By Harry W. Ebert
When discussing Ni alloy filler metals frequently used for piping and pressure vessels by refineries, chemical plants and power plants, one must start with dissimilar joints, such as welding 300 series austenitic stainless steels, to carbon and low alloy steels and also consider weld overlay operations. Initially type E310 (~25%Cr-20%Ni) electrodes were used for such applications. They were easy to use, had weldor appeal and the amount of dilution was not very critical. However, many such welds failed in service since the inherent micro-fissuring of these fully austenitic deposits propagated into cracks when subjected to thermal stresses caused by the large differences of Coefficients of Thermal Expansion (CTEs). By replacing the E310 with E309 (~23%Cr-13%Ni), a stainless steel with some ferrite, the micro-fissuring problem was reduced or even eliminated. These joints are more dilution sensitive. Since they also retain the large differences in CTEs, users were concerned with high stresses and possible thermal fatigue along the ferritic to austenitic steel fusion line when the weld was subjected to a heat treating operation and/or to high temperature (>320ºC [>600ºF]) service.
The introduction of 600 Series Ni-alloy filler metals (~72%Ni, 15%Cr & 8%Fe), which are also known as INCONEL® has a CTE about halfway between ferritic and austenitic steels. This reduces the thermal stresses by dividing them between two fusion lines. They are also less sensitive to dilution problems and micro-fissuring. In the discussion of these Ni-alloys, trade name designations and AWS/ASME Classification have been used. When the latter start with an “E,” they refer to coated electrodes for the SMAW process; when they start with “ER,” they refer to rods and bare wire used for inert gas and submerged arc welding processes.
The use of Inconel filler materials started with INCO-WELD-A® (ENiCrFe-2) and INCO-ROD-A®, now called INCONEL® 92 (ERNiCrFe-6). While both of these materials fulfilled their intended objective, both presented some new problems. The coated electrode had little weldor appeal since its puddle was not very easy to control; this has since been controlled by a modification of the secondary chemicals and the introduction of INCONEL® 182 (ENiCrFe-3). The composition of the INCONEL® 92 bare wire made the deposit subject to age hardening when exposed to heat treatment or service temperatures ≥700ºC [1300ºF], which increased strength but decreased ductility. For most applications, this wire has been replaced by INCONEL® 82 (ERNiCr-3), which does not age harden.
Weld deposits containing high Ni to Cr ratios are more susceptible to sulfur corrosion when subjected to temperatures ≥370ºC [>700ºF]. This ratio and the risk of sulfur corrosion have been lowered by selecting alloys that contain more Cr and/or some Mo such as Alloy 671 with ~44% Cr (ERNiCr-4) and Alloy 625 with ~22%Cr & 9%Mo (ENiCrMo-3) and ERNiCrMo-3). However, at the present time, the Alloy 671 has only been AWS/ASME classified by a bare wire filler metal and the Alloy 625 filler metals should not be used for service at temperatures >540ºC [>1000ºF] since the deposits tend to embrittle with time. For applications up to 1000ºC [1830ºF], Alloy 617 with Co addition (ENiCrCoMo-1 & ERNiCrCoMo-1) has been developed.
Over 60 Ni-alloy filler metals have been classified by AWS/ASME and more are pending. Many are designed to meet specific or special requirements and do not usually apply to piping and pressure vessels. In addition to some of the 600 series alloys, two other types are of concern when dealing with refineries, chemical plants and utilities.
800 Series Ni-alloys, also known as INCOLOYs® (~33%Ni, 21%Cr & Fe), have a number of base metal applications. Since AWS/ASME has not classified a matching filler metal, these alloys are usually welded with one of the INCONELs®, which are quite compatible. However, in Europe a number of Alloy 800 type filler metals have been developed and are accredited by some regulatory agencies.
400 Series Ni-alloys, also known as MONELs® (~ 65%Ni & 30%Cu), are provided with matching filler metals. After years of development, we are now using the 7th composition (ENiCu-7 & ERNiCu-7) to weld Alloy 400 to itself and to steels and other nickel alloys. However, here we must provide a word of caution. One supplier uses the MONEL® for two quite different alloys. In addition to Ni-Cu alloy mentioned above, this supplier also uses it for a copper nickel alloy (~70%Cu & 30%Ni), which AWS/ASME classifies as ECuNi and ERCuNi. To prevent mix-ups, it is suggested the use of the applicable AWS/ASME classification, or renaming the filler metals “NiCu-MONEL” and “CuNi-MONEL.”