JPS61177325A - Improvement of corrosion resistance or stainles steel weld zone - Google Patents

Abstract

PURPOSE:To improve the corrosion resistance of a weld zone at a low cost by irradiating laser light on a weld metal and weld heat affected zone as well as the part near the same thereby remelting said part. CONSTITUTION:The weld metal 2 of steel plates 1, 1' and the weld heat affected zones 3, 3' are sensitized and a part 4 where the corrosion resistance is deteriorated is formed in butt welding of stainless steel flat plates, etc. The laser light 5 is irradiated on the surface of said part 4 and near the same to remelt the only the extreme surface part. The laser light 5 is slightly overlapped and moved in the horizontal direction while said light is irradiated in the vertical direction along the welding direction until the entire required surface is irradiated. The molten surface part is quickly cooled to have the fine solidified structure, by which the precipitation of the chromium carbide at the grain boundary is obviated and the sensitization is annihilated. The corrosion resistance of the stainless steel weld zone is improved with the simple operation by the above- mentioned method.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for improving the corrosion resistance of welds of objects made of stainless steel.

[Prior art]

In general, stainless steel is a steel material whose main purpose is corrosion resistance, but when welding materials made of stainless steel, A.
℃), carbon precipitates as chromium carbide at the grain boundaries, so the amount of chromium around the grain boundaries decreases, corrosion resistance deteriorates, and intergranular corrosion occurs, a so-called sensitization phenomenon.

B. If the heated area exceeds the above temperature range, such as the weld metal or the weld heat-affected zone in the vicinity of the fusion line, it will rather become a solution and not become sensitized at the maximum heating temperature, but in the subsequent cooling process. A sensitization phenomenon occurs.

FIG. 1 represents a cross-sectional view of a stainless steel weld.

In the figure, f1+, (1') is a stainless steel plate, (2
) is the weld metal, (3) and (3') are the weld heat affected zone, and the weld metal (2) and the weld heat affected zone (3), (
3') becomes the sensitization valve. Here, above (3) and (
Table 1 summarizes the phenomenon of B) by specifically illustrating materials.

As a countermeasure for the above-mentioned sensitization phenomenon, the following methods have been conventionally adopted.

a. Material: 1. Reduce the amount of carbon contained to prevent precipitation as chromium carbide at grain boundaries.

2. Adding molybdenum to improve pitting corrosion resistance.

It also makes it difficult for chrome to move.

3. By adding stabilizing elements such as niobium to stabilize carbon, precipitation of chromium carbide is prevented.

b. Welding construction method: 1. Precipitation of chromium carbide is prevented by reducing the welding heat input and increasing the cooling rate.

2. In the case of steel pipes, the welded part is water-cooled from the back side, as in pipe inner water-cooled welding, to speed up the cooling rate and prevent the precipitation of chromium carbides.

3. In the same case as above, a corrosion-resistant material is applied in advance by overlay welding on the inner surface of the tube, and after the groove is processed, butt welding is performed.

C0 heat treatment: Heat treatment is performed after welding to prevent sensitization phenomenon.

[Problem that the invention seeks to solve]

However, each of the above-mentioned measures has advantages and disadvantages, and in particular, each measure requires the use of high-quality materials, and also has the disadvantage of complicating construction and increasing costs.

The present invention eliminates the above-mentioned drawbacks when eliminating the sensitization phenomenon of welded parts of stainless steel, and (1) allows the conventional material to be used as is without upgrading the material of stainless steel.

(2) There is no need to adopt a special welding method.

(3) No heat treatment required after welding.

Therefore, it is an object of the present invention to provide a method for improving the corrosion resistance of stainless steel welds with the aim of reducing costs.

[Means and Effects for Solving the Problems J The method for improving the corrosion resistance of stainless steel welds according to the present invention to solve the above problems will be explained in detail with reference to the drawings listed below. FIG. 2 shows an example of butt welding stainless steel flat plates. (1), (15 are stainless steel flat plates,
These two steel plates were butt welded, and the weld metal at the welded portion was (2).

Also, the stainless steel plate +11 on both sides of the weld metal (2)
, (1') are weld heat affected zones (3) and (3').

Part (4) where weld metal (2) and weld heat affected zone T31, (:3') have become sensitized due to welding and have deteriorated corrosion resistance
becomes. Therefore, the surface of the portion (4) and its vicinity is irradiated with laser light (5) to re-melt only the very surface. The laser beam (5) is irradiated vertically along the welding direction as shown by the arrow in the figure, and is slightly overlapped and moved horizontally to irradiate all the necessary surfaces. A laser is a heat source with high energy density, and by appropriately adjusting the irradiation conditions, it is possible to melt only the surface with low heat input. An enlarged cross-sectional view of the laser is shown in FIG. Since this surface fusion zone is rapidly cooled, it becomes a fine condensed structure, and the precipitation phenomenon of chromium carbide at grain boundaries does not occur, and the sensitization phenomenon disappears.

Note that since lasers are easy to transmit, they can be applied to various types of welded joints. For example, FIG. 4 shows a case where the inner surface of a steel pipe is irradiated with a laser.

That is, the laser beam (5) is focused by a convex lens (6), and is reflected by a flat mirror (7) placed inside the stainless steel pipe 001 as shown in the figure.
Irradiate the inner surface of G. In this case, the steel pipe (101) is rotated and moved gradually forward or backward to re-melt the necessary inner surface of the part.In addition, since the melted area formed in one pass of laser irradiation is narrow, the surface should be thoroughly and uniformly melted. To melt, the irradiation position is gradually moved so that the surface melting parts (9) slightly overlap each other as shown in FIG.

The laser irradiation conditions for obtaining good results are listed below: (1) The amount of heat input by the laser beam is kept within a range that does not sensitize the weld metal or the weld heat-affected zone due to the surface-melted bead.

(2) The bead width for one pass is desirably 1 or more, since if the solution is extremely narrow, it will take too much time to melt the entire required surface, reducing work efficiency.

(3) The shape of the bead depends on the laser irradiation energy, and the energy is expressed by the following relational expression.

Laser irradiation energy: E (KJ/mm3) Laser output P (KW) Laser irradiation speed: v (w/5ee) Focal length of condensing lens: f (am) Diameter of laser incident on condensing lens: D (mm ) Laser irradiation position: h (ms) (Distance measured in the /z direction with the focal point position as 0.) Figure 5 shows the relationship between the laser irradiation positions, and the laser beam (5) is connected to the convex lens (6). The light is focused at (2)
The sample (111) is placed at a distance (hl) from the focal point α2, and the surface of the sample (111) is irradiated with laser light.

[Examples and effects]

In order to clarify that the method of the present invention is significantly effective in improving the corrosion resistance of stainless steel welds compared to conventional methods, the following are the results of various experiments conducted by the present inventor. Details are given below.

(1) First, the appropriate conditions for the laser irradiation energy to obtain good corrosion resistance were determined from experimental results.

That is, a sample αD having the shape shown in Fig. 6 is taken from a 5US430 stainless steel plate (1) with a thickness of 5IIIl, a laser bead (8) is formed in the center of the surface as shown in the figure, and then a JIS G0575 r stainless steel plate is prepared. A test was conducted based on the ``Sulfuric Acid/Copper Sulfate Corrosion Test Method''.

The results are shown in Table 2 below.

As E increases, the amount of heat input naturally increases, and conversely, the cooling rate of the weld metal of the laser bead and the weld heat affected zone decreases, resulting in a sensitization phenomenon.

Therefore, from the results of this test, the following is preferable as an appropriate upper limit value of laser irradiation energy.

On the other hand, the relative relationship between the irradiation energy E and the bead width was investigated, and the results are summarized as shown in FIG. In this case, as already described, if the bead width is narrower than laa, the surface melting operation will take more time and the efficiency will decrease. Therefore, from this graph, the following is preferable as an appropriate lower limit value of laser irradiation energy.

Based on the above test results, the appropriate conditions for the laser irradiation energy E can be expressed by the following relational expression, and it is desirable to apply the present invention within this range.

(2) Next, various conditions constituting the laser irradiation energy E were changed and their effects were investigated.

That is, as shown in Fig. 8, 5US30 with a thickness of 5I1m
4 and 5 US430 stainless steel plate (1), V
Sample OD was manufactured as a butt welded joint using weld metal (2) with a shaped groove, and 5US304 was subjected to heat treatment (650'Q 30Hr, air cooling) after welding.

As shown in Fig. 9, the entire surface of these samples αD was irradiated with laser light under various conditions, and the surface melted area (
After forming 9), the same sulfuric acid/copper sulfate corrosion test as described in the previous section was conducted, and the results are summarized in Table 3 below.

Note that a CO2 laser is used as the laser beam, and D=50
mm f = 190.5 m, and the corrosion results are shown by the following symbols.

○: Good case, ×: Intergranular corrosion occurred To summarize these, JK2.3.4.6.9.12 is 5US304
Good results were obtained with both M5US430 and M5US430.
．． On July 8, 10, and 11, a sensitization phenomenon occurred. Although Comparative Example x1 has good corrosion resistance, it is not suitable for practical use because the bead width is as small as 1 mm or less.

Judging from the above numerous test results, it is clear that the method of preventing sensitization by surface irradiation with laser light based on the present invention has a more significant improvement in corrosion resistance than any of the conventional methods, and is also easy to operate and cost-effective. The contribution made to this is extremely large.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram showing the sensitization of a stainless steel weld, Figure 2 is a perspective view showing laser irradiation on the surface of a steel plate weld, and Figure 3 is a schematic diagram showing the sensitization of a stainless steel weld.
The figure is an enlarged cross-sectional view of the same surface fusion zone, Figure 4 is a cross-sectional schematic diagram when the inner surface of a steel pipe weld is irradiated with a laser, Figure 5 is an explanatory diagram showing the positional relationship of laser irradiation, and Figure 6 is a laser bead attached. Figure 7 is a diagram showing the relative relationship between laser irradiation energy and bead width for the same sample.
Figure 8 is a perspective view of a corrosion test sample that was butt welded;
FIG. 9 is a perspective view showing a state in which the surface of the above sample is irradiated with a laser. 1.1' stainless steel plate, 2: welded metal, 3.3':
Weld heat affected zone, 4: Sensitized area, 5: Laser beam, 6: Condensing convex lens, 7: Plane mirror, 8: Laser bead, 9: Surface fusion zone, 10: Steel pipe, 11: Sample, 12: Focus 1E Figure 2 Figure 3 wA Rattan Figure 7 Laser irradiation energy E (KJ layer) Figure 4

Claims (1)

[Claims] (1) A method for improving the corrosion resistance of a stainless steel weldment, which comprises remelting the weld metal, the weld heat-affected zone, and its surrounding surfaces, which have become sensitized by welding heat input, with a laser beam.