Section 7 — Pressure Vessels

Welding Requirements for Pressure Vessels

IPE Engineering Practice IPE-EP-7-1-5

Document number: IPE-EP-7-1-5 · Section: 7 — Pressure Vessels

SCOPE

This document covers requirements for weld procedure qualifications, and welding process restrictions, used in the assembly of pressure vessels and heat exchangers.
Any deviation from this Practice must be approved by the procedure described in EP 1-1-3.
An asterisk (*) indicates that a decision by the Owner's Engineer or Owner is required, or that additional information is furnished by the Purchaser.
A revision bar indicates all changes made to this Revision.

2.0 REFERENCES

The latest edition of the following standards and publications are referred to herein.

STANDARDS AND PUBLICATIONS

Engineering Practices
EP 1-1-3 Deviations to Engineering Practices EP 7-1-2 Heavy Wall and Special Service Pressure Vessels EP 7-1-4 Supplemental Requirements for Pressure Vessels EP 8-1-1 TEMA Shell and Tube Heat Exchangers EP 10-2-1 Material Requirements for Aggressive Environmental Services EP 10-2-3 Material Hardness Requirements
API Publications
Publ 941 Steels for Hydrogen Service at Elevated Temperatures and Pressures in Petroleum Refineries and Petrochemical Plants.
ASME Codes
Sec VIIIPressure Vessels, Division 1 Sec VIIIPressure Vessels, Alternative Rules, Division 2 Sec IX Welding Qualifications
Welding Research Council Bulletin
342 Stainless Steel Weld Metal: Prediction of Ferrite Content, April 1989

DEFINITIONS

Aggressive Environmental Service (AES) - Process services which result in material degradation such as cracking, scaling, blistering, and severe pitting and/or corrosion. Examples of such services are hydrogen service, wet hydrogen sulfide, cyanides, caustic, amine, and hydrofluoric acid. AES process fluid are defined in EP 10-2-1.

Contractor - Company or business that agrees to furnish materials or perform specified services at a specified price and/or rate to the Owner.
Hydrogen Rich Service - A service defined as a combination of hydrogen partial pressure and temperature at or below the curve for carbon steel per Figure 1 of API Publication 941, latest edition, and with a hydrogen partial pressure greater than 100 psia.
Hydrogen Service - A service defined as a combination of hydrogen partial pressure and temperature above the curve for carbon steel per Figure 1 of API Publication 941, latest edition.
Inspector - A Refining Company appointed engineer or inspector.
Manufacturer - The recipient of a direct or indirect purchase order for materials and/or equipment. In this context, a direct order is one issued to a manufacturer by a contractor or the Owner. An indirect order is one issued to a manufacturer by a vendor (recipient of a direct order) for materials, fabricated components, or subassemblies.
Owner - Engineering Company.
Owner's Engineer - A Engineering Company appointed engineer.
Purchaser - The party placing a direct purchase order. The purchaser is the Owner's designated representative.
Root Pass - The initial weld deposit in a groove or fillet joint, placed at the bottom or "root" of the joint.
Welding Procedures - A document written by a fabricator or manufacturer that references a specific code or standard set of rules and governs both the qualification of welding personnel and the welds they produce.
Welding Process - A specific method of depositing weld metal using a particular set of welding equipment and welding technique.

GENERAL

When conflicts arise between the ASME Code and this Practice, the requirements of the most restrictive document, as determined by the Owner's Engineer, shall govern.
(*)Conflicts between this and other Practices or fabrication codes shall be brought to the attention of the Owner and submitted in writing for review and decision.
Compliance with this practice does not relieve the Manufacturer from meeting the requirements of all governmental regulations or standard codes of practice.

WELDING PROCEDURE REQUIREMENTS

Welding procedure specifications shall conform to the requirements of Section IX of the ASME Code and to the additional requirements of this Practice.
(*)Prior to any welding, one copy of each Welding Procedure Specification (WPS) along with the supporting Procedure Qualification Records (PQR) shall be submitted for written approval by the Owner's Engineer.

Each WPS shall be specified on a weld map (vessel outline sketch) detailing where each welding procedure specification will be used. An alternate method of WPS specification is location of the WPS on the actual fabrication drawings at each weld joint.
Submitted welding procedures shall reference both the item name and the Purchase Order Number.
Procedure qualifications for chrome-moly or chrome alloy steels (ASME Groups P-4, or P-5, 1 Cr - 1/2 Mo thru 9Cr - 1 Mo) shall include brinell hardness tests. Qualified procedures that were not originally brinell hardness tested, may be submitted for approval along with brinell hardness results on a new alloy test plate, welded per the original WPS. Procedure requalification is not required. Hardness testing requirements shall be in accordance with EP 10-2-3.
All welds on pressure containing equipment shall follow an approved WPS.
Duplex stainless steel procedure qualifications shall include the following requirements:
Autogenous (without filler metal) welds are not permitted.
Interpass temperatures shall be 300F maximum.
Ferrite content of the weld shall be within the range of 30% to 60% (or equivalent FN) as measured by a ferritscope or similar instrument.
The average heat input for each welding process used in a PQR shall be recorded. That heat input shall be used to specify a heat input range for any referenced WPS. Unless otherwise approved by the owner's engineer, the limits for any heat input range shall be 13 KJ/in minimum and 64 KJ/in maximum.

WELDING PROCESS REQUIREMENTS

General
Only those processes listed in the ASME Code, Section IX are acceptable for production welding with the additional restrictions as listed in this Practice.
Oxy-acetylene and all other oxy-fuel welding processes are not acceptable.
Submerged Arc Welding (SAW)
Hand-held SAW shall not be used on pressure containing welds unless the completed weld is radiographed 100 percent.
Acid type (Active) or alloying fluxes are prohibited; only neutral type fluxes such as L-Tec 80, Lincoln 860/880/980, or equivalent shall be used.
The fabricator shall list the Manufacturers wire and flux trade names on each WPS to be used for fabrication.
Alloy welding shall be done with an alloy filler wire and a neutral flux. Alloy elements shall not be added to the flux.
Flux used for 300 series stainless steel welding, excluding weld overlay, may contain an alloy addition of 1% to 2% Chrome (i.e. Lincoln ST-100 flux or equivalent) to compensate for alloy burn-out during welding.
The use of recrushed SAW slag as a flux or as a flux addition is not permitted.
Gas Metal Arc Welding (GMAW)

Spray arc and pulsed spray arc Gas Metal Arc Welding Processes are acceptable for all standard shop fabrications.
The Short Circuiting GMAW process is limited to the requirements listed below:
When Short Circuiting GMAW is used for the root pass only in carbon or low alloy steel open butt single side groove welds, the shielding gas used shall be an Argon/CO2 mix.
Short Circuiting GMAW in carbon or low alloy steel may use 100% CO2 as a root pass shielding gas when the backside of the root weld is accessible for backgouging and welding, or backgrinding.
When Short Circuiting GMAW is used for the root pass in stainless steel or nickel based alloys, the shielding gas used shall be a single inert gas, an inert gas mix, or a "Tri-Mix" of He, Ar, and CO2 or O2.
The Short Circuiting GMAW process is acceptable for producing full penetration pressure containing welds when procedures comply with all of the following restrictions:
(*)Written approval from the Owners Engineer is required for each vessel
For carbon steel materials only.
For maximum wall thicknesses less than or equal to 3/8 inch.
Vertical down welding progression is acceptable for the root pass only, all remaining weld passes shall be done in either the flat, rolled or vertical up position.
Shielding Gas for the root pass weld bead shall be an Argon/CO2 (75%-25% typical) mixture. Shielding Gas for the completion of the weld, second pass through the cap pass, shall be 100% CO2.
All weld bevels, including the inside surface of the pipe (or vessel) in the bevel area, shall be ground to clean metal prior to welding.
Wire diameter shall be 0.035 inch max.
All weld bevels shall have a minimum included angle of 70°. Root gap for butt welds shall be held from 3/32 to 5/32 inch, and weld bevel land thickness may vary from a feather edge to 1/32 inch maximum.
Weaving shall be limited to 3/8 inch maximum width.
Weld layer thickness shall be limited to 1/8 inch maximum.
If tack welds are to be included as part of the root pass weld, they shall be deposited with either the GMAW or GTAW process. The ends of each tack shall be tapered by grinding prior to root pass welding. Tack welds deposited with any process other than GMAW or GTAW shall be completely removed before depositing the root pass in that area.
Each weld pass shall be cleaned using a grinder or power wire brush.
Examination of the completed weld shall be done using shear wave ultrasonics; radiography is not an acceptable examination method.
The Globular Transfer GMAW process is limited to the requirements listed below:
For 1G type roll welds; "out of position (5G)" type welds are not acceptable.
For carbon steel materials only.
Shielding gas shall be a minimum of 70% Argon.
Filler wire shall be 0.035 inch maximum.
Minimum voltage required for production welding is 21 volts.
Minimum amperage required for production welding is 180 amps DC.
Flux Core Arc Welding (FCAW)

For all pressure containing applications, the following requirements apply:
The process is not acceptable for root pass welds without backing.
The process is limited to the gas shielded method.
Procedures submitted for approval shall include Vendor trade names for all electrode consumables as well as weld metal Charpy impact tests for all procedures that are Post Weld Heat Treated fabrications. Charpy impact tests shall meet 20 ft-lbs ave/15 ft-lbs min. at -20F.
Carbon Steel Fabrication shall use gas shielded electrodes that meet the requirements of E 7XT-1 or E 7XT-5.
(*)Chrome Moly or Stainless Steel fabrication must have prior approval from the Owner's Engineer. If approved, the process shall use EXXT-1 or EXXT-5 gas shielded electrodes.
For all structural applications, including non-pressure containing attachments such as insulation clips, tray supports and nameplates, the following requirements apply:
Structural attachments welded directly to pressure containing equipment shall use the gas shielded process.
All other structural welding may use the gas shielded or self shielded wires.
Gas Tungsten Arc Welding (GTAW)
An inert gas backing purge shall be used during the welding of the root and second pass of open butt welds containing alloy contents of 2-1/4 Cr - 1 Mo and higher. Adequacy of the purge shall be either measured by an O2 probe or established by purging with a gas volume equal to 4 times that of the vessel or weld area.
Nitrogen is an acceptable backing gas for stainless steels only.
Filler wire type shall be identified by a tag attached to or direct marking on the surface of each length of wire.
Shielded Metal Arc Welding (SMAW)
All SMAW pressure containing welds shall be made using low-hydrogen type electrodes. Cellulosic (E6010) electrodes are acceptable for the root pass only.
Electrodes of the following classifications are not acceptable: E7010-A1, E7010 G, EXX 12, EXX13, EXX14, and EXX2X. However, E6013 may be used for shoe, clip, and pipe support attachments to minimize undercutting.
All low hydrogen, chrome moly electrodes used to fabricate vessels that will operate below the design creep range of the base material shall be "L" grade with a maximum carbon content of 0.05%. All other fabrications shall use standard grade electrodes with a minimum carbon content of 0.05%.
(*)The root pass in groove welds made without backing shall not be made using the SMAW process for any material other than carbon steel without approval from the Owner's Engineer.

WELDER PERFORMANCE QUALIFICATIONS

All welders and welding operators shall be qualified by the bend test or radiographic method in accordance with the ASME Code, Section IX.

For welder performance qualifications with the short-circuiting GMAW process, side bend tests are required if the weld deposit consists of more than one GMAW layer. Qualification by radiography is not an acceptable alternate.
(*)The Owner's Engineer reserves the right to have any welder or welding operator retested at any time his welding abilities become questionable to the Inspector.
All welder and welding operator performance qualification records shall be made available to the Inspector upon request.
The Inspector shall be permitted access to witness any welder or welding operator qualification tests.

WELD JOINT DESIGN

Double welded butt joints (joints welded from both sides) shall be used for rolled plate, forgings and pipe with a diameter greater than 24 inches.
Single bevel groove joints with back welding may be used for thin wall (3/4 inch or less) applications.
Double bevel groove joints (partial bevel made from both sides) should be used for wall thicknesses greater than 3/4 inch.
Vessel closing seams that are inaccessible from the backside may be welded from one side only. The welding process for the root pass shall be GTAW or GMAW.
Tack welds shall be of good quality and sufficient for incorporation into the root pass of the weld or they shall be completely removed. Only qualified welders shall be used for tack welding.
All temporary attachments shall be welded using approved procedures and welders. Upon removal, any gouges shall be repaired and the entire area examined by either the liquid penetrant or magnetic particle method. For Post Weld Heat Treated vessels, all repairs must be done before the vessel is heat-treated.
All weld bevels shall be machined, ground, or thermal cut and then ground to clean metal before welding.

PRODUCTION WELDING REQUIREMENTS

Preheat and Interpass
Minimum preheat temperature shall be 50F prior to any welding unless required otherwise by the WPS. This also applies to tack welds, temporary attachments and thermal gouging.
Whenever material is wet or contains surface moisture or condensate, it shall be dried by heating.
Dissimilar welds between austenitic and ferritic materials shall be welded with preheat determined by the ferritic material. The first layer of weld overlay deposits shall be done using the preheat required for the base plate. Subsequent weld layers may use the preheat required for the alloy deposits.
Preheat shall be determined by temperature-indicating crayons, contact pyrometers or other suitable means. Crayons shall not cause corrosive or other harmful effects.

For all Chrome-Moly alloys, while welding is in progress, the preheat temperature shall be maintained as the minimum interpass temperature until the weld is completed.
Minimum Preheat Temperature for fabrication shall be in accordance with Table 1.
Filler Metals
C-1/2 Mo filler metals or electrodes (ER80S-D2, E7018-A1) are not permitted for vessel fabrication other than for repair of existing C-1/2 Mo equipment, or for the fabrication of multi- piece vessel heads which are subsequently normalized at 1650F or higher.
(*)For dissimilar joints between ferritic and austenitic base materials, the filler material shall meet the requirements of ASME classification ENiCrFe-2 (Inco A); except that ENiCrFe-3 (Inco 182), type 309, and 309L may be used with written approval from the Owner's Engineer.
(*)E 309 consumables shall be used for the attachment of refractory hex mesh or anchors to ferritic materials unless otherwise approved by the Owner's Engineer.
Comparable low carbon austenitic filler materials shall be used when welding "L" grade austenitic base materials.
All low hydrogen type electrodes shall be stored in their original sealed containers; then placed in an electrode holding oven kept at a minimum temperature of 250F once their sealed containers are opened.
When joining similar metals, the deposited weld metal shall match the chemistry and the mechanical properties of the base metal as closely as possible.
Austenitic Stainless Steel welds shall have a Ferrite Number range of 3 to 14 FN based on the Welding Research Council Bulletin Number 342. However, if the vessel design temperature is greater than 1000F or if the vessel is postweld heat treated, the maximum limit shall be 12 FN rather than 14 FN measured before the vessel is heat-treated. Ferrite checks shall be done on a minimum of 5% of production welds.
Welding Technique
All welds, including fillet and buttering welds, except for those listed below, shall be made with a minimum of two weld passes. Single pass welds may be used for the following:
Bellows expansion joint attachment welds when the bellows material is less than or equal to

0.065 inch in thickness.

(*)Seal welds in stainless and high alloy steels when approved by the Owner's Engineer.
Non-pressure containing fillet attachment welds for clips or other structural attachments that are 0.25 inch or less in thickness.
Tube-to-tubesheet welds as stipulated in EP 8-1-1.
All visible defects, flux, scale and spatter shall be removed from each weld pass.
(*)Peening is not permitted unless approved in writing by the Owner's Engineer. The use of pneumatic slag removal tools is not considered peening.
Weld backing is acceptable if both of the following are satisfied:
The weld joint is accessible from the root side.
The backing is either weld metal, a non-metallic material such as flux or ceramic that does not fuse with the root bead, or a metallic strip of equal composition as the base metal and is removed by grinding, or gouging and grinding, before inspection of the weld.

(*)All vertical welding shall be done in the vertical up direction unless otherwise approved by the Owner's Engineer.
For austenitic stainless steels and other high alloy materials:
Grinding wheels shall be of the resinoid aluminum oxide type and shall be new or used previously only on high alloy materials.
Wire brushes shall be made of stainless steel and shall be new or used previously on high alloy materials.
All cleaning solvents, marking crayons or anti-spatter compounds shall be low chloride and suitable for use with austenitic stainless steels.
Welder Identification
Production Quality Control shall include welder identification of fabricated joints by either written documentation or stamping of the joints.
A complete weld map showing the welders identification symbols and corresponding weld joint numbers is an acceptable alternative to weld stamping.
Requirements for weld stamping:
Each welder shall be assigned a symbol and shall identify each joint he welds by stamping his symbol adjacent to the weld.
Only 1/4 or 3/8 inch low stress steel stencils shall be used.
Welder identification shall be stamped approximately 1 inch away from the weld. If more than one welder completes a joint, stamps shall move outward from the weld in chronological order, the welder completing the weld stamping his I.D. furthest away.
Any weld not so identified shall either be removed or 100 percent radiographed at the contractor's expense for acceptance at the discretion of the inspector.
Production Test Plates
Production test plates are not required for vessels built to the requirements of EP 7-1-1.
(*)Production test plates are required for all vessels built to the requirements of EP 7-1-2 unless otherwise approved by Owner's Engineer.
Production Test Plates shall be made during the actual fabrication of the vessel.
For manual welding in the vertical position, a test plate may be made for each procedure.
For automatic and semi-automatic welding in the vertical position, a test plate shall be required for each procedure.
For automatic and semi-automatic longitudinal seam welds in the flat position, one test plate shall be required for each procedure.
Base materials for the test plates shall be the same as the base material for the vessel.
Before required tests (per 9.5.2.7 below) are removed from the production test plates, the test plates shall be subjected to the same heat treatment, including cooling rates and aggregate time at temperature or temperatures as established by the Manufacturer for use in actual vessel fabrication.
Required tests for each Production Test Plate shall be as follows:
Two (2) tensiles tested at room temperature.
One (1) cross section micro hardness specimen testing base metal, HAZ and weld metal at the 1/4 T and the 3/4 T locations.
One (1) weld metal chemistry.

Two (2) sets of Charpy Impact tests, one (1) set all weld metal and one (1) set HAZ. Both sets shall be tested at the specified temperature in EP 7-1-2. Location of both sets shall be per Article T-2 of ASME Sec VIII, Division 2.

POST WELD HEAT TREATMENT(PWHT) - ASME CODE

PWHT shall be in compliance with ASME Section VIII, Division 1 or 2, as applicable and the requirements of this Practice.
For the PWHT cycle, the minimum holding time at temperature shall be 1 hour for any vessel that is up to and including one inch thick.
The holding time at temperature for all vessels greater than 1 inch thick shall be 1 hour for each inch or fraction of an inch; for example 1-1/2 inch thick plate equals 2 hours.
Corrosion allowance shall be considered as material thickness when calculating minimum hold times.
Controlled cooling rates shall not exceed 400F/hr. or 400F/hr. divided by the maximum metal thickness, whichever is slower.
(*)Welding is not permitted on a vessel after it has received its final post weld heat treatment without written approval of the Owner's Engineer. Marking of the vessel to enforce this requirement shall be in accordance with EP 7-1-4.
Post weld heat treatment shall preferably be done in a gas fired furnace. Stacking of components within the furnace shall not be permitted.
Localized post weld heat treatment not done in a furnace may be accomplished by electric resistance or induction heating subject to the following restrictions:
A full circumferential band must be heated to the required temperature.
Heater band shall extend five times the wall thickness on each side of the joint.
Localized PWHT of longitudinal weld seams is not acceptable.
(*)PWHT of a vessel internally by the use of combustion burners shall not be used without written approval of the Owner's Engineer.
(*)Exothermic post weld heat treatment methods shall require written approval of the Owner's Engineer.
Post Weld Heat Treatment Temperatures for ASME Code materials shall be found in Table 2.
The use of reduced PWHT temperatures with an extended hold time (per Table UCS-56.1) is not acceptable.
(*)PWHT requirements for P-5C materials shall be submitted to the Owner's Engineer for review and approval prior to commencement of fabrication.
Vessels fabricated from materials other than P-5B may be slow cooled after completion of welding and post weld heat treated at a later date.

Vessels fabricated of P-5B materials shall be kept at preheat temperature until all welds are complete then immediately post weld heat treated. Alternatively, each weld may be given an intermediate stress relief by heating the completed weld to a minimum temperature of 1000F for 15 minutes, wrapping the weld with insulation, and allowing it to slow cool to ambient. The intermediate stress relief will postpone, but not eliminate the full PWHT required by the Code.

11.0 POST WELD HEAT TREATMENT (PWHT) - AES

Post weld heat treatment requirement for pressure vessel and heat exchangers in Aggressive Environmental Service (AES), see EP 10-2-1, shall be per paragraph 10.0.

12.0 TABLES

TABLE 1

MINIMUM PREHEAT TEMPERATURES FOR WELDING

Base Metal	Thickness (inches)	Minimum Preheat (F)
P - 1 (Carbon Steel)	1 inch or less	50
P - 1 (Carbon Steel)	Greater than 1 inch	200
P - 3 (C - 1/2 M0)	All	200
P - 4(1-1/4 Cr)	All	250
P - 5 A,B,C (2-1/4 Cr thru 9 Cr)	All	300
P - 8 (Stainless Steel)	All	50

TABLE 2

POST WELD HEAT TREATMENT TEMPERATURE

Material	Design Temperature(DT)	PWHT (F)
P-1 (Carbon Steel)	ALL	1150±25
P-3 (Carbon Mo)	ALL	1150±25
P-4 (1-1/4 Cr-1/2 Mo)	DT700F	1250±25
	DT700F	1275±25
P-5 A (2-1/4 Cr-1 Mo)	ALL	1275±25
P-5 B (5 Cr, 9 Cr)	ALL	1350±25
P-42 (Monel - HF Service)	ALL	1050±25

© 2026 Inflection Point Engineering, LLC. All rights reserved. The content of this page — including calculation methods, reference data, written analysis, interactive tools, and source code — is the intellectual property of Inflection Point Engineering, LLC and is protected under applicable copyright, trademark, and trade secret laws. Unauthorized reproduction, redistribution, modification, or derivative use in whole or in part is prohibited without prior written consent.

Disclaimer. This material is provided for informational and educational purposes only and does not constitute professional engineering advice. Calculations, reference data, and methodologies are based on published standards and accepted engineering practice but are not a substitute for engineering judgment, site-specific analysis, or review by a licensed Professional Engineer. Inflection Point Engineering, LLC makes no warranties, express or implied, regarding the accuracy, completeness, or fitness for a particular purpose of any content presented here, and shall not be liable for any direct, indirect, incidental, or consequential damages arising from its use. Users assume all risk associated with applying this content to real-world design, operations, or decisions.