RU2279487C1 - Tubular article heat treatment process - Google Patents

Abstract

FIELD: heat treatment of welded large-size articles such as connecting parts of pipelines or thick-wall large-diameter tubes of low carbon complex-alloy steel.

SUBSTANCE: tubular article is welded of shaped sheet blanks. Before hot shaping sheet blank is heated up to temperature Ac3 +(80 - 190)°C. Ready welded article is finally bulk heated up to Ac3 +(50 -100)°C , then it is cooled and subjected to tempering. Before final bulk heating of ready article it is preliminarily bulk heated up to temperature Ac3 +(80 - 190)°C. After preliminary heating article is cooled in air or it is subjected to accelerated cooling till metal temperature M - 600°C for further cooling in air. After preliminary bulk heating and(or) preliminary final bulk heating cooling may be performed by three stages. At first stage cooling is realized at rate providing crossing of cooling curve with line of starting structural conversion of austenite to ferrite in thermal kinetic diagram. At second stage cooling is realized at less rate or isothermal soaking is performed in temperature range M_n - 550 °C till complete austenite decomposition according to said diagram. At third stage cooling is realized at rate preventing separation of interstitial atoms from supersaturated solid solution till temperature no less than 180°C. Then during preliminary bulk heating article is loaded in furnace for final bulk heating. If only final bulk heating is realized, article cooling is going on at rate no more than 3°C/s. Before preliminary or final bulk heating, additional local heating up to temperature Ac3 +(80 -190)°C may be realized. Accelerated cooling of welded seam metal and adjacent zone whose width does not exceed five thickness values of article wall at each side from center of welded seam till temperature of metal no less than 450°C at rate providing ferrite-bainite or bainite metal structure also may be realized.

EFFECT: optimized parameters of heat treatment of tubular articles.

5 cl, 1 tbl

Description

The invention relates to the field of hardening heat treatment of welded large-sized products such as pipe fittings or thick-walled pipes of large diameter from low-carbon complex alloy steel.

A known method of heat treatment of welded pipes, including the normalization of the seam and the heat-affected space using local heating by high-frequency currents and tempering (see AS USSR No. 802384 in class. C 21 D 9/08, decl. 18.09.78, publ. 02/07/81 "The method of heat treatment of welded pipes").

The specified method for local heating of the welded joint does not allow to obtain high mechanical properties of the product due to the temperature difference between the heated and unheated zones and the resulting residual stresses of large magnitude, which can lead to warpage of the products and the appearance of quenching cracks.

The closest in technical essence, the achieved effect and selected as a prototype is a method of heat treatment of tubular products obtained by welding from molded sheet blanks, including the first volumetric heating above A _s3 ÷ (A _s3 +50) ° C, cooling in a water spray, the second volumetric heating in the intercritical temperature range (A _s1 ÷ A _s3 ) ° C, cooling in a water sprayer and high tempering at a temperature of (550 ÷ A _c1 ) ° C followed by cooling in air (see Cl. RF No. 2096495 according to cl. With 21 D 9/08, decl. 15.12.96, publ. 20.11.97 "Cn GSS heat treatment tube ").

The disadvantage of this method is the insufficiently high austenitization temperature during volumetric heating. This does not allow the full use of alloying elements to increase the complex of mechanical properties of the metal due to their transfer from the bound state to the solid solution. In addition, cooling in the sprayer can be used in the heat treatment of symmetric straight-line cylindrical products such as pipes. For connecting parts of complex geometric shape (tees, transitions, etc.), this method is not feasible or complex and inefficient. It should also be added that the cooling rate provided by the water sprayer is insufficient to obtain the required structural state (ferrite-bainitic or bainitic) in the metal of thick-walled pipe fittings (wall thickness reaches 80 mm) the achievement of high structural reliability of products that have undergone heat treatment in accordance with this method is not ensured.

The objective of the present invention is to increase the structural reliability of thick-walled products from low-carbon complex alloyed steels containing strong carbide and carbon nitride-forming elements, namely increasing strength while ensuring high values of ductility and toughness.

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 tubular products obtained by welding from molded sheet blanks, including volumetric heating of the welded product above A _c3 , cooling and tempering, according to the invention, the heating of sheet blanks carried out for performing the hot forming operation of these blanks, lead to a temperature And _s3 + (80 ÷ 190 ° C), and the final volumetric heating of the finished welded product is carried out to a temperature And _s3 + (50 ÷ 100 ° C).

Prior to the final volumetric heating of the finished welded product, it can additionally be preliminarily volumetric heated to a temperature A _s3 + (80 ÷ 190 ° C), i.e. preliminary normalization.

Cooling after preliminary volumetric heating can be carried out in air or by accelerated cooling to a metal temperature of M _n ÷ 600 ° C followed by cooling in air.

The cooling after preliminary volumetric and / or final volumetric heating can be carried out in three stages, in the first of which it is cooled at a speed that ensures the intersection of the cooling curve with the line of the beginning of the structural transformation of austenite into ferrite of the thermokinetic diagram, in the second stage it is cooled at a slower rate or isothermal holding in the temperature range M _n ÷ 550 ° С until the decomposition of austenite is completed in accordance with the thermokinetic diagram, in the third stage it is cooled at a speed of which prevents the release of interstitial atoms from a supersaturated solid solution to a temperature of not lower than 180 ° C, after which, when carrying out preliminary volumetric heating, the product is put into the furnace for final volumetric heating, and when only final volumetric heating is carried out, the product is continued to cool at a speed of not more than 3 ° C / from.

Prior to the final volumetric heating of the product, additional local heating to a temperature of A _s3 + (80 ÷ 190) ° C and accelerated cooling of the weld metal and the adjacent zone with a width of not more than five product wall thicknesses in each direction from the center of the weld to the metal temperature can be carried out not lower than 450 ° C, at a speed that ensures the production of a ferrite-bainitic or bainitic metal structure.

Studies conducted on the sources of patent and scientific and technical information have shown that the claimed method is not known and should not be explicitly learned from the 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 pipes and fittings for them, i.e. is industrially applicable.

It was experimentally established that heating in the indicated range before forming (stamping or rolling) a sheet blank of low-carbon complex alloyed steel containing strong carbide- and carbonitride-forming elements (vanadium, niobium, titanium) ensures the maximum possible dissolution of particles of the secondary phase (carbides, carbonitrides), in which there is no abnormal growth of austenitic grain. This heating allows you to partially carry out the recrystallization process and to obtain a homogeneous crushed grain at the stage of forming a sheet billet.

Carrying out volumetric heating after welding of the product in the claimed temperature range and cooling according to the indicated scheme allows to obtain complete recrystallization and additional grinding of steel grain. As a result, the strength is increased while providing high values of ductility and toughness.

Conducting additional local heating of the weld and the adjacent zone before preliminary and / or final volume heating of the product improves the mechanical properties of the weld metal by converting the cast microstructure of steel in the weld zone into uniform ferrite-pearlite or ferrite-bainitic with fine grains.

The proposed method of heat treatment of tubular products was tested in industrial conditions when hardening the connecting parts of pipelines, namely die-welded bend with a diameter of 1020 mm and a wall thickness of 28 mm, made of steel 10G2FB. In identical conditions, the prototype method was tested.

Steel sheets intended for the manufacture of the branch, which was supposed to be subjected to heat treatment in accordance with the prototype method, were heated to a standard temperature of 930-950 ° C for stamping after cutting. After stamping, these sheets were also cooled in air; edges were cut on them, assembly and welding of products.

In accordance with the inventive method, steel sheets intended for manufacturing an elbow were subjected to cutting into sheet blanks, heating these blanks to a temperature of 980-1000 ° C for stamping, holding for 20 minutes, stamping, cooling in air, cutting edges, assembly and welding of products. After welding, the outlet was cooled in air.

Further, the taps were subjected either to volumetric heating in a chamber thermal furnace or to local heating by inductors. After that, a vacation was conducted in a hearth furnace. The characteristics of heating and cooling during volumetric, local heating and tempering and the obtained metal properties are presented in table 1.

Critical points for steel 10G2FB (investigated melting):

A _c3 = 890 ° C

A _c1 = 730 ° C

Mn = 425 ° C

An additional improvement in the complex of mechanical properties of the base metal and the weld metal provides tempering by stepwise heating to a temperature below A _c1 . Heating during such tempering is carried out in at least two stages, after each of which isothermal aging and cooling are carried out in the refrigerant [water with a temperature of at least 80 ° C, aqueous solutions of NaCl (3-26% solution), NaOH (10- 50% solution), bischofite with a density of 1.1-1.32 kg / m ³ ] to the stage of cessation of boiling.

Analysis of the data given in the table shows that the proposed method can increase strength compared with the prototype while ensuring high values of ductility and impact strength.

Claims (5)

1. The method of heat treatment of tubular products obtained by welding from molded sheet blanks, including volumetric heating of the product above A _c3 , cooling and tempering, characterized in that the heating of sheet blanks for the operation of hot forming of these blanks is carried out to a temperature of A _c3 + (80- 190) ° C, and the final volumetric heating of the finished welded product is carried out to a temperature of A _s3 + (50-100) ° C. 2. The method according to claim 1, characterized in that prior to the final volumetric heating of the finished welded product, it is additionally preliminarily heated to a temperature A _c3 + (80-190) ° C. 3. The method according to claim 2, characterized in that after preliminary volumetric heating, cooling in air or accelerated cooling to a metal temperature of M _n -600 ° C followed by cooling in air is carried out. 4. The method according to claim 1 or 2, characterized in that the cooling after preliminary volumetric and / or final volumetric heating is carried out in three stages, in the first of which it is cooled at a rate that ensures the intersection of the cooling curve with the line of the beginning of the structural transformation of austenite to thermokinetic ferrite diagrams the second step is cooled at a slower rate or isothermal exposure is carried out at temperatures M _s -550 ° C to complete the decomposition of austenite in accordance with the thermokinetic diagram in a third step they are cooled at a speed that prevents the release of interstitial atoms from a supersaturated solid solution to a temperature of not lower than 180 ° C, after which, during preliminary volumetric heating, the product is put into the furnace for final volumetric heating, and when only final volumetric heating is carried out, the product continues to cool at a speed of no more than 3 ° C / s. 5. The method according to claim 1 or 2, characterized in that prior to the preliminary and / or final volumetric heating of the product, additional local heating is carried out to a temperature A _c3 + (80-190) ° C and accelerated cooling of the weld metal and the adjacent width zone no more than five wall thicknesses of the product in each direction from the center of the weld to the metal temperature of at least 450 ° C, at a speed that ensures the production of a ferrite-bainitic or bainitic metal structure.