Welding of Nickel Alloys

The welding of nickel alloys plays a vital role in many industries. This article outlines the general principles of welding nickel alloys, recommended welding processes and consumables, the effects of alloying elements on weldability, and the requirements for preparation and post-weld heat treatment.

General Description

Nickel alloys are generally welded in much the same way as stainless steels; however, they are ‘heavier,’ less fluid, and produce a weld pool with shallower penetration. For this reason, joint design must be carefully selected. In general, low heat input is recommended, achieved by using lower welding currents and slow movements. Wide passes are not recommended. Weld beads should be slightly convex, while flat or concave beads—which may be acceptable in stainless steel welding—should be avoided.

Methods

Nickel alloys are commonly welded using shielded metal arc welding (SMAW) as well as gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW/TIG). Good results can also be obtained with plasma arc welding (PAW), laser beam welding (LBW), electron beam welding (EBW), and resistance spot welding (RSW). Submerged arc welding (SAW) is generally not recommended because it introduces a large amount of heat into the base material, which promotes hot cracking and the formation of brittle secondary phases.

Heat affected zone HAZ

The heat-affected zone (HAZ) is the region of metal adjacent to the weld that has not melted, but whose properties and microstructure have been changed by the heat of the welding process. The difference in microstructure can be seen in the image below.

Weld and HAZ. Source: ASM Specialty Handbook, Nickel, Cobalt, and Their Alloys

HAZ Hot cracking is a common issue when welding nickel alloys. It is correlated with grain growth. To prevent it:

• choose a different grade,
• solution anneal before the welding to achieve small grain,
• limit heat input during welding.

For example, electron beam welding limits heat input and is a recommended method for welding difficult, age-hardened nickel alloys.

Surface Preparation

Surface. To prevent sulfur, phosphorus and lead compounds contamination related cracking, the surface must be throrougly cleaned before welding. When laying multi-pass welds, it is very important to remove oxides from the surface of each weld, as these usually melt at a much higher temperature than the nickel alloys themselves. It is also recommended to grind the start and end points of the welds.

It is recommended to weld alloys in an annealed state. Particular caution is advised when welding cold worked alloys.

the welding atmosphere should be protected from sulfur compounds, due to the harmful effects of sulfur.

Preheating is generally not necessary as long as the alloy is at room temperature. If the temperature is 2°C or lower, the weld area should be preheated to at least 16°C to prevent condensation. Moisture can cause porosity in the weld.

Welding products

• Alloy 200 / alloy 201: electrode ENi-1; filler metal ERNi-1;
• Alloy 400: electrode ENiCu-7; filler metal ERNiCu-7;
• Alloy 450, copper-nickel alloys with high Cu content: electrode ECuNi; filler metal ERCuNi;
• Alloy 600: electrode ENiCrFe-1, ENiCrFe-3, ENiCrFe-7; filler metal ERNiCr-3, ERNiCrFe-7;
• Alloy 601: electrode ENiCrFe-3; filler metal ERNiCr-3, ERNiCrFe-7;
• Alloy 625: electrode ENiCrMo-3; filler metal ERNiCrMo-3;
• Alloy 617: electrode ENiCrCoMo-1; filler metal ERNiCrCoMo-1;
• Alloy 800HT: electrode ENiCrCoMo-1; filler metal ERNiCrCoMo-1;
• Alloy 825: electrode ENiCrMo-3; filler metal ERNiCrMo-3;
• Alloy 686: electrode ENiCrMo-3; filler metal ERNiCrCoMo-(4 do 10);
• Alloy 622: electrode ENiCrMo-3; filler metal ERNiCrCoMo-(4 do 10);
• Alloy C-276: electrode ENiCrMo-3; filler metal ERNiCrCoMo-(4 do 10);
• Alloy G: electrode ENiCrMo-3, ENiCrMo-9; filler metal ERNiCrCoMo-(4 do 10);
• Alloy G-2: electrode ENiCrMo-3, ENiCrMo-9; filler metal ERNiCrCoMo-(4 do 10);
• Alloy 690: electrode ENiCrFe-3, ENiCrFe-7; filler metal ERNiCr-3, ERNiCrFe-7;
• Alloy 718: filler metal ERNiFeCr-2
• Alloy X-750: filler metal ERNiFeCr-2

Welding products for dissimilar-metal joints

Alloy 200 / Alloy 201:

• with stainless steel: electrode ENiCrFe-2; filler metal ERNiCr-3;
• with carbon / low alloy steel: electrode ENiCrFe-2; filler metal ERNiCr-3;
• with nickel steel5-9% Ni: electrode ERNiCrFe-2; filler metal ERNiCr-3;
• with copper: electrode ENiCu-7; filler metal ERNiCu-7;
• with copper-nickel: electrode ENiCu-7; filler metal ERNiCu-7;

Alloy 400:

• with stainless steel: electrode ENiCrFe-2; filler metal ERNiCr-3;
• with carbon / low alloy steel: electrode ENiCr-7; filler metal ERNi-1;
• with nickel steel5-9% Ni: electrode ENiCr-7; filler metal ERNi-1;
• with copper: electrode ENiCu-7; filler metal ERNiCu-7;
• with copper-nickel: electrode ENiCu-7; filler metal ERNiCu-7;

Alloy 600 / alloy 601 / alloy 690 / alloy 800:

• with stainless steel: electrode ENiCrFe-2; filler metal ERNiCr-3;
• with carbon / low alloy steel: electrode ENiCrFe-2; filler metal ERNiCr-3;
• with nickel steel5-9% Ni: electrode ENiCrFe-2; filler metal ERNiCr-3;
• with copper: electrode ENi-1; filler metal ERNi-1;
• with copper-nickel: electrode ENi-1; filler metal ERNi-1;
  (Due to the high chromium content, when welding alloy 690 with copper and copper-nickel, it is advisable to “coat” the copper part with nickel-based welding metal before welding.)

Alloy 825:

• with stainless steel: electrode ENiCrMo-3; filler metal ERNiCrMo-3;
• with carbon / low alloy steel: electrode ENiCrMo-3; filler metal ERNiCrMo-3;
• with nickel steel5-9% Ni: electrode WE 113; filler metal ERNiCrMo-3;
• with copper: electrode ENi-1; filler metal ERNi-1;
• with copper-nickel: electrode ENi-1; filler metal ERNi-1;

Alloying additions

Pure nickel is easy to weld, though it can produce porous welds.

Copper does not significantly affect the weldability of nickel alloys. Copper-nickel 50Ni-50Cu is welded in much the same way as pure nickel, and 30Ni-70Cu is welded in much the same way as copper.

Chromium reduces the tendency for weld porosity. 15%Cr in combination with 1%Si may increase the tendency for hot cracking. Ni-Cr alloys have a favorably narrow solidification range. Chromium also has a natural affinity for oxygen, nitrogen, and hydrogen. For this reason, protective atmospheres must be used when welding nickel alloys with chromium.

Iron up to 8% does not affect the weldability of nickel-chromium alloys. In alloys containing more than 40% Fe (e.g., alloy 800), iron increases the tendency for hot cracking.

Carbon significantly impairs weldability, especially in pure nickel. Therefore, pure nickel for high-temperature applications (e.g., alloy 201) should have no more than 0.02% C, and nickel for welding should additionally have titanium alloying additions. In nickel alloys, carbon does not impair weldability as much because it binds with copper, chromium, and titanium.

Manganese improves the weldability of nickel alloys. Nickel-based welding electrodes contain up to 9% manganese.

Magnesium protects against hot cracking in the heat-affected zone.

Silicon increases the tendency for hot cracking in the heat-affected zone, especially when its content exceeds 1%. Silicon has the worst effect on Ni-Cr alloys. 

Titanium and aluminum are strengthening additions. Aluminum also increases oxidation resistance. A higher content of these elements significantly exposes the alloy to hot cracking.

Boron is added in amounts of 0.003-0.100% to improve high-temperature properties. Above a concentration of 0.030%, boron significantly impairs weldability, increasing the chance of cracking in the weld and heat-affected zone.

Zirconium has a similar but slightly lesser effect on weldability than boron.

Sulfur is probably the most common cause of problems when welding nickel alloys. At temperatures above 316°C (for pure nickel) or 650°C (for Ni-Cr alloys), it forms harmful, crack-causing precipitates in both the weld zone and the heat-affected zone. Magnesium and manganese alloying additions limit the harmful effects of sulfur. In addition, during welding, extreme care must be taken not to contaminate the alloy with sulfur compounds.

Lead has a similar harmful effect to sulfur, but lead contamination is much less common than sulfur contamination since lead compounds are much less common in workshops.

Phosphorus has a similar effect to lead and sulfur, but the harmful effects of phosphorus begin at temperatures above 870°C, so the problem of cracking is less of an issue in areas affected by heat.

Post-Welding Heat Treatment

Post-Welding Heat Treatment mainly applies to precipitation-hardened nickel alloys (for hardening to be successful, the alloy should generally be in an annealed state. Welding precipitation-hardened alloys can cause strain-age cracking). In other nickel alloys, this is not usually required. Welded joints of nickel alloys with stainless steel should also be annealed. Here are some examples of nickel alloys that require heat treatment after welding.

Alloy 602 CA / 2.4633 - alloy with high aluminum content. Heat treatment after welding is recommended if the welded components are to be used at temperatures between 600 and 750°C. Superheating 1220 °C; air/water cooling + stabilization 950 °C; 3h; air cooling.

Alloy 80a / 2.4952 - heat treatment after welding improves strength properties and corrosion resistance. Treatment parameters for various products in the link.

Alloy 200 / Alloy 201 - heat treatment after welding is recommended if the weld is to be used in a corrosive alkaline environment. Anneal at 700°C for 30 minutes per 25mm sheet thickness, cool with air.

Alloy 600 / 2.4816 - heat treatment after welding is recommended if the weld is to be used in a corrosive environment with an alkaline reaction. Anneal at 900°C for 1 hour, cool with air.

Alloy 400 / 2.4360 - heat treatment after welding is recommended if the weld is to be used in a hydrofluoric acid environment. Anneal at 600°C for 30 minutes, cool with air.

Loss of Strength in the HAZ

Applies to: precipitation-hardened alloys welded in a hardened (aged) state.

Description: The effect of temperature dissolves the strengthening precipitates. The heat-affected zone ceases to be strengthened. In addition, cracks may appear.

Solution: Solution annealing after welding and re-aging.

Strain-Age Cracking

Applies to: precipitation-hardened alloys with a total aluminum and titanium content exceeding 6%, for example:

• Rene 41,
• Alloy 702,
• Alloy 700,
• Unitemp 1753,
• Udimet 500,
• Astroloy,
• Alloy 713,
• B-1900 Alloy
• Waspaloy

Description: During welding, metal passes through the aging temperature range (usually 600-750°C) twice - once during heating and once during cooling. Then, when heated for post-weld heat treatment, as the alloy again passes through the aging temperature, the resulting stresses lead to aging cracks.

Solutions:

• select a precipitation-hardened alloy with a lower Ti+Al content (e.g., alloy 718),
• heat treatment in a vacuum or protective atmosphere,
• welding in an aged state (especially helpful in the case of Rene 41),
• rapid heating during post-weld heat treatment.

In addition, such alloys should always be welded in a stress-relieved state and the amount of heat input should be limited.

Repair Welds

Nickel alloys do not change their chemical composition even after extended use and can be repair welded, particularly if the damage is localized. However, prolonged exposure to sulfiding, carburizing, or oxidizing environments can deteriorate the overall condition of the part to such an extent that repair welding options are limited. 

Welding of Castings

Many nickel alloys intended for machining have a casting equivalent. These alloys usually have a higher silicon content, which makes welding them more difficult.

Sources

Special Metals, Welding Nickel Alloys

Hastelloy International, Welding and Joining Guidelines

ASM Specialty Handbook, Nickel, Cobalt, and Their Alloys