Nace Sp 0472 Pdf Download
Brie Hoffler
Whilst both MR0175 and MR0103 cover a wide range of materials (carbon steels, stainless and duplex steels, nickel alloys and aluminium alloys), SP0472 is only concerned with carbon steels, classified as P1, Group 1 or 2 in ASME IX. These are hot finished carbon steels with a specified ultimate tensile strength less than 485MPa (70,000p.s.i.). Note that the BS EN 10028 steels are now assigned P numbers in ASME IX.
It should be remembered that parent metals may be weld-repaired as part of the plate production process. These base metal repairs must also comply with the NACE requirements with respect to weld metal and HAZ hardness. In addition, although SP0472 is concerned with the results of welding, any thermal cutting process will produce a heat-affected zone, which if not removed or welded over may result in HSC. In these cases, it is generally considered necessary to remove approximately 3mm of material to ensure that there are no areas of unacceptably high hardness.
SP0472 requires that the HAZs of all pressure boundary welds and internal attachment welds as well as repair welds and some external attachment welds in pressure containing equipment comply with a maximum hardness of 248Hv10.
PWHT must be performed correctly, and so a PWHT procedure must give consideration to process, heating and cooling rates, hold times, hot zones, measurement positions and tolerances of all of these. Some guidance on the application of PWHT is given in Appendix D of SP0472.
Welding procedure qualification is the most common method of verifying that the methods put in place to control the hardness generate welds complying with the hardness requirements. It is carried out in accordance with the ASME IX requirements using actual production material or a steel of the same grade but with the maximum carbon equivalent of material to be used. The welding variables are recorded during welding of the test piece and hardness testing is mandatory, the hardness of the test weld HAZ to be less than 248Hv10, that of the weld metal less than an average of 200HBW. Hardness testing surveys are as described in NACE MR0103. In addition to the ASME IX requirements, SP0472 requires butt welds and fillet welds to be qualified separately; although not mandatory, it would also be advisable to qualify separately single and multi-pass fillet welds. The hardness in a single pass fillet weld can easily exceed 300Hv, particularly when welding on thick steel, say over 25mm thick.
Quality control must be based on best practice with well-trained and qualified welders supervised by an adequate number of competent welding foremen and inspectors. Post-weld inspection and NDE will be as required by the construction code. SP0472 does not make hardness testing of production welds a mandatory requirement but since an acceptably low hardness is crucial to satisfactory in-service performance and is sensitive to so many variables, it is advisable to perform some checking of weld and HAZ hardness on completion. This requires the use of portable hardness testing equipment and Job Knowledge articles numbers 74 and 75 discuss some of the methods available.
This paper summarizes the development and objectives of NACE International Standard Recommended Practice RP0472-95, Methods and Controls to Prevent In-Service Environmental Cracking of Carbon Steel Weldments in Corrosive Petroleum Refining Environments. Two types of environmental cracking are included; cracking due to hydrogen charging and stress corrosion cracking (SCC). Welderments are defined to include the weld deposit, base metal heat affected zones (HAZS)and adjacent base metal zones subject to residual stresses from welding.
The foreword of RP0472 contains a good history of the standard?s development, hence this section is primarily excerpts from that. Most petroleum refining equipment and piping is constructed of carbon steel having a minimum specified tensile strength of up to 485 MPa (70,000 psi), and in nearly every case, the equipment is fabricated by welding. ?The welds for refinery equipment and piping are made to conform to various codes, and according to these codes, these carbon steels are classified as P-1, Group 1or 2. Hence, in RP0472, they are referred to as P-1 steels.
From a practical standpoint it is expected that for downstream applications a broad range of users and authors of equipment standards will adopt the new MR0103 standard, in many cases replacing the current application of MR0175. It is expected that use of the environmental guidelines and material requirements of MR0103, together with NACE RP0472 for weld hardness control of P-No. 1 carbon steels, will be broadly applied to piping, valves, process contacted bolting, pumps, and compressors used in the sour service areas of the refinery process units listed in Table 3 in order to prevent SSC.
NACE (formerly the National Association of Corrosion Engineers) has published two specifications that provide guidance on reducing the risk of in-service cracking, one of these also being an ISO standard. The major difference between these two primary specifications is the definition the environmental and service conditions for sour service. The first standard, ANSI/NACE MR0175/ISO 15156, Petroleum and natural gas industries - Materials for use in H2S-containing environments in oil and gas production, is intended for offshore applications and NACE MR0103, Materials Resistant to sulphide stress cracking in corrosive petroleum refining environments, for onshore process plant. The latter specification refers to a recommended practice document for controlling welding activities to ensure a low and acceptable weldment hardness; NACE SP0472, Methods and controls to prevent in-service environmental cracking of carbon steel weldments in corrosive petroleum refining environments.
Whilst both MR0175 and MR0103 cover a wide range of materials (carbon, duplex and stainless steels, nickel and aluminium alloys) SP0472 and this article are concerned with carbon steels only, classified as P1 in ASME IX, ie hot finished carbon steels with a specified ultimate tensile strength less than 480MPa (70,000p.s.i.) and how weldment hardness in these steels can be controlled. Note that the BS EN 10028 steels are now assigned P numbers in ASME IX.
It should be remembered that parent metals may be weld repaired as part of the plate production regime. These base metal repairs must also comply with the NACE requirements with respect to weld metal and HAZ hardness. In addition, although SP0472 is concerned with the results of welding any thermal cutting process will produce a heat affected zone which, if not removed or welded over may result in HSC. It will be necessary to require some 3mm of material to be removed to ensure that there are no areas of unacceptably high hardness.
SP0472 requires that the HAZs of all pressure boundary welds and internal attachment welds in pressure containing equipment comply with a maximum hardness of 248Hv10, as are repair welds and some external attachment welds. Welding processes covered by SP0472 are the more common processes; manual metallic arc (MMA), MAG, FCAW,TIG and submerged arc welding.
Control of the cooling rate requires the time to cool from 800-500OC (t8/5) to be controlled such that hard microstructures are avoided. This cooling rate must be specified for production welding, formulae for the calculation of t8/5 taking into account the relevant variables. Thickness, process heat input, joint configuration, preheat etc are given in Appendix C of SP 0472. The method is qualified by carrying out a pre-production weld test on representative parent material using the fastest cooling rate at which the HAZ hardness is acceptable; several tests may therefore be required. A successful test qualifies all other production welds made with cooling rates slower than that of the test piece, calculated from the formulae in Appendix C. This may require the welders to be specifically trained to deposit weld metal within very tight limits on travel speed, weaving etc and will require close supervision during production welding.
Temper beading is a method of reducing the hardness of HAZs by using the heat input from subsequent weld runs to refine and temper the HAZ of underlying weld passes. Clause QW290 of ASME IX specifies the requirements for temper bead welding, essential variables and weld procedure qualification. Hardness testing is mandatory and the positions of the hardness impressions are given in SP0472; the maximum hardness being required by the NACE specification.
The new welding procedure qualification hardness survey layouts are required for new qualifications, NACE MR0103 2016 is very clear with regards to previously qualified procedures. NACE MR0103 2016 provides clear instructions in clause C7 and clause C8 regarding the validity of hardness surveys in accordance with NACE MR0175 / ISO 15156, previous revisions of NACE MR0103 and NACE SP0472. For more information contact us.
Throughout the course, detailed explanations about the requirements of applicable codes and standards such as API 571, NACE MR0103, NACE TM0177, NACE TM0284, API 934, NACE SP0472, API 970 and more will be provided.
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