Session: MF-20-01 Material Quality and Failure Analysis-1

Paper Number: 152786

152786 - Investigating Low Toughness Events in Sa – 350 Lf2 Cl1 Flanges: A Case Study 

Abstract: 

Low toughness events of SA-350 LF2 Cl1 have become a growing concern in the industry in recent years. The complex relationship between the various factors that can contribute to the deterioration of toughness during the manufacturing process of the flanges and the subsequent fabrication operations at construction sites, where the flanges are integrated into piping systems, prompts EPC Contractors and End Users to adopt conservative positions whenever such events occur. This often leads to the rejection of a large number of flanges without sufficient assessment of the root causes and severity of the issue. This contrasts with the overly optimistic sampling criteria required by SA-350 for the certification of the flanges, which allows the validation of a substantial number of them with the mechanical testing of only one flange, or even just a test block.

This paper summarizes a recent experience during the construction phase of a major capital project where a low toughness event occurred with three SA-350 LF2 Cl1 quenched and tempered flanges. A total of nine flanges supplied by one manufacturer were used for welding procedures qualification. Impact test specimens were extracted from the flanges and yielded values below SA-350 thresholds in three of them. This led to an investigation of the event, which involved an audit of the flange manufacturer and the verification of laboratory practices and procedure qualifications designed at the site.

The three flanges that failed were all 8 inches in diameter and smaller. The audit of the manufacturer’s facilities disclosed that they were produced by closed die forging and certified by means of test blanks. Procedures and practices were found to be in line with relevant SA specifications and ASTM standards. The inquiries at the site found that the test specimens were extracted in the tangential direction, which breaches SA-350 requirements, as it asks for the orientation of the test specimens parallel to the direction of major working, which is the radial direction for closed die forgings. Furthermore, the post-weld heat treatment (PWHT) holding temperatures registered in the procedure qualification records were 620 ºC, quite close to the 640 ºC used by the flange manufacturer in the tempering. Finally, the holding times registered in the charts attached to the procedure qualification records were beyond two hours, which is excessive for the thicknesses of those buttwelds.

PQRs were repeated with flanges from the same manufacturer with successful results. However, they were larger flanges that had been forged by open die. This process provides a more uniform microstructure, similar to test blanks, while in closed-die forging, the ferritic-pearlitic structure shows strong anisotropy. In fact, microstructural verification on failed test specimens confirmed this latter issue, and additionally, SEM analysis showed the presence of carbides in the grain boundaries.

These results pointed to an improper extraction of the test specimens, which had their longitudinal axis transverse to the major working direction, compounded by the lengthy post-weld heat treatment at a temperature just 20 ºC below the one used in tempering, rather than a truly poor toughness of the flanges. This hypothesis was verified by means of six mock-ups, which showed the effect of the PWHT parameters on the toughness when the test specimens were extracted following SA-350 requirements. An optimized PWHT cycle within ASME B31.3 requirements was selected for production welding with the aim of minimizing the deterioration of the toughness, while keeping the hardness below 248 Hv10.

This experience reveals the importance for EPCs to specify supplementary requirements for the fabrication and certification of SA-350 flanges beyond the Code requirements. It also calls for a measured response, rather than overreacting to any impact test failure considering it as a low toughness event.

Presenting Author: Ricardo Hernandez Soto TECNICAS REUNIDAS

Presenting Author Biography: Ricardo Hernandez Soto is the Head of Inspection of TECNICAS REUNIDAS. He holds a M.S in Chemistry at the University of Seville, Spain. He is certified as International Welding Engineer by International Institute of Welding (IIW) and other relevant certifications in the field of welding. He has 20 years of experience with TECNICAS REUNIDAS. He has previously worked as Site Quality Control Manager in different international assignments with TECNICAS REUNIDAS and WOOD for major End Users such as ARAMCO and KNPC.

Authors:

Ricardo Hernandez Soto TECNICAS REUNIDAS
José María Gómez De Salazar Complutense University of Madrid