- International Symposium on NDT Contribution to the Infrastructure Safety Systems, 1999 NOV 22-26 Torres, published by UFSM, Santa Maria, RS, Brazil

TABLE OF CONTENTS

Abstract Introduction Material and experimental technique Experimental results and discussion Conclusions References

Abstract

The use of X-ray diffraction methods provides the wide possibilities of applying of X-ray tensometry to nondestructive control of residual stresses arising after different technologies, especially after welding processes. In this paper the "sin²-y"method of X-ray stress analysis using diffractometer with proportional coordinate sensitive detector is presented. The high sensibility of detector permits to diminish the width of irradiated area up to 0,1 mm and gives possibility to realise the measurements of residual stress distribution in weld region. The study of different causes forming real residual stress distribution in weld region is carried out.

Introduction

Welding is one of the most important technological process that is used in many branches of industry such as industrial engineering, shipbuilding, pipeline fabrication among others. Welding is a complicated process accompanied by shrinkage effects, phase transformations, intensification of corrosion and the creation of residual stresses.
Residual stresses arising after welding exert a considerable influence on the service characteristics of welded equipment and their control permits the avoidance of welded joint failure. The influence of residual stresses on service characteristics of welded equipment is analysed in many original papers and books [1-3]. Different techniques are used to control the stress state and the X-ray diffraction technique is one which is widely used to analyse the origin and residual stress distribution near the welded region. But numerous measurements by X-ray tensometry [4,5] contain contradictory information about the nature and value of weld-induced residual stresses and analyses of stress distribution usually is carried out without consideration of equilibrium conditions.
In this paper the experimental results of residual stress measurements by X-ray tensometry are presented. Analysis of the distribution of both longitudinal and transverse stress components are discussed to permit the non-destructive control of stress state in a seam weld, in the heat-affected zone (HAZ) and in metal near the welded region.

Material and experimental technique

Two A106GrB steel plates were joined by arc welding. The dimensions of each plate were 45 x15cm and the thickness was 13mm. It was necessary to make 9 welding passes to form the normal weld seam. Figure 1 shows the structure of the weld seam and indicates the points of stress measurements located on the seam, in the heat-affected zone and outside the weld region. At each point two stress components were measured; one of them being perpendicular to the weld seam (transverse residual stress) and the other parallel to the seam (longitudinal residual stress).

Fig 1: Scheme of the weld seam structure and points of stress measurements: (a-a), (b-b), (c-c) positions of stress measurements after layer removals.

Fig 2: A schematic diagram of the diffractometer: 1-high voltage source; 2-X-ray tube; 3-collimator; 4-analysed plate; 5-coordinate sensitive detector; 6-eletronics; 7-computer

Stress measurements were made using a special X-ray diffractometer with a coordinate sensitive detector. The construction of the goniometer permits the use of this equipment on samples having considerable weight and offers the possibility of making some of the manipulations foreseen in the stress measurement technique. Slit width for the incident X-ray beam was 0,1x10mm for measurements of transverse stress components and 0,5x2mm for longitudinal stresses. A schematic diagram of the diffractometer is shown in figure 2. X-ray stress measurements were made by sin²y-method [6], using Cr-K_a radiation and (211) reflections.
The formula to determinate stress component in-sin²y-method is based on the well known expression concerning the theory of elasticity for strain in arbitrary diffraction, for angles j and y in a spherical coordinate system:

(1)

where E, n are elastic constants, s₁, s₂ are principal stresses and s_j is the measured stress component.
On the other hand strain e_jy can be expressed using concept of X-ray diffraction theory. According to the differentiation of Bragg's law:

(2)

where d is the interplanar distance and q is diffraction angle, the formula for strain maybe written as

(3)

where d₀ and q₀ are interplanar distance and diffraction angle for unstressed material. After comparing the expressions (2) and (3) the angle q_jy maybe written in the form:

(4)

Here q_jy is linear function of sin²y. If to determine values of diffraction angles q_jy with variation of sin²y, then

(5)

where is tangent of inclination angle. Derivative of q_jywith respect to sin²y may be determined by extrapolation of q_jy to y=0⁰ and y=90⁰.
Therefore, the final formula for stress determination is

(6)

Experimental results and discussion

Experimental results of stress measurements carried out on the front and back faces of welded plates are shown in figure 3 and figure 4. Stress distribution curves both for longitudinal and transverse residual stresses are presented. The stress state on the initial surface is characterised by compressive residual stress at the weld seam and tensile stresses in the HAZ and base metal.
Since the residual stress are self-equilibrium, results presented in the figures 3 and 4 have to satisfy to equilibrium equation:

(7)

where A is the area of cross-section perpendicular to analysed stress component. For longitudinal e transverse residual stresses the equation (7) is transformed to following two equations:

(8)

(9)

To satisfy the equilibrium of residual stresses it is necessary to know the stress variation along Z-coordinate (see figure 1). If one assumes that the stresses within weld seam, HAZ and base metal are the same as on the outer surface then the stress distribution curve for the longitudinal direction is satisfied for the equilibrium equation because of variation of stress sign along X-coordinate. For transverse residual stress (for example at the centre of the weld seam) the equilibrium equation is not satisfied because st can not change in sign along the y-coordinate. In practice, stress measurement at point 11, near the edge of weld seam, show that the value of transverse stress at this point is equal to s= -150Mpa and it is not very different from stress value at the centre of seam.

Fig 3: Residual stress distribution on front face of welded plate: 1-longitudinal stresses; 2-transverse stresses.

Therefore, satisfaction of the equilibrium equation for transverse residual stress requires the modification of the compressive stress acting at the outer surface of the weld seam to a tensile residual stress within interior regions of the seam.. This conclusion means that stress measurements on the surface of a welded joint is not sufficient to examine residual stresses arising after welding. To verify this point of view it is necessary to determine the stress distribution throughout the depth of the analysed plate. This can be accomplished by means of removing of surface layer and carrying out stress measurments on a new surface. It is clear that in the case of layer removal by machining or grinding it is necessary to undertake electropolishing to remove additional surface layer distortion by machining.
Stress measurements after surface removal are presented in figure 5. Curve 1 of this figure shows that compressive residual stresses in the transverse direction acting at the centre of weld seam become tensile stresses that reach s_t = 250 MPa at 2,5 mm from the outer surface. The curve 2 in figure 5 shows the inhomogeneous stress distribution along the weld seam structure. At the centre of the weld pass (x = 0 mm) the stress value is higher than at the border of weld pass (x = 2 mm).

Fig 4: Residual stress distribution on back face of welded plate: 1-longitudinal stresses; 2-transverse stresses.

Fig 5: Stress distributions along Z coordinate (curve 1) and X coordinate (curve 2).

Experimental results presented in figures3-5 do not contradict the theory of the origin of residual stresses after welding. Accordance to [5] the principal sources of residual stresses after welding are:

1. shrinkage;
2. quenching;
3. phase transformation.

Shrinkage provokes the appearance of tensile stresses, whereas quenching and phase transformation cause compressive stresses at the weld seam.
Initial compressive residual stresses in the weld seam indicate that their main sources are quenching or phase transformation. The stress distribution curve presented in figure 5 indicates that quenching is the predominant cause of the creation of a residual stress state. In practice, the stress state of quenched part is characterised by compressive residual stresses on the outer surface and tensile stresses in the interior region.

Conclusion

It has been shown that residual stresses arising after welding are somewhat complicated and are characterised by compressive stresses on the surface of weld seam and tensile stresses in the HAZ and base metal. The non-destructive control of the stress state on the surface made as achieved by X-ray diffraction methods is not sufficient to identify dangerous residual stresses. The grinding of the weld seam and electropolishing of the machined surface does not destroy a weld joint but rather permits one to obtain important information about the tensile residual stresses acting in the sub-superficial layer of the weld seam.

References

1. Masubuchi, K., Analysis of Welded Structures. Pergamon Press, Oxford, N.York, 1980.
2. Macherauch, E., Hauk, V., Residual Stresses, DGM Inform., Verlag, 1986.
3. Residual Stresses and their Effects, The Welding Institute, Abington Hall, Cambridge, 1981.
4. Macherauch, E., Wolfahrt, H., Different Sources of Residual Stresses as a Result of Welding, Residual Stresses in welding Constructions and their Effects, Welding Institute Reprint, 1977, pp. 267-282.
5. Macherauch, E., Kloos, K.H., Origin, Measurements and Evaluation of Residual Stresses, Residual Stresses in Science and Techology. DGM Inform., Verlag, 1987, pp. 3-26.
6. Noyan, I.C., Cohen, J.B., Residual Stress, Measurement by Diffraction and Interpretation, N.York, Berlin, Springer Verlag, 1987.

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