01 Low Temperature Steel Overview
1) The general technical requirements for low-temperature steel are as follows: it must have sufficient strength and broad toughness under low-temperature conditions, along with good weldability, machinability and corrosion resistance.
Among these, low temperature resistance – that is, the ability to prevent and prevent brittle failures under low temperatures – is the most crucial factor. Therefore, each country generally prescribes a certain impact resistance value at the lowest temperatures.
2. Selection of Welding Methods
Low temperature steel can be welded using typical methods such as arc welding, submerged arc welding and gas metal arc welding.
Arc welding is the most commonly used method for low-temperature steel and can be applied to a variety of welding positions. Its heat input is approximately 18~30KJ/cm.
By using low hydrogen electrodes, completely satisfactory solder joints can be achieved, which not only exhibit good mechanical properties but also excellent notch toughness.
In addition, arc welding has the advantages of simple and cheap welding machines, less equipment investment, and no position or direction limitations.
The heat input of submerged arc welding for low temperature steel is about 10 ~ 22 KJ/cm. It is widely used because of its simplicity, high welding efficiency and easy operation.
However, due to the insulating effect of the flow, it delays cooling, leading to a greater tendency for hot cracks to form.
Additionally, impurities and silicon can enter the weld metal from the flux, which can exacerbate this tendency. Therefore, when using submerged arc welding, the choice of wire and flux must be carefully considered, and the operations must be carried out meticulously.
CO 2 gas shielded welding produces joints with lower toughness and is therefore not used in welding low temperature steel.
Tungsten inert gas (TIG) welding is normally operated manually and its heat input is limited to the range of 9 ~ 15 KJ/cm. Although the welding joint produced shows satisfactory performance, this method is not applicable when the steel thickness exceeds 12 mm.
Gas metal arc welding (MIG) is the most widely used automatic or semi-automatic welding method for low-temperature steel, with a heat input of 23 ~ 40KJ/cm.
Based on the droplet transfer method, it can be divided into short-circuit transfer (lower heat input), globular transfer (higher heat input) and pulsed spray transfer (higher heat input). Short-circuit MIG welding may have insufficient fusion depth, which may lead to incomplete fusion defects.
Other modes of MIG welding can also present similar problems, but to varying degrees. To achieve a satisfactory melting depth by making the arc more concentrated, a few to several tens of percent CO2 or O2 can be introduced into the pure argon used as shielding gas.
The appropriate percentage must be determined experimentally, based on the specific type of steel being welded.
3) Selection of welding materials
Welding materials (including electrodes, welding wires and fluxes) should generally be selected according to the adopted welding method, joint shape, groove shape and other required characteristics.
For low-temperature steel, the most important thing is to ensure that the weld metal has low-temperature toughness that matches the parent metal and to minimize the amount of hydrogen diffused.
(1) Aluminum Dead Steel
Aluminum quenched steel is highly sensitive to post-weld cooling speed. The electrodes used in manual arc welding for aluminum quenched steel are typically low hydrogen Si-Mn type or 1.5% Ni type, 2.0% Ni type.
To reduce welding heat input, steel with aluminum generally adopts multilayer welding with 3~3.2mm thin electrodes. This can utilize the secondary thermal cycle of the upper welding pass to refine the grains.
The impact resistance of weld metal welded with Si-Mn electrodes at 50°C will decrease drastically as heat input increases. For example, when the heat input increases from 18KJ/cm to 30KJ/cm, the toughness will lose more than 60%. 1.5% Ni and 2.5% Ni type electrodes are not sensitive to this, so they are the best choice for welding.
Submerged arc welding is a common automatic welding method for aluminum-hardened steel. The best composition for welding wire used in submerged arc welding contains 1.5 ~ 3.5% nickel and 0.5 ~ 1.0% molybdenum.
According to the literature, with the use of 2.5%Ni-0.8%Cr-0.5%Mo or 2%Ni welding wire and the appropriate flux, the average toughness value of the weld metal a - 55°C can reach 56-70J (5.7~ 7.1Kg/fm). Even with 0.5%Mo welding wire and alkaline manganese alloy flux, as long as the heat input is controlled below 26KJ/cm, a 55J (5.6Kg/fm) weld metal can still be made.
When choosing flux, pay attention to the correspondence of Si and Mn in the weld metal. Tests have shown that different Si and Mn contents in the weld metal can greatly affect its toughness. Optimal toughness is obtained with 0.1~0.2% Si and 0.7~1.1% Mn. This must be observed when selecting welding wires and fluxes.
Tungsten inert gas (TIG) and metal inert gas (MIG) welding is less commonly used on aluminum-hardened steel. The welding wires mentioned above for submerged arc welding can also be used for TIG welding.
(2) 2.5Ni Steel and 3.5Ni Steel
For submerged arc welding or MIG welding of 2.5Ni and 3.5Ni steels, welding wires with the same material as the base metal can generally be used. However, as shown in Wilkinson's formula, Mn is a hot cracking inhibitor for low-temperature, low-nickel steels.
Maintaining the manganese content in the weld metal at around 1.2% is beneficial to prevent arc cracking and other hot cracking. This should be prioritized when selecting the welding wire and flux combination.
The temper embrittlement tendency of 3.5Ni steel is high, therefore, after post-welding heat treatment for residual stress relief (e.g. 620℃ × 1 hour, then furnace cooling), the toughness will decrease drastically from 3 .8Kg/fm to 2.1Kg/fm and do not meet specifications.
The temper embrittlement tendency of the weld metal produced by 4.5%Ni-0.2%Mo welding wire is much lower, and the use of this wire can avoid the aforementioned difficulty.
(3) 9Ni steel
9Ni steel is generally subjected to quenching or double tempering heat treatment to maximize its toughness at low temperatures. However, the weld metal of this steel cannot undergo the aforementioned heat treatment.
Therefore, the use of ferrite welding materials makes it difficult to obtain weld metal with low temperature toughness comparable to the parent metal. The most commonly used are welding materials with a high nickel content.
The weld metal of these welding materials is a complete austenitic structure. Despite the disadvantages of lower strength compared to the base metal of 9Ni steel and the high cost, brittle fracture is no longer a serious problem for it.
From the above, we know that:
Because the weld metal is entirely austenitic, the low-temperature toughness of the weld metal welded with the welding electrodes and wires used can fully compete with the parent metal, although its tensile strength and yield point are lower than those of the weld metal. original metal.
Nickel-containing steel has self-hardening characteristics, therefore most welding electrodes and wires have taken measures to limit carbon content to achieve good weldability.
In welding materials, Mo is an important reinforcing element, while Nb, Ta, Ti and W are important reinforcing elements. Its importance has been fully recognized in the selection and configuration of welding materials.
When the same welding wire is used, the strength and toughness of the submerged arc welding metal are slightly lower than those of MIG welding. This may be due to the slower cooling rate of the solder and possible impurities or Si infiltration from the flux.
03 A333-GR6 Low Temperature Steel Pipe Welding
1) Weldability analysis of A333-GR6 steel
A333 – GR6 steel is a low temperature steel, with the lowest using temperature of -70℃, generally delivered in normalized or normalized plus tempered state. A333-GR6 steel has a low carbon content, therefore it has a low tendency to harden and crack when cold, good toughness and plasticity.
It generally does not easily produce hardening defects and cracks and has good weldability.
ER80S-Ni1 argon arc welding wire with W707Ni electrodes can be used, applying combined argon-electric welding, or ER80S-Ni1 argon arc welding wire can be used for full argon arc welding to ensure good welding joint strength.
The brand of argon arc welding wire and electrode may be chosen from products with the same performance, but owner approval must be obtained prior to use.
2) Welding process
During welding, for pipes with a diameter of less than 76.2 mm, type I butt joints and full argon arc welding are used; for pipes with a diameter of more than 76.2 mm, a V-type groove is opened and the method of argon arc root and multi-layer filled argon electric welding or full argon arc welding is used.
Specific practices depend on the pipe diameter and wall thickness approved by the owner.
3) Heat Treatment Process
(1) Preheating before welding
When the ambient temperature is below 5°C, welding preheating is necessary.
The preheating temperature is 100~150°C; the preheating range is 100mm on both sides of the weld; oxyacetylene flame (neutral flame) is used for heating, and the temperature is measured 50 ~ 100 mm away from the center of the weld by a temperature sensing pen, with temperature points evenly distributed for better temperature control.
(2) Post-welding heat treatment
In order to improve the notch toughness of low-temperature steel, the generally used materials have already been quenched and tempered. Inadequate post-welding heat treatment often deteriorates its performance at low temperatures, which should be given sufficient attention.
Therefore, except under conditions where the welding thickness is larger or the constraint conditions are very strict, low-temperature steel generally does not undergo post-welding heat treatment.
For example, welding of the newly added LPG pipeline in CSPC does not require post-welding heat treatment.
If post-welding heat treatment is really necessary in some projects, the heating rate, constant temperature time and cooling rate of post-welding heat treatment should be strictly carried out in accordance with the following provisions:
The constant temperature time must be 1h per 25mm of wall thickness and not less than 15min. The temperature difference between the highest and lowest temperatures during the constant temperature period must be less than 65°C.
After constant temperature, the cooling rate should be no more than 65×25/δ ℃/h and no more than 260 ℃/h. Below 400°C, natural cooling is acceptable. Computer-controlled heat treatment equipment must be used.
4) Precautions
(1) Strict preheating in accordance with regulations, controlling the interlayer temperature within 100 ~ 200 ℃. Each weld must be completed in one go; if interrupted, slow cooling measures must be taken.
(2) Arc scratches on the welding surface are strictly prohibited. After the arc has been extinguished, the crater must be filled and any defects rectified with a grinding wheel. Joints between layers in multilayer welding must be staggered.
(3) Line power must be strictly controlled, using small currents, low voltages and fast welding. For W707Ni electrodes with a diameter of 3.2 mm, the welding length per electrode must exceed 8 cm.
(4) Short arc and flicker-free operation should be adopted.
(5) The full penetration welding process must be used and must strictly follow the requirements of the welding process specifications and welding process card.
(6) The weld reinforcement should be 0~2mm and the weld flare should be ≤2mm on each side.
(7) After passing the weld appearance inspection, non-destructive tests can only be carried out after at least 24 hours. The JB 4730-94 standard must be applied to the pipe butt weld seams.
(8) The standard “Pressure Vessel: Non-Destructive Testing of Pressure Vessels” must be followed and level II qualification must be achieved.
(9) Welding repairs must be carried out before post-welding heat treatment. If repairs are necessary after heat treatment, the weld must be heat treated again after the repair.
(10) If the geometric size of the weld surface does not conform, grinding is permitted as long as the thickness after grinding does not fall below the design requirements.
(11) For general welding defects, a maximum of two repairs are permitted. If it still does not pass after two repairs, the weld must be cut and welded again according to the complete welding process.
