2017 Volume 57 Issue 9 Pages 1637-1644
The characteristics of the γ↔δ phase transformations were observed on the surface of duplex stainless steels with different nitrogen concentrations by the Ultra High Temperature Confocal Laser Scanning Microscope (UHT-CSLM). The effects of the nitrogen content on the phase transformations are discussed based on the experimental results and thermodynamic calculations. It is found that the migration of the δ/γ IB is the main form of the phase transformation and leads to the continuous decline and final disappearance of the retained γ-phase during the γ→δ phase transformation in the high nitrogen steel (N1) and low nitrogen steel (N2). During the δ→γ phase transformation, the γ-cells prefer to precipitate along the δ/δ GBs and develop into the δ-ferrite matrix with finger-like or sword-like patterns. Then the γ-cells also nucleate in the δ-grains with a sword-like pattern and the growing speed in the longitudinal direction is much faster than that in the lateral direction. More importantly, the high nitrogen content can hinder the migration of δ/γ IBs during the γ→δ transformation and also promote the nucleation and growth of γ-phase during the δ→γ transformation by increasing both the starting and finishing temperatures of the phase transformation. Interestingly, the original δ/δ GB which is covered by the precipitation of γ-phase during the δ→γ transformation will reappear first at the same position by the moving of δ/γ IBs during the γ→δ transformation, but it is unstable and migrates fast with further heating.
Duplex stainless steel (DSS) is basically a Fe–Cr–Ni alloy system with a dual phase microstructure comprising nearly equal amounts of ferrite and austenite, and possesses excellent property combinations. Therefore, DSS is replacing conventional austenitic or ferritic stainless steels in many industrial applications because of its combined advantages of better mechanical and corrosion properties.1,2,3,4,5) DSS contains austenite stabilizing elements like Ni, Mn, C, N and Cu as well as ferrite stabilizing elements like Cr, Mo and Si. The recent skyrocketing raw material cost stimulates development of the DSSs with less alloying. So, a new class of DSS in which Ni and Mo are entirely or partially replaced with Mn and N has been developed.6) Nitrogen, as a strong austenite-stabilizing element, has the same effect equivalent to 30 times of nickel and 16 times of Cr at pitting corrosion resistance.7) The austenite characteristics are significantly affected by a small variation of the nitrogen content in N-bearing DSSs.8)
The Confocal Scanning Laser Microscope (CLSM) has recently provided a convenient possibility of making an “in-situ” observation of the phase transformation on the surface of samples at high temperatures. Since the 1990’s, Yin,9,10) Kimura,11) Phelan,12,13) Liu,14) Chen,15) and other researchers16,17,18,19,20,21,22,23) have applied the CLSM to make in situ observations of the phase transformation of steels or alloys at special heating or cooling rates. For example, Yin10) found that the incoherent δ/γ interphase boundaries (abbreviated as IBs hereafter) were always unstable with finger-like morphology during δ→γ transformation, which developed along δ/δ grain boundaries (abbreviated as GBs hereafter) at low supercoolings and even into the δ-ferrite matrix at higher supercoolings for the transformation. Liu14) also showed that the phosphorus content and cooling rates had significant influences on the δ→γ transformation in low carbon steels.
However, so far, few studies have paid more attention on the δ/γ phase transformation during heating, especially in duplex stainless steels. In this paper, the characteristics of the γ→δ and δ→γ phase transformations are observed on the surface of duplex stainless steels with different nitrogen concentrations by the UHT-CSLM. The effects of the nitrogen content on the γ→δ and δ→γ transformations are discussed based on the experimental results and thermodynamic calculation results. Meanwhile, a particular emphasis is placed on the formation and migration of δ/δ GBs during γ→δ phase transformation process.
Two kinds of as-cast duplex stainless steels: High Nitrogen Steel (N1) and Low Nitrogen Steel (N2) were used in this experiment. The chemical compositions of these steels are shown in Table 1. Duplex stainless steels with different nitrogen contents were produced by a ZG-0.01 model vacuum medium frequency induction melting furnace and cooled by air. The test samples were then taken directly from the cast ingots.
| Steel | Cr | Mo | Ni | Si | Mn | C | N | Fe |
|---|---|---|---|---|---|---|---|---|
| N1 | 22.96 | 3.09 | 5.08 | 0.55 | 1.11 | 0.028 | 0.18 | Balance |
| N2 | 22.98 | 3.04 | 5.06 | 0.59 | 1.11 | 0.023 | 0.08 | Balance |
All the samples were machined into a disc (7.6 mm in diameter and 2.6 mm in height) and the surface was mechanically abraded with abrasive papers up to grit number 2000, mirror polished. Finally, in order to directly distinguish the δ phase and γ-phase, the above samples were electro-polished in a solution mixture of 60 ml concentrated sulfuric acid and 40 ml concentrated phosphoric acid at an applied potential of 15 V.
In order to study the characterizations of γ→δ and δ→γ phase transformations, an Ultra High Temperature Confocal Laser Scanning Microscope (UHT-CLSM) was employed for the “in-situ” observation. The sample was heated and cooled in a magnesia crucible (8 mm in inner diameter) under a vacuum condition. And the temperature was measured by using a thermocouple at the bottom of the holder of the crucible. In order to investigate the effects of the nitrogen content on the phase transformation, the heating rate and the cooling rate remain the same. The temperature history used in this experiment is shown in Fig. 1. In the range of high temperature (higher than 900°C), the heating rate was fixed at 50°C/min, and the cooling rate was also set as 50°C/min after holding the samples at 1400°C for 120 s.
The temperature history of samples.
The solid γ→δ transformation sequences during heating at 50°C/min for N1 and N2 are shown in Figs. 2 and 3, respectively. Unlike low carbon steels,9,14) the δ-phase and γ-phase always coexist in duplex stainless steels below the transformation temperature, as shown in Figs. 2(a) and 3(a).
Solid γ/δ transformation sequence in N1 during heating at 50°C/min, the time is counted from the start of transformation. (a–d) before the start of transformation, (e) start of transformation, (n) end of transformation (‘IB’ and ‘GB’ represent interphase boundary and grain boundary, respectively).
Solid γ/δ transformation sequence in N2 during heating at 50°C/min, the time is counted from the start of transformation. (a–b) before the start of transformation, (c) start of transformation, (n) end of transformation (‘IB’ and ‘GB’ represent interphase boundary and grain boundary, respectively).
Before the transformation, the δ-phase, γ-phase and δ/γ IBs are easy to distinguish at 600°C as also shown in Fig. 2(a). The colors of the δ-phase and γ-phase change with further heating before 1100°C, meanwhile, the γ-phase distributes like islands in the δ-ferrite matrix, but at this moment the γ→δ transformation doesn’t occur. The start of the γ→δ transformation (the migration of the δ/γ IB) is observed first from the δ/γ IBs with an obvious boundary layer surrounding the γ-phase as shown in Fig. 2(e). Then the δ/γ IBs continue to move into the γ-phase at the speed of 0.09 μm/s with the temperature increasing, and the area of the retained γ-phase gradually decreases as shown in Figs. 2(f)–2(k). Finally, the residual γ-phase disappears and completely transforms into a single δ-phase. At the same time, the unstable δ/δ GBs start to form. During further heating, the migration of δ/δ GBs proceeds for a while in the observation area and leaves a marked trace behind as shown in Fig. 2(n). The migration of δ/γ IBs and the decrease of the area of the retained γ-phase in the δ-ferrite matrix during the γ→δ transformation at a certain heating rate are summarized schematically in Fig. 4. It is worth noting that there is no δ-grain which forms in the inner part of the γ-phase under the high heating rate (50°C/min).
Schematic diagrams of the γ/δ phase transformation on heating at 50°C/min.
The γ→δ transformation process in the sample N2 as shown in Fig. 3 is similar to that observed in the sample N1. First of all, as can be seen in Fig. 3 that the volume fraction of γ-phase in N2 is less than that in N1. It is believed that the existing γ-phase has precipitated along the original δ/δ GBs during the phase transformation. Then the γ-phase transforms into the δ-phase by the same way, so the area of the γ-phase continues to decline with further heating. Finally, the γ-phase disappears and the new δ/δ GBs appears first nearby as shown in Fig. 3(k). But similarly, the new δ/δ GBs migrate quickly and leave the traces behind as shown in Fig. 3(n).
However, there are still some differences between the two transformation processes. The observed starting temperature of the γ→δ transformation (Ts, at which the δ/γ IBs move first) in the sample N1 at 1130°C is higher than that of the sample N2 at 1080°C. The finishing temperature of the γ→δ transformation (Tf, at which the γ-phase disappears and only δ-phase exists) of the sample N1 at about 1310°C is still higher than that of the sample N2 at 1280°C. The total times for the γ→δ transformation of the sample N1 and N2 are about 217 s and 280 s, respectively. So it is indicated that the transformation process in the sample N1 is hindered compared to that in the sample N2 because of the difference of nitrogen content. But the reason why the total time spent on N1 is less than that spent on N2 is because the high temperature can enhance the diffusion rates of elements (like Cr, Ni and Mo) and promote the γ→δ transformation.
3.2. δ→γ Phase TransformationAfter the above-mentioned γ→δ transformation, once the δ/δ GBs get stagnant, they become much deeper and clearer because the interfacial tension is directed inwardly against the bulk of the sample10) as shown in Figs. 5(a)–5(b) and 6(a)–6(b). After that, the sample was cooled to observe the δ→γ transformation at a certain cooling rate (50°C/min) as shown in Figs. 5 and 6.
Solid δ/γ transformation sequence in N1 during cooling at 50°C/min, the time is counted from the start of transformation. (a–b) before the start of transformation, (c) start of transformation, (n) end of transformation (‘IB’ and ‘GB’ represent interphase boundary and grain boundary, respectively).
Solid δ/γ transformation sequence in N2 during cooling at 50°C/min, the time is counted from the start of transformation. (a–b) before the start of transformation, (c) start of transformation, (n) end of transformation (‘IB’ and ‘GB’ represent interphase boundary and grain boundary, respectively).
With respect to the sample N1, the appearance of γ-phase in the δ-ferrite matrix at the beginning of the δ→γ transformation is observed first on one side of the δ/δ GB at 1158°C as shown in Fig. 5(c). Then the γ-phase gradually spreads along the δ/δ GB and forms a thin layer on the same side of the initial δ/δ GB. Some new γ-cells also form along other δ/δ GBs and grow into the δ-ferrite matrix, first with a curved front, then with the finger-like patterns as shown in Fig. 5(e). Meanwhile, some intragranular γ-cells are also observed in the δ-ferrite grain at 1111°C and are irregular in shape. With further cooling, γ-cells continue to form both along the δ/δ GBs and in the δ-ferrite grains, then grow and merge into bigger ones. Especially, a new γ-cell which has a sword-like pattern and two tips (① and ② as shown in Fig. 5(k)) grows fast in the longitudinal direction at the speed of 0.68 μm/s. But the speed of growth along the longitudinal direction is much quicker than that along the lateral direction.
The sample N2 contained lower nitrogen compared to the sample N1. During the experiment, the heating and cooling rates of the sample N2 are the same as those of the sample N1. The start of the δ→γ transformation is observed first at about 1060°C as shown in Fig. 6(c). The γ-cell precipitates first on one side of the original δ/δ GB and develops into the δ-ferrite matrix with a triangle pattern. At about 1050°C, a new γ-cell which also has a sword-like pattern and two tips forms first in the δ-ferrite grain as shown in Fig. 6(d). Then it grows fast in the longitudinal direction at the speed of 2.08 μm/s and 3.47 μm/s, respectively (① and ② as shown in Fig. 6(e)). Such a great difference in growing speed between the two tips is due to that the growth of the right tip (① in Fig. 6(e)) along the longitudinal direction is hampered by the initial δ/δ GB. Comparing with the sample N1, the growth velocity of this sword-like intragranular γ-cell in the sample N2 is much faster because the precipitation temperature in N2 (about 1050°C) is higher than that in N1 (about 990°C). It is indicated that the growth rate of this intragranular γ-cell is enhanced by the high temperature which leads to the high diffusion rates of elements like Cr, Ni and Mo. At the same time, the growing speed in the longitudinal direction is also much faster than that in the lateral direction. It shows clearly that the δ/γ IBs in the tip of γ-cells are incoherent with high mobility and the δ/γ IBs along the lateral direction are semi-coherent with low mobility.10) So the growth along the longitudinal direction is completed early, then the γ-cells spread in the lateral direction and gradually widened. With further cooling, other sword-like γ-cells are inserted into both sides of the δ-ferrite matrix from the δ/δ GBs along some certain directions7,24) as shown in Fig. 6(i).
Combining with the above observed results, as can be seen that the observed starting temperature of the δ→γ transformation (Ts, at which the γ-phase first appear) in the sample N1 at 1158°C is higher than that of the sample N2 at 1060°C. The finishing temperature of the δ→γ transformation (Tf, at which the δ-phase and γ-phase coexist and don’t change any more) of the sample N1 at about 900°C is still much higher than that of the sample N2 at about 700°C. The total times for the δ→γ transformation of the sample N1 and N2 are about 308 s and 251 s, respectively. It can be inferred that the transformation process in the sample N2 was greatly retarded compared to that in the sample N1, especially in the early stage of the transformation process because of the addition of nitrogen. Of course, the effects of nitrogen in DSS are different from that of phosphorus in low carbon steels.14)
It is worth noting that why the morphologies of γ-phase and δ-phase at the end of the δ→γ transformation shown in Figs. 5 and 6 are quite different from those found in their original ordinary cast microstructures (as shown in Figs. 2(a) and 3(a)). The reasons for the above mentioned phenomenon are considered to be as follows, (1) the observed microstructures shown in Figs. 2(a) and 3(a) belong to the internal part of the sample N1 and N2, and they are not influenced by the surface tension. (2) The observed microstructures shown in Figs. 2(a) and 3(a) are not complete because of the mandatory damage caused by the mechining process. (3) More importantly, the surface tension makes the nucleation and growth of γ-phase only occur on the surface, and it also has a great effect on the shape of γ-phase except for the nitrogen content. In addition, it seems interesting that the volume fraction of γ-phase in the final microstructures (as shown in Figs. 5 and 6) is much smaller when compared to their original ones (as shown in Figs. 2(a) and 3(a)). It results from the surface heating technology used in the in-situ observation. In order to keep the observed surface flat, only the observed surface which instead of the whole sample is heated. The nucleation and growth of γ-phase would be restricted because they can only get supplement of elements below the surface. Meanwhile, the temperature which gradually decreases from the surface to the bottom will reduce the diffusion rate of elements. Finally, the substantial reduction of other γ-phase, which would precipitate in the other positions during the nucleating period and pass the observed surface during the growing period under the normal conditions, also leads to the decrease of γ-phase.
The transformation process mentioned above during continuous cooling agrees partially with the descriptions by Yin,9,10) Phelan12,13) and Liu,14) where these researchers made detailed “in-situ” observations of the δ→γ transformation at some certain cooling rates, but there still exists some differences between the previous investigations with the present work because of the particularity of duplex stainless steels.
As can be seen in Figs. 7 and 8, both the starting and finishing temperatures during the γ→δ transformation and the δ→γ transformation in the sample N1 are higher than that in the sample N2. It indicates that high nitrogen content could greatly increase the starting and finishing temperatures at the same time.
The observed starting and finishing temperature of the γ→δ transformation for N1 and N2 during heating at 50°C/min.
The observed starting and finishing temperature of the δ→γ transformation for N1 and N2 during cooling at 50°C/min.
By using the thermodynamic calculation software (Thermo-Calc 2015), according to the database TCFE7, the mole fractions of δ-phase and γ-phase as a function of temperature under equilibrium condition are shown in Fig. 9. It is also found in Fig. 9 that in the high-temperature region (more than 700°C), the precipitation and decomposition temperatures of γ-phase in the sample N1 are always higher than that in the sample N2.
Mole fraction of ferrite and austenite as a function of temperature by Thermo-Calc.
So the experimental results are consistent with the thermodynamic calculation ones. This further confirms that nitrogen plays a significant role in the precipitation and decomposition of γ-phase, that is to say, the high nitrogen content can hinder the migration of δ/γ IBs during the γ→δ transformation on heating and also promote the nucleation and growth of γ-phase during the δ→γ transformation on cooling by increasing both the starting and finishing temperatures of the phase transformation.
4.2. Formation and Instability of δ/δ GB during γ→δ Phase TransformationThe formation and migration of δ/δ GBs during the γ→δ transformation are summarized schematically in Fig. 10. It can be clearly observed that the δ/γ IBs continue to move and the area of the retained γ-phase decrease with the temperature increasing. Then the γ-phase nearly disappears and looks like lines as shown in Fig. 10(d). As is shown in Fig. 10(e), the retained γ-phase entirely disappears and changes into the new δ/δ GBs (solid line) at 1300°C. But these δ/δ GBs are instable and migrate from the initial position to other ones (dot line and dash dot line) with further heating. Finally, the δ/δ GBs can’t be seen in the observation area of UHT-CLSM and other δ/δ GBs (dash line) also move here.
Schematic diagram of the formation and migration of δ/δ GBs during γ→δ phase transformation on heating at 50°C/min.
So combining with the above observed results during the δ→γ transformation as shown in Figs. 5 and 6, it is found that the precipitation of γ-phase along the δ/δ GBs during cooling will leads to the disappearance of the original δ/δ GBs, and then the decomposition of γ-phase by the migration of δ/γ IBs during heating will make the original δ/δ GBs reappear at the same position which the original δ/δ GBs exist earlier, however, these new δ/δ GBs are unstable. It seems interesting that in the δ→γ→δ transformation process, γ-phase has experienced a process from precipitation to decomposition along the δ/δ GBs, meanwhile, the δ/δ GBs have gone through a contrary process from disappearance to reappear. So, if the instability and migration of the new δ/δ GBs are ignored, it can be inferred that the disappearance (or appearance) of the δ/δ GBs during the δ→γ (or γ→δ) transformation could be considered to be a reversible process but not absolute under the normal conditions. Of course, this phenomenon can’t be found in low carbon steels which only exist a single phase at room temperature.
The solid δ↔γ phase transformations were observed on the surface of duplex stainless steels with different nitrogen contents by the UHT-CLSM at certain heating or cooling rates. The effect of nitrogen on the phase transformation was discussed based on the experimental results and thermodynamic calculations. The main observations and conclusions are as follows.
(1) During the γ→δ phase transformation in the sample N1 and N2, the migration of the δ/γ IB is the main form of the phase transformation and leads to the continuous decline and final disappearance of the retained γ-phase.
(2) During the δ→γ phase transformation, the γ-cells appear first along the δ/δ GBs and develop into the δ-ferrite matrix with finger-like and sword-like patterns. Then the γ-cells also nucleate in the δ-grains with a sword-like pattern and the growing speed in the longitudinal direction is also much faster than that in the lateral direction.
(3) The high nitrogen content can hinder the migration of δ/γ IBs during the γ→δ transformation on heating and also promote the nucleation and growth of γ-phase during the δ→γ transformation on cooling by increasing both the starting and finishing temperatures of the phase transformation.
(4) The original δ/δ GB which is covered by the precipitation of γ-phase during the δ→γ transformation will form again at the same position by the moving of δ/γ IBs during the γ→δ transformation, but it is unstable and migrates fast with further heating.
This work was supported by the National High Technology Research and Development Program of China (863 Program): Contract number 2015AA03A502.
