Welding Electrode Ovens, an Overview
One of the most common joining processes in metal fabrication is welding. Its ubiquity can be attributed to its ability to create strong, quality joints and its versatility. There are numerous types of welding processes. Among the most widely used of these processes is the shielded metal arc welding process. This process involves the use of a flux-coated electrode. The flux coating gives a protective gaseous shield from the atmosphere during welding. Welding occurs through the arc caused by striking the electrode on the parent metal. The coating on these electrodes, especially low hydrogen electrodes such as 7016/7018, is very susceptible to moisture absorption when exposed to the atmosphere. Using electrodes with moisture-affected coatings can lead to various welding defects. It is, therefore, imperative to dry the electrodes properly before usage. This is usually carried out using electrode ovens (aka welding rod ovens).
The Need for Welding Electrode Ovens
The most frequently used electrodes for SMAW are coated with flux. When the flux coating burns, it releases a protective gas that shields the weld pool from contamination. These contaminants include hydrogen, nitrogen, oxygen, etc. The presence of these contaminants produces porosity in welds and can cause cracking (hydrogen cracking). As base metals are cleaned, dried, and even pre-heated, it is also necessary to ensure that the electrodes are dry before usage. When exposed, the flux coating absorbs moisture. During welding, the moisture turns to steam and creates hot air pockets and water. These push away the flux, allowing even more contaminants into the weld. These create weak points that can lead to the failure of the welded part.
The 60xx series welding electrodes, one of the basic electrodes for pipe and plate welding, do not require much heating as their coating already contains moisture, and they are designed to be used with the moisture, though comparatively low. They could be left uncovered and still produce structurally sound welds when used. 6010 and 6011 electrodes have a moisture content of about 4 to 6%. The low-hydrogen welding electrodes, on the other hand, are designed to have a moisture content of <1%. Any increase in the moisture content of these electrodes, such as 7016, 7018, would lead to hydrogen inclusions in the welds. Therefore, these electrodes, after being taken out of their hermetically sealed containers, are expected to be held or stored at a specific temperature (approximately 250F) until they are ready to be used. If the pack is opened and left for more than a day, these electrodes will need to be baked at about 500F for about 4 hours or two before being held at 250F for use.
Welding electrodes are to be rebaked not more than once. Higher tensile strength rods like the 90xx-x do not require baking if they are provided in hermetically sealed containers. If not, they are to be baked for a minimum of 1 hour at about 700F to 800F. All these heating processes are carried out using electrode ovens.
How Welding Electrode Ovens Work
Not all electrode ovens are designed for baking. Some are used for holding the electrodes at specific temperatures depending on the welding requirements. They are available in different sizes depending on the required storage capacity, and they also vary in their temperature ratings. The ovens used for baking generally have higher temperature ratings than those intended for holding electrodes at a particular temperature. However, their principles of operation, to a large degree, are the same.
The oven heats the electrodes by radiation from the heating element. Basically, the heating up process used is resistance heating. When current passes through the heating element, it leads to power loss in the form of heat. This heat gets transferred to the electrodes utilizing radiation. The electrodes get heated from the outside in. This method of heating ensures that the moisture in the flux is removed quickly. To ensure even heating, the heating elements surround the storage compartments in the ovens uniformly.
Components of Welding Electrode Ovens
This is the material that heats up when an electric current is passed through it. The resistance of the material causes power loss as the current flows through it. This power loss is transformed into heat. Metal heating elements can heat up to very high temperatures. These materials have high resistivity and low resistance temperature coefficient. They have small deformation and do not become brittle at high temperatures. Materials commonly used are iron, aluminum, including alloys such as iron-aluminum alloy, nickel-chromium alloys etc., and other metals.
This component is essential for temperature regulation. The thermostat is connected to a long probe that extends into the oven. This probe detects temperature changes. The thermostat is also connected to a control circuit that controls the heat source to the oven. Once the temperature in the oven gets to the set level, the probe detects this and sends a feedback signal to the thermostat. The thermostat then signals the control circuit, and the heating supply is turned off. This is generally noticed when the indicator light periodically goes on and off.
To minimize heat loss to the environment, the body and the lid cover of ovens are insulated. Insulation for ovens have much higher ratings than traditional household insulation. Insulating the oven has two benefits;
i. It ensures even temperature distribution in the oven by eliminating cool spots
ii. It prevents the exterior container from getting too hot as to pose a hazard to those handling it or removing electrodes from it.
The insulation is placed between the metal casing and the heating compartment. The insulation could range from single layers to multiple layers. The density of the insulation is based on the temperature of operation. The higher the temperature, the denser the material. Different insulating materials include fiberglass, cellulose, Rockwool, etc.
Depending on the design, the capacity could range from a few kilograms to more than 100 kilograms. This is made out of metal to ensure unrestricted heat circulation. The compartment could be one large compartment or divided into sub-compartments for effective electrode separation. Other components include; indicator light, handle, wheels for mobility, electric plug, and chord.
Applications of Welding Electrode Ovens
There are two main applications of electrode ovens, namely;
1. Rebaking of Electrode
This takes a relatively shorter time and is done at a higher temperature. Electrodes damaged from moisture absorption are usually rebaked before use.
Electrodes may be held for either short term or long term. Additionally, they could be held at a factory level of dryness. The temperature for holding is based on the type of electrode and the material of the flux coating.
Other applications include
i. Storage of Flux
ii. Preheating of electrodes
iii. Holding and storage of filler rods such as MIG wire and TIG filler rod.
Benefits of Welding Electrode Ovens
- The use of electrode ovens, especially in field applications, prevents hydrogen inclusions in welds and other weld defects.
- It protects the electrodes from absorbing moisture from the atmosphere while also keeping them dry for use.
- Portability for field applications
When hydrogen cracking is a concern, using low hydrogen filler metals is one way to mitigate the risk. Low hydrogen electrodes are specifically formulated to reduce the amount of hydrogen molecules that get into the weld pool, thus reducing the hydrogen available to cause cracking. However, without an electrode oven, these consumables will no longer serve their purpose after a matter of hours once they are exposed to the atmosphere. Electrode rod ovens keep these low hydrogen electrodes from collecting hydrogen by preventing moisture exposure. This allows the electrodes to have a longer usable shelf life.
Limitations of Welding Electrode Ovens
Holding ovens cannot operate at temperatures high enough to rebake the electrodes. Therefore, rebaking is limited to factories or large workshops and not in the field applications.
Welding rod ovens cannot guarantee multiple restorations of electrode integrity more than once. Once they are rebaked, further damage from hydrogen absorption cannot be repaired by baking in the oven, no matter how high the operating temperature or the holding time is.