Welded Steel Pipe

Forming Process: Welded steel pipe is formed by directly rolling steel plates or strips through a forming machine into the shape of a straight seam steel pipe. Generally, the flat steel is first edge-processed, such as beveling, and then bent into a cylindrical shape by a press or roller, so that the two sides of the steel plate are aligned to form a straight seam.

Welding method: When welding straight seams, high-frequency straight seam welding (ERW) or submerged arc welding (SAW) is often used. High-frequency straight seam welding uses the resistance heat generated by high-frequency current on the edge of the steel plate to melt the edge and weld it together under pressure. Submerged arc welding is achieved by covering the welding area with flux, allowing the welding wire and steel plate to melt and fuse under the action of the arc. The flux protects the molten pool and participates in metallurgical reactions.

Weld characteristics: A straight weld runs along the longitudinal direction of the steel pipe, resulting in a regular, simple weld shape. High-frequency straight welds result in a relatively small heat-affected zone and a smooth, neat weld appearance. However, since the weld is linear, the stress distribution at the weld is relatively concentrated when subjected to internal pressure, especially in the case of large diameter and high pressure, which places extremely high demands on the weld quality.

Spiral Welded Pipe

Forming Process: Spiral welded pipe is made from a coiled steel strip. At room temperature, the coiled steel strip is formed into a tube blank using a spiral forming machine at a specific angle (helix angle). As the strip steel moves forward continuously, it is curled and welded at the same time, ultimately forming a steel pipe with a spiral weld.

Welding method: The welding mainly employs double-wire double-sided submerged arc welding. Two welding wires are welded on both sides of the steel pipe at the same time. This method can improve welding speed and welding quality. During welding, the arc, under the protection of the flux, melts and combines the welding wire and the edge of the strip to form a continuous spiral weld.

Weld characteristics: Spiral welds are spirally distributed along the steel pipe. Compared to straight seam steel pipe, the stress concentration at the spiral weld is lower under the same pressure. However, the shape of spiral welds is more complex, and it is slightly more difficult to detect and control the quality than straight welds.

1. Shape: The outer surface of round steel is smooth and circular or tubular, while the outer surface of threaded steel has spiral ribs.
2. Manufacturing process: Round steel is usually made through casting or rolling processes, while rebar is extruded through a die into a shape with spiral ribs during the hot rolling process.
3. Strength: Round steel has relatively low strength and is usually used in the manufacture of building structures and mechanical parts, while threaded steel has higher strength and is usually used in occasions such as bridges and high-rise buildings that require higher load-bearing capacity.
4. Application: Round steel is usually used for reinforcement in reinforced concrete structures, while threaded steel is usually used for connection and reinforcement in steel structures.

1. Different shapes:
Round steel has a smooth outer surface.
Rebar has spiral ribs on its outer surface.

2. Different production standards:
In current standards, round steel refers to HPB235 grade rebar, and its production standard is "Hot-rolled plain steel bars for reinforced concrete" (GB13013).
Rebar generally refers to HRB335 and HRB400 grade rebar, and its production standard is "Hot-rolled ribbed steel bars for reinforced concrete" (GB1499).

3. Strength Differences:
The design strength of round steel (HPB235) is 210 MPa.
Rebar has higher strength than round steel. The design strength of HRB335 is 300 MPa, and the design strength of HRB400 is 360 MPa.

4. Different steel grades (chemical compositions):
Round steel (HPB235) is a carbon steel, grade Q235.
Rebar is a low-alloy steel. HRB335 grade rebar is 20MnSi (20 manganese silicon); HRB400 grade rebar is 20MnSiV, 20MnSiNb, or 20MnTi, etc. 5. Different physical and mechanical properties:
Due to differences in the chemical composition and strength of steel bars, their physical and mechanical properties vary. Round steel has better cold bending performance and can be made into 180-degree hooks, while rebar can only be made into 90-degree straight hooks. Round steel has better weldability and can be made with ordinary carbon welding rods, while rebar must use low-alloy welding rods; rebar has better toughness and fatigue resistance than round steel.

Special-shaped seamless steel pipes refer collectively to seamless steel pipes with cross-sectional shapes other than circular. These pipes are widely used in various structural components, tools, and mechanical parts. Compared to circular pipes, special-shaped pipes generally possess larger moment of inertia and section modulus, offering greater resistance to bending and torsion. They better accommodate the specific demands of usage conditions, significantly reducing structural weight, conserving steel resources, and enhancing labor productivity in component manufacturing.