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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.
Carrier gas hot extraction (CGHE) is a commonly applied technique for determination of hydrogen in welded joints using a thermal conductivity device (TCD) for quantitative measurement. The CGHE is based on the accelerated hydrogen effusion due to thermal activation at elevated temperatures. The ISO 3690 standard suggests different specimen geometries as well as necessary minimum extraction time vs. temperature. They have the biggest influence on precise hydrogen determination. The present study summarizes the results and experience of numerous test runs with different specimen temperatures, geometries and factors that additionally influence hydrogen determination. They are namely: specimen surface (polished/as-welded), limited TCD sensitivity vs. specimen volume, temperature measurement vs. effects of PID-furnace controller as well as errors due to insufficient data assessment. Summarized, the temperature is the driving force of the CGHE. Two different methods are suggested to increase the heating rate up the reach the desired extraction temperature without changing the experimental equipment. Suggestions are made to improve the reliability of hydrogen determination depended on the hydrogen signal stability during extraction accompanied by evaluation of the recorded data. Generally, independent temperature measurement with calibration specimens is useful for further data analysis, especially if this data is used for calculation of trapping kinetics by thermal desorption analysis (TDA).
Neutron cameras allow visualizing hydrogen distributions with radiographic or tomographic imaging methods in iron and steel. The necessary contrast between hydrogen and iron stems from the high difference in the total neutron cross section of both elements. This allows e.g. the in situ measurement of hydrogen mass flow inside cm thick steel samples with a temporal resolution of 20 s using neutron radiography as well as the quantitative measurement of hydrogen accumulations at the crack’s inner surfaces in hydrogen embrittled iron samples with neutron tomography. We could detect directly gaseous hydrogen in the crack cavities and we measured the gas pressure. This new quality of the information on a micrometer scale allows new insights for the analysis of hydrogen-induced damage mechanisms. Further, this method is non-destructive and provides local information in situ and in three dimensions with a spatial resolution of 20-30 µm. In this contribution, we show examples that demonstrate the spatial and temporal resolution of the neutron radiography and tomography methods in order to visualize and quantify hydrogen accumulations at cracks. The measurements were performed at the research reactor BER II of the HZB in Berlin and at the FRM II reactor of the neutron source Heinz Maier-Leibnitz in Garching.
Neutron cameras allow visualizing hydrogen distributions with radiographic or tomographic imaging methods in iron and steel.
The necessary contrast between hydrogen and iron stems from the high difference in the total neutron cross section of both elements. This allows e.g. the in situ measurement of hydrogen mass flow inside cm thick steel samples with a temporal resolution of 20 s using neutron radiography as well as the quantitative measurement of hydrogen accumulations at the crack’s inner surfaces in hydrogen embrittled iron samples with neutron tomography. We could detect directly gaseous hydrogen in the crack cavities and we measured the gas pressure. This new quality of the information on a micrometer scale allows new insights for the analysis of hydrogen-induced damage mechanisms. Further, this method is non-destructive and provides local information in situ and in three dimensions with a spatial resolution of 20-30 μm.
In this contribution, we show examples that demonstrate the spatial and temporal resolution of the neutron radiography and tomography methods in order to visualize and quantify hydrogen accumulations at cracks. The measurements were performed
at the research reactor BER II of the HZB in Berlin and at the FRM II reactor of the neutron source Heinz Maier-Leibnitz in
Garching.
High-strength low-alloyed (HSLA) steels with yield strength ≥ 690 MPa have increasing im-portance 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 sig-nificantly 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 min-imize the incidence of HAC, it is essential to gain knowledge about both the (1) absorbed hydro-gen amount and its distribution in the weld seam and (2) options to lower the amount of intro-duced hydrogen. Existing standards recommend heat treatment procedures (interpass tempera-ture 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 imme-diately 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 concen-trations were measured.
Hydrogen determination in weld seams is standardized in ISO 3690. In accordance to this standard, a defined time for hydrogen collection has to be anticipated for different extraction temperatures. In other words, the temperature is the most important value that has to be monitored in addition to the aimed hydrogen determination.
The specimen geometry has influence on the real sample temperature during CGHE vs. the adjusted furnace temperature. This presentation gives a short summary on possible influences on the "correct" hydrogen determination temperature during carrier gas hot extraction (CGHE) using infrared radiation driven furnace. The main findings are: (1) specimen surface is important in terms of polished or oxidized condition, (2) specimen geometry is important for fast heating, (3) PID-values of control software are a considerable influence to accelerate the heating process depite thick specimens and (4) independent sample temperature determination before CGHE is strongly recommended.
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.