Mechanical Properties of Welded API 5L Gr B Steel for Hydrogen Piping

Abstract

This study was to analyze the factors affecting the mechanical properties of welded joints using API steel. In order to evaluate the safety of the piping, surface replications and hardness measurement, residual stress measurement,continuous indentation test, tensile test, bending tests were performed. Welding ofthis pipe is done by ASME SEC. According to the codes of IX and B31.3, the total welding pass is 4 passes, the first layer is GTAW and the remaining layers are welded with SMAW.Refer to the report of Procedure Qualification Record for current, voltage, welding time, etc. to calculate heat input during circumferential welding.Microstructure observation was performed on the welds of the pipehaving a single pipe length of 55 mm.The size of the heat affected zone by the welding heat is less than 7mm, and it is considered that there is little mutual influence between the two welds in the microstructure.In particular, length of area affected by heat was measured to be within about 7 mm and no effect was observed between neighbors.

The weld hardness was measured up to 235 Hv or less, and it was confirmed that the change in the length of the short pipe had little effect on the hardness.In the case of the parent metal part, it seems that the ferrite and the pearlite are properly mixed, and the weld bead and the acicular ferrite and the bainite are observed in theheat-affected welds.These structures are typical microstructure which are exhibited by typical heat and speed of welding, and are considered to be defects such as creep porosity, graphitization phenomenon and hydrogen embrittlement due to welding heat cycle. 

Keywords: Hydrogen Piping, API steel pipe, Hardness, Residual Stress, Indentation Test, Welding