and then spherodize annealed to create “spherodized annealed at finished size” or “saafs wire”. Spherodizing is thermo-processing or heat treating the wire to change the molecular structure to one which is spheroidal in nature. I like to use the analogy of two bags. One is full of jacks, the other full of marbles. The wire starts out like a bag of jacks and ends up like a bag of marbles. If you nead the bag of jacks the lock up and do not move well. If you nead the bag of marbles they slide well on one another and move very well. This is why the wire is spherodized. In order to allow it to shape easier. “Saafs” wire still requires a final in-line draw or resizing prior to cold forming as the annealing process can result in wire ovality and thus inconsistency in the diameter. The most expensive and highest quality wire is “saip” or spherodized, annealed in process. This process takes the “saafs” wire and runs it through a high speed finishing draw after it is coated. This wire needs no draw and is the most expensive wire available (with the exception of a few exotic options left out for simplicity).
This is a somewhat simplistic look at cold heading wire and meant to provide a general understanding. Hopefully it has informed the uninformed ever so slightly!