A Critical View on the Significance of HAZ Toughness Testing

Abstract

Within the heat affected zone (HAZ) along the weld seam of LSAW linepipes discrete microstructural regions of reduced toughness can not be avoided and are commonly designated with the term Local Brittle Zones. The nature of these LBZ has been intensively investigated and the gathered knowledge is exploited in today’s steel technology, plate processing and pipe manufacturing. The HAZ toughness has been improved in general by reducing MA constituents and by austenite grain refinement. Nevertheless local areas of low toughness within the CGHAZ can not be avoided completely. They are statistically distributed in every pipe. Furthermore it seems to be widely accepted that the structural reliability of LSAW linepipe produced and inspected with state-of-the-art technology is not influenced as these areas of low toughness have a limited size and distribution. This has been demonstrated by numerous investigations including small scale (CVN, CTOD), wide plate and burst tests. The essence of these investigations is that the failure behaviour of linepipe containing part wall defects in the HAZ is toughness independent. So far researcher’s world is clear and in good shape. Nevertheless many linepipe specifications tend to stipulate stringent test requirements with regard to acceptance criteria for the HAZ. In the occurrences of test failures a re-test procedure for test lot acceptance is carried out. As a matter of fact the LBZ are present along the weld seam over limited areas in each pipe. Therefore such a re-test procedure is regarded to be inappropriate in terms of quality inspection as it randomly sorts out pipes just by the statistical chance. With regard to HAZ toughness the pipes failed by this test do not differ from those pipes accepted and released for dispatch. As a final conclusion it can be stated that the existing test procedures for the HAZ toughness testing of the main standards and specifications do not reflect the current developments with regard to improved HAZ toughness achieved by the development of optimised steel composition and with regard to the enhanced defect detection probability along with modern NDT inspection methods. An amendment of the current test procedures in this direction is proposed. Therefore proposals are made as start for a common discussion.