What ASTM A204 Actually Covers
ASTM A204 is the specification for molybdenum alloy steel plates for pressure vessels, published as A204/A204M. It contains three grades, A, B and C, and all three follow the same alloying idea: a low carbon content with about 0.45 to 0.60 percent molybdenum, a controlled manganese level and silicon as a deoxidiser. The molybdenum addition raises elevated temperature strength and creep resistance compared with plain carbon vessel steel, which is why the grade is used in the temperature range between ordinary carbon steel plate and the chromium molybdenum plate family.
In the ASME system the same specification appears as SA-204/SA-204M, so a plate ordered and certified to either document is produced and tested to the same requirements. The grades differ mainly in the specified minimum tensile strength, which rises from Grade A to Grade C because the maximum permitted carbon content is raised from one grade to the next. For most enquiries, Grade B is the grade that is quoted and stocked.
The Real Equivalent of ASTM A204
| System | Designation | Relationship |
|---|---|---|
| ASTM | A204/A204M Grade A, B, C | Base specification |
| ASME | SA-204/SA-204M Grade A, B, C | Identical requirements |
| European | 16Mo3, material number 1.5415, to EN 10028-2 | Closest molybdenum alloy pressure vessel plate |
| Historic German | 16Mo3 | Same molybdenum steel family |
| Chinese pressure vessel practice | Molybdenum bearing plate of the GB/T 713 series | Same service family, different grade designation system |
The important point is that the closest equivalent is a molybdenum steel, not a chromium molybdenum steel. The European grade 16Mo3 is the grade most often quoted as the counterpart of A204 Grade B, and both are used for welded pressure equipment in the moderately hot range, typically up to about 500 C. Above that range the creep behaviour of a molybdenum only steel becomes the limiting factor and the chromium molybdenum grades take over.
Why 15CrMoR and 14Cr1MoR Are Not A204 Equivalents
Lists of A204 equivalents often include 15CrMoR and 14Cr1MoR from the Chinese pressure vessel standard, but those two grades are not substitutes. They are chromium molybdenum steels containing roughly 1 percent and 1.25 percent chromium with about 0.5 percent molybdenum, and they correspond to the ASTM A387 family, Grade 12 and Grade 11 respectively. A204 contains no deliberate chromium addition at all.
The consequence of confusing the two families is not cosmetic. Chromium molybdenum plate has different allowable stresses, a different creep strength curve, a different welding procedure and different post-weld heat treatment requirements, so a substitution changes the design and the fabrication qualification, not just the material certificate. A purchaser comparing quotations should always check the chromium and molybdenum columns of the composition table before accepting an offer described as an equivalent.
Composition and Property Basis
| Element | Typical requirement |
|---|---|
| Molybdenum (Mo) | 0.45 to 0.60 percent |
| Manganese (Mn) | 0.90 percent maximum |
| Silicon (Si) | 0.15 to 0.40 percent |
| Carbon (C) | Limited according to grade, increasing from Grade A to Grade C |
| Phosphorus (P) and Sulfur (S) | 0.035 percent maximum each |
Mechanical requirements are quoted in the specification as a tensile strength band in ksi together with a minimum yield strength, with Grade B carrying a minimum yield strength of about 40 ksi, which is approximately 275 MPa. The exact values are tied to the grade and to the edition of A204/A204M or SA-204/SA-204M that governs the order, so the order document should name the standard, the year of issue, the grade and the heat treatment condition in full. A plate certificate that quotes only the grade letter is not enough to confirm conformity.
Elevated Temperature Service and Design
Molybdenum alloy plate of this type is used for welded pressure vessels, steam drums, heat exchanger shells and similar equipment where the design temperature is above the economic range of carbon steel but below the chromium molybdenum range. Design is not based on room temperature tensile values alone. Allowable stresses are taken from the elevated temperature tables of the applicable construction code, where the values fall as temperature rises, and the creep and stress rupture properties govern at the top of the range.
Because the material is welded into a pressure boundary, the toughness requirement and the welding procedure both matter as much as the chemical composition. The specification is normally ordered with a defined heat treatment condition, and any operation that changes that condition, such as hot forming or post-weld heat treatment above the original temperature, has to be assessed against the design allowable stresses.
Ordering, testing and welding notes
Name the specification, edition, grade and heat treatment condition, and state whether the plate will be used in the as-rolled, normalized or stress relieved condition.
Specify the ultrasonic examination requirement for pressure retaining plate to a straight beam method such as A435 or A578 at the agreed acceptance level, since internal soundness cannot be judged from the certificate alone.
Require the tensile test to report both tensile and yield strength and the elongation, together with the heat treatment applied to the test specimen, because the specimen condition governs the property.
For welding, use a qualified procedure with preheat appropriate to the thickness and restraint, matching filler metal and post-weld heat treatment where the code requires it. Molybdenum steel welds need attention to restraint and to hydrogen control as much as to strength matching.
Track the plate identity through cutting, forming and welding so that the certificate can be matched to the finished component, which is a code requirement in most pressure equipment regimes.
Frequently Asked Questions
Q: What is the equivalent of ASTM A204?
The closest European equivalent is 16Mo3 to EN 10028-2, material number 1.5415, also quoted historically as 16Mo3. In the ASME system the identical specification is SA-204/SA-204M.
Q: What is ASTM A204 Grade B used for?
Grade B is the most commonly ordered grade of the specification. It is used for welded pressure vessels, steam drums and heat exchanger equipment operating in the elevated temperature range up to about 500 C.
Q: Is 15CrMoR an equivalent of ASTM A204?
No. 15CrMoR is a chromium molybdenum steel of the ASTM A387 Grade 12 family. A204 is a molybdenum steel with no deliberate chromium addition, so the two are not interchangeable.
Q: Is SA-204 the same as A204?
Yes. SA-204/SA-204M is the ASME adoption of the ASTM specification and carries the same composition, property and testing requirements for grades A, B and C.
Q: Does ASTM A204 contain chromium?
No deliberate chromium addition is specified. The alloying element that defines the grade is molybdenum at about 0.45 to 0.60 percent, supported by manganese and silicon.
Q: Can ASTM A204 plate be welded?
Yes, with a qualified procedure, suitable preheat, hydrogen controlled consumables and post-weld heat treatment where the construction code requires it. The welding procedure and the heat treatment must satisfy the design code, not only the plate specification.
