Common materials and quality requirements for stainless steel seamless pipes for machinery

Stainless steel seamless pipes (SS seamless pipe) for machinery are widely used in machinery and equipment, automobile manufacturing, chemical equipment, aerospace and other fields. Their quality requirements are strict. Common materials and quality requirements are as follows:

Common materials:

1. Austenitic stainless steel
304 (06Cr19Ni10): General type, corrosion-resistant, high temperature resistant, suitable for general mechanical structures.
316 (06Cr17Ni12Mo2): Contains molybdenum (Mo), resistant to acid, alkali and chloride corrosion, used in chemical and marine environments.

321 (06Cr18Ni11Ti): Contains titanium (Ti), resists intergranular corrosion, and is suitable for high temperature environments (such as heat exchangers).

2. Martensitic stainless steel
420 (20Cr13): Medium strength, wear resistance, used for tools, bearings, etc., but weak corrosion resistance.

3. Ferritic stainless steel
430 (10Cr17): Resistant to oxidizing acids, low cost, but poor toughness, used for general corrosion-resistant parts.

4. Duplex stainless steel
2205 (022Cr23Ni5Mo3N): high strength, stress corrosion resistance, suitable for harsh environments (such as petroleum, chemical equipment).

Quality requirements for seamless pipes for stainless steel machinery:

1. Chemical composition requirements
The chemical composition of seamless stainless steel mechanical tubes directly affects its corrosion resistance, mechanical properties and welding properties, and must comply with relevant standards (such as ASTM A312, EN 10216-5, GB/T 14976, etc.).

Key element content (taking 304 as an example):
Carbon (C): ≤0.08%
Chromium (Cr): 18.0%~20.0%
Nickel (Ni): 8.0%~11.0%
Sulfur (S), phosphorus (P): ≤0.03% (low impurities improve corrosion resistance)

Special requirements:
316 stainless steel needs to contain 2.0%~3.0% molybdenum (Mo) to improve chloride ion corrosion resistance.
321 stainless steel needs to contain titanium (Ti) ≥5×C% to prevent intergranular corrosion.

2. Mechanical properties requirements
The mechanical properties of seamless stainless steel mechanical tubes directly affect their load-bearing capacity and processing performance. The main parameters include: tensile strength (MPa), yield strength (MPa), elongation (%) and hardness (HB) that meet the corresponding standards.

Special requirements:
Low-temperature impact toughness (such as when used in low-temperature equipment, it must meet the ASTM A333 standard).
High-temperature strength (such as when 321 is used in a high-temperature environment, high-temperature creep performance must be guaranteed).

3. Dimensional accuracy requirements
The dimensional tolerance of seamless stainless steel mechanical tubes directly affects the assembly accuracy and processing performance, mainly controlled by:

Outer diameter tolerance: According to GB/T 17395 or ASTM A999, high-precision tubes can reach ±0.1mm.
Wall thickness tolerance: usually ±10%, high-precision requirements ±5%.
Straightness: ≤1.5mm/m, high-precision tubes ≤0.5mm/m.
Ovality: outer diameter ≤±1% (high-precision tubes are stricter).

4. Surface quality requirements
The surface of seamless stainless steel mechanical tubes should be smooth and flat, without obvious bumps, wrinkles, scratches and other defects. Common requirements:
Inner and outer surfaces: no cracks, folds, scratches, oxide scale and other defects, usually require pickling or polishing.

Surface treatment:
Pickling (AP): remove the oxide layer and improve corrosion resistance.
Polishing (BA/MP): mirror or matte treatment, suitable for high cleanliness occasions (such as food, medical equipment).

Special requirements:
Electrolytic polishing (EP): improve corrosion resistance, used in semiconductor and biomedical industries.

5. Process performance requirements
To ensure the reliability of seamless stainless steel mechanical pipes in processing and use, the following tests are required:

a. Flattening test 

Standard requirements: Flatten to H=2/3D without cracks   Applicable scenarios: Test plastic deformation capacity

b. Expansion test 

Standard requirements: Expansion rate ≥15% without cracks    Applicable scenarios: Test pipe end processing performance

c. Bending test 

Standard requirements: Bending 180° without cracks    Applicable scenarios: Test cold bending performance

d. Water pressure test 

Standard requirements: No leakage (pressure ≥ standard value)    Applicable scenarios: Test pressure bearing capacity

e. Non-destructive testing (UT/ET) 

Standard requirements: No excessive defects (such as cracks, pores)    Applicable scenarios: High-demand occasions (such as aviation, petrochemical)

6. Corrosion resistance requirements
The corrosion resistance of stainless steel pipes is a key indicator, and common test methods are:

Salt spray test (ASTM B117): Evaluate atmospheric corrosion resistance.
Intergranular corrosion test (GB/T 4334): Test the resistance to intergranular corrosion during welding or high-temperature use.
Pitting test (ASTM G48): Evaluate the resistance to chloride ion corrosion (such as 316 for seawater environment).

7. Special requirements (depending on the application scenario)
High cleanliness (semiconductor, food industry): control the impurity content, electrolytic polishing of the inner wall.
High temperature/low temperature application: additional testing of high temperature strength or low temperature impact toughness is required.
Welding performance: Low carbon stainless steel (such as 304L, 316L) is suitable for welding occasions.

Application scenario material selection recommendations:

General machinery: 304/316, emphasizing corrosion resistance and formability.
High-strength parts: 2205 duplex steel or martensitic steel.

High temperature environment: 321 or 310S (06Cr25Ni20).

The selection should be based on the working conditions (corrosive media, temperature, pressure) and cost, and the material report and test certificate should be strictly accepted.

Conclusion:

The quality requirements of stainless steel seamless pipes for machinery cover chemical composition, mechanical properties, dimensional accuracy, surface quality, process performance and corrosion resistance. Different application scenarios (such as chemical, aviation, and medical) require targeted selection of materials (such as 304, 316, and 2205) and strict testing. When purchasing, suppliers should be required to provide material reports (MTC) and test certificates (such as UT/ET reports) to ensure compliance with industry standards (such as GB, ASTM, EN, etc.).

Read more: Carbon Steel Mechanical Tube or Cold Drawn Seamless Tube for Mechanical & Hydraulic