WELDING OF MEDIUM CARBON STEEL
- Medium carbon steels are non-alloy steels which contain from 0.30 to 0.55 percent carbon. These steels may be heat treated after fabrication and used for general machining and forging of parts which require surface hardness and strength. They are manufactured in bar form and in the cold rolled or the normalized and annealed condition. When heat treated steels are welded, they should be preheated from 300 to 500°F (149 to 260°C), depending on the carbon content (0.25 to 0.45 percent) and the thickness of the steel. The preheating temperature may be checked by applying a stick of 50-50 solder (melting point 450°F (232°C)) to the plate at the joint, and noting when the solder begins to melt. During welding, the weld zone will become hardened if cooled rapidly, and must be stress relieved after welding. Medium carbon steels may be welded with any of the arc, gas, and resistance welding processes.
- With higher carbon and manganese content, the low-hydrogen type electrodes should be used, particularly in thicker sections. Electrodes of the low-carbon, heavy coated, straight or reverse polarity type, similar to those used for metal-arc welding of low carbon steels, are satisfactory for welding medium carbon steels.
- Small parts should be annealed to induce softness before welding. The parts should be preheated at the joint and welded with a filler rod that produces heat treatable welds. After welding, the entire piece should be heat treated to restore its original properties.
- Either a low carbon or high strength rod can be used for welding medium carbon steels. The welding flame should be adjusted to slightly carburizing, and the puddle of metal kept as small as possible to make a sound joint. Welding with a carburizing flame causes the metal to heat quickly, because heat is given off when steel absorbs carbon. This permits welding at higher speeds.
- Care should be taken to slowly cool the parts after welding to prevent cracking of the weld. The entire welded part should be stress relieved by heating to between 1100 and 1250°F (593 and 677°C) for one hour per inch (25.4 mm) of thickness, and then slowly cooling. Cooling can be accomplished by covering the parts with fire resistant material or sand.
- Medium carbon steels can be brazed by using a preheat of 200 to 400°F (93 to 204°C), a good bronze rod, and a brazing flux. However, these steels are better welded by the metal-arc process with mild steel shielded arc electrodes.
WELDING PROCESSES FOR MEDIUM CARBON STEELS
- The basic welding processes is used unless until the particular process is specified to weld the metal. For higher the carbon content which is more than 0.30%, the welding current and the deposition rate is more.
- For example let us take a metal with carbon content between 0.30% to 0.45% and the welding rod or the electrode of carbon content 0.10%. Let’s consider the welding process to be either oxyacetylene, Shielded Metal Arc, Inert Gas Arc, Vapor Shielded Arc or a Submerged Arc Process.
- The metal deposited by Shielded Arc Process has higher quality of weld. During this process the metal gets heated up and then melted which turns into fluid state. In such fluid state the slag and other nonmetallic particles float out of the deposit.
- Generally in all the Electric Arc process the metal which is to be deposited is being protected by shielding gasses to avoid oxidation taking place in the weld pool. All the processes vary in the amount of heat generated, the rate of heat supplied and the concentration of heat.
- The general factors which can be varied to change the deposition rate and the penetration range are mentioned below
- Melting the welding rod or the welding electrode
- Melting of parent metal
- Distribution of heat in the metal plate
- The distribution of the heat in the weld plate can also be varied. As a result the metallurgical changes which occur during the process are same in all the welding methods.
- The formation of heat varies from process to process where the oxyacetylene welding process forms more heat on the surface and melts less and in the submerged arc welding process the melting is more and the heating of metal is lesser. This is because in oxyacetylene welding process the flame spreads out the heat wherein SAW process the heat is concentrated in one particular point. All the other process lies between these two extremes.
HEATING AND COOLING CYCLE
- Let’s assume an electrode of grade E60xx and a metal plate with carbon content 0.45%. Whenever this metal is subjected to welding the heating and cooling process that occurs seems to be gradual with oxyacetylene torches. If the same welding is done with submerged arc process the temperature change will be faster when compared to oxy acetylene. The temperature gradient varies according to the processes used. In order to harden the steel plate it is heated up to a particular temperature and then quenched rapidly. This sudden heating and cooling process may cause crack in the steel plate. This is because in its hardened condition it does not have the ability to withstand its stresses resulting from unequal heating.
- But usually the metal which is too be welded is not hardened completely and hence forth the defect caused during the cooling or heating process is avoided. During the cooling of the weld the following consideration are to be taken care.
- The amount of heat addition to the weld metal
- Rate at which the heat is added to the weld metal
- Area to which the heat is added
- Size and Shape of the work piece
- Initial temperature of base metal
- Once when the weld plate is heated, later it gets cooled down during the heat leaving process some of the factors that affect are
- Temperature of the air
- Velocity of the air
- Insulating layer of slag over the weld bead
PROCEDURE TO CONTROL HARDNESS
- The thin high hardness zone in the base metal at the weld creates some problem. They can be controlled by following factors
- The heat should be controlled during welding process
- In addition to the heat generated during welding process pre heating the base metal is required.
- Posting heating or tempering the metal after the welding process.
- These are usually done in medium carbon steels. The principle is to heat the base material in order to slow down the rate of cooling and preventing in formation of sharp crystalline structures. The methods to heat the base metal is by Arc welding with high current at low speed. They can also be done by performing multiple passes in arc weld.
- A wider or a U groove can accommodate multiple passes in arc welding. When the welding is done my multiple passes the heat which is formed in each and every pass is adjacent to the heat in the first bead. And during the last pass of the weld, the first bead remains hot still which can prevent in appreciable hardening. The increasing and decreasing of the hardness of the material depends on the following factors.
The factors through which the hardness is reduced,
- The base metal which is fine grained.
- Gradual cooling of the weldment after the welding process.
- Heavy weld deposit during the process of welding with lesser weld speed.
- High welding current.
- The heat transfer during the welding process must be slow.
- Base material which has very low thickness.
- The presence of heat in the weld metal before, during and after the welding.
- Multiple passes must be avoided and single layer deposit is required.
The factors through which the hardness is increased,
- The base metal which is coarse grained.
- Smaller weld deposit during the process of welding with higher weld speed.
- Low welding current
- The heat transfer during the welding process must be faster.
- Base material which has very high thickness
- The work piece is to be kept in room temperature before welding and no extra heat is being applied.
- Faster cooling of the weldment after welding
- Single layer deposit must be avoided and multiple layer deposit is required.
TECHNIQUES TO REMEMBER WHILE WELDING
- The techniques which are to be followed while welding medium carbon steels are shown below,
- The plates should be prepared for welding in a manner similar to that used for welding low carbon steels. When welding with low carbon steel electrodes, the welding heat should be carefully controlled to avoid overheating the weld metal and excessive penetration into the side walls of the joint. This control is accomplished by directing the electrode more toward the previously deposited filler metal adjacent to the side walls than toward the side walls directly. By using this procedure, the weld metal is caused to wash up against the side of the joint and fuse with it without deep or excessive penetration.
- High welding heats will cause large areas of the base metal in the fusion zone adjacent to the welds to become hard and brittle. The area of these hard zones in the base metal can be kept to a minimum by making the weld with a series of small string or weave beads, which will limit the heat input. Each bead or layer of weld metal will refine the grain in the weld immediately beneath it, and will anneal and lessen the hardness produced in the base metal by the previous bead.
- When possible, the finished joint should be heat treated after welding. Stress relieving is normally used when joining mild steel, and high carbon alloys should be annealed.
- In welding medium carbon steels with stainless steel electrodes, the metal should be deposited in string beads in order to prevent cracking of the weld metal in the fusion zone. When depositing weld metal in the upper layers of welds made on heavy sections, the weaving motion of the electrode should not exceed three electrode diameters.
- Each successive bead of weld should be chipped, brushed, and cleaned prior to the laying of another bead.
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