Reading a Steel MTC: Yield Strength, Tensile Strength, Elongation, and Yield Ratio Explained
A mill test certificate lists four numbers that decide whether steel is acceptable, and buyers routinely check only the two they recognise. Yield strength and tensile strength are compared to the grade minimum, the box is ticked, and the material ships. But elongation and the relationship between yield and tensile are where the failures hide — and both are on the certificate you already have.
The other problem is that the same property can be reported several different ways depending on the standard: Rp0.2 or ReL, A5 or A50mm, MPa or ksi. A certificate that looks compliant in one convention may be non-compliant when converted, and a buyer without the conversion in mind cannot tell.
This guide explains what each number means, how the reporting conventions differ by standard, and which combinations should raise a question before you accept the material.
The Four Numbers and What Each One Tells You
| Property | Symbol | What it measures | What it implies |
|---|---|---|---|
| Yield strength | ReH / ReL / Rp0.2 | Stress at which permanent deformation begins | How much load the part carries before it bends |
| Tensile strength | Rm | Maximum stress before fracture | Ultimate load capacity; also correlates with hardness |
| Elongation | A5 / A50mm / A% | Plastic strain at fracture, as a percentage | Ductility — how much the steel stretches before breaking |
| Reduction of area | Z | Cross-section shrinkage at fracture | Ductility in the through-thickness direction; sensitivity to inclusions |
Yield and tensile are the strength numbers. Elongation and reduction of area are the ductility numbers. A steel that meets its strength minimum but misses elongation is too brittle for its grade, regardless of how strong it is — and that is a rejection, not a rounding issue.
Yield Strength: Three Symbols, Three Meanings
This is the first place certificates mislead, because the symbol tells you which measurement was made.
| Symbol | Name | How it is determined | Where used |
|---|---|---|---|
| ReH | Upper yield strength | The peak before the first drop on the stress-strain curve | Steels with a distinct yield point — mild and low-carbon structural steel |
| ReL | Lower yield strength | The plateau stress after the upper yield point | Same steels; often the value used for design |
| Rp0.2 | 0.2% proof stress | Stress at 0.2% permanent (plastic) strain, by offset method | Steels without a distinct yield point — quenched and tempered, cold-worked, stainless, aluminium |
The practical consequence: ReH and Rp0.2 are not interchangeable numbers. For the same steel, ReH is generally higher than Rp0.2 — sometimes by 20–50 MPa — because the upper yield point is a transient peak that depends on testing machine stiffness and specimen alignment. A certificate quoting ReH 355 MPa and one quoting Rp0.2 355 MPa are not describing the same material behaviour.
When comparing to a grade minimum, check which symbol the standard specifies. EN 10025 structural grades are defined on ReH (or ReL for the lower yield). EN 10028 pressure vessel plate and most Q&T grades are defined on Rp0.2. If you convert between them, do not assume equality.
Why some steels have no yield point: a distinct upper/lower yield point appears in annealed or normalized low-carbon steel. Cold working, quenching and tempering, or alloying suppresses it, and the stress-strain curve transitions smoothly — so a proof stress must be used instead. This is why cold-rolled sheet and Q&T plate report Rp0.2, not ReH.
Tensile Strength: The Number Buyers Over-Trust
Tensile strength (Rm) is the maximum engineering stress on the curve. It is easy to measure, hard to argue about, and therefore the number most often checked. Two cautions:
- Higher Rm does not mean better steel. Rm rises with carbon and alloy content and with cold work, but so does brittleness. A heat that runs 60 MPa above the grade minimum is not a better heat — it may be harder, less ductile, and harder to weld.
- Rm is specified as a range, not a minimum, in many standards. EN 10025 gives both a minimum and a maximum for some grades precisely to stop mills from over-strengthening. If a certificate shows Rm above the maximum, that is a non-conformance even though the steel is “stronger.”
The Rm maximum exists because over-strength steel can fail differently: it resists deformation, so it transfers stress to welds and to adjacent components, and it may have inadequate elongation. This is why “stronger is better” is wrong in structural steel procurement.
Elongation: The Gauge Length Trap
Elongation is reported as a percentage, but the percentage depends entirely on the gauge length over which it was measured. Two conventions dominate:
| Symbol | Gauge length | Typical value for the same steel |
|---|---|---|
| A5 (or A5D) | 5 × specimen diameter, round specimens | Higher number |
| A50mm | 50 mm fixed gauge length, often flat specimens | Lower number |
| A80mm / A2in | 80 mm or 2 inch, older conventions | Lower still |
The shorter the gauge length, the higher the reported elongation, because the strain is concentrated in the necked region and averaged over a shorter distance. A steel reporting A5 = 26% might report A50mm = 21% — same material, different conventions.
This matters at goods-in inspection. If the standard requires A50mm ≥ 20% and the certificate reports A5 = 22%, you cannot conclude compliance: converted to an equivalent 50 mm gauge, the value might be 18%. Always check that the gauge length on the certificate matches the gauge length in the standard.
Flat products are usually tested on flat specimens with a 50 mm or 80 mm gauge; round bar and wire on proportional round specimens with A5. If the certificate does not state the gauge length, the elongation figure is not verifiable — request it.
Yield Ratio: The Number Nobody Checks
Yield ratio (also called yield-to-tensile ratio, Y/T or YS/TS) is simply yield strength divided by tensile strength. It is not usually a specified requirement, but it is the most informative single indicator of how the steel will behave.
| Yield ratio | Interpretation | Behaviour |
|---|---|---|
| 0.50 – 0.65 | Low | Large reserve between yield and fracture; good ductility and strain hardening; typical of normalized and annealed steel |
| 0.65 – 0.80 | Normal | Typical of hot-rolled structural steel |
| 0.80 – 0.90 | High | Limited strain hardening; characteristic of cold-worked or Q&T steel |
| > 0.90 | Very high | Little reserve before fracture; brittle-fracture risk under overload; may indicate over-strength or heavy cold work |
Why it matters: a high yield ratio means the steel reaches its yield point and then has very little additional capacity before it breaks. In a structure, that translates to little warning before failure and poor redistribution of stress — the opposite of what ductile design assumes. Some specifications for seismic, pressure, and offshore service set a maximum yield ratio expressly for this reason, commonly 0.85 or 0.80.
You can compute it from the certificate in seconds. If the grade does not specify it but the application is safety-critical, it is worth a look — a heat running at ReH 420 / Rm 470 (ratio 0.89) meets an S355 minimum for both, but behaves very differently from a heat at 380 / 520 (ratio 0.73).
Reading the Certificate: A Checklist
| Check | What to confirm |
|---|---|
| Symbol matches standard | ReH/ReL where the standard requires it; Rp0.2 for Q&T, cold-worked, stainless |
| Units | MPa (N/mm²) vs ksi; 1 ksi = 6.895 MPa — a “50” tensile is 345 MPa, not 50 MPa |
| Gauge length stated | A5 vs A50mm vs A80mm; must match the standard’s requirement |
| Both limits | Is Rm a minimum only, or a min/max range? Check the upper limit too |
| Specimen orientation | Transverse vs longitudinal; transverse is lower, especially for elongation |
| Test standard | ASTM A370, ISO 6892-1, or EN 10002 — procedures differ slightly in rate and extensometer use |
| Heat identification | Values traceable to the heat number on the material |
The units point is not trivial. Certificates from different origins mix MPa and ksi, and a buyer comparing “50” against a 345 MPa minimum without checking units will either reject good steel or accept bad. Convert before comparing.
Where Certificates Commonly Go Wrong
Strength met, elongation missed. The most common real non-conformance. A heat that is over-strength (high carbon, high manganese, or over-cold-worked) will exceed Rm while falling below elongation. It looks compliant if you only check strength.
Wrong symbol for the grade. A certificate reporting Rp0.2 for a grade defined on ReH, or vice versa. Usually not fraud — different labs and different standards use different methods — but it makes direct comparison invalid.
Values at the exact minimum. A certificate showing every property precisely at the minimum, to the MPa, across many heats, is a warning sign. Real testing produces scatter. Uniform exact-minimum values suggest the numbers were transcribed from the standard rather than measured.
Missing test temperature or orientation. Impact values without a stated temperature, or tensile values without orientation, cannot be verified against the standard’s requirement.
Ladle vs product analysis confusion. The MTC normally reports ladle analysis for chemistry and product testing for mechanicals. If a buyer compares a product-analysis chemistry against a ladle-analysis limit, small differences may be within the standard’s permitted product tolerance but look like non-conformance.
FAQ
What is the difference between yield strength and proof stress? Yield strength (ReH/ReL) is read from a distinct yield point on the curve. Proof stress (Rp0.2) is the stress at a defined small plastic strain, used when no distinct yield point exists. They are measured differently and are not numerically interchangeable.
Is higher elongation always better? Generally yes for formability and ductility, but elongation is not a design strength property. Very high elongation usually means lower strength. Match both to the application rather than maximising one.
Can I convert A5 to A50mm? Approximately, using the proportionality relationship between gauge lengths, but the conversion is empirical and standard-dependent. For compliance decisions, require the value in the convention the standard specifies rather than converting.
Why does my certificate show ReH but the standard says Rp0.2? Likely a lab using a different test standard, or a transcription from a different product. Ask for the value in the specified convention; do not assume equivalence.
What yield ratio should I require? Only if your application or standard calls for it. Seismic, pressure vessel, and offshore specifications sometimes cap it at 0.80–0.85. For general structural work it is a useful sanity check rather than a hard requirement.
Does the MTC guarantee the whole heat? It represents the tested samples for that heat, not every plate. For critical applications, specify product testing per piece or per batch, or require third-party witness of sampling.
Why Source Documented Steel from Yihang Metal
We supply carbon, alloy, stainless, galvanized, aluminium, and copper products with mill test certificates that state the measurement convention, so the numbers can actually be checked rather than merely read.
- Symbols stated explicitly — ReH, ReL, or Rp0.2 recorded as measured, with the test standard named, so comparison against the grade is valid.
- Gauge length reported — A5 or A50mm identified on the certificate, removing the elongation conversion trap at goods-in inspection.
- Full property set — yield, tensile, elongation, and reduction of area, plus impact values with test temperature and specimen orientation where the grade requires them.
- Units unambiguous — MPa throughout, with ksi conversions noted where a buyer’s specification uses them.
- Third-party verification available — SGS, BV, Intertek, or your nominated inspector can witness sampling and testing, with reports issued against the same standard.
Send us the grade, the standard, and the properties your design actually depends on, and we will confirm which values the mill can guarantee — and flag where a requirement sits at the edge of what the grade normally produces.
Request a quote or MTC review: contact@yihangmetal.com — include the grade, standard, product form, and the mechanical properties your application requires, and we will respond with availability, mill origin, and documentation scope.