Yield Strength, Tensile Strength and Elongation: What the Mechanical Test Reports

Yield strength is the stress at which a steel stops deforming elastically and begins to deform permanently; tensile strength is the maximum stress the steel carries before it necks and breaks; and elongation is the permanent stretch of the test piece at fracture, expressed as a percentage of its original gauge length. The three are measured in one tensile test and reported together on every mill test certificate, in megapascals for the two strengths and as a percentage for the elongation. For hot-finished annealed 304 bar the specified minimum values are 205 MPa yield, 515 MPa tensile and 40 percent elongation; for a hardened and tempered alloy steel such as EN19 they are about 680 MPa, 850 MPa and 13 percent. Every such figure belongs to one product form, one size range and one delivery condition in one standard, and moving any of those three moves the figure. Designers size a part on the yield strength, because a part that has yielded has changed shape, and use the tensile strength and elongation to judge the margin between yielding and fracture.

The four values defined

Yield strength is the stress at which plastic deformation begins. Mild steel shows a distinct yield point: the load drops slightly and the piece stretches at constant stress before it hardens again, and the upper yield stress, written ReH, is reported. Stainless steels, alloy steels and most non-ferrous metals show no such point; the stress-strain curve bends smoothly from the elastic line into plastic flow, and a proof stress is reported instead. The 0.2 percent proof stress, Rp0.2, is the stress at which the permanent strain reaches 0.2 percent of the gauge length, found by drawing a line parallel to the elastic slope offset by 0.2 percent and reading where it crosses the curve. On a stainless steel certificate the yield strength entry is always this value, and for austenitic grades the 1.0 percent proof stress Rp1.0 is sometimes reported beside it because their curve bends so gradually.

Tensile strength, Rm, also called ultimate tensile strength, is the maximum load reached in the test divided by the original cross-section of the piece. Beyond it the piece necks, the cross-section falls faster than the steel hardens, and the load drops until fracture. Elongation after fracture, A, is the increase in the gauge length measured with the two broken halves fitted together, as a percentage of the original gauge length; it is a measure of ductility and it depends on the gauge length chosen, which is why the standard states it. Reduction of area, Z, is the fall in cross-section at the neck as a percentage of the original, a second measure of ductility that is less sensitive to gauge length. Hardness and impact energy are the other mechanical values a certificate carries, obtained by separate tests: hardness from an indenter, impact from a notched bar broken by a pendulum, usually at a stated temperature.

How the tensile test is done

The test is defined by ISO 6892-1 in Europe and by ASTM A370 and E8 in North America, and the two are close enough that a certificate to either is accepted worldwide. A test piece is machined from the bar: for round bar a cylindrical piece with a reduced parallel length and threaded or plain ends, taken from the mid-radius or, on small bar, the full section. The gauge length is marked on the parallel portion. ISO 6892-1 uses a proportional gauge length of 5.65 times the square root of the cross-section, which is five times the diameter of a round piece; ASTM A370 uses a fixed 50 mm (2 in) gauge on a 12.5 mm piece or four times the diameter. Because elongation rises as the gauge length shortens, a value measured on the ASTM gauge is a little higher than the same steel measured on the ISO gauge, and the two are not compared without conversion.

The piece is gripped in a testing machine and pulled at a controlled rate: a strain rate in the elastic and yield region, about 0.00025 per second under ISO 6892-1 method A, so that the yield value is not raised by loading it too fast, and a faster rate after yield to fracture. An extensometer on the gauge length records strain against the load cell's force, and the machine's software reads the proof stress from the curve, the maximum force for the tensile strength, and the operator measures the broken piece for elongation and reduction of area. The whole test takes a few minutes; the machining of the piece takes longer than the pull.

Specified minima by grade, form and condition

Each row is the minimum a standard requires for one product form in one size range and one delivery condition. A row does not carry over to another form, another size or another condition of the same grade.

Elongation in the ASTM rows is measured on a 50 mm gauge length, in the BS 970 and IS 2062 rows on the proportional gauge of 5.65 times the square root of the cross-section, and the two are not compared without conversion. The editions the table draws on are named in the sources. A standard is revised periodically and an order is filled to the edition it cites, so a value is verified against the edition in force rather than taken from a table of this kind. The strengths in the BS 970 rows fall as the ruling section rises, because a thicker bar hardens less deeply.

These are minima and not expected results. A test on a real bar exceeds them, and hot-finished annealed 304 round bar commonly certifies at 250 to 320 MPa proof stress and 550 to 650 MPa tensile with elongation above 50 percent. The minimum is what the purchaser can rely on and what the designer sizes against; the certified value belongs to that heat and that size alone and is not a property of the grade.

Units and conversions

The SI unit of stress is the pascal, and steel strengths are stated in megapascals, MPa. One megapascal is one newton per square millimetre, so MPa and N/mm² are the same unit written two ways, and certificates in either are read directly. North American standards state strength in thousands of pounds per square inch, ksi: 1 ksi is 6.895 MPa, so 515 MPa is 75 ksi and 205 MPa is 30 ksi, which are the figures ASTM A276 prints. Older European documents use kilograms-force per square millimetre, kgf/mm², where 1 kgf/mm² is 9.807 MPa; 52 kgf/mm² is 510 MPa. Elongation is a percentage in every system, but the gauge length behind it differs, so the certificate states it.

The yield to tensile ratio

The ratio of yield strength to tensile strength describes how much reserve a steel has between the onset of permanent deformation and fracture. Annealed austenitic stainless has a low ratio, about 0.4, which means the part deforms a long way and work hardens before it breaks; this is what makes the grades forgiving in a collision or an overload. Hardened and tempered alloy steels and precipitation hardened stainless have ratios of 0.8 to 0.9: they carry high loads without yielding but give little warning before fracture, so the design margin on them is set with more care. Structural design codes limit the ratio, IS 800 and Eurocode 3 both require it to be no higher than a stated value for the steel to be treated as ductile.

Designers use the yield strength, not the tensile strength, to size a part, because a part that has yielded has taken a permanent set and is out of tolerance even though it has not broken. A shaft, a bolt or a bracket is loaded to a fraction of its yield strength set by the safety factor, and the tensile strength enters only in the check against fracture and in the fatigue estimate, where the endurance limit of steel is roughly half the tensile strength.

Cold work and the yield strength of bright bar

Cold drawing raises the yield strength of a bar far more than its tensile strength. A drawn 304 bright bar in the as-drawn condition can certify at 400 to 600 MPa proof stress, two to three times the annealed minimum, with the tensile strength up by a smaller margin and the elongation down to 20 to 30 percent. The gain is real and it is used, in threaded bar and shafting where a higher yield permits a smaller section. It is also the reason a bright bar certificate must state the condition: the same grade annealed after drawing returns to the Condition A minima in the table, 205 MPa for cold-finished bar over 12.70 mm. Where the annealed properties are required, the bar is ordered annealed after drawing or as peeled and ground, which sizes it without deforming the core. The tolerance classes article sets out which finishing route produces which class; the class fixes the diameter and says nothing about the strength, which the condition and the certificate fix.

How the values appear on the certificate

A mill test certificate to EN 10204 type 3.1 reports the heat number, the grade and standard, the chemical analysis, the delivery condition, and the mechanical test results for the heat and size: the 0.2 percent proof stress, the tensile strength, the elongation with its gauge length, and where the order requires them the reduction of area, the hardness in HBW or HRC, and the impact energy in joules at the test temperature. The test piece direction and location are stated for larger bar. Under ASTM A276 the same values are reported in ksi with a 50 mm or 4D gauge elongation. Because the test is on a sample of the heat in the delivery condition, the certificate for an annealed bar says nothing about the strength of the same bar after the customer hardens it; that value comes from the product standard or a test on the treated part. Testing at Laxcon Steels is described on the quality page.

Bar tested against the specified minima

Laxcon Steels supplies hot rolled round bar from 16 to 125 mm and bright bar from 5 to 115 mm with the proof stress, tensile strength and elongation of each heat and size tested in its own laboratory and reported on the certificate against the standard ordered. The grades supplied and their specified values are listed in the grade reference.

Frequently asked questions

What is the difference between yield strength and tensile strength?

Yield strength is the stress at which the steel begins to deform permanently; tensile strength is the maximum stress it carries before it breaks. Yield always comes first and is always lower. A part is designed against yield, because a part that has yielded has changed shape, and checked against tensile strength for fracture.

What does 0.2 percent proof stress mean?

It is the yield strength of a steel that has no distinct yield point, such as stainless steel. It is the stress at which the permanent strain has reached 0.2 percent of the gauge length, read from the stress-strain curve with an offset line. On a stainless steel certificate the yield entry is this value, written Rp0.2.

Is a higher tensile strength always better?

No. Higher strength usually comes with lower elongation and lower toughness, so a stronger steel gives less warning before fracture and is more sensitive to notches and to hydrogen. The right steel has enough yield strength for the load with the ductility the service needs, which is why annealed 304 at 205 MPa is specified more often than 17-4 PH at 1,170 MPa.

What elongation should stainless steel have?

The specified minimum for hot-finished annealed austenitic bar under ASTM A276 is 40 percent, and 30 percent for the same grade cold finished; tests on real hot-finished bar usually give 50 to 60 percent. Duplex grades specify 25 percent, martensitic grades 15 to 20 percent, and precipitation hardened 17-4 PH in the H900 condition 10 percent. The figure depends on the gauge length used, which the certificate states.

Sources

• ISO 6892-1, Metallic materials, tensile testing, Part 1: method of test at room temperature.
• ASTM A370, Standard Test Methods and Definitions for Mechanical Testing of Steel Products.
• ASTM E8/E8M, Standard Test Methods for Tension Testing of Metallic Materials.
• ASTM A276/A276M-17, Standard Specification for Stainless Steel Bars and Shapes: the stainless rows of the table, Table 2 for hot-finished and cold-finished Condition A bar and Table 3 for hardened and tempered bar.
• ASTM A564/A564M, Standard Specification for Hot-Rolled and Cold-Finished Age-Hardening Stainless Steel Bars and Shapes: the 17-4 PH rows, aged conditions for bar up to 76 mm.
• BS 970-1:1996 and BS 970-3, Wrought steels for mechanical and allied engineering purposes: the EN8 and EN19 rows, by ruling section.
• IS 2062:2011, Hot rolled medium and high tensile structural steel: the E250 row.
• EN 10204, Metallic products: types of inspection documents.