FI77590C - Procedure for mechanical reduction of voltages. - Google Patents

Description

1 77590

This invention relates to a mechanical method for removing residual welding stresses from welded metal-5 strip, heat exposed areas (HAZ) and adjacent parent metal in frame structures, piping and mechanical parts. More specifically, the invention relates to a method for reducing residual weld stresses in a weld metal, a heat exposed region and an adjacent parent metal in steel tubes welded to each other by circumferential welding, whereby mechanical compressive stresses are applied to the weld metal, the heat exposed region and the base metal.

15 When welding pipes, frames and mechanical parts, residual tensile stresses are generated. These tensile stresses tend to promote the formation of stress corrosion fractures, as well as fractures due to fatigue and creep. Thus, it would be highly advantageous to remove these residual hit-20 dry stresses.

Stress reduction by induction heating (IHSI) is one method currently used to remove residual welding stresses. However, the IHSI method used for pipes and frame structures is unable to generate compressive forces in either the radial or circumferential directions. The balance of radial compressive stresses caused by the IHSI method causes radial tensile stresses such that the radial force generated in each cross section is zero-30 la. In fact, large, wall-penetrating stresses are generated by the IHSI method. Welding defects such as inclusions, pores, melting defects, hot tears, etc., as well as stress corrosion or fatigue fractures are thus affected by high tensile stresses during the process. The resulting propagations of stress corrosion fractures, fatigue damage and creep stress fractures reduce the residual strength and durability of the wall and increase the possibility of wall leakage.

U.S. Patent No. 4,018,634 discloses a method of making high strength pipes from a steel sheet by welding. The tubes are work-reinforced by compressing them radially throughout, after which they are heat-treated. This known method presents a compression applied to the entire outer surface of the tubes. Since the treatment reduces the tensile strength of the pipe, it has to be heat treated afterwards.

Our mechanical stress-reducing method is an improved method so that high tensile stresses do not build up in the pipe or tank body during the process, thereby eliminating the potential detrimental effects of these tensile stresses.

We have found that with our method we can reduce residual stresses in weld metal, heat exposed areas and adjacent parent metal, the method being characterized in that the compressive stresses are greater than their individual yield strengths but do not exceed their fracture limit in compression and are sufficient to permanently reduce the circumference of the tube in the weld metal, the heat exposed area, and the adjacent metal, wherein the stresses are applied using annular means distributed circumferentially and forced by external means to cause a permanent reduction in circumference, the compression means being removed after the compression step.

The compressive stresses of our method, when added to the initial residual stresses, exceed the yield strength of the material in the weld metal, in the heat exposed region, and in the adjacent parent metal. This causes a permanent plastic flow of the material when the stresses exceed the yield strength. When the process stresses are then removed, the initial residual tensile stresses are reduced.

Il 3 77590

The mechanical means used to achieve the above result are not critical. In the embodiment for pipes and tank bodies, the mechanical means comprise, for example, ring-5 connectors having an inner surface corresponding to the shape of the outer surface of the pipe or tank body and means for performing a controlled, compression deformation via said ring connectors on the outer surface of the tube or tank body. A method for adjusting the surrounding compression set along the length of a pipe or container body can be obtained by forming openings arranged between adjacent ends of said ring connectors and placing bolts in said openings. Preferably, the initial gap between the adjacent ends 15 of said ring connectors is dimensioned by adding spacers between them. The method for adjusting the desired radial compression set can be obtained by using two pairs of ring connectors, with openings being arranged at adjacent ends of said ring connector and a bolt being placed in each of said openings.

It is obvious that several modifications and variations can be made to the above invention without departing from its spirit and scope, and thus only those limitations which appear from the appended claims may be made.

Claims (1)

4,77590 Claim A method for reducing residual weld stresses in a weld metal, a heat exposed region and 5 adjacent parent metals in steel pipes welded to each other by circumferential welding, whereby mechanical compressive stresses are applied to the weld metal, the heat exposed region, are greater than their individual yield strengths but do not exceed their compressive fracture limit and are sufficient to permanently reduce the circumference of the resulting pipe in the weld metal, heat exposed area and adjacent metal, 15 applying stresses using annular means spaced circumferentially and forced by external means causing a permanent reduction in circumference, whereby the pressing means are removed after the pressing step.