738 Structural Steel: Properties, Applications, Manufacturing Guide

738 structural steel is a high-performance alloy designed for heavy-duty load-bearing applications, blending strength, ductility, and workability. Unlike standard carbon steels, its tailored chemical composition (with alloying elements like chromium and molybdenum) delivers enhanced mechanical properties, making it a top choice for construction, automotive, and heavy equipment industries. In this guide, we’ll break down its key traits, real-world uses, manufacturing processes, and how it compares to other materials—helping you select it for projects that demand reliability under stress.

1. Key Material Properties of 738 Structural Steel

The performance of 738 structural steel starts with its balanced chemical composition, which shapes its robust mechanical properties, reliable physical properties, and practical workability.

Chemical Composition

738 structural steel’s formula is optimized for strength and toughness, with key elements including:

• Carbon content: 0.20-0.25% (balances strength and weldability—higher than low-carbon steels but low enough to avoid brittleness)
• Manganese content: 1.20-1.50% (boosts tensile strength and hardenability without reducing ductility)
• Silicon content: 0.20-0.40% (aids in deoxidation during manufacturing and improves strength)
• Phosphorus content: ≤0.035% (controlled to prevent brittleness, especially in cold environments)
• Sulfur content: ≤0.035% (minimized to maintain ductility and avoid cracking during forming)
• Additional alloying elements: Chromium (0.40-0.60%, enhances corrosion resistance), nickel (0.40-0.60%, improves toughness), and molybdenum (0.15-0.25%, boosts high-temperature strength)

Physical Properties

Mechanical Properties

After standard heat treatment (normalizing or quenching and tempering), 738 structural steel delivers impressive load-bearing performance:

• Tensile strength: 650-750 MPa (higher than standard carbon structural steels like A36)
• Yield strength: 450-550 MPa (2x higher than A36, reducing material thickness for structural parts)
• Elongation: 18-22% (in 50 mm—retains ductility to absorb impact, critical for earthquake-resistant buildings)
• Hardness: 180-220 Brinell, 80-90 Rockwell B, 190-230 Vickers (soft enough for machining, hard enough for load-bearing)
• Fatigue strength: 300-350 MPa (at 10⁷ cycles—ideal for parts under repeated stress, like bridge beams or vehicle frames)
• Impact toughness: 60-80 J (at -20°C—resists cracking in cold weather, unlike brittle high-carbon steels)

Other Critical Properties

• Weldability: Excellent—low carbon and controlled alloy content allow welding via MIG, TIG, or stick methods without preheating (critical for on-site construction).
• Machinability: Good—softer than high-strength alloys like titanium; uses standard high-speed steel (HSS) tools with minimal wear.
• Formability: Very good—can be pressed, bent, or rolled into complex shapes (e.g., curved bridge beams) without cracking.
• Corrosion resistance (relative to carbon steel): Moderate—better than plain carbon steel (thanks to chromium) but requires coating (e.g., galvanizing) for marine or harsh environments.
• Ductility: High—deforms plastically under load before failing, making it safe for structural applications where sudden failure is catastrophic.

2. Real-World Applications of 738 Structural Steel

738 structural steel’s blend of strength, ductility, and workability makes it ideal for applications that carry heavy loads or face variable conditions. Here are its most common uses:

Construction Industry

• Structural beams: Floor and roof beams in high-rise buildings use 738—its high yield strength allows thinner beams, reducing building weight and material costs.
• Columns: Load-bearing columns in commercial buildings (e.g., malls, offices) rely on 738—handles vertical loads of up to 500 kN without buckling.
• Bridges: Highway and railway bridges use 738 for girders—its fatigue strength resists stress from repeated vehicle traffic, and ductility absorbs earthquake energy.
• Buildings: Seismic-resistant buildings in earthquake-prone areas (e.g., California) use 738—its impact toughness prevents collapse during tremors.

Case Example: A construction company used 738 structural steel for a 20-story office building in Tokyo. Compared to standard A36 steel, 738 beams were 20% thinner, reducing overall building weight by 15% and cutting concrete usage (for foundations) by $300,000. The building also passed seismic tests with minimal deformation.

Automotive Industry

• Vehicle frames: Heavy-duty trucks and SUVs use 738 for chassis frames—its strength supports heavy payloads (up to 10 tons) and resists bending from rough roads.
• Suspension components: Truck suspension arms use 738—fatigue strength handles repeated bumps, and ductility prevents breakage during off-road use.
• Axles: Commercial vehicle axles (e.g., delivery trucks) use 738—tensile strength withstands torque from engines, and impact toughness resists pothole damage.

Mechanical Engineering & Heavy Equipment

• Mechanical engineering: Machine frames for industrial presses and manufacturing lines use 738—stiffness minimizes vibration during operation, and machinability allows precise mounting of components.
• Gears and shafts: Large industrial gears (e.g., in factory conveyors) use 738—hardness resists wear, and toughness prevents tooth breakage.
• Heavy equipment:
• Excavators: Bucket arms and boom structures use 738—handles digging loads of up to 20 tons and resists impact from rocks.
• Cranes: Crane booms use 738—high strength-to-weight ratio allows lifting heavy loads (up to 100 tons) without bending.
• Mining equipment: Mine haul truck frames use 738—resists corrosion from mine water (with galvanizing) and handles payloads of 50 tons.

Marine Industry

• Ship structures: Small cargo ship hulls and deck beams use 738—with galvanizing or painting, it resists saltwater corrosion better than plain carbon steel.
• Offshore platforms: Small offshore oil rig support structures use 738—its strength handles wave loads, and weldability allows on-site assembly.

3. Manufacturing Techniques for 738 Structural Steel

Producing 738 structural steel requires precision to maintain its chemical balance and mechanical properties. Here’s the process:

1. Metallurgical Processes (Alloy Control)

• Electric Arc Furnace (EAF): The primary method—scrap steel, iron ore, and alloying elements (chromium, nickel, molybdenum) are melted at 1,600-1,700°C. Sensors monitor chemical composition to ensure carbon, manganese, and alloys meet target ranges.
• Basic Oxygen Furnace (BOF): For large-scale production—molten iron from a blast furnace is mixed with scrap steel, then oxygen is blown to adjust carbon content. Alloys are added post-blowing to fine-tune properties.

2. Rolling Processes

• Hot rolling: The molten alloy is cast into slabs (200-300 mm thick), heated to 1,100-1,200°C, and rolled through a series of mills to form shapes like beams (I-beams, H-beams), plates, or bars. Hot rolling improves grain structure and strength.
• Cold rolling: Used for thin sheets (e.g., for automotive frames)—cold-rolled at room temperature to improve surface finish and dimensional accuracy. Cold rolling increases hardness slightly, so annealing may follow to restore ductility.

3. Heat Treatment (Tailoring Strength)

• Normalizing: Heated to 850-900°C and held for 30-60 minutes, then cooled in air. This refines grain size, balances strength and ductility, and is the most common heat treatment for structural applications.
• Quenching and tempering: For high-strength needs—heated to 820-860°C (austenitizing), quenched in water to harden, then tempered at 550-600°C to reduce brittleness. This boosts tensile strength to 750 MPa (used for heavy equipment axles).
• Annealing: Heated to 700-750°C and cooled slowly—softens the steel for complex forming (e.g., curved bridge beams) or machining.

4. Forming and Surface Treatment

• Forming methods:
• Press forming: Uses hydraulic presses (1,000-5,000 tons) to shape beams or columns into custom profiles (e.g., tapered columns for modern buildings).
• Bending: Uses roll benders to create curved shapes (e.g., bridge arches)—738’s ductility allows bending to radii as small as 5x the material thickness.
• Welding: On-site welding of structural parts (e.g., beam-to-column connections) uses MIG welding with low-carbon filler metal—no preheating needed, saving construction time.
• Machining: CNC mills and lathes shape precision parts (e.g., gear shafts)—uses HSS or carbide tools with cutting fluid to prevent overheating.
• Surface treatment:
• Painting: Industrial paint (e.g., epoxy) is applied to construction steel to prevent rust—common in buildings or inland bridges.
• Galvanizing: Hot-dip galvanizing (zinc coating) is used for marine or outdoor steel (e.g., ship hulls, offshore platforms)—provides corrosion resistance for 20+ years.
• Shot blasting: Blasts steel with tiny metal beads to remove scale and rust—improves paint/galvanizing adhesion.

5. Quality Control (Structural Safety Focus)

• Ultrasonic testing: Checks for internal defects (e.g., cracks, voids) in thick beams or columns—critical for load-bearing parts.
• Radiographic testing: Inspects welds for flaws (e.g., porosity, lack of fusion) in bridge or building connections—ensures welds can carry load.
• Tensile testing: Verifies tensile strength (650-750 MPa) and yield strength (450-550 MPa) to meet 738 specifications.
• Microstructure analysis: Examines the alloy under a microscope to confirm uniform grain structure—no brittle phases (e.g., martensite) that could cause failure.
• Impact testing: Conducts Charpy V-notch tests at -20°C to ensure impact toughness (60-80 J)—critical for cold-weather or seismic applications.

4. Case Study: 738 Structural Steel in Heavy-Duty Crane Booms

A construction equipment manufacturer used standard A572 Grade 50 steel for crane booms. The booms weighed 800 kg and could lift 50 tons—too heavy for mobile cranes, which needed lighter booms to improve maneuverability. They switched to 738 structural steel, with the following results:

• Strength & Weight: The 738 boom was 15% thinner (from 20 mm to 17 mm) and weighed 680 kg (15% lighter) while lifting the same 50 tons—thanks to 738’s higher yield strength.
• Performance: The lighter boom reduced the crane’s total weight by 1.2 tons, improving fuel efficiency by 8% and allowing access to narrow construction sites (where heavier cranes couldn’t fit).
• Cost Savings: The manufacturer saved $500 per boom in material costs (less steel used) and received 20% more orders for the lighter, more agile cranes.

5. 738 Structural Steel vs. Other Materials

How does 738 structural steel compare to other common structural materials? Let’s break it down with a detailed table:

Application Suitability

• High-Rise Buildings: 738 is better than A36/A572 (thinner beams, lower weight) and cheaper than HSLA or composites.
• Mobile Cranes: 738 outperforms A572 (lighter for same strength) and is far cheaper than aluminum or composites.
• Bridges: 738 is superior to A36 (better fatigue strength for traffic) and more cost-effective than HSLA for most projects.
• Marine Structures: 738 (with galvanizing) is cheaper than aluminum and easier to weld than composites, though aluminum has better corrosion resistance.

Yigu Technology’s View on 738 Structural Steel

At Yigu Technology, we see 738 structural steel as a cost-effective workhorse for heavy-duty applications. Its balanced strength, ductility, and weldability make it ideal for our construction, automotive, and heavy equipment clients. We often recommend 738 for high-rise beams, crane booms, and seismic-resistant structures—where it cuts material costs (thinner profiles) and simplifies on-site welding. While it needs coating for harsh environments, its low cost and reliable performance deliver better value than premium HSLA or composite materials, aligning with our goal of sustainable, practical solutions.

FAQ

1. Can 738 structural steel be used in cold weather?

Yes. 738 has excellent impact toughness (60-80 J at -20°C), so it resists cracking in cold climates (e.g., northern Europe, Canada). It’s commonly used for bridges, buildings, and heavy equipment that operate in sub-zero temperatures.

2. Does 738 structural steel need to be coated for outdoor use?

Yes. While 738 has better corrosion resistance than plain carbon steel, it still rusts in rain, snow, or saltwater. For outdoor applications (e.g., bridges, outdoor cranes), use painting (epoxy) or galvanizing (zinc coating) to extend its lifespan to 20+ years.

3. How does 738 compare to HSLA steel for structural projects?

738 has similar yield strength to HSLA Grade 65 but is 10% cheaper and easier to weld (no preheating needed). HSLA has slightly better corrosion resistance, but 738 is a better choice for most construction/heavy equipment projects—offering similar strength at lower cost and simpler fabrication.