Blog / Nickel
23 November 2020 | Nickel

WELDING NICKEL ALLOYS

General

Nickel and its alloys are renowned for their high corrosion resistance and their ability to withstand high temperatures (up to 1’200°C (2’200°F) for some alloys). In its commercially pure form, nickel is primarily used in the manufacture of caustic soda, in the electronics and food industries, and for plating.

Nickel-copper alloys, such as Monel®, provide excellent cavitation and corrosion resistance in reducing environments and salt water. This makes them especially suited for marine applications, chemical industry equipment, hydrocarbon processing, heat exchangers and plumbing hardware and fittings, since they are easy to form and machine.

Nickel-chromium-iron alloys, such as Inconel®, are mostly known for their high temperature and good corrosion resistance. The addition of chromium enhances nickel’s corrosion resistance in oxidizing environments. Typical applications include furnace parts and baskets and turbine parts, and they are used extensively in the chemical, petrochemical and nuclear industries.

Finally, more recently developed nickel-chromium-molybdenum alloys provide particularly high corrosion resistance, specifically with respect to local pitting and crevice corrosion. These are mainly used in the chemical industry, marine applications, pollution treatment equipment and the pulp and paper industry.

Physical properties of nickel and its alloys

The physical properties of nickel and its alloys differ somewhat from those of mild or stainless steel.

Thermal conductivity

Nickel itself has a higher thermal conductivity than steel, but the addition of copper, chromium, iron or molybdenum reduces this value, and most nickel alloys end up with a lower conductivity than steel. On the other hand, nickel-copper alloys conduct heat better than stainless steel, but not as well as mild steel.

Electrical conductivity

The electrical conductivity of nickel and its alloys varies in a similar manner to their thermal conductivity.

Coefficient of thermal expansion

The coefficient of thermal expansion and therefore the amount of expansion and contraction of nickel and its alloys during welding, is somewhere between that of mild steel and stainless steel.

Weldability of nickel alloys

Surface preparation

The presence of impurities such as sulfure, phosphorous, lead, tin, zinc, bismuth, antimony and a few other contaminants, drastically increases the tendency of nickel and its alloys to hot-cracking. It is therefore very important to clean the weld surface well with the appropriate solvents to eliminate any trace of grease, oil, cutting or machining oil, pencil marks, paint, corrosion products, liquid penetrant inspection solution, temperature indicating crayon marks, marks left by lead or brass adjusting hammers, etc.

If the piece does not have to be reheated after welding, it is sufficient to clean 2-3 inches (50-75 mm) along each side of the joint. In addition, unlike steel, the nickel oxide that forms on the surface of the metal melts at a much higher temperature than the base metal and is prone to form inclusions in the welded joint if not removed. This oxide forms a very tough film that cannot be removed with metal brushes – even mechanically driven ones. The surface must either be ground, machined or chemically stripped.

Joint preparation

For plate less than 3/32 inch (2.5 mm) thick, no chamfering is required, but when plate thickness exceeds 3/32 inches (2.5 mm), a ”V”, ”U”, or ”J” groove is needed. In either case, the use of a support plate or a support bead is recommended. For thickness greater than 3/8 inch (10 mm), it is better to make a ”V”, ”U” or ”J” groove on both sides of each plate to minimize the amount of filler metal needed.

Given the low fluidity and penetration of welds made with nickel or its alloys, chamfer angles should be about 30% more open than for steel, and the joint land root face should be narrower to ensure thorough penetration.

Preheating

Generally, it is not necessary to preheat nickel or its alloys provided that the pieces are at room temperature, 60 to 68°F (15-20°C). Interpass temperatures should normally not exceed 210°F (100°C), but this is not as critical as it is for steel, and temperatures of up to 575°F (300°C) have already been used in some applications with no subsequent problems. Clean air can be used to lower the temperature, or the piece can be midly quenched in clean water. Whichever method is used, it is important not to introduce contaminants into the heated area (like oil that may be present in air lines).

Welding method

Nickel and its alloys produce a weld metal that is less fluid than steel and which, as a result, does not wet the joint surfaces as readily. Weld metal fluidity decreases as the percentage of nickel in the alloy increases. The electrode must therefore be manipulated in a way that will deposit the weld metal right where it is needed. Using weaving technique whose extent should not exceed two to three times the electrode diameter, and pausing at the end of each cycle usually produces good results and prevents undercuts.

Weld penetration for nickel and its alloys may be as much as 50% lower than that of steel. Penetration depth should not be increased by using excessive current since doing so could result in a loss of alloy elements, especially those that help remove gasses from the weld bead.

When welding joints in flat position, hold the electrode at an angle of 20° from the vertical (see nickel 1 diagram). Doing so increases the deposition rate, helps prevent slag inclusions, and facilitates penetration, thereby providing better control over the weld pool and arc. The arc must be kept short at all times. For vertical and overhead welds, a lower welding current should be used compared to flat welds. On verticals, the electrode should form a right angle with the weld piece. When welding large plates, it is best to use a back step welding sequences (see nickel 2 diagram) to minimize distortions.

Sodel
Sodel

Many weld spatters occur when one or more of the following conditions exist :

-arc too long ;

-welding current too high ;

-incorrect polarity ;

-magnetic arc blow caused by incorrect ground clamp location;

-moisture in the electrode.

To eliminate the likelihood of electrode re-strike porosity, re-strike the arc in front of the crater at the end of the bead. Then return to the beginning of the crater slowly enough to allow time for the electrode gas shield to form, then move forward again, filling the crater with a slight weaving motion of the electrode. After passing the crater, the back-and-forth motion can be discontinued and the bead can be completed normally. Another technique is to strike the arc on top of the preceding bead about 1/2 to 1 inch (13 to 26 mm) ahead of the crater; this starting point can subsequently be ground off. This technique is used to obtain optimum x-ray inspection results.

Slag cleaning

Solidified slag can be removed with hand tools and a stainless-steel brush. When doing multipass welding, it is essential to remove all traces of slag on the weld before starting another pass. The presence of weld slag is very harmful to corrosion resistance when the piece is used at high temperatures.

Postheating

Generally, nickel and its alloys do not require annealing after welding except for the following reasons :

-to relieve residual stress ;

-to ensure better dimensional stability of the assembly ;

-to prevent stress corrosion of certain alloys, in environments containing hydrochloric acid or sodium hydroxide vapors, for example.

The temperatures required for post-weld annealing depend on the alloy :

-1’525 – 1’600°F (830 – 870°C) for nickel and nickel-copper alloys ;

– 925 – 1’175°F (1’700 – 2’150°C) for nickel-chromium-iron and nickel chromium-molybdenum alloys.

Using temperatures below those recommended by the manufacturer could cause carbide precipitation with consequent lowering of resistance to corrosion, creep, and impact, depending on the alloy, the temperature and time at temperature.

Filler metal

SODEL electrodes are designed to support welding with several combinations of dissimilar metals. Sodel 3500 is a nickel – chromium – iron alloy that can withstand the follwoing dilution rates :

  • 100% nickel ;
  • 15% copper ;
  • 20% chromium ;
  • 35% iron ;
  • 1.0% silicon.

Sodel 3501PC is a nickel – copper alloy that can be used for the following dilution rates :

  • 100% nickel ;
  • 100% copper ;
  • 8% chromium ;
  • 30% iron ;
  • 2% manganese ;
  • 1% silicon ;
  • 0.4% carbon.

When dilution limits are exceeded, the deposited metal will be more susceptible to hot-cracking, or less ductile, depending on the overdiluted element and filler product.

PRACTICAL TIPS FOR WELDING NICKEL ALLOYS

1-Remove the joint oxide layer with a grinder and then eliminate all traces of grease, oil or other hydrocarbons using a rag soaked in acetone. WARNING! Keep the acetone-soaked rag away from the weld area, the vapors given off by this product are very dangerous in the presence of ultraviolet radiation.

2-Use an electrode with as small a diameter as possible, and the lowest recommended current.

3-Avoid weaving motions greater than twice the electrode diameter.

4-Avoid making excessively deep craters at the end of the bead, if there are cracks within the crater, they should be removed with a grinder.

5-Interpass temperatures should not exceed 210°F (100°C) in general.

6-Always use perfectly dry electrodes

7-In multipass welding, the slag must be removed completely after each bead.