Duplex AÇO INOXIDÁVEL

10. SPECIFIC REQUIREMENTS - DUPLEX STAINLESS STEEL

10.1. BASE METAL

Acceptable specifications include:

Stainless steel type 22% Cr duplex.

Alloys with 22% Cr shall be according to UNS S31803 and UNS S32205.

Stainless steel type 25% Cr duplex.

Alloys with 25% Cr and PRE > 40 shall be according to UNS S32550, UNS S32750,UNS S32760 and UNS S39274.

10.2. CONSUMABLES

10.2.1. Duplex stainless steel welding consumables shall be selected to achieve a minimum Pitting Resistance Equivalent (PRE) number of 33 for UNS S31803 / UNS S32205, and 40 for UNS S32550, UNS S32750, UNS S32760 and UNS S39274.

PRE = % Cr + 3.3 x % Mo + 16x % N- (Pitting resistance equivalent)

PREw = %Cr + 3.3x (%Mo +0.5x %W) +16x %N (includes the role of tungsten alloying)

10.2.1.1. A matching consumable with enhanced Ni content compared to the base material shall be used.

10.2.1.2. The S content shall not exceed 0.015%.

10.2.2. The selection of the filler metal shall be the responsibility of MANUFACTURER / PACKAGER, who shall demonstrate that it can meet the mechanical and metallurgical requirements of this Specification. In order to achieve this, over alloying of consumables is required. All welding consumables shall be provided with material certifications.

10.2.3. The alloy elements shall be in the solid rod and not in the flux for covered electrodes, except for SAW.

10.2.4. The MANUFACTURER / PACKAGER shall ensure that each batch of welding consumables is tested prior to use in production. Production welding parameters shall be used to weld up test pieces. The tests shall be carried out by the consumable manufacturer and shall demonstrate that the procedure qualification Charpy V-Notch acceptance criteria (ANNEX J) and ferrite content of between 35% and 65%. The base material ferrite content of duplex stainless steel shall be within the range 35-55%. For weld metal and HAZ, the ferrite content shall be within the range 35-65% can be achieved as per ASTM E 562 - Determining Volume Fraction by Systematic Manual Point Count, and that the analysis is as specified. The test results shall be attached to the relevant welding consumable test certificate.

10.3. WELDING TECHNIQUE

10.3.1. Cleaning and Preparation for Welding

10.3.1.1. Where accessible, the internal side of the welds shall be mechanically cleaned and visually per weld.

10.3.1.2. The weld preparation shall be made using plasma cutting, grinding or machining. Cutting with carbon electrode is not permitted.

10.3.1.3. Tools for slag removal, cleaning, and cutting shall only be used for those materials and shall fulfill the following conditions:

•slag removal and cleaning tools shall be made of stainless steel or covered with that material;

•cutting disks shall be made of aluminum oxide with a nylon or fiber glass core;

•additional precautions shall be taken for cleaning and preparation of the joint to be welded so as to avoid the presence of contaminants.

10.3.1.4. The surface of parts shall be protected against adherence of spatters and other welding projections.

10.3.1.5. The weld preparation shall encompass a 50 mm wide band on both sides of the weld seam. This area shall be cleaned by stainless steel wire brushes and solvent

degreasing shall be used to remove oxides, dirt, grease, paint or other contaminants, which may be deleterious to the weld. Precautions shall be taken to ensure that grinding wheels, files, wire brushes and similar tools are dedicated to stainless steel use only.

10.3.1.6. If tack welds or bridge pieces are used, the MANUFACTURER / PACKAGER shall submit full details of the procedure to be used, including details of materials of the bridges, to the PNBV review.

10.3.1.7. The part of the auxiliary assembly device in contact with or welded to the equipment shall be made of material of the same P number as the base metal (according to the classification of standard ASME Section IX) or, alternatively, covered with the consumable specified for welding of the base metal in deposits of at least two layers.

10.3.1.8. All crater cracks at starts and stops shall be removed before welding the next pass.

10.3.2. Welding Processes

10.3.2.1. The root run of welds in pipework shall be made by the GTAW process. GTAW, SAW, pulsed GMAW or SMAW may be acceptable for fill and cap runs following the root run. The use of other welding processes shall be subject to PNBV approval.

10.3.2.2. The polarity for GTAW shall be DC electrode negative. Electrodes shall be of the 2% Thoriated Tungsten type.

10.3.2.3. The polarity for SMAW shall be as per the manufacturer’s recommendations. Equipment which has been used for welding materials other than stainless steel shall not be used. Components such as gas lenses and ceramic nozzles shall be replaced either with new items, or items used only for stainless steel materials.

10.3.2.4. All welds shall be made with a filler metal. Autogenous welds are not permitted.

10.3.3. Gases for Shielding and Purging

10.3.3.1. Gas or gas mixtures used for shielding or purging shall be inert gases of high purity, oxygen and CO2-free, according to ISO 14175 (group I).

10.3.3.2. Gas mixtures containing hydrogen shall not be used.

10.3.3.3. Gas mixtures containing nitrogen should be used to weld duplex steels to protect against nitrogen loss during welding (maximum of 2% of N2). For root weld using GTAW gas mixture containing nitrogen shall be used.

10.3.3.4. The pipe bore shall be purged prior to and during welding (including tacking): purging shall be maintained until completion of welding unless the MANUFACTURER / PACKAGER can demonstrate that removal of the purge before completion will not affect the bore condition.

10.3.3.5. Addition of oxygen and carbon dioxide to the shielding or purging gas shall be avoided because they reduce the corrosion resistance of the welds.

10.3.3.5.1.