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Heat-resistant austenitic stainless steel having excellent cyclic oxidation resistance

a technology of cyclic oxidation resistance and austenitic stainless steel, which is applied in the field of heat-resistant austenitic stainless steel, can solve the problems of insufficient strength, high cost, and inability to completely prevent oxide formation, so as to improve power generation efficiency due to steam temperature increase, the service life of the heat transfer tube can be extended, and the steel weight loss is reduced.

Inactive Publication Date: 2014-06-05
KOBE STEEL LTD
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  • Summary
  • Abstract
  • Description
  • Claims
  • Application Information

AI Technical Summary

Benefits of technology

The heat-resistant austenitic stainless steel in this invention has several benefits for use in power plants. It reduces weight loss due to oxidation and peeling off of scale, improving power generation efficiency. It also has a longer service life, reducing maintenance costs. Additionally, it minimizes damage to turbines caused by scale scattering.

Problems solved by technology

In such a circumstance, oxides are peeled due to the difference of thermal expansion coefficient to the matrix to result in a problem of insufficiency of strength caused by further development of oxidation and weight loss (thinning) due to peeling off scale.
However, since the stainless steel contains a great amount of expensive Ni, it takes a high cost.
However, while the effect of retarding the growing rate of oxides formed on the surface of steel tubes can be expected, for example, by the addition of Al and Si or formation of the Cr2O3 layer, formation of the oxides cannot be prevented completely and provision of satisfactory cyclic oxidation resistance cannot be expected.
Moreover, the steel containing Al also has a problem that surface defects tend to be caused during manufacture of tubes.
However, since a great amount of Al and Si is contained in this technology, this involves a problem of resulting surface defects of steel tubes or resulting in embrittlement after heat treatment for long time.
In addition, while the literature shows that the addition of REM such as La and Ce including Y exhibits an effect of improving the scale adhesion, this effect is not enough and the technology is not intended for recognition for the improvement of the cyclic oxidation resistance.
However, in the steel using precipitation of a Ti compound as the hardening mechanism such as KA-SUS321J2HTB, it is expected that high temperature strength cannot be ensured unless Ti is added by about 0.1 to 0.25%.
Further, this technology is not intended for the improvement of the cyclic oxidation resistance and it is expected that the cyclic oxidation resistance is low.
However, peening results in another problem of increasing the cost due to increase in the manufacturing process and since the technology is not intended for the improvement of the cyclic oxidation resistance, it is expected that the cyclic oxidation resistance is low.

Method used

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Examples

Experimental program
Comparison scheme
Effect test

example

Example 1

[0051]20 kg ingots prepared by melting various kinds of steels comprising chemical compositions shown in the following Table 1 in a vacuum melting furnace (VIF) were hot-forged each to 120 mm width×20 mm thickness, applied with a heat treatment at 1250° C. and processed by cold rolling to 13 mm thickness. Subsequently, a heat treatment at 1150° C. for 5 min was performed again to provide a master material. A steel material of 20 mm×30 mm×2 mm was cut out from the master material by machining and the surface of the steel material was smoothed and mirror-finished by polishing using emery paper and by buff polishing using diamond abrasive grains to prepare specimens.

[0052]Among the steels shown in the following Table 1, specimens Nos. 1 to 10 are steels that satisfy the requirements defined in the invention (steel of the invention), and specimens Nos. 11 to 16 are steels out of the requirements defined in the invention (comparative steels), in which the specimens Nos. 14, 15, ...

example 2

[0058]For the steels of the invention of specimens Nos. 1 to 6 and the comparative steel of specimen No. 14 shown in Tables 1 and 2, the heat treatment temperature was changed in temperature range of 1125 to 1275° C. after cold working at 35% reduction ratio to prepare specimens of the respective steels with crystal grain size numbers of 4.5 to 10.0. In the cyclic oxidation test, specimens were carried into and out of a furnace at 1100° C. in air at a temperature cycle including furnace heating for 25 min and cooling for 5 min in air, and weight loss (reduction in thickness: mg·cm−2) was determined by comparing the mass of the specimen after 40 cycles with the mass of the specimen in the initial state.

[0059]For the number of cycles, since the weight loss was improved greatly in some steels with addition of Zr and Ce and the weight loss after 20 cycles was about at a level of an allowable error depending on the grain size, heating and cooling were repeated till 40 cycles. The crystal...

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Abstract

A heat-resistant austenitic stainless steel comprising C: 0.05 to 0.2%, Si: 0.1 to 1%, Mn: 0.1 to 2.5%, Cu: 1 to 4%, Ni: 7 to 12%, Cr: 16 to 20%, Nb: 0.1 to 0.6%, Zr: 0.05 to 0.4%, Ce: 0.005 to 0.1%, Ti: 0.1 to 0.6%, B: 0.0005 to 0.005%, N: 0.001 to 0.15%, S: 0.005% or less (not including 0%), and P: 0.05% or less (not including 0%), with the balance of iron and unavoidable impurities.

Description

TECHNICAL FIELD[0001]The present invention concerns a heat-resistant austenitic stainless steel used preferably as materials of heat transfer tubes such as for boilers and it particularly relates to a heat-resistant austenitic stainless steel having excellent cyclic oxidation resistance.BACKGROUND ART[0002]In recent years, for suppressing emission of carbon dioxide as greenhouse gas, improvement in the efficiency of coal-fired thermal power generation plants has been proceeded. For improving the power generation efficiency, it is effective to increase the temperature and the pressure of steam in boilers and, as the materials for heat transfer pipes of boilers, those having excellent high temperature strength and oxidation resistance are used. Further, austenitic stainless steels are used generally as such materials having excellent properties.[0003]The oxidation resistance required for the materials of heat transfer tubes include cyclic oxidation resistance. Since boilers are starte...

Claims

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Application Information

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IPC IPC(8): C22C38/58C22C38/50C22C38/48C22C38/00C22C38/42C22C38/04C22C38/02C22C38/54C22C38/44
CPCC22C38/58C22C38/001C22C38/002C22C38/005C22C38/02C22C38/54C22C38/42C22C38/44C22C38/48C22C38/50C22C38/04C21D6/004
Inventor MIYAMURA, TAKEONAMBA, SHIGENOBUFURUYA, KAZUKI
Owner KOBE STEEL LTD