African Fusion March 2016

Welding of tubular k-joints

An investigation of residual stresses and distortions produced in tubular K-joints G Stix and B Buchmayr: Montanuniversitaet Leoben, Austria. This paper, taken from the proceedings of the 2015 IIW International Conference inHelsinki, Finland, investigates the effects of stresses and distortion on tubular weld joints during the manufacture of lattice boom cranes frommodern high-strength materials.

L attice boom cranes are exposed to heavy loading conditions during service. High inertia forces during lifting and turning lead to high oscillating stresses that are superimposed onto static wind loads. However, the final component performance (fatigue life) is also influenced by the inherent residual stresses, which have to be minimised to achieve better service behaviour. In this paper, themost influencingparameters, suchas geo- metrical factors, heat input and alternativewelding sequences are considered in the prediction of residual stresses inwelded tubular K-joints. Special emphasis is placed on the influence of the design parameters of a K-joint, which consists of one chord and two braces. These parameters are the angle between the chord and the brace, the chord slenderness, the eccentricity of the braces and the welding procedure. For the finite element welding simulation the commercial software Simufact.welding was used. Based on the numeri- cal results, real weldments were done using a robot welding system in order to verify the simulation results. In the weld- ing trials, two tube diameters and wall thicknesses for the braces and one chord diameter and two wall thicknesses are considered. In addition, two different welding sequences (8 and double 3) were compared. By metallographic investiga- tions andmeasurement of the residual stresses using the hole drilling method with strain gauge rosettes, the simulation tests were compared to the results from the residual stress measurements. Introduction For the production of lattice boomcranes, thickermain chords are connected by a high number of welds with thinner braces. Thesewelds have towithstand the resulting forces and torques from the lifting, lowering and turning of loads, [1] and [2]. Hence, it is important to know the implemented residual stresses caused by welding because residual welding stresses can affect the static and dynamic loads during operation.

Figure 1 shows an annotated outline of a tubular K-joint with the most important areas for the following considerations: The smaller brace is described by its diameter (d) and wall thickness (t); the chord, similarly, by its diameter (D) and wall thickness (T). Three geometrical factors, eccentricity ( ε) , angle between brace and chord ( θ) and the gap between the two braces (g L ), are drawn. Moreover the specific areas crown heel, sladder and crown toe are marked [3]. Based on the data in Figure 1, chord slenderness ( γ) is calculated in (1), taking into account the diameter proportion (2). According to references from Kuhlmann et al [3], chord slenderness must be greater than twelve.

(1)

(2)

These tubular K-joints can be welded either in one pass by following a figure 8 pattern or in two passes, in the form of back-to-back figure 3 patterns. Figure 2 shows a reduced outline of a tubular K-joint in top view. The continuous seam begins at the starting point (SP) and runs along the marked arrows back to the starting point (SP). Conversely, the seam path when using the 3s pattern runs from the starting point (SP) to the end point (EP) and from there as a new seam after a reorientation back to the starting point. This reorientation results in a break of a fewseconds, since theweldingwiremust be cut off before thewelding torch is ready to resumewelding.

Figure 2: A top view of a tubular K-joint.

Non-continuous welded K-joints are used by manual welders and in welding stations without rotary tables, that is, in situations with low accessibility. An obvious disadvantage of the non-continuous welded K-joints is the doubling of start and end craters. Due to the number of samples for welding, it was not pos- sible to carry out all experiments twice. A decision was there- fore taken that one welding procedure would be extensively investigated with respect to metallography, while the other would focus on residual stresses. Table 1 shows a complete list of materials and dimensions used. The real experiments were only performed with 20MnV6-TT for the high strength chord and S355-TT for the brace.

Figure 1: An annotated sketch of a tubular K-joint.

14

March 2016

AFRICAN FUSION