Frequently Asked Questions:
Why does granulated submerged arc welding flux need to be rebaked in an oven?
In order to answer this question, it is pertinent to describe the four types of welding fluxes that are commonly available: fused, bonded, agglomerated and pre-mixed fluxes.
Fused — this type of flux is non-hygroscopic (does not absorb moisture from the air). Any surface moisture on the particles can be removed at a low temperature oven setting of 300°F.
Bonded — this type of flux is hygroscopic (absorbs moisture from the air) and is comprised of a combination of dry ingredients that are glued together with a liquid binder, then baked at a low oven temperature.
Agglomerated — this type of flux is hygroscopic and is manufactured the same way as bonded fluxes only a ceramic binder is used instead of a liquid binder.
Pre-Mixed — this type of flux is hygroscopic and is simply a combination of two or more bonded or agglomerated fluxes.
Just as stick welding electrodes readily pick up moisture from the surrounding atmosphere, the same applies to bonded welding fluxes that are comprised mostly of dry, powdered ingredients. As described above, the purpose of flux is to clean and shield the weld area from impurities. If moisture has contaminated the flux, hydrogen is released into the metal when heat is applied. When the weld cools, it can become brittle, crack and/or develop pinholes. Moisture-contaminated flux can also accelerate corrosion to certain metals like aluminum and must be kept dry throughout the welding process.
The rebaking process refers to the short-term, high temperature heating of subarc flux that has been contaminated by atmospheric moisture. The rebaking process “reconditions” the welding flux, meaning it bakes out the moisture that has been absorbed thus restoring the flux so it is suitable for reuse. For specific rebake temperature guidelines, please contact the manufacturer of your consumable.
Keen welding flux holding ovens and rebake ovens are an indispensable addition to any subarc welding operation to help ensure quality welds. Our flux holding ovens and rebake ovens are suitable for almost any flux heating application that is required for today’s professionals.
What terms should I know that are related to poor storage of welding consumables?
Porosity: formed by entrapment of discrete pockets of gas in the solidifying weld pool. The gas can be formed in a variety of ways: poor gas shielding, surface contaminants such as moisture, grease, rust. Porosity can also result from insufficient deoxidants in the parent metal, electrode of filler wire.
Wormholes: A severe form of porosity caused by heavy contamination of the weld pool as a result of surface contamination or welding with damp electrodes. Under radiograph, they appear as elongated pores and are indicative or a large amount of gas that has formed in the weld which is trapped by the solidifying weld metal.
Hydrogen: Contributes to cracking in the solidified weld. In combination with high tensile stresses and sensitive steels, hydrogen can cause cold cracking several hours or days after the weld is complete. For structural welding using high strength steels, consumables that give low hydrogen levels are often used. These types of consumables are prone to moisture pick-up and must be stored at elevated temperatures.
What is the difference between subarc flux holding/storage and subarc flux rebaking/reconditioning?
Generally, there are two procedures involved with the proper maintenance of subarc welding flux: holding (also called storing) and rebaking (also called reconditioning).
The holding process refers to the long-term* heated storage of welding flux to maintain factory-fresh dryness. Storing the flux at elevated temperatures prevents atmospheric moisture contamination
of the hygroscopic granules. There are various temperature requirements according to the type of flux and also that are also set forth by welding codes. For specific holding temperature guidelines, please contact the manufacturer of your welding consumable.
The rebaking process refers to the short-term**, high temperature heating of subarc flux that has been contaminated by atmospheric moisture. The rebaking process “reconditions” the welding flux, meaning it bakes out the moisture that has been absorbed thus restoring the flux so it is suitable for reuse. For specific rebake temperature guidelines, please contact the manufacturer of your consumable.
* - In relation to our products, we consider long-term to mean 24 hours/day 7 days/week.
** - In relation to our products, we consider long-term to mean 24 hours/day 7 days/week.
How is the KHTF-1000 subarc flux rebake oven used in the field?
Subarc flux is top-loaded by way of a hinge-supported lid that is easily lifted manually, and discharged through a slide valve located on the bottom of the unit. Handy, fixed steel bars that stretch across the hopper opening allow the easy emptying of new flux from unopened factory packaging. The flux bags can be placed across these bars, cut open and emptied into the oven hopper. When flux needs to be removed, a container is placed by the user beneath the hopper to catch the granulated flux as it is dispensed. The hopper unit is securely mounted onto a stationary, heavy-duty stand.
The welding flux temperature inside the oven is regulated and controlled by a field-adjustable, digital controller with bright, LED temperature indicating display (user can switch from Celsius and Fahrenheit). This microprocessor is housed in a control box on the side of the flux oven hopper, along with a separate on/off indicating red light. This light enables users to quickly view from a distance if the oven is receiving power or has been shut off.
What can you tell me about performance of Keen KHTF-1000 rebake flux ovens?
The KHTF-1000 is ruggedly designed to withstand years of use in the most demanding industrial environments. Heavy-gauge steel is used throughout the unit and 4 inches of mineral wool in the oven walls and lid keeps the KHTF-1000 flux oven well-insulated and energy efficient. The exterior features a durable powder-coated paint that is highly-resistant to industrial environments and will stand up to years of use. The KHTF-1000’s hopper is has fully welded seams inside and out, and is securely mounted to the wheeled dolly by way of heavy duty screws.
Thorough, energy-efficient and quick heating of welding flux is the hallmark of all Keen flux ovens. The KHTF-1000 rebake flux oven in particular has twenty-seven elements evenly dispersed throughout the flux oven hopper cavity. When loaded with flux, no more than 2” of flux is without direct heating. It is important to note that our flux ovens are designed to reduce watt density to the elements, eliminating the problem of flux fusing to the heating elements which is a common problem seen in competing brands. Fusing of flux often creates a “gumming” effect causing the granulated material surrounding the elements to meld together.
As standard equipment, Keen KHTF-1000 flux ovens feature a UL® approved programmable PID microprocessor with digital temperature control and LED temperature-indicating display. The temperature reading can be displayed in either Fahrenheit or Celsius, and has a ±0.2 degree accuracy. Handy tactile increment/decrement keys on the controller’s water-resistant front panel enable easy temperature set point adjustment. The controller also features an on/off control mode. The standard control unit supplied with the KHTF-1000 flux oven is programmable for standard operation, and an upgraded “ramp & soak” version is available.
What are the key differences between welding flux holding ovens and welding flux rebake ovens?
Keen ovens are specifically designed according to the temperature range of the process, and the amount of subarc flux to be stored. The standard holding ovens are designed to accommodate a maximum temperature of 550F and the rebake ovens are designed to reach 999F. The higher temperature ovens have larger wall thicknesses to accommodate more insulation and digital programmable temperature controllers.
What is the procedure for recycling welding flux and what are slag screens?
After the weld is completed flux that is not consumed can be collected and recycled to the flux oven hopper. However, contained in the non-consumed flux may be some contaminants such as slag, metal, mill scale, grindings, etc. that need to be filtered out before the flux is introduced back into the hopper to mix with new flux. Many manufacturers of sub-arc flux recommend maintaining a ratio of new flux to recycled flux. Some suggest adding 20% new flux by weight to recycled flux, others recommend a rule of thumb that one part new flux be added to three parts reused. It is always best to check with the manufacturer of your consumable for guidelines.
Keen flux oven slag screens are designed to be a sieve for filtering out contaminants in reused flux as it is being poured back into the flux oven hopper. The hole size on our flux oven slag screens is standard, but we can adjust the screen hole size if you have a different requirement. The slag screens are a fixed size according to the flux oven with which they are used.