The Types of Flux-cored Welding Wire

/in /by

    Flux-cored welding wire is more and more popular in engineering practice because of its lower comprehensive cost, faster deposition speed and less splash. According to the manufacturing process, it can be divided into seam flux-cored wire and seamless flux-cored wire. Seam flux-cored wire is a thin steel strip processed into grooves by forming rollers, involved in powder to roll into a tube and then wire drawing, finished wire need surface rust treatment. The seamless flux-cored wire is filled with powder in a pre-formed steel pipe, and then electroplated, wire drawing, can be copper plating, good performance, low cost, is the direction of future development.

    According to the composition of filling powder, flux-cored welding wire can be divided into slag flux-cored wire and metal powder flux-cored wire. The former can be divided into titanium type (acid slag), titanium-calcium type (neutral or weak alkaline slag) and alkaline (alkaline slag) flux-cored wire according to slag basicity. The titanium flux-cored wire has good weld forming and all-position welding operability, but the notch toughness, crack resistance is slightly poor, on the contrary, alkaline flux-cored wire notch toughness, good crack resistance, but poor appearance, molding and weld operation.

    The property of Titanium-calcium flux-cored wires is in between and is rarely used today. In recent years, the new titanium flux-cored wire not only has good welding technology but also has low diffusion hydrogen content and excellent impact toughness. Metal powder flux-cored wire has the characteristics of low slag (little slag production), good crack resistance, and has good welding performance with titanium flux-cored wire, its welding efficiency is higher than titanium flux-cored wire.

    Flux-cored wire can be used for welding low carbon steel, low alloy high strength steel, low-temperature steel, heat resistant steel, stainless steel and wear-resistant surfacing and other steel structures, the most commonly used include:

    • Low carbon steel and high strength steel flux cored wire

    Most of the titanium slag welding wire, good welding process, high productivity, mainly used for shipbuilding flux-cored wire, bridge, construction, vehicle manufacturing and other flux-cored wire with a tensile strength of 490MPa and 590Mpa.

    • Stainless steel flux cored wire

    There are more than 20 kinds of stainless steel flux cored wire, in addition to Cr-Ni stainless steel flux cored wire, and Cr stainless steel flux-cored wire. The diameter of welding wire is 0.8, 1.2, 1.6mm, etc., which can be used to weld stainless steel sheets, medium plate and thick plates. The shielding gas is mostly CO₂ but also can be a mixture of Ar+ (20%~50%) CO₂.

    • Wear-resistant surfacing flux-cored wire

    A certain amount of alloying elements is added into the drug core in order to increase the wear resistance or make the metal surface obtain some special properties. Or by adding alloy elements into sintered flux, the surfacing layer of the corresponding components can be obtained after surfacing. It can be matched with solid core or flux-cored wire to meet different surfacing requirements.

    The available protective gases are CO₂ and Ar+CO₂ mixture gas for flux-cored wire, the former used for the general structure. Therefore, according to the shielding gas, flux-cored wires can be divided into gas shielded flux cored wires and self-shielded flux-cored wires, that is, welding wires that can be arc welded without shielding gas or flux. Common gas shielded flux cored wires are AWS A5.29/5.28 E71T1-C(M), E81T1-K2, E81T1-NI1, E91T1-K2, E101-K3, E111T1-K3, E80C-G, E90C-G, E110C-G, etc. (general diameter 1.2mm-1.6mm). Self-shielding flux-cored welding wire is to put powder and metal powder as slagging, gas making and deoxidation or coated on the surface of the welding wire. During welding, the powder becomes slag and gas under the action of arc and plays slagging and gas making without gas protection. Self-protection flux-cored wire deposition efficiency is higher than electrode obviously, usually under four wind welding, suitable for outdoor or aerial work, mainly used for low carbon steel welded structure, should not be used for welding of high strength steel, and other important structures, it is worth noting that the self-protection welding wire soot is bigger,  ventilated and air change is needed when working in confined space.

    At present, there is no unified standard for the classification of flux-cored wire. According to the type and droplet transition form of flux-cored wire, most countries generally divide flux-cored wire into titanium flux-cored wire, alkali flux-cored wire, metal powder flux-cored wire and self-protection flux-cored wire.

    How to weld titanium and its alloy?

    /in /by

      Titanium metal has been used for various fields due to its unparalleled advantages, such as lightweight, high strength, good resistance to high and low temperature, excellent crack resistance and corrosion resistance in wet chlorine gas. Welding titanium poses an especially significant challenge to many welders since the metal itself is a rather novel one for most industrial sectors. While many materials can be used in welding, none have the combination of durability, flexibility and strength that are found in titanium. This combination of characteristics makes the material extremely difficult to work with and it poses particular challenges even for skilled workers who are trained and experienced in welding. This is what makes titanium welding extremely demanding. Here we will discuss the welding of titanium and its alloy, if interested, please read on!

      Weldability analysis

      • Embrittlement caused by contamination of interstitial elements

      Titanium is an active chemical element at high temperatures. Titanium can absorb hydrogen rapidly above 300℃, absorb oxygen rapidly above 600℃, and absorb nitrogen rapidly above 700℃. If no effective protection is obtained during the welding and post-welding cooling process, plasticity will decrease and brittleness will increase. The carbon of titanium material is generally controlled below 0.1%, because when the carbon exceeds its solubility, it generates hard and brittle TiC with network distribution, which is easy to cause cracks.

      • Hot crack

      Due to the titanium and titanium, alloy impurities content is less, it is not easy to produce hot cracks, which have high-quality requirements for the welding wire, unqualified welding wire will cause cracks, interlayers and other defects, a large number of impurities may cause welding hot cracks.

      • Delayed cracking may occur in the HEAT affected zone

      During welding, the hydrogen in the pool and the base metal in the low-temperature zone diffuses to the HEAT affected zone, which leads to the accumulation of hydrogen in the heat-affected zone and causes cracks under adverse stress conditions.

      • Porosity

      Porosity is the most common defect in welding titanium and titanium alloys. Generally is the weld porosity and fusion line porosity, porosity is generally located near the fusion line when the welding line energy is larger, but mainly in the welding area especially when the welding surface is polluted by water and oil.

      Welding Technology

      • Welding method

      GTAW welding method, direct current connection, using high-frequency arc ignition and attenuation of the arc extinguishing device welding machine.

      • Welding material

      The selection of welding wire should make the tensile strength of the welding seam is not lower than the lower limit of the standard tensile strength of the annealed base metal, the plasticity and corrosion resistance of the welding seam after welding state is not lower than the annealed base metal or similar to the base metal, and the weldability is good.

      The chemical composition of ER Ti-2 wire is shown in the table below.

      Welding wiresTiFeCNH O
      ERTi-2Balance0.30.10.050.0150.25
      Table 1
      • Selection of shielding gas and weld color

      The purity of argon for welding should not be lower than 99.99%, the moisture should be less than 50mL /m³, and the dew point should not be higher than -40℃. It should not be used when the pressure of bottled argon is lower than 0.981MPa. The welding pool and the area where the internal and external surface temperature of the welding joint is higher than 400℃ are protected by argon gas.

      Weld joints colorSilver Light yellowDark yellowPurple (metallic luster)Blue (metallic luster)Off-white, yellow-white
      Argon gas purity99.99%98.7%97.8%97.5%97%96%
      Welding qualityHigh qualityGood QualifiedQualifiedUnqualified Unqualified
      Table 2
      • Weld Preparation

      Effective measures should be taken to avoid mutual dissolving between steel and titanium in the welding process, keep the site clean and avoid using iron tools.

      Groove processing. After cutting the titanium pipe, the grinder is used to polish the groove. The groove Angle is 30°±2.5° on one side and the blunt edge is 0.5 ~ 1.5mm. The processing of the groove should not cause the base metal to produce overheating discoloration. The inner and outer surfaces of the groove and its sides within 25mm shall be cleaned by the following procedure: polishing by polishing machine — polishing by sandpaper wheel — cleaning by acetone. Clean the welding wire with a sponge dipped in acetone, and carefully check whether there are cracks and interlayers near the base metal groove and the welding wire, and wait for the dry end of the groove before operation. If welding cannot be done in time, self-adhesive tape and a plastic sheet should be used to protect the groove. The time from cleaning to welding is not more than 2 hours, welder’s gloves should be clean before use must be cleaned with anhydrous ethanol (or acetone), avoid cotton fiber attached to the surface of the welder.

      • Welding process parameters

      Wall thickness

      Welding layer

      Tungsten electrode diameter

      Welding current

      Wire diameter

      The argon gas flow

      The nozzle diameter

      Welding handle

      Drag cover

      Tube

      3-4

      2

      2.4

      75-95

      2.5

      11-13

      20-22

      11-22

      12

      5-6

      3

      2.4

      90-120

      2.5

      12-15

      20-22

      11-22

      18

      7-8

      3-4

      3.0

      120-160

      3.0

      12-15

      20-22

      11-22

      18

      It is worth noting that, under the condition of ensuring good weld formation, small line energy welding should be selected as far as possible, and the interlayer temperature should not be higher than 200℃ to prevent the grain from growing up for too long at high temperature. The welding process shall be carried out under the protection of argon: the welding torch nozzle shall be used to protect the molten pool, the welding torch drag cover shall be used to protect the hot weld and the outer surface of the near joint area, and the pipe shall be filled with argon to protect the welding seam and the inner surface of the near joint area. When the large-diameter titanium pipe is welded, the welder shall use a gas mask and a hand-held protective cover to protect the back of the welding pool.

      When welding tubes with a small diameter or fixed orifice, the soluble paper should be used at the place where the surface of the titanium tube is 150-300mm away from the groove (a larger value should be taken according to the operability) to prevent the seal soluble paper from being damaged by excessive pressure in the tube, and then argon gas should be filled to exhaust the air in the tube. Argon must be fully precharged before welding, and argon should be delayed after welding to fully cool the high-temperature area and prevent surface oxidation.

      Welding inspection

      The welder shall clean the bead surface to a good appearance.

      The width should be 2mm over the edge of the groove. The height of the fillet weld toe should meet the design requirements and the shape should be smooth. The surface quality shall meet the following requirements: no defects such as edge biting, crack, non-fusion, porosity, slag inclusion and splash are allowed; Weld residual height: when the wall thickness is less than 5mm, 0 ~ 1.5mm; When the wall thickness is greater than 5 mm, it is 1 ~ 2mm; The amount of staggered edge on the surface of c weld shall not be greater than 10% of the wall thickness, and not greater than 1mm.

      The bottom welds shall be penetrant inspected and shall be deemed to be free of cracks and any other surface defects. Check the color of the surface of each weld, which indicates the color change of the surface oxide film at different temperatures, and their mechanical properties are not the same. (See Table 3) Note: The pickling method should be used to distinguish low-temperature oxidation from high-temperature oxidation.

      Tips of welding Austenitic stainless steel

      /in /by

        Austenitic stainless steel is the most widely used type of stainless steel, mainly Cr18-Ni8, Cr25-Ni20, Cr25-Ni35 type. The welding of austenitic stainless steel has obvious characteristics:

        • Welding hot crack.

        Austenitic stainless steel is easy to form a bulky columnar grain structure when the welding joint parts of high temperature and retention time is longer because of small thermal conductivity and large linear expansion coefficient. In the process of solidification, if the content sulphur, phosphorus, tin, antimony, niobium and other impurity element are higher, This leads to the formation of low melting point eutectic between grains. When the welded joint is subjected to high tensile stress, solidification cracks are easy to form in the weld seam and liquefaction cracks are easy to form in the heat-affected zone, which are welding thermal cracks. The most effective method to prevent hot crack is to reduce the impurity elements which are easy to produce low melting point eutectic in steel and welding materials and to make the Cr – Ni austenitic stainless steel contain 4% ~ 12% ferrite structure.

        • Intergranular corrosion.

        According to the theory of chromium depletion, the precipitation of chromium carbide on the intergranular surface, resulting in chromium depletion at the grain boundary, is the main reason for the intergranular corrosion. Therefore, choosing ultra-low carbon grades or welding materials containing stabilized elements such as niobium and titanium are the main measures to prevent intergranular corrosion.

        • Stress corrosion cracking.

        Stress corrosion cracking (SCC) is usually presented as a brittle failure, and the processing time of failure is short and the damage is serious. Welding residual stress is the main cause of stress corrosion cracking in austenitic stainless steel. The microstructure change of the welded joint or the stress concentration of local corrosive media are also the reasons.

        • σ phase embrittlement of welded joints

        σ phase is a kind of brittle intermetallic compound which mainly concentrated in the grain boundary of columnar grains. For Cr-Ni austenitic stainless steel, especially for Ni-Cr-Mo stainless steel, it is prone to the δ-σ phase transition and the change will be more obvious when the δ ferrite content in weld joints by more than 12%, making obvious embrittlement in the weld metal, that is why the delta ferrite quantity of hot wall hydrogenation reactor wall surfacing layer will be controlled in 3%~10%.

        What welding material is suitable for 304 stainless steel welding?

        Type 308 welding material is recommended when welding 304 stainless steel because the additional elements in 308 stainless steel can better stabilize the weld zone. 308L wires are also an acceptable option.

        Low carbon stainless steel carbon content is less than 0.03%, while standard stainless steel can contain up to 0.08% carbon content. Manufacturers should give special consideration to the use of L-carbon welding materials because their low carbon content reduces the tendency for intergranular corrosion. Manufacturers of GMAW welding also use 3XXSi welds such as 308LSi or 316LSi because Si improves the wetting of welds. In cases where the weldment has a high hump or where the pool connection is poor at the toe of the fillet or lap weld, using an air-shielded wire containing Si can moisten the weld and increase the deposit rate. Type 347 welding materials with a small amount of Nb can be selected if carbide precipitation is considered.

        How to weld stainless steel and carbon steel?

        Some structural parts are welded to the surface of carbon steel with a corrosion-resistant layer to reduce costs. When welding carbon steel to alloy base metal, the use of higher alloy content welding material can balance the dilution rate in the weld. For example, when welding carbon steel and 304 or 316 stainless steel, as well as other dissimilar stainless steels, 309L wire or electrode is a suitable choice.

        If you want to get a higher Cr content, use 312 welding material. It should be pointed out that the thermal expansion rate of austenitic stainless steel is 50% higher than that of carbon steel. When welding, the difference in thermal expansion rate will produce internal stress, which will lead to the crack. In this case, it is necessary to select the appropriate welding material or specify the appropriate welding process (Fig. 1). It shows when welding carbon steel and stainless steel, the warping deformation caused by different thermal expansion rates needs more compensation.

        What is the proper pre-weld preparation?

        Before welding, use chlorine-free solvent to remove grease, marks and dust to avoid the corrosion resistance of stainless steel base material from carbon steel. Some companies use separate storage of stainless steel and carbon steel to avoid cross-contamination. When special grinding wheels and brushes with stainless steel are used to clean the area around the bevels, it is sometimes necessary to perform a secondary cleaning of the joints. Because the electrode compensation operation of stainless steel welding is more difficult than that of carbon steel, the joint cleaning is important.

        What is the correct post-weld treatment?

        First of all, let’s recall that the reason why stainless steel does not rust is that Cr and O react on the surface of the material to generate a layer of the dense oxide layer, and play a protective role. Stainless steel rust is caused by the precipitation of carbide and heating during the welding process resulting in the formation of iron oxide on the welding surface. Perfected weldments in the welding state may also produce undercut in the rusted area at the boundary of the welding heat-affected zone within 24 hours. Therefore, in order to regenerate the new chromium oxide, stainless steel needs to be polished, pickled, sanded, or washed after welding.

        How to control carbide precipitation in Austenitic stainless steel?

        When the carbon content exceeds 0.02% at 800-1600℉, C diffuses to Austenitic grain boundaries and reacts with Cr at grain boundaries to form chromium carbides. If a large amount of Cr is cured by element C, the corrosion resistance of stainless steel will decrease, and intergranular corrosion will occur when exposed to a corrosive environment. The experimental results show that intergranular corrosion occurs in the heat-affected zone of welding in the water tank with corrosive media. Using low carbon or special alloy welding materials can reduce the tendency of carbide precipitation and enhance corrosion resistance. Nb and Ti can also be added to solidify C. Compared with Cr, elements Nb and Ti have a greater affinity with C. The grade347 welding material is designed for this purpose.

        Why are stainless steel wires magnetic?

        Stainless steels with full Austenitic structure are non-magnetic. However, the higher welding temperature makes the grains in the microstructure grow larger and the susceptibility to crack increases after welding. To reduce thermal crack sensitivity, the welding consumable manufacturer adds ferrite forming elements to the welding material (Fig. 2). The ferrite phase reduces the austenite grain size and increases the crack resistance. The following picture shows the ferrite phase (gray part) distributed on the austenite matrix in 309L welding material.

        The magnet does not adhere firmly to the Austenitic weld metal, but a slight suction can be felt when thrown. This also leads some users to believe that the product is mislabeled or that the wrong solder material is used (especially when the label is removed from the package). The amount of ferrite in the welding material depends on the service temperature of the application. Excess ferrite, for example, reduces toughness at low temperatures. As a result, the ferrite quantity for grade 308 welding materials used in LNG pipelines is between 3 and 6, while the ferrite count for standard Type 308 welding materials is 8. In short, the welding materials may look similar, but even small differences in composition can sometimes make a big difference.