What Is Shielded Metal Arc Welding?

Arc welding refers to the use of an electric arc as a heat source and the physical phenomenon of air discharge to convert electrical energy into thermal and mechanical energy required for welding, thereby achieving the purpose of connecting metals. The main methods include electrode arc welding, submerged arc welding, gas shielded welding, etc. It is currently the most widely used and most important fusion welding method, accounting for more than 60% of the total welding production. ^[1]

Arc welding

Arc welding refers to the use of an electric arc as a heat source and the physical phenomenon of air discharge to convert electrical energy into thermal and mechanical energy required for welding, thereby achieving the purpose of connecting metals. The main methods include electrode arc welding, submerged arc welding, gas shielded welding, etc. It is currently the most widely used and most important fusion welding method, accounting for more than 60% of the total welding production. ^[1]

It is supplied by welding power.

Gas ionization and cathode electron emission.

Arc welding can be divided into manual arc welding, semi-automatic (arc) welding, and automatic (arc) welding. Automatic (arc) welding usually refers to submerged arc automatic welding-it is covered with protective function

The basic process of manual arc welding is as follows:

a. Clean the welding surface before welding, so as not to affect the arc ignition and the quality of the weld.

b. Prepare the joint type (bevel type).

The function of the bevel is to make the electrode, wire or torch (the nozzle that sprays acetylene-oxygen flame during gas welding) can directly reach the bottom of the bevel to ensure penetration, and it is conducive to slag removal and the necessary swing of the electrode in the bevel. To get good fusion.

The shape and size of the groove is mainly determined by the material to be welded and its specifications (mainly thickness) and the adopted

The most important parameters in arc welding welding specifications are:

Type of electrode (depending on the material of the base material), electrode diameter (depending on the thickness of the weldment, position of the weld, number of layers, welding speed, welding current, etc.), welding current, number of welding layers, welding speed, etc.

In addition to the above-mentioned ordinary arc welding, in order to further improve the welding quality, we also use:

1. There must be no more than 10 meters of welding place

I. Easy to cause electric shock accidents

1. During the welding process, because the welder frequently changes the welding rod and adjusts the welding current, the electrode and plate should be directly contacted during operation, and the welding power source is usually 220V / 380V. When the electrical safety protection device is faulty, labor protection supplies are not qualified, When the operator operates illegally, it may cause an electric shock. The danger of electric shock is even greater when welding in metal containers, pipes, or wet locations.

2. When the welding machine is under no load, the voltage of the secondary winding is generally 60 ~ 90V. Because the voltage is not high, it is easy to be ignored by the welder, but its voltage exceeds the specified safe voltage of 36V, which is still dangerous. It is assumed that the no-load voltage of the welding machine is 70V, and people are working in a high temperature and humid environment. At this time, the human body resistance R is about 1600. If the welder's hand contacts the jaw, the current I passing through the human body is

I = V / R = 70/1600 = 44mA, under the action of this current, the welder's hand will be convulsed, which may easily cause an electric shock accident.

3. Because welding operations are mostly in the open air, welding machines, handlebar wires and power lines are often in high temperature, humid (construction site) and dust environments, and the stove is often overloaded, which easily aging the insulation of power lines and electrical lines. , The insulation performance is reduced, and it is easy to cause leakage accidents.

2. Easy to cause fire and explosion accidents

As arcing or open flames occur during welding, it is extremely easy to cause fire when working in places with flammable materials. Especially in the area of flammable and explosive devices (including pits, trenches, troughs, etc.), the dangers are greater when welding is performed on containers, towers, tanks and pipes that have stored flammable and explosive media.

Third, easy to cause burns

Because arcing and metal slag can occur during the welding process, if the welder does not wear protective welding work clothes, gloves and leather shoes, especially when welding at high places, because of welding sparks, if no protective isolation measures are taken, It is easy to cause skin burns of the welder himself or the construction personnel below the working surface.

Fourth, easily cause electro-optic ophthalmia

Due to the intense fire visible light and a large number of invisible ultraviolet rays during welding, it has a strong irritating and damaging effect on human eyes. Long-term direct exposure can cause eye pain, photophobia, tears, and wind, etc., which can easily cause conjunctiva and Corneal inflammation (commonly known as electrooptic ophthalmitis).

Five, with light radiation

The arc light generated during welding contains infrared, ultraviolet and visible light and has a radiating effect on the human body. Infrared rays have thermal radiation effects, which can easily lead to heat stroke when welding in high-temperature environments. Ultraviolet rays have photochemical effects, which are harmful to human skin. At the same time, long-term exposure to exposed skin can also cause skin peeling, which can cause long-term exposure to visible light. Eye vision is reduced.

Six, easy to produce harmful gases and soot

Because the arc temperature generated during the welding process reaches more than 4200 ° C, the electrode core, the coating and the metal weldment will melt, vaporize, evaporate and condense, which will produce a large amount of manganese chromium oxide and harmful smoke; meanwhile, the arc The high temperature of light and strong radiation can also cause toxic gases such as ozone and nitrogen oxides in the surrounding air. Engaged in electric welding operations under poor ventilation for a long time. These toxic gases and soot are inhaled by the human body and have a certain impact on human health.

Seven, easy to cause fall

Due to construction needs, the welder must often climb up the welding operation. If the high-altitude fall prevention measures are not completed, the scaffolding is not standardized, and it is used without acceptance; the upper and lower cross-operations adopt anti-object strike isolation measures; the welder's personal safety awareness is not strong. Do not wear safety helmets or safety belts when climbing up. Once you encounter accidents such as walking accidentally or the impact of unexpected objects, it may cause a high fall accident.

8. Easy to cause poisoning and suffocation

Welders often enter closed or semi-closed places such as metal containers, equipment, pipelines, towers, storage tanks, etc. for welding. If toxic and harmful media and inert gases have been stored or transported, once the work management is poor, protective measures are not in place. It is easy to cause workers to be poisoned or suffocated by hypoxia. This phenomenon often occurs in oil refining, chemical and other enterprises.

Nine, electric shock prevention measures

The general principle is to take safety measures such as insulation, shielding, isolation, leakage protection and personal protection to prevent humans from touching live objects. The specific methods are:

1. Improve the insulation performance of welding equipment and lines. The electric welding equipment and power cables used must be qualified products, and their electrical insulation performance must be compatible with the used voltage level, surrounding environment and operating conditions; the welding machine should be arranged for special maintenance and maintenance to prevent sun and rain, so as to avoid The electrical insulation performance of the welder is reduced.

2. When the welder fails to repair, move the work place, change the connector or replace the safety device, the power must be cut off before operation.

3. When installing power to the welding machine, don't forget to install a leakage protector at the same time to ensure that people will automatically cut off the power if they get an electric shock. When welding on wet or metal containers, equipment, and components, a leakage protector with a rated operating current not greater than 15 mA and a rated operating time less than 0.1 seconds must be selected.

4. The ends of the butt welder casing and the secondary winding lead should take good protective grounding or zero-connection measures. When the power supply is a three-phase three-wire system or a single-phase system, a protective ground wire should be installed, and its resistance value should not exceed 4; when the power supply is a three-phase four-wire neutral point grounding system, a protective neutral wire should be installed.

5. Strengthen the education of workers' knowledge of electrical safety and self-protection awareness. It is required to wear insulated shoes and special insulated gloves when welding. Welding is prohibited in the open on rainy days; when welding in a particularly humid place, one must stand on dry wooden boards or rubber insulation sheets.

6. It is forbidden to use metal structures, pipelines, tracks and other metal connections for wires. When welding in a metal container or a particularly humid place, the power supply of the running light must use a safe voltage below 12V.

Ten, fire and explosion prevention measures

1. Welding in flammable and explosive places. Prior to welding, a fire work permit must be obtained in accordance with regulations and approved by relevant departments.

Only after agreeing to work, strictly "three fires".

2. Before formal welding, check whether there are flammable and explosive materials under and around the operation, and whether there are anti-corrosive substances such as paint on the working surface. If there is, it should be properly handled in advance. A firewall must be built for welding operations in the vicinity of production equipment areas and oil tank areas; if there are high-altitude welding operations, asbestos plates or iron plates should be used for isolation to prevent Mars from splashing.

3. If welding is performed on containers, equipment or pipelines that have been produced, stored and transported flammable and explosive media, it must be checked whether the connected equipment and pipelines are closed or blocked by blind plates before welding; Purging, cleaning, replacement, sampling and testing, welding can only be performed after passing the analysis.

Eleven, anti-burn measures

1. Welders must wear protective work clothes, insulated gloves and shoes properly for welding. When using high current welding, the welding clamp should be equipped with a protective cover.

2. The as-welded parts should be covered with asbestos boards in time to prevent direct contact between feet and body and cause burns.

3. The electrode heads to be replaced during high-altitude welding should be stacked in a centralized manner, and not to be thrown away, so as not to burn the workers below.

4. Wear protective goggles when cleaning welding slag. When performing overhead welding or horizontal welding at high altitude, due to the serious sparks from Mars, isolation and protection measures should be taken.

Twelve, measures to prevent electro-optic ophthalmitis

According to the size of the welding current, a suitable mask goggle filter should be selected in a timely manner, and other personnel working with the welding industry should wear colored protective eyes when welding.

Thirteen, radiation prevention measures

Welders and surrounding workers should wear labor protection supplies when welding. It is forbidden to directly observe the arc light without wearing a welding mask and colored eye lenses; reduce skin exposure as much as possible. It is forbidden to wear shorts and short gowns for welding operations in summer; if possible, apply UV protective cream to exposed skin.

14. Measures to Prevent Harmful Gases and Smoke

1. Reasonably design the welding process, and try to use the single-sided welding and double-sided forming process to reduce the amount of welding in the metal container.

2. If welding is performed in a small or closed container, forced ventilation measures must be taken to reduce the concentration of harmful gases and smoke in the working space.

3. Where possible, use automatic welding and semi-automatic welding instead of manual welding to reduce the chance of welding personnel exposure to harmful gases and soot.

4. Use low-dust and low-toxicity electrodes to reduce the content of harmful smoke and dust in the working space.

5. When welding, the welder and other people around him should wear dust masks to reduce the inhalation of smoke and dust.

Fifteen, high fall prevention measures

Welders must perform regular physical examinations. Persons with a history of hypertension, heart disease, epilepsy, etc. are prohibited from ascending and welding. Welders must fasten their seat belts and wear safety helmets when climbing up. Before welding, check the ascending operation site and the surrounding environment to see if the foothold is stable and reliable, and whether the safety protection facilities such as scaffolding meet the safety requirements. If necessary, safety nets should be installed under and around the operation. Isolation and protection measures should be taken when crossing up and down.

16.Antipoisoning and suffocation measures

1. Where welding, storage, transportation or production of containers, equipment, pipes, towers, tanks and other closed or semi-enclosed places with toxic and hazardous media, inert gases, all processes and equipment connected to it must be cut off before operation Wash, purge, replace, and apply for equipment operation permits in accordance with regulations. After sampling and analysis, you can enter the operation after passing.

2. Under normal circumstances, analysis should be performed every 4 hours. If conditions change, samples should be analyzed at any time. At the same time, an appropriate amount of air (oxygen) gas respirator should be equipped at the scene for emergency use.

3. Special personnel shall be used for safety monitoring during the operation, and the welder shall rotate the operation regularly. For working equipment with high tightness and prone to hypoxia, oxygen is supplemented by forced ventilation (direct oxygen supply is prohibited) to prevent hypoxia and suffocation.