EQUIPMENT OPERATION AND MAINTENANCE
Learning to arc weld requires you to possess many skills. Among these skills are the abilities to set up, operate, and maintain your welding equipment.
WELDING AREA REQUIREMENTS
In most factory environments, the work is brought to the welder. In the Seabees, the majority of the time the opposite is true. You will be called to the field for welding on buildings, earthmoving equipment, well drilling pipe, ship to shore fuel lines, pontoon causeways, and the list goes on. To accomplish these tasks, you have to become familiar with your equipment and be able to maintain it in the field. It would be impossible to give detailed maintenance information here because of the many different types of equipment found in the field; therefore, only the highlights will be covered.
WELDING MACHINE OPERATION AND MAINTENANCE
You should become familiar with the welding machine that you will be using. Study the manufacturer's literature and check with your senior petty officer or chief on the items that you do not understand. Machine setup involves selecting current type, polarity, and current settings. The current selection depends on the size and type of electrode used, position of the weld, and the properties of the base metal.
Cable size and connections are determined by the distance required to reach the work the size of the machine, and the amperage needed for the weld.
Operator maintenance depends on the type of welding machine used. Transformers and rectifiers require little maintenance compared to engine-driven welding machines. Transformer welders require only to be kept dry and a minimal amount of cleaning. Internal maintenance should only be done by electricians due to the possibilities of electrical shock Engine-driven machines require daily maintenance of the motors. Inmost places you will be required to fill out and turn in a daily inspection form called a "hard card" before starting the engine. This form is a list of items, such as oil level, water level, visible leaks, and other things, that affect the operation of the machine. Transportation departments are the ones who usually handle these forms.
After all of the above items have been checked, you are now ready to start welding.
SHIELDED-METAL ARC WELDING
Before you start to weld, ensure that you have all the required equipment and accessories. Listed below are some additional welding rules that should be followed.
Clear the welding area of all debris and clutter.
Do not use gloves or clothing that contains oil or grease.
Check that all wiring and cables are installed properly.
Ensure that the machine is grounded and dry.
Follow all manufacturer's directions on operating the welding machine.
Have on hand a protective screen to protect others in the welding area from FLASH bums.
Always keep fire-fighting equipment on hand.
Clean rust, scale, paint, or dirt from the joints that are to be welded.
Figure 7-7.-Electrode covering and gaseous shield that protects weld metal from the atmosphere.
In general, all electrodes are classified into five main groups:
1. Mild steel
2. High-carbon steel
3. Special alloy steel
4. Cast iron
The widest range of arc welding is done with electrodes in the mild steel group.
Electrodes are manufactured for use in specific positions and for many different types of metal. They also are specially designed to use with ac or do welding machines. Some manufacturer's electrodes work identically on either ac or dc, while others are best suited for flat-position welding. Another type is made primarily for vertical and overhead welding, and some can be used in any position. As you can see, electrode selection depends on many variables.
Types of Electrodes
Electrodes are classified as either bare or shielded. The original bare electrodes were exactly as their name implied-bare. Today, they have a light covering, but even with this improvement they are rarely used because of their limitations. They are difficult to weld with, produce brittle welds, and have low strength. Just about all welding is done with shielded electrodes.
The shielded electrode has a heavy coating of several chemicals, such as cellulose, titania sodium, lowhydrogen sodium, or iron powder. Each of the chemicals in the coating serves a particular function in the welding process. In general, their main purposes are to induce easier arc starting, stabilize the arc, improve weld appearance and penetration, reduce spatter, and protect
Figure 7-S.-Explanation of AWS classification numbers.
the molten metal from oxidation or contamination by the surrounding atmosphere.
As molten metal is deposited in the welding process, it attracts oxygen and nitrogen. Since the arc stream takes place in the atmosphere, oxidation occurs while the metal passes from the electrode to the work. When this happens, the strength and ductility of the weld are reduced as well as the resistance to corrosion. The coating on the electrode prevents oxidation from taking place. As the electrode melts, the heavy coating releases an inert gas around the molten metal that excludes the atmosphere from the weld (fig. 7-7).
The burning residue of the coating forms a slag over the deposited metal that slows down the cooling rate and produces a more ductile weld. Some coatings include powdered iron that is converted to steel by the intense heat of the arc as it flows into the weld deposit.
Electrodes are often referred to by a manufacturer's trade name. The American Welding Society (AWS) and the American Society foresting and Materials (ASTM) have set up certain requirements for electrodes to assure some degree of uniformity in manufacturing electrodes. Thus different manufacturer's electrodes that are within the classification established by the AWS and ASTM should have the same welding characteristics. (See fig. 7-8.)
In this classification, each type of electrode is assigned a specific symbol, such as E-6010, E-7010, and E-8010. The prefix E identifies the electrode for
Table 7-2.-Electrode Selection Guide
electric-arc welding. The first two digits in the symbol designate the minimum allowable tensile strength in thousands of pounds per square inch of the deposited weld metal. For example, the 60-series electrodes have a minimum tensile strength of 60,000 pounds per square inch, while the 70-series electrodes have a strength of 70,000 pounds per square inch.
The third digit of the symbol indicates the joint position for which the electrode is designed. Two numhers are used for this purpose: 1 and 2. Number 1 designates an electrode that can be used for welding in any position. Number 2 represents an electrode restricted for welding in the horizontal and flat positions only.
The fourth digit of the symbol represents special characteristics of the electrode, such as weld quality, type of current, and amount of penetration. The numbers range from 0 through 8. Since the welding position is dependent on the manufacturer's characteristics of the coating, the third and fourth numbers are often identified together.
Several factors are critical when you choose an electrode for welding. The welding position is particularly significant. Table 7-2 shows the recommended current types and welding positions for the most common electrodes.
As a rule of thumb, you should never use an electrode that has a diameter larger than the thickness of the metal that you are welding. Some operators prefer larger electrodes because they permit faster travel, but this takes a lot of expedience to produce certified welds. Position and the type of joint are also factors in determining the size of the electrode. For example, in a thick-metal section with a narrow vee, a small-diameter electrode is always used to run the frost weld or root pass. This is done to ensure full penetration at the root of the weld. Successive passes are then made with larger electrodes.
For vertical and overhead welding, 3/16 inch is the largest diameter electrode that you should use regardless of plate thickness. Larger electrodes make it too difficult to control the deposited metal. For economy, you should always use the largest electrode that is practical for the work It takes about one half of the time to deposit an equal quantity of weld metal from 1/4-inch electrodes as it does from 3/16-inch electrodes of the same type. The larger sizes not only allow the use of higher currents but also require fewer stops to change electrodes.
Deposit rate and joint preparation are also important in the selection of an electrode. Electrodes for welding mild steel can be classified as fast freeze, fill freeze, and fast fill. FAST-FREEZE electrodes produce a snappy, deep penetrating are and fast-freezing deposits. They are commonly called reverse-polarity electrodes, even though some can be used on ac. These electrodes have little slag and produce flat beads. They are widely used for all-position welding for both fabrication and repair work
FILL-FREEZE electrodes have a moderately forceful are and a deposit rate between those of the fast-freeze and fast-fill electrodes. They are commonly called the straight-polarity electrodes, even though they may be used on ac. These electrodes have complete slag coverage and weld deposits with distinct, even ripples. They are the general-purpose electrode for a production shop and are also widely used for repair work They can be used in all positions, but fast-freeze electrodes are still preferred for vertical and overhead welding.
Among the FAST-FILL electrodes are the heavycoated, iron powder electrodes with a soft arc and fast deposit rate. These electrodes have a heavy slag and produce exceptionally smooth weld deposits. They are generally used for production welding where the work is positioned for flat welding.
Another group of electrodes are the low-hydrogen type that were developed for welding high-sulfur and high-carbon steel. These electrodes produce X-ray quality deposits by reducing the absorption of hydrogen that causes porosity and cracks under the weld bead. Welding stainless steel requires an electrode containing chromium and nickel. All stainless steels have low-thermal conductivity that causes electrode overheating and improper arc action when high currents are used. In the base metal, it causes large temperature differentials between the weld and the rest of the work, which warps the plate. A basic rule in welding stainless steel is to avoid high currents and high heat. Another reason for keeping the weld cool is to avoid carbon corrosion.
There are also many special-purpose electrodes for surfacing and welding copper and copper alloys, aluminum, cast iron, manganese, nickel alloys, and nickelmanganese steels. The composition of these electrodes is designed to match the base metal. The basic rule in selecting electrodes is to pick one that is similar in composition to the base metal.
Electrodes are expensive; therefore, the loss or deterioration through improper handling or storage can become very costly. Always store them in a dry place at room temperature with 50-percent maximum relative humidity. Moisture causes the coating on electrodes to disintegrate and fall off. Low-hydrogen rods are especially sensitive to moisture. After removing these rods from their original packaging, you should store them in a storage space maintained at a temperature between 250°F to 400°F. Portable or stationary drying ovens are used to store and preserve electrodes at specified temperatures. Care should be taken when handling electrodes because bumping or dropping them can cause the coatings to fall off, rendering the rod useless.
Earlier in this chapter, ac and do current was briefly covered. With ac welding machines, polarity is not a problem. When using do welding machines, you can weld with either straight polarity or reverse polarity.
Polarity is the direction of the current flow in a circuit, as shown in figure 7-9. In straight polarity, the electrode is negative and the workpiece positive; the electrons flow from the electrode to the workpiece. In reverse polarity, the electrode is positive and the workpiece negative; the electrons flow from the workpiece to the electrode. To help you remember the difference, think of straight polarity as a SENator and reverse polarity as a REPresentative. Use only the first three letters of each key word. SEN stands for Straight Electrode Negative; REP for Reverse Electrode Positive.
Figure 7-9.-Straight and reverse polarity in electric welding.
On some of the older machines, polarity is changed by switching cables. On many of the newer machines, the polarity can be changed by turning a switch on the machine.
Polarity affects the amount of heat going into the base metal. By changing polarity, you can direct the amount of heat to where it is needed. When you use straight polarity, the majority of the heat is directed toward the workpiece. When you use reverse polarity, the heat is concentrated on the electrode. In some welding situations, it is desirable to have more heat on the workpiece because of its size and the need for more heat to melt the base metal than the electrode; therefore, when making large heavy deposits, you should use STRAIGHT POLARITY.
On the other hand, in overhead welding it is necessary to rapidly freeze the filler metal so the force of gravity will not cause it to fall. When you use REVERSE POLARITY, less heat is concentrated at the workpiece. This allows the filler metal to cool faster, giving it greater holding power. Cast-iron arc welding is another good example of the need to keep the workpiece cool; reverse polarity permits the deposits from the electrode to be applied rapidly while preventing overheating in the base metal.
In general, straight polarity is used for all mild steel, bare, or lightly coated electrodes. With these types of electrodes, the majority of heat is developed at the positive side of the current, the workpiece. However, when heavy-coated electrodes are used, the gases given off in the arc may alter the heat conditions so the
Figure 7-10.-Striking or brushing method of starting the arc.
opposite is true and the greatest heat is produced on the negative side. Electrode coatings affect the heat conditions differently. One type of heavy coating may provide the most desirable heat balance with straight polarity, while another type of coating on the same electrode may provide a more desirable heat balance with reverse polarity.
Reverse polarity is used in the welding of nonferrous metals, such as aluminum, bronze, Monel, and nickel. Reverse polarity is also used with some types of electrodes for making vertical and overhead welds.
You can recognize the proper polarity for a given electrode by the sharp, crackling sound of the arc. The wrong polarity causes the arc to emit a hissing sound, and the welding bead is difficult to control.
One disadvantage of direct-current welding is "arc blow." As stated earlier, arc blow causes the arc to wander while you are welding in corners on heavy metal or when using large-coated electrodes. Direct current flowing through the electrode, workpiece, and ground clamp generates a magnetic field around each of these units. This field can cause the arc to deviate from the intended path. The arc is usually deflected forward or backward along the line of travel and may cause excessive spatter and incomplete fusion. It also has the tendency to pull atmospheric gases into the arc, resulting in porosity.
Arc blow can often be corrected by one of the following methods: by changing the position of the ground clamp, by welding away from the ground clamp, or by changing the position of the workpiece.