WINTER 2014

168 THE DISTRIBUTOR’S

168 THE DISTRIBUTOR’S LINK FASTENER FAILURES continued from page 12 This type of defect on a socket head cap screw may lead to thread strength and shear engagement problems due to its expected high clamp load potential. This condition could be due to a combination of a thread lap and cracking resulting from the heat treatment/process control. The lot needs to be returned to the manufacturer for disposition. Incorrect Material Another interesting type of failure occurs when a manufacturer uses only one type of steel for all fasteners. For instance, the AISI 4140 steel is a versatile composition that can be used for many product grades; from socket head cap screws, SAE Grade 8, ISO 10.9, ASTM A193- B7 bolts and studs and even Grade 5 bolts. It became more cost effective for the manufacturer to use the higher grade of steel for all of his product manufacturing, due to high volume discounts, than to stock several types in different diameters. However, in the following case, the 4140 steel was used for an ASTM A307B fastener. The following depicts a head failure with an A307B fastener using a 10 o wedge during tensile testing. The head was hot formed but the residual stresses from the forming of the steel caused the head to fail where failures are not usually expected with normally specified low carbon steel product. Several samples exhibited hardnesses of Rc 28-31 average, while others were below Rb 100. Hot heading is performed at temperatures above 1000 o F (538 o C). Temperatures, time and cooling rates can change the structure of steels, since iron is an allotropic material and can exist in more than one type of lattice structure (such as a face centered cubic, f.c.c., or body centered cubic, b.c.c.). If not controlled properly, the iron can be in different phases, as evidenced from the wide range in observed hardness readings. This should be the first clue if only hardness is performed and a wedge tensile test is not done as hardness has a direct correlation on tensile strength and brittleness. The fasteners should have been stress relieved. Look for Clues This next photograph shows eight fractured socket head cap screws. Now, clearly these are fatigue fractures, but the patterns can be applied to other application fracture cases as well. Normally, a fastener will fracture at either the head or where the first thread is not engaged. This thread may be either the first thread at the nut which is against the joint surface, or in the case of a tapped hole, the first thread outside the grip zone. This grip zone, where the male and female threads engage to cause the fastener to stretch, depends upon the relative strength-to-hardness ratio between the fastener and the base material of the tapped hole. As with a nut of matching strength and hardness, the depth needs to be only that of the diameter of the fastener. If the material is softer, then the hole must be deeper to provide more threads to carry the load of the cap screw. In the above photograph, the fastener with the head still attached was the last to fail. It broke in tension at the threads in the grip zone. The others failed at the head as the application was supporting a die ring that was under high pressure that cycled. The head on this fastener would have failed also but the others had weakened the connection clamp load so much that this lone fastener was carrying the majority of the load until it failed in tensile. The progression of the fatigue fracture across the surface of the steel will indicate how long the microcrack had been propagating before the tensile failure occurred. For example, the fourth from the left was the first to begin failing. This is because the fatigue fracture had propagated nearly completely across the cross section of the cap screw. As one begins to lose clamping load, the stresses are moved to another fastener, usually the adjacent fasteners, who begins to develop their own stress cracks. Their crack propagation is a little less than the first. And the cycle progresses to the other fasteners in the connection; each one has a little less progression than the previous cap screw. please turn to page 170

THE DISTRIBUTOR’S LINK 169 AZ Lifting Hardware (AZLH) is proud to announce it has teamed up with ProWest Marketing Inc. to represent them in the states of Alaska, Oregon and Washington. ProWest was established in 1989 by George Schneider to bring high quality products to industrial distributors in the northwestern US. Fastener veteran Terry May joined the agency in 1995 to concentrate on fastener and specialty tool distributors. “I’ve known Terry for years and feel very good about our new association.” Says Chuck Smith, president of AZLH. “Our line of industrial lifting and suspension products are a good fit with the other lines ProWest represent.” Terry made clear their decision to represent this line was not taken lightly, “One thing that attracted us to AZLH is their willingness to meet the needs of our customers. Their view on quality and their Specials capability goes well beyond the products in their catalog. Our goal is to insure the distributors in our area have a greater awareness of how AZLH can help.” AZ Lifting Hardware is a wholesale distributor of globally produced industrial lifting hardware such as turnbuckles, eye bolts, shackles, hooks and related wire and chain hardware serving the fastener distribution and industrial supply markets. To request a catalog or more information contact them at Tel: 1-888-936-1466, Fax: 623- 936-8909 or visit their web site www.azliftinghardware.com. The Durham Manufacturing Company introduces two new Bearing Racks. Made using prime cold-rolled steel. Each unit has six full panel cradles holding up to 75 lbs. each. Cradles, when fully extended allow the compartment box to be opened beyond a 90° angle to prevent accidental closure. The racks have a positive closure feature that keep trays from sliding out unintentionally. The units can be purchased with or without a door. Model 321B-95 does not have a door. 321B-95-DR includes the locking door and two keys. Compartment boxes can be easily removed for transport to work area. Base (Part # 304-95) is available as an accessory and the unit has a durable gray powder coat finish. For more information on pricing for this item call 1-800-243-3774 ext. 7300 or send an email to info@durhammfg.com. To print the sell sheets for this item or any of our products, go to: www.durhammfg.com/durham_ catalogs.