I. Classification of Stainless Steel Pipes: System Construction in Multiple Dimensions

(1) Classification by Material Composition

The material differences of stainless steel pipes directly determine their performance. Austenitic stainless steel pipes are a widely used category, such as the 304 and 316 series. They are mainly composed of chromium-nickel alloys and have excellent corrosion resistance, good plasticity and toughness, and good low-temperature performance. They are suitable for food processing, medical devices, decoration, and other scenarios with high requirements for corrosion resistance and formability, such as kitchen water supply and drainage pipes and clean pipes in hospitals, where austenitic stainless steel pipes are often used.

Ferritic stainless steel pipes have a high chromium content (generally 12% - 30%) and low carbon content. They have good thermal conductivity and strong oxidation resistance, but their plasticity is relatively weak, and their weldability is poor. They are often used in building decoration and automotive exhaust systems, such as pipes for building exterior wall decoration, which use their UV resistance and atmospheric corrosion resistance to ensure long-term aesthetics; automotive exhaust pipes use their high-temperature oxidation resistance to adapt to the exhaust gas emission environment.

Martensitic stainless steel pipes have a relatively high carbon content. After heat treatment, their hardness and strength can be significantly improved, but their corrosion resistance is weaker than that of austenitic and ferritic stainless steel. They are commonly used in tools, valves, bearings, and other scenarios with strict requirements for hardness and wear resistance, such as industrial valve cores, which rely on high hardness to resist media scouring and wear.

There are also Duplex stainless steel pipes, which combine austenitic and ferritic phases. They have high strength and excellent corrosion resistance (especially stress corrosion resistance) and are widely used in harsh environments such as chemical and marine engineering, such as seawater desalination equipment pipelines, which cope with high-salt corrosion environments and ensure stable system operation.

(2) Classification by Production Process

Seamless stainless steel pipes are formed in one piece through hot rolling and cold drawing processes, without welds. They have high pressure resistance and high dimensional accuracy, and are the first choice for high-pressure, high-temperature, and high-precision transportation scenarios, such as high-pressure fluid transportation pipelines in the petrochemical field, which require seamless structures to ensure sealing and pressure resistance; precision guide tubes for aerospace use rely on the high precision of seamless pipes to meet equipment assembly requirements.

Welded stainless steel pipes (welded pipes) are made from steel strips through rolling and welding. They have high production efficiency and low cost. According to the welding process, they are divided into high-frequency welded pipes, argon arc welded pipes, etc. They are widely used in building water supply and drainage and general industrial fluid transportation (low-pressure environments), such as the domestic water supply network in residential areas, where welded pipes are a popular choice due to their cost advantages and adaptability.

(3) Classification by Use

Stainless steel pipes for fluid transportation emphasize corrosion resistance and sealing to ensure the stable transportation of water, oil, gas, and other media. They include drinking water pipes, gas pipes, and chemical media transportation pipes. Different media have different requirements for the material and hygiene standards of the pipes. For example, drinking water pipes must meet food-grade hygiene standards and often use 304 austenitic stainless steel pipes.

Structural stainless steel pipes are used to construct load-bearing structures and have high requirements for strength and toughness, such as building steel structure support pipes, bridge guardrail pipes, and mechanical structural component casing pipes. According to the structural design requirements, stainless steel pipes of different materials and specifications are selected. For example, large-span building steel structures may use high-strength Duplex stainless steel pipes or thick-walled seamless pipes.

Decorative stainless steel pipes emphasize appearance and surface quality. After polishing, wire drawing, and plating, they are used in building decoration (such as curtain walls and stair railings) and home decorations (such as furniture frames). Austenitic stainless steel pipes are often selected for their good formability and surface treatment adaptability to create a variety of decorative effects.

II. Hardness Testing of Stainless Steel Pipes: Principles, Methods, and Value

(1) Hardness Testing Principle

Hardness reflects the ability of a material to resist local deformation (such as indentation and scratches) and is a key indicator of the mechanical properties of stainless steel pipes. The test is based on the "indentation method." An indenter (such as a steel ball or diamond cone/pyramid) of a hardness tester is pressed into the surface of the pipe material under a specified test force, and the indentation size (area, depth, etc.) is measured and converted into a hardness value, indirectly reflecting the comprehensive properties of the material, such as strength, wear resistance, and plasticity.

(2) Common Testing Methods

• Brinell hardness test: A hard alloy ball or steel ball of a certain diameter is used to press into the surface of the pipe material with a corresponding test force. After holding for a specified time, the test force is removed, and the indentation diameter is measured. The Brinell hardness value (HBW) is calculated inversely proportional to the indentation surface area. It is suitable for stainless steel pipes with lower hardness and better plasticity (such as austenitic stainless steel pipes). It can intuitively reflect the overall hardness level of the material, but the indentation is large, and the damage to the pipe material surface is obvious. It is not suitable for thin-walled pipes or high-precision finished pipes.

• Rockwell hardness test: A diamond cone (HRA, HRC) or steel ball (HRB) indenter is used. Under the initial test force and the main test force, the difference in indentation depth is measured. The Rockwell hardness value (HR) is calculated according to the indenter type and scale. The test is efficient and the indentation is small. It is suitable for stainless steel pipes in different hardness ranges. For example, the HRC scale is commonly used for martensitic stainless steel pipes (high hardness), and the HRB scale can be used for austenitic stainless steel pipes (medium and low hardness). It is widely used in rapid on-line inspection and finished product quality control.

• Vickers hardness test: A regular tetrahedral diamond indenter is used to press into the surface of the pipe material under the test force, and the diagonal length of the indentation is measured to calculate the hardness (HV). The Vickers hardness test has a wide range of test forces and can test extremely thin pipe walls (such as below 0.1 mm) and small parts. It has high accuracy and can accurately distinguish subtle differences in material hardness. It is often used in scientific research analysis and quality testing of high-precision stainless steel pipes (such as pipes for aerospace use), but its testing efficiency is lower than that of Rockwell hardness, and it has high requirements for equipment and operation.

(3) Key Value of Hardness Testing

• Production quality control: In the production of stainless steel pipes, hardness testing is carried out throughout the entire process, from raw material entry to finished product delivery. By monitoring hardness, rolling and heat treatment process parameters are adjusted to ensure stable pipe material properties. For example, for hot-rolled stainless steel pipes, if the hardness is too high or too low, it reflects abnormal rolling temperature and deformation, and the process needs to be corrected in time to ensure product batch consistency.

• Scene Adaptation Verification: Different application scenarios have significantly different hardness requirements for stainless steel pipes. Duplex stainless steel pipes used in marine engineering require high hardness to ensure wear resistance and to withstand seawater erosion and marine organism attachment; austenitic stainless steel pipes used in medical devices require moderate hardness, balancing plasticity (easy to process and shape) and corrosion resistance (ensuring biocompatibility). Hardness testing provides a quantitative basis for scene adaptation, avoiding the risk of "mismatched materials".

• Failure Analysis Basis: When stainless steel pipes experience wear, cracking, or other failure issues during use, hardness testing can help analyze the causes. If the actual hardness of the pipe material is far lower than the design requirements, it may be due to insufficient material strength leading to deformation failure; if the hardness is abnormally high, it may be due to improper heat treatment causing brittle cracking, providing key clues for troubleshooting and quality traceability.

III. Industry Development and Technological Synergy

With the expansion of stainless steel pipe application scenarios (such as new energy and high-end equipment manufacturing), the requirements for classification accuracy and hardness testing technology continue to improve. In the future, the classification system will become more refined, combining microstructures and special performance requirements to create more specialized stainless steel pipes; hardness testing technology will develop towards intelligence and non-destruction, such as laser hardness testing and ultrasonic hardness testing, ensuring accuracy while reducing damage to the pipes and adapting to online testing and full-process quality control needs.

The classification and hardness testing of stainless steel pipes are not isolated links, but a technological chain that runs through material research and development, production, and application. Only by continuously cultivating the scientific nature of the classification system and improving the accuracy of hardness testing can stainless steel pipes, in diverse scenarios, both "play to their strengths" and "ensure quality control" to safeguard application safety, laying a solid metal "pipeline" foundation for industrial development and people's well-being.