JPS60206568A - Welding method of stainless steel build-up metal - Google Patents

Abstract

PURPOSE:To improve uniformly resistance to a peeling crack by welding a base metal at suitable intervals by a build-up welding method then subjecting the remaining un-welded part to build-up welding by a welding method in which heat input quantity is relatively low. CONSTITUTION:A build-up metal 3 is formed to a base metal 1 by a stainless steel build-up welding method to be executed by using a belt-like electrode. The build-up metals 3 are successively formed at suitable intervals. The un-welded part remaining between the beads is subjected to build-up welding by a welding method in which a filler metal 5 is used and the heat input quantity is relatively low. Bead lap parts 6 are thus formed. The deposition of much intergranular carbide in the part near the boundary with the base metal on the previously welded build-up metal side is averted and the decrease in the hardness in the softened area in the weld heat affected zone of the base metal is suppressed by applying the low heat input welding method to the bead lap parts where the peeling crack is most liable to arise.

Description

[Detailed Description of the Invention] [Field of Industrial Application] This invention is a pressure vessel in which high-temperature, high-pressure hydrogen is present, such as a hydroclara kink reactor, in which an overlay metal of autenitic Nutenre 7 steel is applied to the inner surface of the vessel. This invention relates to a method of welding Nutenrenu steel overlay metal.

[Prior art]

Normally, chromium-molybdenum steel is used as the base material for pressure vessels in which high-temperature, high-pressure hydrogen is present, from the perspective of preventing hydrogen corrosion, and the inner surface is made of autenitic Nutenrenu steel for the purpose of preventing corrosion from hydrogen sulfide. Mold welding is being performed.

When this high-temperature, high-pressure vessel is in operation,
In each of the overlay metal and the base material, hydrogen atoms that have entered from the inner surface of the container diffuse toward the outer surface, but remain in solid solution within the solubility limit. However, when or after the operation of a pressure vessel is stopped, the hydrogen atoms remaining in the overlay metal or base material without being transferred to the outside become considerably supersaturated due to the decrease in hydrogen solubility accompanying the temperature drop. exist in a state. Moreover, as a result of numerical analysis of the hydrogen concentration distribution after cooling, it was found that after cooling, hydrogen atoms were accumulated at a position slightly closer to the inner fill metal side from the boundary between the overlay metal and the base metal, which caused delamination. It is also clear that the generation position of hydrogen almost coincides with the peak position Ig of this hydrogen concentration.

Furthermore, it has been confirmed both in the laboratory and through the analysis of cases where delamination cracks occur that delamination cracks occur only under conditions where this peak hydrogen concentration near the boundary exceeds a certain value. It is believed that the significant local accumulation of hydrogen atoms is the primary cause of delamination cracking.

On the other hand, carbon migrated from the E-phase side during welding or post-weld heat treatment forms carbides at the coarse grain boundaries in the overlaid metal near the boundaries where delamination cracks occur and at the grain boundaries at the boundaries. , microvoids exist around this carbide, and l
/-1 le.

Delamination cracks are formed as a result of hydrogen atoms accumulated in an extremely supersaturated state being molecularized (gasified) in microvoids during cooling, and the resulting pressure forces the microvoids to expand into macroscopic voids. it is conceivable that. The presence of grain boundary carbides is also cited as one of the major causes of delamination cracking.

As described above, the following two phenomena that occur near the boundary between the overlay metal and the base material (on the overlay metal side), which are the occurrence positions, can be cited as causes for the occurrence of delamination cracks.

(1)/l? Significant accumulation of hydrogen atoms remaining during cooling (
2) The amount of hydrogen atoms accumulated in the former grain boundary carbide precipitation that occurs during welding and post-weld heat treatment not only depends on the amount of hydrogen dissolved in the overlay metal or base metal during high-temperature holding; It is also greatly affected by the hardness of the base metal and weld heat affected zone.

Depending on the result, the hydrogen solubility at around room temperature becomes better.

In such a case, during cooling, the amount of hydrogen occluded in one heat-affected zone on the parent phase side increases, and the hydrogen wind that accumulates on the overlay metal side decreases. For this reason, if a construction method is used that increases the hardness of the weld heat affected zone, cracks and delamination will be less likely to occur.

However, the amount of precipitation of the latter grain boundary carbide depends on the subsequent passes that the overlay metal undergoes during welding (it is related to the cycle, and is affected by the temperature and holding time during post-weld heat treatment).

In either case, it is thought that the greater the amount of carbide precipitation, the more it will have a greater effect on the occurrence of delamination cracks.

Next, FIG. 1 shows a construction method in which overlay welding beads are overlapped in parallel, which has been conventionally performed by overlay welding using a strip electrode.

That is, a metal overlay (3) is formed on the base jFA' (1) using a strip electrode (2), and adjacent to the weld bead,
Lay down new weld beads, overlapping them about 5 to 10 tom. This strip electrode (2) usually has a density of 50 to 150
However, the amount of heat input required to weld a unit length is considerably higher than that of other welding methods.

In the overlay metal (3) constructed in this manner, a softened region with low hardness exists in the base metal/weld heat affected zone directly below the adjacent weld bead overlapped portion (4). This softened area is located under the previously placed overlay metal and was formed as a result of the welding heat cycle during post-mackerel bead welding. At the same time, under the influence of the same welding thermal cycle, carbides are produced at the grain boundaries on the side of the overlaid metal on the back side. In this way, on the side of the overlay metal (3) welded to the tip of the bead overlapped part (4), the hardness of the weld heat affected zone is lower than that of other parts, and carbides are formed at the grain boundaries in the overlay metal. The conditions that make it easy for delamination cracks to occur are met.

The present inventor conducted a delamination crack reproduction test using a small overlay test piece using an autoclave. In a test piece taken from the overlapped bead of overlay metal welded using the conventional construction method, microscopic observation of the cut surface after the delamination crack writing test revealed that delamination cracks had occurred at the locations shown in Figure 2. It was confirmed that

Furthermore, it is clear from the numerous experimental takes obtained in such reproduction tests that delamination cracks preferentially occur near the boundary on the side of the overlay metal welded to the tip of the bead overlap. This is also contradicted by the case of delamination cracking reported today.

The welding method shown in Fig. 2 is the welding method using 1-7 easy overlay welding, the strip electrode size is 0.4t x 75wmm, the overlay metal is 347, and the J'WJI '11' condition is 691. ℃x
15311, hydrogen exposure conditions were (454°C, I'aOk
gf/cy! H-2) In X431+, the date is the delamination crack detection position and the number is the delamination crack length (On+).

[Purpose of the invention]

This invention focuses on the overlapped part of the bead where delamination cracking is most likely to occur when overlay weld beads are stacked in parallel using the overlay welding method using a strip electrode, as in the conventional construction method. The purpose is to improve the peeling cracking resistance of the entire overlay metal by increasing the peeling cracking resistance of this part.

[Structure of the invention]

This invention uses a Nutenrenu steel build-up welding method using a strip electrode, and after welding the build-up beads with an appropriate distance between them, the unfinished portions remaining between the beads are welded. This is a method for welding overlay metal of Nutenrenu steel, which is characterized by overlay welding using a welding method with a relatively low amount of L.

〔Effect of the invention〕

Therefore, according to the present invention, welding is first performed at appropriate intervals by an overlay welding method using a strip electrode, and
After that, the welded overlay metal is overlaid with a relatively low heat input welding method on the unwelded part of the crosspiece, and the welded overlay metal has a low heat input even in the center of the weld bead performed using a strip electrode. The peeling cracking resistance is uniformly improved even in the bead overlapped area with the weld metal applied by the welding method. It is possible to eliminate the portion that degrades the peeling and cracking resistance of the plated metal.

〔Example〕

The present invention will now be explained with reference to FIG. 3 showing an embodiment thereof.

The overlay metal (3) is formed on the base material (1) by the Nutenrenu steel overlay welding method using a strip electrode, and the overlay metal (3) is formed in sequence at appropriate intervals, and the beat The unfinished part left between the filler metal (5)
The bead overlapped portion (6) is formed by overlay welding using a welding method with a relatively low heat input.

The spacing left when overlaying welding using a strip electrode depends on the dimensions of the strip electrode and the type of welding method that will be applied to the unwelded parts later.

decide.

In addition, by applying a low heat input welding method to the bead overlapped part where delamination cracking is most likely to occur, this invention prevents the precipitation of many grain boundary carbides near the boundary with the base metal on the previously welded overlay metal side. This prevents the decrease in hardness in the softened region of the base metal and weld heat-affected zone. With these effects, the present invention eliminates local deterioration in the peeling and cracking resistance of the built-up metal in the bead overlapped portion.

The peeling and cracking resistance at the beat overlapping portion to which the present invention is applied, that is, at the end of the overlay metal constructed using the strip electrode, is equivalent to the peeling and cracking resistance at the central portion of the overlay metal. Well, the overlay metal itself welded using the low heat input welding method applied to this invention hardly forms a softened zone in the base metal/weld heat affected zone, and creates carbides at the adjacent overlay metal grain boundaries. Since no precipitation occurs, its resistance to delamination and cracking is even higher than that of overlay metal welded using a strip electrode.

As mentioned above, delamination cracking occurs when hydrogen atoms that have been solidly dissolved in the overlay metal and the base metal in an atmosphere containing high-temperature, high-pressure hydrogen are deposited on the overlay metal side near the boundary after cooling. This is generated during the process in which hydrogen atoms that have accumulated and become extremely supersaturated are gasified around carbides precipitated at grain boundaries of overlay metal.

In the conventional construction method, it is unavoidable that a large amount of carbide precipitates at the grain boundaries of the overlay metal near the boundary on the side of the overlay metal welded at the end of the bead overlapped part, resulting in the influence of the parent phase and welding heat. A softened area is formed in the area. In the softened region of the base metal/heat-affected zone, there are few dislocations that trap hydrogen atoms, and hydrogen solubility near room temperature is low.As a result, the amount of hydrogen atoms that accumulate on the overlay synthesis side after cooling is reduced. It will be increased. For this reason, in conventional construction methods, overlay metal is prone to delamination cracks in the bead overlapped areas, and the presence of these local areas with low delamination cracking resistance may cause delamination cracks in some cases. The structure (plate thickness, etc.) and operating conditions (temperature, etc.) of a pressure vessel that can be operated without

(hydrogen pressure, cooling rate, etc.).

This invention has been achieved by applying a welding method with a relatively low heat input to the bead overlapped part in the conventional construction method.
The amount of carbide precipitated at the grain boundaries of the overlay metal on the previously welded overlay metal side is reduced, suppressing the decrease in hardness of the softened region formed in the base metal/heat affected zone.

By solving the two problems that caused the occurrence of delamination cracks in the bead overlapping portion in this manner, it is possible to improve the delamination crack resistance of the overlay metal in this portion.

Table 1 shows the effect of improving the peeling and cracking resistance of overlay metal when this invention is applied, in comparison with the conventional construction method.

Here, as a low heat input welding method that can be expected to have the effects described above, we will present a delamination cracking reproduction test when using a coated arc welding method (Example 1) and a plasma welding method (Example 2) with additives. Showing results.

Regardless of the welding method, the peeling cracking resistance of the overlaid metal when this invention is applied exceeds the peeling cracking resistance of the overlaid metal/bead overlapped part constructed using the conventional method, and It has the same resistance to peeling and cracking.

Table 1 Results of delamination crack reproduction test 2) Strip electrode dimensions 0.4tX75v' Note 3) Post-weld heat treatment conditions 690'CX16h Note 4) Temperature 4
50'cs holding time゛48h, cooling rate: 400'
c/h Note 5) The presence or absence of cracks is determined by the amount of change in the echo height from the boundary that increases over a period of 5 days from immediately after cooling using an ultrasonic flaw detection test. Exceed.

△ mark Slight cracking...change exceeds 1.0 dB 5
．． 0 dll or less ○ mark No cracking...Change amount 1. o
clB or less

[Brief explanation of the drawing]

FIG. 1 is a perspective view of a conventional welding method, and FIG. 2(a). (b) is a plan view and a front view showing the distribution of cracks detected on the cut surface of the test piece, and FIG. 3 is a perspective view of one embodiment of the welding method for overlay metal of Nutenrene steel of the present invention. (1)... Base material, (3)... Overlay metal, (6)...
・Bead overlap part. Agent: Patent Attorney Ryuta Fujita Figure 1 Figure 3

Claims (1)

[Claims] ■ With the Nutenrenu steel build-up welding method, which uses a strip electrode, after welding the build-up beads with an appropriate distance between them, the unfinished areas left between the beads are relatively inlaid. A method for welding Nutenrenu steel overlay metal, which is characterized by overlay welding using a welding method with a low amount of heat.