JPS63238287A - High-alloy steel pipe having coating layer at end and method for welding same - Google Patents

Abstract

PURPOSE:To prevent the deterioration of the corrosion resistance of a weld zone formed by the butt circular welding of high-alloy steel pipes contg. a specified amt. of Cr by previously forming coating layers based on a metal having lower electric potential than the steel and greater affinity for oxygen than Cr on the insides of the ends of the pipes. CONSTITUTION:When high-alloy steel pipes contg. >=13% Cr are welded by butt circular welding, the insides of the ends of the pipes are previously coated with coating layers based on a metal having lower electric potential than the high-alloy steel and greater affinity for oxygen than C4, e.g., Al, Si or Mn. Then thickness of the coating layers is regulated to about 10-100mum and only the ends of the pipes affected by heat during welding are coated. Thus, the deterioration of the corrosion resistance of weld zone formed by the butt circular welding of the high-alloy steel pipes is prevented.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to steel pipes for line pipes having a coating layer on the pipe ends, particularly high alloy steel pipes and pipes in which the corrosion resistance of the welded part does not deteriorate after pipe end butt welding. Regarding the welding method.

(Conventional technology and its problems) Line pipes, etc. that transport highly corrosive gas and oil,
High-alloy steel pipes with excellent corrosion resistance are used.

Normally called high-alloy steel pipes contain 13% or more of Cr and, if necessary, other alloying components such as Nl and Mo. Although such high-alloy steel pipes themselves have excellent corrosion resistance, when they are butted together and circumferentially welded, a problem arises in that the corrosion resistance (particularly pitting corrosion resistance) of the welded heat affected zone deteriorates. That is, high-alloy steel pipes are entirely ground with a grinder when shipped from the factory, but when they are welded together at the pipeline installation site, scale is generated in the weld heat affected zone. Scale on the outside surface of the tube can be removed by grinding with a grinder, so the welded part on the outside surface has the same corrosion resistance as the base metal, but it is difficult to remove scale on the inside surface of the tube, and it is difficult to remove scale from the inside surface of the tube due to the presence of corrosive gases and oil. This results in an extremely unfavorable situation in which the corrosion resistance of the area near the weld on the inner surface of the tube that is in contact with the area is poor.

The scale in the weld heat affected zone of high alloy steel containing 13% or more of Cr is mainly composed of Cr oxides, and the reason why corrosion resistance, especially pitting corrosion resistance, deteriorates due to the occurrence of such scale is thought to be as follows. Firstly, due to the rapid formation of Cr oxides, Cr, which is essential for maintaining corrosion resistance in the steel underlying the scale, is depleted, and secondly, due to the minuteness of the generated scale, the interaction between the rack and the scale and the underlying steel occurs. The gap is likely to become a starting point for pitting corrosion.

Therefore, in order to ensure the corrosion resistance of the welded part, it is sufficient to remove the generated scale, but it is virtually impossible to remove the scale on the inner surface of the tube by grinding with a grinder or the like after the completion of butt welding.

An object of the present invention is to obtain a welded part having corrosion resistance equal to that of the base metal without performing scale removal treatment after welding and joining.

(Means for Solving the Problems) The basic idea of the present invention is to reduce the scale generated in the weld heat affected zone and to make the generated scale not have an adverse effect on the corrosion resistance of the base metal. be. The present invention, which embodies this idea, has the following points.

(1) A high alloy steel pipe containing 13% or more of Cr,
A steel tube having a coating layer on the inner surface of the tube end mainly consisting of a metal that has a baser potential than the above-mentioned high alloy steel and has a stronger affinity for oxygen than Cr.

(2) In butt welding of high-alloy steel pipes containing 13% or more of Cr, the inner surface of the pipe end is pre-coated with a metal that has a baser potential than the high-alloy steel and has a stronger affinity for oxygen than Cr. A method of welding high alloy steel pipes, which is characterized by butting together and circumferentially welding steel pipes.

Here, the reason why the high alloy steel is defined as containing 13% or more of Cr is because, as mentioned above, the corrosion resistance of welds becomes a problem at 13%, which causes scale mainly composed of Cr oxides to form in the heat-affected zone.
This is because the steel contains the above Cr. A typical example of such high alloy steel is 5tlS430.5tlS
There are things like 316L and 5tlS329J.

Metals that have a more base potential than the above-mentioned high alloy steel (more specifically, a base potential in the galvanic potential hierarchy) and have a stronger affinity for oxygen than Cr include AQ, St, Mn, and the like. The coating layer may be composed of a single substance or an alloy of these metals, or a mixture containing these metals or alloys as main components.Means for forming the coating layer include:
Various methods such as plating, thermal spraying, and painting can be used depending on the type of metal to be coated. For example, for metals with low melting points such as AQ, hot-dip plating or thermal spraying is appropriate, and SIs
For Mn, etc., it is preferable to apply a paint made by suspending its powder in an organic solvent. Although it is sufficient to coat only the inner surface of the tube, there is no problem even if there are coating layers on the inner and outer surfaces.

The coating layer only needs to be provided in the area that is affected by heat during welding of the tube ends. The sub that is usually adopted for circumferential welding? - In the pipe welding method and the TIG welding method, even if the pipe diameter, wall thickness, and welding conditions change, the thermal effect is from the pipe end to about 10011, so it is sufficient to cover this range. The thickness of the coating layer is 1
A value of 0 to 100 μm is appropriate. That is, 10 μm
This is because if the thickness is less than 100 μm, the oxidation will extend to the base material, and if it exceeds 100 μm, the effect will not change and it will be uneconomical. Such a coating can be applied in a steel pipe manufacturing factory, or it can be applied near the site where the vibrating line is installed prior to welding work.

The welding process itself can be carried out using conventional methods such as submerged arc welding or TIG welding, and no special considerations are required.After welding, if necessary, the outer surface scale can be removed by grinding, etc., and the inner surface is There is no problem even if it remains welded.

(Function) When butt welding is performed using the tube end coated steel pipe of the present invention, the metal constituting the coating layer is preferentially oxidized in the weld heat affected zone before oxidation of Cr in the base material. Therefore, the scale that is generated is not mainly composed of Cr oxides,
For example, when coating is performed by AQ thermal spraying, what is generated in the weld heat affected zone is mainly MtOs. Even if such scale remains attached to the inner surface of the pipe, it has no effect on corrosion resistance.The reason is that AQ is preferentially oxidized, so Cr on the steel surface is not depleted, and the oxidized scale This is because since the base is metal AQ, it preferentially melts in a corrosive environment and does not adversely affect the corrosion resistance of high alloy steel.

(Example) Steel pipes with compositions A-H shown in Table 1 (outer diameter 60 l,
The following test was conducted using a wall thickness of 4 dragons.

[Test 1] 100 tube ends of steel pipe A were subjected to the following treatments (a) to (e).

(b) Q hot-dip plating (thickness 10-15μ11) (b) AQ thermal spraying <yt,, 30-50pm) (c) AQ powder coating (thickness 60-10011aa) (
d) Si powder coating (thickness 60-10011tn)
(e) Mn powder coating (thickness: 50 to 80 μn+) Further, the AQ thermal spraying treatment described in (b) above was applied to each of the tube ends of 100 mm of steel pipes B to E. After these tubes were butt-welded and circumferentially welded under the welding conditions shown in Table 2, a test piece with a length of 40 mm was taken from the weld heat-affected zone and subjected to a corrosion test. <The conditions for the corrosion test are also shown in Table 2) Table 1 Table 2 For comparison, steel pipes without pipe end treatment were also welded under the same conditions and subjected to the corrosion test. The results are summarized in Table 3.

Table 3 As shown in Table 3, pitting corrosion occurred in the welded parts of steel pipes without tube end treatment, but in the pipes corresponding to the examples of the present invention, no local corrosion occurred in the welded parts. I haven't. Furthermore, test pieces taken from the welded heat-affected zone of the steel pipes of Tests 6 to 8 were soaked in a 3.5% NaCl aqueous solution at 60°C.
．． When held at a potential of 2V vs SCH for 240 hours,
Although the adhered M was dissolved, no local corrosion was observed in the base metal.

(Effects of the Invention) The present invention has a great effect on preventing deterioration of the corrosion resistance of the inner welded portion in butt welding of high alloy steel pipes.

As shown in the examples, this effect is remarkable regardless of the type of base material high alloy steel, so it can be widely applied not only to the line pipes mentioned above but also to various plants assembled by butt welding. can. In particular, even in cases where the inner surface cannot be cleaned after welding, the present invention has great utility in that the excellent corrosion resistance inherent to high alloy steel pipes can be guaranteed even in the welded parts.

Claims (2)

[Claims] (1) A high-alloy steel pipe containing 13% or more of Cr, which has a coating layer mainly made of a metal that has a more base potential than the above-mentioned high-alloy steel and a stronger affinity for oxygen than Cr on the inner surface of the pipe end. High alloy steel pipe. (2) In butt welding of high-alloy steel pipes containing 13% or more of Cr, the inner surface of the pipe end is coated with a metal that has a baser potential than the above-mentioned high-alloy steel and a stronger affinity for oxygen than Cr. A method for welding high-alloy steel pipes, which comprises butting together and circumferentially welding steel pipes that have been coated in advance.