A high alloy steel resistant to air or chemical corrosive medium. Stainless steel is a kind of steel with beautiful surface and good corrosion resistance. It does not need surface treatment, such as color electroplating, but exerts its inherent surface properties. It is widely used and is usually called stainless steel. 13 Cr steel, 18- Cr-Ni steel and other high alloy steels are representative properties.

From the metallographic point of view, because stainless steel contains chromium, a very thin chromium film is formed on the surface, which isolates the oxygen invading the steel and plays a role in corrosion resistance.

In order to maintain the inherent corrosion resistance of stainless steel, steel must contain more than 12% chromium.

Stainless steel type:

Stainless steel can be roughly classified according to its use, chemical composition and metallographic structure.

Based on the basic composition of austenitic steel 18% Cr -8% Ni, various steels with different element additions were developed.

Classification by chemical composition:

①.Cr series: ferrite series and martensite series.

② Cr-Ni series: austenite series, abnormal series and precipitation hardening series.

Classification of metallographic structure:

① Austenitic stainless steel

②. Ferritic stainless steel

③ Martensitic stainless steel

④ Duplex stainless steel

⑤. Precipitation hardening stainless steel

Identification method of stainless steel

Number and representation of steel

(1) Use international chemical element symbols and national symbols to represent chemical components, and use Arabic letters to represent component content;

Such as: China-Russia 12CrNi3A.

(2) The series or number of steel is represented by a fixed number of digits; Such as: USA, Japan, 300 series, 400 series, 200 series;

(3) with Latin letters and sequence number, only for use.

Numbering rules in China.

(1) Use element symbols

② uses: hanyu pinyin, open hearth steel: p, boiling steel: f, killed steel: b, grade a steel: a, T8: Te8,

GCr 15: ball

◆ Composite steel and spring steel, such as 20CrMnTi 60SiMn (C content is expressed in parts per ten thousand).

◆ Stainless steel and alloy tool steel (C content is expressed in thousandths), such as: 1Cr 18Ni9 thousandths (i.e.

0. 1%C), stainless C≤0.08%, such as 0Cr 18Ni9, and ultra-low carbon C≤0.03%, such as 0Cr 17Ni 13Mo.

International stainless steel marking method

The American Iron and Steel Association uses three numbers to represent various standard grades of malleable stainless steel. These include:

① Austenitic stainless steel is marked with 200 and 300 series numbers,

② Ferritic and martensitic stainless steels are represented by 400 series numbers. For example, some common austenitic stainless steels.

It is marked with 20 1, 304, 3 16, 3 10.

③ ferritic stainless steel is marked with 430 and 446, and martensitic stainless steel is marked with 4 10, 420 and 440C.

Remember, biphasic (austenite-ferrite),

④ Stainless steel, precipitation hardening stainless steel and high alloy with iron content less than 50% are usually named by patent names or trademarks.

4). Classification and grading of standards

4- 1 rating:

① National standard GB

② Industry standard YB

③ Local standards

④ enterprise standard Q/CB

4-2 Classification:

① product standard

② Packaging standard

③ Method standard

④ Basic standards

4-3 standard grades (divided into three grades):

Grade Y: international advanced level.

Class I: international average level.

Class H: domestic advanced level.

4-4 National Standards

GB 1220-84 Stainless Steel Rod (Grade I)

GB424 1-84 stainless steel welding plate (class h)

GB4356-84 Stainless Steel Welded Disc Garden (Grade I)

GB 1270-80 Stainless Steel Pipe (Grade I)

Gb12771-91stainless steel welded pipe (grade y)

GB3280-84 Stainless Steel Cold Plate (Grade I)

GB4237-84 Stainless Steel Hot Plate (Grade I)

GB4239-9 1 stainless steel cold-rolled strip (grade 1)

Stainless steel terminology

Generally speaking, stainless steel is not easy to rust. In fact, some stainless steels have both rust resistance and acid resistance (corrosion resistance). The rust resistance and corrosion resistance of stainless steel are due to the formation of chromium-rich oxide film (passivation film) on its surface. This rust resistance and corrosion resistance are relative. The test shows that the corrosion resistance of steel in weak media such as air and water and oxidizing media such as nitric acid increases with the increase of chromium content in steel. When the chromium content reaches a certain percentage, the corrosion resistance of steel changes suddenly, that is, from easy to difficult to rust, from corrosion resistance to corrosion resistance. Stainless steel can be classified in many ways. According to the structure at room temperature, there are martensite, austenite, ferrite and duplex stainless steel; According to the main chemical composition, it can be basically divided into two systems: chromium stainless steel and chromium-nickel stainless steel. According to the use, there are nitric acid-resistant stainless steel, sulfuric acid-resistant stainless steel and seawater-resistant stainless steel. According to the corrosion type, it can be divided into pitting corrosion resistant stainless steel, stress corrosion resistant stainless steel and intergranular corrosion resistant stainless steel. According to the functional characteristics, it can be divided into nonmagnetic stainless steel, free-cutting stainless steel, low-temperature stainless steel and high-strength stainless steel. Because stainless steel has excellent corrosion resistance, formability, compatibility and toughness in a wide temperature range, it is widely used in heavy industry, light industry, daily necessities industry and architectural decoration industry.

Austenitic stainless steel: stainless steel with austenitic structure at room temperature. When the chromium content is about 18%, the nickel content is about 8% ~ 10%, and the carbon content is about 0. 1%, the steel has a stable austenite structure. Austenitic Cr-Ni stainless steel includes the famous 18Cr-8Ni steel and the high Cr-Ni series steel developed by adding Mo, Cu, Si, Nb and Ti. Austenitic stainless steel is nonmagnetic and has high toughness and plasticity, but its strength is low, so it can not be strengthened by phase transformation, but only by cold working. If sulfur, calcium, selenium, tellurium and other elements are added, it has good cutting performance. In addition to the corrosion resistance of oxidizing acid medium, this steel can also resist the corrosion of sulfuric acid, phosphoric acid, formic acid, acetic acid and urea if it contains elements such as Mo and Cu. If the carbon content in this steel is less than 0.03% or contains Ti and Ni, its intergranular corrosion resistance can be significantly improved. High silicon austenitic stainless steel has good corrosion resistance in concentrated nitric acid. Austenitic stainless steel has been widely used in all walks of life because of its comprehensive and good comprehensive properties.

Ferritic stainless steel: ferritic structural stainless steel in use. The chromium content is 1 1%~30%, and it has a body-centered cubic crystal structure. This kind of steel generally does not contain nickel, and sometimes contains a small amount of elements such as molybdenum, titanium and niobium. This kind of steel has the characteristics of large thermal conductivity, small expansion coefficient, good oxidation resistance and excellent stress corrosion resistance, and is mostly used to manufacture parts resistant to atmospheric, steam, water and oxidative acid corrosion. This kind of steel has some disadvantages, such as poor plasticity, obviously reduced plasticity after welding and corrosion resistance, which limits its application. The application of refining technology outside the furnace (AOD or VOD) can greatly reduce the interstitial elements such as carbon and nitrogen, so this kind of steel is widely used.

Austenite-ferrite duplex stainless steel: it is a kind of stainless steel with about half austenite and half ferrite structure. In the case of low C content, Cr content is 18%~28%, and Ni content is 3%~ 10%. Some steels also contain alloying elements, such as molybdenum, copper, silicon, niobium, titanium and nitrogen. This steel has the characteristics of both austenitic and ferritic stainless steel. Compared with ferritic steel, it has higher plasticity and toughness, no brittleness at room temperature, significantly improved intergranular corrosion resistance and weldability, and maintained the brittleness, high thermal conductivity and superplasticity of ferritic stainless steel at 475℃. Compared with austenitic stainless steel, it has high strength and obviously improved intergranular corrosion resistance and chloride stress corrosion resistance. Duplex stainless steel has excellent pitting corrosion resistance and is also a nickel-saving stainless steel.

Martensite stainless steel: Stainless steel whose mechanical properties can be adjusted by heat treatment. Generally speaking, it is a hardenable stainless steel. Typical brands are Cr 13, such as 2cr 13, 3cr 13, 4cr 13 and so on. After quenching, the hardness is high, and different tempering temperatures have different combinations of strength and toughness, which are mainly used for steam turbine blades, tableware and surgical instruments. According to different chemical compositions, martensitic stainless steel can be divided into martensitic chromium steel and martensitic chromium-nickel steel. According to the different microstructure and strengthening mechanism, it can also be divided into martensite stainless steel, martensite and semi-austenite (or semi-martensite) precipitation hardening stainless steel and martensite aging stainless steel.

Physical, chemical and mechanical properties of stainless steel

The physical properties of stainless steel are mainly shown in the following aspects:

① Thermal expansion coefficient: the change of material measuring element caused by temperature change. 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 specific temperatures is the average thermal expansion coefficient. The expansion coefficient can be expressed by volume or length, usually by length.

② Density: The density of a substance is the mass per unit volume of the substance, and the unit is kg/m3 or 1b/in3.

(3) Elastic modulus: When applying a force at both ends of a unit length can cause a unit change in the length of an object, the force required per unit area is called elastic modulus. The unit is 1b/in3 or N/m3.

④ resistivity: the resistance measured between two opposite sides of cubic material per unit length, in ω? m，μω？ Cm or (discarded) ω/(round mil. Feet).

⑤. Magnetic permeability: dimensionless coefficient, which indicates the degree of easy magnetization of a substance and is the ratio of magnetic induction strength to magnetic field strength.

⑥. Melting temperature range: Determine the temperature at which the alloy starts to solidify and ends to solidify.

⑦. Specific heat: the heat required for the temperature change of a substance per unit mass of 65438 0 degrees. In the British system and the CGs system, the specific calorific value is the same, because the unit of heat (Biu or cal) depends on the heat required for the rise of water per unit mass 1 degree. The numerical value of specific heat in the international system of units is different from that in the English system or CGS system, because the unit of energy (J) is determined according to different definitions. The unit of specific heat is Btu( 1b? 0F) and J/(kg? k).

8. Thermal conductivity: a measure of the thermal conductivity of a substance. When a temperature gradient of 65438 0 degrees per unit length is established on a substance with a unit cross-sectional area, the thermal conductivity is defined as the heat conducted per unit time, and the unit of the thermal conductivity is Btu/(h? ft？ 0F) or w/(m? k).

⑨。 Thermal diffusivity: it is a property that determines the forward speed of temperature inside a substance, and 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) means.

Properties and microstructure of stainless steel

At present, there are more than 100 known chemical elements, and there are about 20 chemical elements that can be encountered in steel materials commonly used in industry. For stainless steel, a special steel series formed by people's long-term struggle against corrosion, there are more than a dozen commonly used elements. Besides iron, the elements that have the greatest influence on the properties and microstructure of stainless steel are carbon, chromium, nickel, manganese, silicon, molybdenum, titanium, niobium, titanium, manganese, nitrogen, copper and cobalt. Except for carbon, silicon and nitrogen, these elements are all elements of the transition group in the periodic table of chemical elements.

In fact, all stainless steels used in industry have several or even a dozen elements at the same time. When several elements * * * exist in the unity of stainless steel, their influence is much more complicated than when they exist alone, because in this case, we should not only consider the role of each element itself, but also pay attention to their mutual influence, so the structure of stainless steel depends on the sum of the influences of various elements.

1). The influence and function of various elements on the properties and microstructure 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, that is, chromium, and each stainless steel contains a certain amount of chromium. So far, there is no stainless steel without chromium. Chromium has become the main element that determines the properties of stainless steel. The fundamental reason is that chromium, as an alloying element, promotes the internal contradictory movement to develop in the direction of corrosion resistance. This change can be explained from the following aspects:

(1) Cr increases the electrode potential of iron-based solid solution.

(2) Chromium absorbs electrons from iron to passivate iron.

Passivation is a phenomenon, and the corrosion resistance of metals and alloys is improved because the anodic reaction is prevented. There are many theories that constitute passivation of metals and alloys, mainly including thin film theory, adsorption theory and electron arrangement theory.

1-2. Duality of carbon in stainless steel

Carbon is one of the main elements in industrial steel, and the properties and microstructure of steel largely depend on the content and distribution of carbon in steel, especially stainless steel. The influence of carbon on the microstructure of stainless steel is mainly manifested in two aspects. On the one hand, carbon is an element that stabilizes austenite and plays a very important role (about 30 times as much as nickel). On the other hand, because of its great affinity with chromium, carbon forms a series of complex carbides with chromium. Therefore, the role of carbon in stainless steel is contradictory in terms of strength and corrosion-resistant candle performance.

Knowing the law of this influence, we can choose stainless steel with different carbon content from different use requirements.

For example, the standard chromium content of five grades of stainless steel -0cl3 ~ 4cr 13, which is the most widely used in industry, is 12 ~ 14%, which is determined by considering the factors that carbon and chromium form chromium carbide, so that the chromium content in solid solution will not be lower than 14% after carbon and chromium are chromized to synthesize chromium carbide.

As far as these five steels are concerned, their strength and corrosion resistance are different because of their different carbon contents. 0cr 13 ~ 2cr 13 steel has good corrosion resistance, but its strength is lower than that of 3Crl3 and 4Cr 13 steel, and it is mostly used to manufacture structural parts. The latter two kinds of steel can obtain high strength because of their high carbon content, and are mostly used to manufacture parts that require high strength and wear resistance, such as springs and knives. For another example, in order to overcome the intergranular corrosion of 18-8 Cr-Ni stainless steel, the carbon content in the steel can be reduced to below 0.03%, or elements (titanium or niobium) with greater affinity than chromium and carbon can be added, so that chromium carbide will not be formed. For another example, when high hardness and wear resistance become the main requirements, we can increase the carbon content of steel and chromium content appropriately, so as to meet the requirements of hardness and wear resistance at the same time. In industry, stainless steel 9Cr 18 and 9Cr 17MoVCo steel are used as bearings, measuring tools and blades. Although the carbon content is as high as 0.85 ~ 0.95%, their chromium content also increases accordingly, so the requirements of corrosion resistance are guaranteed.

Generally speaking, the carbon content of stainless steel used in industry at present is relatively low, most of which are between 0. 1 ~ 0.4%, and the carbon content of acid-resistant steel is between 0. 1 ~ 0.2%. Stainless steel with carbon content greater than 0.4% accounts for only a small part of the total number of steel grades, because under most service conditions, stainless steel always aims at corrosion resistance. In addition, the low carbon content is also due to some technological requirements, such as easy welding and cold deformation.

1-3. The role of nickel in stainless steel is only played after it is combined with chromium.

Nickel is an excellent corrosion-resistant material and an important alloying element of alloy steel. Nickel is the element that forms austenite in steel, but in order to obtain pure austenite structure, the nickel content of low carbon nickel steel should reach 24%. Only when the nickel content is 27%, the corrosion resistance of steel in some media will change significantly. Therefore, nickel itself cannot constitute stainless steel. However, when nickel and chromium coexist in stainless steel, stainless steel containing nickel has many valuable characteristics.

Based on the above, it can be seen that the role of nickel as an alloying element in stainless steel lies in that it changes the structure of high chromium steel, thus improving the corrosion resistance and process performance of stainless steel.

1-4. Manganese and nitrogen can replace nickel in Cr-Ni stainless steel.

Although Cr-Ni austenitic steel has many advantages, in recent decades, due to the extensive development and application of nickel-based heat-resistant alloy and heat-resistant steel with nickel content less than 20%, as well as the increasing demand for stainless steel with the development of chemical industry, there is a contradiction between supply and demand of nickel worldwide. Therefore, in stainless steel and many other alloy fields (such as steel for large castings and forgings, tool steel, heat-resistant steel, etc. ), especially in countries lacking nickel resources, the scientific research and production practice of saving nickel and replacing nickel with other elements have been widely carried out. In this respect, manganese and nitrogen are widely used to replace nickel in stainless steel and heat-resistant steel.

The effect of manganese on austenite is similar to that of nickel. But more precisely, the function 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 keep the austenite formed at high temperature to room temperature. Manganese has little effect on improving the corrosion resistance of steel, such as the change of manganese content in steel from 0 to 10.4%, which has not obviously changed the corrosion resistance of steel in air and acid. This is because manganese has little effect on improving the electrode potential of iron-based solid solution, and the protective effect of oxide film formed is also very low, so although there are austenitic steels alloyed with manganese in industry (such as 40mn 18cr4, 50mn 18cr4wn, ZGMn 13 steel, etc.). ), they cannot be used as stainless steel. The role of manganese in stabilizing austenite in steel is about half that of nickel, that is, the role of 2% nitrogen in steel is also stabilizing austenite, and its role is greater than that of nickel. For example, in order to obtain the austenite structure of steel containing 18% chromium at room temperature, low-nickel stainless steel with manganese and nitrogen instead of nickel and stainless steel with nickel instead of chromium-manganese-nitrogen have been applied in industry, and some of them have 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. Effect of other elements on properties and microstructure of stainless steel

Effects of the above nine main elements on the properties and microstructure of stainless steel Apart from these elements, stainless steel also contains some other elements. Some are common impurity elements like ordinary steel, such as silicon, sulfur and phosphorus, and some are added for certain purposes, such as cobalt, boron, selenium and rare earth elements. As far as the corrosion resistance of stainless steel is concerned, these elements are not the main aspects compared with the nine elements discussed. Even so, they can not be completely ignored, because they will also affect the properties and microstructure of stainless steel.

Silicon is an element that forms ferrite, and it is a constant impurity element in general stainless steel.

As an alloying element, cobalt is more important in other fields (such as high speed steel, cemented carbide, cobalt-based heat-resistant alloy, magnetic steel or hard magnetic alloy, etc.) because of its high price, so it is not widely used in steel. ). Cobalt is rarely added to ordinary stainless steel as an alloying element. Commonly used stainless steel, such as 9 Cr 17 mov co steel (containing cobalt 1.2- 1.8%), is added with cobalt not to improve corrosion resistance, but to improve hardness, because this kind of stainless steel is mainly used for manufacturing microtome blades, scissors and surgical blades.

Boron: Adding 0.005% boron to high chromium ferritic stainless steel Cr 17 mo 2 ti can improve the corrosion resistance in boiling 65% acetic acid. Adding a small amount of boron (0.0006 ~ 0.0007%) can improve the thermoplasticity of austenitic stainless steel. A small amount of boron forms low melting point crystals, which increases the tendency of welding hot cracking of austenitic steel, but it can prevent hot cracking when it contains more boron (0.5 ~ 0.6%). Because when containing 0.5 ~ 0.6% boron, austenite-boride two-phase structure is formed, which reduces the melting point of weld. When the solidification temperature of the molten pool is lower than the semi-melting zone, the tensile stress produced by the base metal during the cooling process is borne by the liquid and solid weld metal, and no cracks will occur at this time. Even if cracks are formed near the seam, they can be filled with liquid-solid molten pool metal. Chromium-nickel austenitic stainless steel containing boron has special uses in the atomic energy industry.

Phosphorus: it is an impurity element in general stainless steel, but its harmfulness in austenitic stainless steel is not as obvious as that in general steel, so the content can be allowed to be higher, and if there are some data, it can reach up to 0.06%, which is beneficial to smelting control. The phosphorus content of individual manganese-containing austenitic steels can reach 0.06% (such as 2 Cr13 NIMNN9 steel) or even 0.08% (such as Cr 14Mnl4Ni steel). Using the strengthening effect of phosphorus on steel, phosphorus is also added as an alloying element of age-hardening stainless steel. The steel with pH 17- 10p (containing 0.25% phosphorus) is pH-HNM steel (containing 0.30 phosphorus).

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, and selenium also has the same effect. Sulfur and selenium improve the machinability 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. 18-8 steel (0.084% C, 18. 15% Cr, 9.25% Ni) has an impact value of 0.31.8kg/cm2; Contains 0. The impact value of 18-8 steel (0.094% C, 18.4% Cr, 9% Ni) containing 22% Se is 3.24 kg/cm2. Sulfur and selenium can 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 applied to stainless steel, and the main purpose at present is to improve the process performance. If a small amount of rare earth elements are added to Cr 17 ti steel and Cr 17Mo2Ti steel, the bubbles caused by hydrogen in the steel ingot can be eliminated and the billet cracks can be reduced. Adding 0.02 ~ 0.5% rare earth elements (Ce-La alloy) to austenitic and austenitic-ferritic stainless steels can significantly improve the forging properties. There was once an austenitic steel containing 19.5% chromium, 23% nickel and molybdenum, copper and manganese. In the past, due to the thermal processing performance, only castings could be produced, and after adding rare earth elements, they could be rolled into various profiles.

2) The classification of stainless steel is based on the metallographic structure and general characteristics of various stainless steels.

According to chemical composition (mainly chromium content) and application, stainless steel can be divided into two categories: stainless steel and acid resistance. In industry, stainless steel is also classified according to the matrix structure type of steel cooled by hot air at high temperature (900- 1 100℃), which is based on the characteristics of the influence of carbon and alloying elements on stainless steel structure discussed above.

Stainless steel used in industry can be divided into three types according to metallographic structure: ferritic stainless steel, martensitic stainless steel and austenitic stainless steel. The characteristics of these three types of stainless steels can be summarized (as shown in the following table), but it should be noted that not all martensitic stainless steels are weldable, but they are limited by certain conditions, such as preheating before welding and tempering at high temperature after welding, which makes the welding process more complicated. In actual production, some martensitic stainless steels such as 1Cr 13, 2Cr 13 and 2Cr 13, 45 are still welded.

Classification, main components and performance comparison of stainless steel

Classification approximate composition (%) quenching, corrosion resistance, machinability, weldability and magnetism.

Carbon-chromium-nickel

Ferrite system below 0.35 16-27- not good, good, and average.

1.20 Markov systems11-kloc-0/5-self-hardening is indispensable.

Austenitic system below 0.25,16,7 or above, no advantages or disadvantages.

The above classification is only based on the matrix structure of steel. Due to the fact that the functions of elements that stabilize austenite and form ferrite in steel cannot be balanced with each other, and because a large amount of chromium shifts the S point of the equilibrium diagram to the left, there are transition multiphase stainless steels such as martensite-ferrite, austenite-ferrite, austenite-martensite and martensite-carbide structure stainless steels in addition to the above three basic types.

2- 1. ferritic steel

Low-carbon chromium stainless steel with chromium 14% or more, chromium stainless steel with chromium 27% or any carbon content, stainless steel with elements such as molybdenum, titanium, niobium, silicon, aluminum, tungsten, vanadium, etc. are added on the basis of the above components, and the elements forming ferrite are absolutely dominant in the chemical composition, and the matrix structure is iron. The microstructure of this steel in quenching (solution) state is ferrite, while a small amount of carbides and intermetallic compounds can be seen in the microstructure in annealing and aging state.

There are Crl7, Cr 17Mo2Ti, Cr25, Cr25Mo3Ti, Cr28, etc. Ferritic stainless steel has good corrosion resistance and oxidation resistance because of its high chromium content, but its mechanical properties and technological properties are poor, so it is mostly used in acid-resistant structures with less stress and as oxidation-resistant steel.

2-2. Ferrite-Martensite Steel

This steel is in y+a (or δ) two-phase state at high temperature, and y-M transformation occurs during rapid cooling, and ferrite remains. The microstructure at room temperature is martensite and ferrite. Due to the different composition and heating temperature, the ferrite content in the microstructure can range from a few percent to dozens. 0Crl3 steel, lCrl3 steel, 2Cr 13 steel with upper chromium limit and lower carbon limit, Cr 17Ni2 steel, Cr 17wn4 steel and many improved 12% chromium heat-resistant steels (also called heat-resistant stainless steel) developed on the basis of ICrl3 steel, such as Cr.

Ferrite-martensite steel can be partially quenched and strengthened, so it can obtain higher mechanical properties. However, their mechanical and technological properties are largely influenced by the ferrite content and distribution in the microstructure. According to the chromium content in the composition, this steel belongs to two series: 12 ~ 14% and 15 ~ 18%. The former has the ability to resist the atmosphere and weakly corrosive media, good shock absorption and 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 it also retains some shortcomings of high chromium ferritic steel to some extent.

2-3. Martensite steel

This steel is in the Y phase region at normal quenching temperature, but its Y phase is stable only at high temperature, and its M point is generally around 300℃, so it transforms into martensite after cooling.

This kind of steel includes 2cr 13, 2cr 13Ni2, 3cr 13 and some modified 12% chromium heat-resistant steels, such as 13cr 14niwvb steel. The mechanical properties, corrosion resistance, technological properties and physical properties of martensitic stainless steel are similar to those of ferritic-martensitic stainless steel containing 12 ~ 14% chromium. Because there is no free ferrite in the structure, its mechanical properties are higher than those of the above steel, but its overheating sensitivity is low during heat treatment.

2-4. Martensite carbide steel

The carbon content of eutectoid point of Fe-C alloy is 0.83%. In stainless steel, the point S is shifted to the left due to chromium. When the steel containing 12% chromium and more than 0.4% carbon (Figure 1 1-3) and the steel containing 18% chromium and more than 0.3% carbon (Figure B) are heated at normal quenching temperature,

There are not many stainless steel brands in this category, but there are some stainless steels with high carbon content, such as 4Crl3, 9Cr 18, 9 Cr 18 mov, 9 Cr 17 mov co steel, etc. This structure may also occur when the upper limit of carbon content is quenched at a lower temperature. The above three kinds of steel, such as 9Cr 18, contain more chromium because of their high carbon content, but their corrosion resistance is only equivalent to that of stainless steel containing 12 ~ 14% germanium. The main uses of this steel are parts that need high hardness and wear resistance, such as knives, bearings, springs and medical devices.

2-5. Austenitic steel

This kind of steel contains a large number of elements that expand Y region and stabilize austenite, and all of them are Y phase at high temperature. Ms point is lower than room temperature when cooling, so it has austenite structure at room temperature. Chromium-nickel stainless steel such as 18-8,12, 25-20, 20-25Mo, and low-nickel stainless steel such as Cr 18mnl0ni5, Cr 13Ni4mn9, Cr 65449.

Austenitic stainless steel has many advantages mentioned above. Although its mechanical properties are relatively low, ferritic stainless steel can not be strengthened by heat treatment, but its strength can be improved by cold working deformation and work hardening. The disadvantage of this steel is that it is sensitive to intergranular corrosion and stress corrosion, which needs to be eliminated by appropriate alloy additives and technological measures.

2-6. Austenite-ferrite steel

This kind of steel is in austenite-ferrite multiphase state because it can't make the steel have pure austenite structure at room temperature or very high temperature because of expanding Y region and stabilizing the action degree of austenite elements, and its ferrite content can also change in a wide range due to the difference of composition and heating temperature.

There are many stainless steels belonging to this category, such as 18-8 Cr-Ni steel with low carbon, 18-8 Cr-Ni steel with titanium, niobium and molybdenum, especially ferrite can be seen in the cast steel. In addition, Cr-Mn stainless steel with carbon content of more than 14 ~ 15% and less than 0.2% (for example, compared with pure austenitic stainless steel, this kind of steel has many advantages, such as high yield strength, strong intergranular corrosion resistance, low stress corrosion sensitivity, difficult to produce hot cracks during welding and good casting fluidity. The disadvantages are poor pressure workability, large pitting tendency, easy to produce C-phase brittleness and weak magnetism under the action of strong magnetic field. All these advantages and disadvantages come from ferrite in the structure.

2-7. Austenite Bowl Martensite Steel

The Ms point of this steel is lower than room temperature, and it has austenite structure after solution treatment, which is easy to form and weld. Generally, two methods can be used to make it undergo martensite transformation. First, after solution treatment, austenite transforms into metastable state when heated at 700 ~ 800℃, Ms point rises above room temperature, and transforms into martensite when cooled. Second, after solution treatment, it is directly cooled to the point between Ms and Mf, so that austenite is transformed into martensite. The latter method can obtain high corrosion resistance, but the interval between solution treatment and cryogenic treatment should not be too long, otherwise the strengthening effect of cryogenic treatment will be reduced due to the aging stability of austenite. After the above treatment, the steel is aged for 400-500 degrees to further strengthen the precipitation of intermetallic compounds. Typical grades of this steel are 17Cr-7Ni-A 1, 15cr-9ni-A 1,/7cr-5ni-mo,15cr-8ni-. This kind of steel is also called austenite-martensite aging stainless steel. Because there are different amounts of ferrite in the structure of this kind of steel besides austenite and martensite, it is also called semi-austenite precipitation hardening stainless steel.

This kind of steel is a new type of stainless steel developed and applied in the late 1950s. Its general characteristics are high strength (c can reach 100- 150) and good thermal strength. However, due to the low chromium content, chromium carbide is precipitated during heat treatment, and its corrosion resistance is lower than that of standard austenitic stainless steel. It can also be said that the high strength of this steel is obtained at the expense of some corrosion resistance and other properties (such as non-magnetism). At present, this kind of steel is mainly used in aviation industry and rocket and missile production, but it is not widely used in general machinery manufacturing and belongs to ultra-high strength steel series.

Corrosion resistance of stainless steel

Types and definitions of corrosion

Stainless steel has good corrosion resistance in many media, but it may be corroded in another media because of its low chemical stability. Therefore, it is impossible for stainless steel to resist the corrosion of all media. In many industrial applications, stainless steel can provide satisfactory corrosion resistance. According to experience, besides mechanical failure, the corrosion of stainless steel is mainly manifested in: a serious corrosion form of stainless steel is local corrosion (i.e. stress corrosion cracking, pitting corrosion, intergranular corrosion, corrosion fatigue and crevice corrosion). The 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.

The corrosion of metals can be divided into special corrosion and chemical corrosion according to the mechanism.