Consumables Corner: What are the requirements for low-hydrogen electrodes?

A: Good question, and the short answer is yes, it is necessary for you to follow the customer’s request.

You stated that the customer specified low-hydrogen electrodes, but did they specify a hydrogen limit? The "8" designator at the end of an SMAW electrode classifies it as low-hydrogen. However, there are different levels of low-hydrogen electrodes, and within industry the most commonly referenced value is H4.

Since we are not familiar with the specifics of the contract, we can only provide insight as to why you would want to use an H4 low-hydrogen electrode. Let's start with an overview of what a low-hydrogen electrode is and why it's an important variable in weld manufacturing.

According to the American Welding Society, an HX designator, with the X being a numerical value of 2, 4, 8, or 16, represents the maximum allowable milliliter amount of diffusible hydrogen per 100 grams of deposited weld metal. An H4 classification means no more than 4 ml of hydrogen per 100 grams of deposited weld metal.

A filler metal that is classified as H4 is typically manufactured to produce deposited weld metal with less than 4 ml of diffusible hydrogen, but over time it will pick up a small amount of moisture that will increase the amount of hydrogen that ends up in the weld metal. However, if you follow recommendations for proper storage and handling and use correct welding practices, the resulting deposited weld metal should not exceed the H4 limit.

From an electrode classification standpoint, any electrode that is designated H16 or lower is considered to be low-hydrogen. For all practical purposes, you are most likely using all low-hydrogen electrodes. If, however, your customer stated H4 as the upper limit, then you would need to make sure all welding electrodes you use on the project have the H4 designator.

The purpose of controlling the amount of hydrogen in weld metal is to reduce the potential for hydrogen-induced cracking (HIC). Moisture in the electrode coating; in flux of the FCAW wire; or contamination in the weld joint from things like paint, grease, or dirt can contribute to the amount of available hydrogen that ends up in the weld metal.

The extremely hot temperature of the welding arc causes disassociation of moisture (H₂O). Since hydrogen is the smallest atom, when present it can migrate freely between the crystalline structures of the weld or base material. It then seeks out areas of low resistance, which happen to be the boundaries of large grains that are typically found in the heat-affected zone of the weld joint. Hydrogen by itself is unstable and will seek out other hydrogen atoms to form molecular hydrogen (H₂). This creates high internal stresses that can lead to cracking if there is a susceptible microstructure such as martensite or too much hydrogen.

Depending on your customer's requirements and the types of base materials you are using for the project, an H4-classified filler metal should adequately assist in preventing HIC.