Is an “Equivalent” Material Really Equivalent?

Counterparts such as “1.2344 = H13” or “42CrMo4 = 4140” are often used in technical documents and sourcing correspondence. These counterparts are a useful guide, but they do not always mean exact equivalence.

Why might they not be exactly the same?

• Chemical composition ranges: Different standards may define different lower and upper limits for the same elements. The ranges may overlap without being identical.
• Mechanical property requirements: One standard may require mechanical values for a given delivery condition while another does not.
• Testing and inspection requirements: Aerospace specifications in particular may include additional testing, melting practice and quality requirements.
• Product form and size coverage: A specification may cover only certain forms or size ranges.

How to approach it in practice

• Use the counterpart as a starting point.
• Compare the property that is critical in the application (chemistry, hardness, toughness, corrosion, testing) in both standards.
• If the drawing or specification names a particular standard, obtain approval for an alternative standard.
• On the certificate, check which standard the material was produced to and which standard it was accepted against.

Conclusion

The word “equivalent” should be verified against the requirements of the application. For critical parts, the standard and revision written in the specification should govern rather than a counterpart.