Stainless steel material knowledge

1. What is stainless steel?

Stainless steel is based on ordinary carbon steel, adding a group of steel with a chromium (wCr) mass fraction of more than 12% alloy element, which maintains metallic luster under the effect of air, that is, it has no rust. This is due to the fact that the presence of a certain amount of chromium alloying elements in such steels enables the formation of a solid oxide film (passivation film) on the steel surface that is insoluble in certain media, so that the metal is isolated from the external substances without chemical effect. In addition to containing more chromium (Cr), some of these steels also match with other alloying elements, such as nickel (Ni), which are very good in air, water, and steam. Chemical stability, but also enough stability in many kinds of acid, alkali, salt aqueous solution, even in the high temperature or low temperature environment, it can still maintain its advantages of corrosion resistance.

2. There are several stainless steel classification methods?

According to the main chemical composition of chromium 316 stainless steel sheet, chromium nickel stainless steel and chromium manganese nitrogen stainless steel, etc.; can also be divided into acid-resistant stainless steel and heat-resistant stainless steel, etc.; usually classified by metallographic organization. Metallographic structure is divided into: ferritic (F) stainless steel, martensitic (M) stainless steel, austenitic – ferrite (A-F) duplex stainless steel, austenite – martensite ( A-M) duplex stainless steel and precipitation hardening (PH) stainless steel.

(1) Ferritic stainless steel F Ferrite has magnetic properties. Its internal microstructure is ferrite, and its chromium content is in the range of 11.5% to 32.0%. However, the carbon content is extremely low at <0.2% and cannot be quenched. With the increase of chromium content, the acid resistance is also improved. After adding molybdenum (Mo), the acid corrosion resistance and stress corrosion resistance can be improved.

(2) Martensitic stainless steel M The microstructure is martensite, and martensitic stainless steel is also magnetic. The mass fraction of chromium in such steel is 11.5%-18.0% but the mass fraction of carbon is up to 0.6%. Increased carbon content increases the strength and hardness of the steel. The small amount of nickel added to this type of steel promotes the formation of martensite while increasing its corrosion resistance. This type of steel has a certain degree of corrosion resistance, good thermal stability, and thermal strength and can be used as a heat-resistant steel that operates at temperatures below 700°C for long periods of time. It is widely used to make parts that require higher toughness and impact toughness, such as turbine blades, internal combustion engine exhaust valves, and medical devices.

(3) Austenitic stainless steel F. Its microstructure is austenite. It is formed by adding appropriate nickel (nickel mass fraction 8%-25%) in high chromium stainless steel (17-26%). With austenitic stainless steel, heat treatment is not used to refine the grain, nor can it be quenched to increase its hardness. This type of steel has a high degree of cold work hardening and is usually non-magnetic. After cold working, a small amount of ferrite or martensite can be precipitated in the steel, and a small amount of magnetism may appear. The austenitic steel sheet has the best overall performance. It has both sufficient strength, excellent plasticity, and low hardness. This is one of the reasons why they are widely used. Austenitic 304L Stainless Steel Bar are similar to most other metal materials. Their tensile strength, yield strength, and hardness increase with decreasing temperature; plasticity decreases with decreasing temperature. Its tensile strength increases rapidly in the temperature range of 15 to 800, and changes slowly when the temperature decreases further, while the increase in yield strength is more uniform. More importantly, as the temperature decreases, its impact toughness decreases slowly, and there is no brittle transition temperature.

(4) Special materials have been developed for the application of other special materials. For example: said austenitic-ferritic double steel, its microstructure is austenite plus ferrite. It contains 18-25% chromium, 4-7% nickel, and 4% molybdenum. Nickel, nickel-copper, nickel-chromium, and other nickel-chromium-based special stainless steels also fall into this category. This special material has special properties, such as ronifer, Nikrofer, AIIoy, ferrotherm, HasteIIoy, IncoIoy, InconeI, MoneI. These materials also have a material number and their chemical composition.

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