JPH0941097A - Austenitic stainless steel for absorption-type air cooling and heating apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the generation of gaseous hydrogen during operation by applying polishing finishing to an austenitic stainless steel as a structural material for a regenerator in an absorption-type air cooling and heating apparatus using an absorbing solution composed essentially of LiBr. SOLUTION: As a structural material for a regenerator in an absorption-type air cooling and heating apparatus using water as refrigerant in a high concentration halide atmosphere and also using an absorbing solution containing LiBr as essential component, an austenitic stainless steel which has a composition containing, by weight, <0.08% C, 1.0-5.0% Si, <1.0% Mn, <0.045% P, <0.005% S, 16.0-25.0% Cr, 6.0-20.0% Ni, 0.5-3.0% Mo, and 0.005-0.35% N or further containing 0.5-3.0% Cu is used. A passivating film on the surface of this stainless steel is subjected to finish polishing at a gauge of >=No.80 at the minimum. By this method, the generation of gaseous hydrogen from a regenerator during operation can be suppressed, and the increase in cost due to the degassing operation for gaseous hydrogen during operation can be prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an austenitic stainless steel used as a structural material for a regenerator or the like in an absorption type cooling / heating equipment which uses an absorption liquid containing LiBr as a main component.

[0002]

2. Description of the Related Art As an air conditioning system that does not use chlorofluorocarbon as a refrigerant, an absorption type air conditioning equipment that uses a highly concentrated alkaline LiBr aqueous solution having a concentration of 50% or more as an absorbent has been attracting attention due to environmental problems. In the absorption cooling and heating system, for example, as shown in FIG. 1 (a), water vapor 5 is generated between the condenser 1 and the regenerator 2 and between the evaporator 3 and the absorber 4.
Is moving. The vapor of the LiBr aqueous solution 6 heated and evaporated from the regenerator 2 is sent to the condenser 1, and the amount of heat is taken out of the system by the circulation medium 7. Pure water 8 is sent from the condenser 1 to the evaporator 3, and cools the brine flowing through the brine transfer pipe 10 that is connected to the cooling room 9. Then, it is sent to the absorber 4 containing the thin aqueous LiBr solution 11 and exchanges heat with the heat medium 12. In this absorption type cooling and heating system, for example, a regenerator having a structure shown in FIG. 1B is used. Rare absorbents and combustion exhaust gas passages are alternately formed inside the frame 12 constituting the regenerator, and the combustion gas is sent as exhaust gas and the rare absorbent is sent as boiling absorbent and steam. At this time, in order to enhance the heat exchange efficiency between the rare absorbent and the combustion exhaust gas, the heat exchanger 13 forms a flow path. Further, the heat exchanger 13 is brazed 14 to the frame 12 in order to prevent the leakage of the diluted absorbing liquid and the mixture of the atmosphere.

In a large-scale commercial machine that has already been put into practical use, a regenerator of an absorption type air conditioner is made of a material such as carbon steel or cupronickel, and a high concentration of alkaline halogen containing LiBr as a main component is used in the regenerator. Absorbing liquid up to 150 ℃
Heating. In order to popularize absorption-type cooling and heating equipment, it is necessary to reduce the size and weight of conventional large-scale equipment designed for business use. In order to reduce the size and weight, it is necessary to change the absorption cooling method from water cooling to air cooling.
Along with that, it becomes necessary to increase the operating efficiency. For this reason, the operating temperature of the regenerator is as high as about 200 ° C. in the air-cooled absorption air conditioner, and carbon steel cannot provide sufficient corrosion resistance. Therefore, a material replacing carbon steel has been studied, and stainless steel is considered to be the most promising material in terms of corrosion resistance and cost. For ferritic stainless steel, Al, C
It is disclosed in Japanese Patent Publication No. 6-6 that excellent corrosion resistance is exhibited even in high-concentration halides by the addition of o, Cr, Ni, Mo, Cu, etc.
-37692, Japanese Patent Publication No. 5-34419, etc. In Japanese Patent Laid-Open No. 2-298237,
Introducing austenitic stainless steel for air conditioners. Further, Japanese Patent Application Laid-Open No. 7-35433 discloses a stainless steel whose corrosion resistance is improved by lowering the spontaneous corrosion potential below the repassivation potential by adding Si.

[0004]

Problems to be Solved by the Invention Al, Co, Cr, N
Addition of i, Mo, Cu, etc. imparts excellent corrosion resistance to a stainless steel in a high-concentration halide, as seen in JP-B-6-37692 and JP-B-5-34419. However, although the action of these additional elements is effective for the acidic absorption liquid, the absorption liquid for absorption cooling and heating is alkaline, and the effectiveness of these alloy components of stainless steel for the alkaline LiBr absorption liquid is unknown. is there. Particularly, since the heat exchange part of the regenerator has a complicated shape, good workability, weldability and brazing property are required. For example, in brazing, Cu brazing which has a problem in corrosion resistance cannot be used, and Ni brazing is used. However, since brazing using Ni brazing is performed at a relatively high temperature, crystal grains become coarse in ferritic stainless steel.

In this respect, the austenitic stainless steel is superior in workability, weldability and brazing property to the ferritic stainless steel and exhibits excellent pitting corrosion resistance and crevice corrosion resistance. However, when austenitic stainless steel is used as equipment for a regenerator, hydrogen gas is generated during the operation of the regenerator, which tends to reduce the cooling and heating capacity. Therefore, when using the regenerator made of austenitic stainless steel, it is necessary to remove the generated hydrogen and maintain the degree of vacuum in the regenerator at an appropriate pressure. However, a dedicated device is required for removing hydrogen gas, and it is necessary to stop the operation of the device during the hydrogen gas removing operation. Therefore, it is an important problem to suppress the gas generation amount in order to put the absorption type air conditioner into practical use. The present invention has been devised to solve such a problem, by controlling the surface finish of austenitic stainless steel having excellent stress corrosion cracking resistance, sufficient workability and brazability. , Suppress the amount of gas generation,
It is an object of the present invention to provide an austenitic stainless steel suitable for an air conditioner using a highly concentrated alkaline LiBr absorbing solution of 50% or more.

[0006]

SUMMARY OF THE INVENTION The austenitic stainless steel of the present invention has a C content of 0.1%.
08 wt% or less, Si: 1.0 to 5.0 wt%, Mn:
1.0% by weight or less, P: 0.045% by weight or less, S:
0.005 wt% or less, Cr: 16.0 to 25.0 wt%, Ni: 6.0 to 20.0 wt%, Mo: 0.5 to
3.0% by weight and N: 0.005 to 0.35% by weight, the balance having a composition of substantially Fe, having a surface polished and finished with a # 80 or higher count, using water as a refrigerant, Li
It is characterized in that it is used as a structural material of an absorption type cooling and heating equipment using an absorption liquid containing Br as a main component. The stainless steel may further contain Cu: 0.5 to 3.0% by weight.

[0007]

In the absorption type air conditioner, a high concentration alkaline LiBr aqueous solution having a concentration of 50% or more is heated to a high temperature of about 200 ° C. Since the inside of the air conditioner is closed and the operation is started after the air is evacuated, hydrogen gas is generated on the surface of the stainless steel due to the corrosion reaction or the cathode reaction during film formation, unlike the normal environment. Since the generated hydrogen gas is a non-condensable gas, the heat exchange efficiency is significantly reduced. As a result of detailed examination of the hydrogen gas generation mechanism, the present inventors have found that the generation of hydrogen gas changes depending on the surface condition of stainless steel. That is, when a passivation film is formed on the surface of austenitic stainless steel by pickling like 2B or 2D finishing, the gas generation amount increases, and when the passivation film generated after pickling is removed by polishing, The generation amount is suppressed. LiBr concentration 55
%, Alkalinity 0.02N, Li ₂ Mo ₄ 300ppm
The absorption liquid of Example 1 was kept at 150 ° C., and it was investigated how the anodic polarization curve changes depending on the surface condition of stainless steel. The anodic polarization curve was measured by the potentiodynamic method at a polarization rate of 20 mV / min. The anodic polarization curve was the curve shown in FIG. 2 for the stainless steel plate of Table 1E finished with 2B and shown in FIG. 3 for the same material polished by No. 600.

As is clear from the comparison between FIG. 2 and FIG.
It can be seen that the material subjected to the polishing treatment has a small passivation sustaining current and a small amount of gas generation. From this, it can be said that polishing finish is an effective means for suppressing gas generation.
However, the polished material is inferior in pitting corrosion resistance as compared with the material subjected to 2B and 2D finishing because defective portions such as non-metallic inclusions in the steel are more likely to be exposed on the steel surface. Therefore, it is preferable that the fine polishing grain is finer than the coarse polishing grain because the substantial surface area can be reduced and the frequency of exposure of defects to the steel surface can be reduced. For this reason, it is necessary to polish with a count of at least 80 or more, and preferably with a count of 240 or more. Also, as the steel type, an alloy design was adopted that provides excellent corrosion resistance in a polished state in an environment in contact with a high-temperature, high-concentration alkaline LiBr aqueous solution. That is, the alloy components contained in the austenitic stainless steel of the present invention and their contents were determined as follows.

C: 0.08% by weight or less It is a strong element that stabilizes austenite, and does not significantly affect stress corrosion cracking resistance and pitting corrosion resistance. However, because it increases the susceptibility to intergranular corrosion at welds,
The upper limit of the C content was set to 0.08% by weight. Si: 1.0 to 5.0 wt% In the austenitic stainless steel of the present invention, it is an alloying element that exhibits an important action, and a content of 1.0 wt% or more forms a stable film on the surface, It significantly improves the pitting corrosion resistance in the vapor phase part and the liquid phase part in a high concentration halide. Further, the compounded addition with Cu enhances the stress corrosion cracking resistance, and even if a fixed amount of Mo is added, the stress corrosion cracking resistance is not impaired and the crevice corrosion resistance is improved. However, since it is a strong ferrite forming element, the upper limit of the Si content is set to 5.0% by weight in order to stabilize the austenite phase in a state where the amount of Ni used is set low.

Mn: 1.0% by weight or less Produces soluble sulfide MnS which easily becomes a starting point of corrosion and deteriorates pitting corrosion resistance and crevice corrosion resistance. Therefore, the smaller the Mn content is, the more preferable. In the present invention, the upper limit is 1.
It was defined as 0% by weight. P: 0.045 wt% or less Since it is a harmful element that deteriorates hot workability, 0.045 wt% allowed for ordinary stainless steel was set as the upper limit of P content. S: 0.005 wt% or less Combined with Mn in steel to form soluble sulfide MnS, which deteriorates pitting corrosion resistance. The lower the S content, the more preferable, and in the present invention, the upper limit is set to 0.005% by weight.

Cr: 16.0 to 25.0% by weight It is an essential alloying element for improving the corrosion resistance in stainless steel, and is 16.0 in the absorbing solution environment containing a high concentration halide which is the object of the present invention. It is necessary to contain Cr by weight% or more. The higher the Cr content, the higher the corrosion resistance, but the Ni content necessary for maintaining the austenite phase increases, and the manufacturability and workability are impaired. Therefore, in the present invention, the upper limit of the Cr content is set to 25.0.
Weight% is set. Ni: 6.0 to 20.0% by weight It is a main alloying element for maintaining the austenite phase, and 6% by weight or more of Ni is necessary to secure a stable austenite phase. However, the Ni content exceeding 20% by weight causes the steel material cost to increase. Further, Ni exhibits an effect of improving the pitting corrosion resistance in the range of 6.0 to 20.0% by weight, and particularly 10% by weight or more of Ni is used as a material used for a portion requiring the pitting corrosion resistance.
Content is preferred.

Mo: 0.5 to 3.0% by weight It is an alloying element effective in improving corrosion resistance, particularly pitting corrosion resistance, in a high concentration halide. Although the passivation film is strengthened by the inclusion of Mo, its action becomes more remarkable by the combined addition with Si. The effect of adding Mo is
It becomes remarkable at 0.5% by weight or more, preferably 0.7% by weight or more. However, the Mo content exceeding 3.0% by weight is
Since a large amount of expensive Mo is consumed, not only the cost of the steel material increases, but also the hardness of the material adversely affects the manufacturability and workability. N: 0.005 to 0.35% by weight It is a strong austenite forming element and an effective alloying element that improves the corrosion resistance of austenitic stainless steel. Such an effect becomes remarkable when the N content is 0.005% by weight or more. However, a large amount of N content exceeding 0.35% by weight remarkably lowers manufacturability and workability. Cu: 0.5 to 3.0 It is an alloy element added as necessary, and improves the pitting corrosion resistance of austenitic stainless steel and the stress corrosion cracking resistance in an aqueous solution containing a halide. C
The effect of adding u becomes remarkable at 0.5% by weight or more, and 3.
Saturate at 0% by weight. Further, when a large amount of Cu is contained in excess of 3.0% by weight, hot workability deteriorates.

[0013]

[Examples] In order to investigate the effect of the polishing finish of stainless steel used in the regenerator of an air conditioner on the amount of gas generated during the operation of the regenerator, various stainless steels whose compositions are shown in Table 1 were tested. 1 for 2B finish, 2D finish and 2B material
Each was polished with No. 20 and was prepared.

[0014]

Each stainless steel has a vapor phase volume of 700c.
A regenerator of c was prepared, and an absorbent having a LiBr concentration of 55%, an alkalinity of 0.02 N and Li ₂ Mo _{4 of} 300 ppm was used, and operated at 190 ° C. for 100 hours. When the amount of gas generated after the operation was measured, as shown in FIG.
In the abrasive material, the amount of gas generated during operation was significantly reduced as compared with the material D. No pitting corrosion, crevice corrosion, stress corrosion cracking, etc. were observed in any of the finished materials. Further, in order to investigate the corrosion resistance of the abrasive, an experiment was conducted assuming the most severe condition in the absorption type cooling and heating machine, that is, the case where air was mixed into the apparatus for some reason. In the corrosion resistance test, the test materials of various stainless steels shown in Table 1
After processing into 50 mm × 30 mm, it was dry-polished with No. 500 emery paper, degreased with an aqueous MgO solution, and then dried overnight or more. Each test piece was semi-immersed in a 60% concentration LiBr aqueous solution so that the depth of the immersed part was 30 mm, and the liquid temperature was kept at 150 ° C. to carry out a semi-immersion test for 168 hours. Then, the corrosion product was removed by immersing the test piece in a 30% nitric acid aqueous solution kept at 80 ° C., and the maximum erosion depth was measured.

The test results are shown in FIG.
The stainless steel containing 5% by weight or more showed good corrosion resistance even under aeration. Also, Mo is about 1% by weight.
Looking at the combined effect of Si on the added steel types B, D and E, as shown in FIG.
The steel added with is superior to the pitting corrosion resistance in the open to the atmosphere as compared with the steel type B added with Mo alone. That is, S
It has been confirmed that the combined addition of i and Mo exhibits the effect of suppressing the occurrence and growth of pitting corrosion even in the most severe corrosive environment considered as the environment of the absorption type cooling and heating machine. From the above results, it is understood that the austenitic stainless steel to which Si and Mo are added in combination is a material suitable for absorption type cooling and heating equipment.

[0017]

As described above, according to the present invention, by polishing the austenitic stainless steel used as the constituent material of the absorption type cooling and heating machine,
By suppressing gas generation during operation of the air conditioner, the work and cost for degassing during operation can be reduced. Furthermore, an appropriate amount of Si
By adding Mo and Mo together, even in the environment where the most severe air is mixed as the environment of the absorption type air conditioner, the damage due to the corrosion in the gas phase part is suppressed to the minimum, and the maintenance cost is greatly reduced.

[Brief description of drawings]

FIG. 1 is an absorption type heating and cooling system (a) and a regenerator (b).

Figure 2: Anodic polarization curve of unpolished austenitic stainless steel

FIG. 3 Anode polarization curve of polished austenitic stainless steel

[Fig. 4] Effect of finish state of austenitic stainless steel on gas generation rate

FIG. 5: Effect of Mo content on pitting corrosion resistance of austenitic stainless steel

FIG. 6 is a graph showing that pitting corrosion resistance is improved by the combined addition of Mo and Si.

Claims (2)

[Claims] 1. C: 0.08 wt% or less, Si: 1.0
~ 5.0 wt%, Mn: 1.0 wt% or less, P: 0.0
45% by weight or less, S: 0.005% by weight or less, Cr: 1
6.0 to 25.0 wt%, Ni: 6.0 to 20.0 wt%, Mo: 0.5 to 3.0 wt% and N: 0.005 to
Containing 0.35% by weight, the balance being substantially Fe composition, having a surface polished with # 80 or more count,
An austenitic stainless steel used as a structural material for an absorption type cooling and heating device, which uses water as a refrigerant and an absorption liquid containing LiBr as a main component. 2. The austenitic stainless steel according to claim 1, further comprising Cu: 0.5 to 3.0% by weight.