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Austenitic heat-resistant steel in boiler applications, the problems and preventive measures


Mechanism of austenitic stainless steel under high temperature long-running role of water vapor from the superheater and reheater tube wall oxidation corrosion, combined with the matrix generated dense oxide layer,[link widoczny dla zalogowanych], the unit parameters, in particular, the unit deactivated prone to oxidation and then start off the skin, accumulate in the elbow or into the steam system. High temperature, the austenite formed on the surface oxidation products were Fe. O a FeCrO a CrO. . On the one hand,[link widoczny dla zalogowanych], oxidation products can lead to deterioration of heat transfer performance of the tube; the other hand, there is the growth of oxide boundary effects, which over time will reach the critical oxide thickness,[link widoczny dla zalogowanych], oxide then began to fall off. Loss is mainly due to the thermal expansion coefficient of oxide and substrate vary greatly, when the operating conditions change, the temperature changes induced by thermal stress may cause local spalling oxide, the oxide scale will be small size, high-speed steam flow into the steam system the rest will be deposited under the action of gravity or accumulate in the lower part of the tube bend, making the heating surface tubes in the flow area reduction tube in the lower part of the steam flow, causing localized overheating of the tube. Deposition serious. Can lead to elbow plug. 3.2 The case of high temperature steam oxidation of a power plant maintenance shutdown boiler 4, found during the high temperature superheater tubes (TP304H, ~ 57mmx9mm) after running about 40000h 150Z. The existence of heating surface oxide peeling management problems, a serious threat to security and stability of the unit. Microscopic analysis showed that the steam side of the residual oxide thickness is 0.19mm, with a double layer structure, two layers there is a wide separation between the cracks. There spalling tendency. Within the oxide layer and metal substrate is not continuous between the interface and into the uneven size of the multilateral shape. Ni Bu TP304H other C of Department of austenitic stainless steel high temperature steam oxidation is the main factor influenced alloys with Cr content and grain size, in general, operating conditions, Cr 22% mass fraction of the oxide layer of steel generation rate is very slow; fine grain high-temperature steam oxidation resistant material is superior to coarse-grained material, in the same conditions, the grain size of 9-class steel grain size of oxide thickness of about 5 to 25 grade steel %. 3.3 Preventive measures to prevent the oxidation of high temperature steam oxidation of the skin peeling of the most important measure is the correct selection, remain over-temperature conditions and to mitigate the operating temperature of 36 units * rate. For the existence of oxide spalling and plug the boiler, the experience can be improved through the following. a. Better use of antioxidant properties of the material. Various materials in the oxidation and spalling somewhat different, Cr content of more than 22% of the Austrian entities, stainless steel and fine grain material has a relatively high antioxidant capacity, reduce the occurrence of spalling oxide, such as the newly developed TH347HFG, HR3C and the T91 and other materials commonly used in high-temperature heating surface oxidation resistance than other materials, the design should be preferred. b. Establish long-term monitoring system of heating surface tubes, including regular oxide measurement, cutting pipe inspection, wall temperature measurement and monitoring. Shutdown the opportunity to use the inner wall heating surface oxide peeling off to inspect the situation and as soon as possible to find out the law. C. During operation, good combustion adjustment, to minimize bending and weld positions, such as the thermal deviation. d. During peak load operation and load movements, strict control of changes in rates of compliance and reduce the oxide spalling. e. Boiler shutdown process,[link widoczny dla zalogowanych], try to take a lower rate of temperature drop; shutdown after the boot process, try to take a faster launch speed, can reduce the formation of oxide deposition the possibility of blockage. 4 Conclusion Department of austenitic heat resistant Cr-Ni stainless steel wide range of applications, and has done a very good cold forming, welding, thermal strength and excellent resistance to gas side corrosion resistance, intergranular corrosion in use , stress corrosion, high temperature steam corrosion, the main problem is its failure. Therefore, in utility boilers should be based on specific environmental conditions, in the selection, design and workmanship to avoid some problems, as long as the proper measures, many problems can be avoided. References: [1] Chen Jigang, Chen Jie, Liang Feng. Austenitic steel boiler tubes in the grain boundary stress corrosion cracking and prevention measures [A]. National Thermal Power Research Institute of Materials Research Corporation. Failure Analysis of key components of thermal power plants and the whole process of life management Proceedings CC3. Beijing: China Electric Power Press ,2000.181-186. (23 Guo Lifeng, Wei Yanxiao .18-8 water vapor oxidation of austenitic stainless steel failure analysis [J]. North China Electric Power Technology, 2005, ( :21-23. [3] Chen Minguang. Of oxide formation prevention and mitigation and peeling method [J]. power construction, 2003,24 (9) :18-19. (Editor Haozhu Jun)
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