Welding – General
Most alloys supplied by NeoNickel are high performance materials and when welded are for use in demanding environments.
As such they require welding with care and in a controlled manner to ensure that the properties of each are maintained for the particular use. The welding characteristics of the alloys are generally satisfactory as long as the correct techniques are used for each grade, ensuring both the metallurgical properties as well as the structural properties are attained. Each group of alloys will have slightly differing requirements and characteristics.
It is necessary to refer to the relevant producer’s safety instructions for each alloy to ensure that the alloys are welded in a safe manner. However most safety instructions for metals are similar and require care with, but not limited to, the use of electrical equipment, good ventilation and extraction when welding and that recommended Personal Protection Equipment (PPE) be worn at all times. Detailed information is available from various sources as well as producers.
Weld Procedure Qualification Test
It is often standard practice for a Weld Procedure Qualification test (WPQR) to be completed to show that an acceptable procedure is in place to produce quality production welds.
This is either required by the end customer or the applicable statutory or industrial code being used. There are a number of certifying authorities who can be used to approve the weld procedure if required and these guidelines will aid with the writing of a procedure and its qualification to satisfy these requirements.
As these alloys can be quite complex metallurgically then it is advisable to use an experienced fabricator for each alloy type as it is the fabricators responsibility to apply the guidelines effectively.
NeoNickel can help and propose some experienced fabricators – please contact us for information. Fabricators should also have a clean area separate from any carbon steel fabrication and that tools are not mixed.
It is essential that base materials are clean, free of oxide and preferably welded in the solution annealed condition as excessive cold work, greater than 7 – 10% can lead to cracking in the weld or HAZ in some alloys.
It is advisable to omit any crevices for corrosion resistant applications and so Butt welds should be full penetration to maintain strength and prevent crevices to obtain optimum corrosion performance. Fillet welds need not be full penetration as long as the required throat thickness is achieved and any crevices are avoided or are sealed.
It is advisable to obtain a good fit up with minimal misalignment and machined preparations and strong tacking can facilitate this.
Thin materials Plasma Arc, Carbon Arc are used then it is essential to grind back to clean metal. Any grease or other contaminant must be removed from the surface prior to welding. Any suitable cleaning method can be used but it is advisable to use chloride free solvent cleaning compounds especially with the stainless steels.
A suitable filler metal should be chosen for the grade of alloy, the process to be used and to be able to achieve the required properties in the weld zone for the intended use. Generally any welding process other than oxy-acetylene welding can be used with high alloy materials. The welding process should be chosen to give the best deposition rate that will also achieve the properties required. Consideration should be given to metallurgical or structural properties and distortion limits.
Generally preheat is not required, only to ensure freedom from any moisture in cold environments. Interpass temperatures are generally recommended with specific guidelines for each alloy and must be adhered to. Care should be taken if using marker crayons to ensure no contamination. Interpass temperature should be the maximum temperature in the weld zone.
Generally low to medium heat input ranges are used and again more specific instructions can be found in the alloy instructions.
The completed weld zone should be inspected in accordance with customer or code requirements and common NDE techniques such as visual, dimensional, surface penetrant, UT, radiography can be used with most alloys.
Post weld heat treatment may need to be considered for certain alloys in line with specific alloy guidelines. Any repairs should also use a qualified weld procedure, ensuring the defect is fully removed prior to welding and the completed weld is re inspected in line with specification requirements.
Post weld cleaning to remove any oxide scale is extremely beneficial for corrosion resistant alloys and if possible carried out. Suitable methods are immersion pickling, pickling paste or light dressing taking care not to produce crevices with heavy rough grinding.
Specific guidelines for each alloy are available in the specific section.