1. Overview of cryogenic steel
1) The technical requirements for low-temperature steel are generally: sufficient strength and sufficient toughness in a low-temperature environment, good welding performance, processing performance and corrosion resistance, etc. Among them, the low temperature toughness, that is, the ability to prevent the occurrence and expansion of brittle fracture at low temperature is the most important factor. Therefore, countries usually stipulate a certain impact toughness value at the lowest temperature.
2) Among the components of low-temperature steel, it is generally believed that elements such as carbon, silicon, phosphorus, sulfur, and nitrogen deteriorate low-temperature toughness, and phosphorus is the most harmful, so early low-temperature dephosphorization should be performed during smelting. Elements such as manganese and nickel can improve 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 low-temperature steel, which also affects the low-temperature toughness of steel. After quenching and tempering treatment, the low temperature toughness is obviously improved.
4) According to the different hot-forming methods, low-temperature steel can be divided into cast steel and rolled steel. 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 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 196°C, such as storage tanks for storage and transportation of liquefied biogas and methane, equipment for storing liquid oxygen, and manufacturing liquid oxygen and liquid nitrogen. Austenitic stainless steel is a very good low-temperature structural material. It has good low-temperature toughness, excellent welding performance, and low thermal conductivity. It is widely used in low-temperature fields, such as transport tankers and storage tanks for liquid hydrogen and liquid oxygen. However, because it contains more chromium and nickel, it is more expensive.
2. Overview of low temperature steel welding construction
When selecting 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 not different in principle from that of ordinary carbon steel, low alloy steel or stainless steel, and can be treated 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 torch. 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 edge can be preheated to 150-200°C before gas cutting, but not more than 200°C.
Gas cutting has no adverse effects on the areas affected by welding heat. However, due to the self-hardening properties of nickel-containing steel, the cut surface will harden. In order to ensure the satisfactory performance of the welded joint, it is best to use a grinding wheel to grind the surface of the cut surface clean before welding.
Arc gouging can be used if the weld bead or base metal is to be removed during welding construction. However, the surface of the notch should still be sanded clean before re-applying.
Oxyacetylene flame gouging should not be used because of the danger of overheating the steel.
2) Selection of welding method
Typical welding methods available for low-temperature steel include arc welding, submerged arc welding, and molten electrode argon arc welding.
Arc welding is the most commonly used welding method for low temperature steel, and it can be welded in various welding positions. The welding heat input is about 18-30KJ/cm. If a low-hydrogen type electrode is used, a completely satisfactory welded joint can be obtained. Not only the mechanical properties are good, but the notch toughness is also quite good. In addition, the arc welding machine is simple and cheap, and the equipment investment is small, and it is not affected by the position and direction. advantages such as limitations.
The heat input of submerged arc welding of low temperature steel is about 10-22KJ/cm. Because of its simple equipment, high welding efficiency and convenient operation, it is widely used. However, due to the heat insulation effect of the flux, the cooling rate will be slowed down, so there is a greater tendency to generate hot cracks. In addition, impurities and Si may often enter the weld metal from the flux, which will further encourage this tendency. Therefore, When using submerged arc welding, pay attention to the selection of welding wire and flux and operate carefully.
The joints welded by CO2 gas shielded welding have low toughness, so they are not used in low temperature steel welding.
Tungsten argon arc welding (TIG welding) is usually performed manually, and its welding heat input is limited to 9-15KJ/cm. Therefore, although welded joints have completely satisfactory properties, they are completely unsuitable when the steel thickness exceeds 12mm.
MIG welding is the most widely used automatic or semi-automatic welding method in low temperature steel welding. Its welding heat input is 23-40KJ/cm. According to 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). Short-circuit transition MIG welding has the problem of insufficient penetration, and the defect of poor fusion may occur. Similar problems exist with other MIG fluxes, but to a different degree. In order to make the arc more concentrated to achieve satisfactory penetration, several percent to tens of percent of CO2 or O2 can be infiltrated into pure argon as a shielding gas. Appropriate percentages shall be determined by testing for the particular steel being welded.
3) Selection of welding materials
Welding materials (including welding rod, welding wire and flux, etc.) should generally be based on the welding method used. Joint form and groove shape and other necessary characteristics to choose. For low-temperature steel, the most important thing to pay attention to 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.
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(1) Aluminum deoxidized steel
Aluminum deoxidized steel is a steel grade that is very sensitive to the influence of cooling rate after welding. Most of the electrodes used in manual arc welding of aluminum deoxidized steel are Si-Mn low-hydrogen electrodes or 1.5% Ni and 2.0% Ni electrodes.
In order to reduce the welding heat input, aluminum deoxidized steel generally only adopts multi-layer welding with thin electrodes of ≤¢3~3.2mm, so that the secondary heat cycle of the upper layer of weld can be used to refine the grains.
The impact toughness of the weld metal welded with Si-Mn series electrode will decrease sharply at 50℃ 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 welding electrodes are not too sensitive to this, so it is best to choose this kind of 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, matched with appropriate flux, the average Charpy toughness value of the weld metal at -55°C can reach 56-70J (5.7 ~7.1Kgf.m). Even when 0.5% Mo welding wire and manganese alloy basic flux are used, as long as the heat input is controlled below 26KJ/cm, weld metal with ν∑-55=55J (5.6Kgf.m) can still be produced.
When selecting flux, attention should be paid to the matching of Si and Mn in the weld metal. Test proof. The 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 value are 0.1~0.2%Si and 0.7~1.1%Mn. When selecting welding wire and Be aware of this when soldering.
Tungsten argon arc welding and metal argon arc welding are less used in aluminum deoxidized steel. The above welding wires for submerged arc welding can also be used for argon arc welding.
(2) 2.5Ni steel and 3.5Ni
The submerged arc welding or MIG welding of 2.5Ni steel and 3.5Ni steel can generally be welded with the same welding wire as the base material. But just as Wilkinson formula (5) shows, Mn is a hot cracking inhibitor element for low-nickel low-temperature steel. Keeping the manganese content in the weld metal at about 1.2% is very beneficial to prevent hot cracks such as arc crater cracks. This should be taken into account when choosing the combination of welding wire and flux.
3.5Ni steel tends to be tempered and embrittled, so after post-weld heat treatment (for example, 620°C×1 hour, then furnace cooling) 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 series welding wire has much smaller tendency of temper embrittlement. Using this welding wire can avoid the above difficulties.
(3) 9Ni steel
9Ni steel is usually heat treated by quenching and tempering or twice normalizing and tempering to maximize its low temperature toughness. But the weld metal of this steel cannot be heat treated as above. Therefore, it is difficult to obtain a weld metal with low-temperature toughness comparable to that of the base metal if iron-based welding consumables are used. At present, high-nickel welding materials are mainly used. The welds deposited by such welding materials will be completely austenitic. Although it has the disadvantages of lower strength than the 9Ni steel base material and very expensive prices, brittle fracture is no longer a serious problem for it.
From the above, it can be known that because 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 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 given full attention in the selection of welding materials.
When the same welding wire is used for welding, the strength and toughness of the weld metal of submerged arc welding are worse than those of MIG welding, which may be caused by the slowdown of weld cooling rate 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 belongs to low-temperature steel, the minimum service temperature is -70 ℃, and it 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 has good toughness and plasticity, it is generally not easy to produce hardening and crack defects, and has good weldability. ER80S-Ni1 argon arc welding wire can be used With W707Ni electrode, use argon-electric joint welding, or use ER80S-Ni1 argon arc welding wire, and use full argon arc welding to ensure good toughness of welded joints. The brand 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 instruction book or WPS. During welding, I-type butt joint and full argon arc welding are adopted for pipes with a diameter less than 76.2 mm; for pipes with a diameter greater than 76.2 mm, V-shaped grooves are made, and the method of argon-electric combination welding with argon arc priming and multi-layer filling is used or The method of full argon arc welding. The specific method is to select the corresponding welding method according to the difference in pipe diameter and pipe wall thickness in the WPS approved by the owner.
3) Heat treatment process
(1) Preheating before welding
When the ambient temperature is lower than 5 °C, the weldment needs to be preheated, and the preheating temperature is 100-150 °C; the preheating range is 100 mm on both sides of the weld; it is heated with an oxyacetylene flame (neutral flame), and the temperature is measured 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, except for the conditions of large weldment thickness or very severe restraint conditions, post-weld heat treatment is usually not carried out for low-temperature steel. 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 in accordance with the following regulations:
When the temperature rises above 400 ℃, the heating rate should not exceed 205 × 25/δ ℃/h, and should not exceed 330 ℃/h. The constant temperature time should be 1 hour per 25 mm wall thickness, and not less than 15 minutes. During the constant temperature period, the temperature difference between the highest and the lowest temperature should be lower than 65 ℃.
After constant temperature, the cooling rate should not be greater than 65 × 25/δ ℃/h, and should not be greater than 260 ℃/h. Natural cooling is allowed below 400 ℃. TS-1 type heat treatment equipment controlled by computer.
4) Precautions
(1) Strictly preheat according to the regulations, and control the interlayer temperature, and the interlayer temperature is controlled at 100-200 ℃. Each welding seam shall be welded at one time, and if it is interrupted, slow cooling measures shall be taken.
(2) The surface of the weldment is strictly prohibited from being scratched by the arc. The arc crater should be filled up and the defects should be ground with a grinding wheel when the arc is closed. 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 welding process card.
(6) The reinforcement of the weld is 0 ~ 2mm, and the width of each side of the weld is ≤ 2mm.
(7) Non-destructive testing can be carried out at least 24 hours after the weld visual inspection is qualified. Pipeline butt welds shall be subject to JB 4730-94.
(8) “Pressure Vessels: Non-destructive Testing of Pressure Vessels” standard, Class II qualified.
(9) Weld repair should be carried out before post-weld heat treatment. If repair is necessary after heat treatment, the weld should be re-heated after repair.
(10) If the geometric dimension of the weld surface exceeds the standard, grinding is allowed, and the thickness after grinding shall not be less than the design requirement.
(11) For general welding defects, a maximum of two repairs are allowed. If the two repairs are still unqualified, the weld must be cut off and re-welded according to the complete welding process.
Post time: Jun-21-2023
