What Are the Different Types of Welding Qualifications?

Welding, also known as welding, butt welding, is a manufacturing process and technology for joining metals or other thermoplastic materials such as plastic by heating, high temperature or high pressure.

Welding achieves the purpose of joining in the following three ways:

1. Fusion welding-heating the workpieces to be joined to locally melt them to form

A method in which two or more surfaces of a thermoplastic article are fused into a single body using heat and pressure or other methods.

Causes of fire and explosion accidents

Welding and cutting operations, especially gas cutting, due to the use of compressed air or oxygen

First, the length of the arc

The length of the arc is related to the type of electrode coating and the thickness of the coating. However, short arcs should be taken whenever possible, especially low hydrogen electrodes. Long arcs can cause pores. The short arc can prevent harmful gases such as O2 and N2 from intruding into the weld metal, forming undesirable impurities such as oxides, and affecting the quality of the weld.

Second, welding speed

The appropriate welding speed is due to the corresponding changes in conditions such as electrode diameter, coating type, welding current, heat capacity of the object to be welded, and the beginning of the structure. Standards cannot be made. Maintain proper welding speed, the slag can cover the molten pool well. Allow the various impurities and gases in the molten pool to fully float out time, and avoid the formation of slag inclusions and pores in the weld. If the speed of moving the rod is too fast during welding, the shrinkage stress will increase when the welded part cools, causing cracks in the weld.

Key points for welding wire selection

The choice of welding wire should be considered based on the type of steel being welded, the quality requirements of the welded parts, welding construction conditions (plate thickness, bevel shape, welding position, welding conditions, post-weld heat treatment and waiting for welding operations), and cost.

The order of welding wire selection is as follows:

Select welding wire according to the steel type of the structure to be welded. For carbon steel and low-alloy high-strength steel, the welding wire that meets the requirements of mechanical properties is mainly selected according to the principle of "equal strength matching". For heat-resistant steel and weather-resistant steel, the main consideration is the consistency of the chemical composition of the weld metal and the base metal to meet the requirements of heat resistance and corrosion resistance.

According to the quality requirements of the welded parts (especially impact toughness), the selection of welding wire is related to the welding conditions, bevel shape, and shielding gas mixing ratio and other process conditions. To ensure the performance of the welded joints, choose to achieve the maximum welding efficiency and reduce Welding materials for welding costs.

Select the diameter of the welding wire according to the plate thickness of the welding position on the spot, determine the current value, and refer to the product introduction data and experience of each manufacturer to select the welding wire grade suitable for the welding position and current. .

Welding process performance includes arc stability, size and number of spatter particles, slag removal, appearance and shape of the weld. For carbon steel and low alloy steel welding (especially semi-automatic welding), the welding method and welding material are mainly selected according to the welding process performance.

2. Selection of solid cored wire

Submerged arc welding wire

Welding wire and flux are consumable materials for submerged arc welding. Welding of various metal materials from carbon steel to high nickel alloys can be performed with submerged arc welding using a combination of wire and flux. The choice of submerged arc welding wire must consider both the influence of flux composition and the influence of base metal. In order to obtain different welding composition and mechanical properties, one flux (mainly smelting flux) can be used with several types of welding wires, and one type of welding wire can be used with several fluxes (mainly sintered fluxes).

High chromium cast iron surfacing welding wire (HS101)

A. Welding wire for low carbon steel and low alloy steel

Commonly used welding wires for submerged arc welding of low carbon steel and low alloy steel are as follows:

Low manganese welding wire (such as H08A) is often used in conjunction with high manganese flux to weld low carbon steel with low strength and low alloy steel.

Medium manganese welding wire (such as H08MnA H10MnSi) is mainly used for welding of low alloy steel, and can also be used with low manganese flux for welding of low carbon steel.

High manganese welding wire (H10Mn2 H08Mn2Si) is used for welding low alloy steel.

B. Welding wire for low alloy high strength steel

The low alloy high strength steel welding wire contains more than 1% Mn and 0.3% -0.8% Mo, such as H08MnMoA, H08Mn2MoA, and is used for welding low strength high alloy steel with high strength. In addition, according to the performance requirements for the composition of low-alloy high-strength steel, elements such as Ni, Cr, V, and RE can also be added to the welding wire to improve weld performance.

590Mpa grade weld metal mostly uses Mn-Mo series welding wire, such as H08MnMoA, H08Mn2MoA, H10Mn2Mo and so on.

C. Welding wire for stainless steel

When welding stainless steel, the composition of the welding wire used should be basically the same as that of the stainless steel being welded. Welding wire such as H0Cr14, H1Cr13, H1Cr17 can be used when welding chrome stainless steel, and H0Cr19Ni9 H0Cr19Ni9Ti can be used when welding chrome-nickel stainless steel; corresponding ultra-low carbon welding wire such as H00Cr19Ni9 should be used when welding ultra-low carbon stainless steel. The flux can be smelted or sintered, and the oxidizing property of the flux is required to reduce the burning loss of alloy elements.

D. Welding rod (electric welding)

J422 is a carbon steel electrode of titanium calcium type coating. AC and DC, can be used for all-position welding. Has excellent welding process performance and good mechanical properties; stable arc, small spatter, easy slag removal, easy arc initiation; beautiful welding seam formation, wide welding wave, narrow, thin, thick, easy welding, efficiency high.

Uses: Used for welding more important low-carbon steel structures and low-alloy steel structures with low strength grades, such as Q235, 09MnV, 09Mn2, etc.

Welding quality standards

1. Welding quality GB6416-1986 Technical factors affecting the quality of steel fusion welding joints

2. Welding quality GB6417-1986 Classification and explanation of welding defects

3. Welding quality TJ12.1-1981 Welding quality regulations for construction machinery

4, welding quality JB / ZQ3679 quality of welding parts

5. Welding quality JB / ZQ3680 appearance quality

6, welding quality CB999-1982 hull weld surface quality inspection method

7, welding quality JB3223-1983 welding rod quality management regulations

8. Welding Standards Abolished in 2005 GB / T 12469-1990 Welding Quality Assurance Requirements and Classification of Defects in Fusion Welded Joints

Welding type

1. Electrode arc welding:

Principle-An arc welding method using manual welding electrodes for welding. The stable burning arc established between the electrode and the welding piece is used to melt the electrode and the welding piece, thereby obtaining a solid welding joint. Gas-slag joint protection.

Main features-flexible operation; low welding head assembly requirements; wide range of weldable metal materials; low welding productivity; strong weld quality dependence (dependent on welder's operating skills and field performance).

Application-widely used in shipbuilding, boilers and pressure vessels, machinery manufacturing, building structures, chemical equipment and other manufacturing and maintenance industries. Suitable for (in the above-mentioned industries) welding of various metal materials, various thicknesses and various structural shapes.

2. Submerged arc welding (automatic welding):

Principle-The arc burns under the flux layer. The heat generated by the arc burning between the welding wire and the welding piece is used to melt the welding wire, the flux and the base material (welding piece) to form a weld. Is a slag protection.

Main features: high welding productivity; good weld quality; low welding cost; good labor conditions; difficult to weld in space; high requirements for welding assembly quality; Bad) and short welds.

Application-Widely used in shipbuilding, boiler, bridge, lifting machinery and metallurgical machinery manufacturing. Submerged arc welding can be used for all weldments where the weld can be kept in a horizontal position or with a small inclination angle. The thickness of the board should be greater than 5 mm (anti-burn through). Welding carbon structural steel, low alloy structural steel, stainless steel, heat-resistant steel, composite steel, etc.

3. Carbon dioxide gas shielded welding (automatic or semi-automatic welding):

Principle: Fused arc welding using carbon dioxide as a shielding gas. Gas protection.

Main features-high welding productivity; low welding cost; small welding deformation (concentrated arc heating); high welding quality; simple operation; large spatter rate; difficult to weld with AC power; poor wind resistance; metal.

Application-mainly welding low carbon steel and low alloy steel. Suitable for all thicknesses. Widely used in automobile manufacturing, locomotive and vehicle manufacturing, chemical machinery, agricultural machinery, mining machinery and other departments.

4. MIG / MAG welding (melt extremely inert gas / active gas shielded welding):

MIG welding principle-an arc welding method that uses an inert gas as a shielding gas and a welding wire as a melting electrode.

The shielding gas is usually argon or helium or a mixture thereof. MIG uses an inert gas, and MAG adds a small amount of active gas, such as oxygen and carbon dioxide gas, to the inert gas.

Main characteristics: good welding quality; high welding productivity; no deoxygenation and dehydrogenation reaction (easy to form welding defects, particularly strict requirements on the surface cleaning of welding materials); poor wind resistance; complex welding equipment.

Application-It can weld almost all metal materials, mainly used for welding non-ferrous metals and their alloys, stainless steel and certain alloy steels (too expensive). The thinnest thickness is about 1 mm, and the large thickness is basically unlimited.

5. TIG welding (tungsten inert gas shielded welding)

Principle-Under the protection of inert gas, the welding method is to use the arc heat generated between the tungsten electrode and the weldment to melt the base metal and fill the welding wire (or without the welding wire) to form a weld. The electrode does not melt during welding.

Main features-strong adaptability (stable arc, no spatter); low welding productivity (poor current carrying capacity of tungsten electrode (anti-tungsten electrode melting and evaporation, anti-tungsten welding)); higher production costs.

Application-Weld almost all metal materials, commonly used for welding stainless steel, super alloy, aluminum, magnesium, titanium and its alloys, refractory active metals (zirconium, tantalum, molybdenum, niobium, etc.) and dissimilar metals. Welding thickness is generally less than 6 mm, or underweight welding of thick parts. The use of small angle grooves (narrow groove technology) can realize automatic welding of narrow gap TIG with a thickness of more than 90mm.

6. Plasma arc welding

PrincipleWith the restraint effect of water-cooled nozzle on the arc, a method of obtaining a high-energy-density plasma arc for welding.

Main features (compared with argon arc welding)- energy concentration, high temperature, can obtain the pinhole effect for most metals within a certain thickness range, can get full penetration, uniformly formed welds on the reverse side. The arc stiffness is good, the plasma arc is basically cylindrical, and the change of the arc length has little effect on the heating area and current density on the weldment. Therefore, the arc length change of plasma arc welding has no significant effect on the weld formation. (3) Welding speed is faster than argon arc welding. Able to weld thinner and thinner machined parts. The equipment is complicated and the cost is high.

application

Penetrating (small-hole) plasma arc welding: Utilizing the characteristics of small plasma arc diameter, high temperature, high energy density, and strong penetrating power, under appropriate process parameters (larger welding current 100A 500A) Plasma arc welding method in which the weldment is completely penetrated, and a small hole penetrating the weldment is formed under the action of plasma flow, and a part of the plasma arc is sprayed from the back of the weldment. It can be welded on one side and on both sides. It is most suitable for butt welding of 3-8mm stainless steel, titanium alloy below 12mm, low carbon steel or low-alloy structural steel and copper, brass, nickel and nickel alloy. (The plate is too thick, it is difficult to form small holes due to the limitation of the plasma arc energy density; the plate is too thin, the small holes cannot be completely closed by the liquid metal, and the small hole welding method cannot be realized.)

Penetration type (dissolution type) plasma arc welding: adopts a smaller welding current (30A ~ 100A) and a lower plasma gas flow rate, and adopts a hybrid plasma arc welding method. No pinhole effect is formed. It is mainly used for welding of thin plates (below 0.5 to 2.5 mm), welding of each layer after the bottom bead of multi-layer welding and welding of fillet welds.

(3) Microbeam plasma arc: Plasma arc welding with welding current below 30A. The nozzle diameter is very small (0.5 1.5mm), and a needle-like fine plasma arc is obtained. It is mainly used for welding ultra-thin, ultra-small, and precision welding parts below 1 mm.

Notes

1. The above are several commonly used fusion welding methods, each with advantages and disadvantages. When choosing a welding method, there are many factors to consider, such as the type of welding material, plate thickness, and position of the weld in space. The principle of selecting a welding method is: on the premise of ensuring the quality of the welded joint, a welding method with a low total cost is used.

Welding temperature control

The temperature of the molten pool directly affects the quality of the welding. The temperature of the molten pool is high, the molten pool is large, the molten iron is good in fluidity, and it is easy to fuse. Welding and forming are also difficult to control, and the plasticity of the joint is reduced, and bending is easy to crack. When the temperature of the molten pool is low, the molten pool is small, the molten iron is dark, the fluidity is poor, and defects such as incomplete penetration, unfused, and slag inclusion are easy to occur.

The temperature of the molten pool is closely related to the welding current, the diameter of the electrode, the angle of the electrode, and the arc burning time. The following measures are taken to control the temperature of the molten pool according to relevant factors.

diameter

1. Welding current and electrode diameter: Welding current and electrode diameter are selected according to the position of the weld seam and the welding level. When welding is started, the selected welding current and electrode diameter are larger, and the vertical and horizontal positions are smaller. For example, the sealing layer of 12mm flat butt welding uses 3.2mm electrode, welding current: 80-85A, filling, cover layer uses 4.0mm electrode, welding current: 165-175A, reasonable selection of welding current and electrode diameter, It is easy to control the temperature of the molten pool, which is the basis of weld formation.

method

2. The method of transporting the strip. The temperature of the circle-shaped transport strip is higher than the temperature of the crescent-shaped transport strip. The temperature of the crescent-shaped transport strip is higher than the temperature of the molten pool of the zigzag transport strip. And, with the amplitude of the swing and the pauses on both sides of the groove, the temperature of the molten pool was effectively controlled, so that the size of the molten holes was basically the same, and the probability of no formation of welding knobs and burn-through at the root of the groove was reduced. The improvement makes the single-sided welding and double-sided forming of the flat butt welding of the plates no longer difficult.

angle

3. Welding rod angle, when the angle between the welding electrode and the welding direction is 90 degrees, the arc is concentrated, the molten pool temperature is high, the included angle is small, the arc is scattered, and the molten pool temperature is low, such as the bottom of a 12mm flat welding seal, the welding rod angle: 50- 70 degrees, which reduces the temperature of the molten pool, and avoids welding bumps or rising on the back. For another example, after changing the welding rod on the bottom of the 12mm plate vertical welding seal, a 90-95 degree welding rod angle is used in the joint to rapidly increase the temperature of the molten pool, the molten hole can be opened smoothly, and the back surface is relatively flat, which effectively controls the inside of the joint point. Concave phenomenon.

time

4. Arc burning time, in the practice teaching of horizontal fixed and vertical fixed welding of 57 × 3.5 pipe, the arc breaking method is used for welding. When sealing the bottom layer, the frequency of arc breaking and the arc burning time directly affect the temperature of the molten pool. The tube wall is thin and the heat capacity of the arc is limited. If you slow down the arc break frequency to reduce the temperature of the molten pool, it is easy to cause shrinkage. Therefore, you can only use the arc burning time to control the temperature of the molten pool. If the temperature of the molten pool is too high, When the weld hole is large, the arc burning time can be reduced to reduce the temperature of the molten pool. At this time, the weld hole becomes smaller, and the internal forming height of the pipe is moderate, so that the internal weld seam of the pipe is not too high or a weld knob is generated.