John R. Worner of Pressform Engineering explains in the white paper, The Welding of Refractory Anchors, how the success or failure of refractory hardware depends on the weld integrity of the anchoring.
This white paper aims to demonstrate to business owners, designers, contractors and maintenance personnel the importance of focusing on the welding of their refractory hardware such as loop based anchors, hexagonal mesh, stud welded anchors, Rod-Lok bases or any form of refractory metal.
There are different types of welding processes including MIG, TIG, stick and stud.
MIG welding is one of the most popular welding processes, and preferred for its speed and efficiency. In this process, the welding consumable is a wire of 0.9mm to 2.00mm diameter. The wire comes off a coil and is fed down the torch held by the operator, achieving an arc to the metals to be welded, shielded by a welding gas.
TIG welding also has a wire as the weld consumable, but is handfed into the arc and weld pool, where a tungsten electrode arcs to the metal, shielded by argon gas.
Stick welding uses flux covered wire as the electrode, which creates the arc and weld pool. There is no shielding gas.
Stud welding relies on a high voltage/amperage power source conducted down the actual refractory stud, which forms an arc between the metal shell and the stud end, melting both sides, forming a weld pool, and simultaneously feeding the stud into the molten substrate.
Selection of the welding process, therefore, requires some thought. Key considerations include metals to be welded, their alloying elements such as carbon content, nickel, chromium and many relevant elements, the shell condition, its age, and identification of shell parent metal, its general condition, and history.
Identification of metals to be welded is the first step. Assuming they are mostly stainless anchors, how will they be identified? What grade are they: 304, 321, 309, 310, 253Ma, Inconel 600 or 601, Incoloy DS or 800 or even 800H? Are there traceability protocols in place? Are the anchors in containers that are inert to the stainless?
How are the containers marked? Is there a competent staff member checking that the correct welding consumable will be used with the anchors to be welded? Is there more than one grade to be welded in different areas?
What grade is the furnace shell? If it’s not a new installation, how old is it? What was its carbon content when new? Has it been subjected to carbon deposition and absorption from say Bunker Fuel firing? Is it a chrome/moly steel? Has it been subjected to temperatures above 225°C at any time? Was it designed to take higher temperatures for short periods? Are there any known hotspots? What does the furnace designer recommend when welding anchors for repairs and maintenance where there are temperature issues?
Whether it is a new furnace or a repair, the shell at the weld area must be freshly ground to shiny metal. The anchor must be free of dust, oil and any contaminants. As far as practicable, the atmosphere should also be clear of dust. An extraction unit should remove the particles put out by the grinding process into the air. If the shell is made from stainless steel, it must be ground with iron-free wheels.
Dissimilar metal welding
Most of the time you will be welding a stainless or nickel alloy refractory anchor to a carbon steel shell. It is imperative that the correct welding consumable is used.
Briefly, austenitic stainless steels such as 304, 321, 309, 310 and 253 Ma require a 309 welding consumable, which must also be molybdenum-free to avoid embrittling phase precipitation during high temperature service. A 309 welding wire often contains moly where it is to be used with marine grades such as 316. These grades are not suitable for high temperature use. If the wrong grade such as 304, 316 or 309Mo is used, a weld will be achieved but it will create a weld pool that will embrittle with time, and may only be 40% of the strength it should be. This is due to the dilution of nickel/chrome ratios in the weld pool, coming from the iron of the parent metal shell, with the resultant composition unable to form an austenitic structure.
The 309 welding consumable must be used to get the extra nickel/chrome contribution. Though the 310 grade can also be used for dissimilar welding, it can result in hot-cracking of the weld bead.
A popular choice, 253Ma’s metallurgy is based on 309, and has added rare earth metals such as Yttrium and Lanthanum, which give it its high temperature strength properties. When it was first made, the makers of 253Ma approved the use of 309. However, 253Ma is not immune to sigma phase embrittlement.
With nickel alloys such as 800, 600 and 601, there are many charts available for the various welding consumables that can be used. These alloys are readily weldable, but behave differently under the arc to austenitic and carbon steels. They have a higher viscosity in the weld pool, and require the services of an experienced welder.
Some of the more sophisticated shell steels used e.g. boiler grades, chrome/molys and so on, may have a requirement for pre- and/or post-weld heating. Incoloy 800 and DS are immune to embrittlement.
The next step is to confirm whether the welding machine has the capabilities to successfully perform the selected process. After selecting the process and the consumable, one will need to check if the equipment can perform the duty cycle. Does it have the controls to select weld power parameters, wire feed speed, voltage and amperage?
Does it have the grunt to push power down long leads if the welding is to be done at a distance from the power source? Is the power source surge and ebb controlled? Is there monitoring equipment such as tong testers to monitor current at the weld pool? Does the procedure call for this? Is there someone qualified to supervise all of this?
Personnel qualifications and safety
Key requirements for the welder include experience and qualifications, good health, drug- and alcohol-free; correct gear; a positive attitude and attention to detail; and good grasp of the required process and procedure.
Independent welding inspectors
Be it a new installation, or a shutdown repair, the facility should have competent independent welding inspectors who are fully informed about an applied procedure, and the interpretation of the particular code to that procedure. The inspector should be available all 24 hours during a shutdown.
A hammer test is a crude way to test welds because it will only show that the anchor has withstood a hammer blow; any inclusion, or micro-crack will not present visually. Since it is not really possible to x-ray, magnetic-particle test or dye-penetrant test fillet welds around anchor bases effectively, visual inspections and close adherence to procedures are the only effective ways to test welds.
Consider the torque tension tester for studs. A variation may be possible for manually welded anchors.
Storage and protection
Humidity, moisture, rain and dust, either airborne or introduced, are sources of undesirable contaminants, which must be prevented from entering into the weld pool under the arc.
Ambient temperature can be a factor if the weather is cold, and the steel is in a ductile/ brittle temperature zone. The procedure may also define a minimum parent metal temperature. If there is sensitivity, arrangements for preheating need to be in place, including keeping consumables at recommended storage temperature, preferably in an oven if it is flux coated stick electrodes.
Weld position will have a marked effect on weld quality. Down hand is best, ably assisted by gravity. Problems occur with positional welding, especially overhead, where gravity tends to drop molten metal out of the weld pool, causing poor penetration, and premature cooling.
Key considerations for selection of consumables include correct grade, history of electrode, exposure to damp or humidity, age, welding gas purity, and source integrity among others. Not all consumables are the same, even though they do comply with grade specification.
The welders must be provided with conditions that allow them to work in safety without distraction. Their working area also needs to be screened off to prevent dangerous eye flash to passing personnel.
Inside a furnace, it may still be too hot to work safely in a repair situation, and the discomfort may affect weld integrity as well. All fumes have to be extracted. Breathing helmets should be supplied with fresh air from a human-safe air source. The welder must have appropriate protective clothing. There should be no inflammable materials in the vicinity. Any x-ray activity should have a provision for isolation. Ensure drinking water is readily available for the welders.
Many factors influence the quality of the final weld; this presentation has outlined the most significant considerations including type of anchor and welding techniques, identification and storage, weld preparation, welding dissimilar metals, personnel and equipment, inspection and procedures, weld testing, consumables, and safety issues.