5.1 Tank Steels, Material Specifications & Historic Standards

5.1 Tank Steels, Material Specifications & Historic Standards

API 653 Core Principle: An inspector cannot evaluate the structural fitness-for-service, permissible fill height, or repair feasibility of an existing tank without establishing the precise metallurgical pedigree, chemical composition, deoxidation practice, and toughness characteristics of its shell plates.

Atmospheric storage tank engineering has progressed through nearly a century of evolving metallurgy, structural design theory, and welding technology. Many tanks operating across global terminals, refineries, and chemical facilities were constructed decades ago under legacy standards. To assess mechanical integrity, calculate minimum allowable shell thickness ($t_{min}$), or execute hot work repairs, the API 653 inspector must understand the historical lineage of tank steels, how vintage plates differ from modern specifications, and how API standards classify materials into toughness groups.

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Evolution of Tank Steel Specifications: API 12C to API 650

The construction of welded steel storage tanks began standardizing in the mid-1930s, transitioning through several distinct historical epochs:

1. The Pre-1960s Era and API Specification 12C

Prior to 1961, the benchmark standard for welded aboveground storage tanks was API Specification 12C (Specification for Welded Oil Storage Tanks), first issued in 1936. Tanks built to API 12C incorporated steels with minimal chemical controls and virtually no mandatory impact toughness testing:

• Historic Plate Specifications: The earliest tanks utilized structural carbon steels originally formulated for bridges and railroad cars, predominantly ASTM A7 and ASTM A113, followed by early formulations of ASTM A283 (Grades C and D) and ASTM A285 (Grades B and C).
• Metallurgical Characteristics: These steels were commonly manufactured using rimmed or semi-killed steelmaking practices. They exhibited coarse ferrite-pearlite grain structures, high variability in carbon and manganese content, and elevated levels of residual elements such as phosphorus and sulfur.
• Weldability Challenges: In 1952, ASTM A373 was introduced to improve field weldability by restricting maximum carbon content ($C \le 0.28%$) and establishing manganese minimums. However, API 12C tanks remained vulnerable to strain aging, notch embrittlement, and catastrophic brittle fracture at low ambient temperatures.

2. The Introduction of API Standard 650 (1961)

In December 1961, the American Petroleum Institute replaced API 12C with the first edition of API Standard 650 (Welded Steel Tanks for Oil Storage). Early editions of API 650 introduced basic design rules for allowable stress and plate thickness formulas, but many 1960s and 1970s tanks still utilized general-purpose structural steels:

• ASTM A36: Introduced in 1960, A36 replaced A7 as the primary structural plate. With a specified minimum yield strength ($S_y$) of 36 ksi (250 MPa) and tensile strength ($S_u$) of 58 to 80 ksi (400 to 550 MPa), early A36 plates were produced without grain size controls or impact requirements, limiting their low-temperature fracture toughness.
• ASTM A283 Grade C: Remained the dominant, low-cost structural carbon steel for smaller and mid-sized tanks ($S_y = 30\text{ ksi}$, $S_u = 55\text{ to }75\text{ ksi}$).
• ASTM A285 Grade C: Widely used in pressure vessels and tanks, but inherently semi-killed or rimmed, making it susceptible to cold temperature embrittlement in thicknesses greater than 0.50 in. (12.5 mm).

3. The Modern Era: Fine-Grain Practice and Quenched & Tempered Alloys

Catastrophic tank ruptures during the 1970s and 1980s prompted major revisions to API 650, leading to the adoption of advanced steelmaking technologies:

• ASTM A516 (Grades 55, 60, 65, 70): The modern industry standard for moderate- and lower-temperature service. A516 is a fully killed carbon steel manufactured to fine austenitic grain size practice (ASTM A20), providing outstanding notch toughness and uniform mechanical properties.
• ASTM A537 (Class 1 and Class 2): Heat-treated carbon-manganese-silicon steel. Class 1 is normalized ($S_y \ge 50\text{ ksi}$), while Class 2 is quenched and tempered (Q&T, $S_y \ge 60\text{ ksi}$), offering superior yield strength and notch toughness for very large tanks.
• ASTM A573 (Grades 58, 65, 70): Structural carbon steel plates of improved notch toughness, killed, made with fine-grain practice.
• Modern TMCP Steels (ASTM A841): Thermo-Mechanical Controlled Processing (TMCP) steels utilize microalloying elements (columbium, vanadium, titanium) and precision rolling/cooling cycles to achieve high strength ($S_y \ge 50\text{ to }65\text{ ksi}$) with exceptionally low carbon equivalents, ensuring crack resistance and weldability.

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Steelmaking Deoxidation Practices & Grain Metallurgy

The resistance of carbon steel to cleavage fracture is governed by its deoxidation practice and microstructure:

+-----------------------------------------------------------------------------------+
|                         STEELMAKING DEOXIDATION SPECTRUM                          |
+-----------------------------------------------------------------------------------+
|  RIMMED STEEL             SEMI-KILLED STEEL         FULLY KILLED (FINE GRAIN)     |
|  -----------------        -----------------         -------------------------     |
|  * No deoxidation         * Partial deoxidation     * Complete deoxidation (Si/Al)|
|  * Heavy segregation      * Moderate segregation    * Homogeneous composition     |
|  * High brittle risk      * Structural steels       * Fine austenitic grain (>= 5)|
|  * Prohibited today       * Older A36 / A283        * ASTM A516 / A537 / Normalized|
+-----------------------------------------------------------------------------------+

Rimmed vs. Semi-Killed vs. Fully Killed Steels

1. Rimmed Steel: Manufactured without adding deoxidizers (such as silicon or aluminum) to the molten bath. During solidification, carbon monoxide gas bubbles evolve vigorously, creating a clean outer iron "rim" but heavily concentrating carbon, sulfur, and phosphorus in the core. Rimmed steel exhibits extreme notch sensitivity and aging embrittlement. It is strictly prohibited for shell plates in modern codes.
2. Semi-Killed Steel: Partially deoxidized with small amounts of silicon. Gas evolution is suppressed just enough to prevent shrinkage cavities. While superior to rimmed steel, semi-killed plates retain moderate chemical segregation and non-uniform toughness.
3. Fully Killed Steel: Fully deoxidized by adding silicon and aluminum before pouring. Gas evolution is completely suppressed, producing a uniform, sound ingot without internal blowholes. Fully killed practice is mandatory for high-toughness tank steels.

Fine Austenitic Grain Practice (ASTM A20)

Fine-grain practice involves adding aluminum (minimum 0.020% total aluminum or 0.015% acid-soluble aluminum) or microalloying elements during steelmaking. The aluminum precipitates as aluminum nitrides ($AlN$), which mechanically pin austenite grain boundaries during high-temperature rolling and heat treatment.

• Steels manufactured to fine-grain practice exhibit an ASTM austenitic grain size number of 5 or finer (higher numbers represent smaller grain diameters per ASTM E112).
• Finer ferrite grain size simultaneously increases yield strength and dramatically depresses the Ductile-to-Brittle Transition Temperature (DBTT), lowering the temperature at which brittle cleavage can initiate by up to 50°F to 80°F (28°C to 44°C).

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API 650 Material Groups (Groups I through VI)

API 650 Table 4.4a/b classifies acceptable shell plate materials into six distinct groups based on chemistry, deoxidation, heat treatment, and notch toughness:

Group I

• Metallurgy: Semi-killed or fully killed carbon steels of structural quality, manufactured without mandatory fine-grain controls.
• Representative Grades: ASTM A283 Gr C, ASTM A285 Gr C, ASTM A36 (thicknesses $t \le 0.75\text{ in.}$ [20 mm] when not made to fine grain), ASTM A131 Gr A.
• Characteristics: Moderate tensile strength ($S_u \approx 55\text{ to }75\text{ ksi}$), lowest notch toughness. Maximum non-impact tested plate thickness is limited to 1.00 in. (25 mm).

Group II

• Metallurgy: Fully killed or semi-killed structural steels with enhanced chemistry controls.
• Representative Grades: ASTM A36 ($t > 0.75\text{ in.}$ to $1.50\text{ in.}$ semi-killed or killed), ASTM A131 Gr B, ASTM A573 Gr 58.
• Characteristics: Improved weldability and uniform yield characteristics ($S_y \ge 32\text{ to }36\text{ ksi}$). Non-impact tested thickness up to 1.25 in. (32 mm).

Group III & Group III A

• Metallurgy: Fully killed carbon steels manufactured strictly to fine austenitic grain practice (ASTM A20). Group III is typically as-rolled, whereas Group III A is normalized or TMCP.
• Representative Grades: ASTM A516 Gr 55 and 60, ASTM A573 Gr 65 and 70, ASTM A662 Gr B.
• Characteristics: High notch toughness at sub-zero temperatures. Normalized Group III A plates provide reliable toughness down to -40°F (-40°C).

Group IV & Group IV A

• Metallurgy: Higher-strength, fully killed carbon-manganese steels made to fine-grain practice, supplied as-rolled (not heat treated). Group IV A covers the higher-yield as-rolled/TMCP grades that API 650 requires to be impact tested as a condition of the group assignment.
• Representative Grades: Group IV — ASTM A573 Gr 70, ASTM A516 Gr 65 and 70, ASTM A662 Gr B. Group IV A — ASTM A662 Gr C, ASTM A737 Gr B, ASTM A841.
• Characteristics: Standard specification for modern large-diameter tanks ($S_y \ge 38\text{ to }40\text{ ksi}$, $S_u \ge 65\text{ to }90\text{ ksi}$).
• Exam trap: the grade alone does not fix the group. A516 Gr 70 supplied as-rolled is Group IV; the identical grade supplied normalized moves to Group V. Always read the heat-treatment condition on the MTR before entering API 650 Figure 4.1a/4.1b.

Group V

• Metallurgy: The normalized (heat-treated) counterparts of the Group IV materials, plus normalized higher-strength carbon-manganese-silicon steels.
• Representative Grades: ASTM A573 Gr 70 (normalized), ASTM A516 Gr 65 and 70 (normalized), ASTM A662 Gr B (normalized), ASTM A537 Class 1, ASTM A633 Gr C and D, ASTM A678 Gr A, ASTM A737 Gr B (normalized).
• Characteristics: Normalizing refines the austenitic grain structure, so Group V plates carry markedly better low-temperature Charpy V-notch energy than the same grade as-rolled and sit farther to the left (colder) on the API 650 Figure 4.1a exemption curves.

Group VI

• Metallurgy: Advanced high-strength, quenched and tempered (Q&T) or TMCP microalloyed steels.
• Representative Grades: ASTM A537 Class 2, ASTM A678 Gr B, ASTM A841.
• Characteristics: Yield strength $S_y \ge 60\text{ ksi}$ (415 MPa), tensile strength $S_u \ge 80\text{ to }100\text{ ksi}$. Used for high-capacity crude oil tanks to minimize shell plate thickness while maintaining extreme fracture toughness.

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Material Identification & Unknown Materials (API 653 Section 7)

When inspecting, repairing, altering, or reconstructing an existing tank, the provenance of the steel must be established per API 653 Section 7.

Unknown Materials Policy

If original construction records, mill test reports (MTRs), or manufacturer nameplates are unavailable, the shell plate material is classified as "unknown." Under API 653 Section 7 and Section 4.3.3.1:

• Default In-Service Stresses: For structural evaluation and $t_{min}$ calculations where materials cannot be verified, the allowable stress must be calculated assuming a default minimum tensile strength $T = 55,000\text{ psi}$ and yield strength $Y = 30,000\text{ psi}$ (API 653 Table 4.1 lists the resulting values: $S = 23,600\text{ psi}$ and $S_t = 26,000\text{ psi}$ for the bottom and second courses; $S = 26,000\text{ psi}$ and $S_t = 27,000\text{ psi}$ for all other courses). Note also that API 653 caps $T$ at $80,000\text{ psi}$ even when a higher tensile strength is documented.
• Brittle Fracture Screening: Steels of unknown material specification are not entered into the API 650 Figure 4.1a/4.1b group curves at all. API 653 5.3 screens them instead against API 653 Figure 5.2, Exemption Curve for Tanks Constructed from Carbon Steel of Unknown Material Specification, using the original nominal thickness of the thickest shell plate.

   +-----------------------------------------------------------------+
   |             API 653 PROTOCOL FOR UNKNOWN SHELL STEELS           |
   |                                                                 |
   |   1. COUPON EXTRACTION: Remove representative test plate        |
   |      (shell trepan or section removed during repair/reconstruction)|
   |                                                                 |
   |   2. MECHANICAL TESTING (ASTM A370):                            |
   |      - Two transverse tensile specimens (determine Sy and Su)   |
   |      - Three Charpy V-notch specimens (if MDMT < 60°F)          |
   |                                                                 |
   |   3. CHEMICAL ANALYSIS:                                         |
   |      - Carbon, Manganese, Silicon, Phosphorus, Sulfur           |
   |      - Calculate Carbon Equivalent (CE) for weldability         |
   |                                                                 |
   |   4. DERIVE PERMISSIBLE STRESSES AND WPS CONTROLS               |
   +-----------------------------------------------------------------+

Chemical Analysis & Carbon Equivalent (CE)

Before performing any hot work (such as installing a shell insert plate, door sheet, or nozzle penetration) on an existing shell plate of unknown chemistry, chemical analysis is critical to determine the Carbon Equivalent (CE):

$\text{CE} = \text{C} + \frac{\text{Mn}}{6} + \frac{\text{Cr} + \text{Mo} + \text{V}}{5} + \frac{\text{Ni} + \text{Cu}}{15}$

• Weldability Threshold: If the calculated CE is $\le 0.40%$, the steel possesses good field weldability under standard ambient conditions.
• Preheat & Consumable Mandates: If $\text{CE} > 0.40%\text{ to }0.45%$, the steel is prone to forming hard, brittle martensite in the heat-affected zone (HAZ) when cooled rapidly. Under these conditions, API 653 Section 7.2 and ASME Section IX require:
  1. Mandatory Preheat: Maintaining a minimum interpass and preheat temperature of $200^\circ\text{F to }300^\circ\text{F}$ ($93^\circ\text{C to }149^\circ\text{C}$) to retard cooling rates.
  2. Low-Hydrogen Consumables: Utilizing strictly low-hydrogen electrodes (e.g., AWS E7018 with H4 or H8 diffusible hydrogen ratings), stored in heated portable rod ovens.

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Shell Plate Thickness Grading & Non-Impact Tested Limits

Under API 650 Section 4.2.9 and API 653 Section 5, carbon steel plates that have not undergone Charpy impact testing are restricted by maximum nominal thickness rules:

• Maximum Thickness Limit for Tank Shells: The maximum allowable plate thickness for any carbon steel tank shell constructed to API 650 is 1.75 in. (45 mm), with certain material grades capped at 1.50 in. or 1.00 in.
• Thickness vs. Triaxial Stress State: Thick plates subject flaws to high triaxial restraint (plane strain conditions), preventing plastic slip and promoting cleavage crack initiation. Consequently, as plate thickness increases, higher toughness grades (fine-grain, normalized) or Charpy impact testing are mandatory.

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API 650 Material Groups Comparison Matrix

The following matrix outlines the metallurgy, heat treatment, representative specifications, and relative toughness of API 650 shell plate groups: