Ultrapure Ferritic Stainless Steel: Effect of Brittleness and Formation on Properties

Ferritic stainless steel usually refers to stainless steel with Cr mass fraction of 12%~30%.

According to different Cr mass fraction, it can be divided into three types: low Cr, medium Cr and high Cr.

Generally speaking, the corrosion resistance of ferritic stainless steel is related to the Cr mass fraction.

The higher the Cr mass fraction, the stronger the corrosion resistance.

In order to improve the comprehensive properties of materials and avoid the adverse effects of Cr carbide and nitride precipitation on the mechanical properties and corrosion resistance of steel, ferritic stainless steel is developing towards low C and N at this stage.

Ultrapure ferritic stainless steel belongs to a kind of ferritic stainless steel.

Its content of C and N elements is extremely low (the sum of the mass fractions of C and N elements generally does not exceed 0.015%) and has medium to high Cr mass fractions.

Because of its good corrosion resistance, thermal conductivity, seismic resistance and processing performance, and its relatively low price compared with Cu, Cu alloys and Ti materials, it is widely used in the automotive industry, kitchen appliances and household appliances, construction industry, petrochemical industry and other fields.

Although the performance of ultra pure ferritic stainless steel is superior, there are also many problems in its production process.

Due to the high mass fraction of Cr element and other alloy components such as Mo and Mn, it is difficult to avoid the inherent problems of high Cr ferrite stainless steel such as σ phase brittleness, 475 ℃ brittleness and high temperature brittleness.

Therefore, the production personnel attach great importance to the harm of these brittleness to the ultra pure ferritic stainless steel, and find that the precipitation of σ phase, χ phase, α ‘phase, Laves phase.

Carbon and nitride and the mass fraction of Cr element are the main reasons for the formation of brittleness.

In this article, the main characteristics and influencing factors of σ-phase brittleness, 475 ℃ brittleness and high temperature brittleness of ultra pure ferritic stainless steel are described in detail, and the effects of the above brittleness on the mechanical properties and corrosion resistance of ultra pure ferritic stainless steel are discussed and analyzed, so as to provide reference for production and users.

1. Main characteristics of brittleness of ultra pure ferritic stainless steel

Ultrapure ferritic stainless steel contains a variety of alloy elements, and it is easy to precipitate different types of intermetallic compounds during hot working, mainly carbon and nitrogen compounds of Cr, Nb and Ti, as well as intermetallic compounds of phase σ, χ, Laves and α ‘.

The characteristics of phase σ, phase χ, phase Laves and phase α ‘are shown in Table 1.

Table 1 Characteristics of Intermetallic Compounds in Ultrapure Ferritic Stainless Steel

Precipitated phaseStructureConfiguration and compositionPrecipitation conditionCharacteristic
σ mutuallyBody centered tetragonal (bct) D8b, 30 atoms/unit cellAB or AxBy, FeCrFeCrMow(Cr)=25%~30%,600-1050℃Hard, brittle, rich in Cr
X phaseBody centered cubic (bcc) A12, 30 atoms/unit cellα- Mn, Fe36Cr12Mo10 or (Fe, Ni) 36Cr18Mo4w(Mo)=15%~25%,600-900℃Hard, brittle, rich in Cr and Mo
Laves phaseClose packed hexagonal (hcp) C14 or C36AB2, Fe2Ti or Fe2Nb or Fe2Mo650-750℃Hard
α’ mutuallyBody centered cubic (bcc)Fe Cr, rich in crw(Cr)>15%,371-550℃(475℃)Hard, brittle, rich in Cr

The precipitation “C” curves of σ, χ and Laves phases of some typical ultra pure ferritic stainless steels are shown in Fig. 1 and Fig. 2.

Due to the difference of alloy composition, the most sensitive temperature for precipitation of these phases is 800~850 ℃.

For 00Cr25Ni4Mo4NbTi (Monit) alloy, phase σ and phase χ precipitate relatively quickly, while Laves phase precipitates most easily at 650 ℃ and takes more time to precipitate.

No matter what kind of brittle precipitates are, excessive precipitation will make the steel brittle, which will lead to a sharp decline in impact properties.

Ultrapure Ferritic Stainless Steel: Effect of Brittleness and Formation on Properties 1

Fig. 1 26% Gr – (1%~4%) Mo – (0~4%) Ni Ferritic Stainless Steel

Ultrapure Ferritic Stainless Steel: Effect of Brittleness and Formation on Properties 2

Fig. 2 TTP Diagram of 00Cr25Ni4Mo4TiNb (Monit) Ferritic Stainless Steel (after solid solution at 1000 ℃)

1.1 Main characteristics of phase σ brittleness

The generation of σ-phase brittleness is mainly due to the precipitation of σ-phase and χ-phase, while Laves phase has a similar precipitation temperature with them, so it is discussed here.

1.1.1  σ mutually

σ-Phase is a size factor compound, its configuration is AB type or AxBy type, and its structure is body centered tetragonal. In ferritic stainless steels, σ- phases are mainly FeCr or FeCrMo.

In general, σ- phase is easy to form under the conditions of w (Cr)=25%~30% and precipitation temperature of 600~1050 ℃, and the formed phase will enrich the Cr element, as shown in Fig. 3.

σ- phase is not magnetic and has high hardness. The Rockwell hardness (HRC) can reach 68HRC. The precipitation process will be accompanied by “volume effect”, and the plasticity of the steel will be reduced.

Ultrapure Ferritic Stainless Steel: Effect of Brittleness and Formation on Properties 3

Fig. 3 Structure and composition of o phase of 447 ferritic stainless steel under EDX linear analysis

σ- precipitation of phase will lead to serious embrittlement of stainless steel, which will reduce its comprehensive properties such as corrosion resistance, impact toughness and mechanical properties.

The precipitation of σ phase can be divided into two stages, namely nucleation and growth.

The place where phase σ starts to nucleate is generally the grain boundary of α/α’, and grows and expands from the grain boundary to the interior of the matrix.

When phase σ grows to a certain size, it will precipitate from the interior of the grain.

1.1.2 Phase χ

Ultrapure ferritic stainless steel will not only precipitate σ phase, but also χ phase when containing a certain amount of Mo element.

The structure of χ phase is body centered cubic, which is of α-Mn type.

In ferritic stainless steel, the χ phase is mainly Fe36Cr12Mo10 or (Fe, Ni) 36Cr18Mo4.

Generally, it will be formed under the condition that w (Mo) is 15%~25% and the temperature is 600~900 ℃.

The toughness of the steel will be significantly reduced when the χ phase is precipitated.

It was found that compared with phase σ, Cr and Mo enriched more rapidly in phase X and precipitated more rapidly in phase χ than in phase σ.

Generally speaking, χ phase has the same structure as ferrite matrix.

Due to its low nucleation potential barrier, nucleation is relatively simple, so χ phase usually precipitates earlier than σ phase, as shown in Fig. 4.

Ultrapure Ferritic Stainless Steel: Effect of Brittleness and Formation on Properties 4

Fig. 4 χ Phase Precipitated from 26Cr Ferritic Stainless Steel Aged at 800 ℃ for 5min

When the χ phase starts to produce, a large amount of Cr and Mo will be enriched in the χ phase, resulting in a decrease in the content of Cr and Mo, which is not enough to nucleate the σ phase, so the formation of the σ phase is relatively difficult at the initial stage.

In addition, χ phase has metastability. Affected by aging time, χ phase will gradually decompose, providing enough Cr and Mo to nucleate σ phase, and finally gradually transform into a stable σ phase.

No matter χ phase or σ phase, the precipitation will reduce the Cr content around the precipitation phase, forming a Cr poor zone, resulting in a decline in its corrosion resistance.

1.1.3 Laves phase

Laves phase is also a size factor compound, its configuration is AB2 type, and its structure is hexagonal, as shown in Fig. 5.

In ferritic stainless steel, Laves phase is mainly Fe2Ti or Fe2Nb or Fe2Mo.

Si element in ferritic stainless steel will be enriched in Laves phase, which plays an important role in its stability.

The precipitation temperature of Laves phase is generally 650-750 ℃ according to the content of alloy composition.

Ultrapure Ferritic Stainless Steel: Effect of Brittleness and Formation on Properties 5

Fig. 5 Laves Phase Precipitated from 27Gr-4Mo-2Ni Ferritic Stainless Steel after Aging at 1050 ℃ for 1h

Andrade T et al. found that after aging at 850 ℃ for 30 min, the ultra pure ferritic stainless steel with the model of DIN 1.4575 can observe the precipitation of Laves phase at the grain boundary, and its size is basically unchanged during aging, mainly because the precipitates at the grain boundary not only contain the same Laves, but also contain σ phase, and the growth rate of σ phase is faster than Laves phase, which will prevent part of Laves phase from growing.

It is found that a large number of Laves phase precipitates are observed in the ferritic stainless steel 11Cr-0.2Ti-0.4Nb aged at 800 ℃ for 24-28h, and the number increases slowly;

When the aging time reaches 96h, the Laves phase transformation is coarse, the number is reduced, and no σ phase is precipitated.

1.2 Main characteristics of 475 ℃ brittleness

The ferritic stainless steel with Cr mass fraction greater than 12% will have a significant increase in hardness and strength, accompanied by a sharp decline in plasticity and impact toughness, after long-term heat preservation in the temperature range of 340~516 ℃, which is mainly caused by the brittleness of the ferritic stainless steel at 475 ℃.

The most sensitive temperature for this property change is 475 ℃.

The precipitation of α ‘phase is the main reason for the 475 ℃ brittleness of ferritic stainless steel.

α ‘phase is a Cr rich brittle phase with a body centered tetragonal structure.

In ferritic stainless steel, α ‘phase is easy to form under the condition that w (Cr) is greater than 15% and precipitation temperature is 371~550 ℃.

α’ phase is a Fe Cr alloy, with Cr content ranging from 61% to 83% and Fe content ranging from 17.5% to 37%.

The literature points out that when the mass fraction of Cr element in steel is less than 12%, there will be no precipitation of α ‘phase, which fundamentally avoids the generation of 475 ℃ brittleness.

In addition, the precipitation behavior of α ‘phase in the dissolution process is a reversible process.

When the temperature of the steel is reheated to above 516 ℃ and rapidly cooled until the temperature is consistent with the room temperature, α’ phase will dissolve into the matrix, and the brittleness at 475 ℃ will not occur again.

1.3 Main characteristics of high temperature brittleness

When the mass fraction of Cr element in ferritic stainless steel is 14% ~ 30%, heating it to above 950 ℃ and then cooling it rapidly will easily lead to the decline of elongation, impact toughness and intergranular corrosion resistance of the steel, which is mainly caused by the high temperature brittleness of ferrite.

The precipitation of carbon and nitrogen compounds of Cr is the main reason for high temperature brittleness.

In addition, in the welding process, when the welding temperature reaches above 950 ℃, Laves phase will also precipitate in the ferritic stainless steel, affecting its comprehensive properties.

This hazard also exists in ultra pure ferritic stainless steel, and because it contains high Cr and Mo elements, it will make it more sensitive to high temperature brittleness.

The hazard of high temperature brittleness can be reduced by reducing the content of C and N elements and adding stable elements.

During welding, high temperature brittleness will cause serious damage to the steel.

On the one hand, because elements C and N precipitate at the grain boundary during welding and react with elements Cr and Mo, forming carbon and nitride rich in Cr and Mo and gradually moving towards the grain boundary;

On the other hand, Laves phase often precipitates when the welding temperature reaches 950 ℃.

These precipitates will appear at dislocations, grain boundaries or within grains, prevent the movement of crystal dislocations and grain boundaries, make local atoms still arrange regularly, improve the strength of steel, and reduce the plasticity and toughness.

2. Influencing factors of brittle precipitates in ultra pure ferritic stainless steel

2.1 Alloy elements

The alloy elements Cr, Mo, Ti, Nb, W and Cu in the ultra pure ferritic stainless steel have certain effects on the brittle precipitates.

The higher the content of Cr element in ferritic stainless steel, the easier it is to be passivated, which will make the surface of ferritic stainless steel not easy to be oxidized, so it has better corrosion resistance, and the resistance to pitting, crevice corrosion and intergranular corrosion will also be improved;

At the same time, the higher the mass fraction of Cr, the faster the formation of brittle phase in ferritic stainless steel.

In addition, the formation and precipitation speed of α ‘and σ phases are also related to the mass fraction of Cr.

The higher the mass fraction of Cr, the faster the precipitation speed.

The precipitation phase will reduce the toughness of the steel and significantly increase the brittle transition temperature.

Mo is the second important element in ferritic stainless steel. When the mass fraction of Mo reaches a certain proportion, the precipitation amount of phase σand phase χ in ferritic stainless steel increases significantly;

Moura L B et al. found that in the ferritic stainless steel of 25Cr-7Mo, the addition of Mo reduced the maximum precipitation temperature of α’phase, reduced the temperature from 475 ℃ to about 400 ℃, and increased the number of α ‘phase.

Kaneko M et al. found that Mo can make Cr accumulate more quickly in the passivation film, improve the stability of the passivation film, and strengthen the corrosion resistance of Cr in steel;

Ma L et al. found that after annealing at 1020 ℃, the 30Cr steel will precipitate Laves phase, which is mainly composed of Fe, Cr, Mo, Si and Nb.

Compared with the base metal, the mass fraction of Nb and Mo in Laves phase is higher. The X-ray energy spectrum analysis of Laves phase of 30Cr steel annealed at 1020 ℃ is shown in Fig. 6.

It can be seen that in 30Cr ultra pure ferritic stainless steel, the increase of Mo content will accelerate the precipitation of Laves phase.

It is pointed out in the literature that, with the increase of Mo content, in addition to σ-phase and Laves phase, there will also be Mo rich χ-phase precipitated in the 26Cr stainless steel after aging, and with the extension of aging time, part of Laves phase will gradually transform into σ-phase.

Ultrapure Ferritic Stainless Steel: Effect of Brittleness and Formation on Properties 6

Fig. 6 X-ray Energy Spectrum Analysis (EDS) of Laves Phase of 30Cr Steel after 1020 ℃ Annealing

(a) EDS analysis of base metal; (b) EDS Analysis of Laves Phase

The combination of stable elements such as Nb and Ti added in steel with C and N will precipitate phases such as TiN, NbC and Fe2Nb, which are distributed in the grain interior and grain boundary, slowing the formation of Cr carbides and nitrides, thus enhancing the intergranular corrosion resistance of ferritic stainless steels;

Anttila S et al. studied the effect of adding Ti and Nb on the welds of 430 ferritic stainless steel. When the welding temperature reaches 950 ℃, Laves phase is easily formed, which leads to the embrittlement of welded joints and the reduction of the impact toughness of joints.

In addition, Naghavi S and other researchers found that the solubility of Nb element in the matrix of ferritic stainless steel decreases with the increase of temperature during high temperature aging, which is easy to lead to coarsening of Laves phase and decrease the tensile strength of ferritic stainless steel.

It is found that the 444 ferritic stainless steel containing W element can significantly improve the high temperature tensile strength of the steel when aged at 1000 ℃, but with the increase of W mass fraction, the Laves phase will be seriously coarsened, the precipitation strengthening effect will be weakened, and the high temperature tensile strength will be reduced.

When the ferritic stainless steel contains Cu, it will precipitate Cu rich phase, which can significantly improve the corrosion resistance of 430 Cu.

Fe Cu binary alloy and Fe Cu Ni ternary alloy containing Cu can improve the strength and toughness of steel.

The Cu rich phase is mainly precipitated at 650 ℃ and 750 ℃.

In the initial aging stage, the Cu rich phase remains spherical.

With the increase of aging temperature and time, it will gradually become elliptical and rod-shaped, as shown in Fig. 7.

Ultrapure Ferritic Stainless Steel: Effect of Brittleness and Formation on Properties 7

Fig. 7 Morphology of Cu rich phase in 17Cr-0.86Si-1.2Cu-0.5Nb ferritic stainless steel aged at 750 °C for 1h

2.2 Rare earth elements

Rare earth element (RE) is highly chemically active, and adding an appropriate amount of RE can effectively optimize the properties of steel.

The TEM test results of precipitates in 27Cr ferritic stainless steel are shown in Fig. 9.

When RE is not added, the precipitated phases in the ferritic stainless steel are more complex.

As shown in Fig. 8 (a), the secondary phase will precipitate at the grain boundary and appear in the ferrite matrix as a chain, mainly including σ phase, M23C6, M6C, and a small amount of M2N and χ phases.

After the addition of RE, the chain precipitated phase gradually decreases, and it often appears in a single form in the matrix, mainly as phase σ.

At the same time, the precipitation of carbon and nitride decreases, as shown in Fig. 8 (b).

When the mass fraction of RE in ultra pure ferritic stainless steel is 0.106%, it will play a better strengthening role.

At this time, the addition of RE will refine the grains, improve the impact energy, and change the impact fracture mechanism from brittleness to toughness;

In addition, the addition of RE can also reduce the mass fraction of S in steel, effectively reduce the source of pitting corrosion, and improve the pitting corrosion resistance.

Ultrapure Ferritic Stainless Steel: Effect of Brittleness and Formation on Properties 8

Fig. 8 TEM Results of Precipitated Phase of 27Cr Ferritic Stainless Steel

(a) Bright field image of 0% RE sample; (b) Bright field image of 0.106% RE sample

2.3 Aging treatment

Different aging treatments have different effects on brittle precipitates.

When super pure ferritic stainless steel produces brittle precipitates, its mechanical properties, impact properties, corrosion resistance and other comprehensive properties will deteriorate.

Aging treatment can not only improve the structure and improve plasticity, but also effectively reduce the precipitation of precipitates and reduce its harm to steel.

LU HH et al. found that when 27Cr-4Mo-2Ni ferritic stainless steel is aged at 600~800 ℃, there are mainly χ phase, Laves phase and σ phase precipitates.

The morphology and distribution of each phase of 27Cr-4Mo-2Ni ferritic stainless steel aged at different temperatures are shown in Fig. 9.

These precipitates will lead to the decrease of impact toughness, tensile strength and plasticity and the increase of hardness.

χ phase is mainly precipitated along the grain boundary after aging at 600~800 ℃.

Laves phase is precipitated in the grain when aging at 700 ℃, while σ phase is generally precipitated at the grain boundary after 750 ℃.

At this time, Laves phase will partially dissolve into the matrix, providing Cr and Mo atoms for the growth of σ phase, coarsening the grain, and causing brittle fracture of steel.

Ultrapure Ferritic Stainless Steel: Effect of Brittleness and Formation on Properties 9

Fig. 9 Morphology and Distribution of 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 SUS444 ultra pure ferritic stainless steel aged at 850 ℃ for 10 min, TiN will slowly transform into a composite structure of TiN/NbC/Nb poor phase.

The interface between the composite structure and the matrix has a high bonding strength, which will greatly improve the impact toughness.

Luo Yi et al. found that when 446 ultra pure ferritic stainless steel was aged at 800 ℃, phase σ would precipitate after 0.5h, and the precipitation of phase σ would slowly increase with the aging time, forming a network like structure.

At the same time, microcracks gradually appeared in phase σ, and these large number of network phase σ would seriously reduce the toughness of the steel.

When Ma Li and others annealed 26% Cr ultra pure ferritic stainless steel, they found that there are mainly three typical precipitates, namely, TiN, NbC and χ, and the harmful phase χ will seriously lead to the brittleness of the steel.

With the increase of the annealing temperature, up to 1020 ℃, the χ phase will gradually decrease to very little.

Therefore, if the χ phase is to disappear, it is necessary to provide a sufficiently high annealing temperature.

For high Cr ferritic stainless steel 27.4Cr-3.8Mo-2.1Ni, QUHP and others found that after aging at 950 ℃ for 0.5h, σ and Laves phases would be precipitated, which improved the hardness of the steel, but decreased its ductility.

These harmful phases can be dissolved into the matrix after solution treatment at 1100 ℃ for 0.5h.

Wu Min et al. found that a large number of Laves phase precipitated when 441 hot-rolled plate was annealed at 900~950 ℃.

As shown in Figure 10, there are two kinds of precipitated phases:

One is the primary phase, which is a composite structure of (Ti, Nb) (C, N) with a size of about 5 μ m;

The other is Laves phase, which is in small point shape, numerous and dense, and uniformly distributed in grain boundaries, subgrain boundaries and grains.

In addition, after the annealing temperature is increased to 1000~1050 ℃, Laves phase can be effectively eliminated, but a small amount of Nb (C, N) phase will be precipitated.

Ultrapure Ferritic Stainless Steel: Effect of Brittleness and Formation on Properties 10

Fig. 10 Laves Phase Morphology of 441 Ferritic Stainless Steel Hot Rolled Plate after Different Annealing Temperatures

(a) Laves phase appearance after annealing at 900 ℃; (b) Laves phase appearance after annealing at 950 ℃.

3. Effect of brittleness on properties of ultra pure ferritic stainless steel

3.1 Effect of brittleness on mechanical properties

The research shows that due to the high content of Cr and Mo and a certain amount of Nb, it is easy to form several brittle intermetallics in the microstructure, such as (Fe Cr Mo) type σ phase, (Fe Cr Mo) type χ phase and Fe2Nb type Laves phase.

These brittle intermetallics will lead to a significant reduction in the plastic toughness and an increase in the hardness of ultra pure ferritic stainless steel.

German scholar Saha R et al. found that due to the low solubility of C element, ferritic stainless steel will precipitate high hardness (Ti, Nb) C during high temperature cooling, and the dispersed (Ti, Nb) C will improve the strength and hardness of ferritic stainless steel.

In addition, it is found that the two-phase particles Cr23C6 and Cr2N in the alloy have a strong impact on the mechanical properties, especially the toughness and ductility, which will lead to the reduction of toughness and ductility and easy to fracture the steel.

Typical α’Phase precipitation will lead to depletion of Cr in ferrite matrix, thus reducing corrosion resistance and toughness of steel and increasing hardness.

It is found that when 444 ferritic stainless steel is aged at 400~475 ℃, α’ phase will precipitate, causing its hardness to increase, and after the aging time at 475 ℃ exceeds 500h, its toughness will drop sharply.

The hardness of 441 ultra pure ferritic stainless steel and the energy absorbed by fracture after aging are shown in Fig. 11.

Ultrapure Ferritic Stainless Steel: Effect of Brittleness and Formation on Properties 11

Fig. 11 Change of hardness and fracture absorbed energy of 441 ultra pure ferritic stainless steel with time after aging at 400 ℃ and 450 ℃

(a) Hardness changes with aging time; (b) The energy absorbed by fracture varies with aging time.

Luo Yi and others found that the tensile strength of 446 ultra pure ferritic stainless steel will be improved to a certain extent when the network structure has not been formed in phase σ after aging treatment;

However, when the precipitation of phase σ forms a network structure, the tensile strength and elongation of the material will decrease significantly, as shown in Fig. 12.

In addition, whether or not a network structure is formed, the precipitation of phase σ will cause serious damage to the impact property of the material, resulting in a decline in its impact property, which cannot meet some requirements for steel.

Ultrapure Ferritic Stainless Steel: Effect of Brittleness and Formation on Properties 12

Fig. 12 Change of tensile strength and elongation of 446 ultra pure ferritic stainless steel with time after aging at 800 ℃

The precipitation of Laves phase has both favorable and unfavorable effects on ultra pure ferritic stainless steel.

It is pointed out in the literature that with the increase of aging time, Fe2Nb phase will gradually precipitate in the steel, resulting in the decrease of toughness and high temperature strength of the steel.

It is found that if Si and Nb elements are added to the Laves phase precipitate, the Laves phase precipitation will help to increase the creep resistance and high temperature strength of the steel.

In addition, if the Laves phase contains W element, it is helpful to improve the high temperature tensile strength of the steel.

As shown in Fig. 13, compared with the non W-type 444 ferritic stainless steel, when the mass fraction of W is 0.5%~1%, the tensile strength is significantly improved.

When aging at 900 ℃, the tensile strength decreases slightly with the increase of aging time, and tends to be stable gradually;

When aging at 1000 ℃, the tensile strength will be greatly reduced, but the initial tensile strength is still higher than that of non W steel.

Ultrapure Ferritic Stainless Steel: Effect of Brittleness and Formation on Properties 13

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 ℃。

Laves phase will be precipitated from 441 ferritic stainless steel during aging at 850 ℃, and it will grow rapidly.

When it is connected into a network structure along the grain boundary, it will reduce the plasticity and impact toughness.

As the number of grain boundaries decreases, the grain size becomes larger, and the precipitation rate will decrease.

The mechanical properties of 19Cr-2Mo Nb Ti ferritic stainless steel at different aging temperatures are shown in Fig. 14.

During the aging process of the steel at 850~1050 ℃, (FeCrSi) 2 (MoNb) and (Fe, Cr) 2 (Nb, Ti) type Laves phases will be transformed into (Nb, Ti) (C, N) precipitates, and the mass fraction of Nb in the solution will increase due to the dissolution and coarsening of the precipitates, leading to the reduction of its tensile strength;

However, after aging treatment at 950 ℃, the homogeneity of recrystallized grains can be improved, and the elongation will increase sharply, up to 37.3%, and finally gradually stabilize at 32.6%.

Ultrapure Ferritic Stainless Steel: Effect of Brittleness and Formation on Properties 14

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 is found that the precipitation of brittle phase will deteriorate the corrosion resistance of steel.

In addition, the literature shows that because of the high Cr mass fraction of 27.4Cr-3.8Mo ultra pure ferritic stainless steel, phase σ and phase χ are easily produced after aging at 950 ℃ for 0.5h, leading to a decrease in pitting resistance.

After aging at 1100 ℃ for 0.5h, phase σ and phase χ will gradually disappear, and the pitting resistance will recover. The pitting potential is shown in Fig. 15.

Ultrapure Ferritic Stainless Steel: Effect of Brittleness and Formation on Properties 15

Fig. 15 Pitting Potential of 24.7Cr-3.4Mo and 27.4cr-3.8Mo Stainless Steel

The content of Cr and Mo in stainless steel has a decisive influence on its corrosion resistance. When the mass fraction of Cr is more than 25% and the temperature is 700~800 ℃, σ phase and χ phase will be precipitated, which will reduce the corrosion resistance of the steel.

In addition, Cr is easy to combine with C and N elements and precipitate at the grain boundary or inside the grain.

On the one hand, Cr rich carbon and nitride will be formed, which will greatly reduce the mass fraction of Cr and reduce the corrosion resistance;

On the other hand, these precipitates are harmful to the passivation film, which will destroy its uniformity, cause its stability to decline, and affect the corrosion resistance of steel.

In corrosive medium, welded joints are prone to intergranular corrosion, pitting corrosion, crevice corrosion and other local corrosion.

Huang Zhitao et al. believed that in the chloride environment, increasing the mass fraction of Mo in the high-purity ferritic stainless steel can delay the precipitation of M23C6 (M is Fe, Cr and Mo) and improve its pitting corrosion resistance;

Zhang Henghua et al. found that if a certain amount of Mo element is added to 26Cr ultra pure ferritic stainless steel, the Cr element in the passivation film can be enriched, and the stability of the passivation film can be strengthened, thus improving the pitting corrosion resistance of the material;

Tong Lihua et al. found that the addition of Nb and Ti elements to the ultra pure ferritic stainless steel can effectively prevent the precipitation of carbon and nitrogen compounds of Cr in the stainless steel, and enhance its intergranular corrosion resistance;

However, other studies have found that in 15Cr ultra pure ferritic stainless steel, if the mass fraction of Ti and N is high enough, it is easy to form TiN, which will accelerate the growth of pitting corrosion, and will be detrimental to its corrosion resistance.

Wen Guojun and others found that when 430Ti ferritic stainless steel is aged at 475 ℃ for 0-100h, its hardness will gradually increase with the increase of aging time, and α ‘phase and α phase will increase, which will seriously reduce the corrosion resistance. Its corrosion resistance is shown in Fig. 16.

Ultrapure Ferritic Stainless Steel: Effect of Brittleness and Formation on Properties 16

Fig. 16 Corrosion Resistance of 430Ti Ferritic Stainless Steel

To sum up, the higher the mass fraction of Cr in the ultra pure ferritic stainless steel, the more likely it is to produce precipitates, which will seriously reduce the corrosion resistance of the steel. Adding a certain amount of Nb, Ti and Mo will improve its corrosion resistance, but the TiN formed by Ti will have a negative impact on the pitting corrosion resistance of the steel.

4. Conclusion and prospect

The main characteristics and influencing factors of σ-phase brittleness, 475 ℃ brittleness and high temperature brittleness of ultra pure ferritic stainless steel are analyzed in this paper.

The following conclusions are drawn:

(1) The brittleness of σ phase in ultra pure ferritic stainless steel is due to the precipitation of σ phase and χ phase rich in Cr and Mo elements;

The brittleness at 475 ℃ is due to the precipitation of Cr rich α ‘phase;

The high temperature brittleness is caused by the precipitation of carbon and nitride of Cr.

(2) Alloying elements, RE and aging treatment in ultra pure ferritic stainless steel have certain effects on precipitated phases, which can inhibit the generation of σ phase brittleness, 475 ℃ brittleness and high temperature brittleness to some extent.

The specific impacts are as follows.

① When the content of Cr and Mo increases, the precipitation amount of α ‘phase, σ phase, χ phase and Laves phase will increase.

In ultra pure ferritic stainless steel, the addition of stabilizing elements can reduce or eliminate the high temperature brittleness when the thin section is used.

High temperature brittleness will not occur if high temperature range is avoided during heat treatment;

In addition, the addition of Ti and Nb can delay the precipitation of σ phase, thus reducing the brittleness of σ phase.

However, the addition of Ti and Nb will lead to the generation of Laves phase, and the high content of Nb will easily lead to the coarsening of Laves phase.

② The addition of RE will reduce the precipitation of carbon and nitride in phase σ and Cr, reduce the brittleness of phase σ and high temperature brittleness, and improve the mechanical properties and pitting resistance of steel.

③ Different aging treatments have different effects on the precipitates. According to the different Cr content, the precipitates will have slight differences.

When aging at 600~800 ℃, a small amount of σ phase, χ phase and Laves phase will precipitate, but when aging at 600 ℃, α ‘phase will re dissolve in the matrix, and the brittleness will disappear at 475 ℃;

During aging at 850~950 ℃, a large number of σ phase, χ phase and Laves phase will precipitate;

When aging at 1000~1100 ℃, the precipitation amount of σ phase, χ phase and Laves phase is obviously reduced or even disappeared.

The brittleness of σ phase can be eliminated by aging treatment above 1000 ℃.

(3) The precipitation of secondary phases such as α ‘phase, σ phase, χ phase and Laves phase in ultra pure ferritic stainless steel will reduce its toughness and plasticity, increase its strength and hardness, and have a great impact on its corrosion resistance;

If Si and W elements are added to Laves phase, its high temperature strength and tensile strength will be enhanced;

In addition, the addition of Cu element will produce Cu rich phase precipitation, which is helpful to improve the toughness of steel.

The domestic Ni resources are scarce.

Once the excessive consumption of Ni resources leads to a shortage of resources, it will have a serious impact on the domestic stainless steel industry.

As a “resource saving” stainless steel, ultra pure ferritic stainless steel has the advantages of high comprehensive performance and low comprehensive cost.

It is an inevitable choice for the domestic stainless steel industry to vigorously promote the 400 series stainless steel with low nickel and little nickel to achieve sustainable and high-quality development.

Now, ultra pure ferritic stainless steel has gradually replaced some austenitic stainless steel in the automotive industry, household appliances and elevator industry.

In addition, it has also achieved phased success in the construction of large building roofs such as airports and stadiums.

The market scale of ultra pure ferritic stainless steel in the future domestic stainless steel products will be very large, with broad prospects for growth.

In the future, it is necessary to focus on the brittleness of ultra pure ferritic stainless steel. In the process of production and use, the generation of σ-phase brittleness, 475 ℃ brittleness and high temperature brittleness need to be effectively restrained, so as to ensure that the steel has good mechanical properties and corrosion resistance, to avoid disadvantages and to give play to its advantages of “resource saving”, so as to make ultra pure ferritic stainless steel obtain greater progress and development in the stainless steel industry.

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