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Today, an expanding application of high-strength steels in modern welded constructions can be observed. The economical use of these steel grades largely depends on the strength and reliability of the weldments. Therefore, the special microstructure and mechanical properties of these grades have to be taken into account by keener working ranges regarding the welding parameters.
However, performance and safety of welded components are strongly affected by the stresses occurring during and after welding fabrication locally in the weld seam and globally in the whole component, especially if the shrinkage and distortion due to welding are restrained. Some extensive studies describe the optimization of the welding stresses and the metallurgical effects regarding an adapted welding heat control. Lower working temperatures revealed to be particularly effective to reduce the local and global welding-induced residual stresses of the complete weld significantly. However, decreased interpass temperatures cause concurrently higher stresses during welding fabrication. This work shows strategies to reduce these in-process stresses.
With help of multi-axial welding stress analyses in component-related weld tests, using a special 2-MN-testing facility, differences in stress build-up are described in detail for root welds, filler layers and subsequent cooling to ambient temperature.
High-strength steels are increasingly applied in modern steel constructions to meet today’s lightweight requirements. Welding of these steels demands a profound knowledge of the interactions between the welding process, cooling conditions, heat input and the resulting metallurgical occurrences in the weld and its vicinity. Additionally, welding stresses may be detrimental for the safety and performance of high-strength steel component welds during fabrication and service, especially due to the high yield ratio. For a development of strategies to adjust welding heat control, all these effects should be considered, to reach a complete exploitation of the high-strength steel potential. In recent researches at BAM, multilayer GMAW experiments were performed with high-strength steels, in which cooling conditions and resulting microstructure were analysed for varied heat control parameters. The application of a unique 3d-operating testing facility and X-ray diffraction measurements allowed the analysis of local stresses in the weld while welding and cooling under component relevant shrinkage restraints. As a result, correlations between material behaviour, welding and cooling condition and the arising multi-axial stresses and forces were found. Based on this study, statements for the development of adapted heat control concepts were derived, which are presented by means of specific analysis examples.
High-strength low-alloyed (HSLA) steels with yield strength ≥ 690 MPa are gaining popularity in civil engineering and construction of heavy vehicles. With increasing yield strength, the susceptibility for degradation of the mechanical properties in presence of diffusible hydrogen, i.e. hydrogen-assisted cracking (HAC) generally increases. HAC is a result of the critical interaction of local microstructure, mechanical load and hydrogen concentration. In existing standards for welding of HSLA steels, recommendations like working temperatures and dehydrogenation heat treatment (DHT) are given to limit the amount of introduced hydrogen during welding. The recommendations are based on investigations with conventional arc welding processes. In the past decade, modern weld technologies were developed to enable welding of narrower weld seams with V-grooves of 30°, e.g. the modified spray arc process. In that connection, a reduced number of weld runs and weld volume are important technical and, hence, economic benefits. In the present study, the hydrogen distribution in S960QL multi-layer welds with thickness of 20 mm was analyzed. The influence of different weld seam opening angles, heat input, working temperature and DHT was investigated. The results show that weldments with narrow groove contained increased diffusible hydrogen amount. Hydrogen concentration has been reduced by decreasing both the heat input and working temperature. Hydrogen free weldments were only achieved via subsequent DHT after welding. Furthermore, hydrogen distribution was experimentally determined across the weld seam thickness in HSLA GMA welded multi-layer welds for the first time.
High-strength steels are increasingly applied in modern steel constructions to meet today’s lightweight requirements. Welding of these steels demands a profound knowledge of the interactions between the welding process, cooling conditions, heat input and the resulting metallurgical occurrences in the weld and its vicinity. Additionally, welding stresses may be detrimental for the safety and performance of high-strength steel component welds during fabrication and service, especially due to the high yield ratio. For a development of strategies to adjust welding heat control, all these effects should be considered, to reach a complete exploitation of the high-strength steel potential. In recent researches at BAM, multilayer GMAW experiments were performed with high-strength steels, in which cooling conditions and resulting microstructure were analysed for varied heat control parameters. The application of a unique 3d-operating testing facility and X-ray diffraction measurements allowed the analysis of local stresses in the weld while welding and cooling under component relevant shrinkage restraints. As a result, correlations between material behaviour, welding and cooling condition and the arising multi-axial stresses and forces were found. Based on this study, statements for the development of adapted heat control concepts were derived, which are presented by means of specific analysis examples.
High-strength low-alloyed (HSLA) steels with yield strength ≥ 690 MPa have increasing im-portance in steel construction and civil engineering. However, weld processing of those steels is a major challenge. The susceptibility for degradation of mechanical properties of weld joints sig-nificantly increases in presence of hydrogen and can result in hydrogen assisted cracking (HAC). Generally, risk for HAC increases with increasing yield strength of HSLA steels. To min-imize the incidence of HAC, it is essential to gain knowledge about both the (1) absorbed hydro-gen amount and its distribution in the weld seam and (2) options to lower the amount of intro-duced hydrogen. Existing standards recommend heat treatment procedures (interpass tempera-ture or post weld heat treatment) to reduce the diffusible hydrogen concentration in weldments. In this context, different weld seam geometries should be considered. For HSLA steel fabrication weld processing with seam opening angles of 45° to 60° is typical. Modern weld technologies allow welding with seam opening angles of 30° - reduced welding time and costs. In the present study, the hydrogen distribution in multi-layer welds of a 960 MPa HSLA steel was analysed. Influence of different seam opening angles as well as heat input, interpass temperature and post weld heat treatments were investigated. The welded samples were quenched in ice water imme-diately after welding and subsequently stored in liquid nitrogen. After defined warming up, small specimens were machined from the weld seam by water jet cutting. The diffusible hydrogen concentration was measured by carrier gas hot extraction with coupled mass spectrometer. The results showed, that low heat input and post weld heat treatment procedures can lower hydrogen concentrations in welds. Furthermore, a gradient of the hydrogen concentration was identified with increasing weld pool depth. By varying the seam opening angles different hydrogen concen-trations were measured.
Three high-strength Nb-,.Ti- and Ti+ V-bearing S690QL steels were welded to investigate and compare the effects of microalloy addition on heat-affected zone (HAZ) toughness. Charpy V notch impact tests from three microalloyed welds under different cooling rates have been performed. Fractographic examination shows that several factors, including large-sized grain, upper bainite or hard second phase, interact to determine brittle fracture and impaired toughness in Nb-bearing weld with high heat input. In contrast to this reduced toughness, Ti-bearing welds exhibits satisfied toughness regardless of at fast or slow cooling. This is attributed to its limited austenite grain and refines favorable intragranular acicular ferrite structure. Moreover, in the case of such refined structure as matrix, TiN particles are found to be irrelevant to the facture process. The crystallographic results also confirm that high-angle boundaries between fine ferrites plates provide effective barriers for crack propagation and contribute to improved toughness.
The phase transformation under various cooling rates and in different HAZ regions for high-strength armour steel was analysed by dilatometry. To develop a continuous cooling transformation (CCT) diagram, the samples were heated up to a peak temperature of 1250 °C to achieve a coarse-grained microstructure and then cooled down with a cooling time t 8/5 varying from 3 to 240 s. Analysis of dilatation curves revealed the austenite decomposition process, during which transformation temperatures were determined. The results showed martensitic transformations for all welding-relevant cooling times. Furthermore, to analyse different heat-affected subzones of the weld, the peak temperature was varied between 550 and 1250 °C at a constant cooling time t 8/5 of 6 s. The simulated coarse-grained heat-affected zone (CGHAZ) and fine-grained-heat affected zone (FGHAZ) showed only martensitic transformations with transformation temperatures below 400 °C. The steel exhibited an inhomogeneous hardness with hardening in the CGHAZ and FGHAZ and softening in the intercritical and subcritical HAZ. The physically simulated microstructure was validated by a real hybrid laser-arc weld microstructure.
In an increasing number of modern steel applications, high-strength structural steel grades are demanded to meet specifications regarding a high load-bearing capacity and a low operating weight. Lightweight design rules enhance the safety requirements, especially for welded joints. Besides a higher cracking risk for high-strength steel welds, the formation of tensile residual stresses might lead to fracture due to overloading or premature failure if not adequately considered. In this study, a stress-strain analysis was conducted at component-related structures from S960QL using digital image correlation while preheating, welding and cooling adjacent to the weld seam. X-ray diffraction analysis of the local residual stresses in the weld seam showed a good comparability with global analyses using either a DIC system or a special testing facility, which allowed in situ measurements of welding loads. By analysing two different seam geometries, it could be shown that lower multi-axial stresses arise if a narrower weld groove is used. Comparative analyses revealed a direct correlation of the local residual stresses in the weld with transverse shrinkage restraint, whereas the residual stress level in the HAZ is significantly affected by the bending restraint of the weld construction and the occurring bending stresses, respectively.
In order to satisfy the growing requirements towards lightweight design and resource efficiency in modern steel constructions, e.g., mobile cranes and bridges, high-strength steels with typical yield strength ≥ 690 MPa are coming into use to an increasing extent. However, these steels require special treatment in welding. The susceptibility for degradation of the mechanical properties
in the presence of hydrogen increases significantly with increasing yield strength. In case of missing knowledge about how and the amount of hydrogen that is uptaken during welding, hydrogen-assisted cracking (HAC) can be a negative consequence.
Moreover, modern weld technology like the modified spray arc process enables welding of narrower weld seams. In this context,
a reduced number of weld beads, volume, and total heat input are technical and economical benefits. This work presents the influence of welding parameters on the diffusible hydrogen content in both (1) single-pass and (2) multi-layer welds. Different
hydrogen concentrations were detected by varied contact tube distance, wire feed speed, arc length, and varied arc type (transitional
arc and modified spray arc). The results show that all welding parameters have significant influence on the diffusible hydrogen concentration in the single-pass welds. By increasing the number of weld beads in case of multi-layer welding, the
hydrogen concentration has been reduced. Whereby, differences in hydrogen concentrations between both arc types are present.