In cement production line and industrial kiln system, the material of expansion joint of tertiary air duct is directly related to the operation stability and service life of equipment. As the key channel connecting the decomposition furnace and the kiln head, the tertiary air duct has been enduring the severe working conditions of high temperature, high dust and high wind speed for a long time. Once the material of its expansion joint is improperly selected, it is easy to lead to cracking, deformation and even unplanned shutdown. This paper will systematically explain how to scientifically select the material of tertiary duct expansion joint from four dimensions: working condition characteristics, material performance comparison, typical failure cases and type selection suggestions.

First, why is the material of tertiary duct expansion joint so critical?

The medium temperature in the tertiary air duct is usually as high as 850℃ – 1100℃, the wind speed is 20 – 35m/s, and it contains a large amount of high-temperature clinker dust. High-temperature oxidation, intergranular corrosion and fatigue cracking can quickly occur in ordinary metal bellows in this environment. The material of the expansion joint of the tertiary air duct not only needs to have excellent high temperature strength, but also must withstand the erosion and erosion of corrosive gases such as alkali, sulfur and chlorine. Once the material deteriorates, the expansion joint may fail within a few weeks, causing hot air leakage, increased energy consumption and even production line paralysis. Therefore, controlling material standards from the source is the core measure to reduce the maintenance cost of the whole life cycle.

2. Material classification and performance comparison of commonly used tertiary air duct expansion joints

At present, the mainstream materials of tertiary duct expansion joints in the industry can be divided into three categories: heat-resistant alloy steel, composite lining structure and new ceramic coated metal.

1. Heat-resistant alloy steel series

• 304/316L stainless steel: It is only suitable for working conditions where the temperature is lower than 600℃. It will quickly oxidize and peel off in the high temperature area of the tertiary air duct, so it is not recommended.
• 310S (0Cr25Ni20): contains 25% chromium, 20% nickel, can operate at 1000℃, short-term tolerance of 1050℃. Good oxidation resistance, but average resistance to chlorine and sulfur corrosion.
• Inconel 625/601: Nickel-based alloy, containing molybdenum, niobium and other elements, which still maintains high strength and corrosion resistance at 1100℃, is the preferred material for high-end expansion joints, but the cost is high.

2. Composite lining structure

In order to take into account both cost and performance, many manufacturers adopt the composite scheme of "metal bellows + internal insulation layer + wear-resistant castable". The outer metal is 309S or 310S with moderate price, and the inner lining is laid with high aluminum fiber blanket or low cement castable with a thickness of 50 – 100mm to reduce the temperature of the metal body below 700℃. This structure can greatly reduce the reliance on top-grade alloys, but attention should be paid to the anchoring mode and construction quality of the lining material to prevent direct exposure of the metal due to shedding.

3. Ceramic Coating and Ceramic Fiber Module

Spraying zirconia-based ceramic coatings on the surface of 310S or RA330 substrates can improve the surface's resistance to thermal shock and dust scour. Another innovative scheme is to adopt all-ceramic fiber module expansion joint, which has no metal parts and has a temperature resistance of 1300℃, but its pressure bearing capacity is low, so it is suitable for micro-negative pressure conditions.

III. Typical failure cases reveal material selection traps

A cement production line with an annual output of 2 million tons, its tertiary air duct expansion joint was originally made of 310S material with a thickness of 2.5mm. After only 8 months of operation, multiple penetrating cracks appeared at the bellows trough. Analysis reasons: the alkali content of clinker dust is high in actual working conditions, and low melting point eutectic is formed at 1050℃, which accelerates intergranular corrosion; At the same time, stress concentration is caused by geometric abrupt change in the trough, and the combined action of thermal fatigue and corrosion leads to early failure. After that, it was replaced with Inconel 625, which was made of the expansion joint of the tertiary air duct, and the thickness was increased to 3.0mm, and the lining castable was added. It has been running stably for 26 months without abnormality. This case shows that the main cause of failure is simply relying on the grade and ignoring the details of working conditions (such as dust chemical composition and temperature fluctuation amplitude).

Fourth, how to scientifically choose the material of tertiary duct expansion joint?

Based on GB/T 12777 and practical experience in the cement industry, it is recommended to follow the following decision-making process:

1. Specify the working condition parameters: maximum continuous temperature, instantaneous overtemperature amplitude, dust concentration and chemical composition (alkali, sulfur and chlorine content), wind speed, axial/transverse compensation requirements.
2. Economic trade-off: Inconel 625 or RA330 is preferred if the temperature is> 1000℃ and contains corrosive components; If the temperature is 850 – 1000℃ and the alkali equivalent in the dust is low, 310S + lining castable is a more cost-effective solution.
3. Structural details optimization: The bellows should adopt multi-layer thin-walled structure (such as two layers of 1.2mm instead of a single layer of 2.5mm), and each layer deforms independently to improve the fatigue resistance life. At the same time, a guide tube is added to avoid high-speed dust directly washing the root of the bellows.
4. Supplier verification: It is required to provide third-party high-temperature durable strength test reports and use cases of similar production lines to avoid the impersonation of low-quality "non-standard materials".