ABSTRACT It has been carried out a test programme to qualify welded super 13%Cr stainless steels for sour service applications. The test programme included weldability trials, weld simulations, mechanical testing and corrosion testing of 13%Cr steels from five different steel mills.
Two of the tested steels have been qualified for use as flowline materials in some parts of new sour service fields. The result shows excellent weldability properties and acceptable corrosion properties. Post weld heat treatment (PWHT) of the welds improved the resistance towards sulphide stress corrosion cracking significantly.
INTRODUCTION This paper gives a summary of the results from a qualification programme performed in 1995/96 for the Asgard and Gullfaks Satellites fields /1-7,9/. The testing of the 13%Cr steels was separated into different phases, starting with weld simulation and thereafter with real welding trials. The qualification programme were performed by Statoil 1 (project leader, mechanical testing, corrosion testing) in cooperation with SINTEF (weld simulation, GTAW welding, mechanical testing, corrosion testing) and Allseas (GMAW welding, mechanical testing, corrosion testing).
EXPERIMENTAL PROCEDURE
13%Cr steels from five steel mills were investigated in this programme. Their chemical compositions were varied as outlined in Table 1. The aim was to vary the composition of the steels such that the influence of the different alloying elements could be studied. Based on knowledge to stainless steels it was expected that the content of C, Cr, Ni and Mo would be the most important elements, however microalloying elements as Ti and N may have an important influence on the weldability properties. These elements may, in turn, influence the transformation behaviour and hence, both the mechanical and the corrosion properties /3/.
Weld simulation studies
The weld simulation tests were carried out by two different techniques:
Resistance heating of specimens with 10.5xl0.5 mm cross section and 100 mm length for subsequent Charpy V notch (CVN) testing. This was done by means of a HAZ microstructure simulator.
Induction heating of specimens with 10-12 mm diameter and 100 mm length for subsequent tensile testing. This was done by means of a high frequency generator.
Welding
An important purpose with the programme was to use commercial welding techniques for welding of pipelines. As Company C the last years have developed their own GMAW system, it was decided to start the welding trials with this system. The welding trials started with a welding parameter study. The trials did document an excellent weldability using GMAW with duplex (22/09) filler wire. Use of superduplex (25/09) filler wire was also tested, but as the actual power supply system did not have any possibility for pulse techniques, the weldability became more poor using superduplex filler wire. Use of pulsed technique is expected to solve the observed problems.
The GTAW welding of steels B3 and E were performed in an equipment which is specially equipped for hyperbaric welding. The work piece was placed in a chamber where the welding is computer controlled. The chamber was filled with argon at an atmospheric pressure.