PROGRESS REPORT FOR AINGRA06184 - arv-offshore.com

PROGRESS REPORT FOR AINGRA06184 PROJECT TITLE Determination of residual stress levels in welded duplex stainless steel

gas pipelines using neutron scattering INVESTIGATOR(S) Institution and Department Chief Investigator Dr Liam Ward Civil Environmental and Chemical Engineering, RMIT University

Other Investigators Dr. Maurice Ripley - ANSTO Dr. Michael Law - ANSTO Dr. Ken Short - ANSTO Mr. G. Biddle - RMIT

Students Barry Gideon - RMIT (This study formed part of the overall Ph.D. program) ANSTO Investigators Dr Maurice Ripley

SCIENTIFIC OBJECTIVES

To (i) measure residual stress levels in welded sections of duplex stainless steel pipelines and to correlate the data with the susceptibility of the welded regions to stress corrosion cracking.

Specific objectives are:

• To measure residual stress levels in welded sections of duplex stainless steel pipelines using neutron scattering techniques and compare the values with those obtained for the parent metal.

• To understand the evolution of residual stresses with welding procedures and the interaction with microstructural changes

• To determine any correlation between the measured residual stress, the local microstructures in the weld and heat affected zone and the propensity for stress corrosion cracking

PROGRESS REPORT and RESEARCH OUTCOMES

Test methods were based on specific objectives as detailed above. The test program was divided into four major parts. Part 1. Welding of Duplex Stainless Steel Pipelines The material welded was a 10 mm wall thickness, 254 mm diameter, duplex stainless steel line-pipe corresponding to UNS 31803. The filler material used was conventional ER2209 AWS A5.9-93 classification. Welding was performed using the manual Gas Tungsten Arc Welding (GTAW) with two different joint configurations (double bevel single V bevel and double bevel single U bevel joint configuration) and two different power levels (high and low heat input conditions). Table 1 describes the chemical composition of the weld and base material.

Date electronic copy received at AINSE: 03 April 07

Part 2. Measurement of Residual Stresses in Girth Welded Pipe Due to time constraints within the testing program for 2006, residual stress levels were determined in only one specimen using neutron diffraction techniques Stress distribution in the various weld regions in the normal, transverse and longitudinal directions and corresponding stress maps are depicted in Figs. 1 (a) to (c) respectively. Measurements were made without cutting the full ring of pipe so as to fully preserve the stress field. The girth-welded rings were measured in a number of areas to cover the parent metal from one side of the weld, the Haz’s and weld themselves, through to the parent metal on the other side of the weld. Sampling was performed at a minimum of 15 positions through-thickness (i.e. from the outer wall to the inner wall)

Normal

y = 1.0467x2 + 32.116x - 106.18R2 = 0.4103

y = -0.426x2 - 19.521x - 61.423R2 = 0.1257

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FerriteAustenitePoly. (Austenite)Poly. (Ferrite)

Fig. 1 (a). Stress in the various weld regions in the Normal direction and corresponding stress Map

Transverse

y = 1.6202x2 + 28.292x - 262.31R2 = 0.3216

y = 0.2671x2 + 2.9612x - 183.19R2 = 0.0336

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Fig. 1 (b). Stress in the various weld regions in the Transverse direction and corresponding stress Map

Longitudinal

y = 0.9921x2 - 78.153x + 1330.5R2 = 0.3424

y = 0.589x2 - 34.636x + 363.93R2 = 0.4164

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Figure 1c. Stress in the various weld regions in the Longitudinal direction and corresponding stress Map The main findings from analysis of the data are:

• There is a variation in the stress/strain fields for both the Austenite and Ferrite in the various areas of the sample i.e. Weld, HAZ and Base Material.

• Both phases tend to be in opposite stress / strain field • The HAZ tends to have the highest stress / strain values • The weld region is mostly in compression • The various layers (Root ,Fill and Cap) of the weld show different stress / strain values

Part 3. Metallurgical and Structural Characterisation Metallographic preparation of the surface and cross section was carried out for subsequent analysis. Magnetic force microscopy (MFM), was carried out to obtain precise topographical information on the structure to supplement structural characterisation using optical and scanning electron microscopy. MFM images of the root section and cap section of a weld are shown in Figs. 2 (a) and (b) respectively. The results from the MFM study reveal that distinct austenite and ferrite regions within both the parent and weld regions were identified. Further, the size shape and distribution of these regions differ; elongated austenite and ferrite regions being observed in the root sectionl while a combination of elongated regions and finer, more equiaxed regions being observed in the weld region.

Fig. 2 (a). MFM on Root Section of Weld Fig. 2 (b). MFM on Cap Section of Weld Optical microscopy was conducted on the duplex stainless steels to determine the metallurgical characteristics. Figs. 3 (a) to 3 (d) show micrographs of the root, fill and cap regions of welded samples taken from two welding conditions, as detailed in Table 2. Scanning electron microscopy and Elemental mapping using EDS have been

used to further characterise the weld regions structure and to identify the distribution of alloy species within the weld regions.

Figure 3 Optical micrographs of the duplex stainless steels at both weld conditions; (a) Root region associated with weld condition 1; (b) Fill region associated with weld condition 1; (c) Fill region associated with weld condition 2, (d) Cap region associated with weld condition 2 (X1000 magnification) Microstructural analysis for both GTAW weld conditions as shown by the optical micrographs in Fig 3 reveals the presence of a two-phase banded structure, typical of such materials. In general, the austenite regions observed in the DSS weld metal is formed from ferrite in three modes, viz., as allotriomorphs at the prior-ferrite grain boundaries, as Widmanstätten side-plates growing into the grains from these allotriomorphs and as intragranular precipitates. In the micrographs, the grain boundary allotriomorphs and Widmanstätten austenite are clearly seen. However, the austenite seen within the grain could be either intragranular precipitates or Widmanstätten austenite intercepted transverse to the long axis. These microstructures, in addition to the presence of discontinuous grain boundary austenite layers and intragranular acicular ferrite are thought to be associated with variations in transformation rates and the degree of undercooling. In summary, these observed microstructures are typical of those formed under such welding conditions. To date metallographic examination, non destructive evaluation, mechanical testing and preliminary intergranular corrosion studies using the double loop electrochemical potentiokinetic reactivation (DL-ERP) have been carried out successfully. Detailed below is a summary of the results (Table 2).

Table 2. Summary of Results

Part 4. Stress Corrosion Cracking Testing of Welded Pipe Sections (mid to late 2007) SCC tests are scheduled to be performed on welded sections of the pipeline mid year 2007; the SCC tests are to be based on slow strain rate testing (SSRT). It is anticipated that tests will be carried out on the four combinations of weld geometry and weld heat input. Further SEM investigation shall be performed after SCC testing. Such studies will allow for investigating the nature and progress of the cracks with respect structural changes in the weld / HAZ / parent metal. Additional Supporting Information This project was conducted in collaboration with Dr. Maurice Ripley, Dr. Michael Law and Dr. Ken Short at ANSTO. Dr. Ripley and Dr. Law have vast experience in determining residual stress analysis in pipe-lines using the technique of neutron scattering. Dr. Short has extensive knowledge in characterisation and evaluation of duplex stainless steels using AFM and MFM techniques. The Chief Investigator has extensive knowledge in the area of materials engineering, physical metallurgy of steels and corrosion behaviour of metals and alloys. The RMIT team will include Mr. Gerry Biddle. Gerry Biddle has considerable expertise in metallurgical materials characterisation, specifically in microscopy and microanalysis techniques including SEM, EDS, EBSD and XRD.

PUBLICATIONS / REPORTS arising as a result of your work.

Gideon B, Ward L P and Biddle G Metallurgical Characterisation of Duplex Stainless Steel and their Susceptibility to Intergranular Corrosion, Proc. European Corrosion Federation (EUROCORR 2006) Conference (Abstract only), Maastricht, Netherlands, 24 – 28 Sep (2006) Session K, pp. 190-191 B. Gideon, L. Ward and G. Biddle, Characterisation of Duplex Stainless Steel Welds and their Susceptibility to Intergranular Corrosion. Paper submitted to Journal of Materials Characterisation (2007) B. Gideon, L.P. Ward, G. Biddle, Characterisation of the Weld Regions within Duplex Stainless Steels using Magnetic Force Microscopy. Paper in preparation. To be submitted to Journal of Materials Characterisation (2007) B. Gideon, L.P. Ward, D. Carr, M. Ripley, K. Short and M. Law, Residual Stress Determination Within the Weld Regions of Duplex Stainless Steels and their Susceptibility to Intergranular Corrosion. Paper to be presented at the Duplex 2007 International Conference and Expo, Grado and Aquileia, Italy, 18 – 20 June (2007)

PhD STUDENTS

PhD student is Barry Gideon Anticipated date of conferment is : 31st March 2008 Title of thesis is : ‘Studies on the Phase Transformations of Duplex Stainless Steel Weldments and Susceptibility to Intergranular Corrosion (IGC) and Stress Corrosion Cracking (SCC)