Welding of Nickel Alloys
Nickel alloys are usually chosen because of their excellent corrosion resistance or high-temperature properties and usually work in extreme environments. It is therefore important that the properties are maintained across the weld zone in order that the alloy performs adequately in service. In this article, we will discuss the welding of nickel alloys, for more information on welding high performance alloys, please click here.
As well as ensuring that properties are maintained across the weld zone, it is also important that the weld is of good quality with the minimum of flaws which could also affect performance in the chosen environment. It is desirable and many applications require the development of a weld procedure specification with qualification to ensure that a weld is fit for its intended service.
Alloying pure nickel
Pure Nickel is utilised but it is more often alloyed with other metals to increase the range of properties achievable. These can be split into two groups, the first of which are the solid solution strengthened alloys of the type: Nickel Copper or Nickel Iron Chromium. These alloys do not undergo any phase change and are not hardenable by heat treatment and are usually used in the as welded condition unless other requirements such as Stress Corrosion Cracking, dimensional stability or Creep etc. are a consideration.
The second group are the Precipitation Hardenable alloys usually containing a combination of Aluminium, Titanium or Niobium in addition to Nickel, Iron and Chromium. These alloys are more complex, having enhanced mechanical properties. It is important to choose the correct filler metal and a post weld heat treatment may be required to achieve matching properties within the weld zone. These alloys are best welded in the solution annealed or aged condition.
All fusion welded processes can be used but Nickel alloys do suffer from solidification or hot cracking and weld bead shape is extremely important with the deposit of small beads preferred. It is recommended to use modest heat inputs with stringer beads and minimal weaving to prevent cracking. A convex weld bead shape is preferable to a flat or concave bead.
The importance of cleanliness
Cleanliness is extremely important as contaminants such as Sulphur, Phosphorus, Lead, Bismuth or Boron which can be present in machining or cutting fluids, greases or oils, corrosion products, marker pens, thermal crayons etc. can lead to solidification cracking.
It is good practice to clean immediately before welding by degreasing the surface, removing any oxide from the weld zone and then degrease again to remove any further contamination from the cleaning process. It is difficult to remove any oxide scale as this is very adherent. It is best to remove it completely from the weld zone by machining or abrasive grinding. Wire brushing is ineffective and can polish the oxide rather than remove it. If not removed fully the oxide, which has a high melting point may not melt on subsequent welding and can be the cause of inclusions or lack of fusion. All tools must be suitable for use and used on Nickel alloys only.
Design of the weld preparation
Nickel alloys are very sluggish and so the design of the weld preparation is important to allow the welder good access for manipulation but not too wide to increase the risk of solidification cracking. A V-preparation with an angle of 70 – 80 ̊ is usually adequate but changing to a double V or U preparation with an angle of 30 – 40 ̊ at sections around 10 mm thick will help. An addition of 10% Hydrogen has been found to help to improve the fluidity of the weld pool. Root faces that are thinner than normal at typically 1.6mm are required due to the difficulty with penetration.
A preheat is not required unless moisture is a concern where a slight warming of the workpiece to remove any condensation is required. Most Nickel alloys usually use an Interpass temperature of 250 ̊C but some alloys such as C276 require 100 ̊C maximum.
Oxygen or Nitrogen from air entrapment or hydrogen from surface contamination can quickly cause porosity within the weld pool. Most consumables contain deoxidants such as Titanium, Aluminium or Niobium to prevent this but careful cleaning and an efficient gas purge and shielding are required to prevent problems with porosity. It is important to ensure that gas hoses are in good condition and there are no draughts that can disturb the gas shield. Grinding of stop starts or other suitable measures are required to prevent porosity at these positions.
Most Nickel alloys will not respond to higher currents for increased penetration, Helium will not be effective until higher levels, typically greater than 40% in the shielding gas.
Matching weld consumables are available for most alloys, but overmatching consumables can also be used to good effect.
Coefficient of expansion
The coefficient of expansion of nickel alloys is similar to that of carbon steel and so it is not as difficult to control distortion as with the austenitic stainless steels. Similar distortion levels to carbon steels are anticipated and so similar controls are effective.
Most applications will require some form of NDE and inspection of the completed joint to prove the integrity of the joint for the intended service. Most NDE is effective apart from Magnetic Particle inspection.
More detailed parameters and information for specific grades is available in the relevant weld consumable data sheet.
TWI Job Knowledge No 22: Weldability of Nickel and Nickel Alloys.
TWI Job Knowledge No 107: Welding of Nickel Alloys, Part 1.
TWI Job Knowledge No 107: Welding of Nickel Alloys, Part 2.
NIDI Reference Book, Series 11_012, 1994: Guidelines for the Fabrication of Nickel Alloys for Corrosion Resistant Service