Selection of welding materials in long-distance pipeline welding

The choice of welding materials is an important aspect to consider for long-distance pipeline welding. It includes welding rods, welding wire, flux and shielding gas in a broad sense. In the narrow sense, it refers to welding rods and welding wires.

1 welding rod

Welding rods for long-distance pipeline welding currently use all-position down-welding electrodes and conventional low-hydrogen welding rods. The implementation standards are GB/T 5117-1995, GB/T 5118-1995, AWS A5.1-91 and AWS A5. .5-96 and so on. The all-position down welding electrode is divided into two categories: one is high cellulose type (based on the low content of pipeline steel C, S, P, can be considered), the welding rod has good welding performance and less slag. Moreover, the blowing force is large, the slag and molten iron are prevented from being smashed, and the penetration capability and the faster deposition speed are large, and the single-sided welding double-sided forming effect is good in various positions, and is suitable for root welding and heat. Welding, representative of BOHLER FOX CEL (AWS A5.1-91 E6010) and BOHLER FOX CEL 85 (AWS A5.5-96 E8010-P1) electrodes produced by Austrian Böhler, FLEETWELD 5P+ produced by Lincoln, USA AWS A5.1-91 E6010) and SHIELD ARC 70+ (AWS A5.5-96 E8010-G) welding rod, SRE425G (AWS A5.1-91 E6010), SRE505 (AWS A5) developed by CSIC 725 .5-96 E7010—G) and SRE555 (AWS A5.5-96 E8010—G) welding rods, etc. The other type is iron powder low-hydrogen type down-welding electrode, which has fast solidification speed, good fluidity and wettability of molten iron, is not easy to squat during full-position welding, has good toughness of weld metal after welding, and has good crack resistance. Down-welding of each layer, representative of BOHLER FOX BVD 85 (AWS A5.5-96 E8018-G) electrode produced by Austria Bole Company, LINCOLN LH D80 (AWS A5.5-96 E8018) produced by Lincoln Company of USA -G) Welding rod. For the traditional low-hydrogen welding rod, due to its general position to root welding, the process performance is general, the arc ignition is difficult, the arc stability is poor, the splash is large, the back surface is poorly formed, and the air hole is easy to be used. Among the repair and rework solder-filled cap welding, CHE507GX (GB/T 5118-1995 E5015) and CHE557GX (GB/T 5118-1995 E5515) are produced by Sichuan Atlantic Company. The specifications of the above-mentioned commonly used high-cellulose electrode are generally Φ3.2mm, Φ4.0mm, the specification of the iron powder low-hydrogen type electrode is generally Φ4.0mm, and the specification of the ordinary low-hydrogen type electrode is generally Φ3.2mm.

Generally speaking, σ0.5(0.2)≤415MPa oil transmission and water pipeline main line welding can choose high-fiber type electrode for welding of each layer; gas pipeline or σ0.5>415Mpa oil pipeline main line welding can adopt high cellulose Type welding electrode root welding, heat welding + low hydrogen type down welding electrode filling, cover surface composite process.

2 welding wire

The long-distance pipeline is divided into two types: solid wire and flux cored wire.

(1) Solid wire

There are two main types of solid wire: one for submerged arc welding and the other for melting active gas shielded welding. Solid welding wire for submerged arc welding is carried out according to GB/T 5293-1999, with low manganese welding wire, such as H08A (such as CHW-SG welding wire produced by Sichuan Atlantic Company) with high manganese type smelting flux for low carbon steel and strength grade. Low pipeline steel welding; medium manganese welding wire, such as H08MnA, H10MnSi, combined with high manganese high silicon low fluorine type smelting flux is mainly used for pipeline steel welding, and can be used with low manganese flux for low carbon steel welding; high manganese welding wire, For example, H08Mn2Si and H08Mn2SiA are used for pipeline steel welding; Mn-Mo welding wire, such as H08MnMoA, HO8MnMoTiB, combined with low-manganese silicon in fluorine-type smelting flux, fluorine-alkaline sintered flux or silicon-calcium sintered flux, mainly used for high strength grade Pipeline steel welding. The wire diameter is generally in the range of 1.6 to 6.4 mm.

The solid welding wire for reactive gas shielded welding has the following standards: GB/T 14947-1994, GB/T 8110-1995, AWS 5.18-93 and AWS 5.28-96. The most commonly used welding wire is H08Mn2SiA (equivalent to GB/T 8110). ER49-1), it has good welding process performance and is suitable for welding pipeline steel with σs≤500MPa. When the steel grade with higher welding strength grade, the welding wire containing Mo should be selected, for example, domestic H10MnSiMo welding wire and JM-58 welding wire, BOHLER SG3-P welding wire produced by Jintai Company which performs American standard AWS 5.18 ER70S-G and Perform Jintai JM-68 welding wire of American standard AWS A5.28 ER80S-G. The specifications of common welding wires are φ0.9mm, φ1.0mm, φ1.2mm, and the like.

(2) Flux cored wire

In recent years, with the development of long-distance pipelines toward high strength, large diameter and thick wall, the traditional manual welding method has been gradually replaced by semi-automatic welding and automatic welding methods, among which semi-automatic welding applications are the fastest growing. What comes with it is the rapid development of flux-cored wire.

The reason why the flux-cored wire can get such attention and development is inseparable from its many characteristics. It is characterized by fast deposition speed and high welding productivity. Compared with solid wire, the flux-cored wire arc is soft. The spatter is small, the welding process performance is good; the penetration depth is large, the molding is beautiful; the comprehensive cost is low.

The flux-cored wire can be divided into gas-protected flux cored wire and self-shielded flux-cored wire according to the different protection methods during welding. The self-shielded flux-cored wire is widely used in long-distance pipelines with its unique superiority. The implementation standard is GB/ T 17493-1998 and AWS A. 29-98. Typical examples include T8-Ni1 type (such as Tianjin Jinqiao JC-29Ni1 Φ2.0mm welding wire, American Haobot HOBART 81N1Φ2.0mm welding wire), T8-Ni2, T8-K6 type (Lincoln NR207 Φ2.0mm welding wire), etc. The welding wire has good operation performance at all positions, and the welding speed is fast, and the weld metal toughness is good, but the appearance of the coarse columnar crystal structure of the weld metal in the welded state makes the weld metal impact toughness in the welded state and the heat treatment. There is a big difference between multi-layer welding and single-pass welding. Therefore, when welding with T8 self-shielded welding wire, the welding specification parameters, heat input amount, welding pass and thickness of each weld layer should be strictly controlled.

3 protective gas

For the automatic welding of long-distance pipelines, carbon dioxide gas shielded welding and oxidizing mixed gas shielded welding are used, that is, the gas used is CO2, CO2+Ar or CO2+Ar+O2. The role of an inert gas (such as Ar) in the gas metal arc welding is to discharge the arc and the air around the molten metal to prevent harmful components in the air from affecting the stability of the arc and contamination of the liquid metal. Other non-inert gases (such as CO2, O2) can also be used as shielding gas for gas metal arc welding. The premise is that although these gases can undergo some metallurgical reaction with the protected liquid metal, conditions can be created during the welding process so that the consequences of these reactions do not cause damage to the welded joint. If CO2 is used as the shielding gas, although CO2 decomposes O2 and CO2 at the high temperature of the arc during the welding process, and then Fe is oxidized to form FeO and may cause pores, this adverse effect can be achieved by adding an appropriate amount of Si to the welding wire. Deoxidation elements such as Mn are solved. The study found that the protective gas composition and flow rate have a certain influence on the weld formation, the composition and flow rate are different, the oxygen content in the weld is different, the weld formation is different, and the defect probability is also different. For example, when pure CO2 is used as the shielding gas for the STT gas shielded root welding and the flow rate is too large, the weld condensation is accelerated due to the decomposition and heat absorption of CO2, and the fluidity of the molten iron is deteriorated, so that the front weld is easy to form a ridge shape, in the subsequent During the welding process, the depressions are likely to cause defects such as unmelting and slag inclusion, and the back welds are liable to cause false melting. This problem is particularly prominent when the welding environment temperature is low and the line energy is low. In addition, welds are prone to venting in the weld due to rapid condensation. If a CO2+Ar mixed gas such as CO2 (15-20%) + Ar (85-80%) is used, the molten iron fluidity can be improved, good weld bead formation, good transition between the base metal and the weld, and low oxygen content in the weld. The weld has good impact toughness. This should be taken into account when choosing the protective gas composition and flow rate.

4 flux

The choice of flux mainly considers the type of flux, the matching characteristics of the flux and the wire, the metallurgical properties of the flux and the process performance. In addition, the particle size, water content, mechanical inclusions, and sulfur content of the flux should also be considered. From the viewpoint of improving the toughness of the weld metal, a high alkalinity flux can be selected. However, it should be noted that when the alkalinity exceeds a certain critical value, increasing the alkalinity will result in a decrease in weld toughness. This is mainly because the welding speed is required for the pipeline steel welding, especially when the thick plate is not opened. Under the condition of no groove and no gap, the process performance deteriorates, and there are pores and pitting on the surface of the weld. The oxide inclusions in the weld are obviously increased, resulting in a decrease in toughness. Therefore, the reasonable choice of flux is of great significance to improve the weld toughness.