JPH0772304B2 - Clad tube heat treatment method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a heat treatment method for a clad pipe used for a line pipe in an oil well or the like in a sour environment with a large H ₂ S content.

(Prior art and its problems) In oil and natural gas wells in recent years, the depth of wells has become deeper. Therefore, the increase of corrosion factors such as chlorine ion and hydrogen sulfide gas and the increase of temperature and pressure. In addition, carbon dioxide gas, seawater, etc. are also injected into the wells to restore the oil well, and the environment for use is severe.

The line pipe material used here is required to have mechanical properties capable of withstanding high temperature and high pressure and excellent corrosion resistance. Regarding this mechanical property, according to the API (American Petroleum Institute) standard, the yield strength is 36.6 kg / mm ² or more, and the yield ratio (proof strength / tensile strength) is 0.
It stipulates that the impact absorption energy at 85 or less, and in some cases at -30 ° C, must have a characteristic of 3.5 kg · m or more. The reason is that if the yield strength is low, the strength of the pipe is weak and the wall thickness of the pipe must be increased, and if the yield ratio is large, slight plastic deformation leads to fracture. The high impact absorption energy at -30 ° C is required because the line pipe is used in a low temperature environment.

Conventionally, a single-layer pipe made of carbon steel or low alloy steel has been used as this pipe material. In order to use it as a line pipe, a welding process for connecting pipes is indispensable, considering weldability, and further, to prevent brittleness due to welding,
I wonder if I had to choose a low-carbon material. However, carbon steel and low alloy steel single-layer pipes have a problem of lacking corrosion resistance.

Therefore, in consideration of the usage conditions as a line pipe, in order to satisfy both the predetermined strength and corrosion resistance characteristics as a line pipe material, a so-called clad pipe formed from carbon steel or low alloy steel on the outside and Ni-based alloy on the inside Although it has been proposed to use it, it was difficult to set proper heat treatment conditions, so it was not possible to secure the characteristics suitable for the material.

That is, in order to impart predetermined strength and toughness to carbon steel or low-alloy steel, it is necessary to perform quenching after heating at about 950 ° C, while in the case of a Cr-Mo-Ni-based Ni-based alloy, it is prescribed. In order to impart the corrosion resistance to the so-called liquefaction heat treatment, the carbon content is solid-soluted in the matrix by heating at about 1050 ° C and then rapidly cooling.

However, after heating to 1050 ℃, water cooling, and after heating to 950 ℃, water cooling, 690
Even if satisfactory results can be obtained in terms of corrosion resistance, yield strength and toughness by performing heat treatment of air cooling after heating to ℃, the yield ratio will be about 0.9 or more and clear the API standard mentioned above. You cannot do it. As a compromise between the heat treatment conditions (heating temperature, cooling method) for the outer layer (carbon steel or low alloy steel) and the inner layer (Ni-based alloy), the cooling rate is slow when heated at 1050 ° C and then air-cooled. It is not possible to obtain sufficient proof stress. As described above, it was extremely difficult to appropriately heat-treat the clad tube due to the difference in material.

In view of the above problems, the present invention provides a novel heat treatment method for a clad tube, which does not raise the quenching heating temperature to an austenitizing temperature, and the quenching heating temperature is a relatively low temperature of ferrite-austenite. It has the greatest feature in heating at a zone temperature.

(Technical Means and Action) The present invention, in% by weight, C: 0.16% or less, Si: 0.50% or less, Mn: 1.55% or less, Ni: 0.60% or less, Ci: 0.20% or less, M
O: contains 0.30% or less, V: 0.10% or less, and has a carbon equivalent value of 0.
42 or less, in the heat treatment of the clad tube consisting of the outer layer of carbon steel or low alloy steel consisting essentially of Fe, and the inner layer of Ni-based alloy, the casting temperature was set to 1050 ± 25 ° C after the casting. The outer layer is subjected to a diffusion treatment and the inner layer is subjected to a solution treatment by heating and then water cooling, and then a quenching treatment is performed by heating at a ferrite-austenite temperature of 820 ± 30 and then water cooling.

However, the carbon equivalent value is represented by the following formula.

After the quenching treatment, if necessary, in order to improve the low temperature toughness, a tempering treatment of heating at 600 ± 30 ° C. and then air cooling is performed.

By carrying out the heat treatment method of the present invention, yield strength 36.6
It is possible to obtain a clad tube composed of an outer layer having characteristics of kg / mm ² or more, a yield ratio of 0.85 or less, and an impact absorption energy at −30 ° C. of 3.5 kg · m or more, and an inner layer having excellent corrosion resistance. it can.

(Example) The clad tube is composed of an outer layer of carbon steel or low alloy steel and an inner layer of Ni-based alloy.

Four types (A, B, C, D) of clad tubes were manufactured as test tubes by the centrifugal casting method. The alloy components are shown in Table 1. The size of the test tube is 168.3 mm in outer diameter and 15.7 m in wall thickness.
m (outer layer 12.7mm, inner layer 3mm), length 5m.

The test tubes Nos. 1 to 8 were heat-treated under various conditions, and the mechanical properties and corrosion resistance after the heat-treatment were examined. The heat treatment conditions are shown in Table 2 and mechanical properties Table 3. Test tube No.1
And 2 are performed for comparison with the method of the present invention, and are heat treatment conditions generally considered by those skilled in the art. Test tube
Nos. 3 to 8 all depended on the heat treatment conditions of the present invention.

As is clear from the results of Table 3, the clad tube produced by the heat treatment method of the present invention has a yield strength of 3 in the above-mentioned API standard.
It has characteristics of 6.6 kg / mm ² or more, yield ratio of 0.85 or less, and impact absorption energy of 3.5 kg · m or more at -30 ° C.
On the other hand, test tube No. 1 has low yield strength, and test tube No. 2 has high yield ratio.

Since the test tubes No. 6 to No. 8 are tempered,
It can be said that the shock absorption energy at a low temperature is larger and the low temperature toughness is particularly excellent.

Furthermore, in order to evaluate the corrosion resistance of the clad tube produced by the heat treatment method of the present invention, a polarization test and an intergranular corrosion test were performed on the test tubes No. 4 and No. 7. The results are shown in Table 4. The polarization test was conducted at 40 ° C. in 3% NaCl, pH 3.90. The intergranular corrosion test was performed according to ASTM A262 Practice C (boiling 65% HNO ₃ ).

In order to evaluate the corrosion resistance, the same material as the inner layer component, IN
For the plate material of 625 materials, when the pitting potential and the corrosion weight loss after the predetermined solution heat treatment were examined, the pitting potential was 758 mV and the corrosion weight loss was 0.45 g / m ² h, respectively, and the method of the present invention was used. It was about the same performance as.

(Effect of the Invention) The clad tube which has been subjected to the heat treatment method of the present invention has high strength, low temperature toughness, and excellent corrosion resistance. Therefore, it is optimal for use as an oil well line pipe member in a sour environment. It can also be applied to the heat treatment of piping materials for chemical equipment that require similar characteristics.

Claims (2)

[Claims] 1. In% by weight, C: 0.16% or less, Si: 0.50% or less, Mn: 1.55% or less, Ni: 0.60% or less, Cr: 0.20% or less, M
O: contains 0.30% or less, V: 0.10% or less, and has a carbon equivalent value of 0.
42 or less, in the heat treatment of the clad tube composed of the outer layer of carbon steel or low alloy steel consisting essentially of Fe and the inner layer of Ni-based alloy, the remainder after casting is heated at 1050 ± 25 ° C and then water-cooled. Then
Next, a heat treatment method for a clad tube, which comprises heating at 820 ± 30 ° C and then water cooling. However, the carbon equivalent value is represented by the following equation. 2. Heating at 1050 ± 25 ° C., cooling with water, and then 820 ± 3
The heat treatment method for a clad tube according to claim 1, wherein after heating at 0 ° C., water cooling, further heating at 600 ± 30 ° C. and then air cooling.