RU2729488C1 - Method of local low-temperature thermal treatment of welded joints of large-size products - Google Patents

Abstract

FIELD: technological processes.

SUBSTANCE: invention relates to a method for local low-temperature heat treatment of welded joints of large-size articles. Welding zone is heated from temperature 5–45 °C to temperature of 400–450 °C at a rate of not more than 15 °C/h. Exposure is carried out at a given temperature for 20–25 hours, and cooling at a specified rate of not more than 20 °C/h to temperature of not more than 250 °C with subsequent cooling in air to temperature of not more than 45 °C.

EFFECT: technical result consists in improvement of strength properties of welded joints of large-size products.

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Description

The invention relates to the field of heat treatment and is intended to improve the strength properties of welded joints by relieving stresses arising during the welding process, during local low-temperature heat treatment, mainly of large-sized products operating in the oil refining, energy, chemical industries.

Welded joints made by arc methods immediately after heat treatment are characterized by inhomogeneity of properties and structure of the weld, heat-affected zone, as well as by the presence of welding stresses in them. One of the main means of solving these problems and increasing the reliability of welded joints is heating or heat treatment, as a result of which the level of welding stresses is reduced, the structure and properties of the joint metal are improved. At present, heat treatment technology uses installations for out-of-furnace local heat treatment of welded joints of large-sized items using heating devices consisting of separate heaters. A feature of the application of the proposed method for heat treatment of welded joints of large-sized products is a significant decrease in labor intensity during heat treatment, including the complex shape of the heated zone.

A known method of heat treatment of welded joints, including heating the zone of the welded joint, holding at a given temperature, cooling at a controlled rate, then cooling in air, (patent RU No. 2299252 C1, IPC C21D9 / 50, C21D1 / 30, priority from 16.11.2005, publ. 20.05.2007).

The disadvantage of this method is the high holding temperature during tempering after welding, which leads to scale formation on the surfaces of the workpiece, significant temperature deformations for devices with complex internal casing devices made of steels of various classes, the impossibility of removing scale on the inner surfaces that is formed during the operation of the product, which can lead to a decrease in the strength properties of the welded joint.

The closest technical solution, chosen as a prototype, is a method of heat treatment of welded joints, including heating the welded joint zone, holding at a given temperature, cooling at a controlled rate, then cooling in air, (patent RU No. 2566241 C1, IPC C21D9 / 50, priority from 07/08/2014, publ. 10/20/2015).

The known method is also associated with thermal deformations during heat treatment of devices with complex internals made of steels of different structural classes, with the formation of unacceptable scale on the internals, as a result of which the strength properties of the welded joint are reduced.

The technical result of the present invention is to relieve stresses arising in the welding process, increase the strength properties of welded joints of large-sized products without the formation of peeling scale on its surfaces.

The technical result is achieved by the fact that the method of local low-temperature heat treatment of welded joints of large-sized products, including heating the zone of the welded joint, holding at a given temperature, cooling at a controlled rate, followed by cooling in air, according to the invention, heating the zone of the welded joint is carried out from a temperature of 5-45 ° C to a temperature of 400-450 ° C at a rate of no more than 15 ° C / hour, holding at a given temperature is performed for 20-25 hours, cooling at a regulated rate is carried out no more than 20 ° C / hour to a temperature of no more than 250 ° C , and subsequent cooling in air is carried out to a temperature not exceeding 45 ° C.

Heating the welded joint zone of large-sized products from a temperature of 5-45 ° to a temperature of 400-450 ° C at a rate of no more than 15 ° C / hour provides an effective low-temperature heat treatment process, which allows relieving stresses arising during the welding process, avoiding temperature deformations and intense scale formation on the surfaces of the workpiece, which makes it possible to increase the strength properties of welded joints and thereby ensure the operational reliability of a large-sized product.

When the welded joint is heated below a temperature of 400-450 ° C and at a rate of more than 15 ° C / hour, the stress arising during the welding process is not relieved, as well as the elimination of temperature deformations, the formation of peeling scale on the surface of the workpiece is possible, which reduces the efficiency of carrying out the process of low-temperature heat treatment and, as a consequence, the strength properties of the welded joint and the operational reliability of the product decrease.

Increasing the holding time at the achieved temperature (400-450 ° C) to 20-25 hours promotes the greatest relief of stresses arising during the welding process, which makes it possible to increase the strength properties of the welded joints of the product.

When holding for less than 20-25 hours, the removal of the necessary stresses arising during the welding process is not ensured, which reduces the operational reliability of the welded joint.

Cooling the product at a regulated rate of no more than 20 ° C / hour to a temperature of no more than 250 ° C, followed by cooling in air, eliminates the possibility of deformations of the processed product.

When the product is cooled at a rate of more than 20 ° C / h to a temperature above 250 ° C, deformations of the workpiece occur, which affects the strength properties of the welded joint.

Local low-temperature heat treatment of welded joints of large-sized joints is carried out after the end of welding operations.

The essence of the proposed method is as follows:

To carry out local low-temperature heat treatment of welded joints of large-sized products, thermoelectric converters (TEC) are preliminarily installed on the welded joint and adjacent to its edges sections of the base metal. Then, flexible heating elements are installed on the heated surface, ensuring their tight fit to the surface of the workpiece, and they are closed from above with thermal insulation. Flexible heating mats are used as heating elements. A paste is applied to the thermocouples installed on the solid metal to isolate thermocouples, the cold ends of the thermocouples are removed for its subsequent connection to compensation wires. Connect the heating elements to power supplies. To control the air temperature inside the product during heat treatment, thermocouples are connected. The correct installation of thermoelectric converters (TEC), heating elements and thermal insulation is monitored to exclude air gaps between heaters and thermal insulation. At the end of the installation of heating elements, TEP and thermal insulation and before connecting the heating elements, the electrical resistance of each heating element is measured in order to identify a possible short circuit to the case. If the results of the measurements of the electrical resistance of the heating elements are satisfactory, their power connection is carried out to the installation for local heat treatment of welded joints, and control (recording) thermoelectric converters with thermocouple connectors to the heating installation.

Upon completion of all operations on installing and fixing heating mats, thermal insulation, the welded joint is heated from a temperature of 5-45 ° C to a temperature of 400-450 ° C at a heating rate of no more than 15 ° C / hour. At this temperature, the stress arising during the welding process relaxes.

When all transducers reach 400-450 ° C, the holding time starts to count. Exposure at a given temperature is provided for 20-25 hours. The start time of holding is counted from the moment the set temperature is reached by all thermocouples installed in the uniform heating zone, as well as the set temperatures in the additional heating zones, if any are assigned to the heat-treatable welded joint. Holding for 20-25 hours contributes to the most complete stress relief and an increase in the reliability of welded joints.

After the end of exposure, the product is cooled at a regulated rate of no more than 20 ° C / hour to a temperature of 250 ° C, which avoids deformation of the processed product.

The required cooling rate is provided by reducing the voltage across the heaters. When heating is turned off during holding, reheating is performed at a regulated rate, the holding time is extended, including the residence time of the welded joint at a given temperature during the cooling process.

Next, air cooling is carried out to a temperature not exceeding 45 ° C.

When performing heat treatment operations and assessing the quality of products, the temperature of the metal in the zone of heat-treated seams, the rate of heating (cooling) to a given temperature, holding temperature, and holding time at a given temperature are controlled.

Temperatures of 250 ° C and 400 ° C are the decision points for further heating. When these temperatures are reached, the take-off according to the readings of all TECs in the heating zone should not exceed the established permissible values during holding. In the event that the run-up along the temperature control points exceeds the set values, the temperature rise is stopped and an exposure is made up to one hour. When the takeoff run enters the set temperature interval, heating is continued. Otherwise, the heating is stopped. If necessary, cool the product by turning off the heaters, identify and eliminate the cause of uneven heating, and only then continue heating. The start time of exposure is counted from the moment the specified minimum exposure temperature (400 ° C) is reached by all TPEs installed on the product. When heating is turned off during holding, reheating is performed at a regulated rate (no more than 15 ° C / hour), holding is continued, including the residence time of the welded joint at a given temperature before cooling. The end time of holding is considered to have reached the minimum holding temperature (20 ° C-25 ° C) during the cooling process by the first thermoelectric converter installed in the uniform heating zone. The heating temperature of the product does not exceed 450 ° C. Physical parameters of heating (speed, temperature, time) are set by the programmer for each of the heating zones individually, based on the technological function of a certain heating zone.

After the heat treatment, the cage is disassembled. To do this, cool the product below 45 ° C. When the product is cooled below 45 ° C, disconnect from the heating elements, remove cable assemblies with connectors and travel cables-separators, dismantle thermal insulation, heating elements and thermoelectric converters in the reverse order to the installation performed. Dismantle thermoelectric converters, heating elements, thermal insulation. The mounting points of thermoelectric converters are cleaned flush with the base metal. Cleaning places are controlled by non-destructive testing. Further, the appearance and condition of the surface of the product are monitored after heat treatment and cleaning of the installation sites of thermoelectric converters, the results of non-destructive testing (by the CD method) are monitored after cleaning the installation sites of thermoelectric converters.

The invention can be illustrated by the following example.

The applicant company carried out local low-temperature heat treatment of the steam generator collectors by local heating of the collector surface with flexible resistance heaters. The product was installed on a roller stand, the places for installing thermoelectric converters on the surface were marked. Next, the fixture sectors were installed with pre-assembled thermal insulation and flexible heating mats. Sectors were fixed with spacer rings, thus ensuring a tight fit to the treated surface of the heating elements and thermal insulation. Thermoelectric converters were installed using a clamping kit for thermocouples, and on the solid metal section using a battery capacitor machine with a discharge energy of not more than 60 J. ... The heaters were connected to Teflon-coated spacer cables. Further, on the side of the collector flanges, we installed clamps with mounted thermal insulation. Free-hanging thermocouples were introduced to control the air temperature during heat treatment.

Heating mats, thermocouples, thermal insulation were mounted on the outer surface of the steam generator housing. We checked the correctness of installation of thermoelectric converters (TEC), heaters and thermal insulation to eliminate air gaps between heaters and thermal insulation. Upon completion of the installation of heating elements, thermal insulation and thermal insulation and before cable connection of the heating elements, the electrical resistance of each heating element was measured in order to identify a possible short circuit to the case. After satisfactory results of the measurements of the electrical resistance of the heating elements, we made their power connection to the installation for local heat treatment of welded joints (model RT 800/6) in accordance with the layout of heating zones and places for installing thermocouples during low-temperature heat treatment of the steam generator. We connected the control (recording) thermoelectric converters using a compensation cable with thermocouple connectors to the corresponding inputs of the RT 800/6 heating unit.

Local heat treatment was carried out according to the schedule: The temperature of the product before heating was 20 ° C. Then the product was heated at a rate of no more than 15 ° C / hour to 400 ° C-450 ° C. The temperature was measured during local heat treatment with thermoelectric thermometers (thermocouples). Thermocouples were located at control points, the number and position of which were selected depending on the size of the heating zone and the number of heating channels. The general zone of controlled heating consists of the main zone of controlled heating and additional zones of controlled heating and includes the welded seam and adjacent areas of the base metal at distances not less than the nominal thicknesses of the welded parts (i.e. the size of the welded seam depends on the nominal diameters and thicknesses of the welded details).

At the reached temperature (400 ° C-450 ° C), the product was kept for 20-25 hours. Then it was cooled at a regulated rate of no more than 20 ° C / hour to a temperature of no more than 250 ° C. Subsequent air cooling was carried out to a temperature not exceeding 45 ° C without dismantling the thermal insulation. The physical parameters of heating (speed, temperature, time) were set by the programmer. After heat treatment, the cage was disassembled. We disconnected the product from the heating elements, removed the cable assemblies with connectors and track separator cables. Next, the thermal insulation, heating elements and thermoelectric converters installed using heat-insulating pins and special devices on the product body were dismantled. The thermoelectric converters, spacer rings were dismantled, the sectors of the device with the heaters and thermal insulation fixed to them were removed. The points of attachment by capacitor welding of thermoelectric converters and heat-insulating pins were cleaned flush with the base metal, the points of cleaning were controlled by non-destructive testing. We checked the appearance and condition of the surface of the product after heat treatment and cleaning of the places for installing thermoelectric converters, places for fixing heat-insulating pins. Controlled by non-destructive testing (CD method) after cleaning the places where thermoelectric converters are installed and places where heat-insulating pins are attached.

Thus, the use of the proposed technical solution makes it possible to optimize the heat treatment process by eliminating all types of losses, to increase the production efficiency during heat treatment of complex curved surfaces of large-sized items with dimensions from 2.0 m to 4.5 m in diameter and to increase the strength properties of welded joints and operational reliability. large-sized products.

Claims (1)

The method of local low-temperature heat treatment of welded joints of large-sized products, including heating the welded joint zone, holding at a given temperature and cooling at a controlled rate, followed by cooling in air, characterized in that the heating of the welded joint zone is carried out from a temperature of 5-45 ° C to a temperature of 400 -450 ° C at a rate of no more than 15 ° C / h, while holding at a given temperature is performed for 20-25 h, cooling is carried out at a regulated rate of no more than 20 ° C / h to a temperature of no more than 250 ° C, and the subsequent air cooling - up to a temperature not exceeding 45 ° С.