This article provides an impression on potentials in applying nowadays welding simulation tools in construction design. This is carried out exemplary on plate girders from two structural steel grades. The calculated residual stresses are compared with measurements by sectioning method. It has been repeatedly stated that present Eurocode models fail to approximate the residual stresses. Especially for high strength steel (HSS) only limited information is available on realistic occurring residual stresses in typical I-girders. The investigations are aimed to give further guidance on these values. A few proposals on advanced models are discussed.
High-strength low-alloyed (HSLA) steels with yield strength ≥ 690 MPa have increasing importance 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 significantly 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 minimize the incidence of HAC, it is essential to gain knowledge about both the (1) absorbed hydrogen amount and its distribution in the weld seam and (2) options to lower
the amount of introduced hydrogen. Existing standards recommend heat treatment procedures (interpass temperature 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 immediately 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 concentrations were
measured.
Today’s light weight design trends lead to a growing application of high-strength structural steels (yield strength ≥ 690 MPa). The mechanical properties of the weld and the component safety have to meet the increased requirements of these steel grades. However, high residual stresses in welded components are detrimental to their safety and integrity. Analyses concerning weld stresses in high-strength steels welded under component related restraint conditions revealed that heat control significantly affects global and local stresses. This occurs especially in highly restrained joints due to superimposing local and global stresses and may cause crack-critical stress-levels. In this study weld tests were performed with plates of high-strength steel in a special test facility. The experimental setup allowed transferring defined restraint conditions to the test welds similarly to real components. Temperature and reaction forces due to restraint were observed online while welding and cooling of multilayer-component MAG-welds. Mobile X-ray diffraction was used for local stress determination in the weld seam areas of the restrained specimens. It was found that interpass temperature has a major influence on the local and global welding forces and stresses. Thus, among the analysed results especially transverse residual stresses of the heat affected zone were strongly affected.
High-strength steels with yield strengths ≥ 690 MPa are becoming increasingly significant for modern steel constructions. Both quenched and tempered (QT) high-strength steels and steels produced by the thermo-mechanically controlled process (TMCP) are currently available. TMCP-steels are usually provided with a typical mill scale. Apart from the manufacturing process these steels differ also in their chemical composition. The influence of these differences are investigated here with focus on the residual stresses formed during TIG welding. Bead on plate welds have shown characteristic residual stress distributions depending on the type of steel as well as on the heat input and preheat temperature. The following work demonstrates that the fluid flow, induced by the elements present in the mill scale, leads to a deeper penetration of the material. This can also lead to increased residual stresses in the surface of the weld and the HAZ. Furthermore, higher heat input and preheat temperature can induce higher tensile residual stresses − which are evident in the present study.
Der verstärkte Einsatz höherfester Stähle und Zusatzwerkstoffe erfordert ein vertieftes Verständnis der Zusammenhänge zwischen Schweißverfahren, Wärmeeinbringung, Abkühlbedingungen sowie den daraus resultierenden metallurgischen Vorgängen in der Schweißnaht und deren Umgebung. Nur aus dem Zusammenwirken aller Einflussfaktoren lassen sich Strategien ableiten, mit deren Hilfe die Festigkeitseigenschaften bzw. die Lebensdauer von Schweißkonstruktionen in einem Maße verbessert werden können, die den Einsatz höherfester Werkstoffe rechtfertigen und somit deren Leichtbaupotenzial voll ausgenutzt werden kann. Vorgestellt werden einige beispielhafte Untersuchungen an Schweißverbindungen aus höherfesten Feinkornbaustählen zu Interaktionen der Haupteinflussgrößen auf die Kaltrissbildung.----------------------------------------------------------------------------------------------------------------------------
The increased utilisation of higher-strength steels and filler materials necessitates more profound understanding of the connections between the welding process, the heat input, the cooling conditions and the resulting metallurgical processes in the weld and its surroundings. Strategies which help to improve the strength properties and service lives of welded structures to such an extent that the utilisation of higher-strength materials can be justified and their lightweight construction potential can thus be exploited to the full can only be derived from the interaction between all the influencing factors. Examples of a few investigations on welded joints between higher- strength fine-grained structural steels with regard to the interactions between the main variables influencing the cold cracking are presented in this article.
High-strength steels with yield strength of 960 MPa are susceptible to hydrogen-assisted cracking (HAC) during welding processing. In the present paper, the implant test is used to study HAC in a quenched and tempered steel S960QL and a high-strength steel produced by thermo-mechanical controlled process S960MC. Welding is performed using the gas metal arc welding process. Furthermore, diffusible hydrogen concentration (HD) in arc weld metal is determined. Based on the implant test results, lower critical stress (LCS) for complete fracture, critical implant stress for crack initiation, and embrittlement index (EI) are determined. At HD of 1.66 ml/100 g, LCS is 605 MPa and 817 MPa for S960QL and S960MC, respectively. EI is 0.30 and 0.46 for S960QL and S960MC, respectively. Fracture surfaces of S960QL show higher degradation with reduced deformation. Both, higher EI of S960MC and fractography show better resistance to HAC in the HAZ of S960MC compared to S960QL.
High-strength structural steels are used in machine, steel, and crane construction with yield strength up to 960 MPa. However, welding of these steels requires profound knowledge of three factors in terms of avoidance of hydrogen-assisted cracking (HAC): the interaction of microstructure, local stress/strain, and local hydrogen concentration. In addition to the three main factors, the used arc process is also important for the performance of the welded joint. In the past, the conventional transitional arc process (Conv. A) was mainly used for welding of high-strength steel grades. In the past decade, the so-called modified spray arc process (Mod. SA) has been increasingly used for welding production. This modified process enables reduced seam opening angles with increased deposition rates compared with the Conv. A. Economic benefits of using this arc type are a reduction of necessary weld beads and required filler material. In the present study, the susceptibility to HAC in the heat-affected zone (HAZ) of the high-strength structural steel S960QL was investigated with the externally loaded implant test. For that purpose, both Conv. A and Mod. SA were used with same heat input at different deposition rates. Both conducted test series showed same embrittlement index “EI” of 0.21 at diffusible hydrogen concentrations of 1.3 to 1.6 ml/100 g of arc weld metal. The fracture occurred in the HAZ or in the weld metal (WM). However, the test series withMod. SA showed a significant extension of the time to failure of several hours compared with tests carried out with Conv. A.
High-strength fine-grained structural steels have great potential for weight-optimized, efficient structures in many modern steel applications. Further advances in efficiency can be achieved through additive manufacturing and bionic design. Commercial high-strength filler materials for wire arc additive manufacturing (WAAM) are already provided by the consumable producers. Today, application would be strictly limited due to absence of quantitative findings or any guidelines for the industry regarding welding-related stresses and component safety during manufacturing and service. Hence, process- and material-related influences and design-related restraint conditions associated with formation of residual stresses and cold cracking risk are investigated. The aim is the accessibility of special WAAM self-restraining cold cracking tests and easy applicable processing recommendations, enabling an economical, fit-for-purpose and crack-safe WAAM of high-strength steels. This first study focuses on determination of interactions between WAAM process parameters, resulting layer geometry, microstructure and residual stresses, analyzed via X-ray diffraction. Defined reference specimens are automated welded using a special WAAM solid wire (yield strength >820 MPa). Geometric properties can be specifically adjusted by wire feed and welding speed, but cannot be varied arbitrarily, since a high heat input causes local overheating, inadmissible changes of microstructure and mechanical properties, defects and comparable high tensile residual stresses.