5.3 Charpy V-Notch Impact Testing & Minimum Design Metal Temperature (MDMT)

5.3 Charpy V-Notch Impact Testing & Minimum Design Metal Temperature (MDMT)

API 653 Core Principle: When an aboveground storage tank's shell plates, replacement inserts, or reconstruction components cannot be exempted from brittle fracture evaluation, toughness must be quantitatively proven through standardized Charpy V-Notch impact testing at or below the governing Minimum Design Metal Temperature (MDMT).

Quantitative fracture control requires measuring the dynamic notch toughness of steel plates, weld metal deposits, and heat-affected zones (HAZ). API Standard 650 (Section 4 and Section 9) and API Standard 653 (Section 7 and Section 11) establish rigorous protocols for Charpy testing, specimen orientation, absorbed energy criteria, and welding procedure qualifications.

---

Minimum Design Metal Temperature (MDMT) Determination

The Minimum Design Metal Temperature (MDMT) is the lowest metal temperature at which an aboveground storage tank shell is designed to safely sustain operational tensile stresses without risk of brittle fracture.

The API 650 15°F Safety Margin Formula

Per API Standard 650 Section 3.8 (the definition of design metal temperature) and Section 4.2.10, the design metal temperature for unheated atmospheric storage tanks is calculated as:

$\text{MDMT} = \text{Lowest One-Day Mean Ambient Temperature} + 15^\circ\text{F} \quad (8^\circ\text{C})$

• Climatic Isothermal Maps: The lowest one-day mean ambient temperature is determined using the official climatic isothermal maps published in API 650 Figure 4.2 (for North America) or local meteorological weather data recorded over a minimum 30-year observation period.
• Engineering Rationale for the $+15^\circ\text{F}$ Margin: Large-volume liquid storage tanks possess immense thermal mass. Because liquids have a high specific heat capacity, the stored product cools much more slowly than fluctuating atmospheric air. During a sudden 24-hour winter cold snap, the bulk product and the wetting liquid boundary layer prevent the steel shell plate from reaching the absolute lowest ambient air spike. The $+15^\circ\text{F}$ ($+8^\circ\text{C}$) addition represents this protective thermal buffer.

   [ Ambient Air Extreme: e.g., -20°F (-29°C) ]
                    |
                    v   (Thermal Lag / Product Mass Buffer: +15°F / +8°C)
   [ Stored Liquid & Shell Plate Minimum MDMT: e.g., -5°F (-21°C) ]

MDMT for Heated Tanks

If an aboveground storage tank is equipped with internal steam coils, electrical heaters, or continuous hot process feed, the MDMT may be established as the lowest controlled operational product temperature, provided:

1. The operating facility enforces strict, automated low-temperature alarms and operational interlocks.
2. In the event of a heating system shutdown during sub-freezing weather, emergency operating procedures prohibit filling or adding hydrostatic load until heating is fully restored.

---

Charpy V-Notch (CVN) Testing per ASTM A370

The standard mechanical test utilized by API 650, API 653, and ASME Section IX to quantify notch toughness is the Charpy V-Notch (CVN) Impact Test, conducted in strict accordance with ASTM A370 (Standard Test Methods and Definitions for Mechanical Testing of Steel Products).

           +------------------- 55 mm -------------------+  
           |                                             |  
      10 mm|                    | |                      | 10 mm
           |                    v v Notch: 2 mm deep     |  
           +-------------------\\   //-------------------+
                                \\_// Root radius: 0.25 mm
                                45° Angle
                                    ^
                                    |  (Striker impacts opposite notch)

Standard Specimen Geometry & Dimensions

• Full-Size Specimen: A rectangular bar measuring exactly $10\text{ mm} \times 10\text{ mm} \times 55\text{ mm}$.
• Notch Detailing: A machined 45° V-notch, $2\text{ mm}$ deep, with a precision root radius of $0.25\text{ mm}$ (±0.025 mm) centered along the 55-mm length.
• Test Execution: The specimen is supported horizontally at both ends as a simple beam. A calibrated swinging pendulum striker impacts the face directly opposite the machined notch with a known kinetic energy (typically 240 to 300 ft-lbs). The pendulum fractures the specimen, and the energy absorbed during rupture is indicated on a calibrated dial or digital encoder.
• Sub-Size Specimens: When the plate thickness is insufficient to extract full-size $10\text{ mm} \times 10\text{ mm}$ bars, ASTM A370 and API 650 permit sub-size specimens ($10\times 7.5\text{ mm}$, $10\times 5.0\text{ mm}$, or $10\times 2.5\text{ mm}$). Minimum absorbed energy values must be adjusted downward by code-specified reduction ratios, or the testing temperature must be lowered accordingly.

Specimen Orientation: Transverse vs. Longitudinal

A fundamental metallurgical requirement in tank plate testing is specimen orientation relative to the final hot-rolling direction of the steel plate:

+-----------------------------------------------------------------------------------+
|                         SPECIMEN ORIENTATION COMPARISON                           |
+-----------------------------------------------------------------------------------+
|  TRANSVERSE SPECIMEN (MANDATORY PER API 650)   |  LONGITUDINAL SPECIMEN           |
|  -------------------------------------------   |  ---------------------           |
|  * Specimen length perpendicular to rolling    |  * Specimen length parallel      |
|  * Notch parallel to rolling direction         |  * Notch perpendicular to rolling|
|  * Crack propagates along rolling inclusions   |  * Crack cuts across inclusions  |
|  * Yields lower, conservative Charpy values    |  * Yields artificially high energy|
|  * MATCHES SHELL CIRCUMFERENTIAL HOOP STRESS   |  * Non-conservative for tanks    |
+-----------------------------------------------------------------------------------+

• Why Transverse Specimens Govern: During plate manufacturing, non-metallic inclusions (primarily manganese sulfides [$MnS$] and silicates) are flattened and elongated into longitudinal "stringers." A transverse specimen has its notch oriented parallel to these stringers, so the fracture path tears along the weakest metallurgical plane. Because hoop stress in a cylindrical tank acts circumferentially—stretching the plate across these longitudinal rolling planes—the transverse orientation reflects the true operational failure mode. API 650 requires transverse specimens for shell plate qualification.

---

Fracture Mechanics & Transition Curve Evaluation

Charpy testing across a temperature spectrum generates the characteristic Ductile-to-Brittle Transition Curve, defining three critical regions:

1. Upper Shelf (Ductile Region): At elevated temperatures, energy absorption is high (e.g., 60 to 120 ft-lbs). Failure occurs by ductile tearing and microvoid coalescence, exhibiting high plastic deformation.
2. Transition Region: As temperature drops, the fracture mode shifts rapidly from ductile shear to brittle cleavage. Slight changes in temperature produce extreme variations in impact resistance.
3. Lower Shelf (Brittle Region): At low temperatures, energy absorption plunges to minimal levels (typically $< 5\text{ to }10\text{ ft-lbs}$). The fracture face is 100% transgranular cleavage.

Absorbed Energy (ft-lbs)
   100 +---------------------------------------------- UPPER SHELF (Ductile Tearing)
       |                                             / 
    80 |                                            /  
       |                                           /   
    60 |                                          /    
       |                                TRANSITION     
    40 |                                  ZONE         
       |                                  /            
    20 +-- MINIMUM CODE THRESHOLD -------/             
       |                                /              
     0 +-------------------------------+-------------- LOWER SHELF (Cleavage)
     -60    -40    -20     0     20    40    60    80
                           Temperature (°F)

The Three Measured CVN Parameters

ASTM A370 and API 650 require recording three distinct parameters for each fractured Charpy specimen:

1. Absorbed Energy: Total energy absorbed during fracture, measured in foot-pounds (ft-lbs) or Joules (J).
2. Lateral Expansion: The measure of plastic ductility opposite the notch, recorded in mils (0.001 in.) or millimeters (mm) using a specialized micrometer gage. API 650 requires a minimum lateral expansion of 15 mils (0.38 mm) for higher-strength alloys.
3. Percent Shear Fracture: Visual examination of the fracture surface under low magnification to determine the percentage of dull, fibrous shear lips versus shiny, crystalline cleavage facets.

---

API 650 Table 4.5a / 4.5b Acceptance Criteria

Under API 650 Section 4.2, base metal plates required to be impact-tested must satisfy the minimum energy criteria outlined in Table 4.5a (US customary units) or Table 4.5b (SI units). Whether impact testing is required at all is decided by plotting the governing thickness against the design metal temperature on API 650 Figure 4.1a/4.1b for the plate's material Group, and the Group itself is read from API 650 Table 4.4a/4.4b.

Exam navigation: the BOK lists these four lookups explicitly — design metal temperature (API 650 3.8, 4.2.10, Figure 4.2), material Group number (Tables 4.4a/4.4b), whether impact testing is required (Figures 4.1a/4.1b), and whether measured impact values are acceptable (Tables 4.5a/4.5b). Learn them as a four-step chain.

Representative Transverse Acceptance Standards (Table 4.5a)

• Test Requirements: A standard test consists of three specimens extracted from a single plate coupon taken from the finished heat-treated plate.
• Group I, II, III Steels ($S_y \le 36\text{ ksi}$ / 250 MPa):
  • Minimum average absorbed energy for 3 specimens: 15 ft-lbs (20 J).
  • Minimum absorbed energy for any single specimen: 12 ft-lbs (16 J).
• Group IV, IV A, V Steels ($36 < S_y \le 50\text{ ksi}$ / 345 MPa):
  • Minimum average absorbed energy for 3 specimens: 20 ft-lbs (27 J).
  • Minimum absorbed energy for any single specimen: 15 ft-lbs (20 J).
• Group VI Steels ($S_y > 50\text{ ksi}$ / Q&T Alloys):
  • Minimum average absorbed energy for 3 specimens: 25 to 30 ft-lbs (34 to 41 J), accompanied by a mandatory minimum lateral expansion of 15 mils (0.38 mm).
• Retest Provisions: If the average energy meets the threshold but one single specimen falls below the individual minimum, or if the average falls just below the limit (by no more than 1 ft-lb), three additional specimens may be tested. All three retest specimens must meet or exceed the required average.

---

Impact Testing Triggers under API 653 (Repairs, Alterations & Reconstruction)

In-service modifications governed by API 653 trigger mandatory Charpy impact testing under several specific engineering conditions:

1. Replacement Shell Plates, Door Sheets & Insert Plates (API 653 Section 7.2)

• When a shell plate section is removed to create an equipment access opening (door sheet) or to replace corroded/damaged plate, the new replacement plate must meet current API 650 impact testing requirements for the specified MDMT if:
  1. The governing shell plate thickness exceeds 0.50 in. (12.5 mm); and
  2. The plate material and design metal temperature combination falls below the applicable exemption curve — API 650 Figure 4.1a/4.1b for an identified steel entered on its material-Group curve, or API 653 Figure 5.2 for carbon steel of unknown material specification.
• If the host shell is an unknown vintage material and an insert is installed, the new plate must match or exceed the toughness of Group III or IV fine-grain steel.

2. Major Alterations and Reconstructed Tanks (API 653 Sections 8 and 12.3)

• Tanks being dismantled and re-erected at a new site must undergo a comprehensive brittle fracture evaluation for the new location's MDMT.
• If a reconstructed tank's shell plates are $> 0.50\text{ in.}$ thick and do not meet the applicable exemption curve (API 650 Figure 4.1a/4.1b, or API 653 Figure 5.2 for unknown material) for the new site's design metal temperature, impact testing of coupons removed from each shell course is mandatory before re-certification.

---

Welding Consumable & Procedure Qualification (ASME Section IX + API 650)

High base metal toughness is entirely useless if the weld metal or adjacent Heat-Affected Zone (HAZ) is brittle. Under ASME Section IX (QW-250) and API 650 Section 9.2, whenever base metal impact testing is required, the Welding Procedure Specification (WPS) must be qualified with impact testing recorded on the Procedure Qualification Record (PQR).

                    PQR CHARPY SPECIMEN EXTRACTION LOCATIONS
               +------------------------------------------------+
               |            WELD METAL CENTERLINE               |
               |  [ 3 Transverse Specimens centered in weld ]   |
               +------------------------------------------------+
               |         HEAT-AFFECTED ZONE (HAZ)               |
               |  [ 3 Specimens: Notch at Fusion Line (FL) ]     |
               |  [ 3 Specimens: Notch at FL + 2 mm ]           |
               |  [ 3 Specimens: Notch at FL + 5 mm ]           |
               +------------------------------------------------+

Scope note for the ICP exam. API's API 653 Body of Knowledge states that the WPS/PQR review on the exam will not include Charpy impact requirements or supplementary essential variables, and that any PQR/WPS presented will not include heat treatment requirements. The material below is included because it governs real repair work on impact-tested tanks; treat it as background rather than as an exam-scored item.

ASME Section IX Supplemental Essential Variables

When impact testing is required, ASME Section IX classifies multiple standard variables into supplemental essential variables (e.g., QW-403.5, QW-406.1, QW-407.2, QW-409.1, QW-410.9). Any of the following changes requires requalification of the WPS:

• Change in Base Metal Group / P-Number / Group Number.
• Maximum Heat Input ($kJ/in$): An increase in welding heat input ($H = \frac{60 E I}{1000 v}$). High heat input slows cooling, causing severe grain coarsening in the coarse-grained HAZ (CGHAZ) and destroying notch toughness.
• Preheat & Interpass Temperature: An increase of more than $100^\circ\text{F}$ ($55^\circ\text{C}$) in the maximum interpass temperature.
• Post-Weld Heat Treatment (PWHT): Any addition or deletion of PWHT, or a change in PWHT soaking time or temperature range.
• Welding Consumable Classification: A change in filler metal AWS classification (e.g., changing from an impact-tested electrode like E7018-1 to standard E7018).

PQR Impact Testing Matrix

The PQR test weldment must yield Charpy specimens from two distinct zones:

1. Weld Metal Centerline: Three specimens extracted with notches positioned completely within deposited weld metal.
2. Heat-Affected Zone (HAZ): Three specimens with notches positioned precisely along the fusion line (FL), and additional specimens at FL + 2 mm and FL + 5 mm if specified.

• Acceptance Criteria: Both the weld deposit and the HAZ specimens must achieve average and single-specimen absorbed energy values equal to or greater than the base metal requirements specified in API 650 Table 4.5a at the design metal temperature.