GB/T 8165 and ASTM A263 Clad Plate Acceptance Criteria and Rejection Thresholds

Literature Overview and Context

Clad plate is a fundamental material for bimetal pressure vessel fabrication, providing corrosion resistance through a metallurgical bond between the cladding layer and the base metal. The acceptance criteria for clad plate are governed by GB/T 8165 (Chinese national standard) and ASTM A263 (American standard), which specify minimum requirements for bond strength, shear strength, and metallurgical integrity. When a clad plate fails to meet these minimum criteria, it triggers a formal rejection review process that may result in rejection of the entire plate or a limited area of the plate.

This study note examines the acceptance lower limits specified in GB/T 8165 and ASTM A263, the testing methods used to verify compliance, and the engineering implications of operating near or below these thresholds.

Acceptance Criteria Comparison

Test Parameter	GB/T 8165 Requirement	ASTM A263 Requirement	Test Method
Bond Strength (Shear)	≥90% of tensile strength of cladding layer	≥90% of tensile strength of cladding layer	Shear test (GB/T 6395 / ASTM A263)
Peel Strength	Not specified as mandatory	Not specified as mandatory	Peel test (optional)
Microstructural Bond	No cracks, no intermetallic compounds	No cracks, no intermetallic compounds	Metallographic examination
Defect Area (per weld)	≤1% of weld area	≤1% of weld area	UT or visual inspection
Total Defect Area (per plate)	≤5% of plate area	≤5% of plate area	UT or visual inspection
Maximum Defect Size	≤6 mm length	≤6 mm length	UT or visual inspection

The bond strength requirement of ≥90% of the cladding layer tensile strength is the critical threshold. For example, if the cladding layer is 304 stainless steel with a tensile strength of 515 MPa, the minimum acceptable bond strength is 463.5 MPa. If the shear test result is below this value, the plate is subject to rejection review.

Testing Methods and Interpretation

Shear Strength Testing

The shear test is the primary method for verifying bond strength. The test specimen is prepared by cutting a coupon from the clad plate, machining it to the specified dimensions, and applying a shear load perpendicular to the clad/base interface. The test is performed in accordance with GB/T 6395 or ASTM A263.

Specimen Dimension	Value	Notes
Specimen Length	100 mm	Minimum
Specimen Width	25 mm	Minimum
Cladding Thickness	As per plate specification	Typically 3–12 mm
Base Metal Thickness	As per plate specification	Typically 10–100 mm
Test Temperature	Room temperature	Unless otherwise specified

The shear strength is calculated as the applied force divided by the bonded area (width × cladding thickness). The result is compared to the tensile strength of the cladding layer material, and the ratio must be ≥90%.

Metallographic Examination

In addition to the shear test, metallographic examination is required to verify the metallurgical quality of the bond. The examination includes:

Macroscopic examination: Visual inspection of the clad/base interface for cracks, voids, or unmelted areas.
Microscopic examination: Examination of the interface at 100× to 500× magnification to detect intermetallic compounds, segregation, or micro-cracks.
Hardness mapping: Hardness measurements across the interface to detect any soft zones or hard intermetallic layers.

The presence of intermetallic compounds (e.g., Fe-Cr intermetallics in stainless steel/carbon steel bonds) is generally unacceptable because they are brittle and can lead to intergranular fracture. The maximum acceptable thickness of intermetallic compound layer is typically 5–10 μm, depending on the specific material combination.

Rejection Review Process

When a clad plate fails to meet the acceptance criteria, a formal rejection review is initiated. The review process includes the following steps:

Defect characterization: The nature, size, and location of the defect are documented. For bond strength failure, the shear test result is recorded along with the material certification.
Root cause analysis: The cause of the failure is investigated. Common causes include insufficient rolling pressure, contamination at the interface, improper heating during explosion cladding, or material incompatibility.
Impact assessment: The impact of the defect on the intended application is evaluated. For example, a bond strength of 85% may be acceptable for a low-pressure storage tank but not for a high-pressure hydrogenation reactor.
Disposition decision: The review committee decides whether to accept the plate with limitations, reject the plate, or require additional testing.
Documentation: The review findings and disposition decision are documented and retained for traceability.

Engineering Practice Cases

Case 1: Bond Strength Failure in an ESW Clad Plate

An electroslag welded clad plate (304/16MnR, 6 mm cladding) failed the shear test with a bond strength of 82% of the cladding layer tensile strength. The root cause investigation revealed that the ESW welding parameters were outside the qualified range, resulting in insufficient fusion at the interface. The plate was rejected because the defect was distributed over a large area, and rework was not feasible.

Case 2: Localized Defect in an Explosion-Clad Plate

An explosion-clad plate (316L/SAE 1020, 3 mm cladding) had a localized area of incomplete bonding detected during UT inspection. The defect area was 2% of the plate area, which is within the acceptable limit of 5%. However, the defect was located at a critical area of the plate (near a nozzle attachment point). The review committee decided to reject the plate because the defect could not be removed without compromising the structural integrity of the nozzle area.

Key Reflections and Recommendations

Supplier qualification is essential: The clad plate supplier must be qualified to produce plates that consistently meet the acceptance criteria. This includes verification of the supplier's welding procedure qualifications, material certifications, and quality control systems.
Incoming inspection must be rigorous: Every clad plate must be inspected upon receipt, including verification of material certifications, dimensional checks, and NDE (UT or MT) of the clad/base interface. The shear test should be performed on a representative sample from each heat number.
Do not accept marginal results: A bond strength of 90% is the minimum acceptable value, not a target. Engineers should aim for a bond strength of ≥95% to provide a safety margin. Marginal results indicate potential process issues that may lead to future failures.
Consider the service environment: The acceptance criteria may need to be tightened for severe service environments. For example, for a vessel exposed to hydrogen service, the bond strength requirement may be increased to ≥95% to account for the potential for hydrogen-induced cracking at the interface.
Maintain traceability: Each clad plate must be traceable to its material certification, heat treatment record, and NDE report. This traceability is essential for investigating any in-service failures and for regulatory compliance.

In conclusion, GB/T 8165 and ASTM A263 provide clear acceptance criteria for clad plate, with the bond strength of ≥90% of the cladding layer tensile strength being the critical threshold. Engineers must rigorously enforce these criteria, investigate any failures thoroughly, and make informed disposition decisions that balance quality, cost, and schedule considerations. The rejection review process is not a bureaucratic exercise but a critical quality gate that ensures the integrity of bimetal pressure vessels.