RU2048542C1 - Low-carbon manganese steels pipes heat treatment method - Google Patents

Abstract

FIELD: mainly pipes of oil products range. SUBSTANCE: low-carbon (C 0.17 0.35 mass) manganese (Mn + 0.35 1.20 mass) steels pipes heat treatment method provides for cooling after passing last stand of rolling mill in air down to the shop temperature. Heating up to temperature of 760 790 C is exercised with water cooling down to the shop air temperature and additional heating up to temperature of 670 - 700 C is exercised with following cooling in the air. The procedure takes off inner tensions, prevents spheroidization of carbides, increases pipes resistivity to sulfide corrosion cracking. EFFECT: increased pieces resistivity to sulfide corrosion cracking under tension in mediums with low concentration of natural and bacterial hydrogen sulfide. 1 tbl

Description

 The invention relates to the field of heat treatment of metals, mainly oil-grade pipes, which increases resistance to sulfide stress cracking in environments with a low content of natural or bacterial hydrogen sulfide.

A known method of heat treatment of pipes of low carbon manganese steels, in which the product is heated to a temperature of 850 ^about C and cooled in air. The method is used in industry and at present (TI 161-T1-1722-89 AOOT "Sinarsky Pipe Plant").

 The disadvantage of this method is that it does not provide the required level of operational properties of pipes, especially resistance to sulfide cracking and to general and pitting corrosion.

Known is a method of processing wire rod comprising rolling and cooling water directly on its exit from the last stand of the mill at a temperature from 650-500 to 1050-1070 ^{° C} at a rate of not less than 1,000 ° ^C / s (AS N 286725, cl. C 21 D 9/52, 1981).

 The disadvantage of this method is that in relation to pipes of low carbon manganese steels, this method does not provide the required level of strength and plastic properties.

The closest in technical essence and attainable effect to the claimed invention is a method of heat treatment of products of low-carbon manganese steel, consisting in that the product is cooled from the rolling heat at the exit of the last mill stand with a temperature ^of 830-870 C by acting on their outer surface water for 0.15-0.30 s with an intensity of 6.0-7.0 l / s per millimeter of wall thickness (RF patent N 2007470, class C 21 D 1/02, 1994). The method is used in the heat treatment of oil assortment pipes with the provision of mechanical properties required by GOST 633-80.

 However, due to the low resistance to sulfide cracking, pipes cannot be used in fields even with a low content of hydrogen sulfide.

 The purpose of the invention is the development of a method for heat treatment of pipes of low carbon manganese steels for use in oil and gas production, the metal of which would have increased resistance to sulfide stress corrosion cracking in environments with a low content of natural or bacterial hydrogen sulfide. This will increase the operational reliability of oil range pipes, i.e. durability of use, resource of work.

The goal is achieved in that the method of heat treatment of low-carbon manganese pipe steels includes their cooling at the exit of the last mill stand in air, whereupon reheat to 760-790 ^{° C} with cooling water to a shop and a third temperature heating to 670-700 ^о С with air cooling.

The heating-pipes under preforms final rolling tubes of low carbon (C 0,17-0,33%) and manganese (Mn 0,35-1,20%) steels to produce 850-920 ^C. pipe exit temperature from the last mill stand of 800-870 ° ^C. in the process of reheating to 760-790 ^C, as shown by metallographic examination, in a metal pipe formed approximately 40% austenite. After cooling with water from this temperature, martensitic transformation of austenite occurs and a dual ferrite-martensitic structure forms. The grain size of the ferrite is 3-5 microns, the size of the martensitic areas 4-7 microns. Increased reheating temperature pipe over 790 ^{° C} increases the amount of austenite in the metal and, consequently, after the cooling water to increase the amount of martensite which increases the strength and decreases the ductility of the tubes. This violates the required ratio of strength and ductility, which is undesirable for oil pipes. Conversely, reheating below 760 ^{° C} leads to reduction in the amount of austenite-forming, and after cooling with water to reduce the strength with increasing plasticity, which is also unacceptable.

The third heating pipes up to 670-700 ^C is an operation that leads to the removal of internal stresses and spheroidization of carbides, thus reducing sensitivity to metal sulfide. Increasing the temperature of the third heating above 700 ^{° C} leads to a weakening of the metal below the required rate, and lowering of the temperature below 670 ^{° C} does not provide spheroidizing carbides.

 The proposed method of heat treatment of pipes is as follows.

Pipes billet heated under the final rolling to a temperature of 850-920 ^C. At the exit of the last mill stand pipe has a temperature in the range 800-870 ° ^C. With the temperature of the pipe was cooled in air to a temperature workshop. Then, the reheating tubes to 760-790 ^{° C} and cooled with water to shop temperature. Finally produce third heating pipes up to 670-700 ^{° C} followed by cooling in air. As a result, pipe metal gains resistance to sulfide stress corrosion cracking, which entails an increase in the resource of their operation in oilfield environments containing hydrogen sulfide.

 It was found that the exclusion of at least one essential feature from the method, starting from the first cooling with rolling heating in air and ending with the third heating and cooling in air, does not provide a solution to the problem of increasing the resistance of pipes to sulfide stress corrosion cracking.

Under industrial conditions, pipes were made of steel containing 0.20% C and 0.65% Mn. The outer diameter of the pipes was 73 mm and the wall thickness was 5.5 mm. The heating-pipes workpieces at the final rolling is conducted at a temperature of 900 ^{° C} at the exit from the last stand pipe mill had a temperature of 840 ^{° C} at which they were cooled in air to a temperature workshop. Reheating pipes produced to a temperature ^of 770 C, at which they were cooled with water, heating was carried out up to the third temperature 680 ^{° C} with air cooling.

Sulfide cracking resistance was determined according to NACE standard TM 01-77-90 (method D, A) on samples of the double-beam type in a test medium consisting of a saturated H ₂ S solution containing 5% NaCl, 0.5% acetic acid and hydrogen sulfide 2400 mg / l. The stress intensity factor K _ISSC and the threshold voltage were _evaluated
σ _th .

 The results of determining the properties of the metal are given in the table.

Claims (1)

METHOD FOR THERMAL TREATMENT OF PIPES FROM SMALL CARBON MANGANOSE STEELS, including cooling in air from the temperature of the end of rolling, heating and cooling, characterized in that the heating is carried out to 760 790 ^o C, cooled in water to a workshop temperature, and then carry out additional heating to 670 700 ^o C and cool in air.

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