RU2256705C1 - Method of thermal treatment of items - Google Patents

Abstract

FIELD: thermal treatment of pipelines coupling pieces made out of alloyed or low-alloy steels.

SUBSTANCE: the invention is pertaining to the field of thermal treatment of pipelines coupling pieces made out of alloyed or low-carbon steels. The technical result is optimization of the treatment parameters. The method of the thermal treatment of items provides for their heating up to the temperature of A_C3 + (50-190°C), aging, cooling in some stages and tempering. At that at the first stage the cooling is conducted at the speed ensuring interception of the cooling curve with the line of the beginning of the structural transformation of austenitic into ferrite on the thermalkinetic diagram below the point A_r1; at the second stage corresponding to the field of formation of ferrite the cooling is conducted at a lower speed or conduct an isothermal aging; and at the third stage the cooling is conducted at the speed ensuring formation of a bainite up to temperature of 250-180°C; then prolong the cooling at the speed of no more than 3°C/s. The temperature increase at tempering may be conducted at least in two stages, after each of which conduct an isothermal aging. At that the first aging is conducted at the temperature of no less than 180°C and the final aging - at the temperature of no more than A_C1-20°C. After the isothermal aging conducted after the first and-or one or several subsequent stages of temperature increase may be conducted a cooling till termination of the stage of boiling in the water at the temperature of no less than 80°C.

EFFECT: the invention ensures optimization of the thermal treatment parameters.

3 cl, 1 dwg, 2 tbl

Description

The invention relates to the field of heat treatment and can be used to increase the complex of mechanical properties of connecting parts of pipelines of alloyed, low alloyed or low carbon steels.

A known method of hardening heat treatment of alloy steel products, including heating to an austenization temperature (860-960 ° C) in an oxidizing atmosphere with a coefficient of excess air supply of 1.25-2.25 for 60-180 minutes, hardening with compressed air for 6 -20 min to a temperature below 320 ° C and high-temperature tempering (see paragraphs of the Russian Federation No. 2111509, class C 21 D 1/18, application. ”).

This method does not allow to obtain a fine-grained ferrite-pearlite or ferrite-bainitic metal structure necessary to obtain a set of mechanical properties that meet the requirements for the connecting parts of pipelines.

The closest in technical essence, the achieved effect and selected as a prototype is a method of heat treatment of products, including heating to a temperature And _c3 + (50-190 ° C), exposure, cooling in several stages and a single tempering in the traditional temperature range. In this case, in the first stage, cooling is carried out at a speed that ensures the intersection of the cooling curve with the start line of the structural transformation of austenite to ferrite of the thermokinetic diagram below the point A _r1 , in the second stage, corresponding to the ferrite formation region, cooling is carried out at a lower rate or isothermal exposure, and of the third stage, cooling is carried out to a temperature of the cooling medium at a rate that ensures the formation of bainite (see AS of the USSR No. 1373735, class C 21 D 1/56, decl. 24.02.86, publ. 15.02.88 “The method is thermal th processing of products ”).

This method allows to obtain metal products with a stronger ferrite-bainitic structure. However, the increased level of residual stresses in the metal, which occurs when the products are cooled in the third stage to the temperature of the cooling medium, does not reduce the warpage of the products to the required level and increase the toughness of the metal.

The objective of the present invention is to reduce the level of warpage of products, increase the toughness of metal products.

The technical result obtained by the implementation of this invention is the optimization of processing parameters.

This problem is solved due to the fact that in the known method of heat treatment of products, including heating to a temperature And _s3 + (50-190 ° C), exposure, cooling in several stages and tempering, in which at the first stage cooling is carried out at a speed that provides the intersection of the cooling curve with the line of the beginning of the structural transformation of austenite to ferrite of the thermokinetic diagram below point A _r1 , in the second stage, corresponding to the region of formation of ferrite, cooling is carried out at a lower speed or isothermal exposure, and in the third stage, cooling is carried out at a rate that ensures the formation of bainite, according to the invention in the third stage, cooling is carried out to a temperature of 250-180 ° C, then cooling is continued at a speed of not more than 3 ° C / s.

The temperature rise during tempering can be carried out in at least two stages, after each of which isothermal exposure is carried out, the first exposure being carried out at a temperature of at least 180 ° C, and the final one at a temperature of not more than A _c1 -20 ° C.

After isothermal holding performed after the first and / or one or more subsequent steps of raising the temperature, cooling can be carried out until the boiling stage in water is stopped at a temperature of at least 80 ° C.

Studies conducted on the sources of patent and scientific and technical information have shown that the claimed method is unknown and does not follow explicitly from the studied prior art, i.e. meets the criteria of novelty and inventive step.

The method can be carried out at any enterprise specializing in this industry, because this requires well-known materials and standard equipment, and is widely used in the manufacture of pipe fittings, i.e. is industrially applicable.

It was experimentally established that the use of accelerated cooling interruption and replacing it with cooling at a speed of no more than 3 ° C / s in the third stage at a temperature below 250-180 ° C creates conditions for reducing the level of residual stresses so that the level of warpage of the products is reduced (in in particular, their ends), the toughness of metal products increases.

The use of a phased increase in temperature followed by isothermal holding, carried out in the temperature range 180 ° C- (A _c1 -20 ° C) after interrupted cooling, allows, while maintaining a reduced level of warping, to improve the conditions of structure formation, which are manifested primarily in obtaining a large number of particles of the secondary phase ( carbides, carbidonitrides, etc.), which improves such mechanical properties of products as impact strength, yield strength, amount of viscous component in fractures of impact samples, and also reduces pores og cold brittle hardware.

Additional cooling after the next stage of raising the temperature with isothermal holding during tempering causes the formation of an additional number of defects in the crystal lattice. Such an operation, like cold plastic deformation or irradiation with high energy particles, leads to additional hardening of the treated metal. In this case, the subsequent stage of raising the temperature to a higher value leads to the formation of a polygonized or cellular substructure, which increases the viscosity and resistance to softening during subsequent heating of the metal, for example, during welding.

In addition, the use of such a tempering scheme reduces the time of isothermal aging during tempering.

The proposed method of heat treatment of products was tested in industrial conditions when hardening the connecting parts of pipelines - die-welded tees with a diameter of 530 mm and a wall thickness of 20 mm from steel 15 HSND. In identical conditions, the prototype method was tested.

Heat treatment was carried out as follows.

After manufacturing, the heat-treated products - tees were placed in a roll-out hearth furnace, heated to 930-980 ° C and kept in the oven at this temperature. Then, using a crane, the products were removed from the furnace hearth and cooled. The cooling characteristics at each stage and the properties of the metal products are presented in table 1.

The invention is illustrated by the drawing, which shows a front view (from the end) on the connecting piece of the pipeline - tee. This drawing shows the position of the measuring points of the diameter of the end of the tee. The measurements were carried out after various modes (the characteristics of the modes are given in table 1) processing to determine the degree of warpage of the tee. The measurement results to determine the degree of warpage of the products by the present method and the prototype method are shown in table 2.

Analysis of the data given in the tables shows that the proposed method can improve the mechanical properties of the product compared to the prototype and reduce the level of warpage of the products.

Table 1 No. p / p processing mode Type of method After quenching cooling circuit Vacation Scheme Product mechanical properties Yield Strength, MPa Tensile strength, MPa Relative extension, % Impact strength KCV ^-20, MD w / m ² 1 Method by A.S. USSR No. 1373375 Stage 1 cooling: the metal temperature at the beginning of the stage is 930 ° C;
the metal temperature at the end of the stage is 690 ° C;
cooling medium - water at 57-60 ° C; cooling rate of 1.5-2.0 ° C / s.
Stage 2 cooling: isothermal exposure for 8 s
3 stage of cooling: cooling medium - water at 90 ° С Heating to 500 ° C
Isothermal exposure 90 min
Air cooling 385 560 26 0.87 2 The proposed method 1 and 2 stages according to claim 1
3 stage cooling:
cooling to a metal surface temperature of 250 ° C (cooling medium - water at
temperature 90 ° С),
further cooling in air at a rate of 1.5 ° C / s Heating to 500 ° C
Isothermal exposure90 min
Air cooling 380 560 28 1,1 3 The proposed method 1 and 2 stages according to claim 1
3 stage of cooling: cooling to a metal surface temperature of 250 ° C
(cooling medium - water at a temperature of 90 ° C), then cooling in air at a speed of 1.5 ° C / s Heating up to 300 ° С
Isothermal exposure 30 min
Heating to 500 ° C
Isothermal exposure 40 min
Air cooling 390 570 26 1,6 4 The proposed method 1 and 2 stages according to claim 1
Stage 3 cooling: cooling to a metal surface temperature of 250 ° C (cooling medium - water at a temperature of 90 ° C), then cooling in air at a speed of 1.5 ° C / s Heating up to 300 ° С Isothermal exposure 30 min
Cooling until cessation of boiling in water at a temperature of 90 ° C
Heating to 500 ° C
Isothermal exposure 40 min
Air cooling 410 600 23 1.3 5 The proposed method 1 and 2 stages according to claim 1
Stage 3 cooling: cooling to a metal surface temperature of 250 ° C (cooling medium - water at a temperature of 90 ° C), then cooling in air at a speed of 1.5 ° C / s Heating up to 300 ° С
Isothermal exposure 30 min
Cooling until cessation of boiling in water at a temperature of 90 ° C
Heating to 500 ° C
Isothermal exposure 30 min
Cooling until cessation of boiling in water at a temperature of 90 ° C
Heating to 550 ° C
Isothermal exposure 30 min
Air cooling 400 590 25 1.4

table 2 Processing mode Absolute dimensions of the diameter of the end of the tee after various processing modes, mm Section 1-1 Section 2-2 Section 3-3 Production + calibration of the ends, i.e. the initial state 530 530 530 Heat treatment according to mode 1, i.e. in accordance with the prototype 526 533 527 Heat treatment mode 2 528 531 529 Heat treatment mode 3 530 530 529 Heat treatment mode 4 529 531 529 Heat treatment mode 5 528 531 529

Claims (3)

1. The method of heat treatment of products, including heating to a temperature A _c3 + (50-190 ° C), exposure, cooling in several stages and tempering, and in the first stage, cooling is carried out at a speed that ensures the intersection of the cooling curve with the line of the beginning of the structural transformation of austenite to ferrite thermokinetic diagram below the a _r1, the second step corresponding to the area of the ferrite formation, are cooled at a slower speed, or performed isothermal exposure, and the third step are cooling at a rate providing boiling bainite formation, characterized in that in a third step cooling is performed to a temperature of 250-180 ° C, then cooling is continued at a rate not exceeding 3 ° C / s. 2. The method according to claim 1, characterized in that the temperature rise during tempering is carried out in at least two stages, after each of which isothermal exposure is carried out, the first exposure being carried out at a temperature of at least 180 ° C, and the final exposure at temperature not more than A _c1 -20 ° C. 3. The method according to claim 2, characterized in that after isothermal aging performed after the first and / or one or more subsequent steps of raising the temperature, cooling is carried out until the boiling stage in water is stopped at a temperature of at least 80 ° C.