The Dangers Posed by Hydrogen
Welding un- and low alloyed steels is subject to crack formation. Appropriate methods exist for reducing the hydrogen input in weld metals, such as selecting suitable welding processes, the proper handling of electrodes and precise welding preparations. In principle, the welding process can be subdivided into the partial systems of parent metal, electrode and ambient air, whereby each partial system can be a source of hydrogen.
Source of moisture: specimen, ambient air, electrodes
Sources of moisture for the specimen are most notably surface layers that have not been subject to any heat treatment to dry the seam edges prior to welding. The effect of the ambient air during arc welding must not be underestimated. The intense energy of the arc can cause moisture in the ambient air, which is partially ionised in the arc into atomic hydrogen, to be absorbed by the weld metal, where it may subsequently lead to damage such as cracking. High temperatures combined with high relative humidities generate a high water vapour partial pressure, which has an effect upon the weld. Resistance to the high water vapour partial pressure of the ambient air can only be maintained through suitable welding procedures and by the electrode itself. It is vital when welding in humid ambient air that an arc that is as short as possible be maintained.
For the welding of un- and low alloyed steels, a variety of electrodes can be employed. However, one of the primary objectives of basic electrodes is to achieve the lowest possible hydrogen content in the weld metal in order to ensure crack-free weld seams even when welding increased-strength and extremely high-strength steels. In the development of stick electrodes, the engineers at Böhler Welding pay special attention to the susceptibility to moisture absorption and the relationship of coating moisture and diffusible hydrogen content in the weld metal.
Böhler Welding was one of the first manufacturers to supply stick electrodes in hermetically sealed tins. This allows the risk of moisture absorption during transport and storage on-site to be excluded up to the moment of welding. The water- and vapour-proof tin ensures ready-to-use electrodes, which can be reliably welded without rebaking. Thanks to an innovative coated electrode design from Böhler Welding, it was possible to substantially lower the moisture absorption, the basic hydrogen content and even the diffusible hydrogen content after removal from storage. Most notably, the susceptibility to moisture absorption in the event of a brief removal from storage was considerably improved.
Identifying the effects of hydrogen input
When considering the potential danger of hydrogen, it is important to estimate the effusion under different conditions in order to be able to introduce countermeasures if hydrogen assisted cracking occurs. Carrying out tests with different welding consumables enables a more accurate description of these effects. In most cases under practical welding conditions, the specimen is only cooled gradually. Particularly with high-strength welds, a certain preheating and interpass temperature needs to be maintained. To evaluate these different conditions and obtain an estimate of the diffusion behaviour in practical welding, Böhler development engineers carried out trial welds with different preheating or interpass temperatures and measured the remaining diffusible hydrogen content in the specimens after various cooling times.
The investigations showed that a higher preheating or interpass temperature appears to be only conditionally suitable for achieving low hydrogen contents in welds, since this is essentially dependent on the added hydrogen content. Nevertheless, hydrogen assisted cracking factors such as residual stress level, joint microstructure and strength are generally positively affected (in terms of preventing cracks) by higher preheating and interpass temperatures.
Effect of multi-layer welds
In practical welding, particularly for thick-walled connections with a high potential risk of hydrogen assisted cracking, multi-layer welds are predominant as ever. Positive effects for avoidance of cracking can be achieved through the thermically activated increased hydrogen effusion and through the recrystallisation of the joint of the deposited layers (beads).
Multi-pass welds are often performed with cellulosic stick electrodes for economic reasons. The high hydrogen input caused by the use of this type of electrode, combined with the usual heavy gauge pipe work connections, requires special measures, such as the correct observance of the required interpass temperature, in order to produce crack-free seams.
Practical analyses have established that when welding a second or third bead, relatively lower diffusible hydrogen values are caused by the cooling of the first or second bead and the additional temperature treatment during depositing. Of course high hydrogen inputs can, in principle, be modified or reduced by selecting a different welding filler material. Using basic vertical-down electrodes instead of cellulosic stick electrodes results in substantially lower hydrogen contents for single- and multi-layer welds.
Minimising the risk of crack formation
Hydrogen assisted cracks occur in the heat-affected zone (HAZ) primarily as longitudinal cracks and are known as underbead cracks. However, hydrogen assisted cracking is also identified in the form of brittle fractures, specifically in high-strength connections. This damage occurs primarily in the upper third of the V-welds, which can be explained by the localised increased tensile stress and the resultant greater amount of embedded hydrogen. In order to make precise forecasts about susceptibility to hydrogen assisted cracking in weld joints, testing methods need to be developed to provide results that are as transferable as possible to on-site conditions.
In the bead bend test, Böhler Schweißtechnik Austria developed a test method that fulfils these criteria for testing weld metals. For this test, plates of the parent metal are fastened to a rigid base plate which should be about four times as thick as the plates. It is possible to control the stress on the weld by altering the restraint length (the distance between the weld and the clamping of the parent metal).
After clamping the plates, the weld can be performed with parameters in step with those in actual practice. To allow for the effects of delayed crack formation, the specimen is extricated from the base plate approximately 24hr after the completion of welding. The hydrogen assisted cracks in the weld are made visible by deforming the specimen through bending. The micro-cracks are expanded and achieve a size that is visible for subsequent evaluation of the specimen.
Due to the simple and inexpensive implementation of this test, it is well suited to researching and developing high-strength and hydrogen-laden weld metals. Moreover, the practical application makes this test suitable for comparative studies, since combination welds with differing electrode types, such as joint welds with cellulosic and basic vertical-down electrodes, can also be tested.
Example of pipeline welding
For the welding of pipelines with cellulosic electrodes, the susceptibility to hydrogen assisted cracking in relation to weld metal strength and joint thickness (pipe wall thickness) was determined with the aid of the bead bend test, enabling recommendations to be made regarding the interpass temperature to be used. In modern pipeline construction, a method is often used that combines cellulosic electrodes for the root and hot pass layers, and basic electrodes for the fill and cap layers.
The bead bend test enables testing of this type of connection for susceptibility to hydrogen assisted cracking. By varying different parameters in the production of these connections, such as the moisture content of the electrode coatings, the susceptibility of the weld metal to cracking is changed.
For a detailed explanation of this method, please request a copy of our special publication ‘The Dangers Posed by Hydrogen’.