Stainless Steel - Sorting and Identification Tests

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Sorting and identification tests are non-destructive, inexpensive and rapid procedures, usually carried out on different grades of stainless steels. Sorting of product procedures is particularly useful when bars of grades 303 and 304 have been accidentally stored together, or when grades 304 and 316 sheet offcuts are mixed.

Although these tests are extremely useful, they have certain limitations too. It is difficult to sort one heat from another of the same grade using these tests. For instance, type 304 cannot be readily sort from 321, 316 from 316L or 304 from 304L.

Apart from these simple tests, there are more complex tests that involve several chemical reagents for product sorting and grade identification, hardness tests for the products, and tests for determining the response of products to heat treatment. Full spectrometric analysis of the product can be carried out if these simple tests are not found to be adequate.

Upon retaining the original product identification, such as grade/heat/specification markings, tags, stickers or product colour codes, these sorting tests may not be required.


The table below provides the tests for the sorting and identification of stainless steels:

Table 1. Tests for the identification and sorting of stainless steels


What Can Be Sorted



Magnetic Response

Austenitic (300 Series) stainless steels can be sorted from other types of stainless steels. All other types of steels such as the precipitation hardening, martensitic, duplex and Ferritic stainless steels are sensitive to magnet fields. The austenitic manganese steels (e.g., “P8”) are the only non-magnetic group of steels.

Observe response when a permanent magnet is made to contact with the steel.

Some austenitic grades, particularly 304, are attracted when cold-worked, e.g. by bending, forming or rolling.

When cold-worked, certain austenitic steels (e.g. grade 304) gain a magnetic response. This response can be removed by stress relieving the steels at cherry-red heat. However, the stress relieving procedure may tend to sensitize the steel. Therefore, care must be taken to avoid stress relief on a materia, that is to be used in corrosive conditions. Full annealing can be performed.

Nitric Acid Reaction

Stainless steels from non-stainless steels.

1. Either, place a drop of strong nitric acid on the steel surface, or the steel in acid solution at room temperature.

2. Test standard samples, such as stainless and non-stainless steel samples, in the same way.
3.When non-stainless steels are affected by nitric reaction, a pungent brown fume is generated. Stainless steels, however, remain unaffected. The results of both the samples can be compared.
4. Samples need to be thoroughly washed.

Strong nitric acid is corrosive and harmful to skin. Hence, clean thoroughly if the skin is brought into contact with the acid. Avoid inhaling brown fume. Safety glasses should be used.

Molybdenum Spot Test


Stainless steels containing a significant amount of molybdenum (Mo) from the steels that are free of Mo. Type 404 steels can also be sorted from type 316 steels. Some of the Mo grades, which provide a positive response to this test, include grade 316, grade 316L, grade 317, grade 317L, grade 444, grade 904L, grade 2205, "6-Mo" grades, grade 4565S and all “super duplex” grades (e.g. S32760 / Zeron 100 / S32750 / 2507 / S32550 / Alloy 255 / S32520 / UR52N+). Other similar grades, having less quantities of Mo, will also show positive response.

1. Clean the surface of the steel using abrasive paper, followed by degreasing and drying if necessary.
2. Use "Decapoli 304/316" solution. Shake well before use, and then place a drop of solution on the steel.
3. Place a drop of solution on standard grades 304 and 316 samples.
4. Presence of Mo in the steel can be confirmed by the darkening of the yellow drop in 2 to 4 min. Compare the indications of test samples with that of standard samples.
5. Wash or wipe the samples.

Avoid contact of test solution on skin and, particularly, eyes.

Avoid exposure of skin and eyes to the test solution. Clean the skin thoroughly in case of contact. Reliable results can be obtained only if all the samples are maintained at the same temperature. Low temperatures should be avoided. Some heats of "Mo-free" stainless steels, such as grade 304, contain small amounts of Mo, sufficient enough to stimulate a slight reaction.

Sulphur Spot Test (S)

Stainless and plain carbon steels having at least 0.1% of sulphur, i.e. free-machining grades. (e.g. grades S1214, S12L14, 303, 416, 430F) can be sorted from non-free-machining steels. Ugima 303 contains high sulphur content and, hence, it will initiate a positive reaction. However, the sulphur content of Ugima 304 and Ugima 316 is less than that of their standard (non-Ugima) equivalents and, hence, no positive reactions can be observed in these grades.

1. Clean the surface of the steel using abrasive paper, followed by degreasing and drying if necessary.
2. Prepare standard samples e.g. known grades CS1020 and S1214, or grades 304 and 303, in the same way.
3. Soak photographic paper in 3% sulphuric acid solution for about 3 min.
4. Press the prepared steel surfaces on the photographic paper surface for 5 s.
5. A dark brown stain indicates the presence of significant quantities of sulphur in the test sample. Compare the indications of test samples with that of standard samples.
6. Wash or clean the samples.

Avoid exposure of skin and eyes to the test solution. Clean the skin thoroughly in case of contact. Safety glasses should be used. Good results can be obtained by maintaining the samples at the same temperature.

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