Chemical composition and heat treatment of duplex stainless steel

Duplex stainless steels require good overall mechanical properties. These properties are achieved by the selection of steel with the appropriate composition and by heat treatment by quenching and high temperature tempering. In the quenched and tempered state, the dual-phase steel has low carbon content, high plasticity and toughness, but low strength; the high carbon content is opposite, the strength is higher, and the plasticity and toughness are lower; if the carbon content is medium, At the same time, it has the proper strength, ductility and toughness. Therefore, in general quenched and tempered parts, most of the carbon content is in the range of 0.25 to 0.55%. The carbon content of carbon steel is biased to the upper limit (0.035-O.055%). This is because the alloying elements in duplex stainless steel have a strengthening effect instead of a part of carbon. The higher the alloying element content, the lower the carbon content. For example, S31803 stainless steel can be used as both quenched and tempered steel and carburized steel.

Effect of Metal Elements Chromium and Nickel on Mechanical Properties, Logistics Properties and Corrosion Characteristics of Duplex Stainless Steel:

Chromium
The addition of chromium is primarily intended to provide corrosion resistance to duplex stainless steels. This is particularly effective in oxidizing environments such as those containing nitric acid. After the addition of chromium, an oxide having a stoichiometric ratio of iron oxide is formed on the surface of the steel. Because the affinity of chromium and oxygen is much higher than that of iron; the presence of chromium increases the stability of this oxide. Steels with a mass fraction of chromium greater than 10.5% are considered to be stainless in the atmosphere. Oxides that are stable in more aggressive environments require higher chromium levels. Chromium is also a ferrite forming element. An Fe-Cr alloy containing more than 12% Cr is an alloy of all ferrite. Increasing the chromium content in Fe-Cr-C or Fe-Cr-Ni-C alloys promotes the formation and retention of ferrite in martensitic stainless steel, austenitic stainless steel and duplex stainless steel. In ferritic alloy steels, chromium is the main alloying element for stabilizing ferrite structure.

Chromium is also a strong carbide-forming element. The most common chromium-rich carbide is M23C6. Among them, M is mainly chromium, but sometimes some iron and molybdenum appear. In duplex stainless steel, the chemical formula of M23C6 is adopted and the default chromium is the main element in M. This carbide is actually found in all stainless steels. Cr7C3 type carbides may also form in duplex stainless steels, although this type of carbide is not common. Other composite carbides and carbonitrides M23CN6 may also form. Chromium can also form nitrides with nitrogen, the most common being Cr2N, which is observed in both ferritic and duplex stainless steels. Chromium is also an important component in the formation of intermetallic compounds, and intermetallic compounds have a tendency to embrittle duplex stainless steels, the most common being phase a, which is a temperature below 815 ° C in Fe-Cr systems. The (Fe,Cr) compound formed underneath. In fact, the sigma phase can be formed in any stainless steel, but is most common in high chromium austenite, ferrite and duplex stainless steel: chromium is also present in the yttrium phase and Laves phase intermetallic compounds. Since chromium can be used as a replacement atom in body-centered cubic (BCC) and face-centered cubic (FCC) lattices, chromium can be solid solution strengthened to some extent from the viewpoint of mechanical properties. However, in ferritic alloy steels, especially when carbon and nitrogen are present in the steel, high chromium content makes the toughness and plasticity poor. High-chromium ferritic steels must be carefully treated or have a low carbon and nitrogen content to achieve acceptable mechanical properties after welding.

The determination of the alloy composition in the duplex stainless steel elbow takes into account the following factors:

  1. To ensure corrosion resistance, especially resistance to pitting corrosion, crevice corrosion resistance, stress corrosion corrosion resistance.
  2. To ensure the characteristics of the dual-phase steel, that is, after the solution treatment can have two phases, and one of the phases is not less than 25%.
  3. Has a certain strength, plastic toughness.
  4. Consider the same or different effects between different alloying elements.
  5. Consider the feasibility of material casting, heat treatment and processing.

  According to the above principle, the main alloying elements in the duplex stainless steel elbow are chromium, nickel, molybdenum, nitrogen, tungsten and the like.
  (1) Chromium. Chromium is the most important element to ensure corrosion resistance. Chromium can form a passivation film and has good self-repairing ability to the damaged passivation film. As the chromium content increases, the passivation ability of the steel increases.
Chromium has a significant solid solution strengthening effect on the matrix. As the amount of chromium increases, the yield strength of the steel increases. However, it causes σ to be precipitated equally, which reduces plasticity and toughness.

Nickel
Nickel is an important alloying element in stainless steel that is second only to chromium (which will be learned later). In order to resist the corrosion of reducing acid and alkali medium, it is not enough to contain only chromium in steel. Nickel must be added to chromium (see Figures 1 and 2). Nickel promotes the stability of the stainless steel passivation film and improves the thermodynamic stability of the stainless steel. Therefore, the coexistence of chromium and nickel in stainless steel can significantly enhance the stainless steel’s rust and corrosion resistance. Nickel is beneficial to the high temperature oxidation resistance of stainless steel, but is harmful to high temperature oxidation resistance. Because nickel and sulfur act to form low melting point sulfides. The formation of low melting point sulfides can significantly reduce the hot workability of steel.

If the nickel content is too low, the ferrite phase content in the steel is too large, and the brittleness is easily increased. If the nickel content is too high and the ferrite content is small, the ferrite is more enriched with alloying elements such as chromium and molybdenum. Promote the precipitation of brittle phase, also reduce the toughness of steel. The low ferrite content, ie the austenite content, will affect the yield strength of steel. Therefore, considering the mechanical properties, the nickel content is suitable between 4.5% and 7.5%.

Quenching and tempering process during heat treatment of duplex stainless steel
In the heat treatment process of duplex stainless steel, the effect of cooling and quenching is very important. It should be correctly selected according to the hardenability of duplex steel, the shape and size of the workpiece and the specific conditions of production, usually under the premise of ensuring hardening. Slow cooling methods should be used to reduce quenching stress, reduce deformation and prevent cracking. Duplex stainless steel has low hardenability and requires oil quenching when stainless steel tubes smaller than 8 mm in size. When a larger diameter duplex stainless steel tube is quenched with two liquids, the residence time in water can be estimated from 3 to 5 mm/sec. In order to reduce quenching stress and deformation and reduce the tendency of quenching, S31803 stainless steel pipe with diameter ≤12 mm can be graded quenching, usually in a hot alkaline bath or molten nitrate salt at 170-200 °C for short time (3~5) Minutes) until silent, then air-cooled. Due to the high hardenability of duplex steel, in addition to water quenching of individual grades, it is generally quenched in oil, and can be quenched in alkali bath or nitrate salt. The residence time at the classification temperature can be extended to 30-60 minutes. The deformation is reduced by appropriately increasing the amount of retained austenite.

After quenching, the duplex stainless steel should be tempered at low temperature to minimize internal stress, improve the plasticity and toughness of the steel, stabilize the performance and size of the steel, and prevent it while maintaining high hardness, high strength and high wear resistance. Grinding cracks. The tempering temperature of the dual-phase steel material is usually 150 to 160 ° C, the hardness after tempering is HRC 61–63, and the tempering degree of the smaller steel tube should be higher (180 ~ 2000 ° C) to increase the strength and keep Sufficient hardness (HRC 60 ~ 61). The tempering temperature of the duplex steel is slightly higher, often 160 to 180 ° C, and has a hardness of HRC 61-63 after tempering.

During the actual operation, the duplex tempering temperature of the duplex stainless steel is higher, and the troostite structure is obtained to improve the toughness. For example, the hardness of S32205 stainless steel after tempering at 300-325 ° C is HRC 52-57; the hardness after tempering at 440–460 ° C is HRC 44-48. The tempering holding time is usually taken for 1 to 2 hours. For duplex stainless steel materials with good wear resistance, it is often necessary to pass tempering at 140-160 ° C for 30-60 minutes after grinding to eliminate the processing stress.

The primary role of alloying elements is to increase the hardenability of duplex stainless steels. Comparing the hardened and unhardened specimens, if the tempering to the same hardness, the tensile strength is very small, while the yield strength and impact toughness of the unhardened specimen are lower. Therefore, all hardened duplex stainless steel materials can achieve high overall mechanical properties after quenching and high temperature tempering, and uniform performance over the entire cross section, while unhardened parts have poor performance. Since the alloying elements improve the hardenability, the mechanical properties of the duplex steel are improved and the process performance is improved. For example, oil quenching can be used to reduce the deformation and cracking tendency after quenching. The larger the cross section, the more pronounced the beneficial effect of the alloying elements. The use of a single element such as chromium, manganese, silicon, boron, can improve the hardenability of duplex steel, but in the quenching and high temperature tempering process, some defects often occur, such as increasing the tendency of overheating or increasing the high temperature Tempered brittleness. To overcome these drawbacks, certain elements such as tungsten, molybdenum, titanium, etc. may be added to the duplex stainless steel.

From the performance of the duplex steel after tempering, in the quenching and tempering temperature range, the obtained tempering stability is higher, and the strength is lower. When tempering to the same strength, the stability is large, the tempering temperature needs to be higher, and the obtained plasticity and toughness are correspondingly higher, that is, the comprehensive mechanical properties are better. The small stability results in poor overall mechanical properties. In order to improve the tempering stability of the duplex stainless steel, alloying elements such as tungsten, molybdenum, vanadium, etc. may be added to the duplex steel. Compared with the carbon steel with the same carbon content, the duplex stainless steel has higher plasticity and toughness at the same strength, and the strength of the duplex steel is higher at the same plasticity and toughness. In addition, since the duplex stainless steel obtains an austenitic structure after high-temperature tempering, in which the ferrite content is large, the properties of the duplex steel can also be affected by changing the properties of the ferrite.

Change process of carbide after tempering of duplex stainless steel
After the tempering treatment of the duplex stainless steel, the a-phase recrystallization process in the material includes two aspects. On the one hand, the phase a and the carbide are in association with each other, so that the phase a is broken; on the other hand, the phase a is broken. Crystallization, forming polygonal ferrite grains. As this process progresses, intergranular stress (ie, the second type of stress) gradually disappears. Therefore, the influence of alloying elements on the above process is also manifested in two aspects, namely, on the one hand, delaying the destruction of the a phase and the carbide, and on the other hand, increasing the recrystallization temperature of the a phase. When discussing the influence of alloying elements on martensite decomposition, it has been pointed out that most of the alloying elements have different degrees of delayed martensite decomposition, ‘this directly proves that the alloying elements destroy the coherent relationship between phase a and carbide. Deferred. Because this process is actually a diffusion process, and the presence of alloying elements hinders the progress of diffusion, so that coherent damage and ferrite fragmentation can occur at higher temperatures. As the tempering temperature increases, the shredded ferrite, ie, the a-phase insert will gradually grow. This is a recrystallization process. This process is also a diffusion process, and the presence of alloying elements not only spreads itself. It is very low, and it also reduces the diffusion rate of iron atoms, thereby increasing the recrystallization temperature of phase a, which can be indirectly confirmed by the influence of alloying elements on the recrystallization temperature after cold deformation processing. For example, when various iron alloys are subjected to 90% cold working deformation, the recrystallization temperature is listed in Table 1-2 for each 1% (atomic) different element. As can be seen from the table, various elements are for ferrite. The body recrystallization temperature is improved to varying degrees. The combined action of several elements at the same time will more significantly increase the recrystallization temperature of phase a. In addition, if the tempering temperature is above the alloying element enrichment temperature, the non-carbide elements will be enriched in the ferrite, causing the ferrite to solid solution and strengthen, and the carbide elements diffusing into the carbide, possibly leading to carbides. The transformation, the precipitation of highly dispersed special carbides to produce dispersion strengthening, can greatly improve the mechanical properties of duplex stainless steel.

In the test duplex stainless steel, alloying elements such as tungsten, molybdenum and niobium have the strongest hindrance to the recrystallization process of phase a, and aluminum, silicon, vanadium, chromium, manganese, nickel, copper and cobalt have less influence. The alloying elements can increase the recrystallization temperature of phase a to varying degrees, leaving the martensite morphology of phase a and the mosaic structure in which it is shredded to a higher temperature, which maintains the strength of the tempered structure and increases the steel. The heat and so on have important contributions. The alloying elements have a great influence on the formation, transformation and aggregation of carbides during tempering, especially the carbide forming elements, and the effect is more remarkable. The alloying elements affect the aggregation growth of carbides in two aspects. On the one hand, due to the aggregation process of carbides, it is essentially a diffusion process. The alloying elements change the diffusion coefficient of carbon in phase a, and thus change. The rate of aggregation of carbides; on the other hand, the aggregation of carbides is carried out by means of small particles dissolved and large particles grow, while alloying elements change the stability of carbides, so that small particles dissolve and large particles grow. The speed has changed, which has changed the rate of carbide accumulation. The non-carbide forming elements nickel and cobalt promote the aggregation of carbides, while the formation of silicon and carbide elements hinders the aggregation and growth of carbides, as shown in the figure for the content of each alloy element and the average of carbides during high temperature tempering. The relationship between the diameters, as can be seen from the figure, the average diameter of carbides increases only with the increase of cobalt and nickel elements, and decreases with the increase of other elements.

In addition, in addition to affecting the aggregation and growth of carbides, alloying elements also produce two new processes for tempering of duplex stainless steels compared to carbon steels, which are described as follows: When carbon phase transitions in duplex stainless steel tempering X-ray analysis and electron microscopy observations show that when tempering duplex stainless steel is tempered, as the tempering temperature increases, during the formation of carbides, there is a process of converting alloy cementite into alloy carbide. A process is a secondary process that is not available when carbon steel is tempered. The transformation process of the alloy carbide can be clearly illustrated by the following diagram. It can be seen from the above diagram that the transformation process of the alloy carbide is also a diffusion process, that is, on the basis of the analysis of the carbide of the martensite fraction, the alloy cementite is formed by the diffusion of iron, carbon and alloy element atoms, and then converted into Alloy carbide. There are two ways for this transformation. One is to form an alloy carbide nucleus at the phase boundary between the alloy cementite and the ferrite, and grow up by the consumption of the alloy cementite; Second, the alloy carbide nucleus is directly precipitated from the ferrite matrix, and grows by relying on the alloy cementite to continuously dissolve into the matrix. This latter transformation will make the alloy carbides highly diffuse and difficult to aggregate and grow because it requires the diffusion of atoms of carbon and alloying elements over long distances.

Effect of alloying elements on mechanical properties of duplex stainless steel
The influence of alloying elements on the mechanical properties of duplex stainless steel is closely related to the heat treatment mode. In the annealed state, the alloying elements mainly act by solid solution strengthening of ferrite and changing the distribution state and relative content of carbides. In the annealed state of duplex stainless steel, the basic constituent phases are ferrite and carbide. The strength of the iron body plays a decisive role in the strength of the steel. Experiments have shown that all alloying elements can strengthen ferrite, but they are not strengthened to the same extent. Silicon, manganese and nickel have a greater strengthening effect. Silicon and manganese have strong effects and are rich in resources. They are the most commonly used elements for strengthening ferrite. China’s ordinary low-alloy duplex stainless steel is made of silicon and manganese as the main alloying elements. Tungsten, molybdenum and vanadium are the second, and chromium and cobalt have the least effect.
Source: China Stainless Steel Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)

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