Correlating weld process conditions, residual strain and stress, microstructure and mechanical properties for high strength steel—the role of neutron diffraction strain scanning

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Abstract

This paper presents the results of neutron strain scanning for a series of welds in high strength steel. Residual strains transverse to the weld were measured at a series of points on planes perpendicular to the weld run, along lines either 1 mm below the plate surface or at the mid-depth. Welds were made using three types of filler metal (undermatched (UM), matched (M) and overmatched (OM) compared with parent plate tensile strength), two heat input values and two plate thicknesses. This work forms part of a programme whose objective of the work was to correlate residual strain and hardness gradients, and their position relative to the weld toe, with the process parameters and with fatigue performance in bending at lives of 105 cycles and 2 × 106 cycles. Fatigue performance was assessed via nominal applied stress, hot spot stress at the weld toe and hot spot strain. The overall aim is to better understand the underlying reasons for fatigue life variability often observed with high strength steels. This will enable development of better life prediction methodologies. Neutron diffraction results show that tensile stresses close to the proof strength of the parent material can occur in the weld and that tensile peaks in the strain profiles correlate with the weld zone width.

Introduction

The work described in this paper exploits neutron diffraction strain scanning to illuminate the interaction between some of the factors that underpin fatigue performance in higher strength steel welds. The major factors of interest are the residual strain and microhardness gradients across the joint, and their relationship with the critical toe position in fusion welds. These factors are strongly influenced by weld and alloy parameters, including heat input, ratio of filler metal to parent plate tensile properties, plate thickness, alloy composition and heat treatment. This paper presents the results of a systematic neutron diffraction investigation of the residual strains transverse to the weld seam, as a function of depth below the plate surface, plate thickness, weld metal strength, and heat input during welding. This work forms part of a project that is considering interactions and relationships among hardness, residual strain and microstructure, with the aim of identifying a parametric approach to fatigue life prediction for welds in higher strength steels.

The underlying hypothesis to be tested was that a cross-correlation among residual strain, hardness and microstructure would show trends in magnitudes and gradients that could be linked to fatigue performance. Fatigue performance in four-point bend for these weld conditions was ranked using nominal applied stress, weld toe hot spot stress and weld toe hot spot strain. Physically meaningful combinations of strain and hardness gradient, in terms of their likely individual effects on fatigue life, could then be developed and assessed through their capability of predicting the observed fatigue ranking order of the welded joints at lives of 105 cycles and 2 × 106 cycles. These results will be reported in a future paper.

Section snippets

Fatigue design of higher strength steels

Weldable higher strength steels (tensile strength > 600 MPa) are often used in low cycle fatigue situations (lives typically less than 105 cycles), where significant engineering benefit derives from their higher tensile strength. The main interest revolves around the use of such steels in tubular structures (e.g. bridges and offshore platforms containing chord/brace joints). Typical tube wall thickness ranges from 8 mm up to >60 mm. The design life of welded steel structures in Europe is based on

Residual strain influences on fatigue behaviour

The difficulty in analysing causes of enhanced fatigue performance is compounded for welded steels by the geometry of the weld toe and the presence of any defects, e.g. nonmetallic weld toe intrusions. As fatigue performance enhancement of the type seen in Fig. 1 occurs with a batch of welds, rather than a single isolated weld it is, however, unlikely that it reflects defect-related events. It is therefore proposed that variability in fatigue behaviour of joints made under nominally similar

Alloy and weld process parameters

The welds were made in RQT701 steel, a roller quenched and tempered alloy with a tensile strength in the range 790–930 MPa, manufactured by CORUS in the UK. Two plate thicknesses of 8 mm and 12 mm were used and the weld runs were made between two plates approximately 1 m long × 150 mm wide. Welds were made using filler metal with a tensile strength that overmatched (OM), matched (M) or undermatched (UM) (respectively indicated in this paper by OM, M and UM) the parent plate tensile strength, and two

Microstructure and mechanical properties

The relative yield strengths of weld metal and parent plate are likely to have a strong influence on the magnitude and distribution of residual strains across the weld zone. Dilution between filler metal and parent plate occurs very quickly in fusion welds and weld metal tensile properties will therefore vary with any changes in weld process parameters or plate thickness. Tensile properties of the weld metal were hence measured for each combination of weld process conditions and plate

Strain measurements by neutron diffraction

Neutron strain scanning was mainly carried out on beam line D20 at the ILL in Grenoble. D20 is a high intensity two-axis diffractometer with variable resolution. Fig. 3 shows a typical welded steel specimen mounted in position, via a standard [8] base plate, on the translation stage of the instrument. For this work (2 1 1) diffraction planes in the Fe body-centred cubic lattice were used, the instrument take-off angle 2θ was 42°, the wavelength of incident radiation was 1.301 Å.

The gauge volume

Results and discussion

A significant issue in making neutron diffraction measurements of strain in welded specimens relates to determining the strain-free lattice parameter d0, which is required to calculate strain. This parameter has been shown to vary across the weld, at least in certain cases, and a useful technique is that of electrodischarge machining small teeth into the edge of the specimen. The free ends of these teeth are free of macrostrains and provide the necessary calibration data [10]. In the present

Conclusions

This systematic examination of residual strain and stress profiles in MIG butt welded plate specimens of 12 mm thick high strength steel has demonstrated the capability of modern neutron diffraction instruments in providing insights into relationships among weld process conditions, weld zone metallurgy and mechanical properties. Parameters such as heat input, filler metal strength level, plate thickness and fusion zone shape cause systematic changes to the positions of tensile and negative peaks

Acknowledgments

This project has been supported by Corus Research and Development, Rotherham through a CASE studentship for S.-P. Ting and this support has been invaluable. The assistance of FaME38 and the allocation of developmental beamtime on the D20 and SALSA instruments by the ILL are also gratefully acknowledged. Terry Richards provided invaluable technical assistance with the tensile testing.

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