ASTM A387 Grade 21 Class 1: specifying the 3Cr-1Mo plate you actually need
A purchase line for Grade 21 Class 1 should answer two separate questions: which chromium-molybdenum grade the drawing requires, and which tensile class is selected. Gangsteel can review an enquiry more effectively when those choices arrive together with the edition, dimensions and acceptance instructions. A similar alloy name or a coinciding strength result is not a reason to change either choice during quotation.
A387/A387M-25 is the current official public edition identified here. Scope 1.2 lists Grade 21 at nominal 3.00% chromium and 1.00% molybdenum. The specification addresses chromium-molybdenum plates primarily intended for welded boilers and pressure vessels designed for elevated-temperature service. Under current Scope 1.3 this grade has two tensile classes; Class 1 is the material selection discussed below.
Equivalent and comparable materials: compare the purchase requirement, not the name
Start with the drawing rather than an equivalence list. For a Grade 21 Class 1 order, the low-carbon 21L sister raises both a carbon question and an edition question. The dated compilations give Grade 21 heat carbon as 0.05–0.15% and product carbon as 0.04–0.15%; the 21L reference instead gives 0.12% maximum in both analyses. These are different requirements, even where individual heats could fall within overlapping chemistry.
Edition coverage matters just as much. Current official Scope 1.2 does not list Grade 21L, while the previous official public Scope listed it alongside Grade 21 at nominal 3Cr-1Mo and provided 21L only as Class 1. The labelled Class 1 tensile figures coincide, but that does not make Grade 21 and Grade 21L equivalent. Any substitution needs a written designation change and confirmation of all governing requirements, not simply a lower reported carbon result.
The current official abstract still names 21L, but official Scope takes precedence over that conflicting grade list. It does not restore 21L to current Scope coverage or establish a second class for it.
Changing from Class 1 to Class 2 is also not an equivalent-material decision: it changes the selected tensile level and requires a separate review of delivery condition and acceptance. Nor is nearby Grade 22 Class 1 a drop-in alternative. Its official nominal chromium is 2.25%, compared with Grade 21 at 3.00%, although both have nominal molybdenum of 1.00%. The alloy requirement on the drawing remains controlling.
Finally, ASTM and ASME are not automatically interchangeable. An ASTM certificate alone is not SA-387 or ASME Code acceptance, and the reverse is not automatic. Identify any dual requirement and the applicable Code edition rather than presuming a current Code year. Related reading is available on Grade 21L Class 1 and the Grade 21L Class 2 designation and availability review. These links support navigation, not substitution approval or technical sourcing from sibling bodies.
Grade 21 chemistry: keep heat and product limits attached to their headings
| Element | Heat analysis | Product analysis |
|---|---|---|
| Carbon | 0.05–0.15 | 0.04–0.15 |
| Manganese | 0.30–0.60 | 0.25–0.66 |
| Silicon, maximum | 0.50 | 0.50 |
| Chromium | 2.75–3.25 | 2.63–3.37 |
| Molybdenum | 0.90–1.10 | 0.85–1.15 |
| Phosphorus, max | 0.025 | 0.025 |
| Sulfur, max | 0.025 | 0.025 |
| Ni, V, Nb, B, N, Al, W |
This is a labelled compilation reference from the Grade 21 Class 1 source modified in September 2026, not official Table 1. The paid full text was not read and that official table was not inspected. Confirm the figures, including phosphorus and sulfur, in the ordered edition. Blank cells are unassigned here, not declarations that an element is absent. Chemistry is organized by grade rather than tensile class.
Nominal 3Cr-1Mo identifies the alloy family; it does not mean every heat must measure exactly 3.00% chromium and 1.00% molybdenum. Similarly, silicon at 0.50% is a maximum in each analysis, not a target value to place on a report. Preserve the separate carbon and alloy bands when comparing documents. Copying a product interval into a heat-analysis column would change the reference instead of clarifying it.
Class 1 mechanical reference and the missing long-gauge minimum
| Property | Reference value | Interpretation |
|---|---|---|
| Tensile strength | 60–85 ksi [415–585 MPa] | Class 1 range |
| Yield strength, minimum | 30 ksi [205 MPa] | Class 1 minimum |
| Elongation, minimum; 8 in. [200 mm] gauge | Not established in the reviewed compilation | |
| Elongation, minimum; 2 in. [50 mm] gauge | 18% | Short-gauge minimum |
The values were independently checked on the dated Grade 21 Class 1 compilation, not taken from a neighbouring supplier page. Official Tensile Requirements were not read. An 8 in. minimum was not established and remains blank; it is not zero and is not permission to borrow a percentage from another grade. Use the complete ordered documents for specimen provisions and any applicable adjustments.
Elongation is a specimen gauge-length measurement, not a thickness band. Keep that basis beside the percentage. Ksi and MPa are independent unit systems, not exact conversions for mixing within an acceptance calculation. State the governing system in the enquiry and use it consistently when checking both tensile range and yield minimum.
Agree thermal condition and inspection scope before comparing offers
The official abstract describes killed and thermally treated steel. It does not establish the detailed route on this public page. A labelled Class 1 compilation restatement describes usual delivery as annealed, or normalized and tempered, or accelerated cooling followed by tempering; annealed plate is Class 1 only. Confirm the required route in the ordered edition. No official heat-treatment clause or processing temperature is reproduced here, and an as-rolled description does not resolve the thermal requirement.
| Item | Information to provide | Why it matters |
|---|---|---|
| Designation | A387 or A387M, Grade 21, Class 1 and edition | Fix the material basis |
| Dimensions | Thickness, width, length, quantity and units | Connect the requirement to each plate line |
| Delivery state | Required thermal condition and project notes | Avoid assumed treatment routes |
| Inspection | Tests, specimen requirements and acceptance criteria | Define what evidence is needed |
| Documents | Certificate form and plate/heat identification | Keep documents traceable to the order |
Scope 1.4 limits thickness only by the composition’s capacity to meet specified mechanical properties. That is not a numerical cap or a rolling-capability promise. Submit the actual size schedule for review. Impact tests, additional examinations and particular certificate forms need express agreement; none is offered as a website default. The grade description also does not establish a welding procedure for a particular vessel.
Buyer questions
Does a 3Cr-1Mo description mean exact chromium and molybdenum results?
No. The nominal identity in official Scope is not the same as the labelled heat and product analysis intervals. Keep the analysis type and ordered-edition limits with the measured results.
Can silicon 0.50% be entered as a required target?
The reviewed Grade 21 compilation presents 0.50% as a maximum in both columns. It is not a prescribed target composition or a substitute for a measured analysis.
Why is the 8 in. elongation minimum blank?
The reviewed Grade 21 Class 1 source does not establish it. Resolve the governing test criterion from the ordered specification; do not complete the cell with another grade’s figure.
May a quotation increase the class without revising the material requirement?
A proposed Class 2 alternative must be identified and reviewed separately. Class 1 and Class 2 are not equivalent, and a commercial revision should not silently alter the selected tensile level or delivery basis.
Which inspection additions belong in the first enquiry?
State any required impact tests, examinations, specimen conditions, acceptance criteria and certificate form. Early agreement makes offers comparable without assuming that every additional test accompanies every plate.
Send a Grade 21 Class 1 plate enquiry
Give Gangsteel the selected edition, dimensions, quantity, governing units and delivery condition. Attach the material schedule and inspection requirements, and identify alternatives separately so the original Grade 21 Class 1 purchase basis stays clear.