Cutting and Welding of Metals Properties of Hydrogen

Effect of hydrogen on weld metal and weld joint

Hydrogen has a significant effect on the properties of metals and on welding processes, which is often underestimated in the design and construction of welding processes. Defects in welded joints caused by the influence of hydrogen are often attributed to other causes, if they are not given due attention.

The influence of hydrogen on properties of welded joints is rather complicated and sometimes has casual character, namely: at a small change in composition of metal (gas saturation) in the welded joint defects suddenly appear (pores, cracks) which then can disappear also at small change of conditions (other party of metal, change of mode of ignition of electrodes, etc.). д.).

At performance of responsible constructions and designs the accidents are inadmissible, therefore last years there was a considerable quantity of works, devoted to research of influence of hydrogen on properties of metals during welding and on durability of welded connections.

The behavior of hydrogen in welding of different metals is different. For example, the allowable amount of hydrogen in steel will be unacceptably high for copper or aluminum alloys.

The complicated process of welded joint formation at fusion welding, having strong thermomechanical influence on welded metal, makes difficult to consider the influence of hydrogen, making unclear the reasons of defects in the weld joint, formed under the influence of a number of factors simultaneously.

Nevertheless, knowing the properties of hydrogen and its influence on fusion welding processes, in most cases it is possible to prevent the appearance of defects such as pores and cracks in the weld metal and in the heat-affected zone, or to reduce the probability of their occurrence.

The dissolution of hydrogen in the weld pool and its incomplete release in the process of crystallization and further cooling of the weld metal can lead to a number of defects, the most important of which are the following:

• a) Pore formation in the weld metal. When liquid weld metal is cooled, particularly during crystallization (the back of the weld pool), it becomes oversaturated with hydrogen, which leads to its evolution by the reaction
• 2[H] = H2.

Since molecular hydrogen is insoluble in the metal, its release from the crystallizing metal may be one of the main factors contributing to the appearance and development of pores in the weld metal.

b) Development of microcracks in the weld metal and fusion zone. The process of evolution of hydrogen does not stop even after the end of the crystallization of the metal. Even at room temperature, it proceeds at a tangible rate. However, the dissolution process is very slow and gets any noticeable development only when the metal is heated to 250300 ° C and above. Therefore, at room temperature not oversaturated with hydrogen the metal is not only released into the external environment, but to a large extent into microscopic and submicroscopic cavities (pores), which are always present in the solid metal. This leads to an increase of the pressure created by molecular hydrogen in them. The pressure level of molecular hydrogen is limited by the K value (equation (2)). Considering that the solubility of hydrogen at 20 ° C and =1 ata is

The effect of hydrogen on cracking of welded joints made of steels with different carbon equivalents is shown in fig. 1 и 2. As can be seen from these figures, increasing the carbon equivalent in the steel being welded significantly increases the effect of hydrogen on the propensity to form cracks.

Properties of Hydrogen | Environmental Chemistry | Chemistry | FuseSchool

d) Cold crack formation around the weld zone. In the arc welding of alloyed steels, accepting hardening, in the zone around the seam, sometimes observed the formation of cracks, located approximately parallel to the fusion boundary This phenomenon is especially common in welding alloyed steels with low-carbon electrodes coated with organic components, and therefore release significant amounts of hydrogen. Formation of cold cracks is often associated with the diffusion of hydrogen from the weld metal to the peri-joint zone. If low-carbon electrodes with hydrogen-containing substances in the coating are used for welding alloyed steels, the weld metal after completion of the crystallization process may contain significant amounts of hydrogen.

Transformation y in the weld metal will occur at an earlier time than in the peri-joint zone (due to the lower degree of alloying of the former and, consequently, less stable austenite in it). Owing to the discontinuous decrease in the solubility of hydrogen during transformations at.- a favorable conditions are created for its diffusion into the circumfabricated zone, which is still in the state of a solid solution. In the further decomposition of austenite in the peri-seam zone, hydrogen concentrates in its residual volumes and, at the end of this process, is “pushed out” into micro-cavities, creating significant pressure. These pressures, combined with structural and thermal stresses, cause the formation of cold cracks in the peri-seam zone.

e) The formation of “fish eyes” (welding flokens). A special defect of welded seams, called fish eyes, is found on the fracture surface of ruptured and bent specimens (fractured along the weld metal). These defects are visible as a light disk of small diameter with a very small cavity in its central part.

The light colored fracture at these defects indicates brittle fracture due to very high hydrogen molecular pressures in the cavity.

“Fish eyes differ from microfractures in that they are found only on fractures in specimens tested under relatively slow plastic deformation conditions. In tests causing rapid plastic deformation (e.g. impact strength test), specimen fractures are not accompanied by “fish-eye formation”. The presence of these defects in the fracture of the samples usually causes a decrease in the plastic characteristics of the weld metal (relative elongation and relative contraction).

According to modern views, at a slow deformation of the metal of the weld, containing in sufficient quantities of hydrogen, there is its enhanced diffusion of relatively large cavities from countless dislocations containing high concentrations of hydrogen, as well as from micropores containing molecular hydrogen at higher pressures than relatively large cavities (small cavities are filled with hydrogen faster than large). This diffusion becomes possible due to a local increase in the temperature of the metal along the planes of slip, along which the movement of hydrogen atoms occurs.

Told about the nature of the formation of “fish eyes” allows us to conclude that the removal of hydrogen from the weld metal by long exposure at room temperature or shorter exposure at 250 ^ 300 ° C allows you to eliminate the tendency to form these defects and thus improve the plastic properties of the weld metal. This is confirmed by the experimental data in Fig. 4. This kind of heat treatment has a favorable effect on the fatigue strength of the weld metal

f) Formation of hot (crystallization) cracks in welding of austenitic steels. Hot cracks in austenitic welds can be caused by hydrogen. So, at welding T-beam samples from steel Kh23N23M3D3 (E11-533) by an electrode wire which composition is close to composition of base metal, under flux AN-23 hot cracks were not observed. But after the electrode wire was artificially hydrogenated by cathodic etching, hot cracks appeared in the seams. The mechanism of hydrogen influence on formation and development of hot (crystallization) cracks has not been studied yet.

Equipment for hydrogen welding

Hydrogen welding equipments, while having different powers, operate from the regular electricity grid. They are equipped with a conventional acetylene torch, through the hose into which the hydrogen-oxygen mixture enters. Adjustment of their flame temperature allows to set it in a wide range (600-2600°C) ºС). The machines can be used for manual as well as automatic welding. Low labor intensity and no need for recharging make their operation uncomplicated.

Compact in dimensions, the equipment can be quite powerful at the same time. It is brought into operation in a few minutes, depending on the temperature at the welding place and the required gas flow. With the possession of basic skills of gas flame treatment, the implementation of hydrogen welding with their own hands will not be difficult, and the performance of the process with the quality of welds will not be worse than with traditional welding.

Unlike traditional welding, which uses acetylene as the main fuel gas, welding using hydrogen instead is not only productive, but also environmentally friendly. Welding with acetylene is fraught with atmospheric air pollution with toxic compounds, while the only product from the combustion process in hydrogen equipment is completely harmless steam.

these units are also absolutely safe to store, transport and operate. Not only welding, but also oxyfuel cutting (hand or machine cutting), brazing, powder surfacing, hardening and powder spraying can be done with them. Several different modes allow you to work in a wide range from joining materials with minimal thickness to cutting thick plates. Despite the small size and low power of these portable devices, they allow to weld and cut parts with thickness up to 2 mm both of ferrous and non-ferrous metals.

Hydrogen and oxygen welding

Highly efficient hydrogen-oxygen flame can serve as a quality alternative to acetylene-oxygen flame in welding, cutting and brazing processes. Partly, hydrogen-oxygen welding can be a substitute for welding in an inert gas medium. This method is practically harmless, because the product of combustion in this process is steam. Hydrogen welding performed with their own hands for performers who know the skills of gas welding with their own hands, does not require long retraining, a short briefing is sufficient

Types of flammable gases used

Gas welding of non-ferrous metals and carbon steels is mostly performed with acetylene. This is due to the high flame temperature and good heat of combustion. Acetylene is a gas with a characteristic odor, which gives it impurities of hydrogen fluoride and hydrogen sulfide. When heated to 500 degrees and at certain concentrations in mixtures with oxygen and air acetylene becomes explosive. Acetylene is formed by the reaction of calcium carbide with water. Calcium carbide itself is formed by fusing burnt lime and coke.

Petroleum gases and pyrolysis gases can be used as combustibles. They are gas mixtures formed during thermal decomposition of oil and its products. Are used for welding, cutting and soldering of steel pieces with thickness not exceeding 3 mm and for welding of non-ferrous metals and their alloys. Natural gas is a product of gas field development and consists of 93-99% methane.

Cylinder

Technical propane and propane-butane mixture are by-products of oil and natural petroleum gas extraction and processing. They are used when welding parts with thickness up to 6 mm, in some cases up to 12 mm. With the help of these gases it is possible to weld and solder cast iron, non-ferrous metals and alloys, to carry out oxygen and oxygen-flux cutting, surfacing, spraying of plastics.

Welding metal at home with hydrogen has become possible thanks to the development of special electrolyzers, which can work from both the home two-phase and three-phase network. These machines decompose water into oxygen and hydrogen in the right proportions for the welding process. Size range of produced electrolysers allows to cover almost all types of gas welding, brazing, cladding, powder spraying, manual and machine oxygen cutting. The units of different capacity allow to perform both micro welding and micro soldering, and cutting of steel sheets with thickness about 300 mm.

Properties of hydrogen

Hydrogen is a flammable gas. In its normal state, it is tasteless, odorless and colorless. Hydrogen is more than 14 times lighter than air, as it is the lightest gas in existence. It also has the highest thermal conductivity. It is highly soluble in many metals, including platinum, nickel and iron. It’s almost insoluble in water. This material is widely found on our planet, and in the universe it is the most common element. This ensures a relatively low material cost. For obtaining hydrogen various chemical methods are used.

Chemical and physical properties of hydrogen allow to use it as the main analog of acetylene. If compared with other welding methods, the use of hydrogen is virtually harmless for soldering, cutting and welding. This feature is due to steam, which is a product of combustion in this process. Almost everyone who works with gas welding can relatively quickly master hydrogen welding as well. Application of this gas is very widely demanded in various industries, and not only in welding.

Hydrogen welding process features

To begin with, hydrogen welding is a type of flame welding. Gas welding with their own hands has been actively used for many years. The combustible gas here is acitelin. Hydrogen welding uses hydrogen instead of acitelene, which is mixed with oxygen. This method has been proved to be more efficient. The result is a thin and high quality seam, but this method has one disadvantage, which is that in the process of welding a lot of slag is formed in the weld pool. To prevent this, small amounts of organic substances are added to the gas mixture, which quench the oxygen. As such substances usually used hydrocarbons, which have a boiling point in the range of 30-80 ° C: gasoline, hexane, heptane, benzene.

Another difficulty encountered in welding with hydrogen was the choice of an efficient gas supply source. Using a hydrogen cylinder is impractical and also very dangerous.

welding machine for hydrogen welding

Liquefied hydrogen in high concentrations can cause symptoms such as choking and dizziness!

Another disadvantage is that the flame of such gas is absolutely invisible during the day. This is why an oxygen welder can work with sensors.

Please note! Hydrogen welding with your own hands can be used to join parts of low-carbon steels, iron. It is not suitable for welding stainless steel products.

When choosing hydrogen welding as a method of joining parts, the user will find that the latter occurs much faster than with the same argon-arc or acetylene. First, water molecules dissociate (disintegrate) into oxygen and hydrogen under the action of high temperatures. Further, the single-atomic hydrogen is converted into two-atomic hydrogen, which provides additional thermal energy to accelerate the bonding process.

The same hydrogen is used to protect the welding zone, so the weld is of high quality. strong and tight. The only exception is copper and its alloys (due to the chemical properties of the material).

The heat released allows you to weld even tungsten (the most refractory metal with a melting point of 3422 degrees). Here again, hydrogen will act as a protective gas, preventing contamination by carbon, nitrogen or oxygen. The arc formed by the torch is quite stable and does not depend on the primary processing of the items being joined.

How to make hydrogen welding with your own hands

A small apparatus for hydrogen welding with his own hands is not difficult to make. The blueprint can be found on the websites, and everything you need can be found at the hardware store.

• Container for alkali solution, hydrogen is released by dissociation. Use a glass container. a jar of 0.5 liters. Two holes are made in the capron cover for the wires going to the electrodes. To seal the structure use “Moment” glue, it is necessary to limit the access of oxygen to the liquid.
• Electrodes are made from stainless steel strips, the recommended height. 4 cm. The plates are fastened with dielectric studs at a great distance from each other, so as to use the entire volume of liquid. Fix the terminals with screws so that the edges were minus, in the center. plus. A current will be injected.
• Diverter fitting for gas output is made from a flexible tube, through it with a syringe injected 10% solution of NaOH. Alkali is diluted in distilled water. In working condition, homemade electrolyzer heats up to 80 ° C.
• Another reservoir filled with water to 1/3rd of its volume serves as a water trap. The end of the outlet nozzle is placed in it.
• Working nozzle, from which combustible gas mixture exits, must be metal. Artisans use medical needles.
• DC current source. battery up to 12V or rectifier. Gas production depends on the current strength. By changing the indicators, adjust the power of the flame. For welding thin-walled workpieces, 3 volts is enough.

Electrolysis begins when the electric circuit is closed, after 2-3 minutes, the jet coming out of the nozzle, ignite. You can start working.

General Information

Electric arc welding. a method of metal joining, based on the use of an electric arc. The arc heats and melts the metal, allowing a weld to form. Can be heated to over 6000 degrees. This is enough to melt most existing types of metals.

Electric arc technology is widely used for welding and cutting of metals. It can be manual, semiautomatic or automatic.

Manual arc welding (aka arc welding). welding with manual labor and an electrode. The welder holds the electrode and guides it into the welding area, forms the weld and follows the process himself. In semi-automatic welding a welding wire is used as an electrode, which is fed into the welding area with a special mechanism. The welder is still monitoring the arc himself. And in automatic welding, both the wire feed and the arc movement are performed by automatic equipment.