Results of Transmission Materials Analysis Presented at Edison Electric Institute

Dr. Zee (Matco Services - Materials Engineering Lab), and representatives from a utility, a tower fabricator and a mill presented the findings of an important materials analysis investigation at the Fall EEI Transmission, Distribution, & Metering Conference held at the Westin Crown Center in Kansas City, MO. The materials analysis was conducted at Matco Services by a team of materials engineers headed-up by Dr. Zee. Senior Engineers, Marty Latona and Ed Larkin worked diligently to resolve the complex materials issue that resulted in cracks in some of the angles on one lattice transmission structure. Technicians supporting their efforts included Charlie Miller, Jose Borge, Glen Phillips and Heather Groll.

e Westin Crown Center in Kansas City, MO. The materials analysis was conducted at Matco Services by a team of materials engineers headed-up by Dr. Zee. Senior Engineers, Marty Latona and Ed Larkin worked diligently to resolve the complex materials issue that resulted in cracks in some of the angles on one lattice transmission structure. Technicians supporting their efforts included Charlie Miller, Jose Borge, Glen Phillips and Heather Groll.

Objectives of Analysis:

• Determine main and root cause of cracking
• Investigate the mechanical properties of lattice tower steel
• Define specification requirements for lattice tower steel
  

   

Observations:

• Cracking along a line of bolt holes
• Cracking was parallel to rolling directions and inclusions
• Mixed mode transgranular cracking found in the analysis
• Cracking initiation at the punched hole
• Chemical and mechanical analysis results confirm the samples meet the requirements for ASTM A572 specification Grade 50
• High sulfur content of 0.033 to 0.042% was detected, which could lead to formation of sulfide stringers that contributes to lower impact resistance in the rolling direction
• Long silicate stinger inclusions
• Brittle failure mechanisms
• Charpy impacts-notch L values always lower than notch T
• Notch L orientation is parallel to crack direction
• Nearly all of the impact specimens (notch T and L) exhibit a brittle cleavage mode of fracture at 70F
• Ductile to brittle transition temperature (DBTT) testing illustrated less than ductile properties of the steel at room temperature

Possible causes of failure:

• LME
• Hydrogen Embrittlement
• Strain Aging
• Fabrication: Punch vs Drill, Edge and Hole Spacing
• Workmanship
• Assembly and Field Erection Techniques

“Out-of-the-Box” thinking by Matco Engineers:

Matco's test protocol included all of the standard tests including chemical analysis, mechanical testing, microstructure evaluation, hardness/micro hardness, charpy impact and transition temperature, bend testing and fracture examination.

In addition to these tests a new, novel bend test was developed to support product acceptance. The bend testing was performed at slow (0.005 and 0.05 ipm) and fast strain rates (1 ipm) to simulate cracking observed in service. The results of Matco's bend test indicate the drilled hole samples provide significantly better bend ductility in comparison to the punched hole samples, and the hole edges are more prone to cracking in comparison to the base material. The same test can be used after fabrication and galvanizing of these steel components to evaluate cracking and embrittlement.  Matco's bend test can also be used as a screening test to identify notch sensitive steels from different lots and different manufacturers.

Photo Above: Matco's bend ductility test results from various lots and different manufactures is shown above.  On the left, samples were tested at .05 inch per minure (ipm) versus samples on the right tested at 1 ipm.  Test results indicate the brittle materials are cracking at a much lower applied load, and much less bend angle.  As a result, this test can identify heats of steel that will crack in service.

Results of Materials Analysis Investigation:

• It was determined the cracking is attributed to the presence of a layer of plastically deformed metal at the punched and shear surfaces of the galvanized steel members used. These locations acted as initiation sites for the formation of cracks.
• Angles should be replaced with materials that have higher charpy values with drilled holes and increased spacing.
• Matco’s new bend test should be considered as an acceptance test for both new and replacement steel.

Matco Recommendations:

• Consideration should be given to include charpy impact testing criteria in steel specification requirements.
• Consideration should be given to include Matco's newly developed bend test as an acceptance test.