Steel A387 Gr 21 Cl.1
Heat-treated state
ASTM A387 Gr 21 · UNS K31545
Cl.1 = 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.
Values for the Cl.1 condition
Other A387 Gr 21 tempers and conditions
A387 Gr 21 is listed in 5 conditions; see the grade overview at A387 Gr 21.
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 |
Cl.1This page 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 Cl.1?
A387 Gr 21 Cl.1 is A387 Gr 21 heat treated to Cl.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. A387 Gr 21 Cl.1 has a yield strength of 300 MPa (43.5 ksi) and a tensile strength of 480 MPa (69.6 ksi) — weaker than 86% of steel grades. Elongation at break is 25% and the elastic modulus is 210 GPa (30.5 Msi). A387 Gr 21 Cl.1 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). Cl.2 is the default condition FabDigit quotes; Cl.1 is available on request or by drawing note.
Advantages
- Tough — resists crack growth — 150 MPa·√m (136.5 ksi·√in), better than 96% of steel grades
- High service temperature — 550°C (1,022 °F), better than 95% of steel grades
- Easy to machine — 62/100, better than 89% of steel grades
- Forms and bends easily — 62/100, better than 78% of steel grades
- Ductile — tolerates forming and impact — 25%, better than 77% of steel grades
Limitations
- Low strength — 300 MPa (43.5 ksi), worse than 86% of steel grades
- Poor heat conductor — 34 W/m·K (19.6 BTU/hr·ft·°F), worse than 77% of steel grades
A387 Gr 21 Cl.1 properties
Typical room-temperature values for A387 Gr 21 Cl.1; 19 are specific to this condition. Other listed values are A387 Gr 21 reference data and may not represent this condition. Bars are the position among steel grades.
Physical3
A387 Gr 21 Cl.1 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 Cl.1 has a yield strength of 300 MPa (43.5 ksi) and a tensile strength of 480 MPa (69.6 ksi) — weaker than 86% of steel grades. Elongation at break is 25% and the elastic modulus is 210 GPa (30.5 Msi).
Thermal6
A387 Gr 21 Cl.1 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 Cl.1 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 Cl.1 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 Cl.1's machinability is good (62/100, better than 89% of steel grades); weldability fair (48/100); formability good (62/100).
Values are nominal handbook figures for design screening. Confirm any required mill or lot documentation when quoting.
Working with A387 Gr 21 Cl.1
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.
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.
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 | — |
A387 Gr 21 Cl.1 — frequently asked
What is A387 Gr 21 Cl.1 used for?
A387 Gr 21 Cl.1 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 Cl.1?
A387 Gr 21 Cl.1 has a typical yield strength of 300 MPa (43.5 ksi) and a tensile strength of 480 MPa (69.6 ksi) — weaker than 86% of steel grades. Strength varies by condition: see the 5 listed tempers.
Is A387 Gr 21 Cl.1 easy to machine?
Reasonably — machinability is rated good (62/100, better than 89% 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 Cl.1 be welded?
With care — weldability is rated fair (48/100). 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 Cl.1?
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 Cl.1 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 Cl.1?
A387 Gr 21 Cl.1 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.
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. Nothing on this page is a certification — request mill certs, CoC or material test reports with your order.