Classification and Standard Analysis of Boiler Stainless Steel Pipes

Boiler steel pipes can be divided into two categories according to their manufacturing methods: seamless steel pipes and electric welded steel pipes; and from the perspective of materials, they include ferritic steel and austenitic stainless steel. In addition, these boiler stainless steel pipes have different uses, such as piping and heat transfer pipes, making their types more diverse. It is worth noting that among all these categories, only austenitic stainless steel seamless pipes are specifically used for heat transfer pipes. In the JIS standard, the Cr content of stainless steel must reach more than 11%, and its grade is represented by SUS plus a numerical serial number.

 

 

Heat transfer pipe function and standard

 

Heat transfer pipes, also known as heat exchange pipes, have the core function of transferring the combustion heat of the boiler to the water flowing inside the heat exchange pipes. Pipes are designed specifically for transporting water and steam and do not participate in the heat exchange process. These two types of steel pipes follow different standards in Japan: the standard for heat transfer pipes is JIS G3463, i.e., "stainless steel pipes for boilers and heat exchangers", while the standard for piping follows JIS G3459, i.e., "stainless steel pipes for piping". Although both standards include general stainless steel SUS304 (18C-8Ni austenitic stainless steel), the two belong to different standard categories.

 

Piping functions and standards

 

Piping is used to transport water and steam and follows the JIS G3459 standard. Although there are overlaps with the heat transfer pipe standard, they belong to different uses. Although there are overlaps in some aspects, such as material selection, their design and manufacturing need to be determined separately according to different working conditions to ensure their reliability and safety.

 

Reasons why ferritic stainless steel is not applicable

 

Characteristics and defects of ferritic stainless steel

Stainless steel is mainly divided into ferritic stainless steel and austenitic stainless steel. Ferritic stainless steel has a body-centered cubic lattice, while austenitic stainless steel is a face-centered cubic metal, which is characterized by the addition of Cr and more Ni and Mn. At room temperature, iron containing C forms a ferrite structure, but in order to obtain austenite, at least 10% Cr and Ni must be incorporated into the steel.

 

The characteristics of ferritic steel include rapid atomic diffusion at high temperatures (about 10 times that of austenitic steel), excellent thermal conductivity, and low thermal expansion. However, it also has some disadvantages, such as precipitation embrittlement, insufficient toughness, and low strength (especially high-temperature creep strength) under high-temperature conditions. Creep refers to the deformation and fracture of metals under load due to high temperature over time, which usually begins to appear when the steel exceeds 400°C.

 

High-temperature environment requirements

 

Boiler Stainless Steel Pipe needs to work in a harsh environment with a maximum steam temperature of about 600°C and a pressure of about 25MPa, so their high-temperature strength, especially creep strength, is extremely high. Unfortunately, ferritic stainless steel containing more than 11% Cr does not perform as well as austenitic stainless steel in this regard. In addition, the rapid atomic diffusion of ferritic stainless steel under a high-temperature environment may lead to the formation of the σ phase (50Fe-50Cr), which reduces the strength of the steel and increases brittleness.

 

However, the rich Ni, N, and Mn elements in austenitic stainless steel can effectively inhibit the formation of the σ phase. Since boiler heat transfer tubes need to have excellent high-temperature structural stability and high-temperature strength, ferritic stainless steel is not suitable for this application scenario.