African Fusion

Welding of tubular k-joints

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

), 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)

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.

An investigation of

residual stresses and

distortions produced in tubular K-joints

G Stix and B Buchmayr: Montanuniversitaet Leoben, Austria.

AFRICAN FUSION

March 2016

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

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

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.

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.