Magnetic test is the simplest method to distinguish annealed austenitic stainless steel from ferritic stainless steel. Austenitic stainless steels are non-magnetic steels, but will be mildly magnetic after cold working at atmospheric pressure: pure chromium steels and low-alloy steels are both ferromagnetic steels.
The table shows the results of trials of magnetic and concentrated nitric acid tests to roughly divide common metal materials.
Nitrate point test
A significant feature of rhenium stainless steel is its inherent resistance to concentrated nitric acid and dilute nitric acid. This property makes it easy to distinguish from most other metals or alloys. However, high-carbon 420 and 440 steels are slightly corroded during the nitric acid point test, and non-ferrous metals are corroded immediately when they encounter concentrated nitric acid. Dilute nitric acid is strongly corrosive to carbon steel.
When austenitic stainless steel is cold-rolled or cold-work hardened under large pressure, as a method of distinguishing stainless steel, the magnetic test method is not applicable. In this case, the specimen should be heated to 1850-2000 ° F (1010 ~ 1093 ° C) and water quenched. After this treatment, the austenitic stainless steel will completely lose its magnetism, and its hardness is usually not greater than
区分 To distinguish 410, 414, 416, and 431 steels from 430, 430F steels, it is necessary to heat the samples to 1750-1850 ° F (955-1010 ° C) and perform oil quenching. The hardness of 430, 430F and 466 steel is less than Rc 24, and the wire of this steel is easy to bend by hand. The hardness of the other four steel grades is Rc36 ~ 43; its wire has strong magnetism and it is difficult to bend by hand.
The hardness of the samples of high carbon 420, 420F, 440A, B, C, and F steels after the above heat treatment is Rc50 ~ 60; if the hardness is Rc50 ~ 55, it may be 420 steel; the quenched 440A and B The hardness of the sample is Rc55 ~ 60; if the Rc value is 60 or above, it is 440C steel.
Copper sulfate point test
铜 The copper sulfate point test is the easiest way to quickly distinguish ordinary carbon steel from all types of stainless steel. The concentration of copper sulfate solution used is 5-10%.
前 Before performing the spot test, the test area should be thoroughly clear of grease or various impurities, and a small area should be polished with a soft abrasive cloth, and then a copper sulfate solution should be dripped into the cleaned area with a drip bottle. Ordinary carbon steel or iron will form a layer of metallic copper in a few seconds, while the surface of stainless steel will not produce copper precipitation or show copper color.
Sulfuric acid test
The sulfuric acid immersion test can distinguish 302 and 304 from 316 and 317. The cut edges of the specimen should be finely ground and then mentioned at a concentration of 20 to 30% and a temperature of 140 to 150 ° F (71 ° C). When 302 and 304 steels are immersed in this hot solution, they are quickly corroded and a large amount is produced. Bubbles, the sample becomes black in a few minutes; while the samples of 316 and 317 steel are not corroded or react slowly (no cheongsam), and the sample does not change color within 10-15 minutes. If a colleague's test is used to make a close comparison with a sample of known composition, the test can be made more accurate.
Cold acid point test
滴 Injecting a 20% sulfuric acid solution into the surface of a sample that has been sparked by grinding, buffing, washing, or rough polishing can distinguish the same type of stainless steel. A few drops of an acid solution were dripped on each activated surface. Under the action of the acid solution, 302 and 304 steels were strongly corroded and turned black, showing brownish black or black, and then formed greenish crystals in the solution; 316 steel was corroded Slowly, and gradually turned brownish yellow, then brownish black, and finally formed some light greenish black crystals in the solution; for 317 steel, the above reaction proceeded more slowly.
Phosphoric acid test
Phosphoric acid test is used to distinguish chromium-nickel stainless steel from chromium-nickel molybdenum stainless steel. Add concentrated phosphoric acid to 0.5% sodium fluoride solution and heat to 140 ~ 150 ° F (60 ~ 66 ° C)
Hydrochloric acid test
The hydrochloric acid test can be used to distinguish low chromium type 403, 410, 416, and 420 steels from higher chromium containing 430, 431, 440, and 446 steels.
溶 Dissolve the same weight of drill cuttings into a 50% volumetric hydrochloric acid solution, and compare the color intensity of the solution. The steel with higher chromium content has a darker green color.
Put a few grams of drill cuttings in each test tube into each test tube, and then apply two equal volumes of 40-50% hydrochloric acid solution to each of them. For 3 minutes, the solution changes are shown in the table. .
Steel type reaction solution color
Quite fast light green blue
Se （Se） Garlic light amber
Sturdy compared with 302 steel, darker green
Quickly emits a smell similar to the smell of rotten eggs Dark green
. Spark test
The spark test is used to distinguish carbon steel, structural alloy steel and tool steel, but it is of little use for distinguishing stainless steel. This spark test method can help experienced operators. Stainless steel is divided into four categories, but it is not easy to distinguish each steel type. The four major types of stainless steel machines have characteristic spark states as follows:
Type: 302, 303, 316 steel, short red sparks with several forks.
Class ：: 308, 309, 310, and 446 steels, producing very few dark red short sparks with several forks.
Class: 410, 414, 416, 430, and 431 steels, producing long white beams of sparks with several forks.
Type ：: 420, 420F and 440A, B, C, F, producing low-light color sparks with obvious flashes or long white sparks.
. Machining test
Comparing the differences between turning chips, you can distinguish between 303, 416, 420F, 430F, and 440F free-cutting steels and standard stainless steels. In contrast to standard stainless steel, which has higher toughness and ductility, chips that are easy to chip are more brittle and more likely to break; this type of chip-friendly steel is usually chipped in a dry state, and is often emitted when it is dictated or sawed quickly foul smell.