Practical knowledge of stainless steel

Boron: The addition of 0.005% boron to the high-chromium ferritic stainless steel Crl7Mo2Ti steel can improve the corrosion resistance in the boiling 65% acetic acid. Adding a small amount of boron (0.0006″0.0007%) can improve the thermal plasticity of austenitic stainless steel. A small amount of boron forms a low melting point eutectic, which increases the tendency of austenitic steel to generate hot cracks during welding, but it contains more When there is more boron (0.5″0.6%), it can prevent thermal cracking. Because when it contains 0.5″0.6% boron, austenite-boride two-phase structure is formed, which reduces the melting point of the weld. When the solidification temperature of the molten pool is lower than the semi-melting zone, the tensile stress generated by the base material during cooling , It is borne by the weld metal in the liquid and solid state, at this time it will not cause cracks, even if a crack is formed in the near-joint zone, it can be filled by the molten pool metal in the liquid-solid state. Boron-containing chromium nickel austenitic Bulk stainless steel has special applications in the atomic energy industry.

Phosphorus: It is an impurity element in general stainless steel, but its hazard in austenitic stainless steel is not as significant as in general steel, so the content can be allowed to be higher, if some data suggest up to 0.06%, in order to facilitate Smelting control. The phosphorus content of individual manganese austenitic steel can reach 0.06% (such as 2Crl3NiMn9 steel) or even 0.08% (such as Cr14Mnl4Ni steel). Using phosphorus to strengthen steel, there is also phosphorus 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) and so on.

Sulfur and selenium: impurity elements are also common in general stainless steel. However, adding 0.2″ 0.4% sulfur to stainless steel can improve the cutting performance of stainless steel. Selenium also has the same effect. Sulfur and selenium improve the cutting performance of stainless steel because they reduce the toughness of stainless steel, such as 18-8 chromium nickel. The impact value of 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/cm2; containing 0.22% selenium The impact value of 18-8 steel (0.094%C, 18.4%Cr, 9%Ni) is 3.24 kg/cm². Both sulfur and selenium reduce the corrosion resistance of stainless steel, so they are actually used as alloying elements of stainless steel. Rarely.

Rare earth elements: Rare earth elements are used in stainless steel, currently mainly to improve process performance. For example, adding a small amount of rare earth elements to Crl7Ti steel and Cr17Mo2Ti steel can eliminate the bubbles caused by hydrogen in the steel ingot and reduce the cracks in the billet. Adding 0.02″0.5% rare earth elements (cerium-lanthanum alloy) to austenitic and austenitic-ferritic stainless steels can significantly improve the forging performance. There was once an austenite containing 19.5% chromium, 23% nickel and molybdenum copper manganese In the past, due to the performance of hot working process, the steel can only be produced as castings. After adding rare earth elements, it can be rolled into various profiles.

2). Classification of stainless steel according to metallographic structure and general characteristics of various types of stainless steel

According to chemical composition (mainly chromium content) and use, stainless steel is divided into two categories: stainless steel and acid resistant. The industry also classifies stainless steel according to the type of the matrix structure of the steel after the high temperature (900-1100 degrees) heating and air cooling. This is based on the characteristics of the influence of carbon and alloy elements on the structure of stainless steel that we discussed above.

The stainless steel used in industry can be divided into three categories according to the metallographic structure: ferritic stainless steel, martensitic stainless steel, and austenitic stainless steel. The characteristics of these three types of stainless steel can be summarized (as shown in the table below), but it should be noted that not all martensitic stainless steels cannot be welded, but are restricted by certain conditions, such as preheating before welding and high temperature tempering after welding. Etc., which makes the welding process more complicated. In actual production, some martensitic stainless steels such as 1Cr13, 2Cr13 and 2Cr13 are welded to 45 steel.

Classification, main components and performance comparison of stainless steel

Classification Approximate composition (%) Hardenability Corrosion resistance Workability Solderability Magnetic

C Cr Ni

Ferritic system below 0.35 16-27-No good Fair good Fair Yes

Markov system 1.20 or less 11-15-self-hardening possible possible not available

Austenitic system 0.25 or less 16 or more 7 or more None Excellent Excellent Excellent None

The above classification is only based on the matrix structure of steel. Because the effects of stabilizing austenite and ferrite forming elements in steel cannot be balanced with each other, and due to the large amount of chromium, the point S of the balance diagram is shifted to the left. Stainless steel used in industry In addition to the three basic types mentioned above, there are also martensite-ferrite, austenite-ferrite, austenite-martensite and other transitional duplex stainless steels, as well as martensite -Stainless steel with carbide structure.