DE102018007766B4 - Process for the cold treatment of stick electrodes - Google Patents

Abstract

Process for the thermal treatment of a consumable stick electrode, in which the stick electrode is subjected to a cold treatment before its intended use, in which the stick electrode is cooled to a lower target temperature (T1) of below minus 50°C in a cooling phase (K) and then in a warming-up phase (W) is heated to an upper target temperature (T2).

Description

The invention relates to a method for the thermal treatment of a consumable stick electrode according to the preamble of patent claim 1.

Various welding processes are available to the welder for connecting metallic components. One of the best-known processes is welding with a consumable stick electrode, which can be manual or automated (gravity welding). Welding with a stick electrode is one of the arc welding processes that is characterized by the fact that an electrically induced arc burns between the stick electrode and the base material during the welding process. Stick electrodes comprise a core stick and a sheath. The core rods, the material of which is adapted to the material of the base material to be welded, are usually made from an endless wire that is drawn to the required wire diameter and then cut to length. In further steps, if necessary, an additional heat treatment takes place, during which the core rod is heated to a temperature of several up to 700° C. and more, and finally the electrode coating is applied to the core rod. This coating, which also melts during use during the welding process, shields the welding area from the surrounding atmosphere due to the gases released and the formation of slag. There are three main types of coating: rutile, basic, cellulosic and mixed types. The quality of the welding process and the welding result are strongly determined by the core thickness, the choice of the electrode coating and the electrical parameters of the power source.

Comparatively uneven weld seams with weld spatter are characteristic of manual welding with stick electrodes. Weld spatter is the term used to describe small, often spherical, melted spots on the surface of the welded workpiece or the weld seam. They result from droplets of liquid metal ejected from the weld pool or the liquid end of the electrode during the welding process. Weld spatter is in most cases harmless to the quality of the welded joint, but sometimes requires considerable, usually manual, reworking of the material surface.

In order to avoid welding spatter, it has hitherto been proposed to change certain parameters in the welding process. For example, in the EP 1 182 001 A1 or the EP 0 273 540 A1 suggested controlling the power supply with a view to minimizing the risk of spattering. Attempts have already been made to reduce the tendency to weld spatter during welding by optimizing the electrode alloy, as for example in DE 2 521 276 A1 suggested.

The measures described in the prior art have proven their worth, but there is still room for improvement with regard to reducing the tendency to spatter. In particular, they require the use of complex electrical control devices and/or comparatively expensive welding filler materials.

The object of the invention is to reduce the occurrence of weld spatter when welding with stick electrodes with little effort.

This object is achieved by a method having the features of patent claim 1 and with a stick electrode having the features of patent claim 9 . Advantageous configurations can be found in the dependent claims.

According to the invention, the stick electrodes undergo a cold treatment after their production and before their intended use as consumable stick electrodes, during which they are cooled to a lower target temperature of below minus 50°C, preferably below minus 100°C. Surprisingly, it has been shown that the welding result is significantly improved when using stick electrodes that have been treated according to the method according to the invention compared to the use of untreated stick electrodes with otherwise the same welding parameters. In particular, regardless of the type of electrode coating, a more stable welding process was observed, making it easier for the welder to achieve a uniform weld. A pronounced and uniform rippling of the weld seam could be achieved in all investigations. In addition, the number of spatters could be reduced, which made it possible to reduce the rework of the weld seam.

In the context of this invention, “cold treatment” is intended to refer to a method in which a workpiece is exposed to temperatures of -50° C. and below. The cooling usually takes place through direct or indirect contact with a cryogenic medium, for example cold gaseous or liquefied nitrogen or an inert gas. The cold treatment is preferably carried out in a closed container (cold chamber) in which a correspondingly low treatment temperature is set by direct or indirect thermal contact between the rod electrode and a refrigerant becomes. For example, the treatment temperature is set by generating a correspondingly tempered atmosphere in the cold chamber. To generate the temperature-controlled atmosphere in the cold chamber, an inert gas, for example nitrogen or an inert gas, is brought to an appropriate temperature outside the cold chamber and then fed to the cold chamber. Cold treatments of metallic workpieces that have become known to date primarily serve to increase hardness and wear resistance. An example is the transformation hardening of steel. The workpiece is first heated to a temperature of over 723°C so that the α-iron (ferrite) present at room temperature is converted into γ-iron (austenite). During subsequent rapid cooling (quenching), the iron atoms arrange themselves into a body-centered cubic crystal lattice that is tetragonally distorted by the carbon, the so-called martensite, and thus into an overall harder structure. The hardness of the workpiece can be further increased by cooling the workpiece to a temperature between minus 70° C. and minus 180° C. after the heat treatment and keeping it at this temperature for a period of 15 hours, for example. During such a cold treatment, any austenite (“residual austenite”) still present in the structure of the workpiece is converted into martensite. Methods of this type for the cold treatment of workpieces made of predominantly ferrous materials are, for example, from U.S. 6,537,396 B1 , the U.S. 3,819,428 A , the EP 184 29 29 A1 and the article by W. Lausecker, "How cool is that - the low-temperature treatment of cutting tools", Werkzeug-Technik 126.15. June 2012 known. In which relates to a method for cryogenic material treatment DE 25 17 147 A1 different, cold-induced rearrangement processes in carbon steel alloys and their effects on wear resistance and corrosion resistance are described.

On the other hand, it was previously unknown that cold treatment of welding consumables leads to a change in the weld quality. In particular, it was not known that cold treatment of stick electrodes during their subsequent use in an arc welding process has a stabilizing effect on the welding process and at the same time reduces the tendency to weld spatter. The cause of these improvements and a possible connection with a structural change induced by the cold treatment is currently unknown.

The improvement in weld quality and the reduced tendency to weld spatter occurs with all electrode materials examined, especially with electrodes intended for welding ferritic or austenitic steels.

The lower target temperature, ie the lowest cooling temperature, is preferably between minus 50° C. and minus 195° C., particularly preferably between minus 100° C. and minus 185° C., with values below minus 150° C. showing particularly good results.

The upper target temperature at the end of the method is a temperature value which is suitable for preventing the condensation of water from an ambient atmosphere present, in particular under normal conditions (20° C.), in order to avoid corrosion on the electrode surface. Until the upper target temperature is reached, the process according to the invention is therefore preferably carried out in an inert atmosphere with a low water content, for example in an atmosphere which consists predominantly of gaseous nitrogen and/or an inert gas. For example, the upper target temperature is between 20°C and 40°C.

A particularly advantageous embodiment of the invention provides that the stick electrode runs through a holding phase after the cooling phase and before the warm-up phase, in which the stick electrode is kept at the lower target temperature for a period of at least 30 s. The duration of the holding phase is preferably between 1 minute and 120 minutes.

The rod electrodes are preferably cooled and heated slowly in order to ensure that there is no temperature shock and that the workpiece is cooled completely and evenly. It is therefore advantageous if the rod electrode is not cooled down in the cooling phase and/or the rod electrode is heated up in the warming-up phase more quickly than with a temperature change (speed) ΔT/Δt of ΔT/Δt ≤ 10 K/min ΔT/Δt between 1 K/min and 10 K/min, particularly preferably between 1 K/min and 3 K/min (interruption times during the cooling or heating process are not included in each case).

In order to ensure uniform cooling or heating, it is also advantageous to interrupt the cooling or heating of the stick electrode several times during the cooling phase and/or the heating process of the warming-up phase and to allow the stick electrode to reach a predefined intermediate temperature for a predefined period of time hold (intermediate hold phase). For example, a break can be inserted at intervals of 5K to 50K the one during which the workpiece is kept essentially at the temperature reached. Following the intermediate holding phase, the cooling down process or the warming up process is continued as before. During the interruptions, rearrangement processes can still take place in the crystal structure of the workpiece material with a comparatively high diffusion rate, which can promote the homogeneity of the material and thus the development of positive material properties. According to the invention, the duration of an intermediate holding phase in the cooling phase and/or the warm-up phase is preferably at least 30 s in each case, preferably between 1 min and 120 min in each case.

A particularly advantageous embodiment of the invention provides for the workpiece to be heated once or several times during the cooling phase and/or the holding phase or an intermediate holding phase and then to be cooled again to the lower target temperature or another temperature. Such an interim warm-up phase can take the place of or in addition to an interim holding phase. The interim warming-up phases reduce stresses in the treated material that can occur in the material due to the temperature changes during the course of the cold treatment. Together with a cooling rate adapted to the respective workpiece, intermediate warm-up phases prevent a deterioration in the quality of the electrode; In particular, in the case of coated stick electrodes, they prevent the coating material from detaching from the electrode core.

The interim warming-up phase includes heating in the cooling-down phase and/or the holding phase by 10 K to 50 K, with the initial temperature (ie the temperature before the start of the cold treatment) preferably not being exceeded. An interim warm-up phase can take place several times in a row and/or be repeated several times during the cold treatment. Following the warm-up phase, there is renewed cooling by the same temperature difference, which is followed by renewed heating, further cooling or maintaining the temperature value reached. For example, during the holding phase at a lower target temperature of, for example, between minus 150°C and minus 195°C, the electrode is heated one or more times to a temperature between minus 140°C and minus 160°C.

The use of rod electrodes treated according to the method according to the invention as consumable rod electrodes leads to an improved welding result and a reduced tendency to weld spatter compared to untreated electrodes with otherwise the same electrode and welding parameters. The method according to the invention is suitable for the treatment of stick electrodes for all welding processes in which consumable stick electrodes are used, in particular manual arc welding or gravity arc welding. The method according to the invention is particularly suitable for stick electrodes with core rods made from unalloyed or high-alloy steel types or nickel alloys. It is also suitable for all types of stick electrodes, i.e. for rutile, acidic, basic or cellulose coated stick electrodes as well as for their mixed types.

In addition to the treatment of the electrode according to the method according to the invention, further measures to reduce the tendency to spatter can be taken when carrying out the welding work, such as are known in particular from the prior art and are described above; however, this does not affect the invention.

An embodiment of the invention will be explained in more detail with reference to the drawing. The drawing shows, in a temperature (T)-time (t) diagram, a schematic representation of the course of the temperature of a rod electrode during treatment using a method according to the invention.

A rod electrode at ambient temperature is fed into a cold chamber, which is then closed. By successively supplying a refrigerant, for example cold gaseous nitrogen at a temperature of minus 190° C., the temperature of the atmosphere inside the cold chamber is slowly lowered, for example at a rate ΔT/Δt between 1 K/min and 10 K/min. As a result, the temperature of the stick electrode drops during a cooling phase K to a lower target temperature T ₁ of minus 150° C., for example. Following the cooling phase K, the stick electrode is held at the lower target temperature T ₁ for a period of, for example, 1 min to 100 min (holding phase H). Following the holding phase H, the rod electrode is gradually heated to an upper temperature, i.e. at a rate comparable to the cooling rate in the cooling phase K, by supplying a gas (e.g. nitrogen) whose temperature is higher than the temperature inside the cold chamber Target temperature T ₂ , warmed up (warm-up phase W). T ₂ corresponds to the ambient temperature, for example.

In order to reduce stresses occurring in the treated material due to the cold treatment, it is advantageous to keep the temperature of the workpiece in the cold chamber during the To increase cooling phase K and / or the holding phase H temporarily.

In these intermediate warm-up phases A ₁ , A ₂ , the temperature increases by, for example, 10K to 50K to a value below the initial temperature. In the exemplary embodiment shown here, a first interim warm-up phase A ₁ takes place after the temperature of the wire electrode has reached a value of T ₅ , and a second warm-up phase A ₂ takes place after the lower target temperature T ₁ has been reached. Following the warm-up phase A ₂ , a further intermediate warm-up phase can follow, or the workpiece remains at the lower target temperature T ₁ for a certain period of time.

During the cooling phase K, the supply of the refrigerant can be stopped one or more times and the cooling of the rod electrode can be slowed down or kept at a predetermined temperature. Likewise, in the warm-up phase W, the supply of warm gas can be interrupted once or several times and the warm-up speed can thus be slowed down or the stick electrode can be kept at a predetermined temperature (intermediate holding phases). In these intermediate holding phases, ΔT/Δt << 1 K/min. In the embodiment shown in the drawing, intermediate holding phases during the cooling phase K at the temperatures T ₃ and T ₄ , with T ₁ <T ₅ <T ₄ <T ₃ <T ₂ , and during the warm-up phase W at a temperature T ₆ , with T ₁ < T ₆ < T ₂ , and loaded again at T ₄ . After the cold treatment, the stick electrode is removed from the cold chamber and is ready for use.

Claims (9)

Process for thermal treatment of a consumable stick electrode, in which the stick electrode is subjected to a cold treatment before its intended use, in which the stick electrode cools down to a lower target temperature (T ₁ ) of below minus 50°C in a cooling phase (K) and then in a Warm-up phase (W) to an upper target temperature (T ₂ ) is heated. procedure after claim 1 , characterized in that the lower target temperature (T ₁ ) is between minus 50°C and minus 195°C, preferably between minus 100°C and minus 185°C. procedure after claim 1 or 2 , characterized in that the upper target temperature (T ₂ ) is between 20°C and 40°C. Method according to one of the preceding claims, characterized in that the stick electrode runs through a holding phase (H) after the cooling phase (K) and before the warm-up phase (W), in which the stick electrode is at the lower target temperature (T ₁ ) is held. Method according to one of the preceding claims, characterized in that the cooling of the stick electrode in the cooling phase (K) and/or the heating of the stick electrode in the warming-up phase (W) at a speed of no faster than 10 K/min, preferably between 1K/ min and 10 K/min. Method according to one of the preceding claims, characterized in that the cooling phase (K) and/or the warming-up phase (W) is interrupted for a predetermined period of time, during which the rod electrode is essentially kept at a temperature that has been reached. Method according to one of the preceding claims, characterized in that the stick electrode is heated several times during the cooling phase (K) and/or holding phase (H) and then cooled again to the lower target temperature (T ₂ ). procedure after claim 7 , characterized in that the rod electrode is heated by 10K to 50K each time it is heated during the cooling phase (K) and/or the holding phase (H) and is then cooled again by this temperature difference. Cold-treated stick electrode, characterized in that a stick electrode has been subjected to a method according to one of the preceding claims.

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