Uncertified Plate for a Lifting Beam
A 12 mm thick, 1500 × 3000 mm steel plate is delivered for a 7.5 m lifting beam. The plate is stencilled ASTM A36, but the mill certificate is missing and the heat number has been painted over. A portable hardness tester gives 210 HBW average. A spark test shows short, bushy sparks with a few bursts, not the long orange stream expected for low-carbon steel. A portable analyzer reports 0.42% C, 0.95% Cr, 0.20% Mo, 0.65% Mn. The beam will be welded with a 1.2 mm solid wire GMAW procedure qualified for A36 and will lift loads up to 5 t. The shop has A36 plate in stock, but it is 10 mm thick. The customer wants the beam by Friday and says 'A36 is A36.'
1. Calculate the carbon equivalent and explain why the chemistry changes the welding decision.
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Using the carbon equivalent formula CE = C + Mn/6 + (Cr + Mo + V)/5 + (Cu + Ni)/15, and treating V, Cu, and Ni as negligible: CE = 0.42 + 0.65/6 + (0.95 + 0.20)/5 = 0.42 + 0.108 + 0.23 = 0.758. That is a high CE for a structural lifting beam. The chemistry is not A36; it indicates an alloy steel with hardenability and cracking risk. The existing A36 GMAW procedure is not qualified for this chemistry, and the beam is a lifting device with a 5 t load. Welding would require a new WPS/PQR with controlled preheat, interpass temperature, low-hydrogen practice, and possibly post-weld heat treatment. The hardness of 210 HBW is consistent with a higher-strength alloy, not ordinary A36.
2. What disposition and testing would you recommend for the plate?
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The plate should be quarantined and marked as unverified. The stencil and spark test are not traceability. I would require a full mill certificate or a documented chemical analysis and tensile test from a qualified laboratory. For a lifting beam, the design engineer must confirm the material properties, allowable stress, and weldability. If the plate is confirmed as an alloy steel such as 4140, it should not be released under the A36 procedure. A new welding procedure qualification, welder qualification, preheat and post-weld heat-treatment instructions, and inspection plan would be required. If traceability cannot be restored, the plate should be rejected for this application regardless of the customer's schedule.
3. The shop wants to substitute the 10 mm A36 stock. What engineering checks are required before that substitution is allowed?
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The substitution cannot be approved only because the material grade is A36. Reducing thickness from 12 mm to 10 mm changes the section modulus, moment capacity, shear capacity, local buckling resistance, deflection, and weld capacity. The lifting lugs, connections, and safety factor for a 5 t lift must be recalculated. The engineer must confirm that the beam still meets the design code and the required safety factor after the thickness change. The shop should also confirm that the 10 mm plate has traceable certification and that the qualified welding procedure covers the thinner section. Until those checks are complete, the 10 mm plate is not an acceptable substitute.