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For decades, automotive connecting rod were fabricated machining separately the cap from the body, with precision to maintain accurate alignment, being that the fixing was by the use of special screws, with grinded body to remain perfectly adjusted in passage holes, aiming this way prevent possible lateral displacements. From the end of the last century, due to new technologies in the production and use of special microalloyed steels such C7056BY, permitted to introduce in the market connecting rods made of a single part, using the method of fracture splitting process to separate the cap from the body of the connecting rod. This technique provided gains as weight reduction and consequently reducing noise and vibration due to the decrease of the oscillating mass from the system. By the literature is estimated cost savings of up to 25%, and better fatigue performance.

The formation of structurally delta ferrite in low carbon alloy steels, used to make high resistance bolts, reduces its mechanical properties, due superficial hardness caused by diffusion of phosphorus during quench and tempering process. This phosphorus is present in the surface of roll material as phosphate bound to zinc to reduce friction coefficient during cold forming operations, as wire drawing and extrusions. In recent years, several recalls have been made by automakers, in order to avoid delayed failure possibility, due embrittlement of the bolts caused by phosphorus diffusion, that became the threaded fasteners susceptible to the retarded fracture for action of hydrogen released in corrosion process when the bolts is simultaneously submitted to tension under corrosion environment.

An increase in the fatigue resistance on threaded elements is obtained by thread rolling after heat treatment. This effect is reached by introducing residual compressive stress gotten from the plastic deformation operation to the formation of the threaded fillets. In critical application bolts, submitted to high work force, as used to assembly connecting rods, main bearing or cylinder head, this additional fatigue resistance is crucial to design joints of low weight and high reliability. In many of these applications, due to corrosion problems, the fasteners receive as a final operation, superficial treatment of the organometallic kind or electroplated coating, which due to the process of curing or to eliminate hydrogen embrittlement are submitted to temperatures between 200° C - 320° C. The objective of this paper is to show the influence of the temperature in the final process in the fatigue resistance to fasteners with thread rolled after the heat treatment.

In the last years, new and precise tightening methods have been introduced in the automotive industry with the purpose of reaching high-precision and low-scatter assembly force. In general, these methods are based on measurement of the angular displacement of a bolt head or nut taking into consideration a fixed reference. The attainment of maximum precision assembly x pre-load is reached at time fasteners are tightened in the elastic-plastic zone, i.e. under permanent deformation. In some applications fasteners are tightened more than once thus resulting in increase in the plastic deformation that add up after each tightening. The purpose of present study is to demonstrate behavior (plastic deformation in 8.8 and 10.9 high strength bolts) whenever bolts are submitted to assembly forces up to 100% of their capability to generate assembly x pre-load, using these results to establish bolting parameters and adequate number of reutilization.