NAME 345 Welding Technology Lecture 07 Shielded Metal Arc Welding (SMAW)

Transcription

1 NAME 345 Welding Technology Lecture 07 Shielded Metal Arc Welding (SMAW) Md. Habibur Rahman Lecturer Department of Naval Architecture & Marine Engineering Bangladesh University of Engineering & Technology Dhaka-1000, Bangladesh

2 Shielded Metal Arc Welding (SMAW) Shielded metal arc welding (SMAW) is one of the simplest, oldest, and most versatile welding methods. It is also referred to as Stick Welding. It is a manual arc welding process that uses a consumable stick shaped electrode coated with chemicals that provide flux and shielding. The filler metal (here the consumable electrode) is usually very close in composition to the metal being welded. The arc comes from a coated electrode tip being touched to the workpiece and then withdrawn to maintain the arc. The heat that is generated melts the tip, coating, and base metal and the weld is formed out of that alloy when it solidifies. Slag that is formed and protects the weld from oxides, inclusions, and nitrides has to be removed after every pass. Current (I): 25A 250A Voltage (V): 15V 35V 2

3 Schematic Diagram of SMAW Schematic Diagram of Shielded Metal Arc Welding (SMAW) 3

4 How to Strike and Establish an Arc A welding arc is maintained when the welding current is forced across a gap between the electrode tip and the base metal. A welder must be able to strike and establish the correct arc easily and quickly. There are two general method to strike the arc: i. Scratching ii. Tapping Scratching Method The scratching method is easier for beginners and when using an AC machine. The electrode is moved across the plate inclined at an angle, as you would strike as match. As the electrode scratches the plate an arc is struck. When the arc has formed, withdraw the electrode momentarily to form an excessively long arc, then return to normal arc length. 4

5 How to Strike and Establish an Arc (Contd.) Tapping Method In the tapping method, the electrode is moved downward to the base metal in a vertical direction. As soon as it touches the metal it is withdrawn momentarily to form an excessively long arc, then returned to normal arc length. 5

6 Procedure of Shielded Metal Arc Welding (SMAW) employs the heat of the arc to melt the base metal and the tip of a consumable covered electrode. The electrode and the work piece are part of an electric circuit. This circuit begins with the electric power source and includes the welding cables, an electrode holder, a workpiece connection, the workpiece (weldment), and an arc welding electrode. One of the two cables from the power source is attached to the work. The other is attached to the electrode holder. Welding commences when an electric arc is struck by making contact between the tip of the electrode and the work. The intense heat of the arc melts the tip of the electrode and the surface of the work close to the arc. Current (I): 25A 250A Voltage (V): 15V 35V 6

7 Procedure of (Contd.) Shielded Metal Arc Welding (SMAW) employs the heat of the arc to melt the base metal and the tip of a consumable covered electrode. The electrode and the work piece are part of an electric circuit. This circuit begins with the electric power source and includes the welding cables, an electrode holder, a workpiece connection, the workpiece (weldment), and an arc welding electrode. One of the two cables from the power source is attached to the work. The other is attached to the electrode holder. Welding commences when an electric arc is struck by making contact between the tip of the electrode and the work. The intense heat of the arc melts the tip of the electrode and the surface of the work close to the arc. Tiny globules of molten metal rapidly form on the tip of the electrode, then transfer through the arc stream into the molten weld pool. In this manner, filler metal is deposited as the electrode is progressively consumed. The arc is moved over the work at an appropriate arc length and travel speed, melting and fusing a portion of the base metal and continuously adding filler metal. 7

8 Procedure of (Contd.) Since the arc is one of the hottest of the commercial sources of heat [temperatures above 9000 F (5000 C) have been measured at its center], melting of the base metal takes place almost instantaneously upon arc initiation. If the welds are made in either the flat or the horizontal position, metal transfer is induced by the force of gravity, gas expansion, electric and electromagnetic forces, and surface tension. For welds in other positions, gravity works against the other forces. 8

9 Electrodes In general, all electrodes are classified into five main groups: i. Mild steel ii. High-carbon steel iii. Special alloy steel iv. Cast iron v. Nonferrous The widest range of arc welding is done with electrodes in the mild steel group. Electrodes are classified as either bare or shielded. The original bare electrodes were exactly as their 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, low-hydrogen sodium, or iron powder. Each of the chemicals in the coating serves a particular function in the welding process. In general, their main purpose are to induce easier arc starting, stabilize the arc, improve weld appearance and penetration, reduce spatter, and protect the molten metal from oxidation or contamination by the surrounding atmosphere. 9

10 Electrode Selection Several factors are critical when you choose all electrode for welding. The welding position is particularly significant. 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. Position and the type of joint are also factors in determining the size of the electrode. 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. Larger electrodes make it too difficult to control the deposited metal. For economy, you should always use the largest electrode. 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. 10

11 Electrode Selection (Contd.) Electrodes for welding mild steel can be classified as fast freeze, fill freeze, and fast fill. FAST-FREEZE electrodes produce a snappy, deep penetrating arc and fastfreezing 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 arc 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. 11

12 Electrode Selection (Contd.) FAST-FILL electrodes are the heavy coated, 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 was 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. The basic rule in selecting electrodes is to pick one that is similar in composition to the base metal. 12

13 Selecting the Type of Current The welding current used for stick welding may be either alternating current or direct current depending on the electrode being used. When using dc 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. If the electrode is connected to positive polarity, the workpiece must be connected to negative polarity. This connection is called 'positive polarity' or electrode positive. In America, the term reverse polarity is used about such a connection. If the electrode is connected to negative polarity, the term used is electrode negative. In the US, the term straight polarity is preferred. 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. 13

14 Selecting the Type of Current 14

15 Selecting the Type of Current (Contd.) 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 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. 15

16 Power Supply The power supply used in SMAW has constant current output, ensuring that the current (and thus the heat) remains relatively constant, even if the arc distance and voltage change. This is important because most applications of SMAW are manual, requiring that an operator hold the torch. Maintaining a suitably steady arc distance is difficult if a constant voltage power source is used instead, since it can cause dramatic heat variations and make welding more difficult. The preferred polarity of the SMAW system depends primarily upon the electrode being used and the desired properties of the weld. Direct current with a negatively charged electrode (DCEN) causes heat to build up on the electrode, increasing the electrode melting rate and decreasing the depth of the weld. Reversing the polarity so that the electrode is positively charged (DCEP) and the workpiece is negatively charged increases the weld penetration. With alternating current the polarity changes over 100 times per second, creating an even heat distribution and providing a balance between electrode melting rate and penetration. 16

17 Advantages The equipment is relatively simple, inexpensive, and portable. The filler metal, and the means of protecting it and the weld metal from harmful oxidation during welding are provided by the covered electrode. Auxiliary gas shielding or granular flux is not required. The process is less sensitive to wind and draft than gas shielded arc welding processes. It can be used in areas of limited access. The process is suitable for most of the commonly used metals and alloys. Limitations The process is discontinuous due to limited length of the electrodes Due to flux coated electrode, the chances of slag entrapment and other related defects are more as compared to MIG and TIG welding. Due to fumes and particles of slag, the arc and metal transfer is not very clear and welding control in this process is a bit difficult as compared to MIG welding. This process uses stick electrodes and thus it is slower as compared to MIG welding. Current limits are lower than for continuous or automatic processes (reduces deposition rate) There is a lot of post-weld cleanup needed if the welded areas are to look presentable. 17

18 Welding Arc Blow Arc blow is the deflection of an electric arc from its normal path due to magnetic forces. It is mainly encountered with dc welding of magnetic materials, such as steel, iron, and nickel, but can also be encountered when welding nonmagnetic materials. It will usually adversely affect appearance of the weld, cause excessive spatter, and can also impair the quality of the weld. It is often encountered when using the shielded metal arc welding process with covered electrodes. It is also a factor in semiautomatic and fully automatic arc welding processes. 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 is the, usually unwanted, deflection of the arc during arc welding. 18

19 Magnetic Arc Blow or Arc Wander Magnetic arc blow or "arc wander" is the deflection of welding filler material within an electric arc deposit by a buildup of magnetic force surrounding the weld pool. Magnetic arc blow is more common in DC welding than in AC welding. It is experienced most when using currents above 200 A or below 40 A. Magnetic arc blow can occur because of: i. Workpiece connection, ii. Joint design, iii. Poor fit-up iv. Improper settings v. Atmospheric conditions Arc blow tends to occur if the material being welded has residual magnetism at a certain level, particularly when the weld root is being made, and the welding current is direct current (DC positive or negative). This is due to interaction between the directional magnetic field of the welding arc and the directional field of the residual magnetism. Magnetic arc blow is popularly attributed to a change in the direction of current as it flows into and through the workpiece. Magnetic arc blow is known to begin at field densities as low as 10 gauss and becomes severe at densities of, equal to or greater than, 40 gauss; it is directional and can be classified as forward or backward moving along the joint, but can occasionally occur to the sides depending on the orientation of the poles to the workpiece. 19

20 Thermal Arc Blow Thermal arc blow is widely attributed to variations in resistance within the base metal created by the weld pool as it is moved across the workpiece. Thermal arc blow occurs because an electric arc requires hot zones on the electrode and workpiece plate to maintain a continuous flow of current in the arc stream. As the electrode advances along the work, the arc tends to lag behind, caused by reluctance of the arc to move to the colder plate. The ionized space between the end of the electrode and the hot surface of the molten crater creates a more conductive path than from the electrode to the colder. Thermal arc blow causes are: i. Improper surface preparation ii. Improper travel speed Thermal arc blow is not as severe as magnetic arc blow, but can still leave undesirable defects in the weld deposit. 20

21 Factors Causing Arc Blow i. Arc blow is caused by magnetic forces. The induced magnetic forces are not symmetrical about the magnetic field surrounding the path of the welding current. The location of magnetic material with respect to the arc creates a magnetic force on the arc which acts toward the easiest magnetic path and is independent of electrode polarity. The location of the easiest magnetic path changes constantly as welding progresses; therefore, the intensity and the direction of the force changes. ii. Welding current will take the easiest path but not always the most direct path through the work to the work lead connection. The resultant magnetic force is opposite in direction to the current from the arc to is independent of welding current polarity. 21

22 Factors Causing Arc Blow iii. Arc blow is not as severe with alternating current because the induction principle creates current flow within the base metal which creates magnetic fields that tend to neutralize the magnetic field affecting the arc. iv. The greatest magnetic force on the arc is caused by the difference resistance of the magnetic path in then the base metal around the arc. The location of the work connection is of secondary importance, but may have an effect on reducing the total magnetic force on the arc. It is best to have the work lead connection at the starting point of the weld. This is particularly true in electro-slag welding where the work lead should be connected to the starting sump. On occasion, the work lead can be changed to the opposite end of the joint. In sane cases, leads can be connected to both ends. 22

23 Minimizing Arc Blow i. If DC current is being used with the shielded metal-arc process - especially at rates above 250 amps - a change to AC current may eliminate problems. ii. Hold as short an arc as possible to help the arc force counteract the arc blow. iii. Reduce the welding current - which may require a reduction in arc speed. iv. Angle the electrode with the work opposite the direction of arc blow. v. Make a heavy tack weld on both ends of the seam; apply frequent tack welds along the seam, especially if the fit-up is not tight. vi. Weld toward a heavy tack or toward a weld already made. vii. Use a back-step welding technique. viii.weld away from the workpiece connection to reduce back blow; weld toward the workpiece connection to reduce forward blow. ix. With processes where a heavy slag is involved, a small amount of back blow may be desirable; to get this, weld toward the workpiece connection. x. Wrap the work cable around the workpiece so that the current returning to the power supply passes through it in such a direction that the magnetic field setup will tend to neutralize the magnetic field causing the arc blow. 23

24 Minimizing Arc Blow 24

25 Comparison chart of welds Undercuts and overlapping in welding Setting the length of an arc 25

26 Common Defects in SMAW Hot cracks Caused by excessive contraction of the metal as it cools. Excessive bead size May also be found at the root of the weld. Slag inclusions Long arc Incomplete removal of slag on multipass welds. Undercutting Improper welding parameters; particularly the travel speed and arc voltage. Porosity Atmospheric contamination or excess gas in the weld pool. Incomplete fusion Microcracks Underbead cracks Toe Cracks Excessive heat and rapid cooling. Underbead cracks Excessive hydrogen in weld pool Microcracks Caused by stresses as weld cools. Incomplete fusion Incorrect welding parameters or welding techniques. Toe cracks 26