42 THE DISTRIBUTOR’S LINK STRESS RELIEF IS CRITICAL FOR MANY UNHARDENED FASTENERS by Joe Greenslade Director of Engineering Technology, IFI Unhardened screws and bolts having a minimum head thickness equal to or less than one-half the nominal screw size are susceptible to delayed failures at the head-to-body juncture if they are not properly stress relieved. Carriage bolts (round head square neck bolts) are the most common standard fasteners in this category of parts. Other commonly used unhardened standard fasteners are round head bolts, step bolts, elevator bolts, and projection weld studs. Unhardened grades of inch screws and bolts having minimum tensile strengths of 74,000 PSI or less are SAE J425, Grades 1 and 2, and ASTM A307 Grade A. Unhardened property classes of metric screws and bolts having minimum tensile strength of 600 MPa or less are 3.6 through 6.8, with the most common property classes of 4.8 and 5.8. Unhardened screws and bolts, with relatively thin heads, are susceptible to delayed failures where the head of the fastener joins its body. This susceptibility is due to the severe compaction of the steel at that location during cold heading or cold forming. This compaction, known as “cold working”, creates what is referred to as residual stresses. Cold working creates residual stresses in all cold headed parts to some extent, but the stresses are not usually detrimental in fasteners having heads thicker than one-half the nominal diameter. Because the material compaction is not as severe when the head is thicker, more volume is available within which the stresses are distributed. Potentially detrimental residual stresses are effectively alleviated by a heat treat process called Industrial Fasteners Institute 636 Oak Tree Blvd. • Independence Ohio 44131 Phone: 216.241.1482 • Fax: 216.241.5901 www.indfast.org “stress relief.” The stress relief process involves heating the cold headed parts to a temperature lower than the material’s annealing temperature (the point at which the material begins to soften) and holding the parts at that temperature for a period of time. Then the material is allowed to slow cool back to room temperature. The stress relief process must be managed as an element of the part’s process control. Unfortunately, after the parts are shipped there is no metallurgical evaluation that can determine if a part has been stress relieved. Sometimes, but not always, the need for stress relief can be determined when the parts are tensile tested before product shipment. Unfortunately, not 100% of all parts in a given lot will fail a tensile test, thus the relatively small required testing sample size may not expose the problem before the parts are put to use. The tell-tale sign that a particular lot of parts needs to be stress relieved is that the screw or bolt fails where the head joins its body, regardless of whether the parts meet the required tensile strength or not. The pictures in this article are classic examples of what tensile failures look like where stress relief is needed. Fasteners that do not have excessive residual stress due to cold working always break in the fastener’s body or thread and never at the head-to-shank juncture. If parts are breaking in the head-to-shank juncture and not meeting the minimum required tensile strength, the mode of failure will change to the body or thread and the tensile values will increase after stress relief is performed. please turn to page 154
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