Material Selection for Square and Rectangular Tubes

The material choice for square tubes and rectangular tubes directly determines their performance and price. Common materials include plain carbon steel, low-alloy steel, and stainless steel. The material selection for square and rectangular tubes should be determined based on the specific application and process requirements.This article discusses common material types for square and rectangular tubes and key considerations when selecting them.

Common Material Types for Square  Tubes and Rectangular Tubes:

1. Plain Carbon Steel

This is the most common and lowest-cost type, used for general structures and non-critical components.

Q235: China's most commonly used plain carbon structural steel, widely used in construction, machining, and other fields. "Q" stands for yield strength, and "235" indicates a yield strength of at least 235 MPa.
Q195 / Q215: Lower in strength and price than Q235, used for lightweight and decorative structures with lower requirements.

ASTM A36:

The most commonly used carbon structural steel in the US standard, with a yield strength of 250 MPa (approximately 36 ksi), equivalent to China's Q235 grade, but with slightly different chemical composition requirements.

2. High-Strength Low-Alloy Steel

Adding a small amount of alloying elements to carbon steel significantly increases its strength and is used in critical load-bearing structures.
Q355 (formerly known as Q345): China's most popular high-strength structural steel, with a yield strength ≥355 MPa. It offers excellent overall performance, high strength, and good weldability. It is a pillar material for steel structures, bridges, and vehicles.

Corresponding International Standards:

American Standards: ASTM A500 Grade B/Grade C (specifically for structural tubing such as square and rectangular tubing), or ASTM A572 Grade 50.

Europe: S355 (EN 10219 standard).

Q390, Q420, Q460:
Higher grades of high-strength steel, used for heavier-load structures such as large stadiums, high-rise buildings, and heavy machinery.

3. High-Quality Carbon Structural Steel

20# Steel (AISI 1020):
High-quality low-carbon steel, with a carbon content of approximately 0.20%. It is the most classic and commonly used low-carbon steel. Its strength is comparable to Q235, but it offers greater plasticity and toughness, and a purer composition.
It is primarily used for shafts, bolts, and structural components. The number represents the average carbon content in parts per thousand.

4. Stainless Steel

Used in environments requiring corrosion resistance, hygiene, or aesthetics.

304 (0Cr18Ni9):
304 stainless steel is the most commonly used stainless steel, offering excellent corrosion resistance, heat resistance, and low-temperature performance.
Applications: Food equipment, kitchen utensils, medical devices, outdoor decoration, and coastal environments.

304L (00Cr19Ni10):
An ultra-low-carbon version of 304, offering enhanced resistance to intergranular corrosion and suitable for components that cannot be heat treated after welding.

316 (0Cr17Ni12Mo2):
316 stainless steel has molybdenum added, offering greater pitting and acid and alkali resistance than 304, making it particularly suitable for chemical and marine environments.

201 / 202:
Nickel-saving stainless steels with poor corrosion resistance and rusting, but are inexpensive. Mostly used for interior decoration where rust resistance is not a high requirement.

Key Points to Consider When Choosing the Right Material for Square and Rectangular Tubes:

1) Identify the use environment

1. Indoor dry environments: such as office furniture and indoor shelving. Minimum corrosion resistance is required. Choose ordinary carbon steel (such as Q235) with ordinary spray paint or electroplated galvanizing.
2. Outdoor atmospheric environments: such as building guardrails and awnings. These must withstand exposure to sunlight and rain. Choose hot-dip galvanized steel, high-strength steel (such as Q355), or weathering steel.
3. High humidity/coastal environments: such as coastal facilities and ship components. These are highly corrosive. Choose hot-dip galvanized steel with sprayed plastic or stainless steel (such as 304 or 316).
4. Chemically corrosive environments: such as chemical plants and electroplating plants. Choose stainless steel (316L is preferred) or special alloys based on the specific corrosive medium.
5. High and low temperature environments: such as cold storage and heating pipes. Consider the material's low-temperature toughness and high-temperature strength and select the appropriate alloy steel grade.

2) Determine Mechanical Property Requirements

Determine the material's strength grade based on the structure's design load.

1. General load-bearing/non-load-bearing structures: such as decorative panels and protective covers. Q235 or A36 are suitable for lower costs.
2. Major load-bearing structures: High-strength steel is essential, such as building beams and columns, equipment frames, and vehicle chassis.
3. Structures subject to dynamic loads or impact: Steel with good toughness and a good impact energy (AkV) value is required, such as crane booms and construction machinery.

3) Evaluate Processing Requirements

Will the material be welded or bent?

Weldability: For structures requiring extensive welding, choose steel with a low carbon equivalent. Carbon equivalent (Ceq) is a key indicator of steel weldability. A higher Ceq increases the likelihood of cold cracking during welding and complicates the process.

Cold bending: The material must exhibit good ductility and toughness.

4) Compliance with Standards and Regulations

Industry Standards: Industries such as construction, bridges, and pressure vessels have mandatory material standards that must be adhered to.
International Standards: If your product is being exported, it must meet the standards of the target market, such as ASTM A500 in the United States and EN 10219 in Europe.
Certification Requirements: Certain industries (such as shipbuilding and nuclear power) may require specific material certification (such as classification society certification).

5) Balancing Costs and Budgets

Material Unit Price: Stainless Steel > Weathering Steel > Alloy Steel > Ordinary Carbon Steel.

Cost Comparison:
Initial Cost: Ordinary carbon steel with anti-corrosion treatment may be cheaper than stainless steel.
Maintenance Cost: Stainless steel or weathering steel is virtually maintenance-free, while ordinary carbon steel requires regular painting and maintenance. In difficult-to-maintain applications (such as high-rise structures), choosing high-end materials is more economical.
Processing Cost: High-strength steel or stainless steel may require specialized welding processes and tooling, increasing processing costs.

6) Appearance and Aesthetic Requirements

Visible Structures: If the rectangular tube is exposed and aesthetics are important, such as shopping mall guardrails or decorative components, choose stainless steel, galvanized and plastic-sprayed tubes, or weathering steel with a sophisticated surface finish.
Concealed Structures: For internal frames, basic rust prevention (such as primer) is sufficient.

How to Calculate the Weight of Square Tubes？

Calculating the weight of a rectangular tube is crucial for costing, structural design, and logistics.

Calculation Formula:
Weight per Unit Length (kg/m) = [(Girth) × Wall Thickness × 7.85] / 1000
The formula is [Volume × Density], where the density of steel is 7.85 g/cm³.

Convenient Tools and Actual Weight:
Theoretical Weight Table: Almost all steel suppliers provide a theoretical weight table for rectangular tubes. You can directly refer to the table based on the specifications.
Online Calculators: There are numerous free steel weight calculators available online; simply enter the dimensions to obtain the result.

Note:
The calculated weight is a theoretical weight. Actual weight may vary slightly due to manufacturing processes and tolerances.

Purchasing and settlement are generally based on the actual weighed weight, but the theoretical weight is an important basis for budgeting and design.

Read more: ERW Square Tube & Rectangular Tube or Square Tubing Size Selection