Fatigue resistance for coupling and connection joint mechanisms

A method of manufacturing one of an integral threaded metallic box or an integral threaded metallic pin of a drilling tool with improved resistance to fatigue failure at a zone of service-induced stress concentration encompassing the location of an initial thread of the box or pin, comprising:

• forming a blank surface on the drilling tool at which the threads of the box or pin will be formed,
• applying tensile force to the material of the drilling tool to obtain an applied tensile stress throughout the integral material of the zone and an area of predetermined size adjacent the zone, the zone extending inward from the blank surface into the material to an innermost boundary and the area extending from the innermost boundary of the zone to even greater depths into the material,
• heating the material of the zone into one of a plstic or liquid state,
• extending the depth of penetration of the plastic or liquid state from the blank surface to a predetermined depth which is greater than the depth of the threads to be formed at the blank surface,
• maintaining the magnitude of applied tensile stress throughout the area within the elastic limits of the metallic material of the box or pin while heating the material of the zone,
• releasing the applied tensile stress in the area after the material of the zone has returned to a nonplastic and nonliquid state,
• selecting the predetermined size of the area for obtaining sufficient residual tensile force therefrom after releasing the applid tensile force to achieve a counterbalancing residual compressive force of sufficient predetermined magnitude in the zone to increase the resistance of the box or pin to fatigue failure from service-induced tension stresses at the zone, and
• forming threads into the blank surface after accomplishment of the foregoing steps.