Clad Pressure vessels should comply with the additional requirements specified herein, in addition to “Specification - Pressure Vessels”, for the design and fabrication.
2.1 Pressure Design
Thickness of the stainless steel cladding or weld overlay should not be included in the pressure design, unless specifically approved by the Buyer.
2.2 Solid Stainless Steel Components
Solid stainless steel materials should not be used for pressure-containing parts, unless specifically detailed on the Buyer vessel drawing or specifically approved by the Buyer.
For clad vessels with a shell thickness less than 2" (50 mm), nozzles and manways should conform to Standard Connection and Welding Details for Cladded Steel Vessels. Stainless steel bosses are permitted only when specifically detailed on the vessel drawing.
For clad or weld overlaid vessels with a shell thickness of 2" (50 mm) or greater, nozzles and manways should be lined or clad in accordance with Standard Connection and Welding Details for Cladded Steel Vessels.
Supports for heavy loads, such as rings supporting beams or catalyst support grids, should be constructed of the same material as the vessel base metal.
- Supports should be welded to the vessel's wall base metal and overlaid with the same alloy as used on the vessel wall.
- An alternate design is a weld build-up support of the same metallurgy as base metal and overlaid with same alloy as vessel overlay.
Light internal attachments may be welded directly to the cladding. Refer to the ASME Code, Section VIII, Div. 2, Part A-D.
3.1 Base Material
Base materials should be in accordance with the requirements of Material Requirements for Carbon Steel Pressure Vessels.
3.2 Cladding Processes
- Roll clad plate should be used for thickness of 4" (100 mm) or less.
- Weld overlay cladding should be used for thicknesses over 4" (100 mm).
- Explosion bonded clad material may be used only with the written approval of the Buyer.
3.3 Cladding Thickness
- Unless specified by Buyer, the minimum allowable thickness of cladding should be 1/8" (3 mm).
- Without specific written Buyer approval, cladding thickness should not exceed 3/8" (10 mm).
3.4 Explosion Bonded Clad Plate
- When approved for use, all explosion bonded clad plate and forgings should be ultrasonically examined to verify bond. Examination method should be in accordance with ASTM Specification SA-435.
- In addition to the acceptance standards of Paragraph 6 of SA-435, two or more defects smaller than described in Paragraph 6.1 should be unacceptable unless separated by a minimum distance equal to the greatest dimension of the larger defect, or unless they may be collectively encompassed by the circle described in Paragraph 6.1.
- Certification of tests should be furnished to Buyer.
3.5 Weld Overlay Cladding
- Overlay should consist of a first pass of 309L alloy with a subsequent pass (or passes) of specified stainless steel.
- A single pass procedure, whereby the filler metal/base metal mix results in an equivalent analysis for the specified minimum thickness, will be considered provided the fabricator can demonstrate the reliability of the proposed procedure and freedom from interface microcracking.
3.6 General Requirements for Roll Bonded and Explosion Bonded Clad Plate
- Austenitic chromium-nickel stainless steel cladding should either meet the extra low carbon (0.03% maximum) specification, or should be a grade stabilized with columbium or titanium, whichever is specified by Buyer.
- Check analysis for carbon content only should be furnished for extra low carbon grades of austenitic stainless steels, cladding or solid, in accordance with the applicable ASME Code requirements and the following additional requirements:
- Check analysis should be required only for alloy pressure containing parts such as nozzles and bosses and for cladding in contact with the contents of the vessel.
- When check analysis is required for cladding material, it should be required for each individual clad plate in contact with the contents of the vessel.
- Clad plate should comply with the requirements of ASTM SA-263 or SA-264.
- Austenitic stainless steel clad plates should be normalized by heating to between 16750F (9130C) and 17250F (9410C) for one hour per inch of thickness, followed by air cooling.
- Average grain size of the base material, per ASTM E112, should be no. 5 or finer in normalized plate, as supplied by the mill.
- Except where prohibited by the ASME Code material specifications, an air blast should be used to cool plates greater than 1-1/2" (38 mm) thick through the temperature range in which carbide precipitation occurs.
- After normalizing, plate should be tempered as necessary for fabrication.
- After cold forming (below 12000F) (6490C) of clad heads, tori-conical transition sections or similar parts, stress relief should be performed.
- If such parts are hot formed between 16000F (8710C) and 17500F (9540C), they should be
- If forming operations are conducted at an initial heating temperature between 12000F (6490C) and 16000F (8710C), or above 17500F (9540C), the parts should be normalized. All heat treatment should be performed prior to welding the part to the shell.
- After such heat treatment, parts may be cold sized, if necessary, to correct for distortion.
- Clad material should not be quenched after cladding.
- Shear testing should be required for all clad plates. Testing procedures and acceptance criteria should be in accordance with the applicable ASME (SA) Specification.
- Ultrasonic examination according to SA-578 should be made of all clad plates within 1T (1x plate thickness) of all edges to be welded, and covering an area extending 1T from all load bearing internal attachments designated on the vessel drawing.
- Cladding should be removed where any lack of bond is detected. The defective area should be repaired by weld overlay cladding (see section “5.2 Weld Overlay” item 3).
- Buyer should be notified in writing of any repair exceeding 10 square inches in area, or of any plate requiring more than three repairs regardless of size.
- All repairs, regardless of area, should be recorded on a scaled sketch.
The requirements of this section apply to all austenitic stainless steel weld overlay cladding, including:
- Weld overlay as the primary method of applying the internal lining
- Weld overlay of groove welds in roll clad or weld overlay clad base materials
- Repair areas of roll clad or explosion bonded clad plate
- After each welding pass, the weld should be cleaned of all flux and slag by power wire brushing, grinding, or hand tools.
- Only stainless steel wire brushes should be used.
- Slag picks and hammers should be tipped with austenitic stainless steel or stellite.
- Argon and argon/helium gas mixtures are required for processes that require shielding.
- Additions of hydrogen, oxygen, or carbon dioxide to the shielding gas are not permitted.
- Details of the weld overlay cladding procedure (WPS), including process, wire, flux, and number of layers, should be established by Fabricator for approval by Buyer before procedure qualification.
4.2 Welding Procedures and Qualifications
- Procedure qualifications should be made using the specified base material and the same type and brand of flux and weld wire to be used in production overlay cladding.
- Welding current and travel speed should be considered essential variables.
- Weld overlay cladding procedures should be qualified in accordance with ASME Code Paragraph UCL-40 and ASME Code Section IX.
- The samples of overlay and backing should be subjected to heat treatment simulating that of the final vessel including intermediate stress relief heat treatments, if used.
- After heat treatment, cladding on the weld overlay procedure qualification test plate should be examined by liquid penetrants in accordance with Code Division 1, Appendix 8, or Code Division 2, Appendix 2, Article 9-2, whichever is applicable.
- After heat treatment, side bend specimens should be machined from the samples.
- The specimen width should be twice the thickness of the overlay.
- Specimens should contain the entire overlay, the bond line, and a thickness of base metal equal to the thickness of the overlay.
- The length of the specimen should be 1-1/2" (38 mm) minimum, and the width of the bend specimen in the direction of bending should be 1/8" (3 mm).
- The finish machined specimens should be bent 180 degrees around a 1/2" (12.5 mm) pin with the “neutral axis” of the bend specimen perpendicular to the bond area.
- After bending, the specimens should have no cracks or open defects exceeding 1/16" (1.5 mm) measured in any direction on the convex surface, except that cracks occurring on the corners of the specimens during testing should not be considered reject-able unless there is definite evidence that they result from slag inclusions or other internal defects.
- The ferrite content of the deposited weld metal should be determined on overlay weld procedure qualification plates and should be between 3% and 10%, as determined by the Schaeffler diagram.
- The qualification test plate should be checked to ensure that the required composition is achieved.
- Samples should be taken from the surface and from the top of the specified minimum allowable thickness of cladding.
- The specimens should be milled and should each represent a total thickness not greater than 1/32" (0.8 mm).
- The composition of each specimen should meet the composition requirements set forth in ASME Code Specification SFA-5.4 for the grade of stainless steel specified for the second pass of the overlay.
- All weld repair procedures should be qualified in accordance with ASME Code Paragraph UCL-40 and ASME Code Section IX and should conform to the additional requirements set forth above for the weld cladding procedures.
4.3 Production Welding
- Weld overlay cladding should be applied to base metal which has been grit blasted to a Steel Structures Painting Council SSPC SP-10 “near white” finish (a SSPC SP-5 “white metal” finish is required for processes that do not use a flux, such as GTAW).
- Weld overlay cladding should be applied after normalizing, but before postweld heat treatment.
- Weld overlay cladding should be applied after heat treatment of the base metal with the exception of the postweld heat treatment.
- Weld overlay cladding should be applied circumferentially. For long or small nozzles where it is impractical to apply the weld overlay circumferentially, it may be applied longitudinally, with Buyer approval.
- The finished weld overlay deposit should have the minimum thickness of undiluted, stainless steel material indicated on the drawings.
- The maximum total cladding thickness should not exceed 3/8" without specific, written Buyer approval.
- The composition of the finished weld overlay deposit from the surface to the specified minimum overlay depth should conform to the requirements set forth in ASME Specification SFA-5.4 for the grade of stainless steel specified on the drawings.
- For austenitic stainless steel weld overlay, the composition of the finished weld overlay deposit from the surface to the specified minimum overlay depth should conform to ASME Specification SFA-5.9 for 316L stainless steel, with the exception that the minimum Molybdenum content should be 1.5%.
- The ferrite content of the deposited weld metal should be between 3 and 10FN as determined by a magnetic instrument calibrated according to AWS A4.2 standard procedure. Ferrite content is to be measured prior to any heat treatment.
- Internal attachments welded to the cladding should be welded with E309L after final PWHT. Low heat input multipass welding with small diameter welding rods should be used.
5.1 Pressure-containing Welds
- At the last stage of fabrication prior to applying the overlay, a magnetic particle examination should be made of the surfaces of all Category A, B, C, and D welds that are to be covered by weld overlay cladding.
- All required radiographic and ultrasonic examinations should be performed on completed weld joints after the weld overlay cladding has been applied.
5.2 Weld Overlay
- The composition of the top layer of the weld overlay should be checked for Cr, Ni, Mo, Cb, and C contents as follows:
- Two locations for each cylindrical shell course and head.
- One location for each nozzle, and for each Category A, B, and D weld seam.
- The content of each specified element should be determined by spectrographic analysis. Portable nondestructive instruments may be used, but the procedures must be submitted to the Buyer for approval.
- When it is necessary to remove a sample from the overlay to make the analysis, the sample should be 1/16" (1.5 mm) thick measured from the top surface.
- If the composition does not conform to the requirements of “4.3 - Production Welding”, shown above, the extent of the nonconformity should be determined by additional chemical analyses, and all nonconforming weld overlay should be removed.
- The ferrite content of the top layer of weld overlay should be determined before postweld heat treatment by a magnetic instrument according to the requirements of “4.3 - Production Welding” and as follows:
- Ten locations per shell course, selected at random
- Ten locations on each vessel head, selected at random
- One location per foot of circumference at both ends of nozzles and manways
- One location at each Category A, B, C, and D weld
- If the ferrite content does not conform to the requirements of “4.3 - Production Welding”, the extent of nonconformity should be determined by additional ferrite measurements, and all nonconforming weld overlay should be removed.
- The completed weld overlay should be checked for bond flaw indications, after postweld heat treatment, by ultrasonic examination in accordance with ASTM SA-578.
- If indications of flaws are detected, the size of the flaws should be determined by additional ultrasonic examination.
- Flaws that exceed acceptance level S6 should be removed.
- Unless indicated otherwise in the vessel data sheet, the coverage of the ultrasonic examination should be:
- One location per lineal foot along the vessel length
- One location per foot along a meridian of each vessel head
- One location per foot of circumference at both ends of nozzles and manways
- One location per foot of length of Category A, B, and D weld seams
- When indicated in the vessel data sheet, the coverage of the ultrasonic examination should be:
- One location per linear foot along the vessel length in each of four quadrants
- One location per foot along a meridian of each vessel head in each of four quadrants
- One location per foot of circumference at both ends of nozzles and manways
- One location per foot of length of Category A, B, and D welds
- The entire surface of all weld overlay cladding should be sufficiently smooth and clean to permit liquid penetrant examination for fissures and cracks.
- Liquid penetrant examination of the entire surface should be performed in accordance with Appendix 8 of ASME Code, Section VIII, Division 1.
- All relevant indications of surface fissures and cracks should be removed.
- All ground areas should be subject to 100% liquid penetrant examination to ensure that the defect has been removed.
- All repairs of weld overlay should comply with all of the requirements of sections “4 - Welding” and “5 - Inspection”.
6. Hydrostatic Pressure Testing
- The hydrostatic test of the completed vessel should be conducted according to Section 7.4 of the “Specification - Pressure Vessels”.
- The cladding thickness should be included in the calculation of the required stress level.
- The chloride content of hydrotest water should not exceed 100 ppm.
- Chemical analysis or certification from water supplier (not older than 30 days) is required.