Elongation of steel pipe: a key indicator for measuring toughness and safety
Date:2025-07-07
In the production, testing and application of steel pipes, elongation is always a crucial mechanical performance parameter. It intuitively reflects the ability of steel pipes to undergo plastic deformation before breaking, and is one of the core indicators for evaluating material toughness, processing performance and service safety.
1. Elongation: Definition and core significance
Elongation refers to the percentage of the plastic elongation of the gauge part of the steel pipe standard specimen after it is broken in the tensile test to the original gauge length. The calculation formula is:
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δ = [(L₁ - L₀) / L₀] × 100%
(Where: δ is the elongation; L₀ is the original gauge length of the specimen; L₁ is the gauge length of the specimen after it is broken.)
The core significance is reflected in:
Plastic deformation ability: The higher the elongation, the greater the plastic deformation that the steel pipe can withstand before breaking, and the material is "softer" and "tougher".
Safety assurance: When the structure is accidentally overloaded, high elongation materials can absorb energy through significant deformation, issue warnings (such as obvious necking), and avoid catastrophic brittle fracture.
Forming performance: For steel pipes (such as automotive oil pipes and furniture pipes) that need to be bent, expanded, flanging and other cold processing, high elongation is a prerequisite for smooth processing and no cracking.
Toughness characterization: Elongation is closely related to impact toughness and is an indirect reflection of the material's ability to resist crack propagation.
2. Determination of elongation: standards and processes
The determination of elongation strictly follows international or national standards (such as GB/T 228.1, ISO 6892-1, ASTM E8/E8M):
Sampling and sample preparation: Cut standard tensile specimens from the specified position of the steel pipe (usually the longitudinal direction of the pipe body), retain the original surface or process it into a specified shape and size.
Gauge length marking: Accurately mark the original gauge length L₀ on the parallel section of the specimen (commonly used proportional gauge lengths such as 5.65√S₀, S₀ is the original cross-sectional area).
Tensile test: The specimen is stretched at a constant speed on a universal material testing machine until it breaks.
Broken butt measurement: Carefully butt the two broken specimens tightly together and measure the post-broken gauge length L₁.
Calculation: The elongation δ is calculated using the formula. The gauge length must be indicated in the report result (e.g. δ₅ represents the elongation at a gauge length of 5 times the diameter or 5.65√S₀).
3. Key factors affecting the elongation of steel pipes
Chemical composition: Basic elements (C, Si, Mn, P, S) have a significant effect. An increase in carbon (C) content usually reduces elongation; high content of impurity elements such as phosphorus (P) and sulfur (S) deteriorates plasticity. Alloy elements such as Cr, Ni, and Mo need to optimize the ratio to maintain sufficient elongation while improving strength.
Microstructure: Pure and uniform fine-grained structures (such as ferrite) usually have higher elongation. Coarse grains and excessive hard and brittle phases (such as coarse cementite, martensite, and bainite) will reduce plasticity.
Manufacturing process:
Rolling: The hot rolling final rolling temperature and deformation amount affect the grain size and microstructure uniformity.
Heat treatment: Annealing, normalizing, quenching + tempering and other processes are key means to control the microstructure and optimize the matching of strength and plasticity. For example, normalizing treatment can usually refine the grains and increase the elongation; as the tempering temperature increases, the elongation often increases accordingly.
Cold working: Although cold rolling and cold drawing can increase strength, they will cause work hardening and significantly reduce the elongation. Annealing is often required to restore plasticity later.
Sample direction: The elongation of the longitudinal (along the rolling direction) sampling is usually higher than that of the transverse (perpendicular to the rolling direction) because of anisotropy.
Test conditions: Temperature (low temperature reduces elongation), loading rate, etc.
4. The core role of elongation in steel pipe applications
Processing formability:
Bending: High elongation steel pipes are not prone to cracking on the outside or wrinkling on the inside when bent.
Flaring/flanging: When flaring or flanging the pipe ends, sufficient elongation is the basis for ensuring crack-free forming.
Cold drawing/cold rolling: The raw pipe must have good plasticity to withstand subsequent large deformation processing.
Structural safety:
Overload resistance: In structures such as buildings, bridges, and pressure pipes, high elongation gives the structure the ability to redistribute stress through plastic deformation (rather than instantaneous fracture) under over-design loads, thereby improving overall safety.
Seismic performance: Under dynamic loads such as earthquakes, the plastic deformation capacity (high elongation) of the material is crucial to absorbing seismic energy.
Avoid brittle fracture: Especially in low temperature or stress concentration areas, sufficient elongation is an important guarantee for resisting low-stress brittle fracture.
Quality judgment basis: Elongation is a mandatory mechanical property requirement item in steel pipe product standards (such as API 5L, GB/T 8162, GB/T 3091, ASTM A53/A106). If it does not reach the lower limit of the standard, it will be judged as a defective product.
V. Examples of requirements for elongation in typical application scenarios
High-pressure boiler tubes/petroleum cracking tubes (such as ASTM A106 Gr.B, GB 5310): require a higher elongation (usually δ₅ ≥ 30%) to ensure good creep resistance and safety under high temperature and high pressure.
Seamless steel pipes for structures (such as GB/T 8162): δ₅ ≥ 24%, to ensure structural bearing and seismic plasticity.
Welded steel pipes for fluid transportation (such as GB/T 3091): δ₅ ≥ 20-23%, to meet general processing and service safety.
Scaffolding pipe (such as GB/T 13793): δ₅ ≥ 14%, meeting basic plasticity and safety margin.
Precision steel pipe for deep processing (such as hydraulic cylinder): The elongation requirement is extremely high (δ₅ is often required to be ≥ 35% or even higher) to ensure the smooth progress of complex cold processing (such as high-power spinning).
