Definition of stainless steel
A high alloy steel that can resist corrosion in air or chemically corrosive media. Stainless steel has a beautiful surface and good corrosion resistance, and does not need to be plated. It is a kind of steel that is used in many aspects and is used in many aspects to bring out the inherent surface properties of stainless steel. It is usually called stainless steel. Representative performance include 13-chromium steel, 18-chromium-nickel steel and other high alloy steels.
From a metallographic perspective, because stainless steel contains chromium, a very thin chromium film is formed on the surface. This film isolates the oxygen that intrudes into the steel and plays a role in corrosion resistance.
In order to maintain the inherent corrosion resistance of stainless steel, the steel must contain more than 12 chromium.
Types of stainless steel:
Stainless steel can be roughly classified according to its use, chemical composition and metallographic structure.
Austenitic steel is basically composed of 18 chromium-8 nickel. The addition amount of each element changes, and steel types for various purposes are developed.
Classified by chemical composition:
①. CR series: ferrite series, martensite series
②. CR-NI series: austenitic series, abnormal series, precipitation hardening series.
Classification based on metallographic structure:
①. Austenitic stainless steel
②． Ferritic stainless steel
③. Martensitic stainless steel
④. Duplex stainless steel
⑤. Precipitation hardened stainless steel
Identification method of stainless steel
Numbering and representation method of steel
① Use the international chemical element symbol and the national symbol to indicate the chemical composition. Arabic letters are used to represent ingredient content:
For example: China, Russia 12CrNi3A
②Use fixed-digit numbers to represent steel series or numbers; such as: United States, Japan, 300 series, 400 series, 200 series;
③ Use Latin letters and sequence to form the serial number, which only indicates the purpose.
my country’s numbering rules
①Use element symbols
②Purpose, Chinese pinyin, open-hearth steel: P, boiling steel: F, killed steel: B, Class A steel: A, T8: Special 8,
GCr15: Balls
◆Combined steel, spring steel, such as: 20CrMnTi 60SiMn, (C content expressed in parts per ten thousand)
◆Stainless steel and alloy tool steel (C content expressed in thousandths), such as: 1Cr18Ni9 thousandths (i.e.
0.1C), stainless steel C≤0.08 such as 0Cr18Ni9, ultra-low carbon C≤0.03 such as 0Cr17Ni13Mo
International Stainless Steel Labeling Method
The American Iron and Steel Institute uses three digits to label various standard grades of forgeable stainless steel. Among them:
①Austenitic stainless steel is represented by 200 and 300 series numbers,
②Ferrite and martensitic stainless steel are represented by 400 series numbers. For example, some of the more common austenitic stainless steels are marked with 201, 304, 316 and 310,
③ Ferritic stainless steels are marked with 430 and 446, Martensitic stainless steel is marked with 410, 420 and 440C.
It is dual-phase (austenite-ferrite),
④ stainless steel, precipitation hardening stainless steel and iron-containing stainless steel High alloys with a weight lower than 50 are usually named by patented names or trademarks.
4). Classification and classification of standards
4-1 classification:
①National standard GB
②Industry standard YB
③Local standard
④Enterprise Standard Q/CB
4-2 Category:
①Product Standard
②Packaging Standard
③Method Standards
④Basic Standards
4-3 Standard Level (divided into three levels):
Y Level: International Advanced Level
I level: international general level
H level: domestic advanced level
4-4 national standard
GB1220-84 stainless steel bar (I level)
GB4241-84 Stainless welding disk (Grade H)
GB4356-84 Stainless welding disk (Grade I)
GB1270-80 Stainless Pipes (Grade I)
GB12771-91 Stainless welded pipe (Grade Y)
GB3280-84 Stainless cold plate (Grade I)
GB4237-84 Stainless hot plate (Grade I)
GB4239-91 Stainless cold strip (Grade I)
Stainless steel professional term
In layman’s terms, stainless steel is not Steel that is prone to rust, in fact some stainless steel, is both stainless and acid-resistant (corrosion-resistant). The rustlessness and corrosion resistance of stainless steel are due to the formation of a chromium-rich oxide film (passivation film) on its surface. This kind of stainlessness and corrosion resistance are relative. Tests have shown that the corrosion resistance of steel in weak media such as the atmosphere, water, and oxidizing media such as nitric acid increases with the increase in the chromium water content in the steel. When the chromium content reaches a certain percentage, the corrosion resistance of the steel deteriorates. Mutation, that is, from easy to rust to not easy to rust, from not resistant to corrosion to corrosion resistant. There are many ways to classify stainless steel. According to the organizational structure at room temperature, there are martensitic, austenitic, ferritic and duplex stainless steels; according to the main chemical composition, they can basically be divided into two major systems: chromium stainless steel and chromium-nickel stainless steel; according to use There are nitric acid-resistant stainless steel, sulfuric acid-resistant stainless steel, seawater-resistant stainless steel, etc. According to the type of corrosion resistance, it can be divided into pitting corrosion-resistant stainless steel, stress corrosion-resistant stainless steel, intergranular corrosion-resistant stainless steel, etc.; according to functional characteristics, it can be divided into none Magnetic stainless steel, free-cutting stainless steel, low-temperature stainless steel, high-strength stainless steel, etc. Because stainless steel has excellent corrosion resistance, formability, compatibility and strength and toughness in a wide temperature range, it has been widely used in heavy industry, light industry, daily necessities industry, building decoration and other industries.
Austenitic stainless steel: Stainless steel with an austenitic structure at room temperature. When the steel contains approximately 18 Cr, 8~10 Ni, and approximately 0.1 C, it will have a stable austenite structure. Austenitic chromium-nickel stainless steel includes the famous 18Cr-8Ni steel and the high Cr-Ni series steel developed by increasing the Cr and Ni content and adding Mo, Cu, Si, Nb, Ti and other elements. Austenitic stainless steel is non-magnetic and has high toughness and plasticity, but its strength is low. It cannot be strengthened through phase transformation and can only be strengthened through cold working. If S, Ca, Se, Te and other elements are added, it will have good machinability. In addition to being resistant to corrosion by oxidizing acid media, this type of steel can also be resistant to corrosion by sulfuric acid, phosphoric acid, formic acid, acetic acid, urea, etc. if it contains elements such as Mo and Cu. If the carbon content in this type of steel is less than 0.03 or contains Ti or Ni, its intergranular corrosion resistance can be significantly improved. High-silicon austenitic stainless steel has good corrosion resistance in concentrated nitric acid. Due to its comprehensive and good comprehensive properties, austenitic stainless steel has been widely used in various industries.
Ferritic stainless steel: Stainless steel with a mainly ferrite structure under use. The chromium content is between 11 and 30, and it has a body-centered cubic crystal structure.
This type of steel generally does not contain nickel, and sometimes contains a small amount of Mo, Ti, Nb and other elements. This type of steel has the characteristics of large thermal conductivity, small expansion coefficient, good oxidation resistance, and excellent stress corrosion resistance. It is mostly used to make atmospheric-resistant steel. , parts corroded by steam, water and oxidizing acids. This type of steel has shortcomings such as poor plasticity, significantly reduced plasticity and corrosion resistance after welding, which limits its application. The application of outside-furnace refining technology (AOD or VOD) can greatly reduce interstitial elements such as carbon and nitrogen, thus making this type of steel widely used.
Austenite-ferrite duplex stainless steel: It is a stainless steel with approximately half austenite and half ferrite structures. In the case of low C content, the Cr content is between 18 and 28, and the Ni content is between 3 and 10. Some steels also contain alloying elements such as Mo, Cu, Si, Nb, Ti, and N. This type of steel has the characteristics of both austenitic and ferritic stainless steel. Compared with ferrite, it has higher plasticity and toughness, no room temperature brittleness, significantly improved intergranular corrosion resistance and welding performance, while maintaining iron content. The solid stainless steel is brittle at 475°C, has high thermal conductivity, and has superplasticity and other characteristics. Compared with austenitic stainless steel, it has high strength and significantly improved resistance to intergranular corrosion and chloride stress corrosion. Duplex stainless steel has excellent pitting corrosion resistance and is also a nickel-saving stainless steel.
Martensitic stainless steel: Stainless steel whose mechanical properties can be adjusted through heat treatment. In layman's terms, it is a type of hardenable stainless steel. Typical grades are Cr13 type, such as 2Cr13, 3Cr13, 4Cr13, etc. The hardness after tempering is high, and different tempering temperatures have different combinations of strength and toughness. It is mainly used in steam turbine blades, tableware, and surgical instruments. According to differences in chemical composition, martensitic stainless steel can be divided into two categories: martensitic chromium steel and martensitic chromium-nickel steel. According to different structures and strengthening mechanisms, it can also be divided into martensitic stainless steel, martensite and semi-austenitic (or semi-martensite) precipitation hardening stainless steel, and maraging stainless steel.
Physical, chemical and mechanical properties of stainless steel
The physical properties of stainless steel are mainly expressed in the following aspects:
①. Coefficient of Thermal Expansion: The change in a substance's measured elements due to changes in temperature. The expansion coefficient is the slope of the expansion-temperature curve, the instantaneous expansion coefficient is the slope at a specific temperature, and the average slope between two specified temperatures is the average thermal expansion coefficient. The expansion coefficient can be expressed in terms of volume or length, usually in terms of length.
②． Density: The density of a substance is the mass per unit volume of the substance, the unit is kg/m3 or 1b/in3.
③. Modulus of elasticity: When applying force to both ends of a unit length edge can cause a unit change in the length of the object, the force required per unit area is called the elastic modulus. The unit is 1b/in3 or N/m3.
④. Resistivity: The resistance measured between two opposite sides of a unit length cube of material, expressed in units of Ω?m, μΩ?cm or (obsolete) Ω/(circular mil.ft).
⑤. Magnetic permeability: a dimensionless coefficient that indicates the degree to which a material is easily magnetized. It is the ratio of magnetic induction intensity to magnetic field intensity.
⑥. Melting temperature range: Determines the temperature at which an alloy begins to solidify and where it completes solidification.
⑦． Specific heat: The amount of heat required to change the temperature of a unit mass of a substance by 1 degree. The values ??of specific heat in the imperial and CGs systems are the same because the unit of heat (Biu or cal) depends on the amount of heat required to raise the unit mass of water by 1 degree. The numerical value of specific heat in the SI system of units is different from that in the Imperial or CGS system because the unit of energy (J) is defined differently. The units of specific heat are Btu(1b?0F) and J/(kg?k).
⑧． Thermal Conductivity: A measure of the rate at which a substance conducts heat. When a temperature gradient of 1 degree per unit length is established on a material with a unit cross-sectional area, then the thermal conductivity is defined as the heat conducted per unit time. The unit of thermal conductivity is Btu/(h?ft?0F) or w/(m ?K).
⑨． Thermal diffusivity: It is a performance that determines the rate of internal temperature transition of a substance. It is the ratio of thermal conductivity to the product of heat and density. The unit of thermal diffusivity is Btu/(h?ft?0F) or w/(m?k )express.
Performance and organization of stainless steel
There are currently more than 100 known chemical elements, and about 20 chemical elements can be encountered in steel materials commonly used in industry. For the special steel series stainless steel formed by people's long-term struggle against corrosion phenomena, there are more than a dozen most commonly used elements. In addition to iron, the basic element that makes up steel, the elements that have the greatest impact on the performance and structure of stainless steel are The elements are: carbon, chromium, nickel, manganese, silicon, molybdenum, titanium, niobium, titanium, manganese, nitrogen, copper, cobalt, etc. Except for carbon, silicon, and nitrogen, these elements are all elements in the transition group of the periodic table of chemical elements.
In fact, the stainless steel used in industry contains several or even more than a dozen elements at the same time. When several elements exist in the unity of stainless steel, their influence is greater than that of individual elements. It is much more complicated when it exists, because in this case not only the role of each element itself must be considered, but also their influence on each other must be paid attention to. Therefore, the structure of stainless steel is determined by the sum of the effects of various elements.
1)． The influence and effect of various elements on the performance and structure of stainless steel
1-1. The decisive role of chromium in stainless steel: There is only one element that determines the properties of stainless steel, which is chromium. Each stainless steel has Contains a certain amount of chromium. To date, there is no chromium-free stainless steel. The fundamental reason why chromium has become the main element that determines the performance of stainless steel is that adding chromium as an alloying element to steel promotes its internal contradictory movements to develop in a direction that is conducive to resisting corrosion damage. This change can be explained from the following aspects:
①Chromium increases the electrode potential of the iron-based solid solution
②Chromium absorbs electrons from iron to passivate the iron
Passivation is a phenomenon in which the corrosion resistance of metals and alloys is improved due to the prevention of anodic reactions. There are many theories that constitute the passivation of metals and alloys, including thin film theory, adsorption theory and electron arrangement theory.
1-2. The duality of carbon in stainless steel
Carbon is one of the main elements in industrial steel. The properties and structure of steel are largely determined by the presence of carbon in steel. The content and distribution form of carbon in stainless steel are particularly significant. The influence of carbon on the structure of stainless steel is mainly reflected in two aspects. On the one hand, carbon is an element that stabilizes austenite and its effect is very large (about 30 times that of nickel). On the other hand, due to the very high affinity between carbon and chromium, Large, and forms with chromium - a series of complex carbides. Therefore, from the perspective of strength and corrosion resistance, the role of carbon in stainless steel is contradictory.
Understanding the law of this influence, we can choose stainless steel with different carbon contents based on different usage requirements.
For example, the standard chromium content of the five steel grades 0Crl3 to 4Cr13, which is the most widely used and the most basic stainless steel in the industry, is 12 to 14%, which means that carbon must be carbonized with chromium. It is decided after taking into account the factor of chromium. The purpose is to ensure that after carbon and chromium are combined to form chromium carbide, the chromium content in the solid solution will not be lower than the minimum chromium content of 11.7%.
As for these five steel grades, due to different carbon contents, the strength and corrosion resistance are also different. The corrosion resistance of 0Cr13~2Crl3 steel is better but the strength is lower than 3Crl3 and 4Cr13 steel. It is mostly used to manufacture structural parts. The latter two steel grades can obtain high strength due to their high carbon content and are mostly used to manufacture springs, knives and other parts that require high strength and wear resistance.
For another example, in order to overcome the intergranular corrosion of 18-8 chromium-nickel stainless steel, the carbon content of the steel can be reduced to less than 0.03%, or an element (titanium or niobium) with a greater affinity than chromium and carbon can be added to prevent it from forming carbonization. Chromium, for example, when high hardness and wear resistance become the main requirements, we can increase the carbon content of the steel while appropriately increasing the chromium content, so as to not only meet the requirements of hardness and wear resistance, but also take into account certain requirements. Corrosion resistance, stainless steel 9Cr18 and 9Cr17MoVCo are used in industry for bearings, measuring tools and cutting tools. Although the carbon content is as high as 0.85~0.95%, their chromium content is also increased accordingly, so they still ensure corrosion resistance. Require.
Generally speaking, the carbon content of stainless steel currently used in industry is relatively low. The carbon content of most stainless steel is between 0.1 and 0.4%. The carbon content of acid-resistant steel is Mostly 0.1~0.2%. Stainless steel with a carbon content greater than 0.4% only accounts for a small part of the total number of steel grades. This is because under most conditions of use, stainless steel always has corrosion resistance as its main purpose. In addition, the lower carbon content is also due to certain process requirements, such as ease of welding and cold deformation.
1-3. The role of nickel in stainless steel is exerted after it is combined with chromium
Nickel is an excellent corrosion-resistant material and an important alloying element in alloy steel. . Nickel is an austenite-forming element in steel, but to obtain pure austenite structure in low-carbon nickel steel, the nickel content must reach 24%; and only when the nickel content is 27% does the steel's resistance to certain media Corrosion properties change significantly. So nickel alone cannot constitute stainless steel. However, when nickel and chromium exist in stainless steel at the same time, nickel-containing stainless steel has many valuable properties.
Based on the above situation, it can be seen that the role of nickel as an alloy element in stainless steel is that it changes the structure of high-chromium steel, thereby improving the corrosion resistance and process performance of stainless steel.
1-4. Manganese and nitrogen can replace nickel in chromium-nickel stainless steel
Although chromium-nickel austenitic steel has many advantages, in recent decades, due to the With the large-scale development and application of hot-strength steel containing less than 20% nickel, as well as the increasing development of the chemical industry, the demand for stainless steel is increasing. However, nickel mineral reserves are small and concentrated in a few areas, so it is very important worldwide. There has been a contradiction between the supply and demand of nickel. Therefore, in the fields of stainless steel and many other alloys (such as steel for large castings and forgings, tool steel, heat-strength steel, etc.), especially in countries where nickel resources are relatively scarce, the science of saving nickel and substituting nickel with other elements has been widely carried out. Research and production practice. In this regard, manganese and nitrogen are used to replace nickel in stainless steel and heat-resistant steel.
Manganese has a similar effect on austenite as nickel. But to be more precise, the role of manganese is not to form austenite, but to reduce the critical quenching speed of steel, increase the stability of austenite during cooling, inhibit the decomposition of austenite, and prevent the formation of austenite at high temperatures. Austenite is maintained at room temperature. In terms of improving the corrosion resistance of steel, manganese has little effect. For example, changing the manganese content in steel from 0 to 10.4 will not significantly change the corrosion resistance of steel in air and acid. This is because manganese has little effect on increasing the electrode potential of iron-based solid solution, and the protective effect of the oxide film formed is also very low. Therefore, although there are austenitic steels alloyed with manganese in industry (such as 40Mn18Cr4, 50Mn18Cr4WN, ZGMn13 steel etc.), but they are not available as stainless steel. The role of manganese in stabilizing austenite in steel is about one-half that of nickel. That is, 2% of nitrogen can also stabilize austenite in steel, and the extent of the role is greater than that of nickel. For example, in order to make steel containing 18% chromium obtain an austenite structure at room temperature, low-nickel stainless steel with manganese and nitrogen substituted for nickel and chromium-manganese-nitrogen stainless steel with elemental nickel are currently used in industry, and some Has successfully replaced the classic 18-8 chromium-nickel stainless steel.
1-5. Titanium or niobium is added to stainless steel to prevent intergranular corrosion.
1-6. Molybdenum and copper can improve the corrosion resistance of some stainless steels.
1-7. The influence of other elements on the properties and structure of stainless steel
The above nine main elements have an impact on the properties and structure of stainless steel. In addition to the influence of these elements on the properties and structure of stainless steel, In addition to the elements that have a greater impact on the structure, stainless steel also contains some other elements. Some of them are common impurity elements like ordinary steel, such as silicon, sulfur, phosphorus, etc. Some are added for specific purposes, such as cobalt, boron, selenium, rare earth elements, etc. From the perspective of the main property of stainless steel's corrosion resistance, these elements are non-main aspects compared to the nine elements that have been discussed. Even so, they cannot be completely ignored because they also affect the performance and structure of stainless steel. Influence.
Silicon is an element that forms ferrite and is a common impurity element in general stainless steel.
As an alloying element, cobalt is not widely used in steel. This is because of its high price and its use in other aspects (such as high-speed steel, cemented carbide, cobalt-based heat-resistant alloys, magnetic steel or hard magnets). Alloys, etc.) have more important uses. There are not many common stainless steels that add cobalt as an alloying element. Commonly used stainless steels such as 9Crl7MoVCo steel (containing 1.2-1.8% cobalt) add cobalt. The purpose is not to improve corrosion resistance but to increase hardness, because the main purpose of this stainless steel is to It manufactures slicing mechanical cutting tools, scissors and surgical blades, etc.
Boron: Adding 0.005% boron to high-chromium ferritic stainless steel Crl7Mo2Ti steel can improve the 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 increases the tendency of hot cracking during welding of austenitic steel due to the formation of low melting point *** crystals. However, when containing more boron (0.5 to 0.6%), it can prevent hot cracking. of production. Because when it contains 0.5 to 0.6% boron, an austenite-boride two-phase structure is formed, which lowers the melting point of the weld. When the solidification temperature of the molten pool is lower than the semi-melted zone, the tensile stress generated by the base material during cooling is in the liquid state. The solid weld metal withstands the stress and will not cause cracks at this time. Even if cracks are formed in the near seam area, they can be filled by the liquid-solid molten pool metal. Boron-containing chromium-nickel austenitic stainless steels have special uses in the atomic energy industry.
Phosphorus: It is an impurity element in ordinary stainless steel, but its harmfulness in austenitic stainless steel is not as significant as in ordinary steel, so the content can be allowed to be higher. If any information proposes It can reach 0.06% to facilitate smelting control. The phosphorus content of individual manganese-containing austenitic steels can reach 0.06% (such as 2Crl3NiMn9 steel) or even 0.08% (such as Cr14Mnl4Ni steel). Taking advantage of the strengthening effect of phosphorus on steel, phosphorus is also added as an alloy element for age-hardened stainless steel, such as PH17-10P steel (containing 0.25% phosphorus) and PH-HNM steel (containing 0.30% phosphorus).
Sulfur and selenium: Impurity elements are also common in general stainless steel. However, adding 0.2 to 0.4% sulfur to stainless steel can improve the cutting performance of stainless steel, and selenium also 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 chromium-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/cm2; the impact value of 18-8 steel containing 0.22% selenium The impact value of -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 rarely used as alloying elements of stainless steel.
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 bubbles caused by hydrogen in the steel ingot and reduce cracks in the steel billet. Adding 0.02 to 0.5% of rare earth elements (cerium-lanthanum alloy) to austenitic and austenitic-ferritic stainless steel can significantly improve the forging performance.
There used to be an austenitic steel containing 19.5% chromium, 23% nickel and molybdenum, copper and manganese. Due to the thermal processing performance, it could only produce castings in the past. 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
Based on chemical composition (mainly chromium content) and uses, stainless steel is divided into two categories: stainless and acid-resistant. In the industry, stainless steel is also classified according to the type of matrix structure of the steel after being heated and air-cooled from high temperature (900-1100 degrees). This is based on the characteristics of the influence of carbon and alloy elements on the structure of stainless steel that we discussed above.
Stainless steel used in industry can be divided into three categories according to its 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 subject to 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 often welded to 45 steel.
Classification, main components and performance comparison of stainless steel
Classification approximate components () Hardenability, corrosion resistance, processability, weldability, magnetism
C Cr Ni
Ferrite system 0.35 or less 16-27 - no good, good or good can be available
Martensitic system 1.20 or less 11-15 - no-bake cocoa can be available
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Austenitic system 0.25 below 16 above 7 above None Excellent Excellent None
The above classification is only based on the matrix structure of steel. Due to the stable austenite and the formation of ferrite in the steel The effects of the solid elements cannot balance each other, and due to the large amount of chromium, the S point of the balance diagram shifts to the left. In addition to the three basic types mentioned above, the structure of stainless steel used in industry also includes martensite-ferrite, austenitic Transitional complex phase stainless steels such as stenite-ferrite, austenite-martensite, etc., and stainless steel with martensite-carbide structure.
2-1. Ferritic steel
Low carbon chromium stainless steel containing more than 14% chromium, chromium stainless steel with any carbon content containing more than 27% chromium, and On the basis of the above ingredients, stainless steel is added with molybdenum, titanium, niobium, silicon, aluminum, tungsten, vanadium and other elements. The elements that form ferrite dominate the chemical composition, and the matrix structure is ferrite. The structure of this type of steel in the quenched (solid solution) state is ferrite, and a small amount of carbides and intermetallic compounds can be seen in the structure of the annealed and aged state.
Belonging to this category are Crl7, Cr17Mo2Ti, Cr25, Cr25Mo3Ti, Cr28, etc. Due to its high chromium content, ferritic stainless steel has relatively good corrosion resistance and oxidation resistance, but poor mechanical and process properties. It is mostly used in acid-resistant structures with little stress and as anti-oxidation steel.
2-2. Ferritic-martensitic steel
This type of steel is in a y a (or δ) two-phase state at high temperatures, and undergoes a y-M transformation during rapid cooling. The element body is still retained, and the normal temperature structure is martensite and ferrite. Due to the difference in composition and heating temperature, the amount of ferrite in the structure can vary from a few percent to tens of percents. 0Crl3 steel, lCrl3 steel, 2Cr13 steel with chromium at the upper limit and carbon at the lower limit, Cr17Ni2 steel, Cr17wn4 steel, and many modified 12% chromium heat-strength steel developed on the basis of ICrl3 steel (this type of steel is also called heat-resistant stainless steel ), many steel grades, such as Cr11MoV, Cr12WMoV, Crl2W4MoV, 18Crl2WMoVNb, etc., belong to this category.
Ferritic-martensitic steel can be partially quenched and strengthened, so it can obtain higher mechanical properties. However, their mechanical properties and process performance are largely affected by the content and distribution form of ferrite in the structure. This type of steel falls into two series according to the chromium content in its composition: 12-14% and 15-18%.
The former has the ability to resist the atmosphere and weakly corrosive media, and has good shock absorption and a small linear expansion coefficient; the corrosion resistance of the latter is equivalent to that of ferritic acid-resistant steel with the same chromium content, but to a certain extent It also retains some of the shortcomings of high-chromium ferritic steel.
2-3. Martensitic steel
This type of steel is in the y phase zone at normal quenching temperature, but their y phase is only stable at high temperatures, and the M point is generally At around 300°C, it transforms into martensite when cooled.
This type of steel includes 2Cr13, 2Cr13Ni2, 3Cr13 and some modified 12% chromium heat-strength steel, such as 13Cr14NiWVBA, Cr11Ni2MoWVB steel, etc. The mechanical properties, corrosion resistance, process performance and physical properties of martensitic stainless steel are similar to ferritic-martensitic stainless steel containing 12 to 14% chromium. Since there is no free ferrite in the structure, the mechanical properties are higher than those of the above steels, but the overheating sensitivity during heat treatment is lower.
2-4. Martensite-carbide steel
The carbon content of the precipitation point of Fe-C alloy is 0.83%. In stainless steel, the S point shifts to the left due to chromium. , steel containing 12% chromium and more than 0.4% carbon (Figure 11-3), and steel containing 18% chromium and more than 0.3% carbon (Figure 1) 3) are all processed steels. When this type of steel is heated at normal quenching temperature, the secondary carbides cannot be completely dissolved in austenite, so the structure after quenching is composed of martensite and carbides.
There are not many stainless steel grades that fall into this category, but they are some stainless steels with relatively high carbon content, such as 4Crl3, 9Cr18, 9Crl8MoV, 9Crl7MoVCo steel, etc. 3Crl3 steel with a carbon content that is towards the upper limit is in the lower range. Such a structure may also appear when quenched at high temperatures. Due to the high carbon content, although the above three steel grades such as 9Cr18 contain more chromium, their corrosion resistance is only equivalent to that of stainless steel containing 12 to 14% germanium. The main uses of this type of steel are parts that require high hardness and wear resistance, such as cutting tools, bearings, springs and medical equipment.
2-5. Austenitic steel
This type of steel contains more elements that expand the y zone and stabilize austenite. It is in the y phase at high temperatures and becomes y phase when cooled. Since the Ms point is below room temperature, it has an austenite structure at room temperature. Chromium-nickel stainless steels such as 18-8, 18-12, 25-20, 20-25Mo, and low-nickel stainless steels that replace part of nickel with manganese and add nitrogen, such as Cr18Mnl0Ni5, Cr13Ni4Mn9, Cr17Ni4Mn9N, Cr14Ni3Mnl4Ti steel, etc., all belong to this category.
Austenitic stainless steel has many advantages as mentioned before. Although its mechanical properties are relatively low and it cannot be strengthened by heat treatment like ferritic stainless steel, it can be deformed by cold working and use work hardening effect. Increase their strength. The disadvantage of this type of steel is that it is sensitive to intergranular corrosion and stress corrosion, which needs to be eliminated through appropriate alloy additives and process measures.
2-6. Austenitic-ferritic steel
This type of steel is not enough to expand the y zone and stabilize the austenite elements at room temperature or It has a pure austenite structure at very high temperatures, so it is an austenite-ferrite complex phase state. The amount of ferrite can also vary within a wide range due to different compositions and heating temperatures.
There are many stainless steels that belong to this category, such as low-carbon 18-8 chromium-nickel steel, 18-8 chromium-nickel steel with titanium, niobium, and molybdenum added, especially in the structure of cast steel. to ferrite, in addition to chromium-manganese stainless steels (such as Cr17Mnll) containing more than 14-15% chromium and less than 0.2% carbon, as well as most chromium-manganese nitrogen stainless steels currently studied and applied. Compared with pure austenitic stainless steel, this type of steel has many advantages, such as higher yield strength, higher resistance to intergranular corrosion, lower sensitivity to stress corrosion, less tendency to produce hot cracks during welding, and good casting fluidity. etc. The disadvantages are poor pressure processing performance, greater pitting corrosion tendency, easy to produce c-phase brittleness, and weak magnetism under the action of a strong magnetic field. All these advantages and disadvantages originate from the ferrite in the structure.
2-7. Austenitic pot-martensitic steel
The Ms point of this type of steel is lower than room temperature. After solution treatment, it becomes an austenitic structure and is easy to form and weld. . There are usually two processes available to achieve martensitic transformation. First, after solid solution treatment and heating at 700 to 800 degrees, austenite transforms into a metastable state due to the precipitation of chromium carbide. The Ms point rises above room temperature and transforms into martensite when cooled; second, after solid solution treatment, it directly Cool to between Ms and Mf points to transform austenite into martensite. The latter method can obtain higher corrosion resistance, but the interval between solution treatment and cryogenic cooling should not be too long, otherwise the strengthening effect of cryogenic cooling will be reduced due to the aging and stabilizing effect of austenite. After the above treatment, the steel is aged at 400 to 500 degrees to further strengthen the precipitated intermetallic compounds. Typical steel grades of this type of steel are 17Cr-7Ni-A1, 15Cr-9Ni-A1, 17Cr-5Ni-Mo, 15Cr-8Ni-Mo-A1, etc. This type of steel is also called austenitic-martensite aging stainless steel, and because in fact there are different amounts of ferrite in the structure of these steels in addition to austenite and martensite, it is also called semi-austenitic. Precipitation hardened stainless steel.
This type of steel is a new type of stainless steel developed and applied in the late 1950s. Their general characteristics are high strength (C can reach 100-150) and good thermal strength. However, due to the low chromium content and Chromium carbide precipitates during heat treatment, so the corrosion resistance is lower than standard austenitic stainless steel. It can also be said that the high strength of this type of steel is obtained at the expense of some corrosion resistance and other properties (such as non-magnetic properties). Currently, this type of steel is mainly used in the aviation industry and rocket and missile production, and is used in general machinery manufacturing. They are not yet common, and they are also classified as a series of ultra-high-strength steels.
Corrosion resistance of stainless steel
Types and definitions of corrosion
A type of stainless steel can have good corrosion resistance in many media, but in another certain In this medium, corrosion may occur due to low chemical stability. Therefore, a kind of stainless steel cannot be corrosion-resistant to all media. In many industrial applications, stainless steel can provide satisfactory corrosion resistance. According to the experience of use, in addition to mechanical failure, the corrosion of stainless steel mainly manifests in: a serious form of corrosion of stainless steel is localized corrosion (i.e., stress corrosion cracking, pitting corrosion, intergranular corrosion, corrosion fatigue and crevice corrosion) . Failure cases caused by these local corrosion account for almost more than half of the failure cases. In fact, many failure accidents can be avoided through reasonable material selection.
Metal corrosion can be divided into special corrosion and chemical corrosion according to the mechanism