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Effect of cooling rate on microstructure and properties of microalloyed HSLA steel weld metals
(2015)
Two high strength Nb/Ti microalloyed S690QL steels were welded with identical filler material, varying welding parameters to obtain three cooling rates: slow, medium and fast cooling. As cooling rate increased, the predominantly acicular ferrite in Nb weld metal (WM) is substituted by bainite, with a consequence of obvious hardness increase, but in Ti WM, no great variation of acicular ferrite at all cooling rates contributed to little increment of hardness. The transition between bainite and acicular ferrite has been analysed from the point view of inclusions characteristics, chemical composition and cooling rate. Excellent Charpy toughness at 233 K was obtained with acicular ferrite as predominantly microstructure. Even with bainite weld of high hardness, the toughness was nearly enough to fulfill the minimal requirements. WM for Ti steel showed to be markedly less sensitive to the variations of cooling rate than that for Nb steel.
Offshore wind turbines (OWT) are a major goal of the energy strategy of Germany encompassing the increase of the installed wind power. OWT components are manufactured from welded steel plates with thicknesses up to 200 mm. The underlying standards and technical recommendations for construction of OWTs encompass specifications of so-called minimum waiting time (MWT) before non-destructive testing of the weld joints is allowed. Reason is the increased risk of time-delayed hydrogen assisted cold cracking as hydrogen diffusion is very slow due to the very thick plates. The strict consideration of those long MWT up to 48 h during the construction of OWTs leads to significant financial burden (like disproportionately high costs for installer ships as well as storage problems (onshore)). In this study, weld joints made of S355 ML were examined in comparison with the offshore steel grade S460 G2+M. The aim was to optimize, i.e., reduce, the MWT before NDT considering varied heat input, hydrogen concentration and using self-restraint weld tests. This would significantly reduce the manufacturing time and costs of OWT construction. To quantify the necessary delay time until hydrogen-assisted cold cracks appear, acoustic emission analysis was applied directly after welding for at least 48 h.
Hydrogen can have an extreme degradation effects in steels, particularly concerning the mechanical properties. These effects can lead to hydrogen-assisted cracking in microalloyed high-strength steels during fabrication and/or operation in industrial applications. In order to study these effects, electrochemically charged tensile specimens were tested to elucidate the degradation of their properties. The carrier gas hot extraction (CGHE) method, which functionally combines a mass spectrometer with a thermal desorption analysis (TDA) process, was used for the detection of ultra-low diffusible hydrogen concentrations in the material specimens. The mass spectrometer provided rapid and automatic determination of hydrogen concentration, whereas the TDA presented the activation energy within the respective test specimen at the specific temperature. Additionally, specimen temperature was carefully monitored to reduce the evaluation error for local effusion peaks. A quenching and deformation dilatometer was used for the analysis of typical heat-affected zones during the welding process for a high reproducibility of the homogenous microstructures that were studied. The present work shows the interaction between hydrogen and lattice defects in different microalloyed materials and heat-affected zones of weldable fine-grained steels. These steels were prepared in a quenched and tempered condition and in a thermo-mechanically rolled condition. These preparations were made according to German standard DIN EN 10025-6 and to DIN EN 10149-2, respectively. The trapping characteristics of two steel grades, S690QL and S700MC, were studied with respect to the activation energy dependent on carbon content and microalloying elements such as Ti, Nb, Mo, Cr, and V. The two steel grades exhibited several types of traps: carbide formations, dislocations, and/or grain boundaries were common, which can influence activation energy and hydrogen solubility. The type and dimension of inclusions or particles also affected the hydrogen trapping behavior. A decrease of carbon and specific alloying elements in thermo-mechanically hot rolled steels led to a change in the activation energy binding the trapped hydrogen. This thermo-mechanically hot rolled steel revealed an increased interaction between hydrogen and precipitations. The higher carbon content in the quenched and tempered steel led to a higher interaction between hydrogen and iron carbide, specifically in the martensitic phase. Furthermore, the trapping behavior in heat-affected zones showed a significant increase in activation energy, especially in the coarse-grained microstructure. These previously mentioned various effects were studied to better understand the degradation of mechanical properties in these two steels.
For higher operational temperatures and pressures required in petrochemical plants, the modified 13CrMoV9-10 steel was developed providing high resistance against creep and compressed hydrogen. Extreme care during the welding procedure is necessary for this steel, attributed to low toughness, high strength in as-welded state, and increased susceptibility to stress relief cracking (SRC) during post-weld heat treatment (PWHT). Previous research of SRC in creep-resistant steels discussed mainly thermal and metallurgical factors. Few previous findings addressed the influences of welding procedure on crack formation during PWHT considering real-life manufacturing conditions. These investigations focus on effects of welding heat control on stresses during welding and subsequent PWHT operations close to realistic restraint and heat dissipation conditions using a special 3D testing facility, which was presented in parts I and II of this contribution. Part III addresses investigations on residual stress evolution affecting crack formation and discusses the transferability of results from large-scale testing to laboratory-scale. Experiments with test set-ups at different scales under diverse rigidity conditions and an assessment of the residual stresses of the weld-specimens using X-ray (surface near) and neutron diffraction analysis (bulk) were performed. This study aims to provide a way of investigating the SRC behaviour considering component-specific residual stresses via small-scale testing concepts instead of expensive weld mock-ups.
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 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 the presence of diffusible hydrogen, i.e., hydrogen-assisted cracking (HAC), generally increases. HAC is a result of the critical interaction between local microstructure, mechanical load, and hydrogen concentration. In existing standards for welding of HSLA-steels, recommendations including working temperatures and dehydrogenation heat treatment (DHT) are given to Limit the amount of introduced hydrogen during welding. These recommendations are based on investigations into 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, 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 were investigated. The results show that weldments with narrow grooves contained an increased amount of diffusible hydrogen. 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 gas metal arc welded multi-layer welds for the first time.
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.
The new alloying concept of multi-element systems with defined entropy (HEA—high-entropy alloy; MEA—medium-entropy alloy) is gaining increasing importance in materials research. Significantly improved properties or combinations of properties are shown by some HEA/MEA systems. Thus, primarily the production and resulting microstructures of HEA, as well as its properties, have been investigated so far. Furthermore, processing is a main issue in transferring HEA systems from the laboratory to real components. Since welding is the most important joining process for metals, it is crucial to investigate the influence of welding to guarantee component integrity. Welding leads to residual stresses, which significantly affect the component integrity. Hence, the focus of this study is the residual stress formation and distribution in a CoCrFeMnNi HEA and ternary CoCrNi MEA using two different welding processes: tungsten inert gas (TIG) welding and solid-state friction stir welding (FSW). As a pathway for the application of HEA in this investigation, for the first time, residual stress analyses in realistic near-component specimens were performed. The residual stresses were determined by X-ray diffraction (XRD) on the surfaces of top and root weld side. The results were correlated with the local welding microstructures. The results show that both FSW and TIG generate significant tensile residual stresses on the weld surfaces in, and transverse to, the welding direction. In the case of FSW of the CoCrFeMnNi HEA, the longitudinal residual stresses are in the range of the yield strength of approx. 260 MPa in the weld zone.
High- and medium-entropy alloys (HEAs) are a quite new class of materials. They have a high potential for applications from low to high temperatures due to the excellent combination of their structural properties.
Concerning their application as components; processing properties, such as machinability, have hardly been investigated so far. Hence, machinability analyses with a focus on the influence of the milling process and its basic parameters (cutting speed, feed per cutting edge) on the resulting surface integrity of specimens from an equiatomic high- (CoCrFeMnNi) and a medium- (CoCrNi) entropy alloy have been carried out. A highly innovative milling process with ultrasonic assistance (USAM) was compared to conventional milling processes.
Recent studies have shown that USAM has a high potential to significantly reduce the mechanical load on the tool and workpiece surface during milling. In this study, the basic machining and ultrasonic parameters were systematically varied. After machining, the surface integrity of the alloys was analyzed in terms of topography, defects, subsurface damage, and residual stresses. It was observed that USAM reduces the cutting forces and increases the surface integrity in terms of lower tensile residual stresses and defect density near the surfaces for the CoCrFeMnNi alloy. It was shown that the cutting forces and the metallurgical influence in the sub surface region are reduced by increasing the cutting speed and reducing the feed rate per cutting edge. With the CoCrNi alloy, the tool revealed severe wear. As a result, for this alloy no influence of the parameters on the machinability could be determined.
Influence of Surface Preparation on Cracking Phenomena in TIG-Welded High and Medium Entropy Alloys
(2022)
Multi-element systems with defined entropy (HEA—high entropy alloy or MEA—medium entropy alloy) are rather new material concepts that are becoming increasingly important in materials research and development. Some HEA systems show significantly improved properties or combinations of properties, e.g., the overcoming of the trade-off between high strength and ductility. Thus, the synthesis, the resulting microstructures, and properties of HEA have been primarily investigated so far. In addition, processing is crucial to achieve a transfer of potential HEA/MEA materials to real applications, e.g., highly stressed components. Since fusion welding is the most important joining process for metals, it is of vital importance to investigate the weldability of these materials. However, this has rarely been the subject of research to date. For that reason, in this work, the weldability depending on the surface preparation of a CoCrFeMnNi HEA and a CoCrNi MEA for TIG welding is investigated. The fusion welding of longer plates is described here for the first time for the CoCrNi alloy. The welds of both materials showed distinct formation of cracks in the heat affected Zone (HAZ). Optical and scanning electron microscopy analysis clearly confirmed an intergranular fracture topography. However, based on the results, the crack mechanism cannot be conclusively identified as either a liquid metal embrittlement (LME) or hot cracking-like liquid film separation.