Casing Material Properties DefinitionIn order to understand strengths of casing tubular materials, it is important to understand the basic terminology and process of manufacture of such materials.
Yield StrengthThe strength is the material property of a steel which is usually indicated by its minimum yield strength or ultimate tensile strength. Casing and tubing are manufactured mostly from ductile steels. Whereas brittle steels fracture without appreciable deformation, ductile steels can withstand significant plastic deformation prior to fracture.
Basic Stress-Strain EquationsStress and strain are common terms used in describing strengths of materials.
If a tensile load (or force) is applied to a test sample cross-sectional area, then the tensile (or axial) stress is found by:
Stress = Force / Area
Axial strain is defined as the ratio of the test sample axial elongation to the original length of the sample:
Axial Strain = Axial Elongation / Original Length
Hooke's Law defines stress as the product of the elastic constant or Young's modulus of elasticity (E) and strain:
Stress = E x strain
Young's Modulus for steel is typically 30 x 10^6 psi.
- Point 'A' represents the yield strength or elastic limit of the steel. If the steel is stressed below the elastic limit, it will return to its original shape upon unstressing or unloading the test sample. Below the elastic limit, the stress-strain curve is linear.
- Point 'B' : The API specifies that the yield stress (yield strength) is the tensile stress required to produce a total elongation of 0.5% of the tensile test sample length. This is shown by point 'B' in the diagram. Stresses greater than the elastic limit cause permanent deformation of the steel and the steel will not return to its original shape when the load is taken away.
- Point 'C': If a steel is stressed beyond its yield strength, it will deform plastically until its ultimate strength is reached as shown by point 'C'. The ultimate strength is the maximum stress that the steel can sustain before it begins to fail. Beyond this point the material will continue to deform plastically (with a reducing stress) until complete failure (breakage) occurs as shown by point 'D'.
Hardness of SteelHardness is a material property which is the measure of a steel's yield point in compression. When a material is required to resist wear, corrosion, erosion or plastic deformation, it may be necessary to specify a specific hardness.
This material property generally increases with increasing material ultimate tensile strength. Very hard materials are brittle and will crack or fracture easily. Hardness is determined by a test where a load is applied with a small ball or pointed object.
The hardness property of the material is then expressed by the depth of the indentation caused by the pointed object. The "Rockwell C", "Brinell", or “Charpy” hardness scales are used to quantify the degree of hardness of an oilfield tubular material. Hardness can be expressed by a Charpy Impact Test, where a weighted pendulum is dropped onto a sample and the amount of impact it takes to break the sample is measured. This amount of impact must exceed a minimum standard.
Mechanical properties of steel such as yield stress, ultimate tensile strength, ductility, or hardness can be achieved by
- Controlling the heat treating portion of the manufacturing process.
- Controlling the chemical composition of the steel.
Heat TreatingHeat treating affects changes in the microstructure, or grain structure of the steel which directly affects its mechanical properties. Heat treating is an operation involving heating and/or cooling the solid steel tubular to develop the desired steel microstructures.
The five basic heat treatments are:
Quenched and TemperedThe steel is heated to 1500-1600 degree F. It is then rapidly quenched (or cooled) in water
or oil to produce a desired microstructure. It is then tempered (or re-heated) at 1000-1300 degree F to produce a desired combination of strength and ductility. This is the preferred method of producing high strength casing and tubing.