{"slug":"/standards/api-577","title":"API RP 577: Welding Inspection and Metallurgy Requirements","description":"API RP 577 covers welding inspection and metallurgy: procedure and performance qualification to ASME IX, PMI of alloy welds, and 200 HB hardness caps.","h1":"API RP 577 welding inspection: what the recommended practice actually requires","answer":"API RP 577 is a recommended practice describing welding processes, inspection activities and metallurgy for the inspector who verifies weld quality. It sets no acceptance criteria of its own. Acceptance always comes from the referencing construction or inspection code — ASME IX for qualification, ASME B31.3, Section VIII or API 510 for weld acceptance — while RP 577 explains how to verify compliance.","expansion":"The practice is written around three linked bodies of knowledge: the welding processes themselves, the documentation that controls them, and the metallurgy that explains why welds fail. In practical terms it tells an inspector how to read a WPS, PQR and WPQ; how to confirm that essential variables have not drifted; what to look at before, during and after welding; and which discontinuities matter. The metallurgy sections exist because the inspector is expected to recognise the conditions that produce hydrogen cracking, sensitisation, sigma phase and hot cracking, not merely to measure a reinforcement height. Most audit findings against RP 577 are not weld defects at all. They are documentation failures: a welder qualified on a process he did not use, a filler metal issued without traceability, a preheat record with no thermocouple location, or a PMI programme that sampled the pipe and forgot the weld.","source":"Sources: API RP 577 Welding Processes, Inspection, and Metallurgy; ASME BPVC Section IX; ASME B31.3 Process Piping; ASME BPVC Section VIII Division 1; API 510; API 570; API 653; AWS D1.1 Structural Welding Code — Steel; NACE MR0103/ISO 17945; NACE MR0175/ISO 15156.","table":{"caption":"API RP 577 subject areas and the documents that actually enforce them","columns":["Subject area","What API RP 577 provides","Where the requirement is enforced","Evidence an auditor asks for"],"rows":[["Procedure qualification","Explains WPS and PQR structure and essential variables","ASME BPVC Section IX; AWS D1.1; ISO 15614","Approved WPS at current revision with its supporting PQR"],["Welder qualification","Explains WPQ ranges and continuity","ASME BPVC Section IX, QW-300 series","WPQ plus six-month continuity record for each process"],["Preheat and PWHT","Explains purpose and the cracking mechanisms behind it","Construction code and the qualified WPS","Calibrated chart, soak record, thermocouple layout"],["Weld acceptance criteria","Describes discontinuities and their causes only","ASME B31.3; ASME VIII Div 1; API 510/570/653","Report naming the acceptance paragraph applied"],["NDE method selection","Compares RT, UT, PAUT, TOFD, MT and PT capability","Owner specification and the construction code","NDE procedure plus in-date technician certification"],["Hardness in sour service","Explains microstructure and testing practice","NACE MR0103; NACE MR0175/ISO 15156","HAZ hardness readings with method and calibration block"],["Alloy weld verification","Points to material verification practice","API RP 578 and the owner's specification","PMI record for the weld deposit, not base metal only"]],"note":"RP 577 is the explanatory layer. In every row, the enforceable number lives in the column to its right."},"facets":[{"q":"Is API RP 577 a mandatory requirement or guidance?","a":"It is a recommended practice, so it carries no legal force on its own. It becomes binding when an owner's written practice, an inspection specification or a purchase order invokes it, and once invoked an auditor can test compliance against it. Many refiners and EPCs do exactly that, which is why fabricators encounter it as a requirement even though API publishes it as guidance."},{"q":"Does API RP 577 contain weld acceptance criteria?","a":"No, and treating it as though it does is a common and losing argument. RP 577 describes discontinuities, their causes and how they are detected. The numerical limits — permitted porosity, undercut depth, indication length — come from the construction code: ASME B31.3 for process piping, ASME Section VIII Division 1 for vessels, AWS D1.1 for structural work, or the in-service codes for repairs. Every inspection report should name the acceptance paragraph applied."},{"q":"How does API RP 577 differ from ASME Section IX?","a":"Section IX is a qualification code: it defines how a welding procedure and a welder are qualified, what the variables are, and what test results are required. API RP 577 is an inspection practice: it explains how to read and verify those Section IX documents, what to watch during production welding, and what the metallurgy behind the requirements means. Section IX creates the evidence; RP 577 teaches the inspector how to interrogate it."},{"q":"Why is NDE deferred on some welds after they have cooled?","a":"Because hydrogen-induced cracking is delayed. Diffusible hydrogen introduced during welding migrates to the hard heat-affected zone and cracks appear hours after the weld has cooled, typically within 24 to 48 hours in crack-sensitive materials such as chrome-moly and higher-strength steels. Examining immediately after cooling produces a clean report of an examination that was performed before the defect formed. The deferral period should be stated in the procedure and enforced as a hold point."},{"q":"Where should weld hardness readings be taken for sour service?","a":"On the heat-affected zone, not the weld cap, because the HAZ is where the hard martensitic microstructure forms and where sulfide stress cracking initiates. NACE MR0103 and MR0175/ISO 15156 impose the limits, commonly expressed as 200 HB or 22 HRC depending on the material and the governing document. Portable hardness testing is sensitive to surface preparation, coupling and curvature, so the procedure must state the method, the calibration block and the number of readings."},{"q":"Which welding records lapse most often on a long project?","a":"Welder continuity. A welder who has not used a process within the preceding six months loses qualification on that process regardless of experience, and long projects with rotating crews accumulate lapses quietly. The others are NDE technician certifications expiring mid-contract, WPS revisions superseded without the shop copy being replaced, and instrument calibration certificates expiring between the test and shipment. All three are found by tracing one weld number end to end."}],"sections":[{"heading":"Scope: what API RP 577 covers and what it deliberately leaves out","paragraphs":["API RP 577 is a recommended practice written for the person who inspects welding rather than for the person who performs it. Its content divides into welding processes and their variables, the documentation that controls production welding, welding metallurgy, weld discontinuities and their causes, the NDE methods used to find them, and a substantial terminology section. It exists to make an inspector competent to judge whether a weld was made the way the paperwork says it was, which is a different question from whether the finished weld looks acceptable to the eye.","What it does not do matters more in practice. RP 577 does not qualify a welding procedure, does not certify a welder, and does not contain acceptance criteria for a single weld discontinuity. Qualification lives in ASME BPVC Section IX or the equivalent AWS or ISO route; acceptance lives in the construction code — ASME B31.3, ASME Section VIII Division 1, AWS D1.1 — or in the in-service code, API 510, API 570 or API 653. Inspectors who cite RP 577 as the basis for rejecting a weld lose that argument every time it is escalated.","RP 577 becomes contractual the moment an owner's written practice, a purchase order or an inspection specification invokes it. At that point an auditor can test compliance clause by clause, and the owner is expected to show a welding inspection programme that reflects it. Building that programme — the written practice, the procedure hierarchy and the inspector qualification matrix — is standard [ASNT Level III consulting](/consulting) work, and it is far cheaper to build before an audit than during one."]},{"heading":"The document set: WPS, PQR and welder qualification, reviewed in order","paragraphs":["The review sequence is fixed, and skipping a step is the most common finding. Start with the code of construction named on the purchase order, then the WPS, then the supporting PQR, then the welder or welding operator qualification records for every person who will deposit metal, then the consumable specification and its storage controls. A WPS reviewed without its PQR is unverifiable, because the PQR carries the actual test values and therefore governs the ranges the WPS is permitted to claim.","On the PQR the inspector checks that the recorded values are real test data rather than figures transcribed from the WPS, and that the mechanical test results — tensile, bend, and where required impact and hardness — meet the code. On the WPQ the inspector checks the process, the position, the backing, the diameter and thickness ranges qualified, the F-number, and critically the continuity record. A welder whose continuity has lapsed beyond six months on a process is not qualified on that process, however experienced he is and however good his X-ray record.","Keeping this current across a contractor population is a records problem, not a welding problem. Sites that manage welder continuity, consumable batches and procedure revisions in a controlled system rather than a shared spreadsheet stop losing days to expired qualifications discovered at the joint; that is one of the standard use cases for [inspection management software](/erp)."]},{"heading":"Essential variables and the ways a qualification quietly goes out of range","paragraphs":["Section IX splits variables into essential, supplementary essential and non-essential. Change an essential variable and the procedure requalifies. Change a supplementary essential variable where impact testing is required and it requalifies. Change a non-essential variable and the WPS is simply revised. RP 577 spends time here because the drift is rarely dramatic: a P-number substitution, a filler metal F-number change, a heat input increase from a higher travel speed, or a preheat dropped below the qualified minimum on a cold morning because the shop was behind.","The traps that recur are thickness range, position and process combination. A procedure qualified on a 10 mm coupon does not automatically cover 40 mm. A welder qualified 6G on pipe covers a wide envelope, but a welder qualified 1G on plate covers very little. And a root run made with GTAW followed by fill and cap with SMAW is two processes; the welder needs qualification on both unless a combination procedure was qualified. Auditors find this by taking one weld number and tracing it back through the weld map to the individual welder.","Where a fabricator argues an interpretation, the resolution is a written technical position referenced to the code paragraph and signed by someone with the standing to hold it. That is what a Level III or a welding engineering authority is for, and it is worth having in place before the coupon is cut rather than after the joint has been welded into a spool."]},{"heading":"Preheat, interpass temperature and post-weld heat treatment","paragraphs":["Preheat exists to slow the cooling rate through the transformation range and to let diffusible hydrogen escape. Its absence produces delayed hydrogen cracking, which is why the practice recommends that final NDE on crack-sensitive materials be deferred, commonly for 24 to 48 hours after welding. Inspecting a 2¼Cr–1Mo weld immediately after it cools and passing it produces a defensible-looking record of an examination that was performed too early to find the defect it existed to find.","For records, the auditor wants the measured value and the location at which it was measured. A temperature-indicating crayon used on the wrong side of the joint, or a thermocouple placed 300 mm from the weld, is not evidence of preheat at the joint. Interpass maximum matters for austenitic and duplex materials, where excessive interpass temperature drives sigma phase and destroys both toughness and corrosion resistance. Duplex stainless is particularly unforgiving, because there the heat input window is bounded on both sides rather than only at the bottom.","PWHT records must show the actual chart, the soak temperature and duration against the code requirement, the heating and cooling rates through the controlled range, and the thermocouple layout. A chart with a soak that dips below the minimum for twenty minutes has not met the requirement, and a reviewer who signs it has created an audit finding that is trivial for anyone to prove years later."]},{"heading":"Consumable control, traceability and PMI of the deposited metal","paragraphs":["Filler metal control is where a welding programme is usually weakest. The requirements are unglamorous: correct AWS classification against the WPS, unbroken traceability from certificate to heat or lot to the joint, controlled issue and return, and hydrogen control for low-hydrogen electrodes — sealed packaging or holding ovens at the manufacturer's stated temperature, with a defined exposure limit once the packet is opened. An E7018 electrode left on a rack overnight is not a low-hydrogen electrode any more, whatever the box says.","For alloy work the deposited weld metal has to be verified as well as the base metal. This is the single most common gap between a nominal PMI programme and a real one: the pipe is tested, the fitting is tested, and the weld between them — which may have been filled with the wrong rod from an unlabelled quiver — is never tested at all. Where alloy piping is in scope, material verification under API RP 578 and welding inspection under RP 577 have to be run as one programme with one record set.","Third-party review of the resulting records finds these gaps faster than the fabricator will. Independent [inspection report validation](/report-validation) against the procedure and the code catches transcription errors, missing continuity, uncertified technicians and NDE performed inside the deferral period, while the item is still in the shop and the correction still costs hours rather than weeks."]},{"heading":"The metallurgy an inspector is expected to recognise on sight","paragraphs":["The metallurgy content in RP 577 is not academic. It exists so the inspector can predict where cracking will appear and time the examination accordingly. Hydrogen-induced cracking in the heat-affected zone of hardenable steels; solidification and reheat cracking; sensitisation of austenitic stainless steel in the 425–815 °C range, producing chromium-depleted grain boundaries and intergranular attack; sigma phase embrittlement in high-chromium alloys; and 475 °C embrittlement in ferritic and duplex grades are all mechanisms an inspector is expected to anticipate rather than discover.","Hardness is the practical proxy for microstructure at the joint, and it is where sour service bites. Where NACE MR0103 or MR0175/ISO 15156 applies, weld and heat-affected zone hardness is capped — commonly 200 HB or 22 HRC depending on the material and the governing document — and the reading has to be taken on the HAZ, not on the weld cap where the fabricator would prefer. Portable hardness testing on a curved surface with poor coupling produces optimistic numbers, so the method and the calibration block matter as much as the limit does.","Recognising these mechanisms is a competence question, and competence has to be demonstrable rather than asserted. Inspector qualification against SNT-TC-1A or ISO 9712 for the NDT methods, plus documented welding inspection training and eye examinations, is what an auditor asks to see; structuring that programme is routine [NDT training and certification](/training) work."]},{"heading":"Discontinuities, NDE selection, and where acceptance actually comes from","paragraphs":["RP 577 catalogues discontinuities by cause: porosity from contamination or loss of shielding, slag inclusions from poor interpass cleaning, incomplete fusion from low heat input or poor manipulation, incomplete penetration from joint geometry or wrong current, undercut, overlap, arc strikes and cracks. The point of the catalogue is diagnostic. A repeated pattern of incomplete fusion on the sidewall of a narrow-gap joint is a procedure or technique problem; repairing it weld by weld without addressing the cause guarantees the same repair rate on the next spool.","Method selection follows the defect you expect and the geometry you have. Radiography images volumetric defects well and finds planar defects poorly unless they are favourably oriented; ultrasonics, and particularly phased array or TOFD, is the better tool for lack of sidewall fusion and for thick sections; magnetic particle finds surface and near-surface indications in ferromagnetic materials; penetrant covers non-magnetic surfaces. Choosing RT for a thick austenitic weld and then arguing about the interpretation is a recurring and entirely avoidable project cost.","Acceptance criteria come from the construction code and nowhere else. ASME B31.3 gives different criteria for normal, category M and severe cyclic fluid service; ASME Section VIII Division 1 governs radiographic examination for vessels; AWS D1.1 gives its own tables for structural work. The inspection report has to name the acceptance paragraph applied, not merely record the word 'accepted' against a weld number and a date."]},{"heading":"Recurring audit findings and procurement traps","paragraphs":["The findings recur with remarkable consistency across owners. A WPS on the shop floor at a revision the client never approved. A welder identification stamp on a joint that does not appear in the welder qualification register. NDE performed by a technician whose certification lapsed mid-project. A hardness survey taken on the weld cap where the specification says heat-affected zone. A PWHT chart accepted with the thermocouple layout undocumented. None of these are welding failures; all are programme failures, cheap to prevent and expensive to discover in a data book review.","The procurement trap is the specification that invokes RP 577 without stating what the fabricator must therefore produce. RP 577 is guidance; if the purchase order does not convert it into deliverables — the document review sequence, the qualification evidence, the hold points, the records that must accompany release — the fabricator will meet the letter of the order and the owner will receive a data book that cannot be audited. Write the deliverable list into the order rather than a bare reference to a recommended practice.","Atlantis provides welding inspection, procedure and qualification review, and welding programme development for owners and fabricators, with inspectors mobilised to the site or the supplier works and deployed for the duration of the contract so that one engineer carries the specification through the campaign. If you need a welding inspection programme built, or an existing one audited against RP 577, [request a consultation](/contact)."]},{"heading":"Where API RP 577 and API RP 578 responsibilities overlap on the same weld","paragraphs":["A single new weld in alloy piping sits inside both API RP 577's welding scope and API RP 578's material verification scope, and the two are frequently owned by different people on the same project, which is where responsibility gaps open up. API RP 577 covers whether the weld was made correctly — WPS, PQR, welder qualification, essential variables, preheat and PWHT. API RP 578 covers whether the deposited metal is actually the alloy the drawing called for, which is a question welding inspection alone does not answer.","The gap shows up when a welding inspector verifies procedure compliance and consumable traceability against the certified WPS, reasonably assumes the filler metal grade stamped on the wire spool is correct, and closes the weld — while the material verification programme under API RP 578 was scoped to check base metal PMI at receiving inspection but was never extended to the deposited weld metal itself. The base metal passes PMI, the weld procedure passes welding inspection, and a wrong-alloy filler metal still makes it into service, because neither check individually was designed to catch it.","The corrective practice is to state explicitly, in the inspection and test plan, which party verifies weld deposit composition and by what method — commonly handheld XRF on the completed weld cap, not merely on the base metal either side of it — and to treat that as a distinct line item rather than an assumed extension of either the welding inspection scope or the base metal PMI scope."]}],"faq":[{"q":"Which edition of API RP 577 applies to my project?","a":"The edition named in the purchase order or the owner's written practice, not simply the newest one available. API has issued RP 577 in successive editions since 2004, and later editions expanded the metallurgy and NDE content considerably. Confirm the edition and any addenda in writing at kick-off, because a fabricator working to an earlier edition and a client auditing against a later one will disagree at the data book review, usually about the extent of documentation."},{"q":"Does API RP 577 require inspectors to hold a particular certification?","a":"RP 577 does not itself certify anyone. Owners typically require a welding inspection qualification such as AWS CWI or CSWIP, plus NDT method certification to SNT-TC-1A or ISO 9712 for any examination the inspector personally performs, and Level III technical authority for procedure approval and interpretation. Atlantis provides NDT training and certification support to ASNT SNT-TC-1A; it does not deliver API inspector certification training."},{"q":"Can one inspector cover everything RP 577 describes?","a":"Rarely. Document review, production welding surveillance, hardness testing, material verification and NDE interpretation span several distinct competencies. In practice a site inspector handles surveillance and verification while a Level III or welding engineering authority handles procedure review, interpretation disputes and non-conformance evaluation. The programme should state explicitly which decisions the site inspector may make alone and which must be escalated, because an undefined boundary is where inconsistent acceptance starts."},{"q":"How is API RP 577 used during a welding programme audit?","a":"As a checklist framework. The auditor takes a small number of completed welds and traces each one end to end: purchase order and code of construction, approved WPS revision, supporting PQR, welder qualification and continuity, consumable certificate and issue record, preheat and PWHT evidence, NDE procedure and technician certification, examination report, and the acceptance paragraph applied. Anything in that chain that cannot be traced becomes the finding, regardless of the weld's actual quality."},{"q":"Does Atlantis provide welding inspection to API RP 577?","a":"Yes. Atlantis provides welding inspection, WPS, PQR and welder qualification review, welding programme development, and independent review of completed weld records, with inspectors mobilised to the site or the supplier works and deployed for the contract duration. Level III technical authority supports the field team for procedure review, interpretation and non-conformance evaluation, so an argument at the joint is resolved with a written position rather than an opinion."}]}