Stainless steel is a kind of high alloy steel that can resist corrosion and have high stability in air or chemically corrosive medium. Stainless steel has good corrosion resistance and has a beautiful and smooth surface. It does not need to undergo surface treatment such as color plating, but it exerts the inherent surface properties of stainless steel and is used in many aspects of steel.
From the metallographic point of view, because stainless steel contains high chromium, chromium forms a very thin chromium film on the surface, which isolates the oxygen intruding into the steel, thus playing a role in corrosion resistance. In order to maintain the inherent corrosion resistance of stainless steel, steel must contain more than 12% chromium (GB/T 20878).
1. Stainless steel detection classification
1. Classification by chemical composition:
①. Cr series stainless steel
②. Cr-Ni series stainless steel
③ .Cr-Ni-Mo stainless steel
④. Cr-Mn-Ni series stainless steel, etc.
2. Classification by metallographic structure:
①.Austenitic stainless steel (200 series, 300 series)
②. Ferritic stainless steel (430, 446, etc.)
③. Martensitic stainless steel (410 series, 420 series and 440C series)
④. Duplex stainless steel (Cr18, Cr23, Cr22 and Cr25)
⑤. Precipitation hardening stainless steel
2. Metallographic inspection of stainless steel
1. Preparation of Specimen
The preparation process of stainless steel metallographic samples is basically the same as that of high alloy steel. Among them, austenitic stainless steel has soft matrix structure, high toughness and easy to produce work hardening. It is difficult to prepare samples, and it is easy to produce mechanical slip during polishing and disturb the metal layer structure, which affects the normal metallographic structure. Analysis and inspection. Improper sample preparation of semi-martensitic steel will transform austenite into martensite, so the preparation of the sample should not cause the sample to generate high heat. The polishing force should not be too large, and the polishing time should not be too long.
2. Chemical attack
Stainless steel has high corrosion resistance, so the erosive agent showing its microstructure must have strong erosiveness in order to make the structure appear clearly. An appropriate etchant should be selected according to the composition and heat treatment state of the steel.
Commonly used etchants are:
Ferric chloride 5g + hydrochloric acid 50ml + water 10ml; (applicable to A stainless steel and F-A stainless steel)
10ml hydrochloric acid + 10ml nitric acid + 100ml alcohol;
Picric acid 4g + hydrochloric acid 5ml + alcohol 100ml; (harder corrosion samples can be heated in water bath)
Hydrofluoric acid (5%) 1ml + nitric acid (5%) 4ml + water 45ml;
In addition, ferrite, austenite, carbide, delta ferrite, and σ may also appear in stainless steel at the same time, which can be distinguished by chemical erosion. The ferrite is often banded or dendritic; after being eroded with red blood salt potassium hydroxide solution (red blood 10g-15g + NaOH 10g-30g + water 100mL, hot boiled at 60-90°C), the ferrite is rose-colored, austenitic The body is bright in color. After being etched with alkaline potassium permanganate, the carbide is light brown and the σ phase is orange-red.
3. Commonly used stainless steel testing standards
1. Non-metallic inclusions: GB/T 10561, ASTM E45
2. Grain size: GB/T 6394, ASTM E112
3. F content: GB/T 13305-2008
4. Precipitation test of σ brittle phase in F-A duplex stainless steel: ASTM A923 Others: GB/4234-2003