JPH07268559A - Bellows excellent in stress corrosion cracking resistance - Google Patents

Abstract

PURPOSE:To produce a bellows by which the generation of cracks caused by stress corrosion cracking is solved, by reducing the contents of C and N in a ferritic stainless steel to specified wt.% or below. CONSTITUTION:The componental compsn. of a ferritic stainless steel is constituted of the one in which the contents of C and N are reduced to, by weight, <=0.02% and <=0.015%, respectively. Furthermore, it is incorporated with Ti by <=0.6% as well as >= four times the total of the C content and N content. The components other than the same in this stainless steel are constituted of 0.1 to 1.5% Si, <=1.0% Mn. 11 to 22% Cr and <=0.1% Al. By this steel, a bellows is formed, and as a result, the bellows capable of sufficiently satisfying the points of high temp. oxidation and molten salt corrosion can be provided.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bellows excellent in stress corrosion cracking resistance. In particular, the present invention relates to a flexible tube used for a high temperature portion such as a connecting portion of an exhaust pipe of an automobile and a flexible tube used for a portion requiring corrosion resistance such as a connecting portion of a high temperature solution pipe.

[0002]

2. Description of the Related Art In various mechanical devices handling gas, liquid or powder, most of them are moved through metal pipes. A bellows is used in the middle of the pipe in order to absorb strain due to thermal expansion or vibration to prevent transmission. Conventionally, copper or austenitic stainless steel has been used for this bellows. The reason for this is that processing into a bellows structure was difficult with other metals. That is, copper or austenitic stainless steel has a large cold elongation, and bulge processing which is formed by the elongation can be applied. On the other hand, other metals such as carbon steel cannot be bulged because they lack ductility.

On the other hand, although the bellows made of austenitic stainless steel is easy to manufacture, it has a drawback that stress corrosion cracking is likely to occur depending on the corrosive solution passing through the inside. This is because the bellows absorbs strains and vibrations by bending the peaks of the convex portion and the valleys of the concave portion of the pipe, so that stress is always applied to the convex portion and the concave portion. In other words, it is a structure and parts in which stress cannot be removed. Nevertheless, austenitic stainless steels are alloys that are highly susceptible to stress corrosion cracking. Therefore, the bellows made of austenitic stainless steel has a drawback that stress corrosion cracking is very likely to occur.

In order to avoid stress corrosion cracking, either a material having low susceptibility to stress corrosion cracking is used, or a structure in which stress corrosion cracking does not easily occur, that is, a structure in which no stress load remains is used. In order to reduce the stress corrosion cracking susceptibility of austenitic stainless steel, for example, as described in JP-A-49-107915, the Ni content is increased and Cr, N, Mo and P are reduced. Is proposed. However, even when such a steel is used, the time until the occurrence of stress corrosion cracking is merely extended and the occurrence of stress corrosion cracking cannot be prevented.

On the other hand, structurally, by increasing the number of concavities and convexities of the bellows and decreasing the bending angle of the concave and convex portions, it is considered that the stress is dispersed and the stress applied to each concave or convex portion is reduced. Has been. However, this method has a drawback in that the cost becomes high, such as the bellows becoming large and long and the device also needs to be large as a result. Moreover, the susceptibility to stress corrosion cracking still cannot be resolved, and stress corrosion cracking has been a problem in some environments.

[0006]

DISCLOSURE OF THE INVENTION The present invention provides a bellows which virtually eliminates stress corrosion cracking susceptibility. That is, in the bellows that cannot completely eliminate the stress application, it is considered that the slight improvement in the stress corrosion cracking resistance of the material is practically ineffective. Therefore, it is necessary to manufacture the bellows with a material that is not substantially susceptible to stress corrosion cracking. The present invention provides a bellows in which stress corrosion cracking susceptibility is virtually eliminated by using a ferritic stainless steel having no stress corrosion cracking sensitivity.

[0007]

In view of the above situation, the present inventors have aimed to apply ferritic stainless steel having extremely low sensitivity to stress corrosion cracking to the bellows.
Generally, a bellows is manufactured by forming a tube by a processing method such as hydraulic bulge. Therefore, if the ductility of the material is insufficient, it will break at the tip of the convex portion of the bellows. In ferritic stainless steel such as SUS430 steel, which is usually used as a general-purpose steel, such fracture occurs and it is extremely difficult to process it into a bellows. Even if you can
Since the difference in diameter between the convex portion and the concave portion of the bellows is small, it is difficult to use the bellows as a bellows. However, the ductility of the material is improved even in the ferritic stainless steel by reducing C and N, which are particularly infiltrating solid solution in the steel as impurity elements, to the utmost limit, and conventionally, it was manufactured by the austenitic stainless steel. It was found that a bellows having a structure comparable to that of the bellows can be manufactured.

The present invention has been made based on this finding. That is, the present invention provides (1)% by weight, C: 0.02% or less and N: 0.0
The first gist is a bellows made of ferritic stainless steel having a stress corrosion cracking resistance reduced to 15% or less. Bellows molded with ferritic stainless steel,
Although the difference in diameter between the convex portion and the concave portion is limited depending on the ductility level of the material, the stress corrosion cracking susceptibility becomes extremely small in any case, and it can be considered that the sensitivity is virtually eliminated. .

By the way, as described above, the bellows is manufactured by forming a tube by a working method such as hydraulic bulge. In this case, for manufacturing the tube, a thin plate is usually rolled into a tubular shape and welded into the tube. Depending on the use environment of the bellows, Cr carbide may be precipitated in the welded portion or the heat affected zone to cause intergranular corrosion, or the general corrosion resistance may be deteriorated. In order to avoid this phenomenon, it is possible to deal with the problem by limiting the welding method to make the fusion zone and the heat affected zone as narrow as possible, for example, by applying a welding method such as laser welding. However, such a method has a cost disadvantage. On the other hand, Ti in the base metal
It was found that it is not necessary to strictly limit the welding method by adding a proper amount of. Based on this finding, the second invention was completed as an embodiment in which the welding method need not be strictly limited. That is, the present invention is (2)% by weight, C: 0.02% or less, Ti: 4 or more times the sum of the content of C and N and 0.6% or less, N:
Including 0.015% or less, other unavoidable impurities and Fe
The second gist is a bellows which is excellent in stress corrosion cracking resistance and is made of ferritic stainless steel.

Further, some bellows are used in an environment of 600 to 900 ° C. like an exhaust system of an automobile.
In this temperature range, the precipitated carbonitride of Cr in the material re-dissolves and re-precipitates again at the grain boundaries, reducing the strength of the material and deteriorating the fatigue characteristics. In this temperature range, oxidation resistance is indispensable, and so-called molten salt corrosion resistance against molten salt corrosion caused by adhesion and melting of various salts existing in the environment is required. Therefore, if fixed as a carbide or nitride that does not dissolve in this temperature range during manufacturing,
There is no reprecipitation at grain boundaries. On the other hand, it has been found that it is effective to add a proper amount of Si to form a stable Si oxide film against molten salt corrosion. In accordance with this idea, the third invention was completed as an embodiment aimed at use in an environment of 600 to 900 ° C. such as an exhaust system of an automobile.

That is, the present invention is (3)% by weight, C: 0.02% or less, Si: 0.1%
1.5% or less, Mn: 1.0% or less, Cr: 11.
Including 0% or more and 22.0% or less, 4 times or more of the sum of Ti: C content and N content and 0.6% or less, Al: 0.1% or less, N: 0.015% or less A third gist is a bellows which is excellent in stress corrosion cracking resistance and is composed of unavoidable impurities and ferritic stainless steel which is Fe.

Next, the limiting conditions of the present invention will be shown. The material used for the bellows is 0.02% or less C and 0.0 N
Limit to ferritic stainless steel reduced to 15% or less. Ferrite stainless steel is Cr in bellows
Regardless of the amount, stress corrosion cracking resistance is overwhelmingly superior to austenitic stainless steel. Therefore, Cr
The amount is not limited because it can be selected depending on the environment used.

If the C content of the material exceeds 0.02%, the bellows processing becomes difficult, and even if it can be processed, the Cr carbide precipitated in the material deteriorates the fatigue properties, resulting in 0.02%. % Is the upper limit. If the N content of the material exceeds 0.02%, the bellows processing becomes difficult as in the case of C, and even if the material can be processed, the fatigue characteristics deteriorate due to the Cr nitride precipitated in the material. The upper limit is 0.02%.

In addition, in the second invention completed as an embodiment which does not require severe limitation of the welding method, the feature is that Ti is added. Ti is the lower limit because it is necessary to fix C and N, which are once solid-dissolved during welding, in an amount equal to or more than 4 times the sum of the C-containing weight and the N-containing weight. However, if a large amount is added, N will be absorbed during welding and nitride will be generated, and a new welding method will have to be limited, so the upper limit was made 0.6%.

Further, like an automobile exhaust system,
The third invention, which is an embodiment aimed at use in an environment of 900 ° C., is characterized by limiting Cr and Al and adding Si. In order to secure the molten salt corrosion resistance, it is necessary for Si to form a stable oxide film of Si in the temperature range of 600 to 900 ° C. For that purpose, addition of 0.1% or more is necessary. Is. However, if added in a large amount, it becomes extremely difficult to process the bellows, and even if the bellows can be processed, the difference in diameter between the convex portion and the concave portion is small and a very long structure is required to secure the function of the bellows. Therefore, the upper limit is 1.5%. As for Mn, although it is necessary to secure the workability of the raw material for the processing into the bellows, since Si is added for the corrosion resistance of the molten salt, the workability is secured by limiting the Mn. The upper limit was set to 1.0%. Cr is 600 to 900
Oxidation resistance is essential in the temperature range of ℃. If it is less than 11%, the oxidation resistance is extremely poor, so the lower limit was made. But,
If added in a large amount, the workability deteriorates, so 22.0% was made the upper limit. Al is an element necessary for deoxidation,
If added in a large amount, not only the processing into the bellows but also the hot workability deteriorates, so the upper limit was made 0.1%.

[0016]

The present invention provides a bellows using ferritic stainless steel. Ferritic stainless steel is
When the bellows process is performed, the susceptibility to stress corrosion cracking, including the welded part, is extremely low, so that the occurrence of cracks due to stress corrosion cracking is greatly reduced. On the other hand, by limiting the content of C and N to a low level, it becomes possible to sufficiently secure the difference in diameter between the convex portion and the concave portion of the bellows.

Further, by adding an appropriate amount of Ti to the base metal, the precipitation of Cr carbide in the weld zone and heat affected zone is eliminated,
Since the occurrence of intergranular corrosion and deterioration of general corrosion resistance are eliminated,
There is no need to take any measures to limit the welding method to make the fusion zone and the heat affected zone as narrow as possible, and it is not necessary to strictly limit the welding method. Further, by adding an appropriate amount of Ti, it becomes 600 to 90.
Even in the environment of 0 ° C., the carbonitride of Cr that has been precipitated in the material is re-dissolved and re-precipitated again at the grain boundary, so that the deterioration of the strength and fatigue characteristics of the material is eliminated. By adding an appropriate amount of Si and limiting the amount of Cr, a stable Si oxide film is formed in addition to the oxide film mainly composed of Cr.
Oxidation resistance is improved, and so-called molten salt corrosion resistance to molten salt corrosion caused by adhesion and melting of various salts existing in the environment is improved. As a result, 60
It is possible to provide a bellows having excellent stress corrosion cracking resistance that can be used even in an environment of 0 to 900 ° C.

[0018]

Example A bellows was prepared from a cold-rolled steel sheet (annealed or solution-treated material) having a thickness of 0.5 mm shown in Table 1, and subjected to a stress corrosion cracking test. In the stress corrosion cracking test, 10% elongation is applied to the entire length of the bellows, covered with gauze, both ends of the gauze are immersed in 0.5% NaCl solution, and the whole is kept in a thermostatic chamber at 80 ° C. It was done by the method. The NaCl solution propagated through the gauze by the capillarity and contacted the surface (outer surface) of the bellows. In this method, it was observed that water vaporized near the surface of the bellows, resulting in the precipitation of NaCl crystals over time.

Table 2 shows the values of the amount of oxidation increase due to stress corrosion cracking of various bellows and high temperature oxidation. The bellows of the ferritic stainless steel of the present invention did not crack for 240 hours or more and stress corrosion cracking did not occur. On the other hand, comparison SUS
The 304 steel bellows cracked in about 20 hours at an early stage. From the observation result of the fracture surface and the cross section in the vicinity thereof, it was judged that the stress corrosion cracking was caused by Cl ⁻ ions.

The steel according to the present invention to which Ti and Si were added showed the same level of increase in oxidation as the SUS304 steel, and showed excellent oxidation resistance.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

According to the present invention, it is possible to provide a bellows in which the generation of cracks due to stress corrosion cracking is almost eliminated. Also, by limiting the composition of the base material, it is no longer necessary to severely limit the welding used to manufacture the tube of material. Furthermore, like the exhaust system of a car, 600-900 ℃
It is now possible to provide bellows that are sufficiently satisfactory not only in stress corrosion cracking, but also in high temperature oxidation and molten salt corrosion even when used in the above environment. As a result, the maintenance of the device using the bellows is extremely simplified, and the life of the entire device is extended. Further, the industrial advantages are great, such as the productivity can be improved by simplifying the maintenance.

Further, considering that conventional austenitic stainless steels containing a large amount of Ni have been used for these bellows, since they are relatively inexpensive ferritic stainless steels, they can be provided at extremely low cost. There are features. The effect obtained by this is not only the economic benefit from the industrial side, but also the temporary leakage of harmful liquids and gases due to the occurrence of stress corrosion cracking is eliminated and environmental pollution is solved. The effect is eliminated, and the social benefits that do not appear in the direct economic effect are enormous.

Claims (3)

[Claims] 1. A bellows excellent in stress corrosion cracking resistance, characterized by comprising ferritic stainless steel in which C: 0.02% or less and N: 0.015% or less by weight. 2. C: 0.02% or less, Ti:
Stress resistance characterized by being composed of ferritic stainless steel which is 4 times or more of the sum of C content and N content and 0.6% or less, N: 0.015% or less, and other unavoidable impurities and Fe. Bellows with excellent corrosion cracking resistance. 3. By weight%, C: 0.02% or less, Si:
0.1% or more and 1.5% or less, Mn: 1.0% or less, C
r: 11.0% or more and 22.0% or less, Ti: 4 or more times the sum of C content and N content and 0.6% or less, Al:
A bellows excellent in stress corrosion cracking resistance, characterized by comprising ferritic stainless steel containing 0.1% or less and N: 0.015% or less and other unavoidable impurities and Fe.