FALL 2024

148 THE DISTRIBUTOR’S

148 THE DISTRIBUTOR’S LINK LAURENCE CLAUS RECOGNIZING COMMON FLAWS IN COLD HEADED FASTENERS from page 128 FIGURE 5: SCREW HEADS EXHIBITING SHEAR BURSTS A unique form of bursts are shear bursts (Figure 5). These are almost always on features that are upset in the heading operation including heads and flanges. They are easily recognizable because the burst is formed along shear planes at a 45° angle. Shear bursts are always a sign of overloading the raw material. This may arise for a multitude of reasons including poor tool design (the tool is attempting to “move” more material than it can), poor material formability due to prior cold working, poor ductility due to coarse grain size, ineffective or insufficient lubrication, rough surface, and overworking the part during pre-forming cold drawing. Forging Cracks are small cracks that appear on the surface of parts, often on sidewalls, bearing surfaces, heads, and sharp corners due to pressure exerted during the forming process or as the result of extremely high localized residual stress concentrations. Figures 6 and 7 show distinct types of forging cracks. Figure 6 shows the crack in the corner of a hex socket recess. The sharp FIGURE 7: EXAMPLE OF ALLOY 718 NUT WITH FORGING CRACKS ALONG SIDE WALLS corners are areas of high residual stress. Figure 7 shows an Alloy 718 nut, a nickel superalloy which is subject to cracking when processed outside of narrow processing margins. Figure 8 shows a part that has succumbed to quench cracking. Quench cracking is not due to forming but rather occurs during heat treatment. It is usually characteristic of larger size fasteners made of alloy or high carbon steels that exhibit excellent hardenability. During the heat-treating process when the parts are quenched the transition from Austenite to Martensite is immediate at the surface but takes a little longer at the core. There is a volume expansion during this transition so when the core expands after the surface, it generates extremely high residual stress. In some instances, those stresses may exceed the parts Ultimate Strength and find a path, usually at a vulnerable spot to initiate and propagate a crack. Often quench cracks such as those illustrated in Figure 8 are large and catastrophic. FIGURE 6: EXAMPLE OF A FORGING CRACK IN THE HEX CORNER (APPROXIMATELY 5 O’CLOCK POSITION) FIGURE 8: ALLOY STEEL BOLTS EXHIBITING QUENCH CRACKS CONTINUED ON PAGE 156

MWFA FSTNR WEEK ’24 71st ANNUAL GOLF OUTING - AUGUST 21, 2024 CONTINUED ON PAGE 151