The majority of problems encountered when welding nickel alloys arise as a result of the different characteristics of nickel alloys compared to CMn or stainless steels. The following comments apply to most of the commonly used nickel alloys. The fluidity of nickel alloys is low, causing a sluggish weld pool which doesn’t wet out easily. To overcome this the weld pool needs to be manipulated or weaved; to enable this to be carried out easily the joint preparation may need to be wider than for CMn or stainless steel welds (~80˚ in a single V). The weave should not be excessive (no more than three times core wire diameter) and there should be a pause at each side of the weave to prevent undercut.

Kiriman dari : jimoc

General Guidance on Welding Nickel Alloys

The majority of problems encountered when welding nickel alloys arise as a result of the different characteristics of nickel alloys compared to CMn or stainless steels. The following comments apply to most of the commonly used nickel alloys.

The fluidity of nickel alloys is low, causing a sluggish weld pool which doesn’t wet out easily. To overcome this the weld pool needs to be manipulated or weaved; to enable this to be carried out easily the joint preparation may need to be wider than for CMn or stainless steel welds (~80˚ in a single V). The weave should not be excessive (no more than three times core wire diameter) and there should be a pause at each side of the weave to prevent undercut.

The penetration achieved with nickel base alloys is also significantly lower than with CMn steel. The penetration can not be altered by increasing the current; this will only result in electrode overheating and eventually porosity. To ensure proper fusion the joint has to be designed appropriately ie. correct joint angle and thinner land or root face.

Hot craking is not a common problem in practice but it is generally recognised that nickel alloy welds are, to some extent, naturally susceptible. The extent of the susceptibility to hot cracking will depend on the situation (such as weld restraint, weld bead profile, alloy type and level of impurities). Solidification cracking is a visibly significant form of hot cracking, typically occurring along the weld centreline or in the weld pool crater. When such cracking occurs it is usually associated with a flat/concave weld profile and can normally be prevented by producing a larger weld bead with a more convex profile. It is also beneficial to ensure that craters are well-filled, and some authorities insist that weld craters are removed by grinding.

Base metal cleaning is very important on nickel base alloys for two reasons – firstly to ensure heavy oxides are removed, secondly to remove partial contaminants eg. S, P, Pb, Zn, Sn. The oxide needs to be removedbecause it wil not melt during welding owing to its very high melting point and can result in radiographic indications, removal is carried out by grinding, abrasive blasting or pickling. The second form of contaminant, which can be present in the form of machining lubricant, marking crayons, temperature indicating crayons, oil, grease, paint, etc, can be removed with appropriate solvents. More resistant paints etc may require proprietary cleaners. Care should be taken to ensure that contaminants are not introduced from grinding wheels, wire brushes or other tools.

Preheating of nickel alloys is not necessary, unless material is below the workshop air temperature, in which case preheating to air temperature will prevent the formation of condensation and potential porosity problems. The majority of standard nickel base alloys do not require post weld heat treatment (PWHT) for normal service conditions. Some specialiazed age-hardenable alloys will require PWHT and some alloys eg. UNS N02200, may require PWHT for optimum stress corrosion resistance in certain environments.

Heat input and interpass temperatures are not as critical as for some alloys but to ensure optimum as welded corrosion resistance those alloys (usually with high Cr + Mo) likely to see corrosive service should be welded with a maximum interpass temperature of 150˚C and maximum heat input of 1.5kj/mm.