Clad Tubesheet SA350 LF2 CL1 + SA240 SS316L

Base Metal SA350 LF2 CL1 and Cladding SA240 SS316L

1. Base Metal: SA350 LF2 CL1 – The Core of Load-Bearing Strength

Material nature: ASTM A350 LF2 CL1 is a low-carbon steel forging specifically designed for low-temperature service. The CL1 grade requires a lower upper strength limit and higher low-temperature impact toughness.

Key performance indicators:

• Yield strength ≥250 MPa, tensile strength 485–655 MPa
• Impact energy ≥27 J at -46 °C, effectively resisting low-temperature brittle fracture
• Forged structure is dense, free from casting defects, capable of withstanding differential pressure across the tubesheet as well as external mechanical loads

Role in the composite tubesheet: Provides overall structural strength, withstands equipment pressure, tube pull-out forces, thermal stresses, and installation loads.

2. Cladding: SA240 SS316L – The Corrosion Barrier

Material nature: ASTM A240 316L is an ultra-low-carbon austenitic stainless steel plate containing 2–3% molybdenum.

Key performance indicators:

• Pitting Resistance Equivalent Number (PREN) ≈ 24–26, offering excellent resistance to chlorides, dilute sulfuric acid, organic acids, etc.
• Carbon content ≤0.03%, no carbide precipitation in the solution-annealed condition, strong resistance to intergranular corrosion in the weld heat-affected zone
• Good work-hardening characteristics, adaptable to subsequent operations such as tube hole expansion and local Weld overlay

Role in the composite tubesheet: Directly contacts corrosive tube-side fluids, protecting the base metal from chemical attack.

Bonding Processes of Clad Tubesheet SA350 LF2 CL1 + SA240 SS316L

1. Explosion Bonding – The Only Feasible Process for SA240 Plate Cladding

Because the cladding material is SA240 SS316L plate, its bonding to the base metal cannot be achieved by weld overlay (weld overlay uses filler metals, not finished plates). The industry standard processes are explosion bonding or explosion + roll bonding.

Typical process sequence:

• Material preparation: SA350 LF2 CL1 forging as base plate (typical thickness 50–200 mm) + SA240 SS316L as cladding plate (thickness 3–10 mm).
• Explosion bonding: The cladding plate is placed parallel to the base plate with a set standoff distance, and explosives are evenly distributed on top. Upon detonation, the explosion generates a high-pressure shockwave that drives the cladding plate at high speed into the base plate. A solid-state metallurgical bond is formed within microseconds, with a wavy interface that provides extremely high strength.
• Post-bond heat treatment (stress relief): After explosion bonding, a stress-relief anneal is performed, typically at 620 ± 10 °C, followed by slow cooling. The sensitisation temperature range of austenitic stainless steel (450–850 °C) must be avoided to prevent chromium carbide precipitation.
• Rolling (optional): If the required tubesheet thickness exceeds the capacity of explosion bonding, the composite billet can be heated and rolled to the target thickness, further densifying the bond interface.
• Tubesheet finishing: The composite plate is then drilled, bored, and tube hole grooves are machined. The inside surface of tube holes naturally retains the stainless steel cladding (if design requires, local weld overlay can be applied to extend protection into the holes).

Key quality indicators:

• Bonding ratio: According to ASME SA-263 or NB/T 47002.1, any unbonded area detected by ultrasonic testing must have a diameter ≤50 mm, with a minimum spacing ≥100 mm, and total bonded area ≥97%.
• Interface shear strength: Typical value ≥200 MPa (for carbon steel – stainless steel combinations).
• Cladding thickness uniformity: Thickness deviation after explosion bonding is generally ≤±0.5 mm.

2. Weld Overlay

Weld overlay is the deposition of a corrosion-resistant layer onto the base metal surface using filler materials (wires or strips) and cannot use SA240 plates. It is commonly applied for:

• Local weld overlay on tube hole bores or gasket seating surfaces of an otherwise explosion-bonded clad tubesheet (because tube holes are too small for plate cladding);
• Repair work or small-batch non‑standard tubesheets.

Key technical points for weld overlay:

• Use a two-layer procedure: E309L buffer layer + E316L cover layer to prevent carbon migration that could form a brittle martensitic zone at the interface.
• Post-weld heat treatment temperature ≤620 °C, with rapid cooling through the sensitisation range.
• Ferrite number (FN) of the overlay should be controlled between 3 and 10 FN to avoid hot cracking or 475 °C embrittlement.

For large-area cladding using SA240 SS316L plate, explosion bonding is the only proper manufacturing method. Weld overlay is only a supplementary technique, and its filler metals are not designated as SA240.

Design Advantages of Clad Tubesheet SA350 LF2 CL1 + SA240 SS316L

Compared with a solid SA240 SS316L forged tubesheet, an explosion-bonded composite tubesheet saves 60–70% of stainless steel consumption. For a DN2000 heat exchanger, the material cost reduction for a single tubesheet is approximately RMB 150,000–300,000

Challenge	Mechanism	Countermeasure
Thermal expansion mismatch	Carbon steel α ≈ 12.2×10⁻⁶/K, stainless steel α ≈ 16.5×10⁻⁶/K; interfacial thermal stresses develop at elevated temperatures	Limit service temperature to ≤400 °C; add a nickelalloy transition layer if necessary
Hydrogeninduced blistering	Hydrogen atoms from the shell side permeate into the base metal and accumulate at the interface, causing cladding blistering	In hydroprocessing service, apply an Inconel 182 weld overlay on the base metal side as a hydrogen barrier
Tube hole machining damage	Cladding edges tend to lift or tear during drilling	Use a step drill: carbide drill first through the cladding, then highspeed steel drill to final bore in the base metal
Sensitisation during heat treatment	If cooling is too slow during stressrelief annealing, the stainless steel cladding may become susceptible to intergranular corrosion	Control cooling rate, or apply solution annealing (requires special equipment)

Typical Engineering Applications of Clad Tubesheet SA350 LF2 CL1 + SA240 SS316L

Industry	Service Conditions	Selection Rationale
LNG liquefaction	Medium: mixed refrigerant (trace H₂S), temperature -50 °C	LF2 provides lowtemperature toughness; 316L resists mild acidic corrosion, preventing pitting leakage
Petrochemical hydroprocessing	Shell side: highpressure hydrogen + H₂S, tube side: hydrocarbons	Base metal carries high pressure; cladding resists sulfide stress corrosion cracking (hydrogen barrier layer required)
Seawater heat exchange	Tube side: seawater (Cl⁻ ≤200 ppm), shell side: process fluid	316L resists chloride pitting; LF2 provides strength at controlled cost
Pharmaceutical reactor	Tube side: organic solvents + dilute acids, shell side: steam (180 °C)	Cladding meets GMP cleanability requirements; base metal withstands jacket pressure

What testing requirements will be done to clad tubesheet?

• Ultrasonic testing (UT): 100% scanning after explosion bonding to evaluate bond ratio and distribution of any unbonded areas.
• Interface shear strength test: At least two specimens per batch; measured value must not be lower than the design requirement (generally ≥200 MPa).
• Cladding chemical composition and ferrite: Perform PMI on the cladding surface; if weld overlay exists, measure FN.
• Integrity of cladding in tube holes: If weld overlay is used to extend protection inside tube holes, perform penetrant testing (PT) to ensure no crater cracks.
• Heat treatment records: The time‑temperature curve for stress‑relief annealing must be complete, and dwell time in the sensitisation range must be verified.