Th. Boellinghaus

Numerical Model for Hydrogen-Assisted Cracking

Abstract Models for environment-assisted cracking (EAC) predominantly consider anodic reaction-based processes for simulation of crack propagation. A limited number of models have been developed to...

Numerical Modelling of Cold Cracking Initiation and Propagation in S 1100 QL Steel Root Welds

Although the phenomenon of hydrogen assisted cold cracking (HACC) and respective avoidance procedures have extensively been investigated in the seventies and eighties, the reasons for recent failures are still a lack of knowledge about the basic hydrogen effects on steel microstructures and, in particular, a lack of welding procedure specifications and standards accounting directly and consistently for cold cracking avoidance in modern high strength structural steels with yield strengths of up to 1 100 MPa. In previous several contributions the consequences of various heat treatment procedures targeted at HACC avoidance have been shown, as for instance their effects on stress-strain build up and on hydrogen diffusion in high strength steel welds. But, a principal interaction of three local influences on hydrogen assisted cold cracking, i.e. local microstructure; local mechanical load and local hydrogen content have not yet been studied in detail for these materials. For this, a numerical model for HACC has been developed, accounting particularly for crack-initiation and crack-propagation criteria, like the hydrogen redistribution during the process of cracking. The numerical model has been used to investigate HACC in such materials, i.e. in the weld microstructures of an S 1100 QL steel, under severe restraint and various hydrogen levels. The results were achieved by in depth thermal and structural finite element simulations combined with numerical hydrogen diffusion modelling. By such procedure, HACC in single-layer welded plates with thickness of 20.0 mm at realistic restraints has been studied. As a particular result, it turned out that the crack-initiation location is typically in the centre of the weld metal (WM), where only a single crack is initiated at hydrogen contents of up to 10.0 Nml/100 g Fe. But, it was evidently shown by such analyses that the crack-initiation location is shifted into the HAZ and that multiple cracking occurs at higher hydrogen contents of up to 15.0 Nml/100 g Fe.

Real Time Imaging of Deuterium in a Duplex Stainless Steel Microstructure by Time-of-Flight SIMS.

For more than one century, hydrogen assisted degradation of metallic microstructures has been identified as origin for severe technical component failures but the mechanisms behind have not yet been completely understood so far. Any in-situ observation of hydrogen transport phenomena in microstructures will provide more details for further elucidation of these degradation mechanisms. A novel experiment is presented which is designed to elucidate the permeation behaviour of deuterium in a microstructure of duplex stainless steel (DSS). A hydrogen permeation cell within a TOF-SIMS instrument enables electrochemical charging with deuterium through the inner surface of the cell made from DSS. The outer surface of the DSS permeation cell exposed to the vacuum has been imaged by TOF-SIMS vs. increasing time of charging with subsequent chemometric treatment of image data. This in-situ experiment showed evidently that deuterium is permeating much faster through the ferrite phase than through the austenite phase. Moreover, a direct proof for deuterium enrichment at the austenite-ferrite interface has been found.

Effects of Preheating and Interpass Temperature on Stresses in S 1100 QL Multi-Pass Butt-Welds

Most of the research on Hydrogen Assisted Cold Cracking (HACC) in high strength steel welds conducted over the last several decades has focused on single-pass welds, especially considering materials with yield strengths about 700 MPa. The guidelines for avoiding cracking that have been developed from such work are therefore useful only where a root pass is the critical event. The well-known guideline is using preheating temperature. Such guideline is very limited when applied to multi-pass welds. In order to support this need, this paper presents the influence of inhomogeneous Hydrogen Removal Heat Treatment (HRHT) procedures, i.e. sole preheating, controlled interpass temperature and combined preheating and controlled interpass temperature, on the residual stresses in multi-pass welds of S 1100 QL. Thereafter, these results are used to identify HACC problems in S 1100 QL and are not reported here. The results were achieved by decent thermal and structural finite element simulations of a five-layer welded 12 mm thick plate at a realistic restraint provided by respective Instrumented Restraint Cracking (IRC) test. The simulations show that the inhomogeneous heat treatment procedures significantly increase the residual stresses as compared to welding without any heat treatment. In contrast to more general anticipations, an increasing controlled interpass temperature does not necessarily lead to a stress reduction, but can even increase the stresses dependent on the location in the multi-pass welds. Maximum residual stresses generally appear in the upper third part of the weld and are not located beneath the top surface where is a typical location used to detect residual stresses in real welded components. If the restraint intensity given to the welded component is not proper, such heat treatment procedures with various temperatures seem to be useful to reduce residual stresses in multi-pass welds.

Numerical modelling of hydrogen-assisted cracking

Hydrogen might be introduced during fabrication welding or might be taken up from an environment during sour service or cathodic protection. Thus, hydrogen assisted stress corrosion and cold cracking is still a major topic regarding the reliability of welded steel components, as for instance offshore platforms and pipelines. In order to support conclusive testing and life time evaluation of welded steel components, a numerical model for hydrogen assisted cracking has been developed, particularly taking into consideration the geometrical effects of crack propagation on the respective hydrogen distribution alongside and ahead of the crack. Numerical calculations were based on finite element analysis of the hydrogen and stress-strain distribution by using a commercially available program. The model has been verified experimentally by slow strain rate experiments of supermartensitic stainless steels which are intended to be used more extensively as materials for welded flowlines in North Sea oil and gas production. As first results of such simulations the influence of the subsurface concentration provided by different H2S saturation levels in the NACE TM 0177-96 standard solution on crack propagation and the effect of crack shape on the hydrogen distribution profile are presented in this contribution.

Vertical-up and -down Laser Plasma Powder Hybrid Welding of a High Nitrogen Austenitic Stainless Steel

Although laser hybrid welding is increasingly introduced into the manufacturing process of several industrial branches, such technologies have up to the present only been qualified for simple horizontal positions. But, particularly in the shipbuilding and offshore as well as in the automotive and transportation industry there is a high demand to incorporate such innovative procedures also to out-of-position welding of assembly sections. Laser plasma powder arc welding (LPPAW) has already been proven to be a stable hybrid process and less susceptible to production related incidents than other hybrid processes. For instance, misalignment and root gap displacements can most smoothly be compensated, due to separation of the filler material feeding from the arc energy input. As a further step towards the application of LPPAW to real production welding, first results of vertical-up and vertical-down welding of 4mm thick high nitrogen austenitic stainless steel (1.4565) plates are presented in the present contribution. Several welding parameters have been varied and their effects on the weld cross sectional shape have been studied with the most important result, that the laser plasma powder process can principally be applied to out-of-position conditions. Similar to previous investigations of LPPAW in the horizontal position, the results of vertical-up and vertical-down welding are significantly dependent on the geometric configuration of the plasma torch towards the laser beam. Aside from these geometric parameters, the influences of the welding speed and of the plasma powder arc welding current have been studied.

Blast resistance of high-strength structural steel welds

As consequence for increasing threats by IEDs (Improvised Explosive Devices) on vehicles, the blast resistance of the welded frames and bodies becomes increasingly important. Considering vehicle welds subjected to blasting, the real configurations of the joints in the structure and the position of the blast loads have to be considered. The present contribution thus focuses on a weld joint at the explosion endangered wheel well of a tactical truck. The high-strength steel welds were subsequently impacted by explosion loads within the upper range from those experienced in practical military operation to cause not only deformation, but also to investigate the ultimate fracture behaviour of the high-strength weld. The interaction between cooling time t8/5 and displacement, crack path as well as fracture surface was analysed. The analyses of the fracture surfaces revealed ductile overload failure and also the size of the dimples was influenced by the cooling time t8/5. As a prominent feature, these investigations showed that the crack path of such high-strength steel welds under blasting is less influenced by the final hardness level in the respective weld microstructures but much more affected by the hardness gradient at the fusion line and inside the Heat Affected Zone (HAZ).

Numerical Investigations on Hydrogen-Assisted Cracking in Duplex Stainless Steel Microstructures

Duplex stainless steels (DSS) are used in various industrial applications, e.g. in offshore constructions as well as in chemical industry. DSS reach higher strength than commercial austenitic stainless steels at still acceptable ductility. Additionally, they exhibit an improved corrosion resistance against pitting corrosion and corrosion cracking in harsh environments. Nevertheless, at specific conditions, as for instance arc welding, cathodic protection or exposure to sour service environments, such materials can take up hydrogen which may cause significant property degradation particularly in terms of ductility losses which, in turn, may entail hydrogen-assisted cracking (HAC). The cracking mechanism in DSS is different from steels having only a single phase, because hydrogen diffusion, stress-strain distribution and crack propagation are different in the austenite or ferrite phase. Therefore, the mechanism of HAC initiation and propagation as well as hydrogen trapping in DSS have not been fully clarified up to the present, as for most of the two-phase microstructures. At this point the numerical simulation can bridge the gap to a better insight in the cracking mechanism regarding the stress-strain distribution as well as hydrogen distribution between the phases, both austenite and ferrite, of the DSS. For that purpose, a two dimensional numerical mesoscale model was created representing the microstructure of the duplex stainless steel 1.4462, consisting of approximately equal portions of austenite and ferrite. Hydrogen assisted cracking was simulated considering stresses and strains as well as hydrogen concentration in both phases. Regarding the mechanical properties of austenite and ferrite different statements can be found in the literature, dependent on chemical composition and thermal treatment. Thus, various stress-strain curves were applied for austenite and ferrite simulating the HAC process in the DSS microstructure. By using the element elimination technique crack critical areas can be identified in both phases of the DSS regarding the local hydrogen concentration and the local mechanical load. The results clearly show different cracking behavior with varying mechanical properties of austenite and ferrite. Comparison of the results of the numerical simulation to those of experimental investigations on DSS will improve understanding of the HAC process in two phase microstructures.

In situ synchrotron X-ray radiation analysis of hydrogen behavior in stainless steel subjected to continuous heating

Hydrogen generally causes lattice distortions and phase transformations when introduced into a metallic crystal lattice. For the investigations reported in this contribution, hydrogen thermal desorption analysis has been carried out to observe the influence of hydrogen desorption on the lattice of super martensitic stainless steel during continuous heating. The lattice expansion parameter and the phase transformations have been monitored during the thermal desorption process, and the influence of hydrogen on such characteristics has been evaluated. It was found that hydrogen has a significant influence on both the lattice parameter and on the thermal expansion. However, hydrogen has no influence on phase transformation during thermal desorption. The hydrogen’s desorption behavior in this process was also observed and it turned out that hydrogen desorbs in two stages, i.e., firstly diffusible hydrogen and trapped hydrogen afterward.

Hydrogen-trapping mechanisms of TIG-welded 316L austenitic stainless steels

The interaction of hydrogen with various tungsten-inert-gas-welded austenitic stainless steels’ (AUSS) microstructure is studied by means of desorption/absorption analysis and microstructure observations. One of the limitations of welding is created by the presence of hydrogen in the weld, which can shorten the steel’s service life. The local hydrogen concentration, trapping, and its distribution along the welded samples were studied by thermal desorption spectrometry and were supported by X-ray diffraction (XRD) and electronic microstructural observations. Hydrogen content demonstrated a dependence on the welding zone. It was found that hydrogen distribution, and accepted microstructure during welding, played a significant role in the trapping mechanism of 316L AUSS. XRD analysis revealed residual stresses which were caused due to the presence of hydrogen in γ-phase. It was shown that the austenite microconstituents inside 316L can have a crucial effect in preventing hydrogen-assisted cracking phenomenon. The effects of AUSS microstructure on hydrogen absorption and desorption behavior are discussed in detail.