Raw material price fluctuations and increasing demand for stainless steels have driven demand for lower cost alloys as alternatives to the traditional “300” series steels. This has been met through a range of existing and new, innovative steels with different properties, performance and availability. But as with the traditional stainless steels you can't tell what they are by looking at them. This article describes a range of test methods available for grade confirmation. The method used depends on the budget, size of job and the potential consequences of having the wrong alloy. Some of these tests are quantitative, giving actual percentages of each element, and others are qualitative tests showing just the presence of absence of an element or property. Some tests are very portable so are ideal for on-site testing, but others require fully-equipped laboratories. The martensitic grades (high hardness, magnetic grades used for making knives and blades) are not considered in detail in this technical note.
Contract documents may require formal test certificates. These are issued by the mill and unless there is reason to doubt them, this is sufficient. However, sometimes a positive material identification (PMI) is required for safety critical items such as LPG valves - this is an individual confirmatory analysis on each finished item.
Some products may be lacking in documentation and traceability; the most common concern is stock mixed in storage or as incoming scrap.
Unexpected poor performance often prompts calls for material testing. Such testing removes one variable in things that might have gone wrong but the cause is more frequently inadequate surface finish or errors in design or fabrication.
Finally, reverse engineering of an existing product often requires detailed materials information - generally more than just composition.
WHAT LEVEL OF TESTING IS REQUIRED?
Simple tests could cover differentiation between carbon and stainless steel, or between 304 and 316, or between 300 series and 200 series, or ferritic/duplex and austenitic grades.
Full laboratory chemical analysis is needed for some cases (such as differentiating between low and standard carbon grades) or when it has become a legal rather than a technical issue.
Full mechanical and metallurgical analyses may also be required if strength or hardness are essential design elements. If the material has undergone cold working or subsequent surface modification such as plasma vapour deposition (pvd) or nitriding, then the required investigation could be extensive - and expensive. The summary table shows results for three tests that can be used to distinguish between grades.
|200 series austenitic
|300 series austenitic
|Some proprietary grades positive
|316, 317, 904L & 6% Mo grades positive
|Not 409 or 430 but 444 and higher grades positive
|2205, super duplex positive
|Positive by definition
|Lean duplex positive