Effect of residual Al in gas shielded welding wire

2022-07-24
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Effect of residual Al in gas shielded welding wire of low alloy steel on hydrogen porosity in weld metal. Under the condition of 80%ar+20%co2 argon rich mixed gas shielded welding, the weld metal of low alloy heat-resistant steel welding wire containing a certain amount of residual Al is prone to produce hydrogen pores

0 preface

porosity is a kind of welding defect often encountered in welding research and production. The existence of pores in the weld metal not only weakens the effective section of the welded structure, but also significantly reduces the strength and toughness of the weld metal. At the same time, it will also cause stress concentration, which is more harmful to the dynamic load strength and fatigue strength. There are a variety of factors that promote weld porosity, such as rust, oil stain or other impurities on the surface of welding wire and welded metal groove, welding rod and flux are not dried or not dried enough before use, welding area is not fully protected, welding parameters or welding materials are improperly selected, etc. Different factors make the weld produce different types and shapes of pores, such as hydrogen pores, nitrogen pores, carbon monoxide pores, etc. The root cause of porosity in weld metal is that when a large number of gas atoms dissolved in molten drops and molten pool metal crystallize in the molten pool under the high temperature of welding arc, due to the sudden decrease of solubility, the gas atoms are in a supersaturated state, and the gas molecules generated by the reaction form bubbles in the liquid metal because they are insoluble in metal. When the speed of bubbles escaping outward is less than the crystallization speed of the molten pool, pores are formed in the weld [1]. For CO2 gas melting electrode shielded welding and argon rich mixed gas shielded welding, if the welding area is well protected and the welding parameters are properly selected, most of the pores in the weld metal are hydrogen pores. This is because the CO2 gas used to protect the welding area from the air is mostly a by-product of the brewery or alcohol plant, which inevitably contains more or less water or other hydrogen containing substances. At the same time, the argon in the mixed gas often contains water. If the total content of moisture and other hydrogen containing substances in the shielding gas exceeds a certain limit, the generation of hydrogen pores in the weld metal will be inevitable [2]. However, if the content of water and other hydrogen containing substances in the shielding gas is controlled within a certain range according to the requirements of relevant standards, hydrogen pores generally do not occur in the weld metal of CO2 gas shielded welding and argon rich mixed gas (80%ar+20%co2) shielded welding. This is because CO2 gas will undergo decomposition reaction (CO2 = CO + O) at high arc temperature. The decomposed atomic oxygen has strong oxidizability and reacts with [h] in the gas phase to generate Oh insoluble in liquid metal, thus effectively preventing the generation of hydrogen pores in the weld [3]. This paper describes that for the 80%ar+20%co2 mixed gas shielded welding of low alloy heat-resistant steel melting electrode, although the content of water or other hydrogen containing substances in the shielding gas is controlled within a certain range according to the requirements of relevant standards, the welding wire and the surface of the welded metal groove are free of rust, oil stain or other impurities, and the welding process parameters are properly selected, if the residual Al content in the welding wire is not strictly controlled, A large number of hydrogen pores will also be generated in the weld metal

1 test conditions

two kinds of shielding gases are used in the test, one is carbon dioxide gas with strong oxidation, and the other is argon rich mixed gas (80%ar+20%co2) with relatively weak oxidation. The carbon dioxide gas used is a commercial gas produced by an alcohol factory and specially used for welding production. It is an excellent product in line with the provisions of gb/t

two types of low alloy steel solid core welding wires of alloy system are selected in the test, one is low alloy heat-resistant steel welding wire with Cr Mo as the main alloy element, and the other is Mn Ni Mo high-strength steel welding wire. These test welding wires contain a certain amount of residual al because necessary control measures are not taken in the smelting process of welding wire steel, but the Al content conforms to the provisions of gb/t "carbon steel and low alloy steel welding wires for gas shielded welding" on residual al of low alloy high strength steel welding wires, i.e. not more than 0.10% [4]

the test method is to use the solid core welding wire with a diameter of 1.2mm of the above two types of alloy systems to carry out single pass automatic welding at horizontal position according to the welding specification parameters shown in Table 1 under two different oxidizing gases, CO2 and 80%ar+20%co2, on the test plate with a polished metal luster, 16Mn material and 12mm thickness. After welding, first observe whether there are visible pores on the weld surface with naked eyes; Then grind the weld and count the number of pores in the weld metal. See Table 2 for the main alloy composition and residual Al content of the six test welding wires with different residual Al content in accordance with the provisions of gb/t. Among them, the five welding wires numbered C1 ~ C5 are low alloy heat-resistant steel welding wires with Cr Mo as the main alloy element, and the welding wires numbered M6 are Mn Ni Mo low alloy high-strength steel welding wires without Cr alloy element. Table 1 welding specification parameters

welding wire diameter welding current arc voltage welding speed dry extension length gas flow 1.2mm270a28v5.5mm/s18mm18l/min

Table 2 main alloy elements and residual Al content of test welding wire (wt.%)

No. C simncrmovwnialc10.0970.791.531.250.560.38//0.008c20.0850.681.491.210.570.34//0.027c30.0790.701.551.230.510.39//0.040c40.0770.771.632.370.670.410.49/0.060c50.0800.80 1.501.260.55//0.080m60.0850.761.470.46//0.800.085

2 test results

residual al contents are different from each other The six test welding wires belonging to two types of alloy systems are welded under the protection of CO2 gas with different oxidation strength and 80%ar+20%co2 argon rich mixed gas. See Table 3 for whether hydrogen pores are generated in the weld metal or the number of hydrogen pores

from table 3, it is not difficult to see the influence of residual Al content of welding wire on hydrogen porosity in weld metal of gas shielded welding. It is not only related to the level of residual Al content of welding wire, but also depends on the alloy system of welding wire, and is also closely related to the oxidation of the used shielding gas. Welding under the protection of carbon dioxide gas with strong oxidizability, whether for low alloy heat-resistant steel welding wire C1 ~ C5 with Cr Mo as the main alloy element, or for Mn Ni Mo low alloy high-strength steel welding wire M6 without Cr alloy element, the presence of residual Al in the welding wire with a content of no more than 0.10% does not lead to the generation of hydrogen pores in the weld metal. However, when welding under the protection of 80%ar+20%co2 rich argon mixed gas with relatively weak oxidizability, for the Mn Ni Mo low alloy high strength steel welding wire M6, the presence of 0.085% residual Al in the welding wire has not caused the weld metal to generate electronic information. In the electrical era, hydrogen pores are rapidly developing with the main characteristics of intelligence, cloud, lightness and high speed, However, for C2 ~ C5 low alloy heat-resistant steel welding wires with Cr Mo as the main alloy element, the existence of a certain amount of residual Al in the welding wire leads to the generation of hydrogen pores in the weld metal, and the number of hydrogen pores in the weld metal increases with the increase of residual Al content in the welding wire, so that a large number of surface hydrogen pores appear in the weld metal of C4 and C5 welding wires with high residual Al content. C1 welding wire with residual Al content not more than 0.010% has no hydrogen pores in the weld metal. Table 3 Effect of residual Al content of welding wire on hydrogen porosity in gas shielded welding seam

welding wire No. welding wire type welding wire Al content (%) CO2 welding 80%ar+20%co2 welding c11.25cr-0.5mo-v0.0082 no porosity c21.25cr-0.5mo-v0.027 no porosity c31.25cr-0.5mo-v0.040 no porosity slightly more porosity c42cr-mowvtib0.060 no porosity more porosity c51.25cr-0.5mo0.080 no porosity large porosity m6mn-si-ni-mo0.085 no porosity no porosity

3 analysis and discussion

we know that under the high temperature of welding arc, The hydrogen in the gas phase of the welding area does not exist completely in the molecular state, and there are quite a lot of atomic hydrogen. Therefore, the solubility of hydrogen in the molten drop and high-temperature molten pool metal is quite high, but when the molten pool metal cools and crystallizes, the solubility of hydrogen decreases sharply. In this way, the supersaturated atomic hydrogen will promote the following reaction: 2[h] = H2, and the molecular hydrogen generated by the reaction will form bubbles in the liquid metal because it is insoluble in the metal. When too many hydrogen bubbles do not escape from the rapidly cooled molten pool metal, the hydrogen bubbles remaining in the weld are called hydrogen pores [1]. As mentioned above, the weld metal of the consumable electrode CO2 gas shielded arc welding generally does not produce hydrogen pores. This is because CO2 gas will undergo decomposition reaction at high arc temperature: CO2 = CO + O, and the decomposed atomic oxygen has strong oxidizability. It reacts with atomic hydrogen in the gas phase to generate Oh insoluble in liquid metal, which can effectively prevent the massive dissolution of atomic hydrogen in the droplet and high-temperature molten pool [3]. For 80%ar+20%co2 argon rich mixed gas shielded welding, although the shielding gas has certain oxidizability under high arc temperature due to the addition of active gas CO2, its oxidizability is relatively weak compared with single CO2 gas, and the tendency of hydrogen pores in weld metal is greater than that of CO2 gas shielded welding

the test results show that the generation of hydrogen pores in weld metal of GMAW and 80%ar+20%co2 argon rich mixed gas shielded welding also depends on the alloy system of weld metal, the residual Al content of welding wire and the oxidation of shielding gas to a large extent. In view of the root causes of this test result, it is discussed as follows: firstly, the viscosity of molten pool metal of low alloy heat-resistant steel with Cr content of more than 1% is large [4], which hinders the floating of bubbles generated therein, thus increasing the tendency of hydrogen pores in the weld; Secondly, Al has a very strong affinity with oxygen, which combines atomic hydrogen in the gas phase with atomic oxygen decomposed from CO2 gas at high arc temperature. The more al remains in the welding wire, the more atomic oxygen it combines with, resulting in the lack of atomic oxygen combined with atomic hydrogen in the gas phase, thus promoting the generation of hydrogen pores in the weld. Therefore, the low alloy heat-resistant steel welding wire with high metal viscosity in the weld pool is welded under the relatively weak oxidizing atmosphere of 80%ar+20%co2 argon rich mixed gas. If the welding wire contains a certain amount of residual Al, the original oxygen decomposed by 20% CO2 gas at high arc temperature is not enough to combine with [h] except for the part that first reacts with al, Moreover, the molten pool metal of the low alloy heat-resistant steel with high viscosity also prevents the floating of hydrogen bubbles, which is bound to lead to the formation of hydrogen pores in the weld. For the Mn Ni Mo low alloy high strength steel welding wire M6 which also contains a certain amount of residual Al, when it is welded under the relatively weak oxidizing 80%ar+20%co2 argon rich mixed gas atmosphere, there is no hydrogen pore in the weld metal. This is because although a small amount of CO2 gas is decomposed under high arc temperature, the atomic oxygen, except the part that first reacts with Al, is not enough to combine with [h], However, the metal viscosity in the molten pool is small, which is conducive to the floating of hydrogen bubbles. When welding wires with two kinds of alloy systems and a certain amount of residual al are welded in a single CO2 protective atmosphere with strong oxidation, there are no hydrogen pores in the weld metal. This is because a large amount of atomic oxygen decomposed from a single CO2 gas at high arc temperature is removed and first combined with Al

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