Kunio Enomoto

Welding residual stresses at the intersection of a small diameter pipe penetrating a thick plate

Abstract This paper investigates welding residual stresses at the juncture of a small diameter pipe penetrating a thick plate. The pipe is welded to the plate, and TIG cladding is melted on the inner surface of the pipe to protect it from stress corrosion cracking due to environmentally-induced changes in nuclear power plant components. Welding residual stresses are calculated by heat conduction and thermal elastoplastic analyses using an assumption of “simplified pass” to save the computing time, and are also measured by the strain-relief technique. Welding residual stresses after TIG cladding are shown to have no corrosive influence on the inner pipe surface, and the residual stresses are compressive enough to protect the pipe against stress corrosion cracking on the outer surface.

Residual stresses in girth butt welded pipes and treatments to modify these

Abstract A number of Type 304 stainless steel pipes are used in the primary cooling systems of nuclear plants. Intergranular stress corrosion cracks (IGSCC) were found at some welded joints in these piping systems due to very high tensile residual stress, sensitization of the material due to welding, and corrosive environment, all occurring simultaneously. Investigations have shown that at least one of the above factors must be eliminated to prevent IGSCC. This report describes experimental results on the temperature variations during pipe welding by conventional techniques and by the heat sink welding (HSW) technique. The mechanism of residual stress generation due to welding is also discussed. The pipe used in these experiments was 4B Sch80 Type 304 stainless steel. It was found that the temperature distribution through the thickness of the pipes was almost uniform for the conventional welding technique, but had a very sharp gradient for HSW. In the pipe axial direction, the temperatures varied sharply for both welding techniques. This implies that the sensitization of metal due to HSW is lighter than that of conventional welding and that the residual stress on the inside surface of the heat sink welded pipe is compressive. The induction heating stress improvement (IHSI) method has been investigated analytically and experimentally. In the IHSI method, a pipe is heated with an induction coil while cold water is pumped through it. This causes a temperature gradient throughout the pipe wall which generates high thermal stresses. This, in turn, generates compressive stresses on the inner surface of the pipe. This method is designed to eliminate tensile residual stresses near the weld heat affected zone on the inner surface. Temperature analysis and subsequent thermoelastic-plastic analysis show that tensile weld residual stresses at a joint were changed into compressive stresses on the inner surface of a pipe. It was confirmed experimentally that these stresses suppressed fatigue crack propagation in the heat affected zone (HAZ) of a welded pipe. Therefore, the IHSI method is effective not only in preventing crack initiation but also in suppressing crack propagation. As for the relaxation of residual stresses, no significant relaxation was measured when external loads were applied at as much as 80% of the yield strength in the experiments.

Development of thermal fatigue testing apparatus with BWR water environment and thermal fatigue strength of austenitic stainless steels

A thermal fatigue testing apparatus was developed in order to clarify the fatigue behavior in BWR environment. Pressurized high and low temperature pure water were alternately supplied into an autoclave with a small cylindrical specimen. Then a fatigue specimen was subjected to homogeneous thermal stress through the wall thickness. Fatigue crack initiation behavior was observed with the replication method and compared with the mechanical fatigue strength performed in air and high temperature water. The thermal fatigue strength of type 304 and 316 nuclear grade (316NG) stainless steels agreed closely with the mechanical fatigue strength, when transforming the nominal stress amplitude to the fictitious stress amplitude by using the mean value of strain amplitudes for room temperature and 288°C.

Optimized Conditions for Tensile Residual Stress Reducing Weld Using Water-Shower Cooling

To reduce tensile residual stress in a welded region, we developed a new cooling method that applies a water shower behind the welding torch. When this method is applied to the welding of austenitic stainless-steel, the welding and cooling conditions mainly determine how much the residual stress can be reduced. To optimize these conditions, we first used FEM to determine the effects of preheating temperature, heat input quantity, and water-shower area on the residual stress, and found that, to decrease tensile residual stress, preheating temperature should be high, heat input low, and the water-shower large. To confirm the effectiveness of these optimized conditions, residual stresses under optimized or non-optimized conditions were experimentally measured. It was found that the residual stresses were tensile under the non-optimized conditions, but compressive under the optimized ones. These measurements agree well with the FEM analysis. It can therefore be concluded that the optimized conditions are valid and appropriate for reducing residual stress in an austenitic stainless-steel weld.Copyright

Residual Stress of Thin-Wall Pipe Subjected to Axisymmetric Plastic Expansion

A straight thin-wall pipe was plastically expanded at one end in the radial direction by inserting a rigid disk. The residual stress measured after withdrawal of the disk at the inner surface in the hoop and in the longitudinal direction shows a strong tensile peak beyond the region where the pipe was directly expanded by the insertion of the disk. The reason why the residual stress reaches its peak at the location far inward of the pipe, not in the region where the pipe was directly expanded, is discussed. From the FE analysis, it is concluded that the residual stress reaches its tensile peak on the inner surface at the plastic region front that was developed during the pipe expansion, and a simple formula for the tensile peak location is proposed. The similarities and differences between the residual stress distribution of the thin-wall pipe and the thick-wall pipe are discussed.Copyright

Changes in Residual Stress in Worked Surface Layer of Type 304 Austenitic Stainless Steel Due to Tensile Deformation

Changes in the residual stress in a worked surface layer of type 304 austenitic stainless steel due to tensile deformation were measured by the X-ray diffraction residual stress measuring method. The compressive residual stresses introduced by end-mill, end-mill side cutter, and grinder were easily changed into tensile stresses when the plate specimens were subjected to tensile stress greater than the yield stress of the solid solution heat-treated material. The residual stresses after the tensile deformation depend on the initial residual stresses and the degree of preliminary working. The behavior of the residual stress changes can be interpreted if the surface-worked material is regarded as a composite made of solid solution heat-treated material and work-hardened material.

Generation of welding residual stress when a pipe penetrates a thick plate

Summary This paper describes a study of the welding residual stress at the intersection of a small‐diameter pipe penetrating a thick plate. The pipe is welded to the plate, and TIG cladding is deposited on its inside surface to protect it from stress corrosion cracking due to prolonged operation in a nuclear power plant. The residual stress is calculated by a heat conduction analysis and tensile elastic‐plastic analysis and also measured by strain gauges. The welding residual stress is shown to have no corrosive effect on the pipe inside surface, and the stress is sufficiently compressive to protect the pipe from stress corrosion cracking on its outside surface. The mechanism of residual stress generation during welding is also investigated.

A Predicting Method of Welding Residual Stress Using Source of Residual Stress. Report 4. Experimental Verification for Predicting Method of Welding Residual Stresses in T-joints Using Inherent Strains.

In this paper, a method of predicting welding residual stress in a T-joint using inherent strain (the source of residual stress) is verified by experiments. The measured inherent strains are compared with the results of thermal elastoplastic analysis and the simplified formulae proposed by some of the authers.Released strains along the welding direction of T-joints were measured by strain gages, and inherent strains were calculated from the released strains. The stress equilibrium across the cutting plane during releasing strains is necessary for determining the inherent strains accurately.Measured inherent strains in T-joints agree well with the results of thermal elastoplastic analysis and also with the simplified formulae which are presented within the scatter of the inherent strain rlictrihntinnThe proposed method of predicting welding residual stresses using the inherent strains is experimentaly validated.