FALL 2024

88 THE DISTRIBUTOR’S

88 THE DISTRIBUTOR’S LINK LAURENCE CLAUS RECOGNIZING COMMON FLAWS IN COLD HEADED FASTENERS from page 8 There are many reasons other than raw material that lead to cracking, exposing a tendency to unfairly or, perhaps, out of indolence, blaming the raw material. Even so, flaws in raw material are a significant source of finished part flaws, and, therefore, shall serve as a starting point for this discussion of flaws commonly found in cold headed fasteners. Wire Seams One of the most common complaints of manufacturers related to the raw material utilized in cold heading is encountering seams in the wire. What exactly is a seam and how does it get there? Wire seams are the result of an unwanted artifact of the melting and casting process. Today’s cold heading raw material comes almost exclusively from mini mills that melt and cast a mix of pure iron and steel scrap. Once refined, the melt is continuously cast into billets which are long bars of round, square, or rectangular cross section. These billets are later reheated and processed through a hot rolling mill which reduces them into long coiled circular rod. During the melting process the mill must undertake a complex process to refine the steel into its desired chemical constituency (steel grade). To obtain the low carbon and low carbon steel alloys necessary for cold heading the mill must reduce the carbon content in the melt. To accomplish this, they lance (inject) pure oxygen into the furnace. The oxygen binds with some of the FIGURE 1: CROSS SECTIONAL VIEW OF A WIRE SEAM FIGURE 2: AXIAL VIEW OF A WIRE SEAM carbon present to form Carbon Monoxide and Carbon Dioxide gases, which bubble out and are recaptured. Injecting pure oxygen into the hot steel generates a violent reaction that at some point needs to be halted. The mill will take steps to do so, however, the reaction may continue to occur for a time and continue to emit these gas bubbles. When the liquid metal is cast into the billet any remaining bubbles that emerge on the surface before solidifying result in a small surface imperfection, like a small crater or pockmark. Although the mills take steps to inspect and remedy these imperfections on the billet prior to hot rolling, they may not always be 100% effective. How do these imperfections, however, turn into a problematic seam? Now imagine for a moment a small round hole in a block of taffy. If the taffy is warmed up and then pulled from each end this round hole will start to stretch and elongate, first becoming egg shaped and eventually totally closing the sides in on one another. Pulling further extends this imperfection but never knits the two sides of the void together. Cutting through this in cross section would reveal a vertical fissure (Figure 1). This is analogous to what occurs with any void or imperfection on the surface of a billet. When hot rolled these imperfections are stretched out into what is called a seam (Figure 2). The problem with wire seams is that they will open into a fully blown crack when exposed to the extreme forming loads imposed during the cold heading process (Figure 3). CONTINUED ON PAGE 128