High temperature hydrogen attack happens in an environment where high temperatures and the presence of hydrogen are combined, mostly to low alloy steels. The high temperature causes some of the hydrogen to change into its atomic form which allows it to migrate into the steel. Inside the steel, the hydrogen will react with carbon and form methane which cannot migrate through the steel. Therefore, it gets trapped in the metal, typically as microscopic methane bubbles at the grain boundaries in the steel (stage 1). As these bubble start to grow, they start to coalesce (stage 2) and eventually lead to fissuring and cracking (stage 3). HTHA occurs preferentially in welds, heat effected zones, and in material that received no heat treatment.