Carbon dioxide gas shielded welding

I. Basic principles

CO2 gas shielded arc welding is an arc welding with fusible metal welding wire as electrode and CO2 gas as protection. It is one of the important welding methods for welding ferrous metals.

Origin of CO2 welding process

The original idea of CO2 welding process originated from the 1920s, but the problem of weld porosity was not solved, which made CO2 welding unusable. Until the early 1950s, the development of welding metallurgy technology solved the metallurgical problems of CO2 welding, and developed Si-Mn series welding wires to make the CO2 welding process practical. Then, according to the performance of structural materials, welding wires with different compositions appeared one after another, which met the diverse needs of CO2 welding.

The practicability of CO2 welding technology has brought great wealth to the society. On the one hand, CO2 gas is cheap and easily available; On the other hand, CO2 welding has high metal deposition efficiency. Taking semi-automatic CO2 welding as an example, its efficiency is 3 ~ 5 times that of manual arc welding. However, the droplet transition of CO2 welding is mostly short circuit transition, which puts forward higher requirements for the stability of CO2 welding process. In addition, since 1950s, spatter in CO2 welding has become one of the main technical problems that restrict the popularization of CO2 welding technology.

Second, the process characteristics

1.CO2 welding has the advantages of deep penetration, high welding current density (100-300A/m2), small deformation, and the production efficiency is three times higher than that of covered electrode arc welding.

2.CO2 gas is cheap, and the cleaning of workpieces before welding can be simplified, and its welding cost is only 40%-50% of that of covered electrode arc welding.

3. The weld has strong rust resistance, low hydrogen content and small cold cracking tendency.

4. There are a lot of metal splashes during welding, especially when the process parameters are not matched.

5. It is impossible to weld easily oxidized metal materials, and the wind resistance is poor. When working in the field or at high altitude, windproof measures need to be taken.

6. Welding arc is strong, pay attention to arc radiation.

Third, metallurgical characteristics.

The welding process of CO2 welding is mainly manifested in metallurgy:

1.CO2 gas is an oxidizing gas, which decomposes at high temperature and has a strong oxidizing effect, burning alloy elements or causing pores and splashing. The measure to solve CO2 oxidation is deoxidation, and the specific method is to add a certain amount of deoxidizer into the welding wire. Practice shows that Si-Mn has the best deoxidation effect, so welding wires such as H08Mn2SiA/H 10Mn2Si are widely used at present.

Metallurgical characteristics of CO2 gas shielded welding

2. 1 oxidation of alloying elements

Carbon dioxide → carbon monoxide+1/2 oxygen

Oxygen → water

CO2 gas is highly oxidizing at high temperature, which will oxidize metals and burn alloy elements.

Fe + O →FeO Si + 2O → SiO2 Mn + O → MnO

FeO + C → Fe + CO

CO expands rapidly at the high temperature of arc, which makes the droplet explode and causes metal splash.

Burning loss of alloy elements, CO blowhole and splash are three main problems in CO2 gas shielded welding, which are all related to the oxidation of CO2 gas.

2.2 Deoxidation and alloying of weld metal

FeO {produces CO→ pores and splash.

Weld [O] =→ Mechanical properties of weld ↓

Deoxidation and alloying: a certain amount of deoxidizer (such as aluminum, titanium, silicon, manganese, etc. ) added to the welding wire, Si and Mn are often combined for deoxidation.

2FeO + Si → 2Fe + SiO2Mn + FeO → Fe + MnO

Part of Si and Mn is used for deoxidation, and the other part is alloyed as alloying elements.

At present, H08MnSiA welding wire is commonly used for welding low carbon steel and H08Mn2SiA welding wire for welding low alloy steel.

2.3 stomata

(1) carbon monoxide pores:

FeO + C → Fe + CO

Adding enough deoxidizer to the welding wire and limiting the carbon content of the welding wire can effectively prevent CO blowholes.

(2) Hydrogen pores

Hydrogen mainly comes from the oil stain and rust of welding wire and workpiece, and the moisture contained in CO2 gas. The oxidizability of CO2 gas can reduce the harm of hydrogen production-H2+CO2→ H2O+Co.

Compared with submerged arc welding and argon arc welding, CO2 gas shielded welding is less sensitive to rust and moisture. Unless the surface of the steel plate has been rusted with a layer of yellow rust, it is generally not necessary to remove rust before welding, but the oil on the surface of the welding wire must be wiped clean with solvents such as gasoline.

(3) Nitrogen hole

Sources of N2: ① Air invades the welding zone; ②CO2 gas is impure (unlikely)

The main reason of N porosity in weld is that the protective gas layer is destroyed and a large amount of air invades the welding zone. The factors causing the failure of protective gas layer are: the flow rate of CO2 gas is too small; Splash partially blocks the nozzle; The distance between the nozzle and the workpiece is too large; There is a crosswind at the welding site. Ensuring the stability and reliability of gas layer is the key to prevent nitrogen looseness in weld.

2.4 droplet transition mode:

In CO2 gas shielded welding, in order to obtain a stable welding process, short-circuit transition and particle transition are usually adopted.

The characteristics of short-circuit transition are: stable arc, small splash, high droplet transition frequency and good weld formation; Suitable for thin plate welding and all-position welding; Short-circuit transition welding mainly adopts thin welding wire, which is generally φ 0.6 ~1.4 mm.

The characteristics of fine particle transition: strong arc penetration and deep base metal penetration, suitable for welding workpieces with medium thickness and large thickness. Mainly thick welding wire, generally φ 1.6, φ2.0mm welding wire.

In CO2 gas shielded welding, DC reverse welding is generally used, because the spatter is small, the arc is stable, the hydrogen content in the weld metal is low, and the penetration depth is large.

Fourth, welding materials.

1. Protective gas CO2

The purity of CO2 gas for welding shall be ≥99.5%. Usually, carbon dioxide is put into a cylinder in liquid form. A standard steel cylinder with a capacity of 40L can be filled with 25Kg of liquid CO2, accounting for about 80% of the cylinder volume, and the remaining 20% of the space is filled with vaporized CO2. The pressure shown on the gas cylinder pressure gauge is the saturation pressure of this part. The pressure is related to the ambient temperature, so the method of weighing the cylinder mass is used to correctly estimate the storage amount of CO2 gas in the bottle. (Note: 1Kg liquid CO2 can vaporize 509LCO2 gas)

The carbon dioxide cylinder is painted black and l is written in yellow.

The commercially available CO2 gas has high water content, and it is easy to produce defects such as blowholes during welding. Measures to reduce field water content are as follows

1) Let the gas cylinder stand upside down 1-2 hours, then open the valve to discharge the water deposited on the bottle mouth, which can be put for 2-3 times, with an interval of 30 minutes each time, and then put the gas cylinder straight.

2) Invert the emptied gas cylinder. Before use, open the valve to release the gas with low purity from the cylinder, and then put the gas pipeline on it.

3) A high-pressure dryer and a low-pressure dryer are arranged in the gas path, and a gas preheating device is also arranged in the gas path to prevent moisture in CO2 gas from freezing in the pressure reducer and blocking the gas path.

2. Welding material (welding wire)

1.) The welding wire should have enough deoxidizing elements.

2.) The carbon content Wc≤0. 1 1% can reduce splashing and blowholes.

3.) It has sufficient mechanical properties and crack resistance.

Welding wire diameter and its tolerance (GB/T 8110-1995)

Allowable deviation of welding wire diameter

φ0.5; φ0.6 +0.0 1,-0.03

φ0.8,φ 1.0

φ 1.2,φ 1.6, +0.0 1,-0.04

φ3.0; φ3.2 +0.0 1,-0.07

Verb (abbreviation of verb) welding equipment (omitted)

Welding process of intransitive verbs

Scope of application of serial number, model, brand and specification

Welding between1er49-1h08mn2siaφ1.2q235.20 #, 20g.2or, 16MnR.

2er50-6/φ1.2q345.16mnr equidistant welding.

3er49-1h08mn2sia φ1.2q235.20 #, 20g. 2 or Q345. 16mnr welding.

Butt welding (I-groove)

Thickness mm welding wire diameter welding current a welding voltage v welding speed Cm/min welding wire dry elongation mm gas flow L/min layers

6 φ 1.2 120- 140 20-22 50-60 10- 12 10- 15 2

8 φ 1.2 130- 150 2 1-23 45-50 10- 12 10- 15 2

10 φ 1.2 200-250 24-26 45-50 10- 12 10- 15 3

14 φ 1.2 280-320 28-34 35-45 10- 12 12- 18 5

20 φ 1.2 360-400 34-38 35-40 10- 12 15-20 7

Thickness of fillet weld mm welding wire diameter welding current a welding voltage v welding speed Cm/min welding wire dry elongation mm gas flow L/min number of layers.

6 φ 1.2 150- 180 22-25 50-60 10- 12 10- 15 1

10 φ 1.2 200-250 24-26 45-50 10- 12 10- 15 2

14 φ 1.2 280-320 28-32 35-45 10- 12 12- 18 2

20 φ 1.2 360-400 34-38 35-40 10- 12 15-20 3

Remarks: the butt joint clearance is1-1.5mm.

Seven. Common defects in CO2 welding and their causes

Causes of defective names

Pore 1. Impure CO2 gas or insufficient gas supply.

2. Air is involved in welding.

3. Preheater does not work

4. The welding area is windy and the gas protection is not good.

5. The nozzle is blocked by spatter, which is not smooth. The distance between the nozzle and the workpiece is too large.

6. The oil stain and corrosion treatment on the surface of weldment is not thorough.

7. The arc is too long and the arc voltage is too high.

8. The content of silicon and manganese in welding wire is insufficient.

Undercut 1. The arc is too long and the arc voltage is too high.

2. The welding speed is too fast and the welding current is too large.

3. The welder swings improperly

Poor welding forming 1 ... inappropriate process parameters.

2. Improper adjustment of welding wire correction mechanism

3. Center deviation of wire feeding wheel

4. The conductive nozzle is loose.

Arc instability 1. Influence of external network voltage

2. Improper adjustment of welding parameters

3. The conductive nozzle is loose.

4. Wire feeding mechanism and contact nozzle are blocked.

Splash 1 .. The adjustment of welding electric parameters does not match.

2. Excessive gas flow

3. The surface of the workpiece is too rough

4. The length of welding wire is too long.

Incomplete penetration 1. Welding current is too small, wire feeding is improper.

2. The welding speed is too fast or too slow

3. The slot angle is too small and the gap is too small.

4. Improper welding wire position and poor neutrality.

5. Welder skill level

Eight. Development direction of CO2 welding

Usually, the main problems in CO2 welding of low carbon steel are welding spatter and weld formation. Solutions to these problems have been described above. However, in order to further popularize CO2 welding technology, its application field should be expanded. Such as: high-efficiency CO2 welding, all-position welding, arc spot welding and automatic welding. These actual welding production requirements have become the development direction of CO2 welding.

1 Efficient CO2 welding?

Modern industrial production puts forward high efficiency requirements for welding production. At present, there are mainly high-speed CO2 welding and high-efficiency MAG welding. High-speed CO2 welding mainly aims at the low welding speed of 0.3 ~ 0.5m/min of traditional CO2 welding. At present, the measures to solve this problem are twin-wire CO2 welding and flux-cored wire CO2 welding. It is difficult to adopt the usual semi-automatic welding method for twin-wire CO2 welding, because two welding wires pass through a welding torch, which makes the welding torch too heavy, so automatic welding can only be adopted, which limits the application of this method. In addition, the application range of flux-cored wire CO2 welding is far from that of solid wire. In fact, solid monofilament CO2 welding wire is the most popular CO2 welding method, and how to solve its high-speed welding process is a concern for everyone. The main problems of monofilament high-speed CO2 welding are undercut and hump welding. These problems are related to the behavior of the molten pool, that is, the stability of the molten pool should be solved from the perspective of welding technology. By controlling the welding arc phenomenon, the welding speed of high-speed CO2 welding has reached 2m/min, even 3m/min. High-speed CO2 welding is mainly used for welding thin workpieces, such as containers.

High-efficiency MAG welding is mainly used to improve the deposition speed, which is beneficial to welding thick plates. Usually the wire feeding speed of CO2 welding is 2 ~ 16m/min, right? 1.2mm welding wire, the maximum welding current can only reach about 350A. If Ar-rich mixed gas shielded welding (CO2+Ar) is used, the rotating jet transition will occur during high-speed wire feeding, resulting in great splash loss. Therefore, Weld Process Company of Canada successfully developed a high-performance MAG welding method in 1980, commonly known as T.I.M.E welding. T.I.M.E gas is a kind of four gases (0.5%O2, 8%CO2, 26.5%He, 65%Ar). Due to the addition of he gas, the lateral rotating splash of the rotating jet transition is limited and becomes a conical rotating jet around the axis of the welding wire. This is a relatively stable droplet transfer form, and good weld penetration can be obtained. The wire feeding speed of T.I.M.E welding can reach 50m/min, and the deposition rate can reach 450g/min. This law has been widely used in Europe and Japan. However, due to the lack of helium in China, thermal induction welding is difficult to popularize in China because of its high price. Therefore, Beijing University of Technology is looking for an efficient non-He MAG welding method, and has successfully achieved a wire feeding speed of 35m/min.

2 All-position CO2 welding

All-position CO2 welding has been applied to pipeline installation, steel structure and shipbuilding. The key of all-position CO2 welding is to prevent the molten iron from flowing in the molten pool. Therefore, the size of the molten pool should not be too large, that is, a small molten pool is used to keep the molten iron in the molten pool by surface tension. Small molten pool requires low welding current, but when connecting thick-walled workpieces under small conditions, it often produces incomplete fusion or slag inclusion. In order to solve the contradiction between small molten pool and penetration depth, the arc-short circuit energy distribution ratio and reasonable welding wire swing mode are often adopted at this time. All-position CO2 welding wire diameter is less than? 1.2mm, and the welding current is about 120 ~ 150a.

3 CO2 automatic welding

Automatic welding has been widely used in developed countries because of its high quality and high efficiency. Taking the welding robot as an example, the ratio of welder to welder in Japan is 1: 2. The advantages of automatic welding are:

(1) The process is stable. Due to the adoption of mechanical devices, the interference of many human factors on the welding process, such as the change of dry elongation caused by hand jitter, is eliminated. The weld produced by automatic CO2 welding is beautiful and the quality is easy to guarantee.

(2) The process has good reproducibility, which is beneficial to mass repeated welding production.

(3) high production efficiency. Compared with CO2 semi-automatic welding, CO2 automatic welding further improves the production efficiency, and the mechanical device can work tirelessly.

(4) It is more beneficial to protect the health of welding operators.

CO2 automatic welding puts forward some special requirements for welding power supply, such as improving the success rate of arc initiation and dropping the ball after welding. At the same time, CO2 automatic welding technology is not a simple combination of robot and welding power supply, it involves more complex technology. Such as: robot trajectory control, attitude control and sensing technology, seam tracking, penetration control, etc. A set of CO2 automatic welding equipment with excellent performance is the perfect combination of these technologies. At present, the application level of CO2 automatic welding technology in China needs to be further improved. Automatic CO2 welding technology represents the future of CO2 welding technology.