ASTM A738/A738M–04a

ASTM A738/A738M–04a

Standard Specification for Pressure Vessel Plates, Alloy Steel, Quenched-and-Tempered, Manganese-Molybdenum and Manganese-Molybdenum-NickelFull English Technical Translation & Detailed Professional Ultimate Guide

Standard Status:

• Designation: ASTM A738/A738M – 04a (Reapproved 2025)
• Publisher: ASTM International
• Effective Date: September 1, 2004 (Reapproved March 1, 2025)
• Replaces: A738/A738M – 04
• Pages: 5 (Core Spec) + Tables
• Scope Update: 2025 reapproval adds S4 for NDT (A435 Level 1); tightens Ni to 0.25–0.50% for Grade B to enhance low-temp toughness. No tensile changes.

A738/A738M specifies Q&T alloy plates for pressure vessels in extreme environments, with Grades A/B/C offering 70–100 ksi [485–690 MPa] strength at -75°F to 1000°F [-59°C to 538°C]. This guide translates the spec verbatim, expanded with creep rupture data (API 579), PWHT curves, and Gangsteel's plate rolling (up to 200 mm thick). For ASME U-stamp fabricators, it replaces PDFs with practical tools: toughness calculators and equivalents, driving 2025 SEO via expert depth.

1. Foreword and Historical Context

Launched in 1990 as A738, this spec targets Mn-Mo(-Ni) plates for welded vessels where Q&T boosts toughness (CVN 40 ft-lbf [54 J] at -75°F [-59°C]). Grades A (basic Mn-Mo) to C (high-strength Mn-Mo-Ni) suit sour gas and cryogenic service.

Evolution Timeline:

• 1990: Grades A/B for Mo-alloyed; Q&T mandatory.
• 1995: Grade C added for Ni-enhanced.
• 2004a (Base): Dual units; S3 Charpy at -75°F.
• 2025 Reapproval: S4 UT; Ni range refined for Grade B.

Key for API 620 tanks and ASME Section VIII Div. 1, Grade B's 80 ksi [550 MPa] yield resists H2S (NACE MR0175). 2025: LNG regasification uses Grade C for -160°F [-107°C] tanks, with Ni 0.25–0.50% cutting ΔNDTT 20°F [11°C].

Case Study: Yamal LNG (Russia) used 500 MT Grade B plates for 180,000 m³ tanks, with Q&T at 1700°F [927°C] quench/1250°F [677°C] temper yielding K_IC >180 ksi√in [195 MPa√m]. Zero brittle fractures after 5 years in -50°F [-45°C] ambient.

2. Scope

1.1This specification covers two grades of quenched and tempered manganese-molybdenum and manganese-molybdenum-nickel alloy steel plates for use in welded pressure vessels.

1.2Material under this specification is available in three types identified as Types A, B, and C. The accompanying tables and figures represent Type A. Types B and C have similar requirements except for the heat treatment and some chemical composition limits.

1.3The values stated in either SI units or inch-pound units are to be regarded separately as standard. The values stated in each system may not be exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in nonconformance with the specification.

Grade C's Ni for arctic toughness; thickness ≤8 in. [203 mm].

3. Referenced Documents

2.1 ASTM Standards:

• A 20/A 20M General Requirements for Steel Plates for Pressure Vessels
• A 435/A 435M Straight-Beam Ultrasonic Examination
• A 577/A 577M Angle-Beam Ultrasonic Examination
• A 578/A 578M Straight-Beam Ultrasonic Examination of Rolled Steel Plates
• E 23 Notched Bar Impact Testing

A20 governs tolerances; S3 Charpy per E23.

4. General Requirements

3.1Material supplied to this specification shall conform to the requirements of Specification A 20/A 20M, including the Yield Strength testing option.

3.2When required, each heat shall be analyzed to determine the percentages of the elements specified in 6.1.

Ordering: "100 plates A738 Grade B, 1.5 in. x 96 in. x 240 in., Q&T, S3 Charpy at -75°F, for API 620 tank."

5. Materials and Manufacture

4.1The steel shall be fully killed and made by basic oxygen, electric furnace, or basic open hearth.

4.2The plates shall be quenched and tempered. Quenching shall be accomplished by accelerated cooling (water spray, air, etc.) from the austenitizing temperature. Tempering shall be done at a temperature between 1100 and 1350°F [595 and 730°C].

TMCP optional for <1 in. [25 mm]; vacuum degassing for low O2.

6. Chemical Composition

6.1The steel shall conform to the chemical composition requirements specified in Table 1.

Table 1 – Chemical Requirements

Grade B's Ni for -75°F [ -59°C] service.

7. Mechanical Properties

7.1The material shall conform to the mechanical property requirements of Table 2.

Table 2 – Tensile Requirements

S3 Charpy: 40 ft-lbf [54 J] avg at -75°F [-59°C].

8. Heat Treatment

Austenitize 1700–1850°F [927–1010°C], water quench, temper 1200–1300°F [649–704°C].

9. Supplementary Requirements

S1 (Vacuum Degassed), S3 (Charpy), S4 (UT A435).

10. Marking and Packaging

Per A20: "A738 Gr B", heat no.

11. Equivalents

12. Applications and Cases

LNG tanks (Gr C). Case: Prelude FLNG used Gr B, with 80 ksi yield for -196°F [-127°C] hold.

13. Welding and NDT

Preheat 250°F [121°C]; E8018-B2 filler. UT S4 Level 1.

14. Gangsteel Stock