Residual stresses measurements at ultrasonic submerged arc welded joints.

Dumitrache, Constantin ; Sabau, Adrian ; Barhalescu, Mihaela 等

1. INTRODUCTION

Ultrasonic welding is a complex procedure that enables substantial improvement of mechanical resistance characteristics of heat affected zone (HAZ) and really step stress gradient. The advantages of this procedure, as compared to normal welding, consist in grain refining by fragmenting primary crystals induced by ultrasonics. In the case of submerged arc welding, current practice involves the introduction of ultrasonics into the liquid metal bath by means of an electrode wire which is at resonance (Susan et al. 2008).

The scheme of ultrasonic submerged arc welding device is illustrated in figure 1 (Dumitrache, 2000). The components are: 1-sustaining pipe of Romanian AST 3 submerged arc welding device; 2-the first support of ultrasonic transducer system; 3-the second support; 4-sustaining element of hub transport electrical current; 5-the element transfer of ultrasonics into the liquid metal bath by using the electrode wire; 6-the electrical cables from ultrasonics generator and hoses from the cooling system.

[FIGURE 1 OMITTED]

The paper presents the residual stresses experimental studies in the proximity of the welded beads, consisting of the following stages:

1) The measurement points are located in the welded area of the specimens with dimensions 720 x 300 x8 x [10.sup.-3] m;

2) Welding was performed both under normal conditions (classical welding-NW) and ultrasonic activation of electrode wire (UW) with amplitude in welding zone A=10 x [10.sup.-6] m;

3) Residual stresses measurements were done by the use of the hole-drilling strain gage method;

4) The values of the residual stress fields were analysed and compared for normal (NW) and ultrasonic welding (UW).

2. EXPERIMENTAL PROCEDURE

Four of residual stress strain gage rosettes (Hottinger RE 21) were installed at the location presented in figure 2. Gage alignment is illustrated in figure 2, where [[epsilon].sub.1] and [[epsilon].sub.3] were lengthwise and across respectively, and [[epsilon].sub.2] was at 45[degrees] for all locations.

[FIGURE 2 OMITTED]

The central hole was introduced and strain measurements were done in accordance with the ASTM Standard Method E 837 (***, 1992). Alignment and drilling were accomplished using milling guide equipped with an ultra-high speed turbine and carbide cutter.

All holes were drilled in successive increments of depth Z, and strain gage measurements were done for each depth increment. The holes were nominally 2,5 mm in diameter.

To obtain the stresses from the measured strains [[epsilon].sub.1], [[epsilon].sub.2] and [[epsilon].sub.3] we used the following procedure (Rendler & Vigness, 1966):

* assign to the three gages identification numbers (1), (2) and (3) in a clockwise order as presented in figure 2. The direction (1) and (3) are mutually perpendicular and (2) coincides with one of bisectors;

* the principal stresses [[sigma].sub.max] are located at an angle [beta] measured clockwise from the direction of gage (1).

Accordingly, the principal stress [[sigma].sub.min] is located at an angle [beta] measured clockwise from the direction of gage (3).

The angle [beta] was calculated with:

[beta] = 1/2 arctan [[[epsilon].sub.3] + [[epsilon].sub.1] - 2 x [[epsilon].sub.2] / [[epsilon].sub.3] - [[epsilon].sub.1] (1)

If [beta] is positive, (ex. [beta] = 60[degrees]) indicates that [[sigma].sub.max] lies 60[degrees] clockwise from the direction of gage (1). A negative value of [beta], (ex. [beta] = -60[degrees]) indicates that [[sigma].sub.max] lies 60[degrees] counter clockwise.

Computational calculation of [[sigma].sub.max] and [[sigma].sub.min] is using:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

The negative square root in this equation is associated with [[sigma].sub.max] because the calibration constants [bar.A], [bar.B] have negative numerical values:

[bar.A] = - 1 + [mu]/2E x [bar.a]; [bar.B] = 1/2E x [bar.b] (3)

[bar.a] = [D.sup.2.sub.0]/2 x GW x ([R.sub.2] - [R.sub.1]} x ([[theta].sub.1] - [[theta].sub.2]); (4)

with E-Young's modulus; [mu]-Poisson's ratio and [bar.a], [bar.b] depends on the geometry of rosettes [R.sub.2], [R.sub.1], GW, [D.sub.0], [[theta].sub.1, 2].

Figure 3 shows the magnitudes of maximum and minimum residual stresses which were determined at all locations, the results being expressed in MPa (Dumitrache et al. 2002).

[FIGURE 3 OMITTED]

3. CONCLUSIONS

Experimental data demonstrates a steep stress gradient in the vicinity of the weld bead which is illustrated in figure 4.

In the near welded region of normal welded plate, there is a compressive stress field, because after the welding process, dilatation of the metal induces plastic deformation in base metal and contraction of weld bead is blocked (Wayne, 1984). In this way, residual stresses from the vicinity of weld bead are compressive stresses. This case occurs at all the thin plates which have low stiffness.

These researches demonstrate that ultrasonic field induced in liquid weld bead refine and change recrystallisation.

All points removed from the weld, have a rapid change to tensile stresses.

The values of residual stresses near the weld bead, in the both cases does not exceed the elastic limit which is 299 MPa for the "A" type naval steel.

[FIGURE 4 OMITTED]

4. AKNOWLEDGEMENT

Part of the results presented in the paper use some of the accomplishments of the "Computer Aided Advanced Studies in Applied Elasticity from an Interdisciplinary Perspective" ID1223 project, the supervisor being the National University Research Council (CNCSIS), Romania, (Oanta et al., 2007).

The future researches of our team are focused to relieve the the interdependences between residual stress field and mechanical characteristics of welded samples.

5. REFERENCES

Dumitrache, C. (2000). Researches about ultrasonic influences on the metal welded structures, PhD Thesis, 182 pages, Field of science: Mechanical Engineering, 2000, 'Gheorge Asachi' University of Iasi, Romania

Dumitrache, C.; Comandar, C.; Susan, M. & Sabau, A. (2002). The influence of ultrasonic energy on the mechanical properties at the welded naval steel, Proceeding of EE&AE'2002 International Scientific Conference, pp. 127-130, ISSN 1311-9974, Rousse, 4-6 April, Bulgaria

Oanta, E.; Panait, C.; Nicolescu, B.; Dinu, S.; Hnatiuc, M.; Pescaru, A.; Nita, A. & Gavrila, G. (2007-2010). Computer Aided Advanced Studies in Applied Elasticity from an Interdisciplinary Perspective, ID1223 Scientific Research Project, under the supervision of the National University Research Council (CNCSIS), Romania

Rendler, N. & Vigness, I.(1966). Hole-Drilling Strain Gage Method of Measuring Residual Stresses, Experimental Mechanics, Vol 6, No. 12, p. 577-586

Susan M.; Bujoreanu G.; Dumitrache C.; Hanganu C. & Baciu C. (2008). A kinematical study of ultrasonic welding based on a system of stationary waves, Journal of Optoelectronics and Advanced Materials, Vol. 10, No. 6, June, p. 1425-1430

Wayne, E. (1984). Weld Induced Residual Stresses Measurements via Hole-Drilling Strain Gage Method, The American Society of Mechanical Engineers, Measurements Group, Inc. Raleigh, NC

*** (1992) Standard Test Method for determining residual stresses by the Hole-Drilling Strain-Gage Method ASTM Standard E837