The Effect of Phosphorus on the Mechanical Properties of X-70 Line Pipe Steel

Abstract

The effect of phosphorus content on the base plate properties and coarse-grained heat affected zone (HAZ) toughness of X-70 Cb-V line pipe steel has been studied. Three laboratory heats were melted with phosphorus contents of 0.001, 0.009, and 0.027%. The ingot sections were control rolled to 13-mm (0.512-in.) thick plates employing a single delay 4T rolling practice. The HAZ toughness was evaluated on specimens subjected to a weld simulation thermal cycle corresponding to 1315°C (2400°F) peak temperature in the HAZ of a submerged arc weld deposited with a heat input of 3 kJ/mm (76 kJ/in.). The as-rolled plates showed no significant effect of phosphorus variation on their microstructure, strength, or impact properties over the range investigated. The steels exhibited similar coarse-grained HAZ microstructures and hardnesses. Although the transition temperatures of the steels were significantly raised in the simulated HAZ condition, as expected, the variation in phosphorus content appeared to have little or no significant effect on toughness. Fractographic examination of the simulated HAZ impact specimens showed similar fracture appearances with no evidence of intergranular fracture. The conclusion is that a specification limit of 0.010% maximum on the phosphorus content in Arctic grade line pipe steels may be unduly restrictive for ensuring superior base plate and HAZ toughness properties, as well as adequate resistance to hydrogen induced cracking (HIC).