Hydrogen Designators Codes

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The AWS D1.1 Structural Welding Code also has several provisions that utilize hydrogen designators (e.g., H8). For example, Category “D” in the minimum preheat and interpass temperaturetable (Table 3.2) allows only “…electrodes or electrode-flux combinations capable of depositing weld metal with a maximum diffusible hydrogen content of 8 mL/100 g (H8).”

This is a good example of properly using the H-designators.

The AWS D1.1 Code also has an alternate method to determine the minimum preheat temperature (Annex XI) that uses three levels of diffusible hydrogen. In Annex XI, category H1 is called an “extra low hydrogen” at less than 5 mL/100g. Category H2 is labeled as “low hydrogen” at less than 10 mL/100g. The third category, H3, is a hydrogen level that is not controlled.

Although category H2 is labeled “low hydrogen,” this does not define low hydrogen electrode as less than 10 mL/100g. The actual diffusible hydrogen value can also be used to calculate the minimum preheat temperature with this method instead of using the H1, H2 and H3 categories. The Fracture Control Plan of the AWS Bridge Welding Code7 (AWS D1.5-95) is another fine example of hydrogen control. This code requires the following for welding Fracture Critical

Members:

• H16, H8 or H4, when the minimum specified yield strength is 50 ksi or less.
• H8 or H4, when the minimum specified yield strength is greater the 50 ksi.

Furthermore, SMAW electrodes can be used for tack welding without preheat if the electrode has an H4 designator, according to AWS D1.5. Other agencies such as the United States Military8 and the American Bureau of Shipping9 also set limits on the diffusible hydrogen levels. Both use limits of 15, 10 and 5 mL/100g, and the military specification has a stricter limit of 2 mL/100g for some applications. Today, a logarithmic system (i.e., H16, H8, H4, and H2) is preferred in the United States.

by R. Scott Funderburk

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