Steel A387 Gr 21
ASTM A387 Gr 21 · UNS K31545
A387 Grade 21 (K31545, SCMV6) is a 3Cr-1Mo low-alloy pressure vessel plate steel supplied as Class 1, Class 2, Class 2 quenched-and-tempered, PWHT and low-carbon 21L.
Reference datasheet — A387 Gr 21 is not in the instant-quote catalog. Upload your part and an engineer will quote it in this grade or suggest the closest stocked equivalent.
Values for the Cl.2 condition
A387 Gr 21 tempers and conditions
A387 Gr 21 is supplied in 5 conditions, soft to hard. Cl.2 is the default FabDigit quotes; the values on this page are for it.
Compare all 5 conditionsHide
| Condition · open for datasheet | Best for | Yield MPa | Tensile MPa | Elong. % | Hardness | Conductivity |
|---|---|---|---|---|---|---|
Grade 21L | Choose the low-carbon 21L modification when weld cracking risk and hardness control dominate and the lower allowable stress is acceptable. | 260 | 450 | 27 | 135 HB | 7.4 % IACS |
Class 1 | Pick when maximum ductility, formability and weld-fabrication ease matter more than allowable stress — thick heads, hot-formed sections, hydrogen service with generous wall. | 300 | 480 | 25 | 145 HB | — |
Post-weld heat treated | Specify when the purchaser requires mechanical tests on coupons carrying the full fabrication PWHT cycle — properties drop slightly versus as-supplied N&T plate. | 345 | 535 | 24 | 160 HB | 7.2 % IACS |
Cl.2Default Class 2 | Standard stocked condition for hydroprocessing reactor and hot-wall vessel shells where higher allowable stress reduces wall thickness. | 380 | 560 | 22 | 170 HB | — |
Class 2, quenched and tempered | Ordered for heavy-wall reactor shells (>100 mm) where a bainitic quench-and-temper structure is needed for through-thickness strength and toughness. | 440 | 600 | 21 | 13 HRC | — |
What is A387 Gr 21?
A387 Grade 21 (K31545, SCMV6) is a 3Cr-1Mo low-alloy pressure vessel plate steel supplied as Class 1, Class 2, Class 2 quenched-and-tempered, PWHT and low-carbon 21L. It provides improved high-temperature hydrogen attack and creep resistance compared with 2.25Cr-1Mo, with 380 MPa yield, 560 MPa tensile, 22 % elongation, 170 HB (88 HRB) and service to 550 °C. Typical forms are heavy-wall plate for refinery hydroprocessing vessels. Weldability (45) is lower than Grade 12, PWHT is mandatory, and temper embrittlement control is required. A387 Gr 21 has a density of 7.85 g/cm³ (0.284 lb/in³), heavier than 56% of steel grades. It melts at 1,425°C (2,597 °F). A387 Gr 21 has a yield strength of 380 MPa (55.1 ksi) and a tensile strength of 560 MPa (81.2 ksi) — weaker than 68% of steel grades. Elongation at break is 22% and the elastic modulus is 210 GPa (30.5 Msi).
What is A387 Gr 21 used for?
- hydrocracker reactor plate
- hydrotreater vessels
- high-temperature/high-pressure reactors
Advantages
- High service temperature — 550°C (1,022 °F), better than 95% of steel grades
- Tough — resists crack growth — 140 MPa·√m (127.4 ksi·√in), better than 94% of steel grades
Limitations
- Poor heat conductor — 34 W/m·K (19.6 BTU/hr·ft·°F), worse than 77% of steel grades
A387 Gr 21 properties
Typical room-temperature values for A387 Gr 21-Cl.2; bars are the position among steel grades.
Physical3
A387 Gr 21 has a density of 7.85 g/cm³ (0.284 lb/in³), heavier than 56% of steel grades. It melts at 1,425°C (2,597 °F).
Mechanical16
A387 Gr 21 has a yield strength of 380 MPa (55.1 ksi) and a tensile strength of 560 MPa (81.2 ksi) — weaker than 68% of steel grades. Elongation at break is 22% and the elastic modulus is 210 GPa (30.5 Msi).
Thermal6
A387 Gr 21 is rated for continuous service to 550°C (1,022 °F). It conducts heat at 34 W/m·K (19.6 BTU/hr·ft·°F), worse than 77% of steel grades. Thermal expansion is 12.2 µm/m·K (6.78 µin/in·°F).
Electrical3
A387 Gr 21 conducts electricity at 7.2 % IACS, worse than 71% of steel grades.
Chemical & Environmental3
Corrosion resistance is poor (20/100), better than 87% of steel grades.
Sustainability4
Producing a kilogram of A387 Gr 21 takes about 25 MJ of energy and emits 2 kg of CO₂ — less than 82% of steel grades. Typical recycled content is 25%.
Manufacturability7
A387 Gr 21's machinability is good (55/100, better than 62% of steel grades); weldability fair (45/100); formability good (55/100).
Common Calculations5
Values are nominal handbook figures for design screening. Certified mill or lot data ships with every FabDigit order on request.
Strength versus weight in the steel family
Yield strength against density for every steel grade FabDigit runs. A387 Gr 21 is highlighted; hover a dot for its name, or open the interactive family chart.
Working with A387 Gr 21
Is A387 Gr 21 easy to machine?
Machines like a medium-hard alloy steel at roughly 55 % of AISI 1212 — use rigid setups, carbide tooling and moderate speeds; the Cr-Mo carbides are abrasive and work-hardening is mild.
Can A387 Gr 21 be welded?
Weldable with matching 3Cr-1Mo (E/ER 90S-B3-type or B9-family per procedure) using 200–250 °C preheat/interpass, low-hydrogen consumables and mandatory 690–720 °C PWHT; control J-factor and X-bar to limit temper embrittlement.
Can A387 Gr 21 be formed, bent or molded?
Hot form at 950–1050 °C followed by re-normalizing and tempering; cold forming is limited by the high yield strength and needs generous bend radii plus a post-form stress relief.
A387 Gr 21 chemical composition (wt %)
Limits by weight percent from the governing specification, written the way the spec states them — a single maximum for impurities, a range for alloying elements, and the base element as balance. Nominal is the typical mid-range value.
| Element | Spec limit (wt %) | Nominal |
|---|---|---|
| C0.10 max for Grade 21L | 0.05 – 0.17 | 0.12 |
| Mn | 0.3 – 0.6 | 0.45 |
| Prestricted (≤0.010) for temper-embrittlement-controlled reactor plate | ≤ 0.04 | 0.01 |
| Stypically ≤0.010 with Ca treatment | ≤ 0.04 | 0.01 |
| Si0.15–0.40 typical for fine-grain practice | ≤ 0.5 | 0.25 |
| Cr | 2.75 – 3.25 | 3 |
| Mo | 0.9 – 1.1 | 1 |
| Fe | balance |
What surface finishes work on A387 Gr 21?
Not anodizable — protect with high-temperature paint, aluminizing or metallizing on the cold face and stainless (309L/347) weld overlay or clad on the process side.
Surface treatments FabDigit runs on steel parts — pick them in the quote configurator or call them out on the drawing. Each links to its datasheet in the surface-finish library (thickness, specs, call-out, material verdicts).
Organic Coating
Black / Phosphate
Thermal Spray
Equivalent designations
Standards and trade names that resolve to Steel A387 Gr 21 in FabDigit's catalog. Equivalence is nominal — check the exact specification when certification matters.
- UNS
- K31545
- ASTM
- A387 Grade 21
- Similar grades
- SCMV6
How much does A387 Gr 21 cost?
A387 Gr 21 is a standard-cost steel ($$), cheaper than about 71% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
A387 Gr 21 — frequently asked
What is A387 Gr 21 used for?
A387 Gr 21 is typically used for hydrocracker reactor plate, hydrotreater vessels and high-temperature/high-pressure reactors. In short: 3Cr-1Mo pressure vessel plate; resists high-temperature hydrogen attack; needs 690–720 °C PWHT.
What is the yield strength of A387 Gr 21?
A387 Gr 21 has a typical yield strength of 380 MPa (55.1 ksi) and a tensile strength of 560 MPa (81.2 ksi) — weaker than 68% of steel grades. Strength varies by condition: see the 5 listed tempers.
Is A387 Gr 21 easy to machine?
Reasonably — machinability is rated good (55/100, better than 62% of steel grades). Machines like a medium-hard alloy steel at roughly 55 % of AISI 1212 — use rigid setups, carbide tooling and moderate speeds; the Cr-Mo carbides are abrasive and work-hardening is mild.
Can A387 Gr 21 be welded?
With care — weldability is rated fair (45/100). Weldable with matching 3Cr-1Mo (E/ER 90S-B3-type or B9-family per procedure) using 200–250 °C preheat/interpass, low-hydrogen consumables and mandatory 690–720 °C PWHT; control J-factor and X-bar to limit temper embrittlement.
What surface finishes work on A387 Gr 21?
Not anodizable — protect with high-temperature paint, aluminizing or metallizing on the cold face and stainless (309L/347) weld overlay or clad on the process side.
Can A387 Gr 21 be formed, bent or molded?
Hot form at 950–1050 °C followed by re-normalizing and tempering; cold forming is limited by the high yield strength and needs generous bend radii plus a post-form stress relief.
What is the maximum service temperature of A387 Gr 21?
A387 Gr 21 is rated for continuous use to about 550°C (1,022 °F). Strength falls off well before that limit — check the elevated-temperature data for load-bearing parts.
What is the difference between A387 Gr 21 Cl.2 and A387 Gr 21 21L?
Cl.2 is the default condition — standard stocked condition for hydroprocessing reactor and hot-wall vessel shells where higher allowable stress reduces wall thickness. 21L: choose the low-carbon 21L modification when weld cracking risk and hardness control dominate and the lower allowable stress is acceptable. Yield strength is 380 MPa in Cl.2 versus 260 MPa in 21L.
How much does A387 Gr 21 cost?
A387 Gr 21 is a standard-cost steel ($$), cheaper than about 71% of the materials FabDigit quotes. Part price depends far more on geometry, tolerance and quantity than on the raw stock — upload a CAD file for a live quote.
Sources
- ASTM A387/A387M — Pressure Vessel Plates, Alloy Steel, Chromium-Molybdenum — ASTM International (2021)
- ASME BPVC Section II Part A, SA-387 and Part D allowable stresses/physical properties — ASME (2023)
- API RP 934-A — Materials and Fabrication of 2¼Cr-1Mo, 3Cr-1Mo and V-modified Steel Heavy-Wall Pressure Vessels — API (2019)
- API RP 941 — Steels for Hydrogen Service (Nelson curves) — API (2016)
- ASM Handbook Vol.1 — Properties and Selection: Irons, Steels and High-Performance Alloys — ASM International (1990)
- JIS G 4109 (SCMV6) / GB 713 (12Cr3Mo1R-type) equivalents — JISC / SAC (2020)
- Mysteel / SMM (2026)
Property data is compiled from published supplier and standards handbooks and normalised for comparison; hardness values on non-Brinell scales are converted approximately for charting only. Manufacturability limits are FabDigit quote-engine defaults and may be relaxed by engineering review. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.