Stresstech Bulletin 15
Text: Jukka Wartiainen, Figures: Stresstech
Modern industry has many requirements like cost efficiency, durability, lightness of manufactured parts and eco-friendliness to name a few. With these requirements in mind it’s crucial to have deep understanding of the manufacturing process and how it affects the finished part.
During manufacturing often two parts are joined together. One method that is commonly used is welding. It is not straightforward to ensure good quality weld seams as the rapid heating and cooling over the weld area create residual stresses and change the quenching properties. Weld seams are identified as critical sections that are prone to mechanical failures. Mechanical failures involve extremely complex interaction of load, time, material, manufacturing processes and environment. The result of welding effect should be compensated in the structural design. As the requirements grow, new materials need to be investigated. Modern high strength steels are problematic as it is difficult to find suitable filler materials.
Due to the melting of the welded material microstructural changes occur near the weld. This results in the development of the heat affected zone (HAZ). For higher yield strength materials HAZ can result in lower hardness and relaxation of beneficial residual stresses. These issues can be controlled by choosing correct size and type of the weld. Very common problem in welded joints is the lack of fusion and undercuts. These defects can act as a potential site for crack initiation. In worst cases due to rapid heating and cooling the residual stresses can change so that there are higher stress concentrations and tensile stresses which decrease fatigue resistance remarkably.
One common method to measure residual stresses in crystalline materials is X-ray diffraction based method. In the discussed example a robotized X-ray diffractometer is used to measure residual stresses over a weld seam. The measurement setup is shown in Fig. 1 and Fig. 2. In this setup a robot makes the movements, which enables measuring residual stresses of virtually any shape or size sample.
The typical residual stress distribution on the surface over the weld seam is shown in Fig. 3. As it can be seen the residual stresses in the weld seam are usually tensile in parallel and especially in the weld toe area in transverse direction to the weld seam.
The influence of HAZ can be minimized by optimizing welding procedure and with post treatment. There are many different post treatment methods available: post weld heat treatment, TIG dressing, hammer peening, burr grinding, high frequency mechanical impact treatment (HFMI) and many more.
As an example, the effect of HFMI is shown in following example of residual stress in welding.
With X-ray diffraction the measurement depth is shallow, only some micrometers from the surface. In order to measure deeper, the material needs to be removed. One way to remove material without affecting the residual stresses is electropolishing. With the electropolishing method, it is possible to measure a depth profile. When X-ray diffraction and electropolishing are combined with the robot a depth profile can be made as a 3D map where X direction is parallel to the weld seam, Y direction is transverse on the sample surface and Z direction is down into the material.
The result can be plotted as a heat map and as an example the effect of welding to residual stresses is shown in Fig. 4. In the example a few different weld seam samples are shown and the weld toe has been post processed with the HFMI method. In Fig. 4 colors from red to blue correspond to changes from tensile to compressive stresses. White line shows the zero stress levels. There are minor differences in the compressive stress areas between different samples. HFMI treatment has created compressive stresses up to about 2 mm depth in the toe area as can been seen in residual stress depth profile distribution in Figure 3 graph D. The net integrated residual stress over the whole sample must be zero, thus deeper in the material residual stresses become tensile.
Stresstech is specialized in the industrial applications of X-ray diffraction based residual stress measurements. Feel free to contact us to learn more about the X-ray residual stress measurements and its applications.
 Lasse Suominen, Mansoor Khurshid, Jari Parantainen. Residual stresses in welded components following post-weld treatment methods. Procedia Engineering 66 (2013) 181 – 191
 Mansoor Khurshid. Static and fatigue analyses of welded steel structures – some aspects towards lightweight design. Doctoral Thesis, Division of Lightweight Structures, Department of Aeronautical and Vehicle Engineering, School of Engineering Sciences, KTH-Royal Institute of Technology