Are 16Mo3 and 15Mo3 the Same Steel?
For practical purposes 16Mo3 and 15Mo3 describe the same molybdenum alloy boiler and pressure vessel steel. They share the material number 1.5415, they have essentially the same composition and the same mechanical properties, and designers treat them as equivalent grades. The difference lies in the standards behind the two designations and in the tightness of the impurity limits: 15Mo3 is the older German designation from DIN 17155, while 16Mo3 is the current European designation from EN 10028-2, and the modern grade usually carries stricter limits on phosphorus and sulphur.
The practical consequence is that a drawing calling for 15Mo3 can generally be satisfied with 16Mo3, which is the grade mills quote today and which offers slightly better high-temperature behaviour because the steel is cleaner.
Standards and History of the Two Designations
The two names exist because European pressure vessel plate standards were restructured. Material previously ordered to the German standard is now ordered to EN 10028-2 as 16Mo3, with the same steel number carried through the documentation.
| Item | 15Mo3 | 16Mo3 |
|---|---|---|
| Origin standard | DIN 17155 | EN 10028-2 |
| Steel number | 1.5415 | 1.5415 |
| Status | Superseded designation | Current designation |
| Relationship | Predecessor grade | Modernised replacement grade |
Because both designations point to the same 1.5415 steel, the changeover did not require requalification of the material concept. What changed was the documentation route and, in practice, the tolerance band accepted for residual elements.
Chemical Composition Compared
The table lists the limits of the two standards. Where a limit is not specified in the older German table, it is shown as not specified.
| Element (%) | 15Mo3 | 16Mo3 |
|---|---|---|
| C | 0.12-0.20 | 0.12-0.20 |
| Si | 0.10-0.35 | max 0.35 |
| Mn | 0.40-0.90 | 0.40-0.90 |
| P | max 0.035 | max 0.025 |
| S | max 0.030 | max 0.010 |
| Cr | max 0.25 | max 0.30 |
| Cu | max 0.30 | max 0.30 |
| Mo | 0.25-0.35 | 0.25-0.35 |
| Ni | not specified | max 0.30 |
| N | not specified | max 0.012 |
The carbon and molybdenum ranges are the ones that define the grade family, and they are unchanged between the two designations. The differences sit at the limits for tramp elements, which is where the modern specification is tighter.
Mechanical Properties Compared
| Property | 15Mo3 | 16Mo3 |
|---|---|---|
| Yield strength (MPa), min | 275 | 275 |
| Tensile strength (MPa) | 440-590 | 440-590 |
| Elongation (%), min | 20 | 22 |
| Typical heat treatment | Normalised | Normalised |
The strength levels are identical, which is what makes substitution practical: a component designed against the yield strength of 15Mo3 passes the same check in 16Mo3. The measurable gain in the newer grade is the minimum elongation, 22% against 20%, together with the tighter impurity limits described below.
Why the Tighter P and S Limits Matter
Phosphorus and sulphur dominate the high-temperature and fabrication behaviour of a molybdenum boiler steel. A lower sulphur limit reduces the number and size of inclusions and improves ductility and toughness in the heat affected zone of welds; a lower phosphorus limit reduces the risk of temper embrittlement during long elevated-temperature service. Because 16Mo3 restricts P to 0.025% and S to 0.010% against 0.035% and 0.030% in 15Mo3, the modern grade is the better starting point for equipment that will run hot for decades.
The alloying concept itself is unchanged. Molybdenum raises creep strength at elevated temperature and improves resistance to pitting corrosion, while chromium improves resistance to high-temperature hydrogen attack and to graphitisation. Together these effects make the grade suitable for service up to around 500 °C in boilers, steam lines and petrochemical equipment.
Substitution, Welding and Application Notes
Direct substitution: where a drawing calls for 15Mo3, 16Mo3 is normally accepted as the modern equivalent, subject to project approval.
Heat treatment: both grades are typically supplied normalised, so no change to the fabrication sequence is needed.
Welding: both show good weldability; the tighter sulphur limit of 16Mo3 gives a cleaner heat affected zone.
Typical service: power plant components, petrochemical plant and other elevated-temperature, elevated-pressure equipment.
Documentation: certificates and pressure equipment files should record the grade actually supplied so that the traceability of the 1.5415 steel remains intact.
Mixing the two designations casually in a datasheet is the most common mistake found during document review, even though the material itself is interchangeable.
Frequently Asked Questions
Q: Is 16Mo3 the same as 15Mo3?
They are equivalent grades of the same 1.5415 steel, with nearly identical composition and mechanical properties. 16Mo3 is the current EN 10028-2 designation; 15Mo3 is the older DIN 17155 designation.
Q: Can 16Mo3 replace 15Mo3 directly?
In most pressure equipment designs, yes. Both are normally supplied normalised with the same yield and tensile ranges, and 16Mo3 offers slightly better elongation and cleaner steel.
Q: Which standard covers each grade?
15Mo3 comes from DIN 17155 and 16Mo3 from EN 10028-2, which replaced the older German standard around the 1990s and 2000s.
Q: What is the difference in phosphorus and sulphur limits?
16Mo3 limits P to 0.025% and S to 0.010%, against 0.035% and 0.030% for 15Mo3, which improves weld toughness and resistance to temper embrittlement.
Q: What is 16Mo3 used for?
Boiler and pressure vessel components, steam lines and petrochemical equipment operating at elevated temperature, typically up to around 500 °C.
Q: What is the molybdenum range in these grades?
Both designations sit in the 0.25-0.35% molybdenum range, with carbon between 0.12% and 0.20%.
