The objective of the paper is to study the influence of the mismatch effect on high-strength steels and to determine fatigue crack propagation design curves. In order for determination and comparison of the fatigue resistance, fatigue crack growth tests were performed on S690QL, S960QL quenched and tempered, and S960M thermomechanically rolled high-strength steels. Fifteen-millimeter- and 30-mm-thick base materials were used for our investigations. Welded joints were made from these base materials, using gas metal arc welding with matching, overmatching, and undermatching filler metals. In the paper, the performance of the welding experiments will be presented, especially with the difficulties of the filler material selection; along with the results of the fatigue crack growth examinations executed on the base materials and its welded joints. Statistical aspects were applied both for the presenting of the possible locations of the cracks in the base materials and the welded joints and for the processing of the measured data. Furthermore, the results will be compared with each other, and the possibility of derivation of fatigue crack propagation design or limit curves will be referred.

With continuous improvements, structural steels are available in even higher strength grades above 1000 MPa yield strength. As the great majority of these steels are used in welded structures, their weldability needs to be taken into account. Several factors can cause difficulties during welding of these steels, but in this paper the softening behavior and the toughness characteristics of the heat-affected zone (HAZ) are examined. As the critical parts of the HAZ in a real welded joint are relatively small, their investigating ability is limited. However, the physical simulation provides a way of evaluating specimens made from a given material to produce the specified HAZ areas in a suitable size range for subsequent testing. In this research work, three different strength categories of high-strength structural steels (with yield strength of 960 MPa, 1100 MPa, and 1300 MPa) are investigated by physical simulation. In the case of different technological variants of gas metal arc welding (GMAW) process, the effect of the cooling time t_8/5 is investigated in different HAZ subzones considered to be critical. The thermal cycles were determined according to the Rykalin 3D model. The investigated cooling times were t_8/5 = 5 s, 15 s, and 30 s. The properties of the selected coarse-grained, intercritical and intercritically reheated coarse-grained zones are analyzed by laser scanning microscope, scanning electron microscope, hardness test, and instrumented Charpy V-notch impact toughness test. Furthermore, additional investigation like JMatPro calculations, electron backscatter diffraction measurements, and prior austenite grain size calculation were carried out. As a result of the tests, the investigated heat-affected subzones indicated higher sensitivity to the welding heat input compared to conventional structural steels. Overall, the results of the tests show that the application of shorter t_8/5 cooling time can be beneficial for the investigated high-strength steel grades, since significant toughness reduction and the risk of softening occur in the whole cooling time range.

The main objective of our ongoing research is data collection for integrity management (IM) tasks, in other words for pipeline integrity management system (PIMS), helping the transporting pipeline operators in different decision situations. Either the reserves of the girth welds after a long-term operation can be identified or the girth welds with different defects should be repaired and/or replaced. From the operators’ point of view, the long-term operation is the cost-effective way; however, from the safety point of view, the managing of the lifecycle of the pipeline is the more reliable way. Unfortunately, inadequate girth welds cause catastrophic damages in transporting pipelines, all over the world. Consequently, the optimal operation of the pipelines is a complex task. The paper introduces our investigation program executed on full-scale pipeline sections. Fatigue (100,000 cycles) and burst tests were executed on different pipeline sections containing girth welds. Long-term operated and replaced gas transporting pipeline sections, furthermore, both appropriate and inadequate girth welds were investigated. The testing results and the damage histories were compared with each other; safety factor (SF) was defined and calculated for the assessment of the reliability of the girth welds.

The lightweight construction of automotive car bodies is the more important to reduce the fuel consumption and costs. High-strength steels and aluminium alloys are suitable for achieving these aims. Recent car bodies contain both materials, therefore necessary to make reliable joints between them. The resistance spot welding (RSW) can be used for joining of car bodies and it is applicable for aluminium/steel hybrid joints, too. High cycle fatigue (HCF) test results can be rarely found in the literature while HCF loading basically determines the lifetime of hybrid joints. 5754-H22, 6082-T6, and DP600 base materials were used for similar and hybrid RSW joints and HCF tests were performed. Number of cycles to failure values, failure modes, furthermore brittle intermetallic compound (IMC) layers were studied and analysed. In both aluminium/steel hybrid joints, the HCF test results showed better endurance limit like concerning aluminium/aluminium similar joints, but worse than steel/steel joints. For 5754-H22 alloy the endurance limit values are 648 N, 939 N, and 1285.5 N, for similar aluminium, hybrid, and similar steel joints, respectively. For 6082-T6 alloy these values are 513 N, 625.5 N, and 1285.5 N, respectively. In case of similar joints only base material fracture happens, but hybrid joint specimens show different failure modes. Base material fracture and shearing after partial base material fracture were typical failure modes in case of 5754-H22/DP600 and 6082-T6/DP600 hybrid joints, respectively. The full and partial plugging as a failure modes appeared for hybrid joints, too. The IMC layer characteristics showed opposite results in cases of hybrid joints, both the layer thicknesses of the shared and plugged joints and the thickness differences between the inner and outer parts of the joints were different.

Three possible ways are known to transport hydrogen by pipeline: (i) blending hydrogen into the existing natural gas pipeline system; (ii) using the natural gas transporting pipeline system to transport (pure) hydrogen, typically after retrofitting or repurposing; and (iii) building a new pipeline system to transport (pure) hydrogen. In all three cases, the used welded or non-welded pipes must be joined with girth welds. Therefore, these low-alloy steel pipelines contain a large number of girth welds, which play a key role in the integrity of the whole pipelines. The own integrity of the girth welds can be interpreted as depending on a number of factors. The factors affecting integrity were systematized and the effect of hydrogen for the natural gas transporting pipelines was interpreted. The influence of material quality was illustrated using the example of two conventional pipeline material (X42 and X52, as older materials) and a high-strength pipeline material (X100, as newer material) that is becoming more widely used. Furthermore, various test results for welded joints were also presented.