JPH09206967A - Manufacture of two-phase stainless steel welded tube - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a two-phase stainless steel welded tube superior in corrosion resistance of a weld zone by welding with a laser beam under a preheating temperature and conditions satisfying some equations. SOLUTION: For the purpose of obtaining a two-phase stainless steel welded tube, preheating the abutted edges of a steel strip within a temperature range of the equation I is more effective than controlling the welding speed, thickness of the steel strip and laser output. In addition, for the purpose of performing complete through welding, it is necessary that a laser output PKw, preheating temperature T deg.C, welding speed Vm/min, thickness of a steel strip tmm and a constant (a) (=0.0006) are satisfied in the equation II. Thus, a two-phase stainless steel welded tube with a welded zone having the same corrosion resistance as the base stock can be manufactured by laser welding at a low cost.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a duplex stainless steel welded pipe excellent in corrosion resistance of a welded portion, which is suitable for application to a line pipe, an oil well pipe or a pipe for chemical machinery.

[0002]

2. Description of the Related Art Duplex stainless steel has a two-phase structure of a ferrite phase and an austenite phase, has excellent resistance to stress corrosion cracking, and also has good toughness and weldability, and is therefore widely used as a material for oil well pipes or seawater. It is used. The proper solution temperature of this duplex stainless steel is 1050 ° C to 1
When the temperature is 100 ° C. and the solution treatment is performed by heating in this temperature range, the volume ratio of the ferrite phase (hereinafter referred to as the ferrite ratio) is usually about 50%. The elements that contribute to the improvement of the corrosion resistance of stainless steel are mainly Cr, Mo and N. However, in duplex stainless steel, Cr and Mo are concentrated in the ferrite phase and N is concentrated in the austenite phase, and the corrosion resistance of both phases increases. I am in balance.

However, when the duplex metal is welded, the structure of the weld metal becomes higher than the above-mentioned appropriate solution temperature range at the time of welding and once becomes a ferrite single phase.
The growth of austenite phase becomes insufficient, and the ferrite ratio greatly exceeds the above-mentioned proper value of 50% of the base metal portion,
% Or more. For this reason, improper distribution of constituent elements and Cr
Nitride precipitation occurs and mechanical properties and corrosion resistance deteriorate.

Conventionally, the welded steel pipe of duplex stainless steel has two
A strip steel made of duplex stainless steel is continuously formed into an open pipe shape by passing it through a forming roll group, and both edges of the strip steel are butted against each other by a squeeze roll, and the butted part is electric resistance welding (hereinafter referred to as ERW) or gas / tungsten. Pipe manufacturing welding was performed by arc welding such as arc welding (hereinafter referred to as GTAW) or submerged arc welding (hereinafter referred to as SAW).

However, since the corrosion resistance of the weld metal deteriorates as described above when the pipe-making welding is performed as it is, in order to bring the ferrite ratio of the weld metal close to an appropriate value, in ERW, a post heat treatment is applied to the welded portion, GTAW, etc. In the arc welding, the ferrite ratio was brought close to an appropriate value by adding a predetermined alloying element to the weld metal using a filler wire.

Further, in a welded pipe manufactured by arc welding such as ERW or GTAW, a heat affected zone (hereinafter referred to as HAZ) is formed adjacent to the weld metal. This HAZ consists of a "high temperature HAZ" heated to 1150 ° C to 1250 ° C adjacent to the weld metal and 700 ° C adjacent to the "high temperature HAZ".
It consists of a "cold HAZ" heated to ~ 950 ° C.
In the “high temperature HAZ”, the ratio of the ferrite phase and the austenite phase is deviated, so that Cr nitride is precipitated, and a Cr deficient layer is formed around the Cr nitride to significantly deteriorate the corrosion resistance. Also,
In the "low temperature HAZ", a stable phase Cr-Mo intermetallic compound in the temperature range of 700 ° C to 900 ° C is deposited, and a Cr-Mo deficient layer is formed around the compound, and the corrosion resistance is significantly deteriorated in this portion as well. . These high temperature HAZ and low temperature HAZ
On the other hand, unlike the weld metal, the structure cannot be optimized by the filler wire, so the post-heat treatment was used exclusively to improve the corrosion resistance.

In recent years, a pipe-making welding method using a laser, which has a higher welding speed than the above-mentioned conventional welding method, has been developed. However, in the conventional method for producing a welded pipe by the laser welding method, since the edge portion of the steel strip is melted and rapidly solidified in the welded portion, the weld metal corresponding to the "high temperature HAZ" in ERW or GTAW as described above is left as-welded. Generate tissue. However, in the laser welding method, it is difficult to add alloying elements to the weld metal using a filler wire due to the equipment. Therefore, as in the case of ERW, by applying an appropriate post heat treatment to the welded portion after laser welding, , We are trying to recover the performance of the welded part. In laser welding,
It does not form Cr-Mo intermetallic compounds in HAZ due to rapid heating and quenching, and ERW or GTAW
There is no "low temperature HAZ" that occurs in.

The improvement of laser welded duplex stainless steel welds by post heat treatment has hitherto not been known. However, regarding the method for producing an austenitic stainless steel pipe, Japanese Patent Laid-Open No. 63-278688 discloses a method for recovering ductility of a weld metal after laser welding.
Proposals have been made that post-heat treatment at ℃ to 600 ℃ is required. Similarly, regarding the method for producing a ferritic stainless steel pipe, JP-A-63-278689 and JP-A-63-278690 both propose to recover the performance of the weld metal by post heat treatment. .

As described above, it is possible to recover the performance of the welded portion to some extent by the post heat treatment. However, the corrosion resistance cannot be restored to the same level as the base metal,
In addition, there is a drawback in that the post-heat treatment causes an increase in manufacturing cost.

[0010]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for producing a two-phase stainless welded pipe having a welded portion having the same corrosion resistance as the base metal as it is by a laser welding pipe making method. To do.

[0011]

[MEANS FOR SOLVING THE PROBLEMS] The laser welding method cools the weld metal much faster than other welding methods.
It is known that deterioration of the corrosion resistance of the weld metal is less likely to occur than that of GTAW and GTAW. However, the effect of welding conditions on the corrosion resistance of the weld metal produced by laser welding of duplex stainless steel is not known in detail. In particular, there is no information on the effect of preheating before welding on the weld metal structure and its corrosion resistance.

Then, the present inventors confirmed the following (a) and (b) as a result of conducting experiments on the duplex stainless steel under various conditions of laser welding.

(B) In order to make the ferrite content of the weld metal equal to that of the base metal, it is necessary to preheat both edges of the abutted steel strips to the temperature range of the following formula, the welding speed, the wall thickness of the steel strip and the laser. It is more effective than controlling output.

900 ≦ T (° C.) ≦ 1100 ... Preheating to this temperature range not only provides a proper ferrite ratio, but also other welding methods and It has been found that it is possible to obtain a weld metal that does not precipitate Cr nitride at all, which cannot be obtained by the conventional laser welding method.

(B) In order to obtain a sound welded joint, it is necessary to perform complete penetration welding, but in order to perform complete penetration welding, laser output P (kW), preheating temperature T (° C),
It was found that it is necessary to satisfy the conditions of the following formula with respect to the welding speed V (m / min) and the wall thickness t (mm) of the steel strip.

0.4 ≦ P · {exp (a · T)} / (V · t) ... In the following description, “P · {exp (a · T)} /
"(Vt)" is called "heating index".

The gist of the present invention is a method for producing a duplex stainless steel welded pipe excellent in corrosion resistance of a welded portion, which is based on the above matters.

(1) In pipe welding of a duplex stainless steel strip steel, preheating is performed at a temperature (T) satisfying the formula, then laser welding is performed under the condition satisfying the formula, and the product is as-welded. And a method for producing a duplex stainless steel welded pipe.

900 ≦ T ≦ 1100 ... 0.4 ≦ P · {exp (a · T)} / (V · t) ... where T: preheating temperature (° C) P: laser output (kW) V: welding speed (m / min) t: wall thickness (mm) of strip steel made of duplex stainless steel a: constant (= 0.0006)

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION The reason why the method of the present invention is limited as described above will be described in detail below.

1. Preheated band steel consisting of duplex stainless steel is continuously formed into an open pipe shape by passing it through a group of forming rolls by a normal method, both edges of the band steel are butted against each other with a squeeze roll, and the butted portion is irradiated with a laser beam. Pipe welding. ERW installed in front of squeeze roll
Both edges of the steel strip are heated by an annular induction heating coil or contact tip that is commonly used to heat the steel strip.
The temperatures of the both edges of the heated strip steel are controlled to a predetermined temperature by the electric power supplied to these induction heating coils and the like.

The preheating temperature T (° C.) of both edges of the steel strip heated by these devices must satisfy the following equation as described above.

900 ≦ T (° C.) ≦ 1100 ... If the duplex stainless steel is preheated to 900 ° C. to 1100 ° C., this temperature range will be maintained during cooling after welding. Since the elapsed time becomes long, the ferrite ratio of the weld metal becomes 60% to 80%, which approaches the appropriate value of 50% of the base metal, and austenite grows sufficiently, so that Cr nitride does not precipitate. This effect depends largely on the preheating temperature, but is almost unaffected by other welding conditions such as power output, welding speed, and strip steel wall thickness. Also, the heat input in laser welding is small,
Since the temperature range of 00 ° C to 900 ° C is rapidly cooled and the elapsed time in this temperature range is short, there is no portion where the Cr-Mo intermetallic compound precipitates. That is, low temperature HAZ in ERW etc. does not occur. Therefore, the corrosion resistance of the weld metal and its vicinity does not deteriorate, and the performance is equivalent to that of the base metal.

FIG. 1 is a drawing showing the effect of preheating temperature on the pitting potential of the welded portion by laser welding. As shown in the figure, it can be seen that the pitting corrosion potential of the welded portion preheated to 900 ° C to 1100 ° C and welded is higher than that in the case of preheating to another temperature range. In addition, since there is no problem of HAZ in the welded part by laser unlike GTAW,
It is the weld metal that determines the corrosion resistance, and the "pitting corrosion potential of the welded part" means the "pitting corrosion potential of the weld metal".
On the other hand, the corrosion resistance of welded parts such as GTOW is HAZ
Determined by

If the preheating temperature is lower than 900 ° C., the weld metal is rapidly cooled, the growth of the austenite phase is insufficient, the ferrite ratio is increased, and the weld metal does not have good corrosion resistance. In addition, the preheated part itself contains Cr
-Mo-based intermetallic compounds are precipitated and corrosion resistance deteriorates.
That is, the same problem as "low temperature HAZ" in ERW or GTAW occurs.

On the other hand, when the preheating temperature exceeds 1100 ° C., the ferrite ratio of the entire preheated portion increases to more than 80%, resulting in improper distribution of elements and precipitation of Cr nitrides, and deterioration of corrosion resistance. To do. When the preheating temperature is set within the above-mentioned appropriate range and the ferrite ratio of the weld metal is brought close to the appropriate value of 50% of the base metal, not only the corrosion resistance but also the toughness and strength become the same as the base metal.

Further, ERW and GT other than the laser welding method
In arc welding such as AW, it is possible to bring the ferrite rate of the weld metal close to a value equivalent to that of the base metal by preheating to around the solution heat treatment temperature of the base metal. However, in the portion corresponding to the above-mentioned "high temperature HAZ", the corrosion resistance is deteriorated due to the precipitation of Cr nitrides, and it is not possible to reduce the corrosion resistance during the welding heat cycle.
The lapse of time at 00 ° C to 900 ° C becomes long, and the Cr-Mo based intermetallic compound precipitates, and the corrosion resistance of "low temperature HAZ" is significantly deteriorated. Therefore, the optimization of the weld metal structure by preheating near the solutionizing temperature of the base material is possible only in the laser welding method with a high cooling rate.

2. Other welding conditions In the present invention, as described above, it is necessary to adjust the welding conditions so as to satisfy the following formula.

0.4 ≦ P · {exp (a · T)} / (V · t) ... When the welding conditions deviate from the conditions of the present invention, that is, the thickness of the strip steel. If the laser output is insufficient or the welding speed is too fast, the heating index “P · {exp (a · T)} / (V
・ T) "is less than 0.4, and it is not possible to completely penetrate the prescribed thickness of the steel strip and obtain a sound welded joint. The upper limit of the heating index is not specified, but the heating index is 5 If the temperature exceeds 5, the weld metal part will melt through and under-beads on the outer surface of the weld part (because the amount of weld metal is insufficient and lower than the surface of the base metal will cause a step), the heating index is 5 or less. Is desirable.

The band steel made of the duplex stainless steel, which is the material, may be any duplex stainless steel having sufficient mechanical properties and corrosion resistance.
For example, JIS standard SUS329J3L or SUS
It is desirable to use a standard material such as 329J4L.

[0031]

EXAMPLES Table 1 is a list showing the chemical composition of the duplex stainless steel strip steel used for the implementation.

Tables 2 and 3 show the welding conditions of laser welding and GTAW in the pipe welding performed on the above strip steel together with the characteristics of the weld zone. In GTAW, a filler wire was used to make the ferrite ratio of the weld metal appropriate. Further, P in the heating index of GTAW was calculated by (current × voltage) used for welding heat input.

[0033]

[Table 1]

The corrosion resistance of these welded parts including the HAZ that was pipe-welded was evaluated by the pitting potential in artificial seawater as follows.

From the welded portion of the as-welded welded pipe, a test piece having an arc cross section having a length of 30 mm in the circumferential direction of the pipe and a length of 10 mm in the axial direction of the pipe and having a weld seam at the center was taken. . In this section, a rectangular portion having an area of 1 cm ² (0.5 cm ² only for GTAW) was defined as a portion for measuring the pitting potential, and the periphery was covered with a thermosetting resin so that only that portion was exposed.

FIG. 2 is a drawing showing an exposed portion of the test piece. Since the pitting potential changes greatly with a slight oxide film, the exposed portion was ground with No. 800 polishing paper immediately before the measurement and used for the test. For the test piece shown in the figure, ASTM-D
In artificial seawater specified in -1141, JIS-G057
The pitting potential was measured based on 7. The potential applied to the test piece was increased from the natural electrode potential at a potential sweep rate of 20 mV / min to measure the pitting corrosion potential. Ag for the reference electrode
/ AgCl electrode, test temperature was 60 ° C, Ar deaeration was performed during measurement, and pitting corrosion occurred at a current density of 100 μA / c
The potential reached m ² . The lower this potential is, the more easily pitting corrosion is likely to occur, resulting in poor corrosion resistance. The pitting corrosion potential of the base material was as follows.

Steel A and Steel B ... 450 to 600 mV Steel C ... 550 to 700 mV Further, whether or not complete penetration welding was performed was visually observed.

In Tables 2 and 3, the column of weld characteristics shows the test results.

[0039]

[Table 2]

[0040]

[Table 3]

Welding conditions according to the method of the present invention (No. 1 in Table 3)
1 to No. 28), the pitting corrosion potential of the weld metal pipe-welded is the same as that of the base metal, and has the same corrosion resistance as the base metal.

Under conditions other than the welding conditions of the method of the present invention, even if laser welding was performed, the corrosion resistance of the weld metal deteriorated or complete penetration welding was impossible. For example, in Nos. 2 and 9 of Table 2, the heating index “P · {exp (a · T)} /
(V · t) ”was smaller than 0.4, so complete penetration welding was impossible. Further, in Table 2, No. 1 and No. 3 to No. 8
In No. 10 and No. 10, since the preheating temperature is low, the corrosion resistance of the weld metal is not good.

The pitting corrosion potentials of Nos. 29 to 31 in the comparative examples in Table 3 are the results for steel C and therefore do not seem to be so much lower than those of the invention example for steel A. Since that of the base material is 550 to 700 mV as described above,
The result is clearly inferior to the base metal.

GTAW using a filler wire
As shown by No. 31 to No. 33 in Table 3, the welded portion of No. 3 shows a lower pitting potential than the base metal, and it can be seen that the pitting potential similar to that of the base metal cannot be obtained depending on GTAW.
In case of GTAW, unlike laser welding, HAZ
The pitting corrosion potential of the welded portion represents the pitting corrosion potential of the HAZ because the corrosion resistance of is lower than that of the weld metal.

[0045]

INDUSTRIAL APPLICABILITY According to the method of the present invention, a duplex stainless steel welded pipe having a welded portion having corrosion resistance equivalent to that of the base material can be produced inexpensively as it is by laser welding, and its industrial value is great.

[Brief description of drawings]

FIG. 1 is a drawing showing the effect of preheating temperature on the pitting potential of a welded portion by laser welding.

FIG. 2 is a drawing showing an exposed portion of a test piece for measuring a pitting potential.

[Explanation of symbols]

 1 ... Welded portion (cross section perpendicular to pipe axis) 2 ... Base material (cross section perpendicular to pipe axis) 3 ... Thermosetting resin 4 ... Epoxy resin 5 ... Coated conductor wire

Claims (1)

[Claims] 1. A method of pipe-welding a duplex stainless steel strip, comprising preheating at a temperature (T) satisfying a formula, laser welding under a condition satisfying the formula, and producing a product as it is welded. Method for producing a duplex stainless steel welded pipe. 900 ≦ T ≦ 1100 ······ 0.4 ≦ P · {exp (a · T)} / (V · t) ··· Here, T: preheating temperature (° C.) P: laser output (kW) V: welding speed (m / min) t: wall thickness (mm) of strip steel made of duplex stainless steel a: constant (= 0.0006) )