The mechanical properties of steel pipes determine their overall operational performance. In fact, all steel products, from carbon steel fittings and flanges to low-temp fittings and flanges to high yield fitting and flanges, possess chemical and mechanical properties that strongly influence their level of performance. The quality of a pipe and/or pipe component is determined by its ability to resist physical stress and fatigue.
Standard pipe, API line pipe and API OCTG line pipe designers and engineers must be knowledgeable of potential physical stresses to properly assess how their constructions will perform during field use. They must pay attention to the mechanical and engineering behavior, as well as the physical and chemical properties, of the desired pipe product(s). Common mechanical properties of steel include yield strength, tensile strength, hardness, ductility and toughness.
The Yield Strength (YS) of steel is sometimes referred to as yield stress. Yield strength describes the maximum amount of stress at which a material permanently deforms, meaning it cannot return to its original shape once the stress is removed.
The Tensile Strength of steel is determined by the maximum stress load a material can endure before it breaks or ruptures. Materials that break without plastic deformation experience brittle failure. Yield strength and tensile strength is established in a laboratory using tensile testing machines.
Steel Hardness defines the material’s level of resistance to indentation, deformation and penetration. Causes of these damages can result from impact, scratching, abrasion, drilling and other operational wear. Hardness tests are non-destructive and easy to perform.
The Ductility of steel determines its ability to withstand tensile stress, or any force that pulls two ends of a material apart. Ductility measures the length a material will stretch before it becomes weaker, more brittle or ruptures.
The Toughness of steel is measured by its ductility under sudden force. It identifies how much the steel material can absorb energy and plastically deform without rupturing.
Operational conditions can also impact the physical degradation of a pipe and pipe components. Three load conditions that may be encountered during use in the field include: 1) external pressure, 2) internal pressure, and 3) axial tension.
1. External Pressure: A material’s Collapse Strength is its ability to resist external pressures.
2. Internal Pressure: A material’s Internal Yield Strength defines its ability to resist internal pressures or burst pressures.
3. Axial Tension: Axial tension is a stress that can elongate a material. The ability to resist axial tension is referred to as Body Yield Strength and Joint Strength (as associated with a particular location of concern).
There are published equations that help pipe designers and engineers predict the performance properties and performance advantages of various pipe products. Each performance property is directly influenced by yield strength and/or tensile strength, outside diameter and wall thicknesses. A pipe’s interior wall condition and remaining wall thickness determines its ability to resist burst, tension, and collapse. In addition, its thread condition indicates its resistance to leaks.
In addition to mechanical properties, pipe designers and engineers must also take into account the numerous circumstances and environmental factors that can affect the performance of a pipe and its components. Additionally, they must consider potential service-induced defects.