Dissimilar Metal Welds and Nickel Alloy Filler Selection for Pipe Fittings
When carbon or low-alloy steel piping transitions to stainless steel or nickel alloy — at equipment nozzles, heat exchanger connections, or material change points — a dissimilar metal weld (DMW) is required. DMWs are among the most complex weld joints in process piping. The differential thermal expansion between the two parent metals, migration of carbon from the ferritic steel into the higher-alloy weld metal, and differing heat treatment requirements create failure mechanisms that don't exist in same-material welds. This guide covers buttering and transition joint configurations, then maps the nickel alloy filler families — ERNiCrMo, ERNiCr/ERNiCrFe and ERNiMo — to their base metals and explains how to state the filler correctly on the WPS and purchase order.
The Carbon Migration Problem
At elevated temperature, carbon diffuses from carbon and low-alloy steel (P11, P22) into adjacent higher-alloy weld metal. The rate of migration depends on temperature and time, becoming significant above about 400°C in long-term service. Carbon depletion on the ferritic side of the fusion line creates a soft, creep-weak carbon-denuded band; carbon enrichment on the austenitic side can lead to carbide precipitation and embrittlement. A stainless filler such as ER309L is acceptable for low- and moderate-temperature DMWs, but for elevated-temperature service the standard solution is a nickel-base filler — ERNiCr-3 (FM82) for GTAW or ENiCrFe-3 (Inconel 182) covered electrode — either as the full weld or as a butter layer applied to the low-alloy steel side before welding. The high-nickel deposit sharply reduces carbon diffusion across the interface, and its coefficient of thermal expansion lies between those of ferritic and austenitic steel.
Standard Transition Joint Configurations
| Transition | Butter Layer | Fill/Cap Filler | PWHT |
|---|---|---|---|
| Carbon steel → 316L (low/moderate temperature) | Normally none | ER309L / E309L; ERNiCr-3 for elevated temperature | Not normally; if the carbon steel needs PWHT for thickness, butter and PWHT the carbon steel side first |
| P11 / P22 → 316L | ERNiCr-3 / ENiCrFe-3 on CrMo side | ERNiCr-3 (ERNiCrMo-3 also used) | Yes — PWHT the buttered CrMo side; no PWHT of the closing weld to SS |
| P91 → austenitic stainless | Nickel-base butter on P91 (ERNiCr-3 or EPRI P87-type) | Same nickel-base filler | Yes — full P91 PWHT after buttering |
| Carbon steel → Inconel 625 | Direct, no butter needed | ERNiCrMo-3 | PWHT if required for the carbon steel thickness; verify with WPS |
Thermal Expansion Mismatch
Carbon steel (CTE ~12–13 µm/m·°C) and austenitic stainless (CTE ~16–17 µm/m·°C) expand at different rates under thermal cycling. Over time, the cyclic stress at the DMW interface fatigue-damages the joint — this is why DMWs in steam and process lines on cycling duty (daily start/stop, batch operation) must be located carefully. Nickel-base weld metal, with an intermediate CTE, places the steepest expansion step at the stronger austenitic side rather than at the ferritic fusion line. Placing the DMW at a low-stress location in the piping layout (away from bends and restraints) and, for severe cycling, using a nickel-alloy transition spool reduces the concentration of differential expansion stress.
Nickel Alloy Filler Designations: How to Read Them
Nickel alloy filler metals are not interchangeable with carbon or stainless steel fillers — and in many cases they are not interchangeable with each other. Using the wrong filler on a Hastelloy C-276 joint can give a weld deposit with lower corrosion resistance than the base metal; using a Ni-Cr-Fe filler where a Ni-Cr-Mo filler is required can leave the weld as the weak point at service temperature or in the corrosive medium. Bare wires and rods are classified to AWS A5.14 (prefix ER, e.g. ERNiCrMo-3) and covered electrodes to AWS A5.11 (prefix E, e.g. ENiCrFe-3). The letters give the main alloying elements and the number distinguishes compositions within that family.
The ERNiCrMo Family
Ni-Cr-Mo fillers are the dominant family for corrosion-resistant nickel alloy welding and for overmatching superaustenitic and 6Mo stainless welds:
| AWS Class | UNS | Base Metal Application | Key Notes |
|---|---|---|---|
| ERNiCrMo-3 | N06625 | Inconel 625, Incoloy 825, 6Mo and 904L stainless, overlay, dissimilar welds | Most widely used Ni-alloy filler; also the common overlay filler. |
| ERNiCrMo-4 | N10276 | Hastelloy C-276 | Do NOT substitute ERNiCrMo-3 — its much lower Mo content reduces corrosion resistance in C-276 service. |
| ERNiCrMo-7 | N06455 | Hastelloy C-4 | Matching filler for the thermally stable C-4 base metal. |
| ERNiCrMo-10 | N06022 | Hastelloy C-22 | Higher Cr than C-276 filler — correct for C-22 base metal; also used to overmatch C-276 in oxidising media. |
| ERNiCrMo-13 | N06059 | Alloy 59 (2.4605) | Very high Cr + Mo — for the most severe mixed acid service. |
| ERNiCrMo-17 | N06200 | Hastelloy C-2000 | Contains Cu — matching filler for C-2000 base metal. |
The ERNiCr and ERNiCrFe Families
Ni-Cr and Ni-Cr-Fe fillers are used for Ni-Cr-Fe base metals (the Inconel 600/601/690 and Incoloy 800 family) and, above all, for dissimilar welds between ferritic steels and austenitic stainless or nickel alloys. The key members:
- ERNiCr-3 (N06082, FM82): Inconel 600, Incoloy 800/800H, and the standard GTAW/GMAW filler and butter for CrMo-to-stainless DMWs.
- ENiCrFe-3 (Inconel 182, covered electrode): the SMAW counterpart, widely used for DMW buttering on the P11/P22 side when joining to 316L.
- ERNiCrFe-7 (N06052, FM52): Inconel 690 (nuclear steam generator and primary circuit work). Susceptible to ductility dip cracking in thick, highly restrained sections; the modified ERNiCrFe-7A (FM52M) is preferred for heavy wall.
- ERNiCrFe-11 (N06601): Inconel 601 — contains Al for oxide scale retention in oxidising high-temperature service.
The ERNiMo Family (B-Family Alloys)
- ERNiMo-7 (N10665): Hastelloy B-2 — largely superseded for new work.
- ERNiMo-10 (N10675): Hastelloy B-3 — the usual filler for new B-family fabrications in hydrochloric and other strongly reducing acid service; used in the as-welded condition.
Ni-Mo fillers have very poor resistance to oxidising conditions and must never be used as a "universal" nickel filler or on dissimilar joints that will see oxidising media.
Procurement — Transition Fittings
For planned dissimilar metal transitions, a factory-buttered fitting is the best approach: the fitting (e.g. a WP22 elbow) is supplied with its EN 10204 3.1 certificate and a pre-applied nickel-base butter layer on the transition end, with the butter PWHT already performed. The field weld then joins butter-to-stainless with the nickel-base filler, with no PWHT required at the field joint. This eliminates field PWHT at the DMW joint — which is difficult to control and document. Specify on the PO: "Transition end to be buttered with ERNiCr-3 / ENiCrFe-3, PWHT per WPS prior to supply — include WPS, PQR, and butter layer certificate."
Stating Filler Metal on the WPS and PO
The WPS must state the AWS classification and the filler UNS number — not just a trade name — and ASME Section IX F-number grouping does not make nickel fillers interchangeable for corrosion purposes. On the PO for shop-fabricated fitting assemblies or buttered fittings, state the filler explicitly, for example: "Filler metal to be ERNiCrMo-[X] per AWS A5.14, UNS [N0XXXX]. Certificate of conformance required." For field welds on critical service, the filler wire heat certificate must be held in the ITP documentation package linked to the fitting heat number, and PMI of the completed weld is a simple check that the correct family was used.