Zhao Jiaxing,Liu Tao,Dong Yongchang

(China Special Equipment Inspection and Research Institution,Beijing 100029)

Abstract:In recent years, with the development of the world economy, the problem of energy consumption has gradually become prominent.Chinese residents need to consume a large amount of energy in their lives and work.In order to solve the energy problem,many power stations have been built in our country.However,utility boilers belong to high-risk infrastructure,which is unstable and relatively difficult to maintain.Therefore,in the process of construction of electric boilers,we should pay attention to the selection of materials,gradually improve the use technology of materials,improve the overall operation stability of utility boilers,and save energy and ensure the safety of power stationsso as to better serve the society.

Keywords:boiler metal;selection;use

Utility boilers are special equipment and are subject to the supervision of national safety supervision.In the selection of metal materials for utility boilers,we should strictly follow the relevant national standards and select materials that meet the specifications to avoid high usage of low materials.When the selected metal materials do not have pressure bearing capacity,serious safety accidents will occur,endangering the safety of people's lives and property.On the other hand,if high materials are used less,it will not only waste materials,but also cause certain potential safety hazards.For example,alloy materials used in carbon steel parts are also prone to hazards.Therefore,scientific selection of metal materials for utility boilers is of great significance for improving the safety and stability of utility boilers.

I.Brief Introduction to Metal Materials

The selection of metal materials for utility boilers is a complicated subject.Relevant maintenance and renovation staff of power stations should master the basic knowledge of metal materials and timely understand the technical development of metal materials at home and abroad.They should be flexible according to different situations and select practical materials to fundamentally improve the stability of utility boilers and provide convenience for maintenance and repair.

Skillful knowledge of metals is a necessary condition to improve the maintenance quality of utility boilers.Only by fully understanding the characteristics of metals can the maintenance quality of boilers be improved.The metal materials commonly used in utility boilers are mainly iron and steel,which are ferrous metals,mainly composed of iron elements,and are crystalline substances.The properties of iron and steel are mainly determined by the composition elements, content, type and microstructure of the composition elements.If the properties of iron and steel need to be improved,it can be heat treated to change its structure and make it suitable for the construction of utility boilers.

II.Application Position of Metal Material for Utility Boiler

Utility boilers are high-risk equipment.In our country's safety inspection,full consideration should be given to the daily maintenance details of boilers,especially in the material selection of components.If the performance of metal materials is mismatched,not only will the work efficiency be reduced,but also there are certain potential safety hazards.Serious accidents may occur,causing casualties.

In order to make the boiler play the greatest role in the use process,the choice of bearing materials inside the boiler must be careful.Selecting the appropriate bearing metal materials can not only better solve the tube explosion accident,but also avoid the waste caused by the limit reduction.In the selection of materials for utility boilers, suitable materials should be selected at different positions.If alloy materials are used to replace carbon steel pipes for connection,the risk factor of boiler explosion will be increased.In order to avoid too long aging time when the boiler is used,attention should be paid to whether the weld position is affected by the parent material.

In addition,the welding of the joint position is a technically difficult link,and the staff should pay special attention to it during the operation,because a little carelessness will increase the risk of boiler explosion.Technicians should not only carefully select the materials when welding metal joints,but also strictly follow the technical requirements and standards to fully exert the performance of the welding materials at the joint positions.

III.Reasonable Selection of Metal Materials

(i)Classification of Metal Materials

The use of metal materials in utility boilers belongs to the category of experimental applied science.Therefore,daily workers need to have a solid grasp of basic theoretical knowledge,make full use of applied chemical knowledge to improve work efficiency and ensure the correct selection of metal materials.Under normal circumstances,steel is a common metal material in utility boilers.It is a metal mixture composed of iron.Other metals are added in the manufacturing process.The steel properties produced according to different types and proportions are also different.At the same time,the properties of steel are also affected by temperature.The properties of steel will change after heat treatment.These changes can change the original metal structure and the function of metal materials.

(ii) Factors Affecting the Selection of Metal Materials

Iron and steel is a mixture with strong metallic properties.Iron is the main element constituting steel and other elements can be added in the forging process.Most of these elements are composed of carbon, followed by sulfur, phosphorus, silicon,manganese and other elements.As the addition of carbon and other elements can often increase the hardness of metal mixture,the corrosion resistance has also been greatly improved.The combination of carbon and other elements is the origin of carbon steel.If there are more other elements,it is called alloy steel,and the staff shall select metal materials with different properties according to different positions of utility boilers[1].

In the composition of steel, sulfur and phosphorus cannot change the properties of steel.In the forging process of steel,these two elements are often impurities.Theoretically,the quality of steel with low impurities is better than that of steel with high impurities.However, in actual forging, the content of impurities should be controlled within a certain range,and too much or too little impurities will have a great impact on the quality and hardness of steel.

(iii)Steel as Crystalline Substance

Steel plays an important role in our daily life.This crystalline substance contains carbon,which is divided into graphite, diamond and amorphous carbon in the form of isomers.The chemical properties of carbon elements are directly affected by the arrangement of carbon atoms.Different arrangement of carbon elements will also affect the crystal properties of formed crystal substances,which will directly lead to very striking differences in physical properties and mechanical properties of element isomers.For example,graphite and diamond have different spatial forms and different functions because of different crystal structures.Graphite has soft properties,not only conductivity,but also can be used to make pencils.Diamond,on the other hand,is often used as cutting material as the hardest substance in the world.

(iv)Composition Elements in Steel

After molten steel is melted into liquid, its constituent elements will generate unique crystals in the solidification process.This reconstituted steel will have obvious physical and chemical properties,which is similar to the isomerization of carbon elements in steel mentioned above.Steel materials used in boilers and pressure vessels can be decomposed into ferrite, austenite, pearlite,martensite and bainite through different heat treatments.

Promote the change of crystal structure,redistribute chemical composition, etc., and can adjust and improve the state of tissue structure to ensure and improve the service performance of the material.

IV.Use of Metal Materials

With the continuous development of energy construction in our country,the requirements for utility construction are gradually raised.Many utility boilers have raised the requirements and standards for raw materials.Some utility boilers use stainless steel materials, usually 18Cr8Ni series.These materials are difficult to process and can only be entrusted to professional factories for manufacturing.However, many stainless steel materials cannot achieve the required performance in heat treatment,and the whole or part of the materials do not meet the standards for solid solution heat treatment.In this case,the service life of the equipment is seriously reduced.For example,austenitic stainless steel is not strong in corrosion resistance and is vulnerable to corrosion during application.The main reason is that chromium element is generated in the internal carbon atoms during the exchange process with grain boundaries, resulting in the lack of chromium element in the stainless steel materials.Especially in high temperature environment,this reaction is more rapid at high temperatures above 400 degrees.Therefore, in actual operation, the temperature should be strictly controlled, although the high temperature residence time is as short as possible,to avoid chromium deficiency reaction.In addition,when welding,it is necessary to select small wire energy as much as possible so that the temperature control can be more accurate.

(i)Trend of Intergranular Corrosion Controlling

Intergranular corrosion seriously damages metal materials, so effective control of intergranular corrosion is necessary to ensure the safe operation of power plant boilers.In order to effectively control intergranular corrosion,staff can propose different use conditions according to austenitic stainless steel,adopt solid solution treatment, and improve the stability of materials by changing the treatment process.Secondly,passivation treatment can be used to reduce the surface activity of metal materials and oxidation with oxidizing acid to form an oxidation protective layer to enhance corrosion resistance.The working temperature of the utility boiler is above 600 degrees,which just reaches the sensitive temperature of the metal material.Intergranular corrosion can still occur in the temperature range of the solid solution treated material again,causing material damage and reducing safety.At this time,the treatment materials should be stabilized, carbon elements should be fixed,and transfer should be avoided,which can significantly improve the service life of the materials[2].During the operation of power plant boilers,austenitic stainless steel products are treated by local solid solution treatment.Theoretically,this treatment method is not scientific enough.Secondly,metal materials are treated by local heat treatment during solid solution treatment.Regardless of solid solution treatment or stabilization treatment,local heat treatment method may cause the material to be in a sensitive temperature range for a long time,thus enhancing the instability of the whole material.Once intergranular chromium deficiency occurs, the performance will decline,forming a potential quality hazard,and this part will be the first to fail during operation[3].

(ii) Metal Materials for High Temperature Section of Utility Boiler Selecting

When selecting the materials for the high temperature section of the power generation boiler,the heat treatment process with stability and integrity should be used,and the finished products of 18Cr—8Ni series stainless steel should be adopted.In order to increase the oxidation corrosion resistance,it is recommended to use the integral inner and outer wall passivation treatment before the heat treatment,followed by the integral heat treatment with stability,but increasing the process also increases the cost.

V.Conclusion

The selection of metal materials for power plant boilers is of great significance for improving the stability of power plant boilers.In the process of maintenance, it should be strictly carried out in accordance with the relevant technical supervision standards of our country.For different welding joints,scientific materials should be selected and strictly checked.In actual work,once the wrong materials are used,the welding joint will be corroded,which will reduce the work efficiency and seriously lead to safety accidents and endanger the safety of people's lives and property.Therefore,utility boiler retrofit personnel should fully understand the knowledge of various metal materials,flexibly select different metal materials,and timely deal with problems arising from actual work, so as to reduce the probability of accidents,improve the boiler operation stability and promote the development of China's power construction.