Even in the most optimum environments, hydrogen makes welding a challenge, particularly when crack-sensitive steel or high-residual stress components are involved. The root cause of cracking and fracturing stems from hydrogen becoming trapped in the weld; a possible result of remaining lubricants on metals, moisture contained in welding electrodes or other consumables, the climate or just moisture within the material itself.
The risk for cracking increases as hydrogen diffuses throughout the steel where it builds up in stress-prone pockets in the weld and the heat-affected zone (HAZ). Hydrogen-assisted cracking, also called cold cracking or delayed cracking, commonly affects heavily restrained parts, thick steel sections and high-strength steels and usually occurs near normal ambient temperatures. Higher hardness materials and lower ductility level components are more prone to hydrogen-related issues. For this reason, managing hydrogen levels in welding is vital when producing offshore structures, transmission pipelines, construction cranes and large-scale steel structures.
While hydrogen-assisted cracking typically occurs near the completion of welding, steel remains susceptible to cracking during the following 48 hours. Even so, delayed cracking can occur weeks or months after welding has been completed. The question becomes: What are the most common causes for high levels of hydrogen in welds and how can the problem be managed?
Causes of hydrogen cracking
While working to minimize, manage or prevent problems related to high weld-hydrogen levels, there are several common causes to keep in mind.
One factor that influences weld-hydrogen levels includes the humidity within a welding environment – a variable that is nearly impossible to regulate. Because atmospheric moisture increases in locations with higher humidity, weld-hydrogen levels spike at locations such as construction sites, shipyards and other manufacturing operations.
Hydrogen can be brought into the welding process from the materials being welded themselves. At times, this residual moisture is the result of processing or servicing of the metals, cleaning fluids or rust stains. Surface contaminants, such as paint, cutting oils, dirt and grease, can also affect hydrogen levels. By cleaning all joint faces, hydrogen cracking can be minimized or avoided altogether.
The wire and gas used – and the moisture within these consumables – can also largely influence the hydrogen levels in the weld deposit. These consumables contain moisture in the flux, which includes the flux in cored wires and the flux used in submerged arc welding as well as the coating of manual metal arc electrodes. Of course, materials, metals and their thickness affect cooling rates and, therefore, play a role in the likelihood of cold cracking.
Another variable affecting hydrogen levels in the welding process includes the type of electrode used during production. Rutile and cellulosic electrodes generate more hydrogen compared to basic SMAW electrodes. Additionally, hydrogen levels are influenced by the specific welding parameters used when producing components or parts, particularly when higher deposition rates are created, which adversely affects hydrogen levels in welds. The handling and storage of such products is also proven to largely affect weld hydrogen.
Reducing hydrogen cracking
It’s hard to quantify all of the sources of hydrogen that impact welds – even more difficult is effectively controlling these influencing factors.
Metal fabrication shops utilize a number of preventative measures to minimize the risk of hydrogen-assisted cracking. While these methods can be effective, they are not without cost; often these methods impede productivity and, in some cases, require the use of more expensive consumables.
One important tactic for minimizing cold cracking includes the proper handling and storage of tubular wire products and SMAW electrodes. For optimum results, materials should be preserved in their original packaging before use and kept dry during slow production periods.
Unfortunately, these requirements may be impractical in the majority of fabrication environments and exposed consumables are significantly more susceptible to moisture absorption as well as the resultant hydrogen accumulation in the weld area that typically follows.
Another approach is the practice of pre-heating and post-heating welds to manage hydrogen levels. Not only can this method be problematic, but it can also be costly and results can vary according to the amount of control and attention dedicated to this process. These methods are traditionally implemented to remove residual moisture on materials, reduce the post-welding cooling rate and minimize the presence of a brittle microstructure.
Working for lower hydrogen levels
Using a filler material – such as SMAW electrodes with a “low-hydrogen” formula – when joining high strength materials can help reduce weld hydrogen levels. These SMAW electrodes are produced in a range of designations, like H4 and H8, in much the same way as flux-cored and metal-cored wires. To achieve the required lower weld hydrogen levels, some shops overlook the reduced performance of these lower hydrogen products.
Within the core of flux-containing wires, solid fluoride compounds can be added to control hydrogen levels, which again, can reduce weldability and performance. During the breakdown of solid fluoride compounds in the arc, fluorine is produced, which reacts with hydrogen before being removed from the arc environment. The breakdown of these compounds often impacts stability, creates spatter and can even result in poor arc control.
Alternately, the process of incorporating a gaseous fluoride to the basic argon-carbon dioxide shielding gas was developed by Hobart/ITW in an effort to minimize weld hydrogen. By adding the gaseous fluoride and eliminating solid fluoride, weldability and performance can be protected while still seeing a reduction of hydrogen levels – a significant accomplishment.
The CF4 solution
In an effort to determine the best strategies to reduce hydrogen-assisted cracking, a variety of tests have been conducted, incorporating a number of consumable material types and formulations in an effort to make fabricators as productive and efficient as possible. One notable example includes an examination of using a gaseous fluoride-supplemented shielding gas mixture with differing welding consumables. The results of the research team’s studies are shown, revealing the use of CF4 (carbon tetrafluoride) as the addition.
CF4, a gaseous material containing fluoride, is non-toxic, has not been linked to any negative long-term health effects and is readily available. It is a simple asphyxiant, like argon; there is no established OSHA Permissible Exposure Limit (PEL) or Threshold Limit Value (TLV) for this gaseous material. Additionally, CF4 can be transported like every other shielding gas without special handling or packaging.
When CF4 was incorporated into the gas blend, analysis revealed an increase in hydrogen fluoride. However, the fume generation rates and balance of fume components were comparable to those created by an electrode used with a traditional shielding gas blend.
Taking those properties and characteristics into consideration, a family of argon-carbon dioxide shielding gases with a CF4 addition has been tested to determine if these gases could repeatedly reduce weld hydrogen. These studies incorporated multiple welding consumables, namely metal-cored wires, solid wire and flux-cored wires with varying fluoride compounds.
These wires were evaluated for the reduction of weld hydrogen levels when using a CF4-supplemented shielding gas mixture. These tests followed the standard AWS protocol for weld hydrogen measurement. The commercially available wires were first tested when used with a standard argon-25 percent CO2 blend and again using the same blend with an additional CF4 supplement.
The results of the testing showed that hydrogen was reduced from 20 to 40 percent (Figure 1) when the CF4-supplemented shielding gas was used. These results were achieved without any undesirable effects to the operability of the wire/gas system in a range of typical applications, both in and out of position. The consensus, taken from the mechanical property testing of the wires with an AR/CO2/CF4 shielding gas, reveals there is no difference between the strength and impact properties of the welds made with or without a CF4 addition to the shielding gas.
The benefits of CF4-modified gas blends continue beyond the expected reduction of hydrogen in that these modified gas blends can do so even when used with wires that have not been properly maintained or stored after being removed from their packaging. Wires that have undergone intentional humidification – a regular procedure for AWS tests – were evaluated to determine the impact of weld hydrogen levels.
As seen in Table 1, the majority of products still showed a dramatic decrease in weld hydrogen of 20 to 40 percent when using the CF4 supplement. With these consistent results, it’s clear that, by using CF4-modified shielding gas blends, shops can achieve lower weld hydrogen levels when using flux-cored wires that have not received proper storage and care.
The management and reduction of residual hydrogen-bearing lubricants on the wire surface may be challenging or time consuming with continuous wire welding consumables. Yet, there is an obvious correlation between weld hydrogen levels and the range of retained surface lubricants on metals.
Tubular wire products also show varying levels of absorbed or chemically combined moisture, which can also contribute to high levels of hydrogen. During analysis of flux-cored wire, weld hydrogen tests were performed with five unique lots of identical E71T1 wire manufactured to an H8 hydrogen specification. And, as the data in Table 2 indicates, the product variability can lead to higher hydrogen levels than one may expect. But by using a CF4-modified gas blend, wire consumables can be made usable once more by bringing their hydrogen down to appropriate levels.
The power of CF4
A “hold time” is often required for welding applications prone to cold cracking before inspection. These periods can last as long as 48 hours, beginning with the completion of the weld before a non-destructive examination of the weld for cracks or stress points is performed. During testing of both flux-cored (Figure 2) and metal-cored (Figure 3) wires, the hydrogen levels of each weld produced with and without the CF4-supplemented gas blend were measured at many hold times following the weld’s completion.
During these tests, hydrogen diffusion was permitted at room temperature between tests. And, as Figures 2 and 3 show, reducing the hold times prior to inspection may be possible when a shielding gas blend containing CF4 is used during welding.
To meet the necessary hydrogen requirements associated with high-strength steel welding, consumables must be selected carefully – especially when there’s a need to achieve high-productivity in welding applications. As for the variety of weld hydrogen levels, these can be directly impacted by the “as received” hydrogen levels of consumables, the way they are handled and stored, and specific welding conditions and parameters.
A CF4-supplemented shielding gas blend can combat the variability of hydrogen levels so that they are properly controlled. By adding these modified gas blends to a shop’s processes, reaching acceptable weld hydrogen levels becomes significantly easier.
One effective product for managing these weld hydrogen levels is Stargon LH (for low hydrogen) shielding gas from Praxair. As a high-quality CF4-supplemented gas, Stargon LH is proven to reduce hydrogen levels in deposited weld metal, particularly in flux-cored and metal-cored wire applications. With Stargon LH, fabricators can not only minimize hydrogen-related cracking, but also enhance the overall productivity of their operation.
