JPS60238423A - Improvement of corrosion resistance in weld zone of two-phase stainless steel - Google Patents

Abstract

PURPOSE:To improve the corrosion resistance of a weld zone by heating and holding the weld zone of a two-phase stainless steel welded as rolled or after a soln. heat treatment or the entire part of the welded structure including the weld zone to and in a prescribed temp. range, thereby effecting the soln. heat treatment. CONSTITUTION:The two-phase austenite-ferrite stainless steel contg. <=0.03% C, 0.1-1.0% Si, 0.1-2.0% Mn, 3-8% Ni, 21-28% Cr, 1-4% Mo and 0.08-0.25% N is welded by the weld metal consisting of the above-described compsn. as rolled or after the soln. heat treatment. The above-described weld zone or the entire part of the welded structure including the weld zone is heated and held within the range enclosed by the four points; (1,000, 20), (1,000, 3,600), (1,100, 0), (1,100, 3,600) in the coordinates shown in the figure showing the relation between temp. ( deg.C) and time (sec), thereby effecting the soln. heat treatment. As a result, the weld zone of the two- phase stainless steel having the improved corrosion resistance including all of general corrosion pitting corrosion, crevice corrosion, integranular corrosion, stress corrosion cracking and hydrogen induced cracking is obtd.

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a method for improving the corrosion resistance of a welded heat-affected zone of duplex stainless steel (austenitic-ferritic duplex stainless steel) and a welded part of weld metal. It is something.

[Prior art]

Oil 45: F Autumn gas supply, etc. 6 uses are becoming more severe, such as prayer gardens. For example, when transporting gas containing a large amount of carbon dioxide or hydrogen sulfide, there is a growing need for steel pipes that are sufficiently corrosion resistant to withstand such usage conditions from the perspectives of stable operation, pollution issues, and safety. .

To meet these needs, the materials used for pipes used under harsh operating conditions tend to be highly alloyed; for example, in heavy sour environments (when the transport gas temperature is around room temperature), austenitic materials is relatively high temperature (100℃
If the pipe contains a large amount of COt and is mixed with Ct (before and after), it is used as a two-phase stainless steel pipe.

Conventionally, welded stainless steel pipes are processed using as-rolled steel plates or plate solution heat treatment (hereinafter referred to as solution treatment).
tment: Abbreviated as ST. ) Wrap steel plate (be/
It was manufactured by post welding (using a rolling roll). In addition, the process of "rUO + welding + tube expansion" has been adopted because it allows long pipes to be manufactured and has excellent dimensional accuracy.

However, in the as-welded state, the weld metal and the weld heat-affected zone near the melting line are subject to high-temperature thermal cycles of over 1300℃ due to welding, so in the case of two-phase (austenite + ferrite) stainless steel, the ferrite ratio becomes extremely large. This has been a problem in the past because it not only deteriorates toughness but also has an adverse effect on the corrosion resistance ratio.

Incidentally, steel equivalent to JIs329J1 with K t = 18- was subjected to latent arc welding on the inner and outer surfaces of 1N with a heat input of 40 KJ/yn, and the cut pieces of the welded part were placed in 10 pieces of Fecls 6H70 solution.
When immersed at 0°C for 24 hours, HAZ's Fusion Li
A bit occurs near the fe stab.

In addition, in the area where the welding heat affected zone exists, where the welding heat cycle is applied to 700 to 900°C, there is a risk that abrasion, σ phase, Cr carbide, etc. will precipitate at the grain boundaries and deteriorate the corrosion resistance.

[Purpose of the invention]

In order to solve the above-mentioned problems of the conventional technology, the present invention has improved the corrosion resistance ratio of welded parts (weld heat-affected zone and weld metal) of two-phase stainless steel, namely general corrosion resistance, pitting corrosion resistance, resistance to hydrogen rusting and cracking. The purpose is to improve corrosion resistance, including all aspects.

There are no other limitations as long as the welded part targeted by the present invention is austenitic-ferritic stainless steel together with the base metal and weld metal. Either a pipe or a curved pipe may be used, and the size (diameter, pipe thickness) is not subject to regulations, but in this specification, relatively large diameter pipes such as flow lines and gearing lines intended for natural gas transportation will be used. I have given an example.

[Summary of the invention]

The present invention has not been completed in order to achieve the above-mentioned objects. The gist of the present invention is to weld duplex stainless steel as rolled or through solution treatment, and to weld the welded part or the welded part including the welded part. The entire structure is heated by heating equipment to maintain the temperature (in coordinates showing the relationship between time and seconds).
1000, 20), (1000, 3600) (1100
, O), (1100, 3600) A method for improving the corrosion resistance of a welded part of duplex stainless steel, characterized by performing solution treatment to maintain heat within the range surrounded by the four points ll'r, iXl, A two-phase stainless steel welded part exhibiting high corrosion resistance under conditions of , high 002-fine H, S-CI-@ can be obtained.

[Structure of the invention]

The present invention consists of the following configuration.

(1) The target is two-phase stainless steel welded parts such as two-phase stainless steel welded steel pipes used in an acidic environment containing Ct ions, such as a high cot-microH2S C1- environment of around 100℃. It must be a structure that has

(2) Corrosion resistance In particular, the conditions for solution treatment of the welded part or the entire welded structure including the welded part should be limited in order to ensure the corrosion resistance of the welded part (weld metal, heat affected zone).

(3) In the present invention, the duplex stainless steel before welding may be in any of the following states: rolled (as hot rolled) or solution treated; Process: For example, the process of forming an incoming plate into a tubular shape in the manufacturing process of welded steel pipes, such as UO bottom forming intervention 1. Senbon (Also, a processing step such as a pipe expansion step may intervene between welding and solution treatment. Further, after the solution immersion treatment, a mild cold working step such as a sizing step may be performed.

Next, the reasons for the limitations of the present invention will be described.

Regarding (1) mentioned above, it is well known that duplex stainless steel is excellent in environments such as forceps, but just to be sure, austenitic stainless steel (e) with the components shown in Table 1 and two-phase stainless steel (A.

B) Fig. 1 shows the results of corrosion tests under two types of high CO2, minute H2S, and C1 environments. Sanf in Figure 1.

All steels were solution-treated at 1050°C for 10 minutes and then water-cooled, and the test conditions were: corrosion bath *-5% NaC7 + 0.5% C) (sCOOH,
Gas: 98% CO, +2% H, S Temperature: 60°C Period: 14 days Compared to the austenitic steel C, the duplex steels A and B have a lower corrosion rate and are superior in corrosion resistance.

Figure 2 shows the above steel A after one layer of latent arc welding on the inner and outer surfaces.
This shows the results of various ST tests performed in the laboratory, followed by shot and pickling tests, followed by pitting corrosion tests. The O mark in the figure is pit
``■'' without ting indicates bit≦2 pieces, bit≦3 pieces, *marked is an electric furnace, and the process was done in a salt system.

The test samples were welded using a welding method of 5AW (as shown in Figure 6).
1 layer on the inner and outer surfaces), heat input: 31KJ/6n on the inner surface and 5 on the outer surface 34
7 cutting, wire cometal, flux: molten type high basicity, proscene: welding → ST by salt pass and electric furnace → shot → pickling, corrosion test is 10% FC3C1s.
A method was used in which the weld metal, HAZ, and base metal were evaluated (inner surface side) after being immersed in a 6H20 solution at 30° C. for 24 hours.

As shown in FIG. 5, as for the influence of temperature, when the temperature is below 1000°C, bits frequently occur in the weld metal and T (AZ), and conversely, even when the temperature exceeds 1100°C, bits occur.

At temperatures below 1000'C, solid solution formation is not sufficient, and -
When the temperature exceeds 100°C, the ferrite ratio increases systematically, and although the detailed cause is unknown, corrosion resistance deteriorates - and the effect of 81 hours is too short to be too short.

If this happens, fine austenite will be broken down and the composition will not be uniform.
This test is specified in ASTM G4B-1!
It is similar to a force test and is considered to be able to strictly evaluate pitting corrosion resistance in a Ct-environment. Therefore, the knowledge obtained from Fig. 2
This can be said to clearly express the influence of the T condition.

From Figure 6, (temperature °C time set・) coordinates (11°0
0゜0) (110rl, 3600) (101110
,20) S surrounded by four points (1000°3600)
The T conditions were limited from the viewpoint of pitting corrosion resistance. Note that exceeding the upper limit of the ST time by 6D is not industrially practical in the case of heat treatment by induction heating.

For atmospheric furnace heating, 81 hours and 30 minutes or more (1800
However, if 81 hours exceeds 60 minutes, the toughness of the weld metal, HAZ, and base metal will deteriorate due to coarsening of austenite grains, so the upper limit of 81 hours is 60 minutes. (3600 seconds).

The reason why the cooling rate was not necessarily limited in the above ST (solution treatment) is that there is no problem as long as the cooling rate is faster than air cooling.

Table 2 shows the influence of the cooling rate of ST on the pitting corrosion resistance of welds, and it changes in the range of 0°5°C/s (equivalent to air cooling) to 60°C/s (water cooling). It turns out that there is no.

Table 2 Effect of ST cooling rate on pitting corrosion resistance〇2N
o pitting Test sample and test conditions are the same as in Figure 5 ST 1
050°C x 5 minutes Cooling rate 0.5°C/S to 60°C/8 Next, regarding the reason for limitation (3), various embodiments were considered, but it is an essential requirement that solution treatment be performed.

Table 3 shows that steel corresponding to steel A in Table 1 (Fig. 1) was submerged arc welded as shown in Fig. 4, and the submerged arc welded part was
The results of a stress corrosion cracking test using small 4-point bending in a 2S-Ct environment are shown.

Table 6 Results of stress corrosion cracking test on welded parts As shown in Table 3, the results of the tests at -60°C and 95°C showed that the as-welded samples were fused in the weld heat-affected zone (HAZ).
While there are cracks near the inine, the solution layer 1050
It can be seen that solution treatment for 6 minutes at °C (cooling rate of 25 °C/!) did not cause cracking, and solution treatment after welding is effective.

Further, FIG. 5 shows the results of simulating a HAZ thermal cycle using steel A and examining its pitting corrosion resistance. Thermal cycle pattern A simulates the plate ST phase, and then I (AZ thermal cycle is applied. Pattern ■ is the HA
The heat cycle equivalent to the Z heat cycle pipe ST is given, and the heat history of each heat cycle pattern is shown in the diagram shown in Figure 6. John test) is 10%Fect11・6H
The test was carried out using 2O solution at 60°C for 5 hrs.

The corrosion rate is small in the thermal cycle pattern ■ which simulates the solution layer pipe ST, whereas the corrosion rate is 1 in the thermal cycle pattern I which simulates the case where the plate is welded after ST.
It can be seen that the corrosion rate is high when the thermal cycle is 300° C., that is, close to the Fuston Line. As shown in FIG. 5 as well, the process of applying pipe ST to the solution layer is effective in improving the corrosion resistance ratio, so performing pipe ST in the solution layer is essential from the standpoint of improving the corrosion resistance of the welded part.

Note that the sizing expansion rate after solution treatment (ST) is not particularly quantitatively limited, but it is desirable to have a smaller one to prevent deterioration of corrosion resistance due to work hardening of the austenite phase. If it is good, it may be possible.

Based on the above findings, it has been discovered that a two-phase stainless steel welded part exhibiting high corrosion resistance can be obtained in a high CO, -fine H2S-C1 environment.

In the present invention, the austenitic-ferritic two-phase stainless steel and the weld metal have the following compositions.

Below C010 1. Si: 0.1-1.0%
, Mn: 0.1-2.0%, Ni: 3-8%, Cr: 2
1-28%, Mo: 1-4%, N: 11nR-11')
Bow ≦ Noriyuki Suzuno r de Mokuchu Natsuki Ayaho r de f In addition to the above composition, CIJ: 2.5 inches or less, V, W: each 1.5 inches or less, Ta, 'ri, Nb, Zr: 1 each.
Less than 0%.

It contains one or more of Ca, Mg: 0.01% or less, B: 0.01% or less, and REM: 0.2% or less.

Also, a heating holding time of 0 seconds means that when the temperature of the welded part or the welded structure including the welded part rises to the relevant temperature due to the induction force aj%, etc., there is no holding time and the process immediately shifts to cooling. be.

[Embodiments of the invention]

Next, examples of the present invention will be shown.

The following ladle components - X and Y are steel plates and this is rUOE
+ Welding + Expansion Welding process to create a steel pipe.

Table 4 shows the steel plate specifications, steel pipe specifications, and mechanical acidity in that case, Table 5 shows the composition of the weld metal, and Table 6 shows the corrosion resistance test results of the steel pipe.

As shown in Table 6, it is clear that both the welded part and the base metal of the duplex stainless steel of the present invention have excellent corrosion resistance ratios.

〔Effect of the invention〕

As mentioned above, according to the method of the present invention, i) Intergranular corrosion test (Hikoi, Strauss) 11
) High CO, - Slight HtS Cf - Stress corrosion test under high temperature environment (4 point bending) iii) It becomes possible to manufacture two-phase stainless steel pipes that exhibit high corrosion resistance for pit surrounds etc. under C6 - .

Of course, in terms of strength and toughness, especially in terms of HAZ toughness, it is possible to obtain better performance after ST than with the tube itself.

The present invention can also be applied to the case where a bent pipe is made by high-frequency bending a two-phase stainless steel welded steel pipe.5.
This is a useful method.

[Brief explanation of the drawing]

Figure 1 is a graph showing the corrosion rate of stainless steel. Figure 2 is a graph showing the relationship between S1 time and ST temperature to determine the optimum conditions for pitting corrosion resistance. Figure 3 is a graph showing the relationship between S1 time and ST temperature to determine the above ST conditions. Figure 4 is a schematic diagram of the welded part subjected to the stress corrosion cracking test of the welded part, Figure 5 is the corrosion rate showing pitting corrosion resistance when HAZ thermal cycle is simulated. The graph in FIG. 6 is an explanatory diagram showing the thermal history for each thermal cycle pattern. Agent Patent Attorney Sanro Kimura Figure 1 Figure 2 Figure 2 57 hours (Support Figure 3 Figure 5 Figure 4 Figure 6

Claims (1)

[Claims] Duplex stainless steel is welded as rolled or after solution treatment, and the welded part or the entire welded structure including the welded part is heated using heating equipment at coordinates (1000, 20 )su(1000°3600), (1
100, O), (1100, 3,600).