HOW TO DETERMINE THE DIFFUSIBLE HYDROGEN OF MILD STEEL WELD FILLER MATERIAL

Although a function of many variables, general hydrogen diffusion rates have been approximated. At 220 deg F, hydrogen diffuses through mild steel weld metal at the rate of one inch in approximately 48 hours. However, at 450 deg F, the diffusion rate increases exponentially, as hydrogen diffuses a distance of approximately one inch per hour. So, if the base weld and base metal remain warm for a longer period of time, it becomes much easier for diffusible hydrogen to escape. Again, a fast rate of diffusion of hydrogen is desirable in most circumstances, but it’s not so desirable when we’re trying to rate the level of hydrogen present in the welding filler material.

All welding procedures should be the same as those used during welding of the conformance test fixture from which tensile and perhaps other test specimens were removed. The only welding variable that may change is the welding technique. Weaving is not permitted under AWS A4.3, so in essence the only parameter that is allowed to differ is the travel speed. The requirement of using stringers only is intuitive, as slowing down the travel speed increases the temperature of the specimen, which as mentioned above can cause hydrogen to escape the weld pool more quickly. This is significant, as a reduction in travel speed can keep the specimen hotter for a longer period of time, skewing the hydrogen content downward.

As an aside, another example of the effect of a slower travel speed becomes apparent when testing the combination of wire and flux submerged arc welding consumables. A slower travel speed in the SAW process can lead to more flux being consumed by the welding arc, which has the potential of increasing the hydrogen content of the weld metal.

Once welding is complete, the specimens should be removed with a pair of vice grips, and immediately dipped into an ice water quench bath. After the specimens have been quenched in ice water and vigorously stirred within for 20 sec, they are then transferred to a holding container with a liquid bath at a temperature less than minus 76 deg F. For illustration, this can be accomplished with a Dewar filled with methanol and cooled with dry ice. A layer of dry ice several inches thick in the bottom of the Dewar should be sufficient. It is imperative that the welded specimens be transferred to this liquid holding tank within 60 sec after the arc is extinguished.

After having been in the liquid bath for at least two minutes, the specimens may be taken out for the purpose of processing the welded specimen. Breaking off of the run-on/run-off tabs and removing any slag, smut, or silicon islands must be performed without contaminating the sample. However, the time spent outside of the bath is to be a maximum of only one minute. At which time the specimen must be placed back into the bath for a minimum of two minutes before processing can continue. The target maximum temperature for the sample during this operation is 32 deg F.

If necessary, the samples may be stored in this holding tank at minus 76 deg F for up to 72 hours before being analyzed for diffusible hydrogen. If the liquid in the Dewar is at cryogenic temperatures (minus 320 deg F) then this holding time may be as long as 500 hours. The idea here is to effectively stop the diffusion of hydrogen out of the weld metal prior to the diffusible hydrogen measurement.

Welded samples are placed in sealed containers and baked to release diffusible hydrogen. The length of time spent in the capturing device should be such that 90 percent of the diffusible hydrogen within the weld metal is captured. Although fairly obvious, care must be exercised to ensure that the measuring apparatus has no leaks. However, realizing that leaks can happen, AWS A4.3 does allow for one of the test results to be scrapped in the event of a leaking eudiometer or isolation chamber.

One of the more common methods used today to actually measure the diffusible hydrogen is the gas chromatography method. The evolved gases are transferred to a gas chromatograph. The gases are then separated with a packed molecular sieve column and analyzed with a thermal conductivity detector (TCD). But regardless of the method of measurement used (Gas Chromatography, Glycerin Method, or Mercury Method) the measured volume in ml of hydrogen must be converted to standard atmospheric temperature and pressure – again for universal comparison purposes.

The sample is then weighed to the nearest 0.1 g. The weight of the blank before welding is subtracted from the weight of the specimen after testing is complete. After this the volume of hydrogen in ml is divided by the weight of weld metal in grams, the result is then is multiplied by 100 to convert to a 100 g basis. This value is then rounded to one decimal point, for example, 3.0 ml hydrogen per 100 g of weld metal.

The entire process from the starting of welding to the final measurement of hydrogen can take up to 24 hours, but it is certainly a worthwhile venture. The resulting data can provide engineers and fabricators with the information necessary to make the appropriate filler metal selection when welding high strength grades of carbon steel.

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