Ferritic stainless steel refers to a type of stainless steel that has a mass fraction of chromium (Cr) between 12% to 30%. It can be further divided into low Cr, medium Cr and high Cr, depending on the mass fraction of Cr.
The corrosion resistance of ferritic stainless steel is proportional to the mass fraction of Cr. The higher the mass fraction of Cr, the greater the corrosion resistance. However, to improve overall properties and reduce the negative impact of Cr carbide and nitride precipitation on mechanical properties and corrosion resistance, the trend in the development of ferritic stainless steel is towards lower levels of carbon (C) and nitrogen. (N).
Ultrapure ferritic stainless steel is a subcategory of ferritic stainless steel that has very low levels of C and N (generally no more than 0.015% combined) and medium to high Cr mass fractions. This type of stainless steel is popular because of its good corrosion resistance, thermal conductivity, seismic resistance, processing performance and affordable price compared with copper, copper alloys and titanium materials. It is widely used in various sectors, including the automotive industry, kitchen and household appliances, construction and petrochemical industries.
However, there are also several challenges in producing ultrapure ferritic stainless steel. Due to its high mass fraction of Cr and the presence of other alloying elements such as molybdenum (Mo) and manganese (Mn), it is difficult to avoid the problems inherent in ferritic stainless steel with high Cr content, such as brittleness in the σ phase, 475℃ brittleness and high temperature brittleness.
Production personnel are therefore aware of the potential damage of these brittleness problems and have found that they are mainly caused by the precipitation of the σ phase, the χ phase, the α' phase, the Laves phase and the mass fraction of the element Cr.
This article provides an in-depth examination of the main characteristics and influencing factors of σ-phase brittleness, 475℃ brittleness and high-temperature brittleness in ultrapure ferritic stainless steel. It also analyzes the effects of these brittleness problems on the mechanical properties and corrosion resistance of ultrapure ferritic stainless steel, serving as a reference for producers and users.
1. Main brittle characteristics of ultrapure ferritic stainless steel
Ultrapure ferritic stainless steel contains various alloying elements and is prone to precipitation of different intermetallic compounds during hot working, mainly carbon and nitrogen compounds of Cr, Nb and Ti, as well as intermetallic compounds of the phases σ, χ, Laves, and α.
The characteristics of the σ, χ, Laves and α' phases are presented in Table 1.
Table 1 Characteristics of Intermetallic Compounds in Ultrapure Ferritic Stainless Steel
| Precipitated phase | Structure | Setup and composition | Precipitation condition | Feature |
| σ mutually | Body centered tetragonal (bct) D8b, 30 atoms/unit cell | AB or AxBy, FeCrFeCrMo | w（Cr）=25%~30%,600-1050℃ | Hard, brittle, rich in Cr |
| Phase X | Body-centered cubic (bcc) A12, 30 atoms/unit cell | α-Mn, Fe36Cr12Mo10 or (Fe, Ni)36Cr18Mo4 | w（Mo）=15%~25%,600-900℃ | Hard, brittle, rich in Cr and Mo |
| Lava Phase | Closed hexagonal (hcp) C14 or C36 | AB2, Fe2Ti or Fe2Nb or Fe2Mo | 650-750℃ | Hard |
| α' mutually | Body-centered cubic (bcc) | Fe Cr, rich in cr | w（Cr）>15%,371-550℃（475℃） | Hard, brittle, rich in Cr |
The “C” precipitation curves for the σ, χ and Laves phases of some typical ultrapure ferritic stainless steels are shown in Figures 1 and 2.
Due to variations in alloy composition, the most sensitive temperature range for precipitation of these phases is between 800 and 850°C.
For the 00Cr25Ni4Mo4NbTi alloy (Monit), the σ and χ phases precipitate relatively quickly, while the Laves phase is more easily precipitated at 650°C and takes longer to form.
Regardless of the type of brittle precipitate, excessive precipitation will make the steel brittle, resulting in a sharp decline in impact properties.
Fig. 1 26% Gr – (1%~4%) Mo – (0~4%) Ni Ferritic Stainless Steel
Fig. 2 TTP diagram of ferritic stainless steel 00Cr25Ni4Mo4TiNb (Monit) (after solid solution at 1000 ℃)
1.1 Main characteristics of the σ fragility phase
The generation of σ-phase brittleness is mainly caused by the precipitation of σ-phase and χ-phase. The Laves phase has a similar precipitation temperature, which is why it is included in the discussion.
1.1.1 σ mutually
The σ phase is a size factor compound with AB or AxBy configuration and a body-centered tetragonal structure. In ferritic stainless steels, the σ phases are mainly composed of FeCr or FeCrMo.
Under conditions where the Cr content (w(Cr)) is between 25% and 30% and the precipitation temperature is between 600 and 1050 ℃, the formation of the σ phase is facilitated. The formed phase enriches the Cr element, as shown in Figure 3.
The σ phase is non-magnetic and has high hardness, with a Rockwell hardness value (HRC) of up to 68. During the precipitation process, a “volume effect” occurs, which reduces the plasticity of the steel.
Fig. 3 Structure and composition of O phase of 447 ferritic stainless steel under linear EDX analysis
Precipitation of the σ phase can seriously weaken stainless steel, decreasing its properties such as corrosion resistance, impact resistance and mechanical properties.
The formation of the σ phase occurs in two stages: nucleation and growth. Nucleation typically begins at the α/α' grain boundary and expands from there into the matrix.
Once the σ phase reaches a certain size, it precipitates within the grain.
1.1.2 χ phase
Ultrapure ferritic stainless steel will not only form σ phase, but also σ phase when it contains a certain amount of Mo element.
The structure of the χ phase is body-centered cubic and of the α-Mn type.
In ferritic stainless steel, the χ phase is mainly composed of Fe36Cr12Mo10 or (Fe, Ni)36Cr18Mo4.
Typically, it forms under conditions where the Mo content (w) is between 15% and 25% and the temperature is between 600 and 900°C.
The toughness of steel decreases significantly when the χ phase is formed.
It was found that, compared with the σ phase, Cr and Mo are enriched faster in the χ phase and precipitate faster in the χ phase than in the σ phase.
Generally, the χ phase has the same structure as the ferrite matrix.
Due to its low nucleation potential barrier, nucleation is relatively simple, and the χ phase generally precipitates earlier than the σ phase, as shown in Fig.
Fig. 4 Precipitated χ phase of 26Cr ferritic stainless steel aged at 800 ℃ for 5 min
When the χ phase begins to form, there will be a significant enrichment of Cr and Mo in the χ phase, leading to a decrease in Cr and Mo content. This decrease is not enough to nucleate the σ phase, making the formation of the σ phase difficult in the early stage.
Furthermore, the χ phase is metastable and its stability decreases with aging time. As the χ phase decays, it will provide sufficient Cr and Mo to nucleate the σ phase, eventually leading to its transformation into a stable σ phase.
Both the χ phase and the σ phase will result in a reduction in the Cr content around the precipitation phase through precipitation, forming a Cr-poor zone and decreasing its corrosion resistance.
1.1.3 Washing Phase
The Laves phase is a size factor compound with AB2 configuration and hexagonal structure, as represented in Figure 5.
In ferritic stainless steel, the Laves phase is typically composed of Fe 2 Ti, Fe 2 Nb or Fe 2 Mo.
The Laves phase in ferritic stainless steel is enriched with Si elements, which play a crucial role in maintaining its stability.
The precipitation temperature of the Laves phase varies from 650-750°C, depending on the alloy composition.
Fig. 5 Precipitated Laves phase of 27Gr-4Mo-2Ni ferritic stainless steel after aging at 1050 ℃ for 1h
Andrade T et al. found that after aging at 850°C for 30 minutes, ultrapure ferritic stainless steel with model DIN 1.4575 shows Laves phase precipitation at the grain boundary, which remains unchanged in size due to the presence of both Laves phases and σ precipitates. The growth rate of the σ phase is faster, preventing the growth of part of the Laves phase.
It has been discovered that 11Cr-0.2Ti-0.4Nb ferritic stainless steel, when aged at 800°C for 24-28 hours, exhibits a large number of Laves phase precipitates that slowly increase over time. However, when the aging time reaches 96 hours, the transformation of the Laves phase becomes coarse and the number decreases, with no precipitation of the σ phase observed.
1.2 Main characteristics of 475 ℃ brittleness
Ferritic stainless steel with chromium mass fraction greater than 12% will experience a significant increase in hardness and strength, accompanied by a marked decrease in plasticity and impact resistance after prolonged exposure to temperatures between 340 and 516℃. This is mainly due to the brittleness that occurs in ferritic stainless steel at 475°C.
The most sensitive temperature for this property change is 475 ℃.
α' phase precipitation is the main reason for the brittleness of ferritic stainless steel at 475℃.
The α' phase is a Cr-rich brittle phase with a body-centered tetragonal structure.
In ferritic stainless steel, the α' phase is easy to form under the condition that w(Cr) is greater than 15% and the precipitation temperature is 371~550℃.
The α' phase is a FeCr alloy, with Cr content ranging from 61% to 83% and Fe content ranging from 17.5% to 37%.
The literature indicates that when the Cr content in steel is less than 12% by mass, there will be no precipitation of the α' phase, thus avoiding the formation of brittleness at 475°C.
Furthermore, precipitation of the α' phase during dissolution is a reversible process.
When steel is reheated above 516°C and then rapidly cooled to room temperature, the α' phase will dissolve back into the matrix and brittleness at 475°C will not occur again.
1.3 Main characteristics of high temperature brittleness
When the Cr content in ferritic stainless steel is between 14% to 30%, rapid cooling after heating the steel above 950°C can result in decreased elongation, impact strength and resistance to intergranular corrosion. This is mainly due to the brittleness of ferrite at high temperature.
The main cause of high-temperature brittleness is the precipitation of Cr-carbon and Cr-nitrogen compounds. Furthermore, during the welding process, precipitation of the Laves phase can occur when the welding temperature exceeds 950°C, impacting the general properties of the steel.
This vulnerability also exists in ultrapure ferritic stainless steel, which is even more sensitive to brittleness at high temperatures due to its high Cr and Mo content.
To reduce the risk of high-temperature brittleness, the C and N content can be reduced and stabilizing elements can be added.
In welding, high-temperature brittleness can result in significant damage to the steel. This occurs because the elements C and N precipitate at the grain boundary during welding and react with Cr and Mo, forming carbon and nitrides rich in Cr and Mo that gradually move toward the grain boundary.
Furthermore, precipitation of the Laves phase at 950°C during welding can lead to precipitation at dislocations, grain boundaries or within grains, inhibiting the movement of crystalline dislocations and grain boundaries. This results in the local arrangement of atoms becoming more regular, increasing the strength of the steel but reducing its plasticity and toughness.
2. Factors influencing brittle precipitates in ultrapure ferritic stainless steel
2.1 Alloy elements
The following elements – Cr, Mo, Ti, Nb, W and Cu – in ultrapure ferritic stainless steel have an impact on the formation of brittle precipitates.
An increased concentration of the Cr element in ferritic stainless steel leads to better passivation, resulting in better resistance to surface oxidation and better resistance to pitting corrosion, crevice corrosion and intergranular corrosion.
However, a higher mass fraction of Cr also leads to faster formation of brittle phases in ferritic stainless steel. The formation and precipitation rate of the α' and σ phases are also influenced by the mass fraction of Cr, with a higher mass fraction leading to a faster precipitation rate. This precipitation phase reduces the toughness of the steel and significantly increases its brittle transition temperature.
Mo is the second most important element in ferritic stainless steel. When its mass fraction reaches a certain level, the amount of precipitation of σ and χ phases in ferritic stainless steel increases significantly.
Research by Moura et al. found that the addition of Mo to 25Cr-7Mo ferritic stainless steel reduced the maximum precipitation temperature of the α' phase, lowering it from 475°C to about 400°C and increasing the number of α' phases.
Kaneko et al. found that Mo contributes to the faster accumulation of Cr in the passivation film, thereby improving film stability and strengthening the corrosion resistance of Cr in steel.
Ma et al. found that annealing 30Cr steel at 1020°C resulted in the precipitation of the Laves phase, which is mainly composed of Fe, Cr, Mo, Si and Nb. The mass fraction of Nb and Mo in the Laves phase was higher in relation to the base metal. The X-ray energy spectrum analysis of the Laves phase of 30Cr steel annealed at 1020°C is shown in Fig.
It was observed that increasing the Mo content in 30Cr ultrapure ferritic stainless steel accelerates the precipitation of the Laves phase. The literature suggests that an increased Mo content leads to the precipitation of the Mo-rich χ phase in 26Cr stainless steel after aging, and with prolonged aging time, part of the Laves phase transforms into the σ phase.
Fig. 6 X-ray energy spectrum (EDS) analysis of Laves phase of 30Cr steel after 1020 ℃ annealing
(a) Base metal EDS analysis; (b) EDS Analysis of the Laves Phase
The addition of stable elements, such as Nb and Ti, to steel combined with C and N results in the precipitation of phases such as TiN, NbC and Fe2Nb. These phases are distributed both within the grain and at grain boundaries, which delay the formation of Cr carbides and nitrides, thus increasing the intergranular corrosion resistance of ferritic stainless steels.
Anttila et al. studied the impact of incorporating Ti and Nb into 430 ferritic stainless steel welds. They found that when the welding temperature reached 950 ℃, the formation of the Laves phase was facilitated, leading to the embrittlement of the welded joints and a decrease in their resistance to impact.
Similarly, Naghavi and other researchers found that the solubility of Nb in the matrix of ferritic stainless steel decreases with increasing temperature during high-temperature aging, causing the Laves phase to thicken and a decrease in the steel's tensile strength.
It is found that the inclusion of W in 444 ferritic stainless steel significantly improves its high-temperature tensile strength when aged at 1000℃. However, as the mass fraction of W increases, the Laves phase becomes coarser, weakening the strengthening effect of precipitation and reducing the high-temperature tensile strength.
The addition of Cu to ferritic stainless steel precipitates a Cu-rich phase, which significantly improves the corrosion resistance of 430 Cu. Binary Fe-Cu alloys and ternary Fe-Cu-Ni alloys containing Cu can improve the strength and toughness of steel.
The Cu-rich phase mainly precipitates at 650℃ and 750℃, and during the initial aging stage, it remains spherical. As the temperature and aging time increase, it gradually transforms into an elliptical and rod-shaped shape, as illustrated in Figure 7.
Fig. 7 Morphology of Cu-rich phase in ferritic stainless steel 17Cr-0.86Si-1.2Cu-0.5Nb aged at 750 °C for 1h
2.2 Rare earth elements
Rare earth elements (REs) are highly chemically reactive and adding the appropriate amount of REs can improve the properties of steel.
The TEM test results of precipitates on 27Cr ferritic stainless steel are shown in Fig.
Without REs, the precipitated phases in ferritic stainless steel are more complex. As illustrated in Figure 8 (a), secondary phases precipitate at the grain boundaries and form chains in the ferrite matrix, mainly consisting of the σ phase, M23C6, M6C and a small amount of M2N and χ phases.
However, after the addition of REs, the precipitated phases of the chain decrease and are often present in unique forms in the matrix, mainly as the σ phase. Furthermore, carbon and nitride precipitation decreases, as shown in Fig. 8(b).
The ideal RE mass fraction in ultrapure ferritic stainless steel was 0.106%, which improves the reinforcing properties. At this concentration, REs refine grain structure, increase impact energy, and change the impact fracture mechanism from brittle to tough.
Furthermore, REs reduce the mass fraction of S in steel, reducing the source of pitting corrosion and improving resistance to pitting corrosion.
Fig. 8 TEM results of the precipitated phase of 27Cr ferritic stainless steel
(a) Bright field image of 0% RE sample; (b) Bright field image of the 0.106% RE sample
2.3 Treatment of aging
Different aging treatments can have varying impacts on the formation of brittle precipitates in materials.
When pure ferritic stainless steel forms brittle precipitates, it can result in a decline in its mechanical properties, impact resistance, corrosion resistance and overall performance.
Aging treatment can help improve the material's structure and increase its plasticity, as well as effectively reduce the formation of precipitates and limit their negative effects on steel.
LU HH et al. discovered that when 27Cr-4Mo-2Ni ferritic stainless steel is aged at temperatures ranging from 600 to 800°C, the main precipitates formed are the χ phase, the Laves phase and the σ phase.
The morphologies and distributions of these phases in 27Cr-4Mo-2Ni ferritic stainless steel aged at different temperatures are represented in Figure 9.
The presence of these precipitates can reduce the impact toughness, tensile strength and plasticity of the material, while increasing its hardness.
After aging at temperatures between 600 and 800°C, the χ phase precipitates mainly along grain boundaries. The Laves phase is precipitated within the grain when the material ages at 700°C, while the σ phase generally forms at the grain boundaries after aging at 750°C.
At this point, the Laves phase partially dissolves into the matrix, providing Cr and Mo atoms for the growth of the σ phase. This coarsening of the grain can lead to brittle fracture of the steel.
Fig. 9 Morphology and distribution of the x phase, Laves phase and o phase of 27Cr-4Mo-2Ni ferritic stainless steel aged at different temperatures
(a) Aging at 650 ℃ for 4h; (b) Aging at 700 ℃ for 4h; (c) Aging at 750℃ for 2h; (d) Aging at 800 ℃ for 4h.
Zhang Jingjing found that when ultrapure ferritic stainless steel SUS444 was aged at 850 ℃ for 10 minutes, TiN transformed into a TiN/NbC/Nb phase-poor composite structure. The bond strength between the composite structure and the matrix is high, which significantly improves the impact resistance.
Luo Yi and colleagues found that when ultrapure ferritic stainless steel 446 was aged at 800℃, the σ phase precipitated after 0.5 hours and increased with aging time, forming a network-like structure. Simultaneously, microcracks appeared in the σ phase and their large number reduced the toughness of the steel.
Ma Li and others annealed ultrapure ferritic stainless steel with 26% Cr and found that there were mainly three precipitates: TiN, NbC and χ. The harmful χ phase seriously led to the brittleness of the steel. With increasing annealing temperature up to 1020°C, the χ phase gradually decreased to a negligible amount. Therefore, to eliminate the χ phase, a high annealing temperature is required.
For high Cr ferritic stainless steel 27.4Cr-3.8Mo-2.1Ni, QUHP and others found that after aging at 950°C for 0.5 hours, the σ and Laves phases precipitated, improving the hardness of the steel, but decreasing its ductility. These harmful phases can be dissolved in the matrix after treating the solution at 1100°C for 0.5 hours.
Wu Min and colleagues found that when the 441 hot-rolled plate was annealed at 900-950 ℃, a large number of Laves phases precipitated. As shown in Figure 10, there are two precipitated phases: (1) the primary phase, which is a composite structure of (Ti, Nb)(C, N) with a size of approximately 5 μm and (2) the Laves phase, which is small, numerous, dense, and evenly distributed at grain boundaries, subgrain boundaries, and grains. Increasing the annealing temperature to 1000-1050°C effectively eliminated the Laves phase, but a small amount of the Nb(C,N) phase precipitated.
Fig. 10 Laves phase morphology of 441 ferritic stainless steel hot-rolled plate after different annealing temperatures
(a) Appearance of Laves phase after annealing at 900 ℃; (b) Appearance of the Laves phase after annealing at 950 ℃.
3. Effect of brittleness on the properties of ultrapure ferritic stainless steel
3.1 Effect of brittleness on mechanical properties
Research shows that high levels of Cr and Mo and a certain amount of Nb in the microstructure can easily lead to the formation of brittle intermetallics such as σ-type phase (Fe Cr Mo), χ-type phase (Fe Cr Mo). , and the Laves phase of the Fe2Nb type. These brittle intermetallics result in a significant decrease in the toughness of the plastic and an increase in the hardness of the ultrapure ferritic stainless steel.
German scholar Saha R and colleagues found that the low solubility of element C causes ferritic stainless steel to precipitate high-hardness C (Ti, Nb) during high-temperature cooling, and dispersed C (Ti, Nb) improves the strength and hardness of steel.
The research also found that the two-phase Cr23C6 and Cr2N particles in the alloy have a strong impact on mechanical properties, particularly toughness and ductility, leading to a reduction in toughness and ductility and an increased risk of fracture.
The typical precipitation of the α' phase leads to the depletion of Cr in the ferrite matrix, reducing the corrosion resistance and toughness of the steel and increasing its hardness.
It was found that when 444 ferritic stainless steel ages at temperatures between 400-475℃, the precipitation of the α' phase leads to an increase in hardness, but after aging for more than 500 hours at 475℃, its toughness drops drastically.
Figure 11 shows the hardness of ultrapure ferritic stainless steel 441 and the energy absorbed by fracture after aging.
Fig. 11 Change in hardness and energy absorbed by fracture of ultra pure ferritic stainless steel 441 with time after aging at 400 ℃ and 450 ℃
(a) Hardness changes with aging time; (b) The energy absorbed by the fracture varies with aging time.
Luo Yi and colleagues found that the tensile strength of ultrapure ferritic stainless steel 446 can be improved to a certain extent when the σ-phase network structure has not formed after aging treatment.
However, when the σ-phase precipitation forms a network structure, the tensile strength and elongation of the material decrease significantly, as illustrated in Figure 12.
Furthermore, regardless of whether a network structure is formed, the precipitation of the σ phase causes severe damage to the impact property of the material, leading to a decrease in its impact property and failing to meet certain requirements for steel.
Fig. 12 Change in tensile strength and elongation of ultra pure ferritic stainless steel 446 with time after aging at 800 ℃
Laves phase precipitation in ultrapure ferritic stainless steel has positive and negative impacts.
According to the literature, with prolonged aging time, the Fe2Nb phase will begin to precipitate in the steel, causing a decrease in its toughness and resistance to high temperatures.
However, the addition of Si and Nb elements to the Laves phase precipitation leads to an increase in the creep strength and high temperature strength of the steel. The presence of W in the Laves phase also helps to improve the tensile strength of steel at high temperatures.
As illustrated in Fig. 13, compared with non-W type 444 ferritic stainless steel, the tensile strength is significantly improved when the mass fraction W is between 0.5% and 1%.
When aging at 900℃, the tensile strength decreases slightly with increasing aging time, but eventually stabilizes. At 1000℃, the tensile strength may decrease significantly, but the initial tensile strength remains higher than that of non-W steel.
Fig. 13 Variation of high temperature tensile strength of 444 ferritic stainless steel with aging time at 900C and 1000°C
（a）900℃; （b）1000℃。
The Laves phase will precipitate from 441 ferritic stainless steel during aging at 850℃ and grow rapidly. When it forms a network structure along the grain boundary, it reduces the plasticity and impact resistance of the steel. As the number of grain boundaries decreases and the grain size increases, the precipitation rate decreases.
The mechanical properties of 19Cr-2Mo Nb Ti ferritic stainless steel at different aging temperatures are shown in Fig. 14. During the steel aging process at temperatures between 850 ℃ and 1050 ℃, the (FeCrSi)2(MoNb) and ( Fe , Laves phases of the Cr)2(Nb, Ti) type will transform into precipitates (Nb, Ti)(C, N). The mass fraction of Nb in the solution will increase due to the dissolution and thickening of the precipitates, leading to a reduction in its tensile strength.
However, after aging treatment at 950 ℃, the homogeneity of recrystallized grains is improved and the elongation increases sharply, reaching 37.3%. It then gradually stabilizes at 32.6%.
Fig. 14 Mechanical properties of 19Cr-2Mo-Nb-Ti ferritic stainless steel at different aging temperatures
3.2 Effect of brittleness on corrosion resistance
It has been found that precipitation of the brittle phase will have a negative impact on the corrosion resistance of steel.
Furthermore, according to the literature, the high Cr mass fraction of ultrapure ferritic stainless steel 27.4Cr-3.8Mo leads to the formation of σ and χ phases after aging at 950°C for 0.5 hours, resulting in a decrease in pitting resistance.
However, aging at 1100°C for 0.5 hours causes the σ and χ phases to gradually disappear and the pitting resistance to recover. The change in corrosion potential is illustrated in Figure 15.
Fig. 15 Corrosion potential of stainless steel 24.7Cr-3.4Mo and 27.4cr-3.8Mo
The chromium (Cr) and molybdenum (Mo) content in stainless steel plays a crucial role in its corrosion resistance. When the mass fraction of Cr exceeds 25% and the temperature is between 700-800°C, precipitation of the σ and χ phases will occur, leading to a decrease in corrosion resistance.
Furthermore, Cr combines easily with carbon (C) and nitrogen (N) elements, causing precipitation at the grain boundary or within the grain. This leads to the formation of Cr-rich carbon and nitride, reducing Cr mass fraction and corrosion resistance. Precipitates also harm the passivation film, causing it to lose its uniformity and stability, affecting the corrosion resistance of the steel.
Welded joints in corrosive environments are prone to intergranular corrosion, pitting corrosion, crevice corrosion, and other types of local corrosion. Researchers such as Huang Zhitao found that increasing the mass fraction of Mo in high-purity ferritic stainless steel in chloride environments can delay the precipitation of M23C6 (where M is Fe, Cr and Mo) and improve pitting corrosion resistance.
Zhang Henghua et al. found that adding a certain amount of Mo to 26Cr ultrapure ferritic stainless steel can enrich Cr in the passivation film and increase its stability, thereby improving the pitting corrosion resistance of the material. Tong Lihua et al. found that adding niobium (Nb) and titanium (Ti) to ultrapure ferritic stainless steel can effectively prevent the precipitation of carbon and nitrogen Cr compounds and increase its resistance to intergranular corrosion.
However, other studies have shown that high levels of Ti and N in 15Cr ultrapure ferritic stainless steel can lead to the formation of TiN, which accelerates the growth of pitting corrosion and negatively impacts the material's corrosion resistance. Wen Guojun and colleagues found that aging 430Ti ferritic stainless steel at 475°C for 0-100 hours leads to an increase in hardness, both α' and α phases, and a significant decrease in corrosion resistance, as shown in Figure 16.
Fig. 16 Corrosion resistance of 430Ti ferritic stainless steel
In conclusion, the higher the mass fraction of Cr in ultrapure ferritic stainless steel, the greater the likelihood of producing precipitates that severely reduce its corrosion resistance. The addition of adequate amounts of niobium (Nb), titanium (Ti) and molybdenum (Mo) can improve the corrosion resistance of steel, however, the formation of TiN from Ti has a negative impact on the pitting corrosion resistance of steel.
4. Conclusion and perspective
The main characteristics and factors influencing σ-phase brittleness, 475°C brittleness and high-temperature brittleness of ultrapure ferritic stainless steel are analyzed in this article. The following conclusions are drawn:
(1) The brittleness of the σ phase in ultrapure ferritic stainless steel is due to the precipitation of the σ phase and χ phase, which are rich in chromium and molybdenum elements. The brittleness at 475°C is due to precipitation of the chromium-rich α' phase. High-temperature brittleness is caused by the precipitation of carbon and chromium nitride.
(2) Alloying elements, rare earth elements (RE) and aging treatments in ultrapure ferritic stainless steel have a certain impact on the precipitated phases, which can, to a certain extent, inhibit the generation of brittleness of the σ phase, 475 °C brittleness and brittleness at high temperature.
The following are the specific impacts:
① The precipitation of α', σ, χ and Laves phases increases when the content of Cr and Mo increases. In ultrapure ferritic stainless steel, the addition of stabilizing elements can reduce or eliminate brittleness at high temperatures in thin sections. High temperature brittleness can be avoided by avoiding high temperatures during heat treatment. The addition of Ti and Nb can also delay the precipitation of the σ phase, reducing its brittleness. However, the addition of Ti and Nb leads to the generation of the Laves phase, and a high Nb content can cause thickening of the Laves phase.
② The addition of RE reduces the precipitation of carbon and nitride in the σ and Cr phases, reducing the brittleness of the σ phase and high-temperature brittleness, and improving the mechanical properties and corrosion resistance of steel.
③ Different aging treatments have varying effects on precipitates. Precipitates may differ slightly based on Cr content. When aging at 600-800 ℃, a small amount of σ, χ and Laves phases precipitate. At 600℃, the α' phase dissolves back into the matrix and the brittleness disappears at 475℃. A large number of σ, χ and Laves phases precipitate when aging at 850-950 ℃. When aging at 1000-1100 ℃, the precipitation of σ, χ and Laves phases is reduced or even disappears. The brittleness of σ phase can be eliminated by aging treatment above 1000℃.
(3) Precipitation of secondary phases such as α', σ, χ and Laves in ultrapure ferritic stainless steel can have a significant impact on its mechanical and corrosion properties. The precipitation of these phases reduces the tenacity and plasticity of the steel, increases its strength and hardness and affects its resistance to corrosion.
The addition of Si and W elements to the Laves phase increases its high temperature resistance and tensile strength. Furthermore, the addition of Cu elements results in the precipitation of the Cu-rich phase, which improves the toughness of the steel.
Domestic Ni resources are scarce and excessive consumption may lead to shortages, which will have a severe impact on the stainless steel industry.
Ultrapure ferritic stainless steel, as a resource-saving steel, has high comprehensive performance and low comprehensive cost, making it an inevitable choice for the domestic stainless steel industry to promote low nickel 400 series stainless steel.
Ultrapure ferritic stainless steel has gradually replaced some austenitic stainless steels in industries such as automotive, household appliances and elevators. It has also been used successfully in the construction of roofs for large buildings, such as airports and stadiums.
The ultrapure ferritic stainless steel market is expected to grow in the future, with large market scale and broad prospects.
In the future, it is crucial to focus on the brittleness of ultrapure ferritic stainless steel. To ensure good mechanical properties and corrosion resistance, it is necessary to effectively restrict the generation of σ-phase brittleness, 475℃ brittleness and high-temperature brittleness during production and use. By doing so, the advantages of “resource saving” can be fully utilized, leading to greater progress and development in the stainless steel industry.
