Hardfacing, as a key process to enhance the wear resistance and service life of metal parts, not only relies on appropriate hardfacing materials and welding techniques but also has strict requirements on the protective gas used. The choice of protective gas directly affects the quality of the hardfacing layer, including its bonding strength, uniformity, and resistance to defects such as porosity and cracks. So, what gas is used for hardfacing? Let's explore in detail.
The core function of protective gas in hardfacing is to isolate the molten pool from the air. During the hardfacing process, the high temperature causes the molten metal to be highly reactive, and it is easy to react with oxygen, nitrogen, and hydrogen in the air, resulting in defects such as oxides, nitrides, and pores in the hardfacing layer, which seriously reduce the performance of the hardfacing part. Therefore, the protective gas needs to have good inertness or appropriate reactivity to form a stable protective atmosphere around the molten pool.
Argon is one of the most commonly used protective gases in hardfacing. As an inert gas, argon does not react with molten metal, which can effectively prevent the molten pool from being contaminated by air. It has a high density, which can form a dense protective layer on the surface of the molten pool, and its protective effect is stable. Argon is especially suitable for hardfacing of non-ferrous metals such as aluminum and copper, as well as some high-alloy steels. For example, when hardfacing aluminum alloy parts, using argon as the protective gas can avoid the formation of aluminum oxide on the surface of the molten pool, ensuring the bonding quality between the hardfacing layer and the base metal.
Carbon dioxide is another commonly used gas in hardfacing, but it is rarely used alone and is often mixed with other gases. Carbon dioxide is an active gas. When used in hardfacing, it can react with the molten metal to a certain extent, which helps to reduce the surface tension of the molten metal, improve the fluidity of the molten pool, and make the hardfacing layer more uniform. However, if carbon dioxide is used alone, it may cause excessive oxidation of the molten metal, resulting in an increase in oxides in the hardfacing layer. Therefore, it is usually mixed with argon to form an argon-carbon dioxide mixed gas.
Argon-carbon dioxide mixed gas is a widely used protective gas in hardfacing of carbon steel and low-alloy steel. The common mixing ratio is 80% argon and 20% carbon dioxide, but it can also be adjusted according to specific hardfacing requirements. This mixed gas combines the advantages of argon and carbon dioxide: argon provides a stable protective effect, and carbon dioxide improves the fluidity of the molten pool. It can not only prevent the molten metal from being oxidized but also ensure that the hardfacing layer has good forming and bonding strength. For example, in the hardfacing of agricultural machinery parts made of low-alloy steel, using this mixed gas can obtain a hardfacing layer with uniform thickness and strong wear resistance.
In some special hardfacing scenarios, such as hardfacing of high-chromium cast iron or other high-alloy materials that are sensitive to nitrogen, pure argon is usually the first choice. Because nitrogen in the air or other gases may react with chromium in the alloy to form chromium nitrides, which will reduce the corrosion resistance and wear resistance of the hardfacing layer. At this time, using high-purity argon (purity above 99.99%) can minimize the intrusion of nitrogen, ensuring the performance of the hardfacing layer.
In addition, for hardfacing processes with higher requirements on the hardness and wear resistance of the hardfacing layer, such as hardfacing of mining machinery parts, a small amount of oxygen may be added to the argon-carbon dioxide mixed gas. The addition of oxygen can further improve the fluidity of the molten metal, reduce the occurrence of defects such as undercut, and make the hardfacing layer more compact. However, the amount of oxygen added must be strictly controlled, usually not exceeding 5%, otherwise, it will lead to excessive oxidation.
It should be noted that the choice of protective gas for hardfacing is not fixed. It needs to consider factors such as the type of base metal, the composition of the hardfacing material, and the hardfacing method. For example, when hardfacing high-nickel alloys, argon with a small amount of helium may be used to improve the heat input of the arc and ensure the melting of the hardfacing material. For manual metal arc hardfacing, since the electrode coating can provide a certain protective effect, the requirement for external protective gas is relatively low, but for gas metal arc hardfacing, the protective gas is particularly important.
In conclusion, there is no one-size-fits-all protective gas for hardfacing. Argon, argon-carbon dioxide mixed gas, and other mixed gases with special ratios are all common choices. The key is to select the appropriate protective gas according to the actual hardfacing conditions to ensure that the hardfacing layer has excellent performance. Before carrying out hardfacing, it is necessary to conduct sufficient tests and adjustments to determine the most suitable protective gas type and parameters.
