Introduction to the Yield Strength of Steel Pipes
Date:2025-07-28
1. Yield strength: the watershed from elasticity to plasticity
Core definition: Yield strength refers to the stress value corresponding to the beginning of significant plastic deformation (i.e., the deformation cannot be fully recovered after unloading) of steel pipe materials when subjected to tensile or compressive loads. It marks the transition point from elastic behavior (recoverable deformation) to plastic behavior (permanent deformation) of the material.
Engineering significance: In structural design, yield strength is regarded as the limit of the material's bearing capacity. Once the stress on the steel pipe exceeds its yield strength, even if it does not break, it will produce irreversible deformation (such as bulging, bending), resulting in structural instability, functional failure, and even catastrophic accidents. Therefore, it is the safety benchmark for engineering design and the main basis for material selection.
2. Key factors affecting the yield strength of steel pipes
The yield strength of steel pipes is by no means fixed. It is shaped by the nature of the material and the manufacturing process:
Chemical composition of the steel itself:
Alloy elements: Carbon is the most effective strengthening element (but it reduces toughness and weldability). Elements such as manganese, silicon, chromium, molybdenum, vanadium, and niobium significantly increase strength by solid solution strengthening, carbide formation, or grain refinement. For example, Q345 (16Mn) steel is stronger than Q235 steel, and the key lies in the manganese content.
Impurity control: Harmful impurities such as sulfur and phosphorus weaken the grain boundary bonding force and usually need to be strictly controlled.
Microstructure:
Grain size: Refining grains is the most effective way to improve strength and toughness at the same time (following the Hall-Page relationship). Modern metallurgical technology (such as controlled rolling and controlled cooling TMCP) uses this principle to produce high-strength steel pipes.
Phase composition: Ferrite is softer, while pearlite (especially cementite), bainite, and martensite have higher strength. High-strength organization can be obtained by heat treatment (such as quenching + tempering) or controlled rolling and controlled cooling process.
Manufacturing and processing technology:
Cold working (cold rolling/cold drawing): The yield strength is significantly improved by work hardening (increased dislocation density). For example, the strength of cold-drawn precision seamless steel pipes is much higher than that of hot-rolled ones. However, it should be noted that cold working reduces plasticity and toughness, and may introduce the Bauschinger effect (yield strength decreases during subsequent reverse loading).
Heat treatment: Normalizing, quenching + tempering and other processes can optimize the organization and obtain the required strength and toughness combination. Quenching and tempering treatment is a common method for producing high-strength steel pipes (such as Q690D for engineering machinery booms).
Welding: The heat-affected zone of welding undergoes complex thermal cycles, which may cause the organization to coarsen or form hard and brittle organizations (such as martensite), so that the yield strength of this area may be higher or lower than that of the parent material, accompanied by a significant decrease in toughness, which is a weak link in the structure.
Size effect (wall thickness):
For thick-walled steel pipes, differences in cooling rates during rolling or heat treatment may lead to uneven microstructure and properties at different locations of the cross section (the core cools slowly and the strength may be lower than the surface). Different yield strength requirements are usually specified in the standard according to the wall thickness range.
Temperature:
As the temperature increases, the atomic activity increases and the yield strength decreases significantly. High temperature environments (such as boiler tubes and thermal pipelines) must consider the high temperature yield strength of the material (usually called the "yield point" or the specified non-proportional extension strength RP0.2).
At low temperatures, some steels may undergo a ductile-brittle transition, with little change in yield strength, but the fracture mode may change from ductile to brittle, which increases the danger dramatically.
Loading rate:
In general, increasing the loading rate will slightly increase the yield strength (strain rate strengthening effect), but the effect is relatively small in conventional engineering analysis.
