CS237813B1 - The method of removing residual strains in welds by ultrasound - Google Patents

Abstract

The invention relates to a method of removing residual stresses in welds by applying ultrasound, where the entire system, namely the backing, both welded parts, filler metal and molten pool, are actively treated with ultrasound in the frequency range of 17 to 213 kHz during the welding process.

Description

The present invention relates to a method of removing residual stresses in welds by ultrasonication.

In welded constructions, due to thermal intervention, an internal unbalanced state of the material structure occurs, which causes their own internal tension, which remains in the weld even after its cooling. The residual stresses that arise during welding belong to the group of those stresses that exist in the weldment even when they are not absorbed by external forces, heat and the like. The sense of residual stresses is decisive for the load-bearing capacity or the service life of the weld. In case of positive orientation and loading by tensile external forces, there is a superposition of tensile stresses, namely residual and loading forces. The result of this superposition of tensions with the same meaning is to reduce the load-carrying capacity of the weld. If this principle is ignored or ignored, the stresses exceed the strength limit and the weld is destroyed without the external tensile load reaching the intended or permissible value, respectively. This is practically reduced by the value of the tensile residual stresses present in the weld introduced by the welding. For this reason, it is necessary to reduce the residual voltage to a minimum.

Currently, residual stresses are most often removed by heat treatment. Specifically, annealing without recrystallization to remove internal stresses. It is carried out by annealing at approx. 550 ° C for two hours. However, the removal of residual stresses by heat treatment is uneconomical both in terms of energy and in terms of volume loss of material by the formation of oxides. It is often not possible to calculate larger welds due to the unavailability of large-scale furnaces.

The above-mentioned drawbacks are eliminated by the method according to the invention, which is based on the fact that both the welded parts, the filler metal and the melting bath are actively sonicated in the frequency range 17 to 213 kHz during the welding process.

When the welding process is affected by ultrasound, residual stresses are reduced. The advantage of this method of reducing residual stresses is that there is no need to supply thermal energy to the weld. j. anneal at approx. 550 ° C for two hours.

Tensometric measurement of the residual stresses by the destruction method revealed in all cases of experiments that the residual stresses were reduced so that the measured values approached zero. Thus, it can be stated that actively the ultrasonic energy propagation during the welding process significantly affects the value of the tensile residual stresses of the weld joint and these are minimized. This decrease in residual stresses in terms of utility stresses of the welded joint results in its higher load-bearing capacity and leads to the elimination of economically demanding processes, i. j. to remove annealing.

Example

According to the method of the invention, a welded ferritic-pearlitic ductile iron having a chemical composition in weight percent was as follows: carbon C - 3.58%, silicon Si = = 2.68%, manganese Mn - 0.50%, sulfur S - = 0.029%, phosphorus P = 0.05%, magnesium Mg = 0.18%, chromium Cr = 0.09%, nickel Ni - 0.05%, molybdenum Mo = 0.136%. Degree of eutecticity S _c = 1.055, at mechanical values: mechanical strength R _m = 531.9 MPa, ductility As = 9.75%, taper Z = 13.95%, Brinell hardness HB = = 186 and notch toughness KCU2 = 6.33 Jcm ^-2 . Welded with ES 722 electrode for welding nickel-based cast iron with nickel content by mass. % Ni -92%. The ultrasonic energy was supplied by an ultrasonic wave generator with a frequency of f = 17.5 kHz. The experimental material was cold welded, intermittent welds and the weld was applied in three layers.

The destruction method was used to determine the surface residual stresses. Electrical resistance strain gauges were glued to the degreased weld and insulated against ambient humidity and mechanical damage. The initial values of strain gauge deformation were measured, the spots were freed by the electro-spark method and the final strain gauge values were measured. Tensions were calculated. Comparing the residual stress values in the ultrasonic and untreated samples, it was found that the affected specimens showed a significant decrease in residual stresses that were close to zero or passed from tensile stresses to compressive stresses. The effect of ultrasonic energy during welding was as follows:

When welding without ultrasonic treatment, R _x = 419.0 MPa and R _y = 243.6 MPa, and with ultrasonic treatment R _x = -19.0 MPa and R _y = 39.7 MPa were measured.

Where R _x is the residual stress in the x-axis direction and R _y is the residual stress in the y-axis direction.

Claims (1)

SUBJECT A method of removing residual stresses in welds by ultrasonic treatment, characterized in that the entire system, namely the substrate, the two welded parts, the additional metal, the melting bath is actively sonicated during the welding process in the frequency range of 17 to 213 kHz.