## Stress Bearing — Clad Layer on a Composite Plate

By Wesley Wang on Thursday, April 17th, 2014

Cladded metal plates (clads) are widely used in the refinery industry. Explosion welding is one of popular joining processes to produce those clads. ASTM codes on the explosion welding of stainless steels and nickel alloys, such as ASTM A263, 264, and A265, allow a cladded layer to be considered to bare structure loads under the following provisions:

(1) The nominal composite gage is less than or equal to 1-1/2 inches.

(2) The specified minimum tensile strength for the base steel is less than or equal to 70,000 psi.

(3) The specified minimum yield strength of the base steel is less than or equal to 40,000 psi.

Otherwise, the clad layer is only to serve the purpose of corrosion resistance. Consequently, when a refinery vessel is designed, an extra base steel thickness has to be considered in order to compensate the disregarded thickness of the cladded layer. By doing that, the material and fabrication costs are increased.

It is well known that stainless steels and nickel alloys have a good compatibility with low alloy carbon steels in welding. Their dissimilar welds with steels are widely used to bear structure loads. By considering the following:

• a welding is defined as a process to achieve an atomic bonding between two mating surfaces;

•an explosion welding does generate the atomic bonding between a clad and a base plate,

it is of significance to explore the feasibility of expanding the scope defined by the above ASTM provisions (the Provisions) so that more clads made by stainless steels or nickel alloys with base steels can be permitted to take structure loads.

In 2005, Prothe and Vargo^{[1]} investigated the tensile stress that is normal to the explosion-cladding mating surface. Sub-sized ASTM tension samples with a diameter of 0.25” were prepared from clads of stainless steel, nickel alloy, copper alloys and titanium on SA 516-70 carbon steel. No fracture was found at bond zones.

After a clad is rolled and seam-welded to become a cylindrical reactor, the circumferential tension stress becomes a major to be considered under a working condition.

As a composite material, the clad satisfies the following stress equation:

A_{total} σ_{work} = A_{1} σ_{carbon steel} + A_{2} σ_{clad metal} +A_{3} σ_{bond }

Considering the deformation accommodation factor of the composite:

A_{total} σ_{work} = k_{1} A_{1} σ_{carbon steel} + k_{2} A_{2} σ_{clad metal} + k_{3}A_{3} σ_{bond}

Within the elasticity of the clad:

σ_{work} =_{ }[k_{1} (A1/A_{total})E_{carbon steel} + k_{2} (A2/A_{total}) E_{clad metal }+k_{3} (A3/A_{total})E_{bond}]ε

σ_{work }= E’ ε (equation [1])

By changing the area ratio between a base steel and clad metal on tension samples extracted out from a clad, k_{1}, k_{2}, k_{3} and E_{bond }constants can be determined from multiple tension experiments. Assuming a clad passes the bending test per ASTM A263, A264 or A265, equation [1] can be used to support the following discussion on clads with a thickness less than 1-1/2”.

Case 1 – a designed working stress is lower than the yield strength of a clad metal

It is reasonable to assume that the yield strength of the bond zone is between those of the clad metal and base steel. The Hook’s equation [1] is still valid in this case. Therefore, it should not be a problem to include the clad metal layer to bare the work load regardless to the yield and tensile strengths of a base steel.

Case 2 – a designed working stress is no greater than the yield strength of a clad whose base steel has a yield strength higher than 40,000 psi

It is a usual engineering practice to consider the yield strength, instead of a tensile strength, as a limit of a stress bearing in structure design. Also, the Provisions regard the 40ksi yield strength as a stress threshold of integrating the clad metal to bear work loads.

In this case, the Hook’s equation [1] is valid too. Since stainless steel or nickel alloys usually have a tensile strength around 70ksi, there is a 30ksi work-hardening capacity with respect to the 40ksi stress. As long as the yield strength of the clad is less than 40 ksi, the clad metal should be capable of bearing the loading stress regardless to the yield and tensile strengths of a base steel.

If the yield strength of the clad exceeds 40ksi, then a strength safety factor on a clad metal needs to be determined prior to any stress bearing considerations.

To conclude, as long as the Hook’s equation [1] is valid on a clad, the clad metal layer can be considered to take structure work loads.

References:

[1] Prothe, C., Vargo, A., 2007, “Comparative Tensile Strength and Shear Strength of Detaclad Explosion Clad Products”, Proceedings of Corrosion Solutions Conference 2007, ATI Wah Chang, Albany, OR, USA.

[2] ASTM/ASME A263

[3] ASTM/ASME A264

[4] ASTM/ASME A265

Dr. Wesley Wang