JPS63140795A - Welding material for high-si austenitic stainless steel - Google Patents

Abstract

PURPOSE:To improve the nitric acid resistance of a weld zone by controlling C, Si, Mn, Cr, Ni, Nb, and N respectively to specific weight %, specifying the content of Nb so as to satisfy the prescribed formula and maintaining Ni-bal in a prescribed range. CONSTITUTION:The component compsn. of the welding material is controlled, by weight %, to <=0.04% C, 2.5-5.0% Si, <=5% Mn, 15-20% Cr, 10-20% Ni, 0.05-0.3% Nb, and <=0.025 N. The Nb component is so specified as to satisfy formula I. The Ni-bal expressed by formula II is limited in a -4--2 range by which the welding material for austenitic stainless steels is formed. Since the content of Nb is controlled and the Ni-bal value is limited in the range, delta ferrite is formed and the high-temp. weld crack sensitivity is degraded. Weld cracks are, therefore, decreased and the nitric acid resistance of the weld zone is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to welding materials, particularly welding materials for welding high-Si austenitic stainless steels that have excellent corrosion resistance against nitric acid.

(Prior art) The main components of a nitric acid production plant are in a high-temperature, highly concentrated nitric acid environment, and this environment is extremely oxidizing, causing stainless steel to become hyperpassive. cannot be maintained.

Furthermore, it is a harsh corrosive environment with extremely high intergranular corrosion.

Traditionally, glass lining materials have been used because silica has durability in such highly oxidizing environments, but such materials cannot be welded, making it difficult to manufacture large-scale equipment. It also had serious problems in terms of plant maintenance, such as being brittle and susceptible to impact.

Recent metal materials that are durable in this type of environment include 17%Cr-14%Ni-4% containing approximately 4% Si.
Si-based austenitic stainless steel is used, which forms a Sing-based oxide layer on the surface in a strongly oxidizing environment, which significantly increases the service life and reliability of plant equipment. I was able to improve it.

For example, the material disclosed in Japanese Patent Publication No. 57-37669 has Si: 2, 5 to 5%, and in addition to Cr and Ni, Ta or Zr is added in an amount of 10×C% to 2.5% (partially replaced with Nb). This is a stainless steel for concentrated nitric acid with a composition of (acceptable). This type of nitric acid-resistant stainless steel has been widely used to date due to its excellent properties, and has become a highly regarded material in the industry.

(Problems to be Solved by the Invention) However, as the material has been used for a long time, even with materials exhibiting such excellent nitric acid resistance, the problem of corrosion perforation in the weld bead has become more prevalent.

That is, during use, one part of the welding bead in particular undergoes severe corrosion with unevenness as shown in FIG. 3, and the preferentially corroded area penetrates, causing -a2! ! This may lead to serious accidents. In addition, as shown in FIG. 3 (al and (bl) will be described later,
It is a micrograph of the metal ML weave of the corroded part in a conventional example. Therefore, we are currently taking measures such as shortening the inspection period.

However, such measures are only symptomatic and do not provide a fundamental solution, and they are also expensive in terms of costs, as operations often have to be interrupted for inspection. .

Thus, an object of the present invention is to provide a welding material that prevents the occurrence of corrosion perforation at the weld bead when welding nitric acid-resistant stainless steel as described above and has a nitric acid resistance equivalent to that of the base metal. That's true.

(Means for solving the problem) In order to solve the problem of corrosion perforation of the weld bead as described above, various research studies were conducted and the following knowledge was obtained.

(1) Welding materials that have been used for welding the nitric acid-resistant stainless steel mentioned above have C≦0.015%, Si:
5-7%, N≦0.02%, C+N≦0.03%, N
b+Ta≧15x (C+N)%, which means that it is low in carbon and nitrogen, and contains a relatively large amount of Si. Also, N
b. Regarding the amount of Ta, it was conventionally thought that it was necessary to add a relatively large amount relative to the amount of (C+N).

(2) Therefore, when welding the nitric acid-resistant stainless steel using such a welding material, the corrosion was caused by intermetallic compounds with high Si content (Ni-5t) precipitated as a result of solidification segregation in the weld zone. -Nb system).

(3) In other words, with the precipitation of the high-SI content intermetallic compound as described above, the Si content in the surrounding matrix, which is important for exhibiting excellent corrosion resistance in nitric acid, decreases, resulting in preferential corrosion of the matrix. This causes pitting-like corrosion.

As a result of various studies on measures to prevent such pitting corrosion, the present inventors found that 17%Cr -14%N
I-4% Nb welding material (Nb: 0.5 to 1°2%), which is commonly used as a welding material for Si-based materials.
In addition to controlling the Nb content of (addition) to 0.3% or less, 2X (C + N) to 6 × (C
+N), it is possible to significantly reduce the amount of intermetallic compound precipitation mentioned above, and thereby the perforation corrosion of the bead portion can be effectively prevented, and the present invention completed.

Therefore, the gist of the present invention is that C≦0 in weight%.
．． 04%, Si:2.5-5.0%, MnS2.0
%, Cr:15-20%, Ni:10
~20%, Nb: 0.05~0.3%, further 2X(C
+N)≦Nb≦6
The present invention is a welding material for high-Si austenitic stainless steel having excellent corrosion resistance against nitric acid, characterized in that the corrosion resistance is -2.

Ni-bal, -%Ni+30(%C+%N)+0.5
%Mn-1,L(%Cr+1.5%Si+0.5%
Nb)+8.2 Note that at least a part of the Nb content is T
You can change your stomach with a.

Normally, the amount of Nb added in austenitic Nb-stabilized steel is viscous when it is 8X(C+N)% or more, which shows a different tendency from the results of the present invention, but this can be explained as follows. I can think.

That is, conventionally, Nb, which is a stabilizing element, is
Carbon and nitrogen are added to fix carbon and nitrogen in order to suppress the precipitation of Cr carbides as much as possible and secure the amount of solid solution Cr, and are added in relatively large amounts taking into consideration the yield and other factors. It was.

However, strong positive segregation of St is observed in the grain boundaries of the weld metal of the steel that is the object of welding in the present invention due to the concentration effect during weld solidification. Therefore, multiple thermal cycles occur during welding, but when the amount of Nb added is small as in the present invention, even if Cr carbides are generated at the weld metal grain boundaries and a Cr-depleted region is formed, Si in this Cr-depleted region is Since the concentration is high, it is thought that there is no decrease in nitric acid resistance.

The steel type targeted for welding in the present invention is specifically 17%Cr-14%N+-4%54 type austenitic stainless steel, preferably containing about 4% Si and adding Nb. However, the present invention is not limited to this, and in general, the present invention can be applied to any high-Si austenitic stainless steel.

In addition, since such steel types have high Si content, they generally have an inherently high welding hot cracking susceptibility.In order to reduce this cracking susceptibility, it is effective to adjust the composition so that δ ferrite is generated in the weld metal & [ ] weave. By producing 5 to 15% of δ ferrite in the weld metal, it is necessary to reduce the weld hot cracking susceptibility to a level that causes no practical problems.

Therefore, in the present invention, the above-mentioned Ni-bal is -
It is limited to 4 to -2.

(Function) Next, the reason for limiting each component in the welding material according to the present invention will be described below.

C: In order to improve corrosion resistance, especially in order to prevent the precipitation of Cr carbides in a high-temperature concentrated nitric acid environment with high intergranular corrosion, it is preferable that C be as low as possible; however, in the case of a coated arc welded ring, Since C is unavoidably mixed during welding, the upper limit of the C content is set to 0.04%. However, in the welding material according to the present invention, the Ni-5i-
In order to prevent Nb-based precipitates from forming in the weld metal and reducing corrosion resistance, the Nb content for C stabilization is kept low. Therefore, it is preferable that the C content is as low as possible. T.I.G.

With MIG welding rods, there is almost no increase in Cl during welding using a coated rod, but as mentioned above, N
Since b1t is small, it is desirable to set C50°015%.

Si: St forms Si on the steel surface in a highly oxidizing environment.
Sil is the main element that imparts nitric acid resistance to the entire G-based coating, and Sil of the same level (2.5 to 5.0%) as the base material is essential. However, since adding more than 5% of Si deteriorates the manufacturability of the welding rod, the upper limit is set to 5%.

Mn: Cr, Si, as austenite forming elements
It contributes to the austenitization of steel in balance with elements such as Nb, and has the effect of fixing S, an impurity element in steel, and reducing susceptibility to weld cracking.It is also used as a deoxidizing agent in industrial production. 6 As a normal deoxidizing agent,
Mn of 2% or less is sufficient, but addition of 1 to 5% is effective in preventing weld cracking susceptibility, especially weld reheat cracking. Generally it is 5.0% or less.

Note that addition of more than 5% Mn deteriorates the resistance to concentrated nitric acid, so the upper limit is set to 5%.

15% or more of Cr is required to provide general corrosion resistance to Cr and corrosion resistance to low and medium concentrations of nitric acid. However, if it exceeds 20%, a two-phase Mn weave in which a large amount of ferrite is formed in the austenite matrix will appear, impairing workability and accelerating sigma brittleness, so in the present invention, it is limited to 15 to 20%.

Ni: Nt is the main austenite-forming element,
In order to maintain an austenitic structure in balance with Cr, Si, Nb, etc., and obtain a welded joint with good workability, 10 to 20% is required.

Nb: lib is an element that fixes carbon in steel and imparts intergranular corrosion resistance to the steel.

It is generally thought that Nb needs to be added in an amount of 8X (C+N)% or more, and in the case of the base material, it is approximately 18X (C+N)%.
% is added. However, the material according to the present invention is a welding material for high-Si austenitic steel that imparts nitric acid resistance by forming a SiO□ film on the steel surface in a highly oxidizing environment. This shows a different tendency from intergranular corrosion caused by Cr-depleted layers at grain boundaries.

That is, in the case of the present invention, positive segregation of Si is strongly observed at the grain boundaries of the weld metal due to the concentration phenomenon during weld solidification. Therefore, even if Cr carbide is generated at the grain boundaries of the weld metal due to multiple thermal cycles during welding, and a Cr-depleted region is formed in the surrounding matrix, the Si concentration in this Cr-depleted region is high, resulting in a high resistance to nitric acid. No decline occurs.

To obtain such an effect, Nb 2 O, 0.5% or more is required. In addition, if the weld metal has a large amount of Nb added, -1Ni-
5i-Nb-based intermetallic compounds precipitate, reducing corrosion resistance and causing perforation corrosion in the bead. 17%Cr-14%N
In the case of i-4%Si type, when the Nb content exceeds 0.3%, weld cracking susceptibility increases rapidly. Therefore, from the characteristics of the above-mentioned high SL weld metal, the lower limit of the amount of Nb added is set at 0.05% and the upper limit is set at 0.3%.

As shown graphically in FIG. 1 below, the maximum erosion depth exhibits a particularly significant decrease when the ratio (Nb+Ta)/(C+N) is in the range of 2 to 4.

In this way, Nb1l, which may be partially replaced with Ta, is 2X
If it is less than (C+N)%, corrosion resistance is insufficient, while if it exceeds 6 X (C+N)%, corrosion perforation resistance is insufficient. Therefore, in the present invention, 2
N)%≦Nb%≦6×(C+N)%.

N: It is preferable that N has a strong affinity with Nb and is as low as possible since it inhibits the fixation of C by Nb, but the upper limit is set to 0.602% in consideration of manufacturability and economic efficiency.

Note that the welding material according to the present invention is austenitic steel and S
Since the i content is extremely high and Nb is included, the susceptibility to solidification cracking during welding is high. Therefore, in the present invention, in order to reduce this susceptibility to solidification cracking, 5 to 15% of the weld metal is added.
The above Ni-bal to produce δ ferrite,
control in the range of -4 to -2* Ni-ba
When the value of l is more than 12, the amount of δ ferrite produced is small and the effect of reducing cracking susceptibility is insufficient. On the other hand, when the value of l is less than -4, the amount of δ ferrite produced is too large, degrading the toughness of the weld metal. In addition, if it is less than -4, hot workability is also significantly deteriorated and manufacturability is deteriorated.

Next, the present invention will be explained in more detail with reference to examples thereof.

Table 1 of Examples shows the chemical composition of each welding material of the present invention example and comparative example.
A post-TIG welding corrosion test was conducted on a -14%Ni-4%Si high SL austenitic steel. Part of the test results
They are summarized in the table and FIG.

Furthermore, the relationship between the weld cracking susceptibility and the Nb content of these welding materials is summarized in FIG. 2.

That is, Table 2 shows the weld metal (
In an actual nitric acid manufacturing plant (98% IIN (h, boiling state and NO
Fig. 1 shows the results of a corrosion test for 3,624 hours in a 8-gas atmosphere), and Fig. 1 summarizes the results in a graph.

As can be seen from these results, C below 0.04%
In the landscape, (Nb + Ta) / (C + N) is 2 to 6
It can be seen that the range of is the best. Further, when the amount of C is about 0.05%, grain boundary corrosion occurs violently in addition to drilling corrosion, which is insufficient for practical use regardless of (Nb + Ta) / (C + N).

FIG. 2 shows the influence of the amount of Nb on cracking susceptibility during TIG welding, and it can be seen that when Nbi exceeds 0.3%, cracking susceptibility increases significantly.

The cracking index is 0: no cracking, 1: bit occurrence,
2: Slight cracking, 3: Slight cracking, 4:
There are severe cracks.

In the figure, the numbers in parentheses indicate the test list.

Figure 4 +11 and (bl are microscopic metal mw & photographs showing the state of perforation corrosion of the corrosion test piece under the above-mentioned environment for the steel of Test-10 in Table 2, which is a comparative example. (BL is an enlarged version of Figure 3 (al), and the pitting-like corrosion can be clearly seen.

Next, in order to examine the effect of Si concentration, (Nb + Ta
) amount is on the low side within the range of the present invention, and each of Table 2, test sample 5, where the amount of (Nb + Ta) is on the low side within the range of the present invention, and is contained in a large amount exceeding the upper limit of the present invention. For each sample material, the entire X&IL* degree map near the weld metal corrosion area was measured using an IEPMA (X-ray microanalyzer). Secondary electron beam images of the test materials of the comparative example and the invention example are shown in Figures 4+11 to 5) and Figure 5(1), respectively.
This is briefly shown as 5).

The elements written in the bottom of each figure have their respective distributions,
Indicates the state of concentration.

From FIG. 4, the existence of a Si--Ni--Nb enriched area and a Si-poor area can be clearly recognized. On the other hand, from Figure 5, the fourth
Strong enrichment of St as shown in the figure is not observed.

Next, the significance of Ni-bal in the welding material according to the present invention was evaluated in terms of weld cracking resistance, toughness, and hot workability.

First, using each specimen with dimensions of 20t x 150w x 2001 (all), hot-rolled to 8t by 4-pass rolling while heating to 1200°C, visually observing processing cracks that occurred at the mill edge, and then The ear cracking index was evaluated on a five-point scale.

The test results are summarized in a graph in FIG. 6. In the figure, the numbers in parentheses indicate the results of the test.

0: No cracking 1: Pit formation 2: Slight cracking 3: Slight cracking 4: Severe cracking Next, 5 x 10 x with a 211-V notch.
TIG welding was performed on each sample material with a size of 55 mm,
The weld cracking index was determined as described above, and a Charpy impact test was conducted on each of the obtained weld metal parts at 0°C. The test results are shown in graphs in FIGS. 6 and 7. In the figures, the numbers in parentheses indicate the number of test points.

As is clear from the results shown in Figure 6, Nt-bal,
If it is less than -4, the absorbed energy becomes extremely low and the cracking on the edges becomes worse.

Furthermore, as is clear from the results shown in FIG. 7, weld cracking significantly worsens when the temperature exceeds -2.

Table 2 (Effects of the Invention) According to the present invention, as can be seen from the above explanation, Nb
content below 0.3% and 2X(C+N)≦Nb(+
By limiting Ta) ≦6X(C+N), the weld cracking resistance is significantly improved, and therefore the 4! li! The welding assembly of S can be easily carried out, and the benefits of the present invention are large.

[Brief explanation of the drawing]

Figure 1 shows the maximum erosion depth of weld metal and Nb + Ta
/C+Nil; Figure 2 shows the relationship between the welding material's welding crack susceptibility and Nb(±Ta)
Graph showing the relationship with content; Figure 3 is a microscopic metallographic photograph showing the state of perforation corrosion seen in the conventional example; Figures 4 and 5 are X-ray microscopic photographs of the comparative example and the inventive example, respectively. A photo of the metal Mi weave of the manalizer; and FIGS. 6 and 7 are graphs showing the influence of Ni-bat on material properties. Figure 1 0 2 4 6 B To 12 14
16 18 20Figure 2 OQ,! CAoh Q88 No. 3 S436 Procedural Owner's Manual (Method) May 25, 1988 Patent Application No. 288284 of 1988 2, Title of Invention: High Sj Welding Material for Austenitic Stainless Steel 3, Person Making Amendment Case and Relationship Patent applicant Address: 2, 8-8 Honshio-cho, Shinjuku-ku, Tokyo Name: Nippon Stainless Steel Co., Ltd. (and one other person) 4, Agent: 10
1. Contents of the γ correction (1) The statement from line 6 to line 19 on page 15 of the specification is corrected as follows. r Next, in order to see the effect of converting S i 'IQ, (N
b + Ta) amount is on the lower side within the range of the present invention.
EI' in the vicinity of the capacitive metal corrosion area for each of the test materials in table test 5 and experiment 9 in Table 2 in which (Nb + Ta) ffi is contained in a large amount exceeding the upper limit of the present invention) X by MA (X-ray microanalyzer)
A line intensity map was created. Secondary electron beam images were created for the test materials of the present invention example and the comparative example, and the distribution of each element and the agricultural state were examined. In the case of the comparative example, S
The presence of an i-Ni-NMffi-formed part and a Si-poor part was clearly recognized, but in the case of the present invention example, the strong Si quaternization as in the comparative example is due to L? 1. I couldn't get it. J (2) The following sections of the detailed description are corrected as follows. ,■ Oka 4ya revised JJ1 series Tsujiya 16 8
Fig. 6 Fig. 4 and hill Original description Anal injury measuring scale M1620
Figure 6 Figure 4 Figure 7
Figure 5 17 3 Figure 6 Figure 4 6 Figure 7 Figure 5 2015-17 Delete 18
Fig. 6 Fig. 4 Fig. 7 Fig. 5 (3) ``Fig. 4'' and ``Fig. 5'' of the attached drawings have been added by 11 times, and ``Fig. 6'' and ``Fig. 7'' have been added respectively. As shown in the drawings attached to the main text, “Figure 4” and “
Figure 5” is corrected. 8. Catalog of attached documents Drawings with corrected drawing numbers (Figures 4 and 5) and above

Claims (2)

[Claims] (1) In weight%, C≦0.04%, Si: 2.5 to 5.0%, Mn≦5.0%, Cr: 15 to 20%, Ni: 10 to 20%, Nb: 0. 05 to 0.3%, N≦0.02%, further 2×(C+N)≦Nb≦6×(C+N), and the balance consists of Fe and inevitable impurities, and the Ni-bal. ga-4
A welding material for high-Si austenitic stainless steel having excellent corrosion resistance against nitric acid, characterized in that the corrosion resistance is -2. Ni-bal. =%Ni+30(%C+%N)+0.5
%Mn-1.1(%Cr+1.5%Si+0.5%Nb
)+8.2 (2) The welding material for high-Si austenitic stainless steel according to claim 1, wherein at least a part of the Nb content is replaced with Ta.