JP3131597B2 - Nitrate resistant austenitic stainless steel and weld metal - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nitric acid-resistant austenitic stainless steel used for equipment and piping for handling nitric acid and a weld metal for joining the same, and particularly to a weld having excellent pitting corrosion resistance and crevice corrosion resistance. About the material.

[0002]

2. Description of the Related Art Stainless steel is widely used as a main structural material of equipment for handling nitric acid, etc. In order to ensure excellent corrosion resistance in a nitric acid environment, it is necessary to devise components of the stainless steel, etc. ing. From the viewpoint of nitric acid corrosion resistance, SUS304 which is a low carbon stainless steel is used.
L or SUS316L, or an ultra-low carbon stainless steel with a lower carbon content than that is excellent. In addition to nitric acid resistance, excellent properties against pitting and crevice corrosion are also required depending on the use environment. From the viewpoint of nitric acid resistance alone, the 304 series containing no Mo is superior to the 316 series, but if pitting corrosion resistance and crevice corrosion resistance are also required, M
A 316 system containing o is suitable. Especially when the material is sensitized by the heat effect during welding,
In the molten re-solidified portion and the weld metal portion, intergranular corrosion easily occurs in nitric acid.

[0003] However, even ultra-low carbon stainless steel satisfying the SUS316L standard and having a lower carbon content may be inferior in nitric acid resistance when subjected to sensitization heat treatment assuming the heat affected zone of welding. Also, the nitric acid resistance of the weld metal may be poor. This is presumed to be due to the precipitation of intermetallic compounds containing Mo at a high concentration at the grain boundaries of the 316 stainless steel, which are easily dissolved in nitric acid.

[0004] On the other hand, various measures have been studied for improving the nitric acid resistance of such stainless steel welds, although not limited to 316 stainless steel. Representative methods are described in JP-A-1-268848 and JP-A-2-99.
No. 294 proposes that the ratio of Cr equivalent to Ni equivalent be specified to be 1.6 or more so that the δ-ferrite phase is first crystallized when the weld is solidified.
However, according to the study results of the present inventors, in order to improve the nitric acid resistance of 316 stainless steel, it is always essential that the δ-ferrite phase is first crystallized when the weld is solidified. It is not considered. That is, there is a region where the corrosion resistance is sufficiently good even in the component range in which the austenite phase is first crystallized when the weld is solidified.
-It is because it was found that some of the regions where the ferrite phase crystallized first had low corrosion resistance. Rather,
It is considered more important to suppress the precipitation of the intermetallic compound containing Mo at a high concentration, which is easily dissolved in nitric acid as described above.

[0005]

As described above, even if the carbon content is extremely low in 316 series stainless steel containing Mo, particularly, the welding heat affected zone affected by heat during welding, There is a problem that the solidified portion and the weld metal may have poor nitric acid resistance and pitting corrosion resistance.

An object of the present invention is to provide a nitrate-resistant austenitic stainless steel and a weld metal, which are excellent in pitting corrosion resistance and crevice corrosion resistance and are suitable as a constituent material of a welded structure used in a nitric acid environment. .

[0007]

In order to obtain a welded structure having excellent corrosion resistance used in a nitric acid environment, stainless steel constituting a base material of the welded structure is affected by heat input during welding. It is essential that each of the weld heat affected zone, the molten re-solidified zone, and the weld metal has excellent corrosion resistance. Here, assuming that stainless steel is joined by the TIG welding method,
The weld heat-affected zone refers to the base metal portion that has been heated by the welding arc to change the metal structure, the molten re-solidified portion refers to the base metal portion that has been melted and solidified by the welding arc without a welding wire, and the weld metal portion refers to the welding wire. And a portion substantially consisting of the composition of the welding wire.

[0008] Therefore, in order to maintain good nitric acid resistance of the base metal which is the stainless steel itself used for the welded structure, the welded portion of the stainless steel, and the weld metal constituting the welded portion, the components of the base material are required. This is achieved by appropriately regulating the range and the composition range of the weld metal. The nitric acid-resistant austenitic stainless steel of the present invention has a C content of 0.03% or less, a Cr content of 16 to 20%, a Ni content of 8 to 16%, and a Mo content of 4%.
%, Si: 1% or less, Mn: 2% or less, balance Fe and unavoidable impurities, and the weld metal of the present invention is C: 0.03% or less, Cr: 18 to 22%, Ni: 8 to 8%.
15%, Mo: 4% or less, Si: 0.65% or less, M
n: 1 to 2.5%, the balance being Fe and inevitable impurities, and the contents of C, Cr, Ni, Mo, Si, and Mn are regulated as described below.

First, the nitric acid resistance of the Mo-containing stainless steel constituting the welded structure will be examined. FIG. 1 shows the results of a study on the corrosion resistance of Mo-containing stainless steels of various components, which were subjected to sensitization heat treatment assuming the heat affected zone of welding. FIG. 1 shows the relationship between the component ranges of Cr, Ni, Mo, and Si and the nitric acid resistance. The total content of Cr and Ni (Cr + Ni), which are the basic elements of stainless steel, and Mo, an element that affects corrosion resistance and intergranular corrosion, are shown. And Si content (M
o + Si). Here, as the nitric acid resistance, the intergranular corrosion rate in a 5-cycle corrosion test (JIS G 0573) in which the cycle is 48 hours in 65% boiling nitric acid and converted to a corrosion rate per year is shown. is there. From the conventional knowledge, the corrosion rate of the SUS316L-based steel is set to be the corrosion rate obtained under the test conditions described above, which is equal to 0.1.
When it is specified that 5 mm / y or less shows good nitric acid resistance,
In order to improve the nitric acid resistance of the sensitized heat-treated material, Cr, N
It can be seen that this is achieved by regulating the component ranges of i, Mo, and Si in terms of% by weight as in the following equation.

Cr + Ni ≧ Mo + Si + 28.4 (1) Equation (1) can also be expressed as follows.

[0011] Next, the nitric acid resistance of the welded portion of the structural stainless steel will be described. When a TIG welding method is used as a welding method and a stainless steel member is welded with a weld joint having a U groove or a V groove, the groove root is often melted with a welding arc without adding a welding wire to form a first layer. Is formed, and then the second and subsequent layers are formed using a filler wire. In such a case, the composition of the first layer greatly depends on the composition of the stainless steel itself, that is, the base metal, and the composition of the second and subsequent layers greatly depends on the composition of the filler wire, that is, the weld metal. In the first layer, the base material is substantially melted and re-solidified, while in the second and subsequent layers, although there is some dilution of the base material, mainly the weld metal constituting the filler wire is used. . Therefore, regarding the corrosion resistance of the welded portion, it is necessary to define the composition of both the base material and the weld metal in consideration of the corrosion resistance of the portion where the base material has been melted and re-solidified (the molten re-solidified portion).

FIG. 2 shows a molten re-solidified portion obtained by melting and solidifying a 316 stainless steel material by TIG welding without a filler wire at a groove root portion as described above, and a welded portion obtained by adding a welding wire and TIG welding. FIG. 3 is a view showing nitric acid resistance of the present invention. Here, the welded portion refers to a portion composed of a molten re-solidified portion and a weld metal portion. The nitric acid resistance of the 316 stainless steel weld shown in FIG. 2 has a large effect on the solidification structure of the austenitic stainless steel, which is Cr equivalent (Creq) and Ni equivalent (Nie).
q) and the total content of Mo and Si (Mo + Si) as intergranular corrosion affecting elements. As the nitric acid resistance, the grain boundary erosion rate in a 5-cycle corrosion test in 65% boiling nitric acid with 48 hours as one cycle is arranged as a parameter. Here, in consideration of the fact that the intergranular erosion is particularly likely to occur in the welded portion, the limit of the corrosion resistance allowed for the welded portion is set at a grain boundary erosion speed of 3 mm / y per year, and the corrosion speed is smaller than this. The region is a preferable range. The range of good nitric acid resistance in the molten re-solidified part is Cr,
It can be seen that the component ranges of Ni, Mo, and Si are in a range satisfying the following expression by weight%.

Creq / Nieq> 1.5 or Creq / Nieq <1.3 (2) where Creq = Cr + Mo + 1.5Si + 0.5Nb Nieq = Ni + 30C + 0.5MnMo + Si <2.8 (3) In consideration of the nitric acid resistance of the solidified portion, not only the JIS standard of 316 stainless steel and the formula (1) are satisfied, but also the ranges of the components are as described in (2) and (3).
It is necessary to regulate to satisfy the expression (3).

On the other hand, for the weld metal, it is necessary to prevent the occurrence of weld cracks during welding, and the prevention of the occurrence of weld cracks can be dealt with by including at least 2% of δ-ferrite. If Creq / Nieq is 1.3 or less, it becomes difficult to contain δ-ferrite due to the composition of the components.
Creeq / Nieq <1.3 cannot be regulated for weld metal. Therefore, for the weld metal, SUSY316L
In addition to satisfying the JIS standards such as the above, it is necessary to further regulate the component range as in the following formula.

Creq / Nieq> 1.5 (4) Mo + Si <2.8 (5)

[0016]

The nitric acid resistance of the stainless steel having the composition described in the above means is improved because the intermetallic compound mainly composed of Fe, Cr and Mo, which is easily dissolved in nitric acid, hardly precipitates at the grain boundary. it is conceivable that.

The influence of the content of the main elements of the stainless steel (base material) on the precipitation of such easily soluble intermetallic compounds and the nitric acid resistance is considered as follows.

If the content of C is high, Cr carbide in the weld heat affected zone precipitates at the grain boundary, and a Cr deficient layer is formed, so that the corrosion resistance is remarkably reduced. However, if it is 0.03% or less, this can be prevented.

Cr is a basic element for maintaining nitric acid resistance, and it is widely known that 16% or more is required for this purpose. On the other hand, even if it is too large, it is necessary to add a large amount of expensive Ni in order to obtain a uniform austenite structure.

Ni is an element necessary for maintaining the austenite structure, and also contributes to corrosion resistance. In order to maintain the austenite structure after containing 16% or more of Cr, 8% or more is necessary. On the other hand, even if the Ni content is too high, the effect is saturated and does not change much, and it is very expensive. From the viewpoint of a necessary and sufficient amount, the content is set to 16% or less.

Mo is an element necessary for improving pitting corrosion resistance, but 4% or less is sufficient except in a very special environment containing a large amount of halogen ions.

Si is an element necessary for obtaining a normal material having few inclusions such as oxides. For that purpose, a small amount of Si may be added, and if it is too high, the toughness is reduced. For this reason, 1% or less is sufficient.

Mn stabilizes the austenite structure and acts as a deoxidizing and desulfurizing agent, and is a useful element. However, if its content is large, it reduces the corrosion resistance. Is not desirable.

Next, the constituent elements of the welding wire used for TIG welding the above stainless steel base material will be described. The reason for limiting the upper and lower limits of the action and the range of the constituent elements of the welding wire is almost the same as that of the base metal, but the range differs for some elements in order to maintain good characteristics of the welded portion.

First, as a special feature of the welded portion, if the metal structure of the welded portion is completely austenitic, a weld crack is likely to occur during welding. Therefore, δ-ferrite must be at least 2% or more. Therefore, the ferrite forming element, Cr, is 18 to 22 higher than the base metal.
%, And Ni, which is an austenite forming element, is desirably set to 8 to 15%, which is slightly lower than that of the base material. Also, with respect to Si, when this content is large, it is easily embrittled, and the effect is more likely to appear at the welded portion than at the base metal. For this reason, the lower .0.
65% or less. Regarding Mn, unlike the base metal at the welded portion, it is necessary that the alloy component itself has a high deoxidizing and desulfurizing effect. Therefore, the content of Mn having the deoxidizing and desulfurizing effect is higher than that of the base material by 1-2. It is desirable to set it to 5%.

The results of an experimental study of the influence of the alloy components on the base material were arranged by parameters based on the above-described concept. The results show that the component composition described in the above-described means improves the nitric acid resistance. It was found to be optimal.

On the other hand, it is necessary to suppress the precipitation of the same intermetallic compound as in the base metal also in the welded portion, but in the case of the welded portion, it is necessary to consider the solidification structure. The solidification structure of austenitic stainless steel is most affected by the ratio of Cr equivalent to Ni equivalent (Creq / Nieq). The component range having excellent nitric acid resistance has two regions, a region having a high ratio and a region having a low ratio. That is, the low region (Creq / Nieq <1.3) is a region in which the austenite phase is stable, Cr and Mo are dissolved stably, and the intermetallic compound is unlikely to precipitate. On the other hand, the high region (Creq / Nieq> 1.5) is a region where the ferrite phase considerably coexists. If the ferrite phase can coexist stably, Cr and Mo are absorbed in this phase. It is considered that the intermetallic compound is unlikely to precipitate. C
It is necessary to avoid the region where req / Nieq is 1.3 to 1.5, since the intermetallic compound is most easily precipitated. Such a welded portion is not necessarily composed only of the weld metal. For example, the first layer of the butt welded portion with a U groove, a V groove, or the like is formed by melting and re-solidifying the base material. Therefore, in order to improve the nitric acid resistance of the welded portion, it is necessary to regulate the component range of the base metal as well as the weld metal within the above range. If the amount of ferrite in the weld metal structure is extremely reduced by lowering the Creq / Nieq of the weld metal, welding cracks are likely to occur.
The region of Creq / Nieq> 1.5 is desirable. On the other hand, even if Creq / Nieq is restricted to a desirable range, if the Mo content is too high, the precipitation of the intermetallic compound of Mo cannot be suppressed, and high nitric acid resistance cannot be maintained. Further, Si seems to promote the precipitation of the intermetallic compound of Mo, and it is necessary to suppress this too. Therefore, it is desirable to suppress the content of these two elements, that is, to satisfy Mo + Si <2.8%.

[0028]

【Example】

[Example 1] Austenitic stainless steel SUS316
For L, a material having a component range satisfying the following equation (1):
Three types of materials which were out of the range of the type and the component range of the formula (1) were melted. When the sensitization treatment is performed, a material satisfying the formula (1) has good nitric acid resistance, whereas a material not satisfying the formula (1) has poor nitric acid resistance.

Cr + Ni ≧ Mo + Si + 28.4 (1) or In Table 1, materials satisfying the expression (1) are referred to as test materials A to D.
Materials that do not satisfy the formula (1) are indicated by test materials EG and their chemical components are shown.

[0030]

[Table 1]

Each of the melted materials was hot-rolled into a 10 mm thick plate, and then subjected to a solution treatment at 1090 ° C. for 15 minutes. After heat-sensitizing each of these materials at 650 ° C. for 2 hours, the nitric acid resistance was examined.

The nitric acid resistance was evaluated by a 5-cycle corrosion test (JIS G) in 48% boiling nitric acid with 48 hours as one cycle.
0573). The corrosion rate was determined by measuring the weight of the specimen before and after the corrosion test.
Table 1 shows the corrosion rate obtained by the corrosion test in numerical values converted per year, and FIG. 3 shows a graph. Although the test materials A to D satisfying the expression (1) have a small corrosion rate of 0.5 mm / y or less, the corrosion rates of the test materials EG of the comparative materials not satisfying the expression (1) are remarkably high.

FIG. 4 is a diagram in which the results of the corrosion test are arranged using Cr + Ni and Mo + Si as parameters. It can be seen that those having chemical components within the component range defined by the formula (1) have excellent nitric acid resistance.

Example 2 For the purpose of examining the corrosion resistance of a weld metal, four types of SUS316L welding wires were newly melted, and the welding metals shown in Table 2 were produced by TIG welding using these welding wires. In order to obtain a weld metal, a wide groove was formed in the test piece, and the weld metal was collected only from the portion of the first layer of the base material melted and welded to the second and subsequent layers not including the solidified portion.

[0035]

[Table 2]

In Table 2, the test materials H and I required the following (2) and (3) required for good nitric acid resistance of the weld metal.
The component ranges of the formula are satisfied, and the test materials J and K are out of this component range.

Creq / Nieq> 1.5 (2) Here, Creq = Cr + Mo + 1.5Si + 0.5Nb Nieq = Ni + 30C + 0.5MnMo + Si <2.8 (3) A test piece was collected from these welded metal parts and 65% boiling nitric acid In the test, a corrosion test was carried out for 5 cycles with 48 hours as one cycle. After the corrosion test, the cross section was observed, and the grain boundary erosion rate was determined by measuring the grain boundary erosion depth. The results are shown numerically in Table 2 by converting the grain boundary erosion rate obtained in the corrosion test per year, and are shown graphically in FIG. Specimens H and I satisfying the equations (2) and (3) have a grain boundary erosion rate of 1.
Although it is small at 0 mm / y or less, the grain boundary erosion rates of the test materials J and K which do not satisfy the expressions (2) and (3) are extremely large.

FIG. 6 shows the results of the above-mentioned corrosion tests arranged using Creq / Nieq and Mo + Si as parameters. It can be seen that those having chemical components within the specified component range have excellent nitric acid resistance.

[Example 3] Using the 316 stainless steel sheets (A to G) used in Example 1, the base material was melted by TIG non-filler welding to produce a test piece for the molten and re-solidified portion. , And the nitric acid resistance was examined. Table 3 shows the chemical components of the test materials and the results of the corrosion test. The chemical components are the same as those shown in Table 1.

[0040]

[Table 3]

Of these, test materials A, C, and E satisfy the following component ranges, and are considered to have good nitric acid resistance in the melt-resolidification portion. Test materials B, D, and F , G are out of this component range.

Creq / Nieq> 1.5 or Creq / Nieq <1.3 (2) where Creq = Cr + Mo + 1.5Si + 0.5Nb Nieq = Ni + 30C + 0.5MnMo + Si <2.8 (3) In 65% boiling nitric acid, 48
A four-cycle corrosion test was performed with the time as one cycle. After the corrosion test, the cross section was observed, and the grain boundary erosion rate was determined by measuring the grain boundary erosion depth. The results are shown in Table 3 as numerical values in terms of the grain boundary erosion rate obtained in the corrosion test per year, and as a graph in FIG. Specimens A, C and E satisfying the equations (2) and (3) have a grain boundary erosion rate of 1.0 mm /
The grain boundary erosion rates of the test materials B, D, F and G, which are small below y but do not satisfy the equations (2) and (3), are remarkably large.

FIG. 8 is a diagram in which these results are arranged using Creq / Nieq and Mo + Si as parameters. It can be seen that those having chemical components within the specified component range have excellent nitric acid resistance.

Example 4 Using the test materials A and F of the 316 stainless steel plate used in Example 1 and the test materials H and J of the welding wires used in Example 2, Four kinds of butt welding test pieces in which a base material and a welding wire were combined were manufactured, and the nitric acid resistance was examined. The test material A (base material) and the test material H (welding wire) each satisfy the chemical composition required for high nitric acid resistance, but the test material F (base material) and the test material Material J (welding wire) deviates from the required chemical composition. Table 4 shows combinations of the base material and the welding wire of each welding test piece. Welding is TI
The groove was narrowed by G welding.

[0045]

[Table 4]

From these butt weld test pieces, welded joint test pieces L to O including a welded portion and a base metal portion were sampled, and subjected to a 5-cycle corrosion test in 65% boiling nitric acid with 48 hours as one cycle. After the corrosion test, the surface was observed to check for abnormal corrosion. Table 4 shows the observation results after the corrosion test.
No abnormal corrosion was observed in the welded joint test piece L composed of the base metal A and the welding wire H satisfying the chemical components required for high nitric acid resistance, but either the base metal or the welding wire used was required. Severe intergranular corrosion was observed in the weld metal or in the molten re-solidified portion (weld first layer portion) of the welded joint test pieces M, N, O deviating from the chemical composition composition to be performed.

[0047]

According to the present invention, the chemical composition of austenitic stainless steel containing Mo is changed to Cr.
+ Ni ≧ Mo + Si + 28.4, Creq / Nieq> 1.5
Or Creq / Nieq <1.3 and Mo + Si <2.8, and the composition of the weld metal is Creq.
/Nieq>1.5 and Mo + Si <2.8 are effective in improving the nitric acid resistance of the austenitic stainless steel base material and the welded portion. As a result, it can be greatly expected that the corrosion resistance of equipment handling nitric acid and the like will be improved.

[Brief description of the drawings]

FIG. 1 is a view showing the effect of alloying elements on the nitric acid resistance of a heat treatment material for sensitizing Mo-containing austenitic stainless steel.

FIG. 2 is a diagram showing the effect of alloying elements on the nitric acid resistance of a weld metal joining a Mo-containing austenitic stainless steel and a molten re-solidified portion.

FIG. 3 is a graph showing a nitric acid corrosion test result of a heat treatment material for sensitizing Mo-containing austenitic stainless steel.

FIG. 4 shows the results of a nitric acid corrosion test of a heat treatment material for sensitizing Mo-containing austenitic stainless steel on Cr + Ni and Mo + S.
FIG. 9 is a diagram in which i is arranged as a parameter.

FIG. 5 is a graph showing the results of a nitric acid corrosion test of a weld metal of Mo-containing austenitic stainless steel.

FIG. 6 is a diagram in which the results of a nitric acid corrosion test on a weld metal of Mo-containing austenitic stainless steel are arranged using Creq / Nieq and Mo + Si as parameters.

FIG. 7 is a graph showing the results of a nitric acid corrosion test of a molten re-solidified portion of Mo-containing austenitic stainless steel.

FIG. 8 shows the results of a nitric acid corrosion test of the molten re-solidified portion of Mo-containing austenitic stainless steel by Creq / Nieq and M
It is the figure arranged using o + Si as a parameter.

Continuation of the front page (72) Inventor Masayuki Yukawa 3-1-1 Kochi-cho, Hitachi-shi, Ibaraki Pref. Hitachi, Ltd. Hitachi factory inspector Kengo Koyanagi (56) References JP-A 64-62278 (JP, A) JP-A-4-143214 (JP, A) JP-A-5-271752 (JP, A) JP-A-55-107762 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00 302 B23K 35/30 340 C22C 38/44

Claims (3)

(57) [Claims] 1. A weight ratio of C: 0.03% or less, Cr: 1
6-20%, Ni: 8-16%, Mo: 4% or less, S
i: 1% or less, Mn: 2% or less, the balance being Fe and unavoidable impurities, and the contents of C, Cr, Ni, Mo, and Si are determined by the following formulas (1), (2), and (3). Regulated nitrate resistant austenitic stainless steel. Cr + Ni ≧ Mo + Si + 28.4 (1) Creq / Nieq> 1.5 or Creq / Nieq <1.3 (2) where Creq = Cr + Mo + 1.5Si + 0.5Nb Nieq = Ni + 30C + 0.5MnMo + Si <2.8 (3) 2. A method for welding nitric acid-resistant austenitic stainless steel according to claim 1, wherein C: 0.03% by weight.
Hereinafter, Cr: 18 to 22%, Ni: 8 to 15%, Mo:
4% or less, Si: 0.65% or less, Mn: 1 to 2.5
%, The balance being Fe and unavoidable impurities, and C, C
The contents of r, Ni, Mo, Si, and Mn were determined in the following (4),
A weld metal characterized by the formula (5). Creq / Nieq> 1.5 (4) where Creq = Cr + Mo + 1.5Si + 0.5Nb Nieq = Ni + 30C + 0.5MnMo + Si <2.8 (5) 3. A welded structure, wherein the nitrate-resistant austenitic stainless steel according to claim 1 is welded using the weld metal according to claim 2.