Residual stresses measurement by neutron diffraction and theoretical estimation in a single weld bead

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Abstract

Welding residual stresses are important in pressure vessel and structural applications. However, residual stress remains the single largest unknown in industrial damage situations. They are difficult to measure or theoretically estimate and are often significant when compared with the in-service stresses on which they superimpose. High residual stresses lead to loss of performance in corrosion, fatigue and fracture.

In this research, a measurement of residual stress by the neutron diffraction technique is compared to an analysis of the same geometry by theoretical finite element procedures. The results indicate good agreement but scope for further understanding of the details of modelling the welding heat source, heat transfer and variation of material properties with temperature.

Introduction

Residual stresses are formed in weld structures primarily as the result of differential contractions, which occur as the weld metal solidifies and cools to ambient temperature. These stresses can have important consequences on the performance of engineering components [1]. Weld residual stresses have a significant effect on corrosion, fracture resistance and corrosion/fatigue performance [2] and a reduction of these stresses is desirable.

There are several ways of directly measuring residual stresses in small volumes. The most common ones involve mechanical invasive methods (e.g. hole drilling or cutting [3], [4]) and non-destructive methods using radiation such as X-ray (laboratory or synchrotron) or neutron diffraction [5], [6], [7]. Of these, only neutron beams can establish stresses in the interior of components of a metallic material and have a small volume of measurement (1 mm3).

In this paper, experimental measurements of weld stresses generated by a single bead-on-plate of low-carbon steel using MIG welding are presented. In this work, we have concentrated on the influence of restraint on the residual stresses behaviour with the intention of providing key data for the validation of design and fitness for purpose methodologies and finite element tools.

Section snippets

Material and welding procedure

The material used in this study was a low-carbon steel [8]. The chemical composition of the parent material and weld metal are shown in Table 1. The dimensions of the plates were 200×100×12 mm. Typical mechanical properties of parent and weld metal are shown in Table 2.

Sample I was unrestrained and Sample II was fully restrained. Restraint was achieved by welding Sample II to a very thick steel plate, which was cut off after cool down. Distortion of Sample I was overall approximately 1° in

Finite element modelling

The objective here was to perform three-dimensional finite element modelling of the bead-on-plate experiment to calibrate the welding procedure. A relatively uncommon but powerful commercial FEA package called Sysweld+ was used in this attempt. The parent and the weld material were assumed to have the same mechanical and thermal properties, as was provided in the software database for the material S355J2G3 with chemical composition as follows: C≤0.20%, Mn≤1.60%, Si≤0.55%, S≤0.035%, and

Discussion

Transverse and normal stresses are low in the unrestrained Sample I, because the sample deformed during welding. However, for Sample II transverse and normal stresses are raised at all points of measurements as shown in Fig. 5. Longitudinal stress also generally increases but there are some reductions around the toe of the weld.

The highest increase between the unrestrained and restrained specimen was observed in middle of the weld where the normal and transverse stresses change from compressive

Conclusions

The use of a neutron beam as a non-destructive method of measuring residual stress due to welding has been explored. Experimental investigation showed that restraint of the plate during welding has a significant effect.

The analysis of the same weld geometry following the procedures of SYSWELD+ were used. The handbook suggested method of analysing the weld pool, heat transfer and material properties were used. The results of the comparison were promising, but further improvement in the modelling

Acknowledgements

This work was conducted with the assistance of an Australian Research Council grant supported by the Welding Technology Institute of Australia (WTIA). Other assistance has been received from the Monash University Research Fund, the Australian Nuclear Science and Technology Organisation (ANSTO) and an Australian Institute of Nuclear Science and Engineering (AINSE) grant.

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