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The effect and function of various elements on the properties and microstructure of stainless steel

● Adding titanium or niobium to stainless steel is to prevent intergranular corrosion.

● Molybdenum and copper can improve the corrosion resistance of some stainless steels.

● Effects of other elements on the properties and microstructure of stainless steel

The effects of the above nine major elements on the properties and texture of stainless steel, in addition to the elements that have a greater impact on the properties and texture of stainless steel, stainless steel also contains some other elements. Some are common impurity elements like general steel, such as silicon, sulfur, phosphorus, etc. There are also some added for certain purposes, such as cobalt, boron, selenium, rare earth elements and the like. From the main properties of the corrosion resistance of stainless steel, these elements are non-primary in relation to the nine elements discussed. However, they cannot be completely ignored because they also occur in the properties and organization of stainless steel. influences.

Silicon is an element that forms ferrite and is an ordinary impurity element in general stainless steel.

Cobalt is not used as an alloying element in steel because the price of cobalt is high and it is more important in other aspects (such as high speed steel, hard alloy, cobalt-based heat resistant alloy, magnetic steel or hard magnetic alloy). use. In general stainless steel, there are not many cobalt added as alloying elements. Commonly used stainless steel such as 9Crl7MoVCo steel (including 1.2-1.8% cobalt) plus cobalt is not intended to improve corrosion resistance but to increase hardness because of the main use of this stainless steel. It is the manufacture of slicing mechanical cutting tools, scissors and surgical blades.

Boron: Adding 0.005% boron to high-chromium ferritic stainless steel Crl7Mo2Ti steel improves corrosion resistance in boiling 65% acetic acid. Adding a trace amount of boron (0.0006~0.0007%) can improve the hot plasticity of austenitic stainless steel. A small amount of boron forms a low-melting eutectic, which increases the tendency of hot cracking when austenitic steel is welded. However, when boron is contained in a large amount (0.5 to 0.6%), thermal cracking can be prevented. . Since the austenite-boride two-phase structure is formed when 0.5 to 0.6% of boron is contained, the melting point of the weld is lowered. When the solidification temperature of the molten pool is lower than that of the semi-melting zone, the tensile stress generated by the base metal during cooling is in the liquid state. The solid weld metal is not damaged at this time, and even if a crack is formed in the near seam region, it can be filled with a molten metal in a liquid-solid state. Boron-containing chromium-nickel austenitic stainless steels have special applications in the atomic energy industry.

Phosphorus: It is an impurity element in general stainless steel, but its harm in austenitic stainless steel is not as significant as in ordinary steel, so the content can be allowed to be higher, if some data is proposed to reach 0.06%. To facilitate smelting control. Individual manganese-containing austenitic steels may have a phosphorus content of 0.06% (e.g., 2Crl3NiMn9 steel) or 0.08% (e.g., Cr14Mnl4Ni steel). Phosphorus is used to strengthen steel, and phosphorus is added as an alloying element for age hardening stainless steel. PH17-10P steel (containing 0.25% phosphorus) is PH-HNM steel (containing 0.30 phosphorus).

Sulfur and selenium: It is also often an impurity element in general stainless steel. However, adding 0.2~0.4% sulfur to stainless steel can improve the cutting performance of stainless steel, and selenium has the same effect. Sulfur and selenium improve the cutting performance of stainless steel because they reduce the toughness of stainless steel. For example, the impact value of general 18-8 chrome-nickel stainless steel can reach 30 kg/cm2. The impact value of 18-8 steel (0.084% C, 18.15% Cr, 9.25% Ni) containing 0.31% sulfur is 1.8 kg/cm 2 ; 18 containing 0.22% selenium The impact value of -8 steel (0.094% C, 18.4% Cr, 9% Ni) was 3.24 kg/cm 2 . Both sulfur and selenium reduce the corrosion resistance of stainless steel, so they are rarely used as alloying elements for stainless steel.

Rare earth elements: Rare earth elements are used in stainless steel, and currently mainly in improving process performance. For example, adding a small amount of rare earth elements to Crl7Ti steel and Cr17Mo2Ti steel can eliminate bubbles caused by hydrogen in the ingot and reduce cracks in the billet. Austenitic and austenitic-ferritic stainless steels with 0.02-0.5% rare earth elements (yttrium alloy) can significantly improve the forging properties. There has been an austenitic steel containing 19.5% chromium, 23% nickel, and molybdenum copper manganese. In the past, only the castings were produced due to the hot work process performance, and the rare earth elements were added to various shapes.

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