SU1749256A1 - Method of thermally treating cold-deformed workpieces - Google Patents

Abstract

Use: manufacture of pipes from chromic stainless steels of semi-ferritic grade for heat exchange equipment of power plants. The essence of the invention: the products are heated to 850-870 ° C, kept at this temperature for 3-7 minutes, heated to 900-920 ° C, kept for 10-15 minutes, cooled with the furnace to 680-700 ° C, maintained at this temperature of 1.5-2.0 hours and finally cooled in air. 4 tab.

Description

The invention relates to metallurgy and mechanical engineering, in particular to the manufacture of semi-ferritic chromium stainless steel pipes for connecting a pipe to a pipe board of heat exchanging equipment of thermal power plants, nuclear power plants.

The closest to the technical essence of the invention is a method of heat treatment of pipes made of steel 08KH14MF, including heating to 770 ° C, holding for 3 hours and cooling the air.

This mode does not provide for obtaining the required level of plasticity, the yield strength of hundred, 2 40 kg / mm2 and the ratio of hundred, 2 / av 0.7,

The aim of the invention is to increase the operational reliability of products by increasing the density

This goal is achieved by the fact that according to the method of heat treatment of cold deformed products, mainly pipes, including heating of the product, aging at a temperature

heating and cooling in air, heating is carried out to 900-920 ° C, exposure is carried out for 10-15 minutes, and in the process of heating is carried out isothermal exposure at 850-870 ° C for 3-7 minutes, and cooling to 680 -700 ° C is carried out with the furnace and maintained at this temperature for 1.5-2.0 hours.

When heated to 900–920 ° C, d-ferrite of 08H14MF steel is converted, which leads to additional grain refinement and, as a result, carbides leave the grain boundaries. Heating to temperatures below 900 ° C does not allow phase recrystallization, heating above 920 ° C leads to the dissolution of the carbide phase and grain growth. This deteriorates the ductility.

A shutter speed of less than 10 minutes is insufficient for phase recrystallization; a shutter speed of more than 15 minutes causes grain growth.

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Isothermal aging at 850-870 ° C ensures recrystallization in the steel structure. Heating to temperatures below 850 ° C does not ensure complete recrystallization, heating to temperatures above 870 ° C may lead to a transition of the plant critical point and austenite inheritance of increased defect density (Act 820-830 ° C; Haz - 890-900 ° C)

The exposure temperature in the lower temperature range of 680–700 ° C is due to the location of the ferritic-carbide decomposition region.

A holding at a temperature below 680 and above 700 ° C does not ensure the complete decomposition of austenite into a ferritic carbide mixture at the indicated time of 1.5 h, which increases the duration of the heat treatment cycle.

Exposure in the range of ferritic-carbide decomposition should be not less than 1.5 h, which ensures that the conversion proceeds completely. When cooled after exposure for less than 1.5 hours, martensite is formed in the structure; the necessary complex of mechanical properties is not achieved. Exposure of more than 2 hours is impractical from the point of view of saving time.

After processing by the proposed method, the metal of the tubes has a fully crystallized 5-ferrite structure with evenly located MazSe carbides in the ferrite. In this case, the required complex of properties ffo, 2 26-35 kg / mm2, (5–35–40%, (Go.2 / 08 0.6) is achieved.

The proposed method is implemented in the manufacture of cold-deformed pipes with dimensions of 16x2, t6x 1.2, 14 x 1.2 mm from stainless steel of ferrite grade 0.8X14MF.

The pipes are manufactured using mechanical-thermal processing, which includes the operations of rolling, drawing and heat treatment. The rolling is carried out as follows: the initial billet with a size of 74 x 7.5 mm is rolled to a size of 38 x 3.9 mm and heat treated, pipes of 38 x 3.9 mm rolled to a size of 19 x 2 mm and heat treated, then drawing of pipes of 19 x 2 mm to a size of 16 x 2 mm is carried out and heat treated.

Heat treatment was carried out in a 7-zone pass-through roller furnace with temperature control in the zones:

I zone 850-870 ° C, shutter speed 3-7 min:

Zone II 900–920 ° C, holding 10–15 min; III-YH zones 680-700 ° C, shutter speed 1.5-2.0 h.

The mechanical properties of 15 packages of pipes processed according to the proposed technology are listed in Tables 1-4, the yield strength after heat treatment is 26-33 kg / mm.

Compared with the base object, the proposed heat treatment method allows improving manufacturability in the manufacture (distribution, bending, stamping) of cold-deformed pipes and a sheet of semi-ferrite grade stainless steel by increasing ductility from 25-30

to 35–40% and a decrease in the ratio of 7 °, 2 / GB ° t 0.7–0.9 to 0.5–0.6 (the test results are shown in Table 1-4).

The developed method allows to increase the reliability of the equipment of thermal power plants,

Nuclear power plants, which is achieved by increasing the density of valche connections pipe - tube

Claims (1)

The method of heat treatment of cold-deformed products, mainly pipes, including heating of products, holding at heating temperature and cooling in air, characterized in that, in order to increase the operational reliability of products by increasing the density, they heat up to 900-920 ° C , the exposure is carried out for 10–15 min, and in the process of heating, isothermal aging is carried out at 850–70 ° С for 3–7 min, and cooling to 680–700 ° С is carried out with the furnace and maintained at this temperature within 1,5-2,0 h Table 1 Comparative mechanical properties of cold-formed pipes of steel 08H14MF, treated according to the known regime (tempering 770 ° C, 3 hours) and the proposed methods Note. The amendment leads to an increase of 2 by 10 kg / mm. The mechanical properties of cold-formed pipes and sheets of steel 08H14MF, processed by a known method (tempering 770 ° С, Зч), degree strain 15%. Table 2 The mechanical properties of cold-formed pipes of steel 08x14MF, processed by the proposed method (the degree of deformation is 15%) The mechanical properties of cold-formed pipes of steel 08H14MF, processed in a 7-zone continuous furnace according to the proposed method Table3 Table4 Continuation of table 4