Regardless of whether stainless steel plates or heat-resistant steel plates, austenitic steel plates have the best overall performance, with sufficient strength, excellent plasticity and low hardness, which is one of the reasons why they are widely used. Austenitic stainless steel is similar to most other metal materials. Its tensile strength, yield strength and hardness increase as the temperature decreases; the plasticity decreases as the temperature decreases. Its tensile strength increases evenly within the temperature range of 15~80°C. More importantly, as the temperature decreases, its impact toughness decreases slowly, and there is no brittle transition temperature. Therefore, stainless steel can maintain sufficient plasticity and toughness at low temperatures. The heat resistance of stainless steel refers to the resistance to oxidation or gas medium corrosion at high temperatures, that is, thermal stability.
The effect of chromium
Chromium is the most important alloying element in austenitic stainless steel. The rust resistance and corrosion resistance of austenitic stainless steel are mainly due to the fact that chromium promotes the passivation of steel and keeps the steel in a stable passivation state. the result of. ○1 The influence of chromium on the structure: in austenitic stainless steel, chromium is an element that strongly forms and stabilizes ferrite, reducing the austenite zone. As the content of steel increases, ferrite may appear in austenitic stainless steel ( δ) Organization. Research shows that in chromium-nickel austenitic stainless steel, when the carbon content is 0.1% and the chromium content is 18%, in order to obtain a stable single austenitic structure, the minimum nickel content is required, about 8% In this regard, the commonly used 18Cr-8Ni type chromium-nickel austenitic stainless steel is the most suitable one containing chromium and nickel. In austenitic stainless steels, as the chromium content increases, the tendency to form some intermetallic phases (such as δ phase) increases. When the steel contains molybdenum, the chromium content will increase and χ will be formed equal, as before As mentioned, the precipitation of σ, χ phase not only significantly reduces the plasticity and toughness of steel, but also reduces the corrosion resistance of steel under some conditions. The increase in the content of chromium in austenitic stainless steel can make the martensite conversion temperature (Ms ) Decrease, thereby improving the stability of the austenite matrix. Therefore, high chromium (for example, more than 20%) austenitic stainless steel is difficult to obtain martensitic structure even after cold working and low temperature treatment.
Chromium is a strong carbide forming element, and it is no exception in austenitic stainless steel. The common chromium carbide in austenitic stainless steel is Cr23C6; when the steel contains molybdenum or chromium, carbides such as expired Cr6C can also be seen. The formation of steel will have an important impact on the performance of steel under certain conditions. ○2 Effect of chromium on performance: Generally speaking, as long as the austenitic stainless steel maintains a completely austenitic structure without the formation of δ ferrite, only increasing the chromium content in the steel will not have a significant impact on the mechanical properties. The performance of austenitic stainless steel has the greatest impact on corrosion resistance, which is mainly manifested as: chromium improves the performance of steel's oxidation resistance medium and acid chloride medium; under the combined action of nickel, molybdenum and copper, chromium improves steel resistance to some reduction Medium, organic acid, urea and alkali medium performance; chromium also improves the resistance of steel to local corrosion, such as intergranular corrosion. Pitting corrosion, crevice corrosion and stress corrosion performance under certain conditions. The factor that has the greatest influence on the intergranular corrosion sensitivity of austenitic stainless steel is the carbon content in the steel. The effect of other elements on the intergranular corrosion is mainly determined by its influence on the dissolution and precipitation behavior of carbides. In austenitic stainless steel Chromium can increase the solubility of carbon and reduce the degree of chromium depletion. Therefore, increasing the chromium content is beneficial to the intergranular corrosion resistance of austenitic stainless steel. Chromium is very effective in improving the pitting corrosion and crevice corrosion resistance of austenitic stainless steel. When there are molybdenum or molybdenum and nitrogen in the steel at the same time, the effectiveness of chromium is greatly enhanced. Although according to the research, the resistance of molybdenum to pitting corrosion and crevice corrosion is about 3 times that of chromium, and nitrogen is 30 times that of chromium. However, a lot of research shows that if there is no chromium in austenitic stainless steel or the chromium content is low, the pitting and crevice corrosion resistance of molybdenum and nitrogen will be lost or not significant enough.
The effect of chromium on the stress corrosion resistance of austenitic stainless steel varies with the experimental medium conditions and the actual use environment. In the MgCl2 boiling solution, the effect of chromium is generally harmful, but in the water medium containing Cl- and oxygen, Under high temperature and high pressure water and pitting corrosion as the origin of stress corrosion conditions, increasing the chromium content in steel is beneficial to stress corrosion resistance. At the same time, chromium can also prevent the intergranular type that is easy to appear in austenitic stainless steels and alloys due to the increase in nickel content. The tendency of stress corrosion is also beneficial to cracking (NaOH) stress corrosion. The role of chromium is also beneficial. In addition to having an important influence on the corrosion resistance of austenitic stainless steel, chromium can also significantly improve the oxidation resistance, sulfidation resistance and Resistance to melting salt corrosion and other properties.
The impact of nickel
1 The impact of nickel on the organization
Nickel is an element that strongly stabilizes austenite and expands the austenite phase region. In order to obtain a single austenite structure, the minimum nickel content required when the steel contains 0.1% carbon and 18% chromium is about 8%. It is the basic component of the most famous 18-8 chromium-nickel austenitic stainless steel. In austenitic stainless steel, with the increase of nickel content, the residual ferrite can be completely eliminated, and the tendency of σ phase formation is significantly reduced; The temperature of transforming from tensite to hydrocarbon is lowered, and the λ→M phase transformation may not even occur. However, the increase of nickel content will reduce the solubility of carbon in austenitic stainless steel, thereby increasing the tendency of carbide precipitation.
2 The impact of nickel on performance
The influence of nickel on the mechanical properties of austenitic stainless steel, especially chromium-nickel austenitic stainless steel, is mainly determined by the influence of nickel on the stability of austenite. Within the range of nickel content where martensite transformation may occur in the steel, As the nickel content increases, the strength of the steel decreases and the plasticity increases. The chromium-nickel austenitic stainless steel with stable austenitic structure has excellent toughness (including extremely low temperature toughness), so it can be used as low temperature steel, which is well known. For the chromium-manganese austenitic stainless steel with stable austenitic structure, the addition of nickel can further improve its toughness. Nickel can also significantly reduce the cold work hardening tendency of austenitic stainless steels. This is mainly due to the increased stability of austenite, which reduces or even eliminates the martensite transformation during cold working. At the same time, it has no effect on the cold work hardening of austenite itself. Too obvious, the influence of stainless steel cold work hardening tendency, nickel reduces the cold work hardening rate of austenitic stainless steel, and reduces the room temperature and low temperature strength of steel, and improves the effect of plasticity, which determines that the increase in nickel content is beneficial to the cold forming of austenite stainless steel. Performance. Increasing the nickel content can also reduce or even eliminate the δ ferrite in 18-8 and 17-14-2 chromium-nickel austenitic stainless steels, thereby improving its hot workability. However, the reduction of δ ferrite will affect these The unfavorable weldability of steel grades will increase the tendency of welding hot cracks. In addition, nickel can significantly improve the hot workability of chromium-manganese-nitrogen (chromium-manganese-nickel-nitrogen) austenitic stainless steel, thereby significantly increasing the yield of steel. In the austenitic stainless steel, the addition of nickel and the increase in the nickel content lead to the increase of the thermodynamic stability of the steel. Therefore, the austenitic stainless steel has better rust resistance and resistance to oxidizing media. Increase, the performance of resistance to reducing media is further improved. It is worth pointing out that nickel is also the only important element to improve the resistance of austenite stainless to many media trans-crystalline stress corrosion. In various acid media, nickel is resistant to corrosion of austenitic stainless steel. The effect of performance needs to be pointed out. Under some conditions in high temperature and high pressure water, the increase in nickel content leads to an increase in the intergranular stress corrosion sensitivity of steel and alloys, but this adverse effect will be due to the increase in chromium content in steel and alloys. Reduced or inhibited. With the increase of nickel content in magnetic card austenitic stainless steel, the critical carbon content for intergranular corrosion decreases, that is, the intergranular corrosion sensitivity of steel increases, and the resistance to pitting corrosion and crevices of austenitic stainless steel The corrosion performance of nickel is not significant. In addition, nickel also improves the high temperature oxidation resistance of austenitic stainless steel. This is mainly due to the improvement of the composition of the chromium oxide film, the structure and performance of the nickel, and the higher the nickel content, the more Harmful, this is mainly due to the low melting point nickel sulfide at the grain boundaries in the steel. Generally speaking, simple chromium-nickel (and chromium-manganese-nitrogen) austenitic stainless steels are only used for media that require rust resistance and oxidation resistance (such as Under the use conditions of nitric acid, etc., molybdenum, as an important alloying element in austenitic stainless steel, is added to the steel to further expand its use range.
The influence of molybdenum
1 The influence of molybdenum on the organization
Both molybdenum and chromium are elements that form and stabilize ferrite and expand the ferrite phase region, and the ability of molybdenum to form ferrite is equivalent to that of chromium. Molybdenum also promotes the precipitation of intermetallic phases in austenitic stainless steels, such as σ phase, κ phase, and Laves equivalent, which will adversely affect the corrosion resistance and mechanical properties of the steel, especially resulting in a decrease in plasticity and toughness. In order to maintain a single austenite structure in austenitic stainless steel, as the content of molybdenum in the steel increases, the content of austenite-forming elements (nickel, nitrogen, manganese, etc.) should also be increased correspondingly to maintain the ferrite and ferrite content in the steel. Austenite forms a balance between elements.
2 The effect of molybdenum on performance
The oxidation effect of molybdenum on austenitic stainless steel is not significant, so when the chromium-nickel austenitic stainless steel maintains a single austenitic structure and no intermetallic precipitation, the addition of molybdenum has little effect on its room temperature mechanical properties, but, with With the increase of molybdenum content, the high-temperature strength of steel is improved, such as durability, creep and other properties are greatly improved. Therefore, molybdenum-containing stainless steel is often used at high temperatures. However, the addition of molybdenum increases the high-temperature deformation resistance of steel, and There is often a small amount of δ ferrite in steel, so the hot workability of molybdenum-containing stainless steel is worse than that of molybdenum-free steel, and the higher the molybdenum content, the worse the hot workability. In addition, molybdenum-containing austenitic stainless steel is prone to κ ( σ) phase precipitation, which will significantly deteriorate the plasticity and toughness of steel. Therefore, in the production, equipment manufacturing and application of molybdenum-containing austenitic stainless steel, attention should be paid to prevent the formation of intermetallic phases in steel, although molybdenum acts as an alloying element The reasons for the resistance of austenitic stainless steel to reducing media, surface pitting corrosion and crevice corrosion are not fully understood, but a large number of experiments have pointed out that the corrosion resistance of molybdenum is only effective when the steel contains a relatively high amount of chromium. Molybdenum is mainly It strengthens the corrosion resistance of chromium in steel. At the same time, the corrosion inhibition effect of molybdenum after the formation of salt has also been confirmed by experiments. In terms of resistance to stress corrosion of high-concentration chloride solutions, although molybdenum is an alloying element for austenite The reasons for the resistance of bulk stainless steel to reducing media, pitting corrosion and crevice corrosion are not fully understood, but a large number of experiments have pointed out that the role of molybdenum is only effective when the steel contains a higher amount of chromium. Molybdenum is mainly used to strengthen the chromium in the steel. At the same time, the buffering effect of molybdenum after the formation of molybdate has been confirmed by experiments. In terms of resistance to stress corrosion caused by high concentration of chlorides, this experiment refers to the same. Molybdenum below 3# is harmful to the stress corrosion resistance of austenitic stainless steel, but because common chromium-nickel austenitic stainless steels are mostly used in aqueous media containing trace chlorides and saturated oxygen, their stress corrosion is also originated from pitting corrosion Therefore, chromium-nickel-molybdenum austenitic stainless steel containing molybdenum has higher resistance to pitting corrosion, so in practical applications it often has better resistance to chloride stress corrosion than steel without molybdenum.