Hydrogen Cracking in an API 5L X70 Steel Welded with Cellulosic Electrodes of Different Levels of Mechanical Resistance

Abstract

Steels classified according to the American Petroleum Institute (API) standard face the problem of cold cracking due to welding. In this work, the effect of the mechanical resistance of the weld metal on the susceptibility of an API-5L-X70 steel to the cracking by hydrogen was studied, through the implant test. The welds were made with gravity welding device, using cellulosic electrodes, AWS-E6010 and AWS-E8010. The diffusible hydrogen of the electrodes was determined by gas chromatography. As a result of the implant test, a greater cracking tendency was observed when the AWS-E6010 electrode was used. This can be related to the lower mechanical resistance presented by its weld metal and also to the fact that the electrode is less chemically bonded. Thus, the weld metal may have been transformed in higher temperatures, which led to a greater migration of the hydrogen to the HAZ. It was also observed that, even welds that didn’t fail showed a big amount of cracks. This may be due to the cooling condition of the weld bead, which led to the appearance of brittle and susceptible cracking microstructures. For the smaller loads used (0,8.σ_e), however, these cracks did not reach a critical size for the final break of the specimen test being tested.

Key-words: Hydrogen cracking; API 5L X70; Implant test; Diffusible hydrogen