Buttering Technique for Dissimilar Metal Welds: Procedure, Filler Selection, and Applications

Buttering deposits a compatible weld layer onto one side of a dissimilar joint before making the actual joint weld. This creates a chemical buffer zone between two incompatible base metals, controls dilution, and allows separate PWHT of each side when the two base metals need different heat treatment temperatures. It’s the standard approach for dissimilar joints in pressure vessels, power plants, and petrochemical piping where dilution control and PWHT sequencing are mandatory.

The most common buttering application is depositing ERNiCr-3 (Inconel 82) or ERNiCrMo-3 (Inconel 625) onto the bevel face of carbon steel or low-alloy steel before joining it to stainless steel or nickel alloy piping.

Why Buttering Exists

Every dissimilar weld has a dilution problem. When you melt filler and base metal together, the base metal chemistry contaminates the weld deposit. On a dissimilar joint, both base metals contribute different chemistries, and the deposit sits somewhere in between. If the resulting chemistry falls into a crack-sensitive or corrosion-susceptible range, the joint fails.

Buttering solves this by breaking the joint into two steps:

1. Step 1 (Buttering): Deposit filler onto one base metal. The first layer has high dilution from that base metal. The second layer has much less dilution. By the time you’ve deposited two or three layers, the surface chemistry is essentially the same as undiluted filler metal.

2. Step 2 (Joint weld): Join the buttered surface to the other base metal. Now the joint weld is between the butter layer (which has controlled chemistry) and the other base metal. Dilution from the first base metal is eliminated because the butter layer acts as a barrier.

Standard Buttering Procedure

Step 1: Prepare the Base Metal

Machine or grind the bevel on the side to be buttered. Standard groove prep per the WPS (typically 30-37.5 degrees for a single V-groove). Clean the bevel face with acetone and a stainless steel brush.

Step 2: Deposit the Butter Layer

TIG weld the butter filler onto the entire bevel face in flat or horizontal position if possible. SMAW (stick) is also common for buttering, especially on thick sections.

First layer:

• Use the butter filler (typically ERNiCr-3 or ERNiCrMo-3)
• Run stringer beads across the bevel face
• This layer has 20-40% dilution from the base metal
• Bead thickness: approximately 1/16 to 3/32 inch per layer

Second layer:

• Same filler, stringer beads perpendicular to or at an angle to the first layer
• Dilution from the base metal drops to approximately 5-10%
• The surface chemistry is now close to the undiluted filler

Third layer (if specified):

• Brings surface chemistry to essentially pure filler composition
• Required by some critical specifications (nuclear, high-temperature service)

Step 3: PWHT the Buttered Side (If Required)

If the buttered base metal requires PWHT (for example, P91 chromoly at 1375-1425F, or P22 at 1250-1325F), do it now, before making the joint. The butter layer, being nickel-based, tolerates the PWHT temperature without degradation.

This is the key advantage of buttering: the carbon steel or chromoly side gets its required PWHT without exposing the stainless or nickel alloy on the other side to that temperature. Stainless steel exposed to 1375F for extended periods sensitizes (chromium carbide precipitation), which destroys corrosion resistance. Buttering eliminates this problem.

Step 4: Machine the Butter Surface

After PWHT (if applicable), machine or grind the butter layer back to the required bevel profile and dimensions. This removes any oxide scale from PWHT and provides a clean, flat surface for the joint weld fit-up.

Step 5: Fit Up and Weld the Joint

Align the buttered piece with the other base metal. The joint weld is now between the butter layer surface and the other base metal. Use the appropriate filler for this combination:

Step 6: Final PWHT (If Required)

If the specification requires a joint PWHT (lower temperature than the base metal PWHT), it can be done now. The buttered side has already received its full PWHT, so the joint PWHT is only for the stainless or nickel alloy side and the joint weld. Typical joint PWHT temperatures for dissimilar welds are much lower (or often not required at all) than the base metal PWHT.

Common Buttering Applications

Power Plant Dissimilar Metal Welds (DMWs)

The classic application. High-temperature headers and piping in fossil fuel and nuclear power plants often transition from P91 or P22 chromoly steel to 304H or 316H stainless steel. The chromoly side needs PWHT at 1250-1425F. The stainless side must not see those temperatures.

Solution: Butter the chromoly bevel with ERNiCr-3, PWHT the buttered assembly at the required chromoly temperature, machine the butter surface, then make the joint weld to the stainless steel. The nickel butter layer:

• Tolerates the chromoly PWHT without degradation
• Acts as a carbon migration barrier (slows carbon diffusion from chromoly into stainless during high-temp service)
• Creates a ductile interface that accommodates CTE mismatch

This DMW configuration has been in service at hundreds of power plants worldwide since the 1970s.

Pressure Vessel Nozzles

Pressure vessels made from carbon or low-alloy steel frequently have nozzles or piping connections in stainless steel or nickel alloy. The nozzle-to-shell weld is a dissimilar joint that often requires buttering because:

• The vessel shell may need PWHT per ASME Section VIII
• Nozzle alloy (stainless or nickel) doesn’t need or can’t tolerate that PWHT
• Buttering allows the shell PWHT before the nozzle is installed

Petrochemical Piping

Process piping per ASME B31.3 routinely transitions between carbon steel (process side) and stainless or nickel alloy (corrosion-resistant side). Buttering is specified when:

• The carbon steel side requires PWHT per the piping class
• The dissimilar joint is in high-temperature service where carbon migration is a concern
• Owner specifications require controlled dilution at the fusion line

Filler Selection for Buttering

ERNiCr-3 (Inconel 82) is the default butter filler for most applications. ERNiCrMo-3 (Inconel 625) is used when the service environment requires additional molybdenum for corrosion resistance (chloride service, reducing acids).

Technique for Depositing the Butter Layer

TIG Buttering

• DCEN, 100% argon, 18-22 CFH
• Stringer beads, maximum 2.5 times filler rod width
• Overlap each bead by 30-50% to ensure full coverage
• Run beads parallel to the long dimension of the bevel for uniform coverage
• Maintain interpass temperature below 300F (nickel filler)
• Two layers minimum; second layer beads offset from first layer to eliminate any thin spots

Stick Buttering

• DCEP, short arc length
• Small electrodes (3/32 or 1/8 inch) for the first layer to minimize dilution
• Can step up to 5/32 inch on subsequent layers
• Peel slag completely between every pass (nickel slag sticks harder than carbon steel slag)
• Same interpass and overlap requirements as TIG

Critical Control Points

Dilution of the first layer is the single most important variable. Lower dilution means the butter surface reaches pure filler chemistry faster (fewer layers needed). Control dilution by:

• Using low amperage on the first layer
• Fast travel speed (spreads heat without deep penetration)
• Small-diameter filler or electrode
• Aiming the arc at the previously deposited bead, not the base metal, wherever possible on subsequent layers

Preheat of the base metal before buttering depends on the base metal grade. Carbon steel and low-alloy steels that need preheat for same-metal welding also need it for buttering. P91 typically requires 400-500F preheat. Plain carbon steel may need none.

When Buttering Is Not Required

Buttering adds cost, time, and complexity. Skip it when:

• Both base metals can tolerate the same PWHT (or neither needs PWHT)
• The filler metal alone can manage dilution from both sides (e.g., ER309L on carbon-steel-to-stainless in mild service)
• The service temperature is low (below 600F) so carbon migration isn’t a concern
• The code doesn’t require it and the owner specification allows direct welding

For example, a carbon steel pipe welded to 316 stainless with ER309L filler in a 200F water service line doesn’t need buttering. The ER309L handles the dilution, no PWHT is required, and the temperature is too low for carbon migration to matter.

But a P91 header welded to 304H stainless in a 1050F steam line always needs buttering because of the PWHT mismatch, carbon migration risk, and code requirements.

Common Mistakes

Skipping the second butter layer. One layer of buttering has too much base metal dilution on the surface. The joint weld then fuses into a mixed-chemistry layer that may not have the intended corrosion resistance or ductility. Two layers minimum.

Wrong PWHT sequence. PWHT-ing both sides together at the chromoly temperature exposes the stainless to sensitization. Butter first, PWHT the buttered side alone, then make the joint weld.

Grinding through the butter layer during fit-up. After machining the butter to a flat bevel, don’t grind deeper during fit-up adjustments. You’ll expose the high-dilution first layer or even the base metal. Machine the butter with sufficient extra thickness (add 1/16 inch to the target profile) to allow for fit-up grinding.

Insufficient overlap between beads. Gaps in the butter coverage expose base metal at the surface, creating dilution paths in the joint weld. Overlap beads by at least 30% to ensure full coverage.

For the overview of dissimilar joint failure mechanisms and filler selection strategy, see the dissimilar metal welding guide. For bimetallic transition inserts used when fusion welding isn’t possible, see the transition joints guide.

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