Investigation on the effect of ball burnishing on fracture toughness in spiral API X70 pipeline steel

The present work is an investigation on the effect of mechanical surface treatment using ball burnishing technique on fracture toughness in spiral API X70 pipeline steel. The burnishing operation has been carried out on three series of impact API 5L 45 Ed specimens that have been prepared according to the location of the notch towards the weld bed: BM notched specimens, FZ notched specimens, and HAZ notched specimens. In the untreated specimens, the absorbed energy in BM specimens is higher than the energy found in FZ and HAZ through the testing temperature range of 50 to −40 °C. In surface-treated specimens, there has been a significant change in the three curves of absorbed energy versus temperature. This is observed when testing temperature decrease from 20 to −40 °C since the energy in BM specimens dropped drastically reaching values lower than that obtained for FZ and HAZ specimens. At the temperature range 50 to 20 °C, the absorbed energy in ball-burnished specimens is relatively higher. At lower testing temperatures below −40 °C, the absorbed energy converged to the same value. At room temperature, hardness tests indicate that where the absorbed energy is higher, hardness is lower.