FALL 2024

128 THE DISTRIBUTOR’S

128 THE DISTRIBUTOR’S LINK LAURENCE CLAUS RECOGNIZING COMMON FLAWS IN COLD HEADED FASTENERS from page 88 surface are points of vulnerability and can be a crack initiation site. FIGURE 3: FINISHED PART EXHIBITING WIRE SEAM Although seams are the most notorious, it is important to understand that other imperfections that appear like seams can occur. If there are misalignment problems with the rollers during the hot rolling process, a lap can occur. Often the surface appearance and potential consequences of a lap are identical to a seam. Commonly hot drawn material is further cold drawn for improved dimensional tolerances. The hot drawn rod is pulled through a round draw die. If there is any kind of hard debris that gets lodged in the die or the polish on the die begins to deteriorate, a scratch on the wire surface may form. Once again, this may appear like a seam and result in similar problems during part forming. Inclusions Inclusions are unwanted non-metallic structures or debris trapped inside the steel. They perform no useful purpose and, in fact, can be deleterious if too large, near the surface, or in too significant a quantity. There are two types of inclusions, Endogenous Inclusions and Exogenous Inclusions. Endogenous Inclusions are formed when small residual (or tramp) elements are present during the melting phase. These undergo reactions with other elements to form non-metallic compounds. Exogenous inclusions are the entrapment of nonmetallic debris, usually slag or a piece of refractory surface (ceramic lining that helps protect the walls of a furnace or ladle). Inclusions can negatively impact material properties, such as formability and toughness. Large inclusions (macro inclusions), especially near the Forming Flaws One of the most common forming problems is cracking, particularly in the head or flange. Of course, this makes sense when you consider the cold heading process, and the substantial loading exerted on the raw material to form it. In fact, some of these machines are exerting many tons of force to move the material around. Any concentration of stress or existing vulnerability becomes a likely site to initiate a crack. In cold heading cracks can be characterized into three general categories: Bursts, Forging Cracks, and Quench Cracks. Bursts can be categorized into two types: Bursts and Shear Bursts. Bursts are most frequently found at the periphery of the head or flange and follow the direction of the axis of the part. They may be open or closed and almost always form from a vulnerability in that spot such as a seam, lap, scratch, or other surface defect. Figure 4 illustrates a classic burst in the head and Figure 3 illustrates a burst arising from a wire seam. In addition to the obvious sources like wire seams and scratches, these can be triggered by worn out tools, large macro inclusions near the surface, or any surface condition that results in localized stress concentrations. FIGURE 4: EXAMPLES OF BURSTS IN SCREW HEAD CONTINUED ON PAGE 148