Thomas Kannengiesser

The preservation and strengthening of existing steel structures is gaining importance, as many bridges and buildings erected between 1880 and 1940 from old mild steels remain in service today. Unlike modern structural steels, these historical materials, produced by the Thomas, Bessemer, or Siemens-Martin processes, often exhibit pronounced chemical and metallurgical inhomogeneities, including centreline segregation and elevated phosphorus, sulphur, and nitrogen contents, which may impair their weldability. Reliable guidance for joining such steels by gas-shielded arc welding, particularly for full-penetration joints, is still lacking. This work examines the weldability of an early twentieth-century Siemens-Martin steel joined to the fine-grained structural steels S355J2 + N and S355ML. The combination with S355J2 + N was welded with the austenitic solid wire G 18 8 Mn by gas metal arc welding (GMAW), and the combination with S355ML with the basic flux-cored wire T42 6 1Ni B M21 1 H5 by flux-cored arc welding (FCAW). Both combinations were welded at three energy inputs per unit length (5, 10 and 15 kJ/cm) and characterised by non-destructive testing, metallography, hardness, tensile, and Charpy impact testing. With the basic flux-cored wire, sound full-penetration joints were obtained. With the austenitic solid wire, radiographic testing and macro-sections revealed inclusions and lack of fusion exceeding the acceptance limits as well as isolated pores, indicating that the slag-free filler metal is less tolerant of the impurities of the old steel. The heat-affected zone (HAZ) of the old steel underwent marked grain refinement (from approximately 29.80 to 8 µm), and only a slight hardness increase occurred, remaining well below a critical hardness. Although the Mn/S ratio in the segregation zone of the old steel was below the recommended minimum of 4, no hot cracks occurred, which is attributed to the manganese supplied by the filler metals. In tensile testing, failure consistently occurred in the unaffected base metal rather than in the weld, while the Charpy impact energy of the old-steel HAZ at − 20 °C increased to 1.5 to 4 times that of the base metal. FCAW with a basic-flux-cored wire represents a viable technique for repairing historical steel structures, provided that careful material assessment and adapted process control are ensured.

Dilatometry was used to generate continuous cooling transformation (CCT) diagrams for two different L485ME pipeline steels for hydrogen transportation. One material originated from an in-service pipeline, while the other, newer material was taken from plate material. To generate the CCT diagrams, specimens were heated up to peak temperatures of 1250°C using dilatometry to generate coarse-grained microstructure and subsequently cooled with t _8/5 cooling times varying from 2 to 500 s. In addition, heat-affected zone (HAZ) subregions were thermophysically simulated using peak temperatures between 700 and 1250°C at t _8/5 cooling times of 6 s and 15 s. The newer plate material exhibited hardening across the entire investigated HAZ region, whereas the in-service material showed significant hardening only at peak temperatures above 1100°C (15 s) and 1000°C (6 s). These findings provide a basis for understanding the influence of cooling conditions on microstructural evolution and hardness in pipeline steels relevant to hydrogen transport applications.

In-service welding of pipelines without interrupting the internal gas flow is nowadays state of the art for the maintenance and extension of the grid. However, due to access restrictions, it is quite challenging or even impossible to measure local temperatures during welding and subsequent cooling. To address this issue, in the present work welding experiments were performed that consider representative weld configurations and process parameters relevant for in-service welding of gas pipelines. A flat steel sheet was multi-pass welded onto the outer surface of a straight steel pipe segment by manual shielded metal arc welding (SMAW) or by manual gas tungsten arc welding (GTAW). Thermocouples were used for measuring local temperatures at defined positions next to the weld seam. The influences of joint type, wall thickness of the sheet and pipe and heat input on the temperature distribution and, in particular, on the local peak temperatures and cooling rates were studied. Moreover, a finite element (FE) model of a selected weld configuration was created and validated for calculating the temperatures at positions that were inaccessible for direct measurements. The results of the measurements showed that the peak temperature at the inner surface of the pipe decreases with increasing wall thickness. However, independently from the wall thickness, the results of the simulations indicated austenitization next to the weld seam. The low cooling rates did not promote any martensite transformation, as confirmed by the moderate increase of the hardness and by microstructure characterization. The results are of practical interest for pipelines used to transport hydrogen gas, as the temperature-dependent composition of the microstructure and the temperature itself influence the absorption, diffusion and solubility of hydrogen in steel.

Two high-strength, low-alloy Gas Metal Arc Welding (GMAW) wires were evaluated for their susceptibility to Solidification Cracking (SC) using a modified version of the Transverse Motion Weldability (TMW) test1). In the original TMW test, the Transverse Displacement Speed (TDS) is varied iteratively, and crack lengths are measured in each iteration. The modified version eliminates the need for multiple runs by continuously reducing the TDS over a single run. Rather than emphasizing crack lengths, this method assesses the reaction force to the TDS, thereby determining a Critical TDS (CTDS). This critical point directly correlates with the cessation of SC growth.CTDS values were measured under a range of welding parameters, thereby demonstrating the influence of welding speed and arc energy (U × I/welding speed) on SC susceptibility for the tested filler metals. The CTDS data obtained from this testing method have the potential to provide valuable insights for practical applications in the field.

Hydrogen is the energy carrier for a sustainable future without fossil fuels. As this requires a reliable transportation infrastructure, the conversion of existing natural gas (NG) grids is an essential part of the worldwide individual national hydrogen strategies, in addition to newly erected pipelines. In view of the known effect of hydrogen embrittlement, the compatibility of the materials already in use (typically low-alloy steels in a wide range of strengths and thicknesses) must be investigated. Initial comprehensive studies on the hydrogen compatibility of pipeline materials indicate that these materials can be used to a certain extent. Nevertheless, the material compatibility for hydrogen service is currently of great importance. However, pipelines require frequent maintenance and repair work. In some cases, it is necessary to carry out welding work on pipelines while they are under pressure, e.g., the well-known tapping of NG grids. This in-service welding brings additional challenges for hydrogen operations in terms of additional hydrogen absorption during welding and material compatibility. The challenge can be roughly divided into two parts: (1) the possible austenitization of the inner piping material exposed to hydrogen, which can lead to additional hydrogen absorption, and (2) the welding itself causes an increased temperature range. Both lead to a significantly increased hydrogen solubility in the respective materials compared to room temperature. In that connection, the knowledge on hot tapping on hydrogen pipelines is rare so far due to the missing service experiences. Fundamental experimental investigations are required to investigate the possible transferability of the state-of-the-art concepts from NG to hydrogen pipeline grids. This is necessary to ensure that no critical material degradation occurs due to the potentially increased hydrogen uptake. For this reason, the paper introduces the state of the art in pipeline hot tapping, encompassing current research projects and their individual solution strategies for the problems that may arise for future hydrogen service. Methods of material testing, their limitations, and possible solutions will be presented and discussed.

Creep-resistant steels such as 13CrMoV9-10 are utilized in the manufacture of thick-walled pressure vessels and are typically joined by submerged arc welding (SAW). However, these materials are susceptible to stress relief cracking (SRC) if the required post weld heat treatment (PWHT) is not applied correctly. Existing PWHT guidelines, encompassing heating rate and dwell (or holding) time at a given temperature, are derived from a synthesis of empirical knowledge and typically free-shrinkage weld experiments to assess the susceptibility to SRC. Therefore, this study discusses the combined effect of the PWHT heating rate under free-shrinkage compared to restrained shrinkage. Welding experiments were conducted (using plates with a thickness of 25 mm) for both shrinkage conditions for a variety of heating rates and maximum temperatures. In-situ acoustic emission analysis was used to locate propagating SRCs during PWHT. Hardness measurements, mechanical property characterization (Charpy impact strength), and microstructure correlation were used to evaluate the SRC susceptibility. The results suggested that the influence of heating rate could not be directly related to SRC formation and that the initial weld microstructure prior to PWHT was more relevant in terms of very high hardness in the coarse grain heat affected zone, especially that of the last beads in the top layer of the welding sequence. This was seen in the form of random, unexpected SRC occurrence in only one specimen at a heating rate commonly used in welding practice (approximately 100 K/h). In this context, the additional effect of an external shrinkage restraint on SRC must be considered in the form of increasing mechanical loads during welding, which are typically not within the scope of welding practice. To mitigate the probability of SRC during PWHT, it is imperative to reduce the welding heat input and to restrict the structural shrinkage restraint of the weld joint.

In addition to chemical composition, metallurgy, and welding parameters, the intensity of restraint is one of the key variables influencing solidification cracking (SC). Due to their high strength-to-density ratio, many modern lightweight steel constructions increasingly rely on high-strength steel. Given the theoretical framework of solidification cracking theory, tests tend to focus on the effects of strain rate. Externally restrained tests have provided valuable insights into solidification crack susceptibility. In practice, most welded structures are self-restrained; therefore, self-restraint tests more accurately reflect real-world applications. By varying the plate thickness in controlled thermal severity (CTS) tests conducted on S1100 QL, it was possible to adjust the intensity of restraint on fillet welds at a high level. Testing was performed using four different filler wires for gas metal arc welding (GMAW), including three solid wires and one metal-cored wire. Additionally, two sets of welding parameters were evaluated. High arc energy (U × I/welding speed) and increased welding speed were found to be more prone to solidification cracking compared to the parameter set with lower arc energy and welding speed. The results indicate a correlation between increasing restraint severity and a higher incidence of solidification cracking.

The ISO 3690 standard “Determination of hydrogen content in arc weld metal” requires a thermal activation of the diffusible hydrogen in a piece of weld metal for the subsequent ex situ concentration measurement by carrier gas hot extraction CGHE or thermal desorption spectroscopy (TCD). Laser-induced breakdown spectroscopy (LIBS) offers a time and spatially resolved, almost non-destructive, in situ measurement of hydrogen at surfaces without sample preparation. We measured hydrogen in steels, which were charged either electrochemically or by high-pressure hydrogen gas, and compared the results. Further, the feasibility of quantitative hydrogen line scan measurements with LIBS was demonstrated by measuring hydrogen at water jet cut surfaces. The hydrogen concentrations measured with the help of LIBS were compared with CGHE measurements. It was observed that hydrogen can be reliably measured with LIBS for concentrations larger than 2 wt.-ppm. The maximum hydrogen concentration achieved using electrochemical charging was 85.1 ppm. The results show that LIBS is a promising technique for time- and spatially resolved measurements of hydrogen in steels.