1. Overview of low-temperature steel

1) The technical requirements for low-temperature steel are generally: sufficient strength and sufficient toughness in a low-temperature environment, good welding process performance, processing performance and corrosion resistance, etc. Among them, low-temperature toughness, that is, the ability to prevent the occurrence and expansion of brittle failure at low temperature is the most important factor. Therefore, countries usually stipulate a certain impact toughness value at the lowest temperature.

2) In the composition of low-temperature steel, it is generally believed that carbon, silicon, phosphorus, sulfur, nitrogen and other elements deteriorate the low-temperature toughness, and phosphorus is the most harmful, so it should be dephosphorized at low temperature in the early stage of smelting. Elements such as manganese and nickel can improve the low-temperature toughness. For every 1% increase in nickel content, the brittle critical transition temperature can be reduced by about 20 °C.

3) The heat treatment process has a decisive influence on the metallographic structure and grain size of the low-temperature steel, which also affects the low-temperature toughness of the steel. The low-temperature toughness after quenching and tempering treatment is obviously improved.

4) According to the different hot forming methods, low-temperature steel can be divided into two types: cast steel and rolled material. According to the difference of composition and metallographic structure, low-temperature steel can be divided into: low alloy steel, 6% nickel steel, 9% nickel steel, chromium-manganese or chromium-manganese-nickel austenitic steel and chromium-nickel austenitic stainless steel Wait. Low-alloy steel is generally used in a temperature range of about 100 ° C for the manufacture of refrigeration equipment, transportation equipment, vinyl above-ground storage rooms and petrochemical equipment. In the United States, the United Kingdom, Japan and other countries, 9% nickel steel is widely used in low-temperature structures at a temperature of 196 ° C, such as storage tanks for storage and transportation of liquefied biogas and methane, storage of liquid oxygen, and equipment for manufacturing liquid oxygen and liquid nitrogen. Austenitic stainless steel is a very good structural material for low temperatures. It has good low-temperature toughness, excellent welding performance and low thermal conductivity. However, because it contains a lot of chromium and nickel, it is more expensive.

2. Overview of low-temperature steel welding construction

When choosing the welding construction method and construction conditions of low-temperature steel, the focus of the problem is on the following two aspects: preventing the deterioration of the low-temperature toughness of the welded joint and preventing the occurrence of welding cracks.

1) Bevel processing

The groove form of low-temperature steel welded joints is no different from ordinary carbon steel, low alloy steel or stainless steel in principle, and can be handled as usual. But for 9Ni Gang, the opening angle of the groove is preferably not less than 70 degrees, and the blunt edge is preferably not less than 3mm.

All low-temperature steels can be cut with an oxyacetylene flame. It’s just that the cutting speed is slightly slower when gas cutting 9Ni steel than when gas cutting ordinary carbon structural steel. If the thickness of the steel exceeds 100mm, the cutting can be preheated to 150-200°C before gas cutting, but should not exceed 200°C.

Gas cutting has no adverse effect on areas affected by welding heat. However, due to the self-hardening properties of nickel-containing steel, the surface of the cut will harden. In order to ensure satisfactory performance of welded joints, it is best to use a grinding wheel to smooth and clean the surface of the cut before welding.

If the weld bead or base metal is to be removed during welding, arc gouging can be used. However, the notch surface should still be sanded clean before re-applying.

Oxyacetylene flame gouging cannot be used because of the risk of overheating the steel.

2) Selection of welding method

Typical welding methods available for low-temperature steel are arc welding, submerged arc welding, melting electrode argon arc welding, etc.

Arc welding is the most common welding method for low-temperature steels, and it can be applied in a variety of welding positions. The welding heat input is about 18-30KJ/cm. If a low-hydrogen type electrode is used, a fully satisfactory welded joint can be obtained, which not only has good mechanical properties but also has excellent notch toughness. In addition, arc welding and welding machines are simple and cheap, with low equipment investment, and can be independent of location and direction. limitations, etc.

The heat input of low-temperature steel submerged arc welding is about 10-22KJ/cm. Because of its simple equipment, high welding efficiency and convenient operation, it is widely used. However, due to the thermal insulation effect of the flux, the cooling rate will be slowed down, so the tendency to generate hot cracks is also greater. In addition, impurities and Si may often enter the weld metal from the flux, which will further promote this tendency. When using submerged arc welding, pay attention to the selection of welding wires and fluxes and operate them carefully.

The toughness of the joint welded by CO2 gas shielded welding is low, so it is not used in the welding of low-temperature steel.

Gas tungsten arc welding (TIG welding) is usually operated manually, and the welding heat input is limited to the range of 9-15KJ/cm. Therefore, while the welded joint has completely satisfactory properties, it is completely unsuitable when the steel thickness exceeds 12mm.

Melting electrode argon arc welding (MIG welding) is currently the most widely used automatic or semi-automatic welding method in low-temperature steel welding. Its welding heat input is 23-40KJ/cm. The droplet transfer method, it can be divided into three types: short-circuit transfer process (lower heat input), jet transfer process (higher heat input) and pulse jet transfer process (highest heat input). The short-circuit transition MIG welding has the problem of insufficient penetration, and there may be defects of poor fusion. Other methods of MIG solder have similar problems but to a different degree. In order to make the arc more concentrated to obtain satisfactory penetration, pure argon can be infiltrated as a shielding gas with a few percent to several tens of percent of CO2 or O2. The appropriate percentage should be determined experimentally for the specific steel being welded.

3) Selection of welding materials

Welding consumables (including electrodes, wires and fluxes, etc.) should generally be based on the welding method used. The joint form and groove shape and other necessary characteristics are selected. For low temperature steel, the most important thing is to make the weld metal have low-temperature toughness enough to match the base metal, and minimize the content of diffusible hydrogen in it.

(1) Aluminum deoxidized steel

Aluminum deoxidized steel is a steel grade that is very sensitive to the influence of post-weld cooling rate. Most of the electrodes used in manual arc welding of aluminum deoxidized steel are Si-Mn series low hydrogen electrodes or 1.5% Ni series and 2.0% Ni series electrodes.

In order to reduce the amount of welding heat input, aluminum deoxidized steel generally only uses thin electrodes of ≤¢3 ~ 3.2mm for multi-layer welding, so that the secondary thermal cycle of the upper weld bead can be used to refine the grains.

The impact toughness of the weld metal coated with Si-Mn electrode at 50℃ will decrease sharply with the increase of heat input. For example, when the heat input increases from 18KJ/cm to 30KJ/cm, the toughness will lose more than 60%. 1.5%Ni series and 2.5%Ni series electrodes are not very sensitive to this, so it is best to use this electrode for welding.

Submerged arc welding is a commonly used automatic welding method for aluminum deoxidized steel. The welding wire used in submerged arc welding is preferably the kind containing 1.5~3.5% nickel and 0.5~1.0% molybdenum.

According to the literature, with 2.5%Ni-0.8%Cr-0.5%Mo or 2%Ni welding wire, with the appropriate flux, the Charpy toughness value of the weld metal at -55℃ can reach an average of 56-70J (5.7 ~7.1Kgf.m). Even when using 0.5% Mo wire and manganese alloy basic flux, as long as the heat input is controlled below 26KJ/cm, the weld metal with ν∑-55=55J (5.6Kgf.m) can still be produced.

When choosing a flux, pay attention to the matching of Si and Mn in the weld metal. Test proof. Different Si and Mn contents in the weld metal will greatly change the Charpy toughness value. The Si and Mn contents with the best toughness values ​​are 0.1~0.2%Si and 0.7~1.1%Mn. This should be noted when fluxing.

TIG welding and melting electrode argon arc welding are less used in aluminum deoxidized steel. The above-mentioned welding wire for submerged arc welding can also be used for argon arc welding.

(2) 2.5Ni steel and 3.5Ni

Submerged arc welding or MIG welding of 2.5Ni steel and 3.5Ni can generally be performed with the same welding wire as the base metal. But as shown in Wilkinson’s formula (5), Mn is a hot cracking inhibitor for low nickel and low-temperature steels. Keeping the manganese content in the weld metal at about 1.2% is very beneficial to prevent hot cracks such as crater cracks. This should be taken into account when choosing the combination of wire and flux.

3.5Ni steel tends to be brittle during tempering, so after post-weld heat treatment (such as 620℃×1 hour, then furnace cooling) in order to eliminate residual stress, ν∑-100 will drop sharply from 3.8 Kgf.m to 2.1Kgf.m can no longer meet the requirements. The weld metal formed by welding with 4.5%Ni-0.2%Mo welding wire has a much smaller tendency to temper embrittlement. The above difficulties can be avoided by using this welding wire.

(3) 9Ni steel

9Ni steel is usually heat treated by quenching and tempering or twice normalizing and tempering to maximize its low-temperature toughness. However, the weld metal of this steel cannot be heat treated as described above. Therefore, it is difficult to obtain a weld metal with a low-temperature toughness that is comparable to that of the base metal if the iron system is used for welding materials. At present, mainly high-nickel welding materials are used. The welded seam of such welding material is completely austenitic. Although it has the disadvantages of lower strength than 9Ni steel base metal and very expensive price, for it, brittle fracture is no longer a serious problem.

It can be seen from the above that since the weld metal is completely austenitic, the low-temperature toughness of the weld metal used for welding with electrodes and wires is completely comparable to that of the base metal, but the tensile strength and yield point are lower than those of the base metal. Nickel-containing steel is self-hardening, so most electrodes and wires pay attention to limiting the carbon content in order to achieve good weldability.

 Mo is an important strengthening element in welding materials, while Nb, Ta, Ti and W are important toughening elements, which have been fully paid attention to in the selection of welding materials.

 When the same wire is used for welding, the strength and toughness of the submerged arc welding weld metal are worse than that of MIG welding, which may be caused by the slow cooling rate of the weld and the possible infiltration of impurities or Si from the flux. of.

3. A333-GR6 low-temperature steel pipe welding

1) Weldability analysis of A333-GR6 steel

A333–GR6 steel is a low-temperature steel with a minimum service temperature of -70 °C and is usually supplied in normalized or normalized and tempered state. A333-GR6 steel has low carbon content, so the hardening tendency and cold cracking tendency are relatively small, the material toughness and plasticity are good, generally hardening and crack defects are not easy to produce, and the weldability is good, ER80S-Ni1 argon arc welding wire can be used With W707Ni electrode, use argon electric welding, or choose ER80S-Ni1 argon arc welding wire, use full argon arc welding to ensure good toughness of welded joints. Brands of argon arc welding wire and electrode can also choose products with the same performance, but they can only be used with the consent of the owner.

2) Welding process

For detailed welding process methods, please refer to the Welding Process Instructions or WPS. In welding, the pipes with a diameter less than 76.2 mm shall adopt I-port butt joint and full argon arc welding; for pipes with a diameter greater than 76.2 mm, the V-shaped groove shall be made by argon arc backing and multi-layer filling argon electric welding method or Full argon arc welding method. The specific method is to select the corresponding welding method according to the different pipe diameters and pipe wall thicknesses in the WPS approved by the owner.

3) Heat treatment process

(1) Preheating before welding

When the ambient temperature is lower than 5 ℃, it is necessary to preheat the weldment, and the preheating temperature is 100 ~ 150 ℃; the preheating range is 100 mm on both sides of the weld; The pen measures the temperature at a distance of 50-100 mm from the center of the weld, and the temperature measurement points are evenly distributed to better control the temperature.

(2) Post-weld heat treatment

In order to improve the notch toughness of low-temperature steel, the materials generally used have been quenched and tempered. Improper post-weld heat treatment often deteriorates its low-temperature performance, which should be paid enough attention to. Therefore, low-temperature steels are usually not subjected to post-weld heat treatment except for the conditions where the thickness of the weldment is large or the restraint conditions are very severe. For example, the welding of new LPG pipelines in CSPC does not require post-weld heat treatment. If post-weld heat treatment is indeed required in some projects, the heating rate, constant temperature time and cooling rate of post-weld heat treatment must be strictly implemented in accordance with the following regulations:

When the temperature rises above 400 °C, the heating rate shall not be greater than 205 × 25/δ °C/h, and shall not be greater than 330 °C/h.  The constant temperature time shall be 1 h per 25 mm wall thickness, and shall not be less than 15 min. During the constant temperature period, the difference between the highest and lowest temperature shall be lower than 65 ℃.

The cooling rate after constant temperature shall not be greater than 65 × 25/δ °C/h, and shall not be greater than 260 °C/h, and it can be cooled naturally below 400 °C. Adopt computer-controlled TS-1 heat treatment equipment.

4) Precautions

(1) Strictly preheat according to regulations, control the temperature between layers, and control the temperature between layers at 100 ~ 200 ℃. Each weld should be finished at one time, and if it is interrupted, slow cooling measures should be taken.

(2) The surface of the weldment is strictly prohibited from being scratched by the arc, and the arc crater should be filled with a grinding wheel to remove the defects. The joints between layers of multi-layer welding should be staggered.

(3) Strictly control the line energy, adopt small current, low voltage and fast welding. The welding length of each W707Ni electrode with a diameter of 3.2 mm must be greater than 8 cm.

(4) The operation mode of short arc and no swing must be adopted.

(5) The full penetration process must be adopted, and it must be carried out in strict accordance with the requirements of the welding process specification and the welding process card.

(6) The excess height of the weld is 0 to 2mm, and the width of each side of the weld is ≤2mm.

(7) Non-destructive testing can be carried out after at least 24 hours after the weld appearance inspection is qualified. Pipe butt welds execute JB 4730-94.

(8) Standard of “Pressure Vessel: Nondestructive Testing of Pressure Vessels”, grade II qualified.

(9) Weld repair shall be carried out before post-weld heat treatment. If repair is required after heat treatment, the weld shall be heat treated again after repair.

(10) If the geometrical dimension of the weld surface forming exceeds the standard, grinding is allowed, and the thickness after grinding shall not be less than the design requirements.

(11) For general welding defects, up to two repairs are allowed. If the two repairs are still unqualified, the weld should be cut off and re-welded according to the complete welding process.

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