AP: anneal treatment & passage / (acid pickling) MP: mechanical polishing
Ba: bright annealing
Here I know the pipeline material processing flow, for reference
EP: bright annealing — nondestructive testing — Chemical Polishing — electropolishing; BA: bright annealing — nondestructive testing — primary cleaning; CP / AP: heat treatment — nondestructive testing — chemical polishing.
What does stainless steel plate 2B and BA plate mean？
1. Cold rolling annealing pickling leveling
Cold rolling bright annealing temper
The main difference is annealing, 2b is annealing, pickling, the surface has a certain degree of oxidation, darker; BA bright annealing, the surface does not oxidize, brighter.
2. Bright surface: BA is a product obtained by cold rolling, bright annealing and leveling. Excellent surface gloss, high reflectivity. Like the surface of a mirror. Used for household appliances, mirrors, kitchen equipment, decorative materials, etc.
Fog surface: no.2b cold rolled, heat-treated, acid washed, and then finish rolled to make the surface moderately bright. Because the surface is smooth, easy to grind, so that the surface is more bright, widely used, such as tableware, building materials, etc. After adopting surface treatment to improve mechanical properties, it can meet almost all applications.
304j1 is a kind of stainless steel plate material. It is 304 copper bearing material with better deep drawing performance than 304. It is mainly used for deep processing and belongs to nickel saving series.
All stainless steel plates are divided into 2B (cold rolling) and No.1 (hot rolling), which is the difference of plate thickness and surface degree. The thickness of 2B plate is less than 3.0, the thickness of No.1 plate is more than 3.0, and the thickness of 304j1 plate is the same, which can be divided into 304j1 / 2B and 304j1 / No.1. The surface of 2B plate is brighter than No.1 plate, which can be used for wire drawing, mirror processing, but No.1 plate can not, Of course, 2b is definitely more expensive than No.1.
The chemical content of the same material, no matter 2B or No.1, is the same.
Introduction of stainless steel grade: SUS 304
1. Summary
The most basic 18Cr-8Ni stainless steel is representative of 300 series stainless steel. The most widely used steel in stainless steel.
2. Characteristics
1) Excellent processing performance; 2) excellent welding performance
3) It has strong corrosion resistance to oxidizing acid, and also has certain corrosion resistance to alkali liquor, most organic acids and inorganic acids; 4) it can be used in a wide range of temperatures (- 196 C0 ~ 800 C0); 5) it is non-magnetic in solid solution state; 4) it has good corrosion resistance to alkaline liquor, most organic acids and inorganic acids
6) Cold rolled products have good gloss and beautiful appearance. 2. Scope of application
Household products (Class 1 and 2 tableware, kitchen cabinet, cooker, indoor pipeline, water heater, boiler, bathtub), auto parts (windshield wiper, muffler), medical appliances, architectural decoration, chemistry, chemical industry, food and beverage, agriculture, marine machinery, etc.
3, Scope of application
Household products (Class 1 and 2 tableware, kitchen cabinet, cooker, indoor pipeline, water heater, boiler, bathtub), auto parts (windshield wiper, muffler), medical appliances, architectural decoration, chemistry, chemical industry, food and beverage, agriculture, marine machinery, etc.
4, The below table shows the chemical composition of the used for 304 Grade/www.widesteel.com
| STAINLESS STEEL 304 GRADES | UNS | C | Mn | P | S | Si | Cr | Ni | Mo | Ti | Nb | N |
| TP304 | S30400 | 0.08 | 2 | 0.045 | 0.03 | 1 | 18.0-20.0 | 8.0-11.0 |
5, 304 STAINLESS STEEL PIPE PROPERTIES www.widesteel.com
STAINLESS STEEL 304 PIPE MECHANICAL PROPERTIES
| Material | Heat | Temperure | Tensile Strength | Yield Strength | Elongation %, Min |
| Treatment | Min. | Ksi (MPa), Min. | Ksi (MPa), Min. | ||
| º F(º C) | |||||
| TP304 | Solution | 1900 (1040) | 75(515) | 30(205) | 35 |
6, STAINLESS STEEL 304 PIPES PHYSICAL PROPERTIES
| Grade | Density (kg/m3) | Elastic Modulus (GPa) | Mean Coefficient of Thermal Expansion (m/m/0C) | Thermal Conductivity (W/m.K) | Specific Heat 0-1000C (J/kg.K) | Electrical Resistivity (n.m) | |||
| 0-1000C | 0-3150C | 0-5380C | at 1000C | at 5000C | |||||
| 304 | 8000 | 193 | 17.2 | 17.8 | 18.4 | 16.2 | 21.5 | 500 | 720 |
7, EQUIVALENT GRADES FOR 304 STAINLESS STEEL PIPE www.widesteel.com
| Grade | UNS No | Old British | Euronorm | Swedish SS | Japanese JIS | ||
| BS | En | No | Name | ||||
| 304 | S30400 | 304S31 | 58E | 1.4301 | X5CrNi18-10 | 2332 | SUS 304 |
8. Heat treatment www.widesteel.com
Melting point: 1400 ~ 14500c; solution treatment: 1010 ~ 11500c.
Service condition (1) annealing solution condition:
No.1, 2D, 2b, N0.4, HL, Ba, mirror, and various other surface treatment states (2) rolled hard state. 8. Precautions for use
-Magnetic phenomenon will appear after cold working
-Pitting corrosion and stress corrosion cracking are easy to occur in Cl – environment
-Due to the sensitization between 475 and 850 0C, the grain boundary corrosion is easy to occur in the welding affected zone. Grain boundary corrosion should be avoided by rapid cooling or heat treatment after welding.
-Magnetic phenomenon will appear after cold working
-Pitting corrosion and stress corrosion cracking are easy to occur in Cl – environment
-Due to the sensitization between 475 and 850 0C, the grain boundary corrosion is easy to occur in the welding affected zone. Grain boundary corrosion should be avoided by rapid cooling or heat treatment after welding.
Hardness conversion formula and measurement range
Hardness conversion formula
1. Shore hardness (HS) = Bhn / 10 + 12 2. Shore hardness (HS) = Rockwell hardness (HRC) + 15 3. Rockwell hardness (HV) 4. Rockwell hardness (HRC) = Bhn / 10-3
Hardness measurement range: HS < 100, Hb < 500, HRC < 70
Properties and microstructure of stainless steel,
At present, there are more than 100 kinds of chemical elements known, and about 20 kinds of chemical elements 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, the most commonly used elements are more than ten kinds. In addition to the basic element iron, the elements that have the greatest impact on the performance and structure of stainless steel are: carbon, chromium, nickel, manganese, silicon, molybdenum, titanium, niobium, titanium, manganese, nitrogen, copper, cobalt, etc. Except for carbon, silicon and nitrogen, all of these elements belong to the transition group in the periodic table of chemical elements.
In fact, there are several or even more than a dozen elements in stainless steel used in industry at the same time. When several elements coexist in the unity of stainless steel, their influence is much more complex than when they exist alone, because in this case, not only the role of each element itself should be considered, but also the influence of each other should be paid attention to. Therefore, the structure of stainless steel is determined by the temperature The sum of the effects of various elements.
1. The effect of various elements on the properties and structure of stainless steel 1-1. The decisive role of chromium in stainless steel
There is only one element that determines the property of stainless steel, which is chromium. Each stainless steel contains a certain amount of chromium. So far, there is no stainless steel without chromium. The reason why chromium has become the main element to determine the properties of stainless steel is that adding chromium to the steel as an alloying element promotes the internal contradictory movement to develop in favor of corrosion resistance. This change can be explained from the following aspects:
The results show that: (1) chromium increases the electrode potential of iron-based solid solution; (2) chromium absorbs electrons of iron and passivates iron
Passivation is a phenomenon that the corrosion resistance of metals and alloys is improved due to the inhibition of anodic reaction. There are many theories about passivation of metals and alloys, 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. The properties and microstructure of steel are largely determined by the content and distribution of carbon in steel, especially in stainless steel. The effect of carbon on the structure of stainless steel is mainly manifested in two aspects. On the one hand, carbon is the element that stabilizes austenite and plays a great role (about 30 times as much as nickel). On the other hand, due to the great affinity between carbon and chromium, it forms a series of complex carbides with chromium. Therefore, from the strength and corrosion resistance, the role of carbon in stainless steel is contradictory.
Knowing the law of this effect, we can choose stainless steel with different carbon content from different requirements. For example, the standard chromium content of stainless steel 0crl3-4cr13, which is the most widely used and the least used in industry, is 12-14%, which is determined after considering the factors that carbon and chromium form chromium carbide. The purpose is to make the chromium content in solid solution not less than the minimum of 11.7% after carbon and chromium combine to form chromium carbide Chromium content.
For these five steel grades, due to different carbon content, the strength and corrosion resistance are also different. 0Cr13 ~ 2Crl3 steel has better corrosion resistance, but the strength is lower than 3Crl3 and 4Cr13 steel, which are mostly used for manufacturing structural parts. The latter two steel grades can obtain high strength due to high carbon content, and are mostly used for manufacturing spring, cutting tool and other parts requiring high strength and wear resistance. For example, in order to overcome the intergranular corrosion of 18-8 Cr Ni stainless steel, the carbon content of the steel can be reduced to less than 0.03%, or elements (titanium or niobium) with greater affinity than chromium and carbon can be added to prevent the formation of chromium carbide. For another 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 meet the hardness and wear resistance requirements In industry, it is used as bearing, measuring tool and cutting edge. It has stainless steel 9Cr18 and 9cr17movco. Although the carbon content is as high as 0.85-0.95%, their chromium content is also increased correspondingly, so the requirement of corrosion resistance is still guaranteed.
Generally speaking, the carbon content of stainless steel used in industry is relatively low. The carbon content of most stainless steel ranges from 0.1% to 0.4%, while that of acid resistant steel ranges from 0.1% to 0.2%. Stainless steel with more than 0.4% carbon content only accounts for a small part of the total number of steel grades. This is because in most service conditions, the main purpose of stainless steel is corrosion resistance. In addition, the lower carbon content is also due to some technological requirements, such as easy welding and cold deformation.
1-3. The function of nickel in stainless steel is developed after cooperating with chromium
Nickel is an excellent corrosion-resistant material and an important alloying element in alloy steel. Nickel is the element to form austenite in steel, but for low carbon nickel steel to obtain pure austenite structure, the content of nickel must reach 24%, and only when the content of nickel is 27%, the corrosion resistance of steel in some medium will be significantly changed. Therefore, nickel cannot form stainless steel alone. However, when nickel and chromium exist in stainless steel at the same time, stainless steel containing nickel has many valuable properties.
Based on the above situation, 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, so as to improve the corrosion resistance and process performance of stainless steel. 1-4. Mn and N can replace Ni in Cr Ni stainless steel
Although chromium nickel austenitic steel has many advantages, in recent decades, due to the development and application of nickel based heat-resistant alloy and hot strength steel with nickel content less than 20%, as well as the growing development of chemical industry, the demand for stainless steel is increasing, and the nickel deposit is less and concentrated in a few areas, so there is a contradiction between supply and demand of nickel in the world. Therefore, in stainless steel and many other alloy fields (such as heavy casting and forging steel, tool steel, hot strength steel, etc.), especially in the countries lacking nickel resources, the scientific research and production practice of saving nickel and substituting other elements for nickel have been widely carried out. In this aspect, manganese and nitrogen are used to replace nickel in stainless steel and heat-resistant steel.
The effect of manganese on austenite is similar to that of nickel. But to be exact, 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 at room temperature. In the aspect of improving the corrosion resistance of steel, manganese has little effect, such as the change of manganese content from 0% to 10.4%, and the corrosion resistance of steel in air and acid does not change obviously. This is because manganese has little effect on increasing the electrode potential of iron-based solid solution, and the protective effect of the formed oxide film is also very low
But they can not be used as stainless steel. The effect of manganese on stabilizing austenite in steel is about half of that of nickel. That is to say, 2% of nitrogen in steel also stabilizes austenite, and the effect is greater than that of nickel. For example, in order to obtain austenite structure of steel containing 18% chromium at room temperature, low nickel stainless steel with manganese and nitrogen instead of nickel and chromium manganese nitrogen free steel with nickel have been applied in industry, and some 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
The influence of the above nine main elements on the properties and structure of stainless steel, in addition to these elements have a greater impact on the properties and structure of stainless steel, stainless steel also contains some other elements. Some of them are common impurity elements, such as silicon, sulfur, phosphorus and so on, as well as some of them are added for some specific purposes, such as cobalt, boron, selenium, rare earth elements and so on. In terms of the corrosion resistance of stainless steel, these elements are non main compared with the nine elements discussed. However, they can not be completely ignored, because they also affect the properties and microstructure of stainless steel.
Silicon is an element forming ferrite, which is a common impurity element in general stainless steel.
As an alloying element, cobalt is seldom used in steel because of its high price and more important application in other fields (such as high speed steel, cemented carbide, cobalt based heat resistant alloy, magnetic steel or hard magnetic alloy). In general, there are not many stainless steels with cobalt as alloying element,
There is only one element that determines the property of stainless steel, which is chromium. Each stainless steel contains a certain amount of chromium. So far, there is no stainless steel without chromium. The reason why chromium has become the main element to determine the properties of stainless steel is that adding chromium to the steel as an alloying element promotes the internal contradictory movement to develop in favor of corrosion resistance. This change can be explained from the following aspects:
The results show that: (1) chromium increases the electrode potential of iron-based solid solution; (2) chromium absorbs electrons of iron and passivates iron
Passivation is a phenomenon that the corrosion resistance of metals and alloys is improved due to the inhibition of anodic reaction. There are many theories about passivation of metals and alloys, 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. The properties and microstructure of steel are largely determined by the content and distribution of carbon in steel, especially in stainless steel. The effect of carbon on the structure of stainless steel is mainly manifested in two aspects. On the one hand, carbon is the element that stabilizes austenite and plays a great role (about 30 times as much as nickel). On the other hand, due to the great affinity between carbon and chromium, it forms a series of complex carbides with chromium. Therefore, from the strength and corrosion resistance, the role of carbon in stainless steel is contradictory.
Knowing the law of this effect, we can choose stainless steel with different carbon content from different requirements. For example, the standard chromium content of stainless steel 0crl3-4cr13, which is the most widely used and the least used in industry, is 12-14%, which is determined after considering the factors that carbon and chromium form chromium carbide. The purpose is to make the chromium content in solid solution not less than the minimum of 11.7% after carbon and chromium combine to form chromium carbide Chromium content.
For these five steel grades, due to different carbon content, the strength and corrosion resistance are also different. 0Cr13 ~ 2Crl3 steel has better corrosion resistance, but the strength is lower than 3Crl3 and 4Cr13 steel, which are mostly used for manufacturing structural parts. The latter two steel grades can obtain high strength due to high carbon content, and are mostly used for manufacturing spring, cutting tool and other parts requiring high strength and wear resistance. For example, in order to overcome the intergranular corrosion of 18-8 Cr Ni stainless steel, the carbon content of the steel can be reduced to less than 0.03%, or elements (titanium or niobium) with greater affinity than chromium and carbon can be added to prevent the formation of chromium carbide. For another 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 meet the hardness and wear resistance requirements In industry, it is used as bearing, measuring tool and cutting edge. It has stainless steel 9Cr18 and 9cr17movco. Although the carbon content is as high as 0.85-0.95%, their chromium content is also increased correspondingly, so the requirement of corrosion resistance is still guaranteed.
Generally speaking, the carbon content of stainless steel used in industry is relatively low. The carbon content of most stainless steel ranges from 0.1% to 0.4%, while that of acid resistant steel ranges from 0.1% to 0.2%. Stainless steel with more than 0.4% carbon content only accounts for a small part of the total number of steel grades. This is because in most service conditions, the main purpose of stainless steel is corrosion resistance. In addition, the lower carbon content is also due to some technological requirements, such as easy welding and cold deformation.
1-3. The function of nickel in stainless steel is developed after cooperating with chromium
Nickel is an excellent corrosion-resistant material and an important alloying element in alloy steel. Nickel is the element to form austenite in steel, but for low carbon nickel steel to obtain pure austenite structure, the content of nickel must reach 24%, and only when the content of nickel is 27%, the corrosion resistance of steel in some medium will be significantly changed. Therefore, nickel cannot form stainless steel alone. However, when nickel and chromium exist in stainless steel at the same time, stainless steel containing nickel has many valuable properties.
Based on the above situation, 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, so as to improve the corrosion resistance and process performance of stainless steel. 1-4. Mn and N can replace Ni in Cr Ni stainless steel
Although chromium nickel austenitic steel has many advantages, in recent decades, due to the development and application of nickel based heat-resistant alloy and hot strength steel with nickel content less than 20%, as well as the growing development of chemical industry, the demand for stainless steel is increasing, and the nickel deposit is less and concentrated in a few areas, so there is a contradiction between supply and demand of nickel in the world. Therefore, in stainless steel and many other alloy fields (such as heavy casting and forging steel, tool steel, hot strength steel, etc.), especially in the countries lacking nickel resources, the scientific research and production practice of saving nickel and substituting other elements for nickel have been widely carried out. In this aspect, manganese and nitrogen are used to replace nickel in stainless steel and heat-resistant steel.
The effect of manganese on austenite is similar to that of nickel. But to be exact, 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 at room temperature. In the aspect of improving the corrosion resistance of steel, manganese has little effect, such as the change of manganese content from 0% to 10.4%, and the corrosion resistance of steel in air and acid does not change obviously. This is because manganese has little effect on increasing the electrode potential of iron-based solid solution, and the protective effect of the formed oxide film is also very low
But they can not be used as stainless steel. The effect of manganese on stabilizing austenite in steel is about half of that of nickel. That is to say, 2% of nitrogen in steel also stabilizes austenite, and the effect is greater than that of nickel. For example, in order to obtain austenite structure of steel containing 18% chromium at room temperature, low nickel stainless steel with manganese and nitrogen instead of nickel and chromium manganese nitrogen free steel with nickel have been applied in industry, and some 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
The influence of the above nine main elements on the properties and structure of stainless steel, in addition to these elements have a greater impact on the properties and structure of stainless steel, stainless steel also contains some other elements. Some of them are common impurity elements, such as silicon, sulfur, phosphorus and so on, as well as some of them are added for some specific purposes, such as cobalt, boron, selenium, rare earth elements and so on. In terms of the corrosion resistance of stainless steel, these elements are non main compared with the nine elements discussed. However, they can not be completely ignored, because they also affect the properties and microstructure of stainless steel.
Silicon is an element forming ferrite, which is a common impurity element in general stainless steel.
As an alloying element, cobalt is seldom used in steel because of its high price and more important application in other fields (such as high speed steel, cemented carbide, cobalt based heat resistant alloy, magnetic steel or hard magnetic alloy). In general, there are not many stainless steels with cobalt as alloying element,
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
The influence of the above nine main elements on the properties and structure of stainless steel, in addition to these elements have a greater impact on the properties and structure of stainless steel, stainless steel also contains some other elements. Some of them are common impurity elements, such as silicon, sulfur, phosphorus and so on, as well as some of them are added for some specific purposes, such as cobalt, boron, selenium, rare earth elements and so on. In terms of the corrosion resistance of stainless steel, these elements are non main compared with the nine elements discussed. However, they can not be completely ignored, because they also affect the properties and microstructure of stainless steel.
Silicon is an element forming ferrite, which is a common impurity element in general stainless steel.
As an alloying element, cobalt is seldom used in steel because of its high price and more important application in other fields (such as high speed steel, cemented carbide, cobalt based heat resistant alloy, magnetic steel or hard magnetic alloy). In general, there are not many stainless steels with cobalt as alloying element,
Common stainless steel such as 9crl7movco steel (containing 1.2-1.8% cobalt) with cobalt is not to improve the corrosion resistance, but to improve the hardness, because the main purpose of this stainless steel is to manufacture slicing machine tools, scissors and surgical blades.
Adding 0.005% boron to crl7mo2ti steel can improve the corrosion resistance in boiling 65% acetic acid. The addition of trace boron (0.0006-0.0007%) can improve the hot ductility of austenitic stainless steel. A small amount of boron forms eutectic with low melting point, which increases the tendency of producing hot cracks in austenitic steel during welding. However, when there is a large amount of boron (0.5-0.6%), the hot cracks can be prevented. Because when 0.5-0.6% boron is contained, 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 that of the semi melting zone, the tensile stress produced by the base metal during cooling is borne by the weld metal in the liquid and solid state, which will not cause cracks. Even if cracks are formed near the weld zone, they can also be filled by the molten pool metal in the liquid and solid state. Chromium nickel austenitic stainless steel containing boron has a special application in atomic energy industry.
Phosphorus is an impurity element in general stainless steel, but its harmfulness in austenitic stainless steel is not as obvious as that in general steel. Therefore, the content of phosphorus in austenitic stainless steel can be allowed to be higher, which can reach 0.06% according to some data, so as to facilitate smelting control. The phosphorus content of some austenitic steels containing manganese can reach 0.06% (such as 2crl3nim9 steel) to 0.08% (such as cr14mnl4ni steel). The strengthening effect of phosphorus on steel can also be used as alloying element of age hardening stainless steel. Ph17-10p steel (containing 0.25% phosphorus) is ph-hnm steel (containing 0.30 phosphorus).
Sulfur and selenium are often impurity elements 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 machinability of stainless steel because they reduce the toughness of stainless steel. For example, the impact value of 18-8 Cr Ni stainless steel can reach 30 kg / cm2. The impact value of 18-8 steel containing 0.31% sulfur (0.084% C, 18.15% Cr, 9.25% Ni) is 1.8 kg / cm2, and the impact value of 18-8 steel containing 0.31% sulfur (0.084% C, 18.15% Cr, 9.25% Ni) is 0. The impact value of 18-8 steel with 22% se (0.094% C, 18.4% Cr, 9% Ni) is 3.24 kg / cm2. Sulfur and selenium reduce the corrosion resistance of stainless steel, so they are rarely used as alloying elements in stainless steel.
At present, the application of rare earth elements in stainless steel mainly lies in improving process performance. If a small amount of rare earth elements are added to crl7ti and cr17mo2ti steel, the bubbles caused by hydrogen in ingot can be eliminated and the cracks in billet can be reduced. The addition of 0.02-0.5% rare earth elements (CE La alloy) to austenitic and austenitic ferritic stainless steels can significantly improve the forging properties. There used to be an austenitic steel containing 19.5% chromium, 23% nickel and molybdenum, copper and manganese. Due to its hot working properties, only castings could be produced in the past. After adding rare earth elements, various profiles could be rolled.
2. Classification of stainless steel according to metallographic structure and general characteristics of all kinds of stainless steel
According to the chemical composition (mainly chromium content) and application, stainless steel is divided into two categories: stainless steel and acid resistant. In industry, stainless steels are classified according to the type of matrix structure of the steel after heating and air cooling at high temperature (900-1100 ℃), which is determined by the characteristics of the influence of carbon and alloy elements on the structure of stainless steels discussed above.
According to the metallographic structure, stainless steels used in industry can be divided into three categories: ferritic stainless steels, martensitic stainless steels and austenitic stainless steels. The characteristics of these three types of stainless steels can be summarized (as shown in the table below), but it should be noted that not all martensitic stainless steels can be welded, but they are limited by some conditions, such as preheating before welding and tempering after welding, which makes the welding process more complicated. In actual production, some martensitic stainless steels such as 1Cr13, 2Cr13 and 2Cr13 are welded with 45 steel more often.
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
The influence of the above nine main elements on the properties and structure of stainless steel, in addition to these elements have a greater impact on the properties and structure of stainless steel, stainless steel also contains some other elements. Some of them are common impurity elements, such as silicon, sulfur, phosphorus and so on, as well as some of them are added for some specific purposes, such as cobalt, boron, selenium, rare earth elements and so on. In terms of the corrosion resistance of stainless steel, these elements are non main compared with the nine elements discussed. However, they can not be completely ignored, because they also affect the properties and microstructure of stainless steel.
Silicon is an element forming ferrite, which is a common impurity element in general stainless steel.
As an alloying element, cobalt is seldom used in steel because of its high price and more important application in other fields (such as high speed steel, cemented carbide, cobalt based heat resistant alloy, magnetic steel or hard magnetic alloy). In general, there are not many stainless steels with cobalt as alloying element,
Common stainless steel such as 9crl7movco steel (containing 1.2-1.8% cobalt) with cobalt is not to improve the corrosion resistance, but to improve the hardness, because the main purpose of this stainless steel is to manufacture slicing machine tools, scissors and surgical blades.
Adding 0.005% boron to crl7mo2ti steel can improve the corrosion resistance in boiling 65% acetic acid. The addition of trace boron (0.0006-0.0007%) can improve the hot ductility of austenitic stainless steel. A small amount of boron forms eutectic with low melting point, which increases the tendency of producing hot cracks in austenitic steel during welding. However, when there is a large amount of boron (0.5-0.6%), the hot cracks can be prevented. Because when 0.5-0.6% boron is contained, 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 that of the semi melting zone, the tensile stress produced by the base metal during cooling is borne by the weld metal in the liquid and solid state, which will not cause cracks. Even if cracks are formed near the weld zone, they can also be filled by the molten pool metal in the liquid and solid state. Chromium nickel austenitic stainless steel containing boron has a special application in atomic energy industry.
Phosphorus is an impurity element in general stainless steel, but its harmfulness in austenitic stainless steel is not as obvious as that in general steel. Therefore, the content of phosphorus in austenitic stainless steel can be allowed to be higher, which can reach 0.06% according to some data, so as to facilitate smelting control. The phosphorus content of some austenitic steels containing manganese can reach 0.06% (such as 2crl3nim9 steel) to 0.08% (such as cr14mnl4ni steel). The strengthening effect of phosphorus on steel can also be used as alloying element of age hardening stainless steel. Ph17-10p steel (containing 0.25% phosphorus) is ph-hnm steel (containing 0.30 phosphorus).
Sulfur and selenium are often impurity elements 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 machinability of stainless steel because they reduce the toughness of stainless steel. For example, the impact value of 18-8 Cr Ni stainless steel can reach 30 kg / cm2. The impact value of 18-8 steel containing 0.31% sulfur (0.084% C, 18.15% Cr, 9.25% Ni) is 1.8 kg / cm2, and the impact value of 18-8 steel containing 0.31% sulfur (0.084% C, 18.15% Cr, 9.25% Ni) is 0. The impact value of 18-8 steel with 22% se (0.094% C, 18.4% Cr, 9% Ni) is 3.24 kg / cm2. Sulfur and selenium reduce the corrosion resistance of stainless steel, so they are rarely used as alloying elements in stainless steel.
At present, the application of rare earth elements in stainless steel mainly lies in improving process performance. If a small amount of rare earth elements are added to crl7ti and cr17mo2ti steel, the bubbles caused by hydrogen in ingot can be eliminated and the cracks in billet can be reduced. The addition of 0.02-0.5% rare earth elements (CE La alloy) to austenitic and austenitic ferritic stainless steels can significantly improve the forging properties. There used to be an austenitic steel containing 19.5% chromium, 23% nickel and molybdenum, copper and manganese. Due to its hot working properties, only castings could be produced in the past. After adding rare earth elements, various profiles could be rolled.
2. Classification of stainless steel according to metallographic structure and general characteristics of all kinds of stainless steel
According to the chemical composition (mainly chromium content) and application, stainless steel is divided into two categories: stainless steel and acid resistant. In industry, stainless steels are classified according to the type of matrix structure of the steel after heating and air cooling at high temperature (900-1100 ℃), which is determined by the characteristics of the influence of carbon and alloy elements on the structure of stainless steels discussed above.
According to the metallographic structure, stainless steels used in industry can be divided into three categories: ferritic stainless steels, martensitic stainless steels and austenitic stainless steels. The characteristics of these three types of stainless steels can be summarized (as shown in the table below), but it should be noted that not all martensitic stainless steels can be welded, but they are limited by some conditions, such as preheating before welding and tempering after welding, which makes the welding process more complicated. In actual production, some martensitic stainless steels such as 1Cr13, 2Cr13 and 2Cr13 are welded with 45 steel more often.
According to the matrix structure of steel, due to the fact that the elements that stabilize austenite and form ferrite in the steel can not balance each other, and the large amount of chromium makes the s point in the balance diagram move to the left, the structure of stainless steel used in industry, in addition to the three basic types mentioned above, also has martensite ferrite, austenite ferrite, austenite martensite and other transitional duplex stainless steel And stainless steel with martensite carbide structure
2-1. Ferritic steel
For low carbon chromium stainless steel containing more than 14% chromium, any carbon content of chromium stainless steel containing more than 27% chromium, and stainless steel added with molybdenum, titanium, niobium, silicon, aluminum, tungsten, vanadium and other elements on the basis of the above composition, the element forming ferrite is absolutely dominant in chemical composition, and the matrix structure is ferrite. The microstructure of this kind of steel is ferrite in quenched (solution) state, but a small amount of carbides and intermetallic compounds can be seen in annealed and aged state.
Crl7, cr17mo2ti, Cr25, cr25mo3ti, Cr28 and so on belong to this category. Ferritic stainless steel has good corrosion resistance and oxidation resistance due to its high chromium content, but its mechanical and technological properties are poor. It is mostly used in acid resistant structure with little stress and as oxidation resistant steel.
Ferritic martensitic steel
This kind of steel is y + a (or δ) two-phase state at high temperature, y-m transformation occurs at rapid cooling, ferrite is still retained, the normal temperature structure is martensite and ferrite, due to the different composition and heating temperature, the amount of ferrite in the structure can change in the range of several percent to dozens. 0crl3 steel, lCrl3 steel, 2Cr13 steel, Cr17Ni2 steel, cr17wn4 steel with upper chromium limit and lower carbon limit, and many steel grades of 12% chromium heat-resistant steel (also called heat-resistant stainless steel) developed on the basis of icrl3 steel, such as cr11mov, cr12wmov, crl2w4mov, 18crl2wmovnb, all belong to dry type.
Ferritic martensitic steel can be partially quenched, so it can obtain higher mechanical properties. However, their mechanical and technological properties are largely affected by the content and distribution of ferrite. According to the chromium content in the composition, this kind of steel belongs to two series: 12-14% and 15-18%. The former has the ability to resist the atmosphere and weak corrosive medium, and has good shock absorption and small coefficient of linear expansion; the latter has the same corrosion resistance as the ferritic acid resistant steel with the same chromium content, but also retains some shortcomings of high chromium ferritic steel to a certain extent. 2-3. Martensitic steel
These steels are in the y-phase region at normal quenching temperature, but their y-phase is stable only at high temperature, and the m-point is generally about 3oo ℃, so they transform into martensite when they are cooled.
These steels include 2Cr13, 2Cr13Ni2, 3Cr13 and some modified 12% Cr hot strength steels, such as 13cr14niwvba,
Cr11ni2mowvb steel, etc. 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, the mechanical properties of the steel are higher than those of the steel mentioned above, but the overheat sensitivity is lower during heat treatment. 2-4. Martensite carbide steel
The carbon content of the eutectoid point of Fe-C alloy is 0.83%. In stainless steel, the s point moves to the left due to chromium. The steel containing 12% chromium and more than 0.4% carbon (Fig. 11-3), and the steel containing 18% chromium and more than 0.3% carbon (Fig. 3) all belong to hypereutectoid steel. When this kind of steel is heated at normal quenching temperature, the secondary carbide can not be completely dissolved in austenite, so the structure after quenching is composed of martensite and carbide.
There are not many stainless steels in this category, but some stainless steels with high carbon content, such as 4Crl3, 9Cr18, 9cr18mov, 9cr17movco and so on. 3Crl3 steel with higher carbon content may also have such structure when quenched at lower temperature. Due to the high carbon content, the above three grades of steel, such as 9Cr18, contain more chromium, but their corrosion resistance is only equivalent to that of stainless steel containing 12-14% germanium. This kind of steel is mainly used for parts requiring high hardness and wear resistance, such as cutting tools, bearings, springs and medical devices. 2-5. Austenitic steel
This kind of steel contains more elements of expanding y-zone and stabilizing austenite. It is y-phase at high temperature and has austenite structure at room temperature because MS point is below room temperature. Chromium nickel stainless steels, such as 18-8, 18-12, 25-20, 20-25mo, and low nickel stainless steels, such as cr18mnl0ni5, cr13ni4mn9, cr17ni4mn9n, cr14ni3mnl4ti, are all in this category.
Austenitic stainless steel has many advantages mentioned above. Although the mechanical properties of austenitic stainless steel and ferritic stainless steel are relatively low and can not be strengthened by heat treatment, their strength can be improved by cold work deformation and work hardening. The disadvantage of this kind of steel is that it is sensitive to intergranular corrosion and stress corrosion. 2-6. AUSTENITIC FERRITIC STEEL
Because of enlarging the y-zone and stabilizing the austenite element, this kind of steel is not enough to have pure austenite structure at room temperature or very high temperature, so it is in austenite ferrite multiphase state, and its ferrite content can vary in a large range due to different composition and heating temperature.
There are many stainless steels in this category, such as low carbon 18-8 chromium nickel steel, 18-8 chromium nickel steel with titanium, niobium and molybdenum, especially ferrite can be seen in the structure of cast steel. In addition, chromium manganese stainless steel (such as cr17mnll) containing more than 14-15% chromium and less than 0.2% carbon, and most of chromium manganese nitrogen stainless steels researched and applied at present are also included. Compared with pure austenitic stainless steel, this kind of steel has many advantages, such as high yield strength, high resistance to intergranular corrosion, low sensitivity to stress corrosion, small tendency to produce hot cracks during welding, good casting fluidity and so on. The disadvantages are that the pressure processing performance is poor, the tendency of pitting corrosion is large, c-phase brittleness is easy to occur, and weak magnetism is shown under the action of strong magnetic field. All these advantages and disadvantages come from the ferrite in the structure. 2-7. Austenite martensite steel
The MS point of this kind of steel is lower than room temperature, and it is austenite structure after solution treatment, so it is easy to form and weld. In general, two processes can be used to induce martensitic transformation. One is that after solution treatment, austenite transforms into metastable state due to precipitation of chromium carbide, Ms point rises above room temperature and transforms into martensite when cooled; the other is that austenite transforms into martensite when cooled directly between MS and MF points after solution treatment. The latter method can obtain higher corrosion resistance, but the interval between solution treatment and cryogenic treatment should not be too long, otherwise the cryogenic strengthening effect will be reduced due to the aging and stabilization of austenite. After the above treatment, the steel is aged at 400-500 degrees to further strengthen the precipitated intermetallic compounds. The typical grades of these steels are 17cr-7ni-a1, 15cr-9ni-a1, 17cr-5ni-mo, 15cr-8ni-mo-a1, etc. This kind of steel is also called austenite maraging stainless steel. In fact, in addition to austenite and martensite, there are different amounts of ferrite in these steels, so 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 heat strength. However, due to the low chromium content and the precipitation of chromium carbide during heat treatment, its corrosion resistance is lower than that of standard Austenitic stainless steel. It can also be said that the high strength of this kind of steel is obtained at the expense of some corrosion resistance and other properties (such as non-magnetic). At present, this kind of steel is mainly used in aviation industry and rocket missile production, but it is not widely used in general machinery manufacturing, and there is also a series of ultra-high strength steel in classification.
For low carbon chromium stainless steel containing more than 14% chromium, any carbon content of chromium stainless steel containing more than 27% chromium, and stainless steel added with molybdenum, titanium, niobium, silicon, aluminum, tungsten, vanadium and other elements on the basis of the above composition, the element forming ferrite is absolutely dominant in chemical composition, and the matrix structure is ferrite. The microstructure of this kind of steel is ferrite in quenched (solution) state, but a small amount of carbides and intermetallic compounds can be seen in annealed and aged state.
Crl7, cr17mo2ti, Cr25, cr25mo3ti, Cr28 and so on belong to this category. Ferritic stainless steel has good corrosion resistance and oxidation resistance due to its high chromium content, but its mechanical and technological properties are poor. It is mostly used in acid resistant structure with little stress and as oxidation resistant steel.
Ferritic martensitic steel
This kind of steel is y + a (or δ) two-phase state at high temperature, y-m transformation occurs at rapid cooling, ferrite is still retained, the normal temperature structure is martensite and ferrite, due to the different composition and heating temperature, the amount of ferrite in the structure can change in the range of several percent to dozens. 0crl3 steel, lCrl3 steel, 2Cr13 steel, Cr17Ni2 steel, cr17wn4 steel with upper chromium limit and lower carbon limit, and many steel grades of 12% chromium heat-resistant steel (also called heat-resistant stainless steel) developed on the basis of icrl3 steel, such as cr11mov, cr12wmov, crl2w4mov, 18crl2wmovnb, all belong to dry type.
Ferritic martensitic steel can be partially quenched, so it can obtain higher mechanical properties. However, their mechanical and technological properties are largely affected by the content and distribution of ferrite. According to the chromium content in the composition, this kind of steel belongs to two series: 12-14% and 15-18%. The former has the ability to resist the atmosphere and weak corrosive medium, and has good shock absorption and small coefficient of linear expansion; the latter has the same corrosion resistance as the ferritic acid resistant steel with the same chromium content, but also retains some shortcomings of high chromium ferritic steel to a certain extent. 2-3. Martensitic steel
These steels are in the y-phase region at normal quenching temperature, but their y-phase is stable only at high temperature, and the m-point is generally about 3oo ℃, so they transform into martensite when they are cooled.
These steels include 2Cr13, 2Cr13Ni2, 3Cr13 and some modified 12% Cr hot strength steels, such as 13cr14niwvba,
Cr11ni2mowvb steel, etc. 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, the mechanical properties of the steel are higher than those of the steel mentioned above, but the overheat sensitivity is lower during heat treatment. 2-4. Martensite carbide steel
The carbon content of the eutectoid point of Fe-C alloy is 0.83%. In stainless steel, the s point moves to the left due to chromium. The steel containing 12% chromium and more than 0.4% carbon (Fig. 11-3), and the steel containing 18% chromium and more than 0.3% carbon (Fig. 3) all belong to hypereutectoid steel. When this kind of steel is heated at normal quenching temperature, the secondary carbide can not be completely dissolved in austenite, so the structure after quenching is composed of martensite and carbide.
There are not many stainless steels in this category, but some stainless steels with high carbon content, such as 4Crl3, 9Cr18, 9cr18mov, 9cr17movco and so on. 3Crl3 steel with higher carbon content may also have such structure when quenched at lower temperature. Due to the high carbon content, the above three grades of steel, such as 9Cr18, contain more chromium, but their corrosion resistance is only equivalent to that of stainless steel containing 12-14% germanium. This kind of steel is mainly used for parts requiring high hardness and wear resistance, such as cutting tools, bearings, springs and medical devices. 2-5. Austenitic steel
This kind of steel contains more elements of expanding y-zone and stabilizing austenite. It is y-phase at high temperature and has austenite structure at room temperature because MS point is below room temperature. Chromium nickel stainless steels, such as 18-8, 18-12, 25-20, 20-25mo, and low nickel stainless steels, such as cr18mnl0ni5, cr13ni4mn9, cr17ni4mn9n, cr14ni3mnl4ti, are all in this category.
Austenitic stainless steel has many advantages mentioned above. Although the mechanical properties of austenitic stainless steel and ferritic stainless steel are relatively low and can not be strengthened by heat treatment, their strength can be improved by cold work deformation and work hardening. The disadvantage of this kind of steel is that it is sensitive to intergranular corrosion and stress corrosion. 2-6. AUSTENITIC FERRITIC STEEL
Because of enlarging the y-zone and stabilizing the austenite element, this kind of steel is not enough to have pure austenite structure at room temperature or very high temperature, so it is in austenite ferrite multiphase state, and its ferrite content can vary in a large range due to different composition and heating temperature.
There are many stainless steels in this category, such as low carbon 18-8 chromium nickel steel, 18-8 chromium nickel steel with titanium, niobium and molybdenum, especially ferrite can be seen in the structure of cast steel. In addition, chromium manganese stainless steel (such as cr17mnll) containing more than 14-15% chromium and less than 0.2% carbon, and most of chromium manganese nitrogen stainless steels researched and applied at present are also included. Compared with pure austenitic stainless steel, this kind of steel has many advantages, such as high yield strength, high resistance to intergranular corrosion, low sensitivity to stress corrosion, small tendency to produce hot cracks during welding, good casting fluidity and so on. The disadvantages are that the pressure processing performance is poor, the tendency of pitting corrosion is large, c-phase brittleness is easy to occur, and weak magnetism is shown under the action of strong magnetic field. All these advantages and disadvantages come from the ferrite in the structure. 2-7. Austenite martensite steel
The MS point of this kind of steel is lower than room temperature, and it is austenite structure after solution treatment, so it is easy to form and weld. In general, two processes can be used to induce martensitic transformation. One is that after solution treatment, austenite transforms into metastable state due to precipitation of chromium carbide, Ms point rises above room temperature and transforms into martensite when cooled; the other is that austenite transforms into martensite when cooled directly between MS and MF points after solution treatment. The latter method can obtain higher corrosion resistance, but the interval between solution treatment and cryogenic treatment should not be too long, otherwise the cryogenic strengthening effect will be reduced due to the aging and stabilization of austenite. After the above treatment, the steel is aged at 400-500 degrees to further strengthen the precipitated intermetallic compounds. The typical grades of these steels are 17cr-7ni-a1, 15cr-9ni-a1, 17cr-5ni-mo, 15cr-8ni-mo-a1, etc. This kind of steel is also called austenite maraging stainless steel. In fact, in addition to austenite and martensite, there are different amounts of ferrite in these steels, so 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 heat strength. However, due to the low chromium content and the precipitation of chromium carbide during heat treatment, its corrosion resistance is lower than that of standard Austenitic stainless steel. It can also be said that the high strength of this kind of steel is obtained at the expense of some corrosion resistance and other properties (such as non-magnetic). At present, this kind of steel is mainly used in aviation industry and rocket missile production, but it is not widely used in general machinery manufacturing, and there is also a series of ultra-high strength steel in classification.