Some Questions Regarding Pressure Vessel Design (III)

This is based on the allowable stress ranges of the materials listed in GB150. For example, 0Cr18Ni9 can be used in a temperature range of -196 to 700 degrees, and there are corresponding allowable stress values for it.

In my opinion, devices within this range are relatively common, while those operating at high or extremely low temperatures are less so; such devices require some additional requirements beyond those that apply to normal conditions. It can also be seen from the materials provided in GB150 that most of them are conventional materials.

It is based on the fact that it is safe to use within this temperature range.

It is considered based on the allowable range of commonly used materials.

In my opinion, devices within this range are relatively common, while those operating at high or extremely low temperatures are less so; such devices require some additional requirements beyond those that apply to normal conditions. It can also be seen from the materials provided in GB150 that most of them are conventional materials. Agree with this view.

It is still considered based on the allowable stress of materials specified in GB150.

The design temperature specified in GB150 is determined with consideration for the mechanical properties of the material; beyond this temperature, the microstructure of the metal changes, which in turn results in changes in its mechanical properties. Above this temperature, design can be carried out using analytical design and experimental methods.

GB-150 is primarily a reference standard for conventional design, providing the corresponding allowable stress values at operating temperatures for common materials. If the temperature is too high or too low, it will cause changes in the properties of the material. Therefore, if the allowable temperature range of GB-150 is exceeded, analytical design and materials other than 150 are used.

This is determined by the material requirements of GB150; if the temperature exceeds the specified range, JB4732 may need to be used for the design

The design temperature specified in GB150 takes into account the mechanical properties of the material and the operating temperature; exceeding this temperature causes significant changes in the material’s performance. Above this temperature, analytical design methods can be used for design.

The design temperature specified in GB150 takes into account factors such as the mechanical properties of the material at its design temperature: (1) High-temperature performance. For design temperatures exceeding a certain limit, this limit is 700°C for 18-8 stainless steel, while it is lower for other common materials. For pressure vessels that operate at high temperatures for extended periods, it is necessary to evaluate the high-temperature endurance strength and creep strength of the steel. At room temperature, when a metal material is subjected to external forces, if the stress is below the elastic limit, the component undergoes only elastic deformation; if the stress reaches the yield limit, in addition to elastic deformation, the component also experiences some plastic deformation. These deformation values remain constant as long as the force applied does not change, and they do not alter over time. However, under high-temperature conditions, the situation is different: on top of the deformation caused by external forces, the amount of plastic deformation continues to increase over time. The longer the time passes, the greater the cumulative plastic deformation, until failure occurs. At a certain temperature, metal materials will undergo slow permanent plastic deformation over time even when the stress applied to them is below the yield point; this phenomenon is known as creep. For carbon steel, creep must be considered above 300°C. That is, the creep limit refers to the stress value at which a certain amount of total deformation occurs in the specimen within a specified period of use, at a given operating temperature. In practice, the conditional creep limit is defined as a total deformation of 1% after 100,000 hours ； Persistent strength of steel: The stress at which a steel material reaches a certain steady creep rate at a given temperature can be determined through creep tests. However, it cannot determine the time at which the material fractures at that temperature, nor the total deformation of the specimen at the moment of fracture. Furthermore, it cannot reflect the possible changes in the microstructure that may occur during the entire creep process of the specimen before fracture, and such changes in microstructure often accelerate the creep process, leading to earlier fracture of the specimen. In summary, since creep tests cannot directly reflect the fracture behavior of materials at high temperatures, it is insufficient to rely solely on the results of creep tests as a basis for the strength design of high-temperature components. For this reason, in recent years it has been replaced by a strength parameter that better reflects the failure characteristics of high-temperature components—the high-temperature endurance strength. (1) Low-temperature performance: At low temperatures, the failure of steel used in pressure vessels is generally due to the metal’s cold brittleness at low temperatures. That is, at certain low temperatures, metals quickly change from a ductile state to a brittle state. The main requirement for materials in low-temperature pressure vessels is the ability to resist cracking under operating temperature and load conditions, as well as the ability to prevent crack propagation once cracking occurs; the aim is to avoid brittle failure of the vessel due to low stresses. 18-8 stainless steel is a face-centered cubic lattice metal that retains a high degree of deformability even at extremely low temperatures, although there is a limit to this capacity; GB150 specifies this limit as -196°C. The main factors affecting the properties of low-temperature steel and its applications are the chemical composition of the steel, grain size, and heat treatment condition. Above this temperature, design methods such as analytical design can be employed.

Temperatures above or below this value fall outside the GB150 range; there may be separate design specifications for low-temperature or high-temperature vessels