Various stainless steel round steel types, characteristics list
One of the simplest ways to differentiate between austenitic stainless steel and ferritic or martensitic stainless steel is through a magnetic test. Austenitic stainless steels are typically non-magnetic in their standard condition, but they can develop slight magnetism after heavy cold working. On the other hand, ferritic and martensitic stainless steels are generally magnetic. This makes the magnetic test a quick and effective method for identifying the type of stainless steel being used.
To assess the corrosion resistance of a stainless steel tube, a nitric acid spot test is often used. This method helps distinguish stainless steel from other metals or alloys. However, it's important to note that certain grades like 420 and 440, which are high-carbon types, may not show significant corrosion. In general, stainless steel will only be slightly affected during this test, while other materials such as non-ferrous metals react more strongly. For instance, carbon steel tends to corrode quickly when exposed to dilute nitric acid.
Another common identification technique involves the copper sulfate spot test. This test can help differentiate between ordinary carbon steel and various types of stainless steel. The process begins by cleaning the test area thoroughly, removing any grease or impurities, and polishing a small section with a soft cloth. A drop of copper sulfate solution is then applied. If the material is a stainless steel tube, no copper plating will appear, and the surface won't take on a copper color. In contrast, carbon steel or iron will produce a visible layer of metallic copper.
For more specific identification, such as distinguishing between 302 and 304, or 316 and 317 stainless steels, a sulfuric acid test can be useful. Prepare a nitric acid solution with a volume concentration of 20–30% at a temperature of 60–66°C. Grind the sample finely, clean it in the nitric acid, and allow it to passivate for about an hour. Then, immerse the samples in a 10% sulfuric acid solution heated to 71°C. 302 and 304 stainless steels will corrode rapidly, producing a lot of bubbles and turning black. Meanwhile, 316 and 317 grades will either remain uncorroded or react very slowly under the same conditions. This method is particularly helpful in identifying different grades based on their chemical composition and performance characteristics.
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