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An approach to develop an arc sensor for gap-width estimation during automated NG-GMAW with a weaving electrode motion is introduced by combining arc sensor readings with optical measurement of the groove shape to allow precise analyses of the process. The two test specimen welded for this study were designed to feature a variable groove geometry in order to maximize efficiency of the conducted experimental efforts, resulting in 1696 individual weaving cycle records with associated arc sensor measurements, process parameters and groove shape information. Gap width was varied from 18 to 25 mm and wire feed rates in the range of 9 to 13 m/min were used in the course of this study. Artificial neural networks were used as a modelling tool to derive an arc sensor for estimation of gap width suitable for online process control that can adapt to changes in process parameters as well as changes in the weaving motion of the electrode. Wire feed rate, weaving current, sidewall dwell currents and angles were used as inputs to calculate the gap width. Evaluation the proposed arc sensor model show very good estimation capabilities for parameters sufficiently covered during experiments.
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.
For a significant increase in the strength of quenched and tempered, high-strength structural steels with a yield strength > 690 MPa, the addition of micro-alloying elements such as Nb and Ti is essential. The standard specifications for the chemical composition of these steels (e.g. in DIN EN 10025-6) often only specify limit contents for the manufacturer to achieve the specified properties. The effect of the alloying elements or their carbides and/or nitrides is sometimes completely contrary, especially in the case of dissolution and re-precipitation in the HAZ with identical filler metal. This makes it difficult to adequately predict the batch dependency regarding the weldability and the load-bearing behaviour of the welded joint. In addition, modern welding processes, such as GMAW with modified spray arc, can negatively influence the phase transformation in the heat-affected zone due to their high heat input. An undesirable property is the softening of the HAZ, as well as the opposite effect of hardening. Against this background, microalloying routes with varying Ti and Nb contents of the high-strength and quenched and tempered structural steel S690QL are systematically investigated within the framework of a DFG project. For this purpose, GMAW welding with modified spray arc is used, which is characterised by high deposition rate and allows narrower joint opening angles (α = 30°). The effect of metallurgical composition in combination with high heat exposure on the formation of a critical HAZ microstructure zone with softening and/or excessive hardening is investigated on these welds. Special attention is paid to the phase transformations and precipitation behaviour in the microstructure of the heat-affected zone and weld metal. In addition to extensive metallographic investigations of individual HAZ areas, thermodynamic phase calculations were carried out using ThermoCalc while varying the chemical composition. This provides an understanding of phase transformation and precipitation growth and dissolution during welding as a function of temperature and cooling conditions. The aim is to show the influence of the heat effect on the microstructure formation in the HAZ by means of initial investigations. Particular attention is paid here to the effect of the different microalloying concepts (Ti or Nb).
In many industrial steel construction branches, like mobile cranes and the offshore sector, high-strength fine-grained structural steels with a yield strength of over 690 MPa have long been used. To guarantee the necessary load-bearing capacity, the welding processing of these steels requires reliable knowledge of the complex interaction between the welding process, its underlying heat input and corresponding cooling conditions, chemical composition of base and filler materials, and resulting metallurgical phenomena in the weld seam and HAZ. Microalloying elements such as Ti and Nb make an indispensable contribution to increasing the strength by precipitation hardening. Previous investigation has shown that significant influence of the microalloying elements on the phase transformation can be assumed regarding the hardening and/or softening of the heat-affected zone. The standard specs for the chemical composition according to DIN EN 10025-6 often just specify chemical limitations for the manufacturer to achieve the desired mechanical properties. The effect of the alloying elements and the corresponding precipitates (carbides and/or nitrides) is sometimes entirely contrary, especially in case of dissolution and re-precipitation in the heat-affected zone (HAZ) with identical filler metal. This makes an adequate prediction of the batch dependency with regard to weldability and the load-bearing capacity of the welded joint difficult. The joining of these steels is mainly carried out by gas metal arc welding (GMAW). Modern inverter welding sources and micro-electronics control technology enabled the use of controlled arc variants, such as the modified spray arc (Mod. SA). Several characteristics from this arc variant e.g. reduced weld seam opening angles and increased deposition rates. Whereby this high heat exposure can have a negative effect on the phase transformation in the HAZ. An undesirable property is the softening of the HAZ, as well as the opposite effect of hardening.