A Study of Metallurgical Factors for Defect Formation in Electric Resistance Welded API Steel Pipes

Abstract

A critical assessment has been made for the electric resistance welded API-J55 and P110 steel pipes to clarify the metallurgical factors crucial to the occurrence of welding defects. Electric resistance welding (ERW) is widely accepted due to its low cost and high efficiency of production as a conventional manufacturing technology for the steel pipes. However, ERW pipes are vulnerable to the defect formation because its welding zone has different characteristics compared to the base material. It has been found that there were two major crack types in the investigated steels: surface crack and hook crack (J-shaped crack). Macroscopic examinations suggested that the causes and occurrences of the cracks were distinct among the investigated steels. The small surface cracks were largely occurred in the API-J55 steel pipes. The microstructure in the vicinity of crack was identical to the matrix, but it was found that the formation of the surface cracks was attributed to the sulfur and oxide inclusions. The energy dispersive X-ray spectroscopy (EDS) analysis showed that the cracks were associated with hydrogen and clusters of complex oxide inclusions with calcium such as Al-Ca-O and Fe-Ca-O. Moreover, sulfur was found to be the major culprit for the surface crack formation in the statistical evaluation. On the other hand, most of the hook cracks were large in size and occurred in the API-P110 steel pipes even though the sulfur level was very low, where the phosphorous was critical to the occurrence of hook crack. Although the EDS analysis showed the similar oxides compared to the case of surface cracks, B and P segregation were found in secondary ion mass spectrometry and electron probe micro analyzer analyses. In the vicinity of the hook cracks, martensite (locally hardened microstructure) was formed because the segregation enhances the hardenability. Eventually, the crack propagates along the martensite which was the band of ferrite and pearlite. It is postulated that the hook crack in API-P110 steel pipes is initiated from the oxides with hydrogen and propagated along the banded microstructure with the prior austenite grain boundaries where culprit elements can be segregated.

Investigation of penetrator defect formation during high frequency induction welding in pipeline steels

“…This phenomenon is amplified in the weld region, where the mechanical stresses are also accompanied by the thermal stresses imposed by welding. Since the stresses are greater in the outer surface of the pipeline, the cracks tend to propagate from the center to the outer surface of the pipeline [11]. The case of the elongated morphology that was detected in sample 2 fits well the above analysis.…”

“…Existing literature [11] suggests that the main causes for the formation of cracks in this type of welds is inhomogeneity in the microstructure between the different welding zones, segregation of elements such as Mn and oxide clusters. It is suggested [11] that oxide inclusions serve as stress concertation spots and lead to the nucleation of voids.…”

“…Existing literature [11] suggests that the main causes for the formation of cracks in this type of welds is inhomogeneity in the microstructure between the different welding zones, segregation of elements such as Mn and oxide clusters. It is suggested [11] that oxide inclusions serve as stress concertation spots and lead to the nucleation of voids. Due to the high deformation stresses, in which the steel strip is subjected to during the forming process, cracks can form at the penetrators and propagate in the neighborhood of the oxide clusters.…”

Numerical Modeling of the Inclusion Behavior during AC Flash Butt Welding

“…Shajan et al 18,19) reported that the toughness quality of the flash welding joint deteriorates due to the presence of inclusions. Yu et al 20) and Joo et al 21) also reported that inclusions cause the crack formation in welded joints. Hence, the removal of oxide inclusions from welded joints is a critical issue associated with the flash welding process.…”

Non-metallic inclusions and hook cracks of high-frequency induction welded pipes

“…There are descriptions of hook-shaped defects in few foreign publications [3][4][5][6][7][8][9][10][11][12][13][14][15]. National standards GOST 31447-2012 [16] and GOST 59496-2021 [17] provide only the most common defects of welded joints without description of specific types typical for HFIW pipe joints.…”