Inconel 625 Weld Overlay: Dilution and Quality
Date: 2026年10月2日 Categories: News Views: 322
By Shanghai Hangbo Alloy Group Co., Ltd. - China Supplier | ISO 9001:2015 | Est. 2012
Contact: sales@hangboalloy.com | hangbo@nickel-alloy.com | WhatsApp (Lisa): +86 13611656360
Quick Answer: What decides the quality of an Inconel 625 overlay?
The iron content of the deposited layer decides it, and the iron content is decided by dilution from the base metal. Specify the process, the number of layers, the minimum deposit thickness and the maximum iron content at a defined depth, qualify the procedure to ASME Section IX, and verify the chemistry on a production coupon, not a separate one.
Key Takeaways
- Dilution is the whole engineering problem. The first layer of an overlay is a mixture of filler metal and melted base metal, and the iron that enters through that mixture is what degrades the corrosion performance of the deposit.
- Chemistry must be specified at a depth, not at the surface. An analysis taken at the surface of a two-layer deposit describes the second layer; the layer that governs performance is the one adjacent to the base metal.
- The process chosen sets the achievable dilution. Hot wire GTAW and laser cladding dilute less than spray arc or submerged arc, and strip cladding trades dilution against deposition rate; the choice follows the geometry and the required chemistry.
- Procedure qualification for overlay is a separate qualification from a butt weld. ASME Section IX treats corrosion-resistant overlay as its own variable set, and a qualified butt-weld procedure does not qualify an overlay.
- Base metal heat treatment and overlay chemistry pull in opposite directions. Stress relief required for the carbon or low-alloy steel base can precipitate phases in the overlay that reduce its corrosion resistance, so the cycle has to be chosen deliberately.
- Inspection has to test the right thing in the right place. Surface examination, volumetric examination, thickness measurement and chemistry verification each address a different failure mode of the overlay.
Why Overlay Instead of Solid Alloy
A weld overlay puts a thin layer of corrosion-resistant alloy on a structural base metal. The economics are the reason it exists: a pressure vessel made from solid Inconel 625 plate costs several times as much as the same vessel made from carbon steel with a 625 overlay on its wetted surface, and in most services the corrosion resistance is needed only at the surface. The engineering reason it works is that the base metal carries the pressure and the structural loads while the overlay carries the corrosion duty, with each material doing what it is cheapest at.
The decision between solid alloy, clad plate and weld overlay follows from the size and geometry of the component. Solid alloy is used for small components where the fabrication cost dominates and where the thickness of the alloy is not a large fraction of the total; clad plate, produced by roll bonding or explosion bonding and covered by standards such as ASTM A265, is used for large flat areas where a mill can supply the composite; and weld overlay is used where neither is practical — on the inside of nozzles, on valve seats, on the ends and knuckles of formed heads, on weld repairs, and increasingly on the internal surfaces of large vessels that are fabricated from carbon steel plate and clad after assembly.
The distinction between the three routes matters because they produce different interface characteristics. Clad plate has a metallurgical bond produced by the mill with a defined bond strength and a defined minimum cladding thickness, and its chemistry is verified at the mill. A weld overlay has a fusion boundary produced by welding, with a dilution zone whose chemistry depends on the process and the procedure, and its quality depends on the fabricator rather than on the mill. That is why an overlay specification has to state chemistry requirements at a defined position, and why inspection has to verify the deposit rather than the intention.
Inconel 625 is the most widely used overlay material in the process industries for three reasons. It resists both oxidising and reducing media, so it covers a wide range of process streams without a change of alloy; it is highly resistant to chloride pitting and to chloride stress-corrosion cracking, which is the failure mode that governs most wetted surfaces in refining and chemical service; and it is tolerant of dilution, in the sense that an overlay substantially richer in iron than the parent alloy still performs acceptably in many services, which makes the overlaying operation forgiving in a way that an alloy such as C-276 is not. Our nickel-chromium-molybdenum range covers the C-276 alternatives for services in which the dilution tolerance of 625 is not enough.
The last point deserves qualification, because it is where most overlay specifications go wrong. A 625 deposit containing 5 % iron performs very differently from one containing 25 % iron, and the difference appears in the most demanding service — in hot chloride environments, in sour service with an aqueous phase, and where crevices exist. A specification that simply says "Inconel 625 overlay" without a chemistry requirement at a defined depth has not specified the corrosion performance at all; it has specified a process. The sections that follow set out how to close that gap.
The Overlay Specification: What Has to Be Defined
An overlay specification becomes enforceable when it defines six things: the base metal, the overlay material, the process, the number of layers, the minimum deposit thickness after machining, and the chemistry of the deposit at a defined depth with a defined maximum for the elements that degrade performance. Omitting any one of them transfers the decision to the fabricator, who will make it on the basis of production convenience.
| Parameter | What it controls | Typical requirement | Consequence if not stated |
|---|---|---|---|
| Base metal | dilution chemistry and heat treatment constraints | carbon or low-alloy steel, grade named | overlay chemistry is unpredictable |
| Overlay material and filler designation | deposit chemistry | ERNiCrMo-3 wire to AWS A5.14, or ENiCrMo-3 to AWS A5.11 | an unspecified nickel filler may be substituted |
| Process | achievable dilution and deposition rate | process and current mode named | dilution may exceed the design assumption |
| Number of layers | iron content of the layer adjacent to the base | two layers minimum for wetted surfaces | first-layer chemistry becomes the service exposure |
| Minimum deposit thickness after machining | corrosion allowance and full coverage | 3 mm minimum, or per the design | local thin areas and incomplete coverage |
| Chemistry at depth, with maximum iron | corrosion performance of the controlling layer | iron maximum at 1 mm above the fusion line, or per the design | acceptance becomes a matter of opinion |
| Base metal heat treatment | hardness of the heat-affected zone, and overlay phase stability | cycle, temperature and duration stated | conflicting requirements resolved by guesswork |
| Acceptance and inspection | verification of chemistry, thickness and soundness | methods and standards named | the deposit is never verified |
Table note: The parameters listed are those that must appear in a purchase specification or on a drawing for an overlay to be verifiable; the specific values depend on the service, and the chemistry-at-depth requirement is the one most often omitted because it requires the specifier to decide where the analysis will be taken. The general practice is to specify a maximum iron content and a minimum nickel, chromium, molybdenum and niobium content at a defined depth below the finished surface, typically a nominated distance above the fusion line, so that the analysis describes the layer that will see the process rather than the cosmetic layer above it. Our Inconel product range covers both the solid alloy and the filler metals used for overlaying.
The base metal heat treatment constraint is the parameter that causes the most difficulty, and it deserves to be decided at the design stage rather than at the fabrication stage. Carbon and low-alloy steels used for pressure equipment generally require post-weld heat treatment after welding, and in sour service the hardness of the heat-affected zone is limited, typically to 22 HRC, by the material requirements of NACE MR0175 and ISO 15156. Both requirements push the fabricator towards a stress-relief cycle in the region of 620 to 690 °C. Inconel 625 held in that temperature range can precipitate phases that reduce its corrosion resistance and its ductility, and the effect increases with time and temperature. The resolution is to choose the base metal so that the required stress relief can be performed at the lower end of the range, to limit the time at temperature, and to verify the overlay chemistry and corrosion performance after the heat treatment rather than before it — because an overlay that passes acceptance in the as-welded condition may fail the same acceptance after the vessel has been stress relieved.
Filler Metals and Processes
The process determines the dilution, and the dilution determines how much iron from the base metal enters the deposit. The table below sets out the processes used for 625 overlaying with their typical characteristics; the figures are indicative because dilution depends on current, travel speed, electrode angle and the base metal, but the ranking between processes is consistent.
| Process | Filler form and designation | Typical dilution | Deposition rate | Where it is used |
|---|---|---|---|---|
| GTAW, cold wire | ERNiCrMo-3 wire to AWS A5.14 | lowest of the arc processes | low | small areas, nozzle bores, repairs, root passes |
| GTAW, hot wire | ERNiCrMo-3 wire | low | moderate | large internal surfaces, mechanised overlaying |
| GMAW, spray transfer | ERNiCrMo-3 wire | moderate | high | flat and horizontal surfaces, vessel internals |
| Submerged arc, wire | ERNiCrMo-3 wire with basic flux | moderate to high | high | large flat areas, longitudinal seams |
| Submerged arc, strip | strip electrode, typically 60 mm wide | moderate | highest | large cylindrical shells |
| Electroslag strip cladding | strip electrode with flux | moderate | highest | vertical and circumferential surfaces |
| FCAW, gas shielded | flux cored nickel alloy wire | moderate to high | high | site work, repairs, difficult positions |
| SMAW | ENiCrMo-3 covered electrode to AWS A5.11 | high | low | repairs, local areas, small components |
| Laser cladding with powder | atomised 625 powder | lowest | low to moderate | precise, low-distortion components, valve trim |
| PTA with powder | atomised 625 powder | low | moderate | valve seats, seals, small areas at high volume |
Table note: Dilution figures are indicative ranges for each process family and are intended to show their relative ranking rather than to fix a value; the actual dilution depends on parameters and must be established by procedure qualification for the specific application, because it is the measured chemistry of the qualification test piece that demonstrates compliance. Filler metals should be certified to the relevant AWS specification — AWS A5.14 for bare nickel alloy wire, AWS A5.11 for covered nickel alloy electrodes — with the classification and the manufacturer's heat number stated on the package. The practice for welding the nickel alloys in general, and the differences between filler metals for the Ni-Cr-Mo family in particular, apply to overlay work as they do to joints.
Two filler-metal points are worth stating because they cause recurring problems. The first is that the electrodes and fluxes used for nickel alloy overlaying are hygroscopic to a degree that steel consumables are not always assumed to be, and moisture introduced with the consumable produces porosity that is detected only after the deposit is machined. Consumables should be stored and handled to the manufacturer's instructions, and rebaking regimes should follow the manufacturer's data rather than those used for steel electrodes. The second is copper pickup: contact tips, liners and drive rolls that are worn or that are used for copper-coated steel wire can transfer copper into a nickel alloy deposit, and copper is an element that is limited in the specification of the alloy and is not removed by the process. Consumable handling for nickel alloys is best kept separate from the steel welding side of the shop for this reason.
The choice among processes is normally made on three grounds: the geometry of the surface, the chemistry the service requires, and the deposition rate that the schedule allows. Hot wire GTAW and strip cladding are the two processes that cover most large-surface work, with strip cladding offering a much higher deposition rate at the cost of a wider and slightly more dilute deposit, and hot wire GTAW offering tighter control on complex geometry. Laser and PTA powder processes are used where dilution must be minimal or where the component is small and distortion must be controlled, such as valve trim and sealing surfaces.
Welding Procedure Qualification for Overlay
Overlay work requires its own procedure qualification, and the mistaken assumption that a qualified butt weld qualifies an overlay is a common cause of rejected fabrications. ASME Boiler and Pressure Vessel Code Section IX treats corrosion-resistant overlay welding as a distinct qualification with its own essential variables and its own test requirements, and the qualification must demonstrate the properties of the deposit as well as the soundness of the bond.
| Qualification element | Requirement | Reference |
|---|---|---|
| Procedure specification | written and qualified before production | ASME Section IX QW-200 series |
| Overlay qualification test | deposit and bond assessed on a qualified test piece | ASME Section IX QW-453 |
| Essential variables | process, filler, position, base metal group, layer count, thickness | ASME Section IX QW-250 series |
| Filler classification | classification and certificate stated | AWS A5.14, AWS A5.11, ASME Section II Part C |
| Test piece chemistry | iron and alloying elements at a defined depth | project specification, PMI to ASTM E1476 |
| Hardness survey | base metal heat-affected zone, and overlay where required | NACE MR0175 / ISO 15156 where applicable |
| Mechanical or bend testing | bond integrity and ductility of the deposit | ASME Section IX as applicable to the qualification |
| Welder performance | welders qualified for the process and position used | ASME Section IX QW-300 series |
| European route | procedure qualification to the welding procedure standard | ISO 15614-1 together with the application standard |
| Field practice guidance | welding guidelines for process industry fabrications | API RP 582 |
Table note: The references given are the conventional ones for pressure equipment and process industry work, and the governing document is the code or standard named in the project specification; where several apply, the qualification must satisfy all of them. The essential variables for overlay qualification include variables that do not exist in a butt-weld qualification — the number of layers, for example, and the thickness of the deposit — because both change the chemistry of the layer that matters. Where a fabricator proposes to use a previously qualified procedure for a new vessel, the comparison should be made against these variables rather than against the base metal and filler alone. Our inspection equipment and PMI practice covers how the chemistry of a qualified test piece is verified.
The qualification test piece should be prepared as closely as possible to production practice, and this is where a well-written specification earns its cost. Dilution depends on the process parameters, so a test piece welded at a lower current with more passes than the production procedure will show a lower iron content than the production deposit, and the qualification will not represent what is actually built. The same applies to positions: a deposit welded flat in the shop and vertically in the field will not have the same chemistry, and both should be covered by the qualification if both will occur. Where a fabricator is asked to qualify a procedure, the useful questions are which parameters are fixed and which may vary, and whether the qualification test piece was welded at the upper end of the permitted ranges rather than the middle.
There is also a documentation requirement that buyers should state explicitly. The qualification record should identify the procedure, the test piece, the measured chemistry at the defined depth, the results of the inspections performed, and the base metal and its heat number, so that the qualification can be related to the production work it covers. Where the overlay is produced in more than one shop or by more than one team, the qualification should be traceable to the shop and to the equipment used, because a change of power source or of positioner is a change of a variable that affects dilution.
Chemistry Control in the Overlay: Iron, Niobium and Molybdenum
The chemistry requirement for an overlay is stated in a form that has no equivalent in a mill product specification: a maximum for the elements that come from the base metal, a minimum for the elements that provide corrosion resistance, and a position at which the analysis is taken. The reason is that the deposit is not a homogeneous material; it is a gradient from base metal to nearly pure filler metal, and only the top of the gradient sees the process.
| Element | Direction of concern | Typical overlay requirement | Effect if the requirement is not met |
|---|---|---|---|
| Fe | comes from the base metal; must be limited | maximum typically 5 % at the defined depth, higher permitted in mild service | reduced pitting and crevice resistance; loss of performance in chloride service |
| Ni | from the filler metal; establishes the matrix | minimum close to the filler metal content | matrix becomes iron-rich; corrosion resistance falls |
| Cr | from the filler metal; forms the passive film | minimum at the low end of the filler specification | loss of oxidation and pitting resistance |
| Mo | from the filler metal; pitting and crevice resistance | minimum at the low end of the filler specification | loss of resistance in chloride and reducing acid service |
| Nb + Ta | from the filler metal; stabilises the deposit | minimum consistent with the filler classification | reduced elevated-temperature strength; higher susceptibility to sensitisation |
| C | from both; affects sensitisation | controlled low | chromium carbide precipitation at elevated service temperature |
| Cu | contamination from tooling and handling | maximum per the alloy specification | not removable by the process; requires rework |
| S and P | from base metal dilution and consumables | controlled low | hot cracking and reduced corrosion resistance |
| Si | from flux and consumables | controlled | affects the fluxing behaviour and may reduce corrosion resistance |
Table note: The typical requirements shown are the forms in which overlay chemistry is normally specified and the direction in which each element moves performance; the numeric limits in a particular specification depend on the service and on the design, and where a maximum iron content is stated it must be accompanied by the depth at which the analysis is taken, because the iron content falls steeply with distance from the fusion line. Verification is normally performed by taking drillings or by a chemistry slug welded onto the component or onto a representative test piece, analysed to ASTM E572, and supplemented by PMI to ASTM E1476 for the elements that portable instruments resolve. Our other materials testing articles cover the verification practice in more detail.
The single most useful decision a specifier can make is where to take the analysis. If the requirement says only "iron maximum 5 %", the analysis can be taken at the finished surface of a two-layer deposit and will satisfy the requirement while the layer immediately above the fusion line contains 15 % iron or more. In a mildly corrosive service that may be acceptable; in hot chloride service or in an aqueous sour environment it is not, because the attack that terminates the life of an overlay usually begins at a defect or a crevice that penetrates to the diluted zone. Stating the depth — as a nominated distance above the fusion line, or as a requirement on the analysis of the layer adjacent to the base metal — makes the specification mean what it intends.
The second useful decision is the number of layers. Two layers of 2 mm produce a total deposit of about 4 mm before machining and a first-layer chemistry that is substantially diluted; three layers produce a more uniform deposit and consume more filler metal and time. For most wetted surfaces in refining and petrochemical service, the convention of two layers with a defined minimum total thickness is the standard practice, and the first layer is treated as a bonding layer whose chemistry is not required to reach the full alloy content. Where the service is severe, the specification often requires a minimum thickness of undiluted material specified by chemistry rather than by number of layers, which leaves the fabricator free to choose the layering that achieves it — a formulation that is both more demanding and more honest about what is being bought.
Base Metal Heat Treatment and the Overlay
The interaction between base metal heat treatment and overlay performance is the least understood part of overlay specification, and it is where a technically compliant fabrication can still fail in service. The base metal governs the requirement; the overlay governs the constraint; and the resolution has to be written into the specification rather than left to the fabricator.
| Heat treatment situation | Requirement driving it | Effect on the 625 overlay | Practical resolution |
|---|---|---|---|
| Post-weld heat treatment of carbon steel base | stress relief of the base metal and its weldments | exposure in the 620–690 °C range can precipitate phases that reduce corrosion resistance | choose a base metal and thickness that permit the lower end of the range; limit time at temperature |
| Hardness limitation for sour service | typically 22 HRC maximum in the heat-affected zone | the same stress relief that reduces hardness also ages the overlay | verify overlay chemistry and corrosion performance after heat treatment |
| No post-weld heat treatment permitted | avoidance of overlay ageing | residual stress remains in the base metal | acceptable only where the base metal and service permit it |
| Overlay applied after the vessel is stress relieved | base metal already treated | overlay remains in the as-welded condition | preferred where geometry allows, since the overlay sees no ageing |
| Local stress relief of nozzle welds | localised base metal requirement | localised ageing of the overlay near the affected zone | restrict the treated area and verify the overlay outside it |
| Service temperature in the ageing range | process duty at 600–700 °C | progressive precipitation in service | select a different overlay alloy, or accept a reduced design life |
| Re-aging after field repair | restoration of local properties | repeated exposure accumulates | qualify a repair procedure and verify chemistry after repair |
| Verification sequence | confirming the delivered condition | results before and after heat treatment can differ | require chemistry and corrosion testing after final heat treatment |
Table note: The table states the interaction rather than prescribing a cycle; the mandatory treatment is that required by the design code and by the service, and the overlay requirement must be written so that it can be met after that treatment. The practical implication for a specification is that any overlay acceptance test — chemistry at depth, hardness, or a corrosion test to ASTM G48 — should be required after the final heat treatment rather than in the as-welded condition, because that is the condition in which the material will enter service. Our corrosion test buyer's guide covers how to state such a test with a numerical acceptance criterion.
The commercial consequence of ignoring this interaction is a fabrication that passes all its inspections in the shop and exhibits corrosion damage in service, typically in the form of localised attack adjacent to welds and in crevices, where the deposit is thinnest and the base metal dilution is greatest. Because the failure appears after commissioning, the cost of resolution includes the process interruption as well as the repair, and the repair of an aged overlay in an installed vessel is a considerably more difficult operation than the original fabrication. Deciding the heat treatment sequence at the design stage, and specifying where the overlay sits in that sequence, is inexpensive at that point and very expensive afterwards.
There is one option that resolves the conflict cleanly where geometry allows it, and it is worth asking for: overlaying after the base metal has been heat treated. Where the vessel can be stress relieved before the overlay is applied — which requires that the overlay itself does not need stress relief and that the subsequent welding of the overlay does not reintroduce an unacceptable stress state in the base metal — the overlay enters service in the as-welded condition with its full corrosion resistance intact. This sequence is not always admissible, because the fabrication sequence may require welding after heat treatment, but where it is admissible it removes the problem entirely and should be considered before the alternative is accepted.
Inspection and Acceptance of the Overlay
An overlay can fail in four distinct ways: it can be the wrong chemistry, it can be too thin, it can be unbonded, and it can contain defects. Each of these requires a different inspection method, and a specification that requires only one of them leaves the others unverified.
| Inspection | Purpose | Method or standard | Typical acceptance |
|---|---|---|---|
| Visual and dimensional | coverage, surface condition, extent of the overlay | visual examination, measurement | full coverage of the specified area, no exposed base metal |
| Chemical verification | iron content and alloying elements at the defined depth | drillings or chemistry slug analysed to ASTM E572, PMI to ASTM E1476 | within the specified maxima and minima at the stated depth |
| Thickness measurement | corrosion allowance and minimum remaining alloy | ultrasonic thickness measurement, or measurement after machining | minimum thickness after machining as specified |
| Surface examination | surface-breaking defects, cracks, porosity, incomplete fusion at edges | liquid penetrant testing, ASME Section V | acceptance criteria of the governing code |
| Volumetric examination | lack of bond between overlay and base metal | ultrasonic examination, ASME Section V Article 4 | bond continuous over the specified area |
| Hardness survey | base metal heat-affected zone, and overlay where the service requires it | portable hardness testing to ASTM A956 or equivalent | limits per the governing service specification |
| Corrosion performance | the property the overlay exists to provide | ASTM G48 or the specified test, on a test piece taken through the final heat treatment | numerical criterion agreed on the order |
| Ferrite or phase check | where the overlay composition or the base metal makes it relevant | metallographic examination to ASTM E3 and E112 | as specified |
Table note: The inspection methods listed address different failure modes and are complementary; the acceptance criteria are those of the governing code and of the project specification, and the standard methods named are the conventional references. Two sequencing points are worth stating in the specification: the chemistry verification should be performed on material that has been through the final heat treatment, and the thickness measurement should be performed after machining, because the thickness that matters is the alloy thickness that remains when the vessel is complete. Our quality inspection practice describes the equipment used for the chemical and hardness verification.
The most frequently neglected of these is the bond examination, and the omission is understandable because it requires ultrasonic examination of a surface rather than a weld and the technique is less familiar to inspectors trained on joints. A lack of bond between overlay and base metal is a failure that develops into a blister or a localised disbond in service, and once disbonded, the base metal behind the overlay is exposed to the process in a geometry that concentrates corrosion. Ultrasonic examination of the bond is practical, and where the overlay is applied to a pressure boundary and the consequence of a disbond is significant, the specification should require it with an agreed acceptance criterion rather than leaving the decision to the fabricator.
The chemistry verification deserves a procedural note as well. A chemistry slug welded onto a separate test piece demonstrates the chemistry that the procedure can achieve, not the chemistry that the component received, and the two can differ if the parameters drifted or if the position changed. Where the requirement matters, the analysis should be taken from a coupon attached to the component and welded as part of the production sequence, or from a location on the component itself where a sample can be removed without affecting the design, and the removal should be repaired and inspected afterwards. The cost of doing this on a large vessel is trivial; the value of a verified overlay chemistry is that it converts the corrosion resistance of the wetted surface from an assumption into a record.
Common Defects and How They Arise
Overlay defects are usually procedural rather than metallurgical, and almost all of them can be traced to a parameter that drifted or to a step that was omitted. The table below lists the defects we see in practice with their causes and their prevention.
| Defect | Root cause | Consequence | Prevention |
|---|---|---|---|
| High iron content in the deposit | excessive dilution from high current or low travel speed | loss of corrosion resistance | qualify at the upper end of the parameter range; verify chemistry at depth |
| Porosity | moisture in flux or electrode, contaminated shielding gas, surface contamination | through-thickness paths for the process | consumable control and storage to the manufacturer's instructions; clean base metal |
| Incomplete fusion or lack of bond at edges | electrode angle and travel speed at the boundary of the overlay | localised disbond and corrosion behind the deposit | edge procedures qualified and included in the qualification |
| Copper pickup | worn contact tips, liners or drive rolls used with copper-coated steel wire | chemistry outside specification; rework | consumable path dedicated to nickel alloys; inspect and replace components |
| Transverse cracking | high restraint and high heat input in a thick section | crack initiation site in the deposit or at the boundary | procedure qualification on representative restraint; control of heat input |
| Local thin areas after machining | inadequate allowance for distortion and machining | insufficient alloy thickness in service | specify minimum thickness after machining and measure it there |
| Surface iron contamination | carbon steel tooling, wire brushes, grinding wheels and scaffolding | rust staining and localised attack on the passive film | dedicated stainless or nickel alloy tooling; cleaning verification |
| Arc strikes and spatter outside the overlay | careless manipulation and inadequate protection | localised hardening and corrosion initiation on the base metal | procedure discipline; inspection of the surrounding surface |
| Sensitisation or phase precipitation in the overlay | excessive time in the 620–690 °C range during heat treatment or service | reduced corrosion resistance and ductility | sequence the heat treatment deliberately; verify properties after treatment |
| Rust staining on the finished deposit | external iron contamination rather than a defect in the alloy | cosmetic and diagnostic confusion | ferroxyl or equivalent test, dedicated tooling, and cleaning procedures |
Table note: The defects and causes listed are those encountered in nickel alloy overlay work on carbon and low-alloy steel; the acceptance criteria and the inspection methods for each are those of the governing code and the project specification, and the preventive measures are procedural rather than metallurgical. Our machining guide covers the tooling and handling practices that prevent the contamination-related defects, and the general rules for welding nickel alloys apply to overlay work as well.
The observation worth carrying into planning is that the defects with the highest consequence are the ones that are hardest to detect after the fact. High iron content and a localised lack of bond both leave a deposit that looks correct from the surface and passes a visual examination, and both reduce the corrosion performance of the surface that the customer is buying. That is why the verification requirements concentrate on chemistry at depth and on bond examination rather than on the surface condition that is easiest to inspect, and why a specification that requires only penetrant testing of the overlay is a specification that verifies appearance.
Cost Reference and Ordering (2026, EXW Shanghai)
Overlay work is quoted by area, layer count and inspection scope rather than by weight, so the material prices below are given for the consumables and for the alternative solid and clad products that a buyer may compare against. The comparison that matters commercially is not the price per kilogram of the overlay material but the price of the finished wetted surface.
| Item | Form or scope | Reference, 2026, EXW Shanghai | Note |
|---|---|---|---|
| Solid Inconel 625 plate | 3–20 mm | USD 40–62/kg | the alternative to overlaying |
| Inconel 625 bar | 20–100 mm | USD 42–68/kg | for components machined from solid |
| ERNiCrMo-3 welding wire | 1.2 mm and 1.6 mm spools | USD 48–78/kg | AWS A5.14, certified, per spool |
| ENiCrMo-3 covered electrode | 3.2 mm and 4.0 mm | USD 55–90/kg | AWS A5.11 |
| 625 strip electrode for cladding | 60 mm width | quotation by coil | used with a matching flux |
| Clad plate, 625 on carbon steel | 3 mm cladding, roll bonded | USD 22–40/kg of composite | ASTM A265 route |
| Overlay service | per square metre, two layers | quotation by area, layer count and inspection | deposition rate and inspection scope govern |
| Solid 316L plate, for comparison | 3–20 mm | USD 4–9/kg | shows the cost of the corrosion requirement |
Table note: Material figures are reference ranges only, 2026, EXW Shanghai, USD/kg, subject to the LME nickel price and to the molybdenum and niobium markets, and they are not a quotation; overlay work is quoted by area, layer count, process and inspection scope, and a price per kilogram for overlay is not a meaningful measure because it omits the deposition rate and the inspection. The commercial comparison to make is between the installed cost of a clad or overlayed surface and the installed cost of solid alloy, including the inspection required by each route. Our Inconel range covers the solid alloy, the filler metals and the overlay service, and our price benchmark article puts the material figures in context.
The ordering point that follows from this is to specify the inspection scope with the overlay rather than after it, because the inspection is a substantial part of the cost of the finished surface and a scope that is agreed late is a scope that is either overpriced or inadequate. The scope should state the extents of the overlay, the minimum thickness after machining, the chemistry requirement with its depth, the examination methods with their acceptance criteria, and the point in the fabrication sequence at which each is performed. A quotation that states all of these can be compared with another quotation on the same basis; a quotation that says only "625 overlay" cannot.
Standard Index
| Standard | Title / scope | Covers | Form |
|---|---|---|---|
| ASME BPVC Section IX | Welding, brazing and fusing qualifications | procedure and performance qualification | all forms |
| ASME BPVC Section IX QW-453 | Corrosion-resistant overlay welding qualification | overlay specific requirements | overlay |
| ASME BPVC Section II Part C | Specifications for welding rods, electrodes and filler metals | filler metal requirements | consumables |
| ASME BPVC Section V | Nondestructive examination | examination methods and acceptance | all forms |
| ASME BPVC Section VIII | Rules for construction of pressure vessels | design and fabrication requirements | pressure vessels |
| ASME B31.3 | Process piping | fabrication and examination requirements | piping |
| AWS A5.14 | Nickel and nickel alloy bare welding electrodes and rods | filler classification | wire |
| AWS A5.11 | Nickel and nickel alloy covered welding electrodes | filler classification | electrode |
| ASTM A265 | Nickel and nickel-base alloy clad steel plate | clad plate requirements | clad plate |
| ISO 15614-1 | Specification and qualification of welding procedures for metallic materials | procedure qualification | all forms |
| API RP 582 | Welding guidelines for the chemical, oil and gas industries | good practice guidance | fabrication |
| NACE MR0175 / ISO 15156 | Materials for use in H2S-containing environments in oil and gas production | hardness and material requirements | all forms |
| ASTM E1476 | Metals identification, grade verification and sorting | PMI practice | all forms |
| ASTM E572 | Analysis of stainless steel and nickel alloys by X-ray spectrometry | chemistry verification | overlay, bar, plate |
| ASTM G48 | Pitting and crevice corrosion resistance in ferric chloride solution | corrosion testing | test method |
| ASTM E3 / E112 | Metallographic specimen preparation and grain size determination | metallography | test method |
| ASTM A956 | Leeb hardness testing of steel products | hardness testing | test method |
| EN 10204 | Metallic products — types of inspection documents | inspection documents | all forms |
Table note: Standards are listed by number and scope; where an edition year is not quoted the latest edition applies, and the controlling edition is the one named on the purchase order. The distinction between a qualification standard, a filler metal specification and a product specification should be kept clear in an overlay requirement, because each answers a different question: qualification standards define how the procedure is proven, filler metal specifications define what is consumed, and product specifications define the chemistry and properties of the deposited alloy.
FAQ
Q1: What is an Inconel 625 weld overlay used for?
It is used to put a corrosion-resistant surface on a structural base metal, most commonly carbon steel or low-alloy steel, where making the whole component from solid alloy would be uneconomic. Typical applications are the wetted surfaces of pressure vessels in refining and petrochemical service, hydroprocessing and hydrocracking reactors, nozzles and their internal bores, valve bodies and seats, heat exchanger channel heads, and repairs to components that have lost their original protection. In these applications the base metal carries pressure and structural loads while the overlay provides resistance to chlorides, to aqueous sour service and to a wide range of oxidising and reducing process streams. The alternative routes are solid alloy, which is used for small components, and clad plate produced by roll or explosion bonding to ASTM A265, which is used for large flat areas where the mill can supply the composite. Our Inconel range covers the solid alloy, the filler metals and the overlay service.
Q2: Why does dilution matter so much in an overlay?
Because dilution determines how much base metal iron enters the deposit, and iron is the element that most reduces the corrosion resistance of a nickel-chromium-molybdenum alloy. The first layer of any fusion overlay is a mixture of filler metal and melted base metal, and in a carbon steel component that mixture can contain 15 to 30 % iron depending on the process and the parameters, compared with the fraction of a percent in the solid alloy. Iron dilutes the chromium, molybdenum and niobium contents and reduces the passivity and the pitting resistance of the deposit, and the effect increases with the severity of the service. Dilution is not a defect; it is an unavoidable consequence of fusion, and the engineering response is to control it through process choice and procedure, to limit its effect by using more than one layer, and to verify the chemistry of the layer that will see the process.
Q3: How many layers should an overlay have?
Two layers is the standard convention for wetted surfaces, and it exists because the first layer is diluted and the second layer is deposited on a surface that is already largely nickel alloy. The second layer therefore contains much less iron than the first and provides the chemistry that the service requires, and the first layer serves as a bonding layer between the base metal and the alloy deposit. Where the service is severe, requiring three layers or requiring a specified thickness of material that meets a chemistry requirement is more protective than requiring a number of layers, because a chemistry-based requirement describes the outcome while a layer count describes only the method. Where the service is mild, a single layer may be adequate, but the chemistry should then be verified at the depth that will be exposed rather than assumed.
Q4: Which welding process should be specified for overlay?
The process follows from the geometry, the required chemistry and the schedule. Hot wire gas tungsten arc welding and strip cladding cover most large surface work: hot wire offers better control and lower dilution on complex geometry, while strip cladding offers a much higher deposition rate on large cylindrical surfaces at the cost of a slightly more dilute deposit. Spray transfer gas metal arc welding is used for flat and horizontal surfaces at high productivity, and submerged arc with wire or strip is used for large areas where rate dominates. Laser and plasma transferred arc cladding with powder are used where dilution must be minimal or where the component is small and distortion must be controlled, such as valve trim and sealing surfaces. Covered electrode welding is normally reserved for repairs and small local areas. Whatever is chosen, the procedure must be qualified with that process and those parameters.
Q5: Does a qualified butt weld procedure qualify an overlay?
No. Corrosion-resistant overlay welding is treated as a separate qualification with its own essential variables and test requirements, and a butt-weld procedure qualification does not cover it. Among the variables that exist for overlay and have no equivalent in a butt weld are the number of layers and the thickness of the deposit, and both change the chemistry of the layer that governs the corrosion performance. The qualification must also cover the positions actually used and the base metal group, and it must demonstrate the deposit chemistry at the defined depth as well as the soundness of the bond. Where a fabricator offers an existing procedure for a new application, the comparison should be made against the overlay variables rather than against the base metal and the filler metal alone.
Q6: Why is post-weld heat treatment a problem for a 625 overlay?
Because the temperatures required to stress relieve carbon and low-alloy steel, generally in the region of 620 to 690 °C, are also in the range in which nickel-chromium-molybdenum alloys can precipitate secondary phases that reduce their corrosion resistance and ductility. The requirement comes from the base metal and the constraint falls on the overlay, so the two have to be reconciled deliberately. The practical approaches are to select a base metal and thickness that allow stress relief at the lower end of the permitted range, to limit the time at temperature, to apply the overlay after the base metal has been heat treated where the fabrication sequence permits, and to require the overlay chemistry and any corrosion test to be verified after the final heat treatment rather than in the as-welded condition. The last point is the one most often missed.
Q7: How is the chemistry of an overlay verified?
By taking a sample at a defined depth and analysing it, with the depth stated in the specification. In practice this is done either by welding a coupon attached to the component as part of the production sequence and taking drillings or a chemistry slug from it, or by removing a sample from a location on the component where the removal is acceptable and repairing it afterwards. The analysis is performed to ASTM E572 or an equivalent method, and it is supplemented by PMI to ASTM E1476 for the elements that portable instruments resolve, principally iron, chromium, molybdenum and niobium. An analysis taken only at the finished surface describes the top layer and can pass while the layer adjacent to the base metal is heavily diluted, which is why the depth requirement is essential; our inspection practice describes how this is carried out.
Q8: What inspection should be required on an overlay?
At minimum, visual and dimensional verification of coverage, a thickness measurement after final machining, and a chemical verification at the specified depth. Beyond those, liquid penetrant examination detects surface-breaking defects and is normally required where the code or the project specification calls for it, and ultrasonic examination of the bond detects lack of fusion or disbonding that surface examination cannot see. Where the service is sour, a hardness survey of the base metal heat-affected zone is required by the applicable material specification, and where the overlay is relied on for corrosion resistance in a demanding service, a corrosion test to an agreed criterion is the check that verifies the property the overlay exists to provide. The scope should be agreed at enquiry stage so that it is priced with the work.
Q9: What causes high iron content in a deposit?
Excessive dilution, which usually means a combination of parameters rather than a single error: current too high, travel speed too low, electrode angle that directs the arc into the base metal, too few layers, or a process chosen for deposition rate when the geometry called for something tighter. Base metal chemistry also contributes, because a low-alloy steel contributes more alloying elements than a plain carbon steel at the same dilution. The remedy is procedural and it is established during qualification: the procedure should be qualified at the upper end of the permitted parameter ranges so that the iron content measured on the qualification test piece represents the worst case that production can produce, and production should then be verified against that qualification.
Q10: How do I compare an overlay quotation with a clad plate price?
On the basis of the finished wetted surface, not the material weight. Clad plate is priced per kilogram of composite and the cladding thickness is fixed by the production method, while overlay work is priced per unit area and depends on the number of layers, the process and the inspection scope. The comparison should include the fabrication consequences: clad plate requires welding of the cladding at every joint with matching filler and the associated examination, while an overlay applied after fabrication avoids cladding weld preparation but consumes more expensive filler metal and time. For large flat surfaces clad plate is usually cheaper; for nozzles, complex geometry, repairs and small areas overlay is the only practical route. Our price benchmark gives reference material figures for the comparison.
Q11: What documentation should come with an overlayed component?
The procedure qualification record for the overlay, the welder performance qualification for the operators who performed it, the filler metal certificates with heat numbers traceable to the consumables used, the parameters actually used for each area, the heat treatment records showing the cycles applied to the base metal and the overlay, the results of the chemistry verification with the depth at which the sample was taken, the thickness measurements after machining, the results of the nondestructive examinations with the acceptance criteria applied, and the inspection document to EN 10204 stating the conformity of these results. A fabrication that cannot produce the chemistry result at the specified depth has not demonstrated the property the customer purchased. We supply the certificates for the material and the consumables, and we apply the same documentation requirement to the overlay work before a component is released.
Q12: Can you supply the overlay work as well as the material?
Yes. We supply solid Inconel 625 in bar, plate, sheet, tube and pipe, the filler metals ERNiCrMo-3 to AWS A5.14 and ENiCrMo-3 to AWS A5.11 with certificates traceable to the heat, and clad plate to the ASTM A265 route, and we can quote overlay work by area with a stated layer count, process and inspection scope where the geometry is suitable for the processes described in this article. Where a buyer is comparing routes, we will set out the installed cost of each with the inspection included, so that the comparison is between finished surfaces rather than between material prices. Send your drawing, the service conditions and the inspection requirement through our contact page and we will confirm the route and the scope.
Conclusion and Selection Rules
An Inconel 625 weld overlay is a specified product rather than a generic service, and the specification has to define the base metal, the filler metal, the process, the number of layers, the minimum thickness after machining, and the chemistry at a defined depth. Of these, the chemistry requirement with its depth is the one that converts the overlay from a process into a corrosion-resistant surface, because it is the only one that describes the material that will see the process. Everything else is a means of achieving it.
Three rules are worth applying to every overlay order. Specify the chemistry at the depth that matters and verify it after the final heat treatment, because the acceptance of an overlay in the as-welded condition says nothing about the deposit that enters service. Qualify the procedure as an overlay rather than relying on a butt-weld qualification, because the variables that govern dilution are not represented in a joint qualification. And decide the heat treatment sequence at the design stage, because the conflict between base metal stress relief and overlay phase stability cannot be resolved at the fabrication stage without cost.
Shanghai Hangbo Alloy Group Co., Ltd. supplies Inconel 625 in bar, plate, sheet, tube, pipe and wire, filler metals to AWS A5.14 and AWS A5.11 with heat traceability, and overlay and cladding work with qualified procedures, chemistry verification at depth by ASTM E572 and PMI to ASTM E1476, hardness surveys, liquid penetrant and ultrasonic examination, corrosion testing to an agreed criterion, and third party inspection by SGS, BV or TUV. Send your drawing requirements through our contact page and we will confirm the overlay scope, the inspection requirement and the acceptance criteria before quoting.
Contact & Complete Product Range
Shanghai Hangbo Alloy Group Co., Ltd. - China Supplier
Email: sales@hangboalloy.com | WhatsApp (Lisa): +86 13611656360 | www.nickel-alloy.com
All Grades
Ni: NI200-N6 | Monel: 400-502, K-500, R-405 | Inconel: 600-740H, X-750, 725 | Incoloy: 800-27-7MO | Hastelloy: C-276-HYBRID-BC1, N | Super Austenitic: 904L-AL-6XN | Chromium-Rich: Alloy 33 | High-Strength Stainless: Nitronic 50/60, PH13-8Mo | Chlorination: Alloy 45 | Nimonic: 75-263 | Haynes: 25-HR-160 | RA: 330/333 | Precision: 1J22-4J45 | High-Strength: 18Ni250-AerMet100 | Heating: Cr20Ni80/60 | Specialty: Alloy 20-602CA
Product Forms: Bars(6-500mm) | Tubes(OD 6-610mm) | Plates(0.1-100mm) | Forgings | Welded Pipes | Fittings | Wire(0.05-10mm) | Strip(0.02-4.0mm)
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