Effect of Normalizing High Frequency Resistance-Welded Line Pipe

Client was interested in the effect of a post weld normalizing on the weld joint properties of high-frequency (450kc) and low-frequency (10kc) resistance-welded API 5LX-52 line pipe. The 450 kc power supply results in a weld with a narrow heat-affected zone, usually considered beneficial in most welding processes. The faster cooling associated with this process could, with some steels, result in the formation of martensite. This is considered unacceptable and a post-weld normalizing treatment is usually performed to temper or transform any martensite.

The 10 kc power supply on the other hand results in a wider heat-affected zone which cools slower and may not result in the formation of martensite and hence a post-weld normalizing treatment may not be required.

Several lengths of 8" diameter by 0.156" wall thickness pipe were made using both processes, with and without post-weld normalizing. All pipe was tested to assure conformance to the API 5LX-52 Specification prior to detailed evaluation to determine the effect of the normalizing treatment. Evaluation included :

• Tensile Testing - longitudinal and transverse, base metal and weld metal.
• Drop-Weight-Tear Test - base metal and weld metal
• Charpy Impact - base metal and weld metal
• Metallographic Examination - base metal and weld metal

Tensile test result showed the ultimate and yield strengths of un-normalized welds from both the high and low frequency welded pipe to be higher than the base metal in both the transverse and longitudinal directions. This corresponded to a small but acceptable decrease in ductility. The ultimate and yield strengths were either unaffected or slightly reduced by normalizing with a corresponding slight increase in ductility.

Drop-Weight-Tear Test results for the high frequency welded pipe showed normalizing resulted in improved fracture toughness properties by the lowering of the transition temperature to close to that of the base metal. Likewise, improved fracture toughness properties also resulted from normalizing the low frequency welded pipe.

The results of the Charpy impact test supported the improved fracture toughness determined from the drop-weight-tear test results.

Metallographic examination results of cross through the as welded high and low frequency resistance welded pipe were as expected but with no evidence of martensite. Normalizing modified the microstructure in the weld heat affected zone of pipe made using both processes with a corresponding small decrease in hardness of 20-40 KHN (Knoop Hardness Number).

In conclusion, although martensite was not found in the microstructure of the as welded high-frequency resistance welded pipe and pipe made using both welding processes met the API Specification for 5LX-52 pipe, the normalizing heat treatment improved the fracture toughness as demonstrated by the decrease in transition temperature in the weld joint. Normalizing was therefore considered beneficial and an important consideration for pipe exposed to lower service temperatures.

To see the resume of the expert associated with this case study, see the link below.