JPS6277422A - Solution heat treatment for two phase stainless weld steel pipe - Google Patents

Abstract

PURPOSE:To manufacture weld steel pipe having superior corrosion resistance and good mechanical property, by applying heat treatment at suitable temp. and cooling rate to weld steel pipe in which base metal and welded metal are both made of austenitic and ferritic stainless steel. CONSTITUTION:Welded steel pipe in which base metal and welded metal are both made of austenitic-ferritic two phases stainless steel is soln. heat treated. When heat treating condition is exhibited by a coordinate indicating one and the other axes with heat treating temp. deg.C and cooling rate deg.C/sec respectively, the soln. heat treatment is carried out in the domain surrounded by connecting six plotted points of 1,200 deg.C-0.5 deg.C/sec, 1,200 deg.C-1 deg.C/sec, 1,100 deg.C-25 deg.C/sec, 1,100 deg.C-200 deg.C/sec, 1,000 deg.C-0.5 deg.C/sec, 1,000 deg.C-200 deg.C/sec. By the treatment, the titled steel pipe having superior corrosion resistance such as resistances for pitting corrosion, crevice corrosion, intergranular corrosion, stress corrosion cracking, further having good mechanical property such as strength, toughness is obtd.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] This invention provides an austenite-ferrite two-phase system having excellent corrosion resistance such as pitting corrosion resistance, crevice corrosion resistance, intergranular corrosion resistance, and stress corrosion cracking resistance. The present invention relates to a method of solution heat treatment of a two-phase welded stainless steel pipe to obtain a welded stainless steel pipe.

[Prior art and its problems]

The operating conditions for oil and natural gas transmission pipes have become increasingly severe in recent years. For example, when the transport gas has a relatively high temperature such as about 100°C and contains a large amount of CO2,
If the material is carbon steel, it cannot be used because corrosion will occur on the entire surface. In addition, if chlorine ions are mixed in the transport gas, stress corrosion cracking will occur, so JIS
Stainless steels such as SUS 304 and SUS 316 specified in the above cannot be used.

Therefore, austenitic-ferritic two-phase stainless steel is often used for welded steel pipes used in the above-mentioned corrosive environment. Such a welded steel pipe is manufactured by forming a two-phase stainless steel plate as rolled or after solution heat treatment, seam welding it into a steel pipe, and subjecting it to solution heat treatment. It is desirable to subject welded steel pipes to solution heat treatment in order to ensure the corrosion resistance of weld metal and weld heat affected zone.

Solution heat treatment of duplex stainless steel is performed according to, for example, JISG
According to 4304, it is prescribed to heat to 950 to 1100°C and then cool with water.

However, simply performing the solution heat treatment as described above is insufficient, and corrosion resistance may deteriorate.

In other words, if the heat treatment temperature is low, the corrosion resistance will deteriorate due to insufficient solution treatment, whereas if the heat treatment temperature is high but the cooling rate is fast, there will be a problem of pitting corrosion occurring in the base material. arise.

[Purpose of the invention]

Therefore, the object of the present invention is to provide a solution heat treatment method for a welded duplex stainless steel pipe to obtain a welded austenitic-ferritic duplex stainless steel pipe that has excellent corrosion resistance and good mechanical properties such as strength and toughness. Our goal is to provide the following.

[Summary of the invention]

This invention shows solution heat treatment of an austenitic e-ferritic two-phase stainless steel welded steel pipe whose base material and weld metal are both austenitic-ferritic two-phase stainless steel, at a heat treatment temperature of 1000 to 1200°C on one axis, When heat treatment conditions are expressed by coordinates of 0.5 to b on the other axis, the heat treatment temperature (°C) on the one axis and the cooling rate (°C/°C) on the other axis
5ee) Next 6 plot points connecting 120
0℃-0.5℃/see, 1200℃-1℃/s
ee, 1100℃-25℃/sec, 1100
℃-200℃/sec, 1000℃-0.5℃/s
ee, 1000° C.-200° C./Be (!).

[Structure of the invention]

Next, the reason why the solution heat treatment conditions are limited as described above in this invention will be explained.

Figure 2 shows the temperature at 950 to 1250°C for a test piece of a two-phase welded stainless steel pipe having the composition shown in Table 1 below.
It is a graph showing the pitting corrosion test results when heat treatment temperature, holding time of 1 to 30 minutes, and solidification heat treatment of 0.5 to b.

As shown in Fig. 3, the test piece was made by submerged arc welding (SAW) on the inner and outer surfaces of a duplex stainless steel plate with a length a of 60W, a width of 3QIEL, and a thickness t of 14.3I under the following conditions. ) was used, which was welded in one layer.

(1) Heat input: Inner surface 31 KJ/cIn Outer surface 34 KJ/Cm (2) Welding wire: Same material as the test piece (3) Welding flux: Melting type high basic pitting test was conducted under C/- environment. In order to strictly evaluate the pitting corrosion resistance of the specimen, the test piece subjected to the solution heat treatment as described above was subjected to shot blasting, pickling, and 1O% FeCl3-6H2 at a temperature of 30°C.
The weld metal rod, the weld heat affected zone (HAZ), and the inner surface of the base metal were examined for pitting corrosion. This test is based on the test specified in JIS G 0578.

In FIG. 2, the upper half of the circle indicates the base metal part, and the lower half of the circle indicates the weld zone (weld metal and weld heat affected zone). Also, 1
The mark indicates 2 or less pitting corrosion, the ecological circle indicates 3 or more pitting corrosion, and the half-white circle indicates no pitting corrosion. As can be seen from the drawings, when the solution heat treatment temperature is less than 1000° C., the welded portion is not sufficiently solutionized and a lot of pitting corrosion occurs in the weld metal.

On the other hand, when the solution heat treatment temperature exceeds 1200°C, pitting corrosion occurs in the base material, and between over 1100°C and 1200°C, pitting corrosion occurs in the base metal depending on the cooling rate.

In the region where the solution heat treatment temperature is 1100°C or less,
No effect of cooling rate on pitting corrosion resistance was observed. However, in a region where the solution heat treatment temperature exceeds 1100° C., the pitting corrosion resistance of the base metal portion deteriorates if the cooling rate is fast. One of the causes is that Cr
As shown in the layer schematic diagram, this is because when the cooling rate is fast, precipitates mainly consisting of Cr nitrides are generated at ferrite grain boundaries, and the amount of Cr in the vicinity decreases, creating a Cr-deficient region. Conceivable. If the cooling rate is slow even when the solution heat treatment temperature exceeds 1100 °C, Figure 4 (b) shows a schematic diagram of the Cr layer when the solution heat treatment temperature is 1150 °C and the cooling rate is 0.5 °C/S. As shown in the figure, even if precipitates made of Cr nitride are generated, there is time for Cr to diffuse.
As a result of filling the deficient layer, pitting corrosion does not occur.

In the region where the solution heat treatment temperature is 1000 to 1100°C, it is not necessarily necessary to limit the cooling rate, but the reason why the lower limit of the cooling rate is set to 0.5°C/SeC in this invention is that this cooling rate is different from air cooling. The reason for setting the upper limit to 200° C./see is that this cooling rate is the maximum cooling rate that can be obtained industrially by water cooling at present.

From the above, in this invention, as shown in Fig. 1, the heat treatment conditions are expressed by coordinates where one axis indicates the heat treatment temperature of 1000 to 1200 °C, and the other axis indicates the cooling rate of 0.5 to b. Sometimes, the following six plot points connecting the heat treatment temperature (°C) on one axis and the cooling rate (°C/5ec) on the other axis, namely 1200°C-〇, 5°C/see, 1200
℃--1℃/see, 1100℃-25℃/see
, 1100℃--200℃/see, lOO
0°C-0.5°C/8eCT1000°C-200°0.
The area surrounded by /sec was defined as the solution heat treatment condition.

The composition range of the austenite-ferrite two-phase stainless steel welded steel pipe and weld metal in the present invention is as follows.

C: 0.03 wt@% or less, Si: 0.1 = 1.0 wt, Chi, Mn
: 0.1~2. Owt, qb.

Ni: 3-8 wt Mamoru, Cr: 21-28 wt, qb.

Mo: 1 to 4 wt, %, N: 0.08 to 0.25 wt, %, iron and inevitable impurities; remainder.

Or, in addition to the above component composition, one or two of the following
Ingredients containing seeds or more.

Cu: 2.5 wt, % or less; V, W: each 1.5 wt, % or less; Ta, Ti, Nb, Zr: each 1.5 wt, % or less; Qwt, 4 or less, C
alMP: each 0.01 wt, 4 or less, B: 0.
01 wt, 1 or less, REM: 0.2 wt, 4 or less.

[Embodiments of the invention]

Next, the present invention will be explained using examples. Steel plates A-1 to 3 and B-1 to 3 shown in Table 3 were prepared using steels A and B having two ladle compositions shown in Table 2.

Next, the above-mentioned steel plates were formed into pipes by the UOE process, seam welded using matching metal wire, and solution heat treated by the method of the present invention to produce welded steel pipes A-1, A-2, and B-1.
, B-2, and welded steel pipes A-3 and B-3 which were subjected to solution heat treatment by a conventional method were prepared. Table 4 shows the dimensions, welding conditions, and solution heat treatment conditions of each welded steel pipe.

Table 5 shows the mechanical properties of each test piece shown in Table 4. In Table 5, [)epo means weld metal, and HAZ means weld heat affected zone. The fracture location in the weld tensile test was the base metal. In addition,
Charpy testing of AB-1 and AB 2 specimens was conducted on 1/2 size.

Table 6 shows the corrosion resistance test results for the base metal and welded parts of each test piece shown in Table 4. In Table 6, welded part 4
The point bending SCC test uses CO2 at 9.95 atm and H2S.
5% of the temperature of 90℃ melted at 0.05 atm.
This was done by examining the stress corrosion cracking of a test piece in a four-point bending state with a stress of 80 inches σYS applied in a NaC7 solution. In addition, the pitting corrosion test is carried out by testing the test piece at 10°C at a temperature of 30°C.
% F'eCA3s6H2+ aqueous solution for 24 hours, and the state of pitting corrosion was investigated.

As is clear from Tables 5 and 6, welded steel pipes subjected to solution heat treatment by the method of the present invention have a base metal part,
Both welded parts had good strength and toughness, and no intergranular corrosion or pitting corrosion occurred, resulting in excellent corrosion resistance.

The welded steel pipe to which the method of this invention is applied may be a straight pipe or a curved pipe, as long as the base metal and weld metal are both austenitic-ferritic duplex stainless steel, and any other material such as a clad steel pipe may be used. It can be applied to objects of various shapes and sizes.

〔Effect of the invention〕

As described above, according to the method of the present invention, the material has excellent pitting corrosion resistance, crevice corrosion resistance, single grain boundary corrosion resistance, stress corrosion cracking resistance, etc., and has excellent mechanical properties such as strength and toughness. An industrially excellent effect is brought about by making it possible to obtain a good quality austenite-ferrite two-phase stainless steel welded steel pipe.

[Brief explanation of drawings]

Fig. 1 is a graph showing the range of solution heat treatment temperature and cooling rate in the method of this invention, Fig. 2 is a graph showing the pitting corrosion test results, Fig. 3 is a perspective view showing the shape of the test piece, and Fig. 4 (
(a) and (b) are schematic diagrams of a Cr layer.

Claims (1)

[Claims] Solution heat treatment for austenitic-ferritic dual-phase stainless steel welded steel pipes in which both the base metal and weld metal are made of austenitic-ferritic dual-phase stainless steel,
One axis is shown at a heat treatment temperature of 1000 to 1200°C,
When heat treatment conditions are expressed by coordinates with the other axis showing a cooling rate of 0.5 to 200°C/sec, the heat treatment temperature (°C) of the one axis and the cooling rate (°C) of the other axis are expressed as follows:
/sec) and the next six plot points, i.e. 1200
℃-0.5℃/sec, 1200℃-1℃/sec, 1
100℃-25℃/sec, 1100℃-200℃/s
ec, 1000℃-0.5℃/sec, 1000℃-2
A method for solution heat treatment of a two-phase stainless steel welded steel pipe, characterized in that the solution heat treatment is carried out within a region surrounded by 00° C./sec.