Comprehensive analysis of stainless steel materials: history, types, properties and applications
Table of Contents
1.The history of stainless steel
The birth of stainless steel can be traced back to 1912, when the German Maura first proposed its concept. Subsequently, the British Henry Brierley further discovered and began to apply this material. After more than a century of continuous development and evolution, more than 100 types of stainless steel have been successfully developed for industrial use. Stainless steel, the name refers generally to those steel grades that are resistant to weak corrosive media such as air, steam, water, etc., while those steel grades that are resistant to erosion by chemically corrosive media (e.g., acids, alkalis, salts, etc.) are known as acid-resistant steels. These two types of steel in the chemical composition of the difference determines their corrosion resistance are different. Ordinary stainless steels are usually unable to resist the corrosion of chemical media, while acid-resistant stainless steels usually show excellent stainlessness. The development of stainless steels for different applications is centered around their specific properties, which are commonly characterized by the presence of 17 to 22% chromium. In addition, other elements, such as Ni, Mo, Nb, Ti, etc., are added according to specific needs to fully utilize the rust and corrosion resistance properties of stainless steel.
2.Main types of stainless steel
Stainless steel can be classified according to its state of organization, mainly including ferritic steel, austenitic steel, austenitic-ferritic duplex steel, martensitic steel and precipitation hardening stainless steel. At the same time, according to its composition characteristics, stainless steel can be divided into chromium stainless steel, chromium-nickel stainless steel and chromium-manganese-nitrogen stainless steel and other categories. In addition, there are specialized stainless steel designed for pressure vessels and pressurized equipment, such as stainless steel plate and strip.
2.1 Ferritic stainless steel
The chromium content of this type of stainless steel is between 15% and 30%, and its corrosion resistance, toughness and weldability will be improved with the increase of chromium content. Particularly noteworthy is its resistance to chloride stress corrosion is better than other types of stainless steel. For example, CrlCr17Mo2Ti, Cr25, Cr25Mo3Ti and Cr28 belong to this category. Due to the high chromium content of ferritic stainless steel, it not only has good corrosion and oxidation resistance, but also can resist the erosion of the atmosphere, nitric acid and brine solution. In addition, this type of steel also has high temperature oxidation resistance, small coefficient of thermal expansion and other characteristics, is very suitable for nitric acid and food factory equipment manufacturing, but also can be used to make high temperature environment working parts, such as gas turbine parts.
2.2 Austenitic stainless steel
This type of stainless steel contains more than 18% chromium, and also incorporates about 8% nickel and a small amount of molybdenum, titanium, nitrogen and other elements. This makes the austenitic stainless steel in the comprehensive performance is excellent, can resist a variety of media corrosion. Commonly used austenitic stainless steel grades include 1Cr18Ni9 and 0Cr19Ni9, etc., of which 0Cr19Ni9 steel has a carbon content of less than 0.08%, and is therefore labeled as “0” in the steel grade. This type of steel is rich in Cr and Ni elements, austenitic at room temperature, showing superior plasticity, toughness, weldability and corrosion resistance. In addition, it is non-magnetic or weakly magnetic and exhibits good corrosion resistance in both oxidizing and reducing media. Therefore, austenitic stainless steel is very suitable for the production of acid-resistant equipment, such as corrosion-resistant containers, equipment lining, piping and nitric acid-resistant equipment parts. At the same time, it can also be used as a stainless steel watch jewelry and other body of the production materials. Austenitic stainless steel is usually solid solution treated, that is, the steel is heated to 1050 ~ 1150 ℃, and then water-cooled or air-cooled, in order to obtain a single-phase austenitic organization.
2.3 Austenitic-ferritic duplex stainless steels
This type of stainless steel skillfully combines the advantages of austenitic and ferritic stainless steel, and shows superplasticity. Its organization, austenite and ferrite each accounted for half, in the case of low carbon content, chromium content is controlled at 18% ~ 28%, while adding 3% ~ 10% of nickel. In addition, may also contain molybdenum, copper, silicon, niobium, titanium and nitrogen and other alloying elements. This duplex stainless steel not only combines the properties of austenitic and ferritic stainless steels, but also overcomes the room temperature brittleness of ferritic stainless steels, and significantly improves intergranular corrosion resistance and weldability. At the same time, it also retains the 475 ℃ brittleness of ferritic stainless steel as well as excellent thermal conductivity and superplasticity. Compared with austenitic stainless steels, it is stronger, and significantly more resistant to intergranular corrosion and chloride stress corrosion. Duplex stainless steel also has excellent resistance to pore corrosion, is a nickel-saving stainless steel.
2.4 Martensitic stainless steel
This type of stainless steel is characterized by high strength, but plasticity, weldability is relatively poor. Commonly used martensitic stainless steel grades include 1Cr13 and 3Cr13 and so on. Due to the higher carbon content, they have excellent strength, hardness and wear resistance, but slightly less corrosion resistance. Martensitic stainless steels are mainly used for parts requiring high mechanical properties and general corrosion resistance, such as springs, turbine blades and hydraulic press valves. This type of steel after quenching and tempering treatment is better, forging and stamping need to be annealed.
2.5 precipitation hardening stainless steel
This type of stainless steel to austenite or martensite as the matrix, through the precipitation hardening treatment to enhance its hardness and strength. Commonly used grades such as 04Cr13Ni8Mo2Al. This treatment can significantly improve the hardness and strength of stainless steel, while maintaining its good plasticity and toughness.
2.6 Other specialized stainless steels, such as steel plates and strips for pressurized equipment, etc.
These stainless steels are designed for specific applications, such as steel plates for pressure vessels. They must meet a series of strict requirements, including classification, size, shape, technical requirements, test methods. Commonly used grades such as 06Cr19Ni10 and 022Cr17Ni12Mo2, etc. These steel grades are mainly used in the manufacture of sanitary equipment such as food machinery and pharmaceutical machinery.
Stainless steel products in the production process, polishing process is indispensable, in addition to a few such as water heaters, water dispensers liner and other special products, most of them need to go through this treatment to enhance the surface finish. However, the performance of polishing is affected by a variety of factors, including raw material surface defects, the hardness of the material and the amount of deep tensile deformation. If the surface of the raw material has scratches, pockmarks or over pickling, etc., it will directly affect its polishing effect. At the same time, the hardness of the raw material is also a key factor, the hardness is too low will lead to polishing is difficult to achieve high brightness, and in the deep drawing process is prone to produce orange peel phenomenon, thus impairing the BQ (i.e., good surface quality and processing performance). After deep drawing products, the deformation of the area of the surface of the great amount of small black spots and Ridging (line bumps) phenomenon may also occur, which will also damage the BQ. In addition, the heat resistance of stainless steel is also critical, which determines the ability of the material to maintain its excellent physical and mechanical properties at high temperatures.
3. Composition of stainless steel
The corrosion resistance of stainless steel will gradually weaken with the increase of carbon content. Therefore, in order to ensure the corrosion resistance of stainless steel, its carbon content is usually controlled at a low level, generally not more than 2%, and even some stainless steel carbon content (ωc) is as low as 0.03% or less, for example, 00Cr12. Chromium (Cr) is the key alloying element in stainless steel, and only when the chromium content reaches a certain level, the steel will have corrosion resistance. Typically, stainless steel has a chromium content of at least 5%. In addition, stainless steel may contain other elements such as nickel (Ni), titanium (Ti), manganese (Mn), nitrogen (N), niobium (Nb), molybdenum (Mo), silicon (Si) and copper (Cu).
4.Applicable materials of stainless steel
Stainless steel, as an important metal material, has a wide range of applications in many fields. Its excellent corrosion resistance and mechanical properties make it ideal for making a variety of durable products. From kitchen utensils, medical devices to chemical equipment, stainless steel plays an indispensable role. Meanwhile, as technology continues to advance, stainless steel materials are constantly being innovated and optimized to meet the demands of increasingly demanding applications.
The processing and raw material quality requirements for each product vary depending on the application. Usually, the raw material thickness tolerance of stainless steel products varies according to their types. For example, the thickness tolerance of second class cutlery and thermos cups is higher, usually between ±3~5%; while the thickness tolerance of first class cutlery is usually within -5%. The thickness tolerance of steel tube products is more stringent, reaching -10%. As for the hotel with a cooler with the material, the thickness tolerance needs to be controlled within -8%. In addition, the thickness tolerance requirements of the distributor also vary from market to market, usually between -4% to 6%. It is worth noting that export products customers often have higher requirements for thickness tolerance, while domestic sales enterprises are relatively low, some customers even accept -15% thickness tolerance.
In addition, stainless steel material is also divided into two categories: DDQ (deep drawing material) and general material. DDQ material, i.e. deep drawing material, is known for its high elongation and low hardness characteristics, and is suitable for products that require a high degree of deep-drawing performance. Its internal grain size ranges from 0 to 0, which ensures excellent deep-drawing performance. However, if the elongation of the material is not up to standard, cracks or pull-throughs may occur in deep-drawn products, affecting the yield of the finished product.
In contrast, general materials are mainly used for products that can be molded without deep-drawing, such as spoons, ladles, and forks for tableware, as well as electrical appliances and steel pipes. It is characterized by relatively low elongation, but slightly higher hardness, and internal grain size grades between 0 and 0. Although its deep-drawn performance is slightly inferior to that of DDQ material, it is affordable and has relatively good BQ, so it has its unique market in practical applications.
In addition, stainless steel sheet as a common material, despite the price of pro-people, but the customer’s requirements for its surface quality is extremely strict. In the production process, a variety of surface defects such as scratches, pitting, sand holes, etc. will inevitably appear, these defects not only affect the appearance of the product quality, but also may affect its use performance. Therefore, the degree and frequency of various surface defects need to be strictly controlled during the production process to ensure that the products can meet the requirements of customers.
5. Physical properties of stainless steel
Ferritic and martensitic stainless steel has the following significant features compared with carbon steel:
5.1.1 Density: the density of carbon steel is slightly higher than ferritic and martensitic stainless steel, but lower than austenitic stainless steel.
5.1.2 Resistivity: the resistivity increases in the order of carbon steel, ferritic type, martensitic type, while the austenitic type stainless steel is located at the end.
5.1.3 Coefficient of Linear Expansion: The order of magnitude of this coefficient is similar to resistivity, with austenitic stainless steels having the highest coefficient of linear expansion and carbon steels the lowest.
5.1.4 Magnetic properties: Carbon, ferritic and martensitic stainless steels are magnetic, while austenitic stainless steels are non-magnetic. It is worth noting that the austenitic stainless steel in the cold work hardening to generate martensitic phase transition will produce magnetic, but through heat treatment methods can eliminate this martensitic organization, thus restoring its non-magnetic.
Austenitic type stainless steel is unique compared to carbon steel:
5.2.1 High resistivity: the resistivity of austenitic stainless steel is about five times that of carbon steel.
5.2.2 Large Coefficient of Linear Expansion: Its coefficient of linear expansion is 40% higher than that of carbon steel, and increases accordingly with increasing temperature.
5.2.3 Low thermal conductivity: the thermal conductivity of austenitic stainless steel is about 1/3 of carbon steel.
6. Main products of stainless steel
Stainless steel can be divided into Cr-Mn-Ni (200 series), Cr-Ni system (300 series), Cr system (400 series), heat-resistant chromium alloy steel (500 series) and precipitation hardening system (600 series) according to the composition. Among them, the 200 series to manganese instead of nickel, corrosion resistance is poor, often as a cheap alternative to the 300 series. 300 series of chromium – nickel austenitic stainless steel, such as 304 stainless steel, versatile, widely used in corrosion-resistant containers, tableware and so on. In addition, there are 30310 and other high-temperature oxidation resistance products, as well as 316 and other corrosion-resistant products, commonly used in the food industry, the pharmaceutical industry and so on.
347 Stainless Steel: With the addition of the stabilizing element niobium, this type is particularly suitable for welding aerospace appliance parts and chemical equipment.
400 series stainless steel: Containing ferritic and martensitic types, these products are free of manganese and can therefore replace 304 stainless steel in some applications.
408 stainless steel: excellent heat resistance, but weak corrosion resistance, its composition contains 11% Cr and 8% Ni.
409 Stainless Steel: Often regarded as the cheapest type, especially in the Anglo-American market, it is often used to make automobile exhaust pipes and belongs to the category of ferritic stainless steel.
410 stainless steel: as a martensitic high-strength chromium steel, it exhibits good wear resistance, but relatively poor corrosion resistance.
416 Stainless Steel: By adding sulfur, it improves the machinability of the material.
420 Stainless Steel: A “cutlery grade” martensitic steel, similar to Brinell high-chromium steel, this early stainless steel was widely used for surgical knives, and its surface can be polished to a very bright finish.
Stainless Steel 430: A ferritic stainless steel often used for decorative purposes, such as automotive jewelry. It has good formability, but is slightly less resistant to temperature and corrosion.
440 Stainless Steel: High-strength sharpening steel with a slightly higher carbon content. After appropriate heat treatment, it can obtain high yield strength and hardness, reaching 58HRC, making it one of the hardest stainless steels. It is often used to make “razor blades” and is available in a wide range of models.
500 Series: Designed for heat-resistant chromium-alloyed steels.
600 Series: Contains martensitic precipitation-hardening stainless steels.
7. Stainless steel surface processing grade
Original surface: the surface after hot rolling, heat treatment and pickling, commonly used in cold rolled materials, industrial tanks and chemical installations, the thickness range between 0MM and 0MM.
Blunt: NO.2D cold rolled after heat treatment and pickling, soft material, silver-white glossy surface, especially suitable for deep stamping process, such as automobile components and water pipes.
Matte: NO.2B is cold rolled, also heat treated and pickled, and then finish rolled to give a moderately glossy surface. This smooth surface is easy to regrind and is suitable for a wide range of applications such as tableware and building materials.
Coarse Grit: NO.3 products are ground by 100-120 abrasive belts to give a glossy surface with discontinuous coarse grain and are commonly used for building interior and exterior decoration, electrical appliances and kitchen equipment.
Fine Grit: NO.4 products are processed by No.150-180 abrasive belt, which also has glossiness and discontinuous coarse grain, but with finer streaks, and is suitable for many fields such as baths, interior and exterior building decoration.
#320: Treated with #320 abrasive tape, similar to NO.4 but with finer stripes, also suitable for a wide range of applications.
HAIRLINE: HLNO.4 products are continuously abraded with appropriate grit polishing belts to produce a unique abrasive pattern, especially suitable for architectural decoration, elevator and door panels, etc.
BRIGHT SURFACE: BA products are cold rolled, bright annealed and flattened to obtain excellent gloss and reflectivity, just like a mirror, suitable for home appliances, mirrors and decorative materials.
8. Areas of application
8.1
The surface finish of stainless steel is critical in the construction field. Smooth surfaces are less susceptible to dirt accumulation, and dirt deposits may lead to rust and corrosion of stainless steel. In the hall, elevator decorative panels are often used stainless steel, although handprints can be erased but affect the aesthetic, so it is important to choose the right surface treatment to prevent handprints.
Hygienic conditions are critical for many industries, such as food processing, catering, brewing and chemical industries. In these areas, stainless steel stands out for its excellent resistance to chemical cleaners. The unique finish of the stainless steel surface makes it easy to cope with everyday cleaning and even removes dirt such as scribbles. Its unidirectional grain design makes the cleaning process more efficient and convenient.
Stainless steel also performs well in situations where hygiene is critical, such as hospitals. It is not only easy to clean, but also effective in preventing bacterial growth, and its resistance to rust and corrosion can even be compared with glass and ceramics.
However, stainless steel also faces some challenges. One of them is corrosion. Although the surface of stainless steel forms a stable layer of chromium-rich oxide film, which provides protection against rust and corrosion, but this protection is not invulnerable. Once this film is damaged, the metal surface can corrode. In addition, the presence of chloride ions can accelerate the corrosion reaction process.
Therefore, in the process of using stainless steel, we need to carry out regular cleaning and maintenance to maintain the quality of its surface and extend its service life. When cleaning, tools and washing liquids that may scratch the surface should be avoided, and care should be taken to rinse thoroughly to get rid of washing liquid residues.
Chlorine ions are widely found in salt, sweat, seawater, sea air and soil. Stainless steel is highly susceptible to corrosion in a chlorine ionized environment, even at a rate greater than that of ordinary mild steel. Therefore, the use of stainless steel need to pay attention to its environment, and regular cleaning to keep clean and dry. There have been U.S. companies with oak containers containing chlorine ion solution for nearly 100 years, but replaced with stainless steel after only 16 days due to corrosion leakage, highlighting the vulnerability of stainless steel in the chlorine ion environment.
In addition, the corrosion of stainless steel may also be affected by other factors. For example, stainless steel without solid solution treatment, its alloying elements are not sufficiently dissolved into the matrix, resulting in poor corrosion resistance. Stainless steel materials that do not contain titanium and niobium, on the other hand, present a risk of intergranular corrosion, but this risk can be reduced by the addition of these elements in conjunction with stabilization treatments. During fabrication, equipment surfaces can be contaminated with dust, which can be removed with water or an alkaline solution, but adherent dirt and grime require high-pressure water or steam for cleaning. Also, the presence of free iron can cause corrosion of stainless steel and must therefore be completely removed.
In order to prevent the process lubricants or generators and dirt accumulation, scratches and other rough surfaces need to be mechanically cleaned, usually using stainless steel special polishing machine for removal. In addition, rust spots must be removed before the equipment is put into service, and thoroughly cleaned surfaces should be inspected by iron and/or water tests. Finally, rough grinding and machining may also affect the performance and appearance of stainless steel, so care should be taken to avoid.
During grinding and machining processes, metal surfaces can develop defects such as roughness, grooves, overlaps and burrs, all of which can damage the metal to a certain depth and cannot be completely removed by pickling, electropolishing or shot peening. Rough surfaces are not only an easy source of corrosion and deposits, but also affect the quality of the weld. Therefore, when cleaning weld defects or removing excess weld prior to rewelding, fine rather than coarse abrasives should be used for grinding.
In addition, the welding process can produce some potential sources of corrosion, such as welding arc spotting, welding spatter, and welding defects. Welding lead arcs can cause rough defects on the metal surface, destroying the protective film and thus leaving a corrosion hazard. To avoid this, the welder should lead the arc on the side of the welded weld path or weld joint and fuse the arc lead marks into the weld.
At the same time, welding spatter is also a factor that affects the quality of welding. When welding processes such as GMAW and FCAW are used, a large amount of spatter will be produced if the parameters are not used properly. To solve this problem, spatter inhibitor can be applied on both sides of the joint before welding to eliminate the adhesion of spatter. After welding, the anti-spatter agent and spatter are then easily cleaned off to avoid surface damage.
In addition, the welding process will produce some welding defects such as nibbling edge, unwelded through, dense porosity and cracks. These defects will not only reduce the welding strength, but also become the source of crevice corrosion. To improve this situation, a thorough cleaning operation is required. At the same time, organic substances such as oil, grease, etc. can also be a source of localized corrosion and must be thoroughly removed using melting agents and/or acidic chemical cleaners.
Finally, residual adhesives are also a problem that requires attention. Adhesives may be used during machining and assembly. However, if not thoroughly removed, residual adhesives may cause corrosion of the metal. Therefore, it is important to ensure that all adhesives have been thoroughly removed from the final product.
Adhesives are often used to join or secure parts during machining and assembly. However, if not thoroughly removed, adhesives left on stainless steel surfaces will gradually harden in light or air, forming a source of crevice corrosion that can seriously affect the metal’s corrosion resistance. Therefore, these residues must be cleaned up in a timely manner. When the adhesive has not yet solidified and hardened, you can use organic melts to dissolve and remove; and if it has hardened, you need to use fine abrasives for mechanical cleanup to ensure the integrity and corrosion resistance of the stainless steel surface.
In addition, contaminants such as paint pen marks can have a similar effect on stainless steel. These contaminants must be removed in a timely manner to avoid long-term damage to the metal. Removal methods include washing with clean water or alkaline cleaners, or rinsing with high pressure water or steam to ensure a clean and corrosion resistant stainless steel surface.
8.2
The properties of austenitic stainless steel, as an important metallic material, are influenced by a number of factors. Among them, chromium, as the main alloying element, has a significant effect on the properties of stainless steel. The addition of chromium not only promotes the passivation of steel, but also keeps the steel in a stable passivated state, thus improving the stainless steel’s stainlessness and corrosion resistance. However, the increase in chromium content will also bring some negative effects, such as increasing the tendency of intermetallic phase formation, reducing the plasticity and toughness of steel, as well as reducing the corrosion resistance of steel under certain conditions. Therefore, when formulating the composition of stainless steel, various factors need to be considered in order to obtain the stainless steel material with optimal performance.
The effect of chromium on performance: chromium is a crucial alloying element in austenitic stainless steel. Under the premise of maintaining a fully austenitic organization, increasing the chromium content in steel does not significantly affect its mechanical properties, but will produce a significant increase in corrosion resistance. Chromium significantly improves the corrosion resistance of steel in oxidizing and acidic chloride media. In combination with nickel as well as other elements, chromium also enhances the resistance of steel to reducing media, organic acids, urea and alkaline media. In addition, chromium for improving the local corrosion properties of steel, such as intergranular corrosion, pitting corrosion, crevice corrosion, and stress corrosion properties under certain conditions, also has a significant effect.
Austenitic stainless steel intergranular corrosion susceptibility to the most influential factor is the carbon content in steel, and chromium can increase the solubility of carbon in austenite and reduce its depletion, which is beneficial to improve the intergranular corrosion resistance of steel. At the same time, chromium is also effective in improving the resistance of austenitic stainless steels to pitting and crevice corrosion, an effect that is even more pronounced when molybdenum or both molybdenum and nitrogen are present in the steel. Although molybdenum and nitrogen also have a certain ability in the resistance to pitting corrosion and crevice corrosion, but the study shows that if the austenitic stainless steel lacks chromium or chromium content is low, molybdenum and nitrogen of these effects will become less significant.
In addition, chromium has an important influence on the stress corrosion resistance of austenitic stainless steel. In specific experimental media conditions or actual use of the environment, such as aqueous media containing Cl- and oxygen, high temperature and high pressure water, as well as pitting corrosion as the origin of the stress corrosion conditions, improve the chromium content of the steel will be beneficial to enhance its stress corrosion resistance. At the same time, chromium also prevents intergranular-type stress corrosion tendencies that may occur in austenitic stainless steels as a result of increased nickel content, and exhibits beneficial effects on cracking (e.g., NaOH) stress corrosion. In addition to the significant effect on corrosion resistance, chromium can also significantly improve the austenitic stainless steel oxidation resistance, sulfidation resistance and resistance to molten salt corrosion and other properties.
8.3
Next, we will look at the effects of nickel, another key element in stainless steel.
Nickel is another key element in austenitic stainless steels and serves to stabilize the austenite and expand its phase region. To ensure that only austenitic organization exists in the steel, the minimum nickel content required is about 8% when the carbon content is 1% and the chromium content is 18%. This is precisely the classical composition of 18-8 chromium-nickel austenitic stainless steels. As the nickel content increases in austenitic stainless steels, residual ferrite can be completely eliminated, while the possibility of σ (ferrite) phase formation is significantly reduced. In addition, the martensitic transition temperature (Ms) decreases as well. However, it is worth noting that the elevated nickel content decreases the solubility of carbon in austenite, which in turn increases the tendency of carbide precipitation. Next, we will further explore the specific effects of nickel on the properties of austenitic stainless steels.
Effect of molybdenum on stainless steel
8.3.1 The effect of molybdenum on the organization
Molybdenum, in conjunction with chromium, forms and stabilizes ferrite and enlarges its phase region. Although molybdenum’s ability to form ferrite is comparable to that of chromium, it also promotes the precipitation of intermetallic phases in austenitic stainless steels, which may adversely affect the corrosion resistance and mechanical properties of the steel, in particular leading to a decrease in plasticity and toughness. In order to maintain the single austenitic organization of austenitic stainless steel, with the increase of molybdenum content in the steel, it is necessary to correspondingly increase the content of austenite-forming elements (such as nickel, nitrogen and manganese, etc.), in order to maintain the balance between ferrite and austenite-forming elements.
8.3.2 Effect of molybdenum on properties
The oxidizing effect of molybdenum on austenitic stainless steel is not significant. In the case of chromium-nickel austenitic stainless steels that maintain a single austenitic organization and have no intermetallic precipitation, the addition of molybdenum does not have a significant effect on their room temperature mechanical properties. However, as the molybdenum content increases, the high-temperature strength of the steel, such as durability and creep properties, will be significantly improved, which makes the molybdenum-containing stainless steel in high-temperature environments in a wide range of applications. It is worth noting, however, that the addition of molybdenum also increases the steel’s resistance to deformation at high temperatures. Together with the small amount of δ-ferrite that may exist in the steel, the hot working performance of molybdenum-containing stainless steels is usually poorer than that of molybdenum-free steels, and the higher the molybdenum content, the poorer the hot working performance. In addition, molybdenum-containing austenitic stainless steels are prone to phase precipitation, which may significantly deteriorate the plasticity and toughness of the steel. Therefore, special attention should be paid to preventing the formation of intermetallic phases in steel during production, equipment manufacture and application. Although molybdenum strengthens the corrosion resistance of chromium in steel, and the formation of acid salts with corrosion inhibition, but common chromium-nickel austenitic stainless steel containing traces of chlorides and saturated with oxygen when used in aqueous media, its stress corrosion is still pitting corrosion as the origin. Therefore, molybdenum-containing chromium-nickel-molybdenum austenitic stainless steel due to the high resistance to pitting corrosion, in practice, often show superior resistance to chloride stress corrosion than molybdenum-free steel.
Carbon plays an important role in austenitic stainless steels as it is an element that strongly forms, stabilizes and enlarges the austenite zone. Compared to nickel, carbon is up to 30 times more capable of forming austenite. As an interstitial element, carbon significantly increases the strength of austenitic stainless steels through solid solution strengthening. In addition, it enhances the stress corrosion resistance of austenitic stainless steels in highly concentrated chloride environments.
8.4
However, carbon may adversely affect austenitic stainless steels under certain conditions. In particular, when welded or heated by 450 ~ 850 ℃, carbon may react with chromium in steel to form a high-chromium Cr23C6-type carbon compounds, resulting in localized chromium depletion, which in turn undermines the steel’s corrosion resistance, in particular, intergranular corrosion resistance. Therefore, since the 1960s, the new development of chromium-nickel austenitic stainless steel is mostly carbon content of less than 03% or 02% of the ultra-low carbon type. With the reduction of carbon content, the intergranular corrosion susceptibility of steel is gradually reduced, and the effect is most obvious when the carbon content is less than 02%. At the same time, experiments have also shown that carbon increases the tendency of chromium to pitting corrosion in austenitic stainless steels. In view of the harmful effects of carbon, not only in the smelting process need to strictly control the carbon content, but also in the subsequent hot and cold working and heat treatment process to prevent the surface of stainless steel carbonization, in order to avoid chromium carbide precipitation.
