SA387 Gr11 Cl2 Chrome-Moly Steel Plate: Complete Technical Guide

SA387 Gr11 Cl2 is a chromium-molybdenum (Cr-Mo) alloy steel plate specifically developed for welded pressure vessels and boilers operating at elevated temperatures. Every part of the designation carries engineering meaning:

• ASTM A387/A387M - "Standard Specification for Pressure Vessel Plates, Alloy Steel, Chromium-Molybdenum" - the base product specification covering Grades 2, 5, 9, 11, 12, 21, 22 and 91.
• ASME SA-387/SA-387M - code edition of the same specification, permitted for construction under ASME Boiler & Pressure Vessel Code Section VIII Divisions 1 and 2.
• Design basis - allowable stresses for elevated temperature are given in ASME Section II Part D; the material is listed in ASME Section II Part A (SA-387).
• Code data - UNS K11789, P-number 4, Group 1. The specification sets no fixed maximum thickness: plates are limited only by the composition's ability to meet the required mechanical properties; practical commercial supply typically extends to ~300 mm (12 in) by mill agreement.

Why these elements:

• Chromium (1.00–1.50%) - provides high-temperature oxidation and scaling resistance, and resistance to graphitization.
• Molybdenum (0.45–0.60%) - raises elevated-temperature creep strength and resists high-temperature hydrogen attack (HTHA).
• Carbon (0.05–0.17%) - controlled to balance strength with weldability; combined with low S/P for toughness.

Elongation measured over 200 mm (8 in) gauge length; on a 50 mm (2 in) gauge, the minimum is 22% for both classes. Hardness is not a mandatory requirement of the base specification; purchaser specifications commonly impose a ceiling of ≤197 HB for Class 2. Impact testing is likewise not mandatory but is frequently specified for low-temperature or hydrogen service.

On a 50 mm (2 in) gauge, the minimum elongation is 22% for both classes; the 200 mm gauge values differ (19% Class 1, 18% Class 2).

Selection rule: choose Class 2 when design stress, wall-thickness reduction or hydrogen-service integrity is the driver; choose Class 1 when formability and ductility dominate. For modern refinery and petrochemical hydrogen service, Class 2 is the prevailing choice.

• Class 2 mandatory condition: normalized and tempered (N+T). Normalizing is typically performed at ~900–950°C, followed by tempering at a minimum of 620°C (1150°F) to develop the tempered bainite/martensite microstructure that balances strength with toughness.
• Alternative condition: for very thick sections or special toughness requirements, quenched and tempered (Q+T) may be specified by agreement.
• Inspection: plates are supplied with full mechanical testing (tensile per ASTM A370), and ultrasonic examination per ASTM A578 is commonly specified for critical hydrogen-service vessels.

The engineering identity of SA387 Gr11 Cl2 is defined by three elevated-temperature failure modes it is designed to defeat:

• High-temperature hydrogen attack (HTHA) - carbon steels decarburize and fissure in hydrogen service above ~200–350°C; the Cr-Mo alloy system raises the safe hydrogen-service envelope and delays methane-forming internal damage.
• Creep deformation - molybdenum provides the elevated-temperature creep strength that keeps vessels and headers dimensionally stable at continuous service temperatures of 350–600°C.
• Oxidation, scaling and graphitization - chromium stabilizes carbides and forms protective oxide layers, preventing graphitization embrittlement that limits carbon-steel service life.

Service envelope: continuous operating temperatures roughly 350–600°C. For sour-service (H₂S) or high-pressure hydrogen duties, purchasers commonly impose supplementary requirements aligned with NACE MR0175 / ISO 15156 and API 934 to control sulfide stress cracking (SSC) and hydrogen-induced cracking (HIC) risks.

Equivalence is compositional/mechanical in nature; formal substitution on a code-stamped vessel requires verification against the governing code edition and the purchaser's specification.

Cr-Mo steels are air-hardening: the weld metal and heat-affected zone (HAZ) harden readily, creating severe cold-cracking risk in thick sections. The following are code-level engineering practices; production work must follow a qualified WPS:

• Preheating: minimum 150°C, typically 150–250°C depending on thickness and restraint; hold preheat continuously through welding for thick-wall vessels.
• Consumables: low-hydrogen Cr-Mo matching consumables (e.g. AWS E8018-B2 / ER80S-B2 class); store and bake per manufacturer requirements to keep diffusible hydrogen low.
• Interpass temperature: generally maintained between 200°C and 300°C - high enough to avoid HAZ cracking, low enough to avoid over-softening.
• Post-weld heat treatment (PWHT) - mandatory: required per ASME Section VIII Division 1, typically at 650–760°C (1200–1400°F), with holding time based on thickness. PWHT relieves residual stress and tempers the HAZ to restore toughness and prevent delayed cracking.
• Cooling control: after welding and before PWHT, components should not be allowed to cool below ~200°C in high-restraint conditions; use controlled cooling or immediate PWHT scheduling.
• Disbonding control (hydrogen service): for thick-wall hydrogen reactors, strict hydrogen control during welding and proper PWHT minimize the risk of hydrogen-induced disbonding of the weld overlay interface.

• Hydroprocessing reactors: hydrodesulfurization (HDS) and hydrocracking reactor shells in refineries - the flagship application of 1.25Cr-0.5Mo steel.
• Catalytic units: catalytic cracking, reforming and coking vessel shells, hot-wall reactors and internals supports.
• High-pressure steam equipment: steam headers, boiler drums and superheater components in power and utility plants.
• Petrochemical equipment: desulfurization towers, amine regenerators, heat-exchanger shells and channel heads in hydrogen and sour service.
• Liquefied petroleum gas (LPG) and ammonia plants: vessels subject to elevated temperature and hydrogen partial pressure.
• Heavy-wall cladded vessels: base plates for stainless or alloy weld-overlaid reactor walls in severe service.

• Specification identity: order as "SA387 Gr11 Cl2" with the governing edition (e.g. ASTM A387/A387M and ASME SA-387/SA-387M); confirm Class 2 and the required delivery condition (N+T).
• Inspection documents: require a mill test certificate (MTC) per EN 10204 3.1 (or equivalent) with heat analysis, heat-treatment record and mechanical test results.
• Non-destructive testing: ultrasonic examination per ASTM A578 (common levels: 100% UT, class B or C) for hydrogen-service and thick-wall plates.
• Supplementary tests: PMI (positive material identification) of Cr/Mo on every plate; hardness survey ≤197 HB; Charpy impact per project specification where required; NACE MR0175 / API 934 conformance for sour service.
• Dimensional control: thickness tolerances per A20/A20M (the general requirements standard referenced by A387); verify flatness, width, length and edge condition.
• Stock range: plates are commonly available 6–100 mm thick; heavier gauges (up to ~300 mm) are mill-rolled to order for reactor fabrication.
• Weight calculation: density ≈ 7.85 g/cm³; use actual dimensions and the mill certificate thickness for order-weight and freight estimates.

None in technical content - they are the same specification issued by two organizations. ASME SA-387 is the code-referenced edition used for vessels built to the ASME Boiler & Pressure Vessel Code; ASTM A387 is the base specification. Materials satisfying either are acceptable to the other when the edition matches.

Q2: Why is it called 1.25Cr-0.5Mo?

It refers to the nominal alloy content: approximately 1.25% chromium (actual range 1.00–1.50%) and 0.5% molybdenum (actual range 0.45–0.60%). Chromium provides oxidation/scaling resistance, molybdenum provides creep strength and resistance to high-temperature hydrogen attack.

Q3: What service temperature range is SA387 Gr11 Cl2 suitable for?

Continuous elevated-temperature service is typically 350–600°C, where the alloy's creep strength, oxidation resistance and HTHA resistance exceed the capability of carbon steels. Exact allowable stresses are taken from ASME Section II Part D for code design.

Q4: Is post-weld heat treatment (PWHT) mandatory for SA387 Gr11 Cl2?

Yes for code construction - PWHT at 650–760°C is required per ASME Section VIII Division 1. It relieves residual stress and tempers the hardened heat-affected zone, preventing delayed hydrogen cracking and restoring toughness in the welded joint.

Q5: Can SA387 Gr11 Cl2 be used in sour (H₂S) hydrogen service?

Yes, with supplementary requirements. For sour service, purchasers typically impose NACE MR0175 / ISO 15156 and API 934 criteria - including hardness control, PWHT compliance and low residual elements - to prevent sulfide stress cracking and hydrogen-induced cracking.

Q6: SA387 Gr11 Cl2 or SA516 Gr.70 - how to choose?

Choose SA516 Gr.70 carbon steel for ambient and moderate-temperature general pressure vessel duty where cost dominates and hydrogen is absent. Choose SA387 Gr11 Cl2 when continuous service exceeds roughly 350–400°C, when the design involves hydrogen partial pressure (HTHA risk), or when creep and oxidation resistance are required. The Cr-Mo plate costs more but is the code-accepted answer for elevated-temperature and hydrogen service.