Hot gas circulation test of zirconium-titanium steel composite plate

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To evaluate the long-term service performance of the zirconium or titanium welds in composite panels, a hot gas cycling test can also be conducted. Advanced foreign countries commonly use hot gas cycle tests for comparison, but the situation in China is exactly the opposite. It is rarely used mainly due to the high difficulty and cost of hot air circulation tests. Hydrostatic tests conducted at room temperature on titanium equipment, especially those with titanium linings, often do not reflect the conditions under high temperatures. This is because titanium and steel expand to different degrees at operating temperatures, which results in an additional thermal stress σ1 at the welds. At high temperatures, the sum of this thermal stress σ1 and the stress generated by the operating pressure σ2 may exceed the allowable stress for the welds, that is, σ1+σ2＞[σ]. The magnitude of σ1 depends on factors such as the structural design of the welded joint, the shape of the weld root, and the number and nature of any defects in the welds. The purpose of the hot gas cycle test is to evaluate the strength and stress levels of the weld under operating temperature and pressure conditions. Typically, this test is repeated twice, which can effectively compensate for the limitations of pressure tests at room temperature. It is of great importance for ensuring the quality of titanium equipment. With the rapid development of the petrochemical industry, rare and refractory precious metals such as titanium, zirconium, tantalum, and niobium are increasingly used in the manufacture of pressure vessels. To save costs, thick-walled pressure vessels are usually manufactured using composite plates. Due to the significant difference in properties between the cladding and the base layer of composite panels, certain simulation tests are required to evaluate the performance of composite panel containers. The hot gas circulation test is a simulation method that has been widely used in recent years for testing pressure vessels made of composite panels. There are currently no unified standard specifications for conducting hot gas circulation tests. Recently, I saw that the NB/T 47011-202X Zirconium Pressure Vessels (draft for review) specifies the requirements for hot gas cycle tests. The hot gas circulation test should be conducted after the pressure resistance test is passed and before the final leak test. Operation of the hot gas circulation test: The hot gas circulation test generally uses dry and clean air, nitrogen, or other inert gases as the testing medium. For the hot gas circulation test, combinations of design temperature and design pressure or operating temperature and operating pressure can be used as test parameters ； The sequence and rate of pressure increase, temperature rise, pressure decrease, and temperature drop should be determined in accordance with the requirements for operating the container. Among them, the heating and cooling rates should not be lower than the requirements for container operation. For multi-chamber containers, the test parameters should be determined based on the principle that the coating stress during testing is equivalent to the stress during operation. Steps for the hot gas circulation test: First, heat the test container to the specified test temperature at a prescribed rate of temperature increase, and maintain that temperature for a specified period of time ； Then, the test medium is filled into the test container at the specified pressure increase rate until the test pressure is reached ； Maintain at the test pressure and test temperature for a specified period of time ； First, release the test medium to atmospheric pressure (or slightly positive pressure) at the specified pressure reduction rate ； Then, cool the test container to room temperature at the specified cooling rate. During the cooling process, it is necessary to prevent a negative pressure from forming inside the container, as this could cause damage to the structure.