JPS60165323A - Prevention of weld decay in stainless steel welded part - Google Patents

Abstract

PURPOSE:To prevent the deterioration in corrosion resistance by inhibiting the formation of weld decay, by rapidly heating the surface of the weld decay formed region in the periphery of the welded part of stainless steel by the irradiation of laser beam to form the solid solution of contained Cr-carbide in the matrix. CONSTITUTION:When stainless steel pipe materials are welded so as to form a welded part 2, the peripheral part 3 of the welded part 2 is heated to 500- 850 deg.C and Cr23C6 is precipitated in a grain boundary surface to generate a Cr defect layer and weld decay deteriorated in corrosion resistance is generated to said part. In this case, the weld decay part 3 is irradiated with laser beam 4 by a rotary reflective mirror 5 and rapidly heated to form the solid solution of Cr23C6 in the stainless steel matrix. The heated part 3 is cooled itself by heat transfer to both sides shown by arrows D or cooled at an accelerated speed by water cooling or forcible air cooling to eliminate the weld decay and the deterioration in the corrosion resistance in the vicinity of the welded part is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for preventing weld decay in stainless steel welds.

Conventionally, chromium carbide (or s
s Oe ) is precipitated and a chromium-deficient layer is formed, resulting in deterioration of corrosion resistance in some places. This phenomenon is well known as weld decay in austenitic stainless steels.

To prevent this weld decay, either suppress the precipitation of 0r1806 or reheat and rapidly cool the 0r1806 at 1000°C or higher.
The lI806 must be redissolved in the matrix to achieve a uniform ROM concentration. To this end, conventional techniques include l) using steel materials with a particularly low carbon concentration of 0.08% or less (ultra-low carbon stainless steel), and (2) adding carbide stabilizing elements such as Ti and Nb to prevent the precipitation of chromium carbides. or 8) solution treatment after welding is required.

However, ultra-low carbon stainless steel has a lower yield strength, increases the thickness of the steel material used, and is also expensive to manufacture.

Stabilized stainless steel is susceptible to corrosion called knife-line attack and hot cracking at welded joints.

The solution treatment after welding uniformly coats the entire welded structure to 100%
It is not effective unless it is heated to 0° C. or higher and then rapidly cooled. Although this is possible for small parts, it is generally difficult for large parts and is likely to cause deformation.

The present invention aims to overcome the drawbacks of these conventional methods.

The present invention irradiates the surface of the weld decay generation area of a stainless steel welded part with a laser beam to rapidly heat only the surface, solidify the chromium carbide in the surface layer in the mud and lithics, and self-cool the stainless steel by heat conduction. Alternatively, it is characterized by accelerated cooling.

Stainless steel is ordinary stainless steel, such as 5US80.
4 austenitic stainless steel, and the weld decay generation area of the weld is, for example, a weld width of about 500 to 850.
The area is heated to ℃.

Accelerated cooling is water cooling or forced air cooling. Rapid heating and cooling restores the corrosion resistance of the weld.

The advantage obtained by the present invention is that corrosion resistance can be restored and maintained by localized rapid treatment of ordinary stainless steel welds with O,OS%C. It is possible to eliminate the need to use steel or stabilized stainless steel, and it can also be applied to large parts of stainless steel, resulting in extremely large technical and economical effects.

Also, compared to heating and quenching the entire body such as solution heat treatment, the deformation after laser beam treatment is far less.

The present invention can be used in general stainless steel i-welded products where corrosion resistance is a problem.

・Specifically, since the laser uses optical energy, by devising an optical system, it is possible to perform laser beam heat treatment on the inner surface of a tube or in a narrow space of a complex structure.

The method according to the invention will now be explained in more detail with reference to the drawings.

Weld decay is approximately 500 to 850°C due to the welding heat cycle at the part of the butt or fillet welded joint of stainless steel 1 and 1 shown in FIG. 1A or B that is away from the weld metal 2.
occurs in region 8 which is heated to When this region 8 is heated with a laser beam, only the surface of the region V28 is rapidly cooled because the laser is a light with high energy density and is accompanied by reflection. This cooling is 0.08%0 180r-8Ni
In the case of stainless steel, it must be sufficiently fast in the range outside the curve shown by the solid line in FIG. 2, for example as shown by the Fl line AB. Otherwise, the dissolved chromium carbide will precipitate again. Only lasers are capable of rapidly heating the metal surface and rapidly cooling it through heat conduction through the metal itself. In FIG. 2, marks e indicate points at which chromium carbide is precipitated, and marks marked ○ indicate points at which no chromium carbide is precipitated.・・・
・Since a laser beam is a type of light, it can be transmitted to narrow areas) ・Inner surfaces of pipes using an optical system. In particular, the corrosion resistance of the inner surface of the pipe becomes a problem when corrosive substances are transported in the chemical industry or the like.

In this case, the method of the present invention is used as shown in FIG.

Laser beam treatment of the inner surface of the pipe becomes possible.

That is, the laser beam 4 transmitted to the inner surface of the vibro by the reflecting mirror 5 or the like is irradiated onto the weld decay generation regions 8 on both sides of the weld metal 2. Since the reflecting mirror 5 has a structure that allows rotation and horizontal movement, continuous laser irradiation treatment to the weld decay generation region 8 is possible. Weld decay generation area 8 rapidly heated by radar beam
is rapidly cooled by self-cooling due to heat conduction in both directions indicated by arrow D, thereby preventing weld decay.

Example 1 5US804 austenitic stainless steel with a plate thickness of 14u was used. The chemical components are shown in Table 1 below.

Table 1 Chemical composition of stainless steel This test material was subjected to sensitization heat treatment at 650°C for 2 hours.
It was in a state of sensitization. The plate surface was irradiated with a laser beam under various conditions, and the conditions were determined so that the carbides precipitated at the grain boundaries were dissolved in the IJ sock. The surface of the test material is +
One was polished with 120-grid merry paper, and the other was coated with graphite coating (trade name: AerO500) on the surface.

Then, the corrosion test of the laser beam irradiated material was carried out by JISG
A 10% oxalic acid etch test using O571 was performed.

Beam output is 2. OKW% beam diameter was between 6.0. The results are shown in Figure 4. In the same figure, the O mark indicates the one polished with ÷120 emery paper (12° grid), and the * mark indicates the one coated with graphite. The width of the part that is not etched by the oxalic acid etch test is determined by JISGO5
71 defines a step-like structure, the vertical axis in FIG. 4 shows a step-like structure obtained by laser beam irradiation, and the region where the corrosion resistance is improved is shown as the corrosion resistance improvement width.

According to the figure, the corrosion resistance improvement width decreases as the irradiation speed (irradiation movement speed) increases. Regarding the surface condition, it can be seen that the irradiation rate is higher with graphite coating than with emery polishing even for the same corrosion resistance improvement range.

In addition, in emery polishing, approximately 2.5 m/min or more, gra]□
・For phytocoating, at an irradiation rate of approximately Q m7 minutes or more, the laser beam irradiation has no effect, and the corrosion resistance is the same as that of the non-irradiated weld decay generation part0.Example 2 Next, the center spacing of the laser beam is changed. By doing so, we attempted to widen the scope of improvement in corrosion resistance.

The results are shown in Figure 5. According to the same figure, for both the Na120 emery polished one (○ mark) and the graphite coated one (◇ mark), if the beam center spacing is 2 degrees or less, the irradiation area is substantially continuous and the beam is partial. This shows that the range of improvement in corrosion resistance can be expanded. If the distance between the beam centers is larger than that, the beams will separate from each other, causing variations in irradiation, and sufficient improvement in corrosion resistance cannot be expected.

In the figure, marks ◆ and ◆ indicate ÷120 emery polishing and graphite coating, respectively, when the irradiation area is not continuous. Although the improvement in corrosion resistance is large, there is variation in irradiation, and the Difficult to achieve irradiation effect.

Table 2 and FIG. 6 show examples of laser beam treatment of actual stainless steel M I G welded joints. The following Table 2 shows the laser beam processing conditions, and Figure 6 shows the products as shown in Table 2 (C) and 120 emery polished products treated with the laser beam (A). 8% hydrofluoric acid + 1O% nitric acid was applied to 8 bodies, including the graphite-coated one and the laser beam treated one (B).

A corrosion test (JISGO574) was carried out at 70°C for 4 hours, and the corrosion weight loss was shown.

Compared to the as-welded specimen (0), the laser beam-treated specimens (A, B) both showed smaller corrosion loss and improved corrosion resistance. 1. After the corrosion test, the appearance is the 7th one.
As shown in Figures 9 to 9, in the as-welded specimen (0), there is a significant corrosion zone in the weld decay generation region 8 on the surface. However, no corrosion was observed on the surface of the laser beam treated test piece (A I B ). However, corrosion was observed in the thickness direction of the plate I, which was not irradiated with the laser beam. Note that the numbers in FIGS. 7 to 9 are assigned reference numbers in the same manner as in FIG. 1.

Therefore, the value of corrosion loss shown in No. 61\'I is a value in which the corrosion loss in the thickness direction of the plate is also measured, so it is the value of the plate surface. The present invention produces the following excellent effects.

1) Grain boundary carbides in the weld decay generation region of the weld are uniformly dissolved in the matrix to make the chromium concentration in the matrix uniform and restore corrosion resistance to -7.

+1) Since localized rapid heating and cooling is performed, deformation and residual stress generation can be lowered compared to the solution treatment method that heats the entire area.

l11) Ultra-low carbon stainless steel or 1. for corrosion-resistant welded structures.
There is no need to use 1-stabilized stainless steel, and restrictions on welding work can be reduced.

lv) For the purpose of helping the absorption of the laser beam on the surface of the treated material, by performing phosphoric acid coating treatment, chemical conversion treatment, graphite coating, paint application, etc.
This processing can be speeded up.

Although the present invention has been described above with reference to specific examples and numerical values, the present invention is not limited thereto, and changes and modifications can be made without departing from the broad spirit and scope of the present invention. Of course.

[Brief explanation of the drawing]

Figures 1A and B are diagrammatic side views showing a butt weld joint and a corner weld, respectively. Figure 2 shows an example of the state of chromium carbide precipitation in stainless steel in relation to cooling temperature and cooling time. Figure 8 is a diagrammatic longitudinal cross-sectional view explaining the principle of laser beam treatment of the inner surface of stainless steel pipes; Figure 4 is a characteristic diagram illustrating the influence of irradiation speed on the corrosion resistance improvement width; Figure 5 is a characteristic diagram illustrating the influence of the laser beam center spacing that does not reach the extent of improvement in corrosion resistance. Figure 6 is a characteristic diagram illustrating the relationship between corrosion time and corrosion reduction it of a welded part. Figures 7 to 9 FIG. 2 is a diagrammatic perspective view showing the corrosion state of a weld decay generation area. 1... Stainless steel 2... Welded part 8... Weld decay generation area 4... Laser beam 5... Reflector 6... Stainless pipe patent applicant Nagoya University President's representative patent attorney Sugi Akira Mura Competence Patent Attorney Oki Sugimura Figure 1 A Haruka Figure 1 B Figure 2 II Nowh'I (4≦T Figure 3 Figure 5 Laser Bee A Middle 'CI' / e'l r System (fight) Figure 6 Corrosion time (8ft 2

Claims (1)

[Claims] L Weld decay of stainless steel mi contact parts. Stainless steel welding that is characterized by irradiating a laser beam onto the surface of the target region to rapidly heat only the surface, causing the surface layer ROM carbide to form a solid solution in the matrix, and self-cooling or accelerated cooling of the stainless steel by heat conduction. How to prevent welding from occurring. λ The method according to claim 1, wherein the accelerated cooling is water cooling or forced air cooling. & The method according to claim 1, wherein the weld decay generation region is a region heated to a weld width of about 500 to 850°C. Table: A method according to claim 1, in which the corrosion resistance of the welded part is restored by cooling.翫 The method of claim 4 in which the stainless steel weld is irradiated at a processing rate slow enough to restore corrosion resistance. a. The method according to claim 1, in which the irradiation area is substantially continuous.