Semi-Elliptical Pressure Vessel Head ASTM A516 GR. 70 Carbon Steel ASTM B171 C71500 Cu-Ni Clad 2:1

ASTM A516 GR. 70 +ASTM B171 C71500 CARBON STEEL CLAD DISHED SPHERICAL HEAD FOR EQUIPMENT 

Overview – Clad Elliptical Head as Pressure Vessel Closure Component

A clad elliptical head is a formed pressure vessel end closure consisting of a carbon steel or alloy steel base plate with a corrosion-resistant alloy (CRA) layer metallurgically bonded to the process-facing surface. The 2:1 semi-elliptical shape is the most common torispherical/ellipsoidal head configuration in shell and tube heat exchangers, pressure vessels, reactors, and storage tanks, providing an optimal balance between pressure containment efficiency and forming cost.

The cladding layer provides corrosion resistance to the process fluid, while the base material provides structural strength and pressure integrity. This product is designed and manufactured per ASME Section VIII Division 1 or Division 2, and GB/T 25198 (Pressure Vessel Heads) for domestic applications.

Geometric Definition – 2:1 Elliptical Head

The 2:1 semi-elliptical head has an inside depth (at center) equal to one-quarter of the inside diameter (ID) of the shell to which it is attached:

• Major axis (D) = Shell ID

• Minor axis (h) = D/4 (inside depth from tangent line to crown center)

• Knuckle radius (r) ≈ 0.17 × D (transition zone between crown and straight flange)

• Crown radius (R) ≈ 0.90 × D (spherical central portion)

Cladding Material and Base Material Combinations

The clad elliptical head is manufactured by applying a corrosion-resistant alloy (CRA) layer to a carbon steel or alloy steel base plate through one of the following methods:

• Explosion-bonded clad plate: Detonation of controlled explosive charge creates a metallurgical bond between CRA and base material (per ASME SA-263 / SA-264 / SA-265)

• Roll-bonded clad plate: CRA and base material are hot-rolled together under pressure to form a metallurgical bond

• Weld overlay (strip cladding): CRA material is weld-deposited onto the base plate surface in overlapping strips (per ASME IX)

Base Material Options

Cladding Material Options

Cladding Thickness (CRA Layer)

• Standard clad thickness: 2mm – 6mm (typically 3mm minimum per ASME VIII-1 UW-13)

• Clad thickness tolerance: +0.5mm / -0.0mm (no under tolerance)

• Clad ratio: Clad thickness as a percentage of total thickness (clad + base). For corrosive service, clad thickness is selected based on required corrosion allowance.

Total Nominal Thickness

• Base thickness: 6mm – 80mm (selected per ASME VIII-1 UG-27 / UG-32 for pressure containment)

• Total thickness = Base thickness + Clad thickness (clad included in pressure calculation only if metallurgically bonded; per ASME VIII-1 Appendix AA)

• Minimum formed thickness after forming: ≥ 90% of nominal thickness (per GB/T 25198 / ASME VIII-1 UG-79)

Forming Process – Hot or Cold Forming

Clad elliptical heads are formed from clad plate using one of the following methods. Process selection depends on cladding material, total thickness, and required precision.

Hot Forming

• Forming temperature: 900°C – 1,050°C (for carbon steel base + stainless clad)

• Cladding side protection: Oxidizing atmosphere controlled to prevent excessive scaling; inert gas purge or protective coating applied to clad surface

• Heating rate: ≤ 150°C/hour to prevent thermal shock and cladding delamination

• Quench / cool down: Air cooling or controlled furnace cooling (for alloy base materials requiring PWHT)

Cold Forming

• Forming temperature: Ambient (20°C – 40°C)

• Forming equipment: Press brake or dishing press with matched dies

• Springback compensation: Die dimensions adjusted for cold-formed springback (typically 0.5% – 1.5% of crown radius per head diameter and material yield strength)

• Maximum cold forming strain: Limited to prevent clad delamination and base material cracking. For carbon steel base with stainless clad, recommended forming strain ≤ 15% (per manufacturer's forming procedure).

• After cold forming: Full post-weld heat treatment (PWHT) required per ASME VIII-1 UCS-56 if base material thickness > 38mm or if service requires stress relief.

Forming Tolerances (per ASME VIII-1 UG-81 / GB/T 25198)

Cladding Integrity – Bond Quality and Testing

The metallurgical bond between clad and base material must be verified per ASME VIII-1 UW-13 and ASME SA-263 / SA-264 / SA-265.

Bond Integrity Criteria

• Shear strength (clad-to-base bond): ≥ 140 MPa (for explosion-bonded plate) per ASTM B898

• Bond area: ≥ 99% of total clad surface (clad surface area that is metallurgically bonded)

• Unbonded area allowance: Individual unbonded patches ≤ 100mm²; total unbonded area ≤ 1% of surface (per ASME VIII-1)

Non-Destructive Examination (NDE) of Clad Surface

• Ultrasonic testing (UT) – 100% scan of clad-to-base interface for bond defects and delamination (per ASME V, Article 5)

• Acceptance criteria: No linear indications > 20mm; no clustered indications; no indication with amplitude ≥ 50% of reference level

• Liquid penetrant (PT) – 100% of clad surface and weld joints for surface cracks (per ASME V, Article 6)

• Acceptance: No cracks, no porosity larger than 0.8mm, no clustered porosity

Welding to Shell – Joint Design and Preparation

The clad elliptical head is welded to the shell or flange at the tangent line. The joint must maintain both structural integrity and corrosion resistance across the clad interface.

Joint Preparation (Per ASME VIII-1 UW-13)

• Bevel angle: 30° – 37.5° (single V or double V)

• Root gap: 2mm – 4mm

• Clad layer step-back: The cladding material is cut back approximately 2mm – 3mm from the weld groove edge to allow the first pass (base material weld) to be deposited without melting the clad layer into the weld

• Transition weld: A buttering layer of cladding material is deposited on the weld groove surface (if required) to achieve a corrosion-resistant weld surface

Weld Sequence (Recommended)

1. Base material weld (carbon steel / alloy) – deposited first, 100% RT or UT per ASME VIII-1 UW-52

2. Clad-side weld (stainless or CRA) – deposited after base weld is completed and inspected

3. Clad overlay on weld surface – final layer of CRA material deposited to restore corrosion resistance across the joint

4. Final PT inspection of clad surface and weld overlay – 100%

PWHT (Post-Weld Heat Treatment)

• Required per ASME VIII-1 UCS-56 when:

  • Base material thickness > 38mm

  • Service contains wet H₂S (NACE MR0175 / ISO 15156)

  • Base material is low-alloy steel (SA-387) requiring tempering

• PWHT temperature: 620°C ± 15°C for carbon steel; 680°C – 760°C for low-alloy steel (per UCS-56 tables)

• Hold time: 1 hour per 25mm of base thickness; minimum 1 hour

• Cooling rate: ≤ 50°C/hour to 400°C (for carbon steel) to prevent carbide precipitation or stress cracking

• Clad protection during PWHT: Inert gas purge (argon) applied to clad surface to prevent oxidation; thermal shields installed if required to minimize clad surface scaling

Inspection and Testing per Head

Dimensional Inspection

• Crown radius: Measured with templates or 3D scanning – tolerance ±1.5%

• Inside depth (h): Measured from tangent line to crown center – tolerance ±1.0%

• Shell ID matching at tangent line: Out-of-roundness ≤ 1.0% of shell ID

• Straight flange length: Typically 25mm – 100mm (per design); tolerance ±2mm

• Clad thickness: Ultrasonic thickness gauge at 10% of surface area – minimum thickness ≥ nominal clad thickness – 0.5mm

Non-Destructive Examination (NDE)

Pressure Testing (when welded to vessel)

• Hydrostatic test: 1.3 × design pressure × (S at test temp / S at design temp) per ASME VIII-1 UG-99

• Hold time: 30 minutes minimum

• Acceptance: Zero pressure drop; no visible leakage at weld joints or clad interface

• Clad surface after hydrotest: Visual inspection for blistering or delamination – no visible blistering allowed

Documentation per Shipment

• Material test certificates (EN 10204 3.1 or 3.2) – base plate and cladding material

• Clad plate manufacturing report (explosion-bond or roll-bond) – including bond shear strength test results

• ASME U-stamp data report (if applicable)

• GB/T 25198 data sheet (if domestic standard)

• Dimensional inspection report (crown radius, depth, thickness, out-of-roundness)

• NDE reports – UT bond scan, PT surface scan, RT/UT weld scan (if welded)

• PWHT chart recording (time-temperature curve) – if performed

• Forming procedure specification (FPS) and forming report (hot/cold forming records)

• Clad thickness measurement report (ultrasonic thickness gauge records)

Selection Checklist – Clad Elliptical Head

1. Shell ID (inside diameter) – determines head size

2. Design pressure – determines base thickness per ASME VIII-1 UG-32

3. Design temperature – determines base material grade and PWHT requirement

4. Process fluid corrosivity – determines cladding material and minimum clad thickness

5. Cladding material required – 304L / 316L / Duplex / Ti / Alloy 625 / Alloy C276

6. Clad thickness required – based on corrosion allowance (typically 3mm minimum)

7. Base material required – CS SA-516 Gr.70 / Q345R / SA-387 Gr.11/22

8. Forming method preference – hot forming or cold forming

9. PWHT requirement – based on base thickness and service

10. NDE requirement – per ASME VIII-1 or per project specification (e.g., 100% UT bond scan, 100% PT)

11. Flange or nozzle attachments – if already welded to head, provide weld joint details

12. Paint/coating specification – for external surface (carbon steel base) if not insulated

Design Limitation Statement – Clad Elliptical Head

The clad elliptical head is not applicable for:

• Services where operating temperature causes cladding delamination due to thermal expansion mismatch (clad and base materials have different coefficients of thermal expansion). Per ASME VIII-1 Appendix AA, thermal stress between clad and base must be included in design calculations.

• Severe thermal cycling (> 1,000 cycles/year) where clad-to-base bond may fatigue. Clad thickness and bond integrity should be re-evaluated per fatigue analysis per ASME VIII-2.

• Very high temperature (> 540°C) where cladding material loses corrosion resistance or undergoes phase transformation. Alloy 625 or Alloy 800 is recommended for such service.

• Services with rapid pressure cycling (pulsating service) – clad bond may be subject to peel stress; consult manufacturer for additional bond testing.