Chemical material types and general performance

The abbreviation of the engineering materials required for the construction of chemical plants. The chemical machinery, chemical instrumentation, pipelines and structures that make up the chemical production facility are operated at different temperatures, pressures and mechanical loads, and the materials in contact with them are highly corrosive. Therefore, in addition to the properties of general engineering materials, chemical materials should also have excellent corrosion resistance. If the corrosion resistance is poor, it will not only directly affect the life of the device, but it may also cause fire, explosion and other accidents, as well as affect the product's output and quality. In addition, depending on the application and the conditions of use, chemical materials are sometimes required to have special properties such as high or low temperature resistance, heat conduction, or heat insulation. Due to the different materials and process conditions in chemical production, there are many types of chemical materials, and the models and specifications are complex. They include both general engineering materials and materials with special properties and special purposes that are unique to chemical plants.

Type Chemical materials are divided into two major categories: metallic materials and non-metallic materials. Chemical metal materials can be divided into black metal materials and non-ferrous metal materials. Black metal materials mainly refer to iron and steel, and most chemical machinery and equipment are made of cast iron and carbon steel. High-alloyed irons, high-alloyed steels (such as high-silicon-iron, high-nickel-iron, and various types of stainless steel) and nickel, copper, aluminum, titanium, zirconium, and their alloys are also widely used in chemical production.

The use of non-metallic materials in chemical industry has become increasingly widespread, mainly including plastics, rubber, glass, ceramics, enamels, impervious graphite, and the like. Rapid development of plastics, excellent corrosion resistance, the most widely used; impermeable graphite heat and acid, also has a high thermal conductivity, the amount is also more; ceramics, glass and other acid resistance is good, but crisp, can not be made Large-volume equipment, so its limited use, but can be used to rake, lining the surface of the metal, made of corrosion-resistant enamel equipment, enamel (or lining) glass tanks, tanks and so on.

Between the metal and non-metal, there is also an amorphous alloy, a structure with a supercooled liquid, a strong corrosion resistance, and some close to the niobium and precious metals, and is a developing material.

In order to take advantage of the advantages of different materials, a variety of composite materials have also been developed. Among them are: 1 metal and metal composites, such as sandwich plates, using high-strength, low-corrosion materials (such as carbon steel, aluminum alloy) as the bottom layer, resistant to The highly corrosive materials (such as pure aluminum, nickel, titanium, stainless steel, etc.) are used as the surface layer for the contact corrosion media. 2 Non-metal and non-metal composites, such as reinforced plastics. 3 Non-metal and metal composites, such as alloy ceramics, have both strength and high temperature resistance.

Selection principles Due to different conditions of use, the selection principles of chemical materials mainly include:

The mechanical properties of the material The chemical machinery generally receives a certain pressure, tensile force or impact force, so the chemical material must first meet the requirements of mechanical properties, mainly tensile strength, yield strength, impact strength, creep strength, elastic modulus, elongation Rate, hardness, fatigue limit and so on. In addition, chemical machinery operates in highly corrosive environments and mediums. For example, the strength standard value (or nominal strength, ie, the average value of tests in air) in the manual may sometimes be damaged when the stress is far below the maximum value. This is especially true in environments where stress corrosion cracking or corrosion fatigue may occur (see Corrosion and Corrosion Prevention). Therefore, consider the critical strength value of stress corrosion cracking in environments where stress corrosion cracking may occur.

Corrosion resistance of materials Different materials have different corrosion resistance for various corrosive environments. The so-called excellent corrosion resistant materials are only relative concepts. For example, stainless steel is generally considered to be superior to ordinary carbon steel corrosion resistant materials, but this is only in atmospheric, water and oxidizing environments; in reducing acids, both are also corroded; in more than 70% concentration * * Carbon steel is better than stainless steel; in water containing chloride ions, austenitic stainless steels may produce stress corrosion cracks, whereas carbon steels do not have this danger. The corrosion data of various materials in various environments (acids, alkalis, salts, gases, inorganics, organic solvents, atmosphere, soil, water, seawater, industrial products, foods, vegetable oils, etc.) are usually included in the manual. If the manual does not have the required data, a corrosion test should be performed.

The standard for evaluating the corrosion resistance of metals is divided into four grades according to the size of the uniform corrosion rate (see corrosion resistance grades of the table metals and alloys). For precision parts and high-voltage, hazardous equipment, the first-class material with a very low corrosion rate is used. In general containers and equipment, two or three grades of materials are available, and the wall thickness is appropriately increased to ensure a long life. For metallic materials, not only uniform corrosion but also local corrosion should be observed. When the manual states that this “material-environment” system has the potential to cause pitting, intergranular corrosion, and stress corrosion cracking, avoid using it or take necessary protective measures.

Non-metallic materials have not yet been well evaluated. For glass, ceramics, graphite, etc., metal standards can be used. The applicable standards for plastics and rubbers can refer to the following figures: the decrease in bending strength should be less than 25%; the change in weight or size should be less than 5%; the change in hardness (Rockwell M) should be less than 30%.

In the material selection, it is also necessary to consider whether the reaction between the material and the environment will have an adverse effect on the environment or production, such as whether it will pollute the product quality, affect the process flow, cause adverse reactions, reduce the yield, and even cause fires, explosions, etc. For example, although the corrosion rate of some materials is very low, it can degrade the color, aroma, and taste of products, and even produce toxicity (such as trace lead compounds). This cannot be used for industries such as food, medicine, fiber, and cosmetics.

Material processing properties such as casting, cutting, welding, forging, and heat treatment should meet the requirements. Welding performance is particularly important for chemical equipment.

General physical properties of the material such as density, melting point, thermal conductivity, coefficient of thermal expansion, etc. should also be considered. For example: to use heat-conductive materials to make heat exchangers; use materials with the lowest thermal conductivity as insulation materials.

Economic Benefits The purpose of selection is to avoid premature damage to equipment and buildings and to ensure the normal operation of production. This is essentially an economic problem. It is hoped that the materials chosen will have the best corrosion resistance and the lowest price, but the materials with the strongest corrosion resistance are very expensive. It is usually necessary to comprehensively consider factors such as material prices, equipment life, maintenance intervals, maintenance costs, lost production, accidents, and scrap values ​​to select the most suitable material. General short-term operation of the intermediate test equipment, small, intermittent operation equipment and easy to replace parts, can choose the material with low price and corrosion resistance; large-scale continuous production equipment, it requires long life, less maintenance, more use of price High and corrosion resistant materials.