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Mechanics of Materials
Effect of Thermal History on Hot Workability of Fe–Cr Multiphase Steel during Hot Working
Shunsuke SasakiTatsuro KatsumuraHiroki Ota
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2021 年 62 巻 2 号 p. 177-184

詳細
Abstract

In the hot working of multiphase steels, the thermal history before hot working affects the phase balance. The aim of this study is to investigate the effect of the thermal history immediately before hot working on hot workability by performing hot uniaxial tensile tests. Duplex stainless steel was chosen as the test material, as this material has a typical dual-phase structure consisting of δ ferrite and austenite, and its phase balance changes with the working temperature. The phase ratio of δ ferrite is higher at high temperatures, and the ratio of the austenite phase increases as the temperature decreases. To clarify the effect of the cooling rate immediately before hot working on hot workability, a hot tensile test was carried out with two extremely different cooling rates (0.3°C/s, 10.0°C/s) from the heating temperature of 1250°C. The hot working test temperature was in the range of 650°C to 1150°C. Hot workability was higher under the rapid cooling condition than under the slow cooling condition in the hot working temperature range of 850°C to 1150°C, but this tendency was eliminated or reversed at temperatures below 750°C. The reason for this change was clarified by investigating both the phase balance immediately before hot working, which depends on the cooling rate, and the strength of each single phase. Therefore, in order to accurately measure the hot workability of a multiphase material, it is important to consider the thermal history immediately before hot working.

 

This Paper was Originally Published in Japanese in J. JSTP 60 (2019) 340–345.

1. Introduction

Duplex stainless steels are widely used in structures and piping for chemical plants and petroleum extraction, which are exposed to severe corrosive environments, because of their excellent corrosion resistance and high yield strength. Hot working is used to shape the material as required for each application, but hot working of duplex stainless steel is difficult, as it displays high hot deformation resistance and low hot workability due to its large content of corrosion-resistant alloy elements. In order to prevent defects such as cracks and holes in the product during hot forming, it is important to clarify the change in hot workability in the hot working temperature range and to use that knowledge to manage manufacturing conditions. The results of several investigations of the hot workability of duplex stainless steel have been reported. Faccoli et al.1) conducted experiments at various processing temperatures and strain rates to investigate changes in flow stress and hot workability. Barteri et al.2) observed the structure immediately after hot working and reported that recrystallization behavior and hot workability change depending on the strain rate. In addition, Blum3) and Fan4) reported that the initial heating temperature affected hot workability at the same hot working temperature, and considered that phenomenon to be affected by the initial grain size and the distribution of each phase.

Since the processing temperature, strain rate, and initial heating temperature change to various conditions at the manufacturing site, it is important to clarify the relationship between the processing conditions and the hot workability indicated in these reports. However, to the best of our knowledge, no studies have focused on the temperature history immediately before hot working, which changes at the manufacturing site. At a manufacturing site, even assuming the hot working temperature is the same, the cooling rate history immediately before hot working changes depending on the relationship between the volume and surface area of the material. In addition, the surface and edge portions of the material have a relatively fast cooling rate compared to the center of the material, and the cooling rate also increase in case of extended contact with a die or cooling water immediately before hot working. The authors5,6) investigated the effect of rapid temperature change before hot working, and clarified the phenomenon in which hot flow stress is greatly reduced by rapid cooling immediately before hot working in duplex stainless steel. This study also clarified the fact that this phenomenon is caused by a non-equilibrium state, which is affected by changes in the temperature history immediately before hot working. The non-equilibrium state due to rapid temperature change immediately before hot working is considered to have a great influence on hot workability.

In this paper, a hot uniaxial tensile test was used to investigate the effect of differences in the cooling rate immediately before hot working on hot workability. In addition, the mechanism of the change in hot workability was examined by observing the microstructural change after hot working.

2. Experimental Method

2.1 Chemical composition and equilibrium diagram of specimen material

The specimen material used in this study was a 25%Cr duplex stainless steel with a δ ferrite phase and an austenite phase in the hot working temperature range. Table 1 shows the chemical composition of the specimen material (UNS: S31260). The calculated equilibrium diagram for that chemical composition is shown in Fig. 1. In this chemical composition system, the δ ferrite phase is the main phase at high temperatures, but the phase fraction changes continuously during cooling, as part of the δ ferrite phase transforms to the austenite phase. In addition, the δ ferrite phase transforms to the sigma (σ) phase when held isothermally at a temperature lower than 990°C for an extended time.

Table 1 Chemical composition of specimen (mass%).
Fig. 1

Equilibrium diagram of specimen.

2.2 Hot tensile test

Figure 2 shows the geometry of the hot uniaxial tensile specimen. The specimen was machined to a parallel part diameter of 10 mm and a length of 120 mm. The temperature history of the specimen was controlled by a thermocouple attached to the outer surface at the center of the specimen length. Heating was performed by a direct current heating method, in which the specimen was energized by clamping its two ends with copper dies. Cooling was performed by injecting N2 gas from two injection ports arranged in the center of the specimen length. The soaking time before hot working was 3.0 s. This soaking time was determined by confirming that the internal temperature of the specimen after rapid cooling was comparable to the external surface temperature in preliminary experiments. In order to prevent overheating due to reduction of the tensile (cross-sectional) area during hot working, energization was discontinued 1.0 s before hot working. Figure 3 shows the temperature history in these hot working experiments. The initial heating temperature was constant at 1250°C, and the hot working temperature was set to various temperatures between 650°C and 1150°C at 100°C intervals. As the cooling rate before hot working, two conditions were used, (a) 0.3°C/s (slow cooling) and (b) 10.0°C/s (rapid cooling). In addition, three experiments were conducted under the same conditions. The cooling rate in condition (a) was determined based on the calculated CCT diagram using the chemical composition of the specimen, and was determined as a cooling rate at which the σ phase was not formed during continuous cooling from the initial heating temperature. Condition (b) was set to the fastest possible cooling rate while controlling the temperature distribution of the specimen. The extension speed was 10.0 mm/s. However, only for the hot working temperature of 1050°C, conditions of 0.1, 1.0, and 100.0 mm/s were added in addition to the extension speed of 10.0 mm/s in order to investigate the effect of the extension speed. Under the slower extension speeds of 0.1 and 1.0 mm/s, hot working was carried out while continuing direct electrifying heating because the temperature drop during hot working was excessive when heating was stopped before hot working. The thermocouple for temperature control was attached to the center of the specimen from the start of the test until just before the fracture, and it was confirmed that the temperature control was carried out correctly. After the hot tensile test, hot workability was evaluated by the cross-sectional reduction of area RA. RA was calculated as (A0A)/A0 × 100 when the cross-sectional area A after fracture was obtained by image analysis and the initial cross-sectional area A0 was assumed. Immediately after hot working, the microstructure was frozen by rapid cooling. After rapid cooling, the central part was cut parallel to the axial direction of the specimen, and EBSD (Electron Back Scatter Diffraction) observation was carried out in the vicinity of the fracture surface to investigate the volume fraction of the austenite phase and the phase distribution. The measurement range was 800 × 800 µm, and measurements were made at a 0.3 µm pitch.

Fig. 2

Shape of the hot uniaxial tensile test specimen.

Fig. 3

Experimental conditions for tensile test.

3. Experimental Results

3.1 Change in hot workability depending on cooling rate immediately before hot working

Figure 4 shows the changes in reduction of area RA and tensile strength TS for each working temperature at a constant extension speed of 10.0 mm/s. Figure 5 shows optical micrographs of the fracture surface under the experimental conditions of 650°C, 950°C, and 1150°C. Under all conditions, the fracture surface had an elliptical shape, which was due to the structural anisotropy of the duplex stainless steel. In the plots without error bars, all experimental results were similar values. Under condition (b), that is, rapid cooling immediately before hot working, RA improved greatly in the temperature range from 850°C to 1150°C compared to the slow cooling condition (a). On the other hand, substantially the same values of RA were obtained at 750°C regardless of the cooling condition, and at 650°C, the above-mentioned relationship was partially reversed. A tendency in which TS increased as the hot working temperature decreased regardless of the cooling condition was confirmed. TS showed higher values under the slow cooling condition (a) at processing temperatures from 750°C to 1150°C, but was higher under the fast cooling condition (b) at the processing temperature of 650°C.

Fig. 4

Hot workability RA and tensile strength TS. Testing temperature range: 650 to 1150°C. (a) Cooling rate: 0.3°C/s, (b) Cooling rate: 10.0°C/s.

Fig. 5

Overview of fracture surfaces of hot worked specimens. Testing temperature: 650°C, 950°C, 1150°C. (a) Cooling rate: 0.3°C/s, (b) Cooling rate: 10.0°C/s.

3.2 Change in austenite phase ratio immediately after hot working

Figure 6 shows the austenite phase fraction immediately after hot working at the various hot working temperatures at the extension speed of 10.0 mm/s. Under condition (a), slow cooling immediately before hot working, the austenite phase fraction increased as the processing temperature decreased and became saturated at about 40% below 950°C. Under condition (b), rapid cooling immediately before hot working, the austenite phase fractions were lower than those under condition (a) at all hot working temperatures, and was around 20% or less in all cases. This low austenite phase fraction is attributed to the rapid cooling rate, which made it possible to maintain the δ ferrite phase fraction from the initial heating temperature until the hot working temperature was reached. Figure 7 shows the phase distributions immediately after processing at 650°C, 950°C, and 1150°C measured by EBSD. Under the slow cooling condition (a), the austenite phase fraction increased due to the growth of austenite grains at the grain boundaries and in the grains of the δ ferrite grain as the temperature decreased. On the other hand, under the rapid cooling condition (b), austenite grains did not grow even when the hot working temperature decreased, and a low austenite phase fraction was maintained. The σ phase was not confirmed at any hot working temperature under conditions (a) and (b). Maehara7) and Ohmori et al.8) reported the σ phase is formed by slow cooling and isothermal holding of duplex stainless steel. In this experiment, the formation of the σ phase was suppressed by controlling the chemical composition and the cooling rate. From those results, the hot workability of the two-phase structure of δ ferrite and austenite phase was evaluated at all hot working temperatures.

Fig. 6

Volume fraction of austenite phase after hot working at 650°C to 1150°C with cooling rates of (a) 0.3°C/s and (b) 10.0°C/s just before hot working.

Fig. 7

Phase distribution of austenite and δ ferrite phases after hot tensile test. Testing temperature: 650°C, 950°C, 1150°C. (a) Cooling rate: 0.3°C/s, (b) Cooling rate: 10.0°C/s.

3.3 Effect of extension speed on hot workability

Figure 8 shows the change in RA when the extension speed was changed to 0.1, 1.0, 10.0, and 100.0 mm/s at the hot working temperature of 1050°C. Regardless of the cooling rate condition, RA also decreased as the extension speed decreased, but at the same extension speed, RA was higher under the rapid cooling condition (b) than under the slow cooling condition (a). However, under condition (b), RA decreased as the extension speed became slower, and as a result, the difference between RA under condition (a) and condition (b) became smaller under low extension speed conditions. Figure 9 shows the EBSD measurement results of the phase distribution and phase fraction immediately after the tensile test for the extension speeds of 0.1 and 100.0 mm/s. Under the slow cooling condition (a), there was no significant difference in the phase fraction depending on the extension speed, and the austenite grains were coarse and elongated under all extension speed conditions. On the other hand, under the rapid cooling condition (b), a large microstructure change was observed with changes in the extension speed. Under condition (b) with the high extension speed of 100.0 mm/s, a low austenite phase fraction was observed immediately after hot working, which means that hot working was conducted while maintaining a high δ ferrite phase fraction. In contrast, at the low extension speed, the austenite transformation progressed during hot working, and the austenite phase fraction had increased after hot working. Moreover, the observed austenite grains were finer than those under condition (a), in which slow cooling was performed immediately before hot working. Thus, it is thought that these fine grains were not the form generated by slow cooling before hot working, as under condition (a), but were formed by a phase transformation which occurred during hot working.

Fig. 8

Hot workability RA at 1050°C with extension speeds of 0.1 to 100.0 mm/s. (a) Cooling rate: 0.3°C, (b) Cooling rate: 10.0°C/s.

Fig. 9

Phase distribution and volume fraction of austenite phase just after hot working at 1050°C with extension speeds of 0.1 and 100.0 mm/s. (a) Cooling rate: 0.3°C/s, Cooling rate (b): 10.0°C/s.

4. Consideration

4.1 Mechanism of change in hot workability by change in cooling rate before hot working

The measurement results of the austenite phase fraction change immediately after hot working shown in Fig. 6 confirmed that the phase fraction during hot working was greatly influenced by the cooling rate immediately before working. The difference in the phase fraction is thought to affect hot workability. However, under the rapid cooling condition (b), the phase fraction did not change greatly between 650°C and 950°C, but hot workability at the hot working temperature of 850°C was the lowest among all test temperatures. In addition, at the hot working temperatures of 650°C and 750°C under conditions (a) and (b), hot workability RA and tensile strength TS were approximately the same or their tendencies was partially reversed, even though the phase fraction was greatly different. These behaviors cannot be explained simply by changes in the phase fraction. Tamura9) and Tomita et al.10,11) investigated the tensile strength when the fraction of each phase and the hardness of the hard phase are changed using a steel having a two-phase structure, and explained that strength did not follow the mixing rule when the hardness difference between the two phases increased. However, those studies were carried out at room temperature, and not under hot conditions. In studies using the finite element method, Karlsson,12) Jinoch,13) Tomita,14) and Hernandes15) visualized and explained the fact that the change in strength characteristics is caused by the strain distribution behavior associated with the difference in the strength of each phase. Furthermore, Tomota16) and Gurland17) explained that the shape and distribution of the two phases also affected strain distribution behavior and caused changes in strength characteristics and hot workability. The single-phase mechanical properties of duplex stainless steel, which are thought to affect strength and hot workability, were investigated by Floreen,18) Hayden,19) and Decker20) at temperatures lower than room temperature, and it was reported that the strength of the δ ferrite phase exceeded that of the austenite phase. On the other hand, McQueen et al.21) reported that the strength of the austenite phase was higher than that of the δ ferrite phase under hot conditions at temperatures above 1000°C.

In the present study, changes in the phase fraction during hot working were confirmed from Figs. 6 and 9, but the single-phase strength change in each hot working temperature range, which is thought to affect strength characteristics and hot workability, was unclear. In past surveys, there were no results of detailed investigation of the strength of each single phase from temperatures of 1000°C and lower to room temperature. Therefore, in this study, single-phase steels were prepared and a hot uniaxial tensile test was conducted in order to measure the strength of the single phases in the temperature range from 650°C to 1250°C. The chemical components were determined so as to be in a single-phase state during hot working by measuring the amounts of the elements in each phase with an EPMA (Electron Probe Micro Analyzer).

Table 2 shows the chemical composition of each single phase. The steel ingots were rolled into a plate shape and machined into the shape of the hot uniaxial tensile specimen shown in Fig. 2. The test conditions were a temperature range of 650°C to 1150°C and an extension speed of 1.0 mm/s. Figure 10 shows the results of the change in tensile strength TS at each test temperature and the strength ratio of the austenite phase to the δ ferrite phase. In the hot uniaxial tensile test of the single phase steel, TS could be measured, but RA could not be accurately measured because non-uniform necking deformation occurred just before fracture. Therefore, the results of TS in Fig. 10 are shown, and the changes in hot workability are considered.

Table 2 Chemical compositions of δ ferrite and austenite (mass%).
Fig. 10

Tensile strength TS and TS ratio in δ ferrite and austenite single phase steels. Testing temperature range: 650°C to 1250°C.

From the results, TS of each single-phase steel increased as the hot working temperature decreased. On the other hand, although the strength ratio shows that the strength of the austenite phase was approximately twice as high as that of the δ ferrite phase in the range of 850°C to 1250°C, at 750°C, the strength ratio decreased rapidly as the hot working temperature decreased and was sometimes reversed at 650°C. In other words, at 850°C or higher, where the strength ratio is large, a large amount of strain was distributed to the relatively soft δ ferrite phase, but when the hot working temperature was 750°C or lower, the average distribution of strain existed in each phase. As the reason why hot workability RA was lower under the slow cooling condition (a) in the temperature range of 850°C or higher than that under the rapid cooling condition (b), based on the results described above, it can be concluded that the fraction of the relatively soft δ ferrite phase decreased as a result of slow cooling, and strain concentrated in the smaller amount of δ ferrite phase.

In addition, the results in Fig. 4, which evaluated the hot-workability of the two-phase state, show that the decrease of RA from 950°C to 850°C is large under both conditions (a) and (b). As shown in Fig. 6, there is no change in the phase fraction due to the difference in the hot working temperature in this temperature range. Since the strength ratio of the single phase steel did not change, it is considered that strain was concentrated in the soft δ ferrite phase. Figure 4 and Fig. 6 show that condition (b) displays a large RA drop from 950°C to 850°C during hot working in spite of the large fraction of the δ ferrite phase. In other words, RA of the δ ferrite phase was greatly reduced in this temperature range. From this result, it can be explained that the decrease in RA under both condition (a) and condition (b) was large in the range of 950°C to 850°C. However, under condition (b), the fraction of the δ ferrite phase in which strain was concentrated was large, resulting in a larger improvement of RA in comparison with condition (a).

On the other hand, RA recovered in the temperature range below 750°C because the strength ratio approached 1.0, and the effect of the phase fraction on strain concentration was reduced (that is, the average strain was introduced in each phase). In addition, at the hot working temperature of 650°C, a partially improved tendency of RA was also observed under condition (a) in comparison with condition (b). This can be explained as follows: Because the strength ratio becomes smaller than 1.0 at 650°C and the austenite phase becomes a relatively soft phase, strain concentration occurred in the relatively soft austenite phase at 650°C under condition (b), in which the phase fraction of austenite is small.

4.2 Mechanism of hot workability change depending on strain rate

From the results shown in Fig. 8, hot workability decreased due to a decrease in the extension speed at the hot working temperature of 1050°C, and the decrease of RA was greater under condition (b), that is, when rapid cooling was conducted immediately before hot working. It is thought that this result was also influenced by the changes in the strength and phase fraction of each single phase during hot working. Therefore, first, the tensile strength TS of the δ ferrite and austenitic single-phase steels and the strength ratio of the δ ferrite phase to the austenite phase were measured at different extension speeds at the hot working temperature of 1050°C, as in Section 4.1, in order to consider the effect of single-phase strength.

Experiments were conducted under the four tensile extension speed conditions of 0.1, 1.0, 10.0, and 100.0 mm/s. Figure 11 shows the results of the change in tensile strength TS and the strength ratio of the δ ferrite phase to the austenite phase. From the experimental results, as the extension speed decreased, decreases in TS were seen in both phases, but on the other hand, the strength ratio increased. In other words, with this material and hot working temperature, the strength of the relatively soft δ ferrite phase decreases compared to that of the austenite phase as the extension speed decreases, and strain concentration occurs strongly in the δ ferrite phase. Therefore, it can be concluded that RA decreased due to the reduction in extension speed. When the strain rate in hot working decreases, hot workability improves due to strain recovery. However, in this material, which has a large content of alloy elements, the effect of strain concentration was greater than the effect of strain recovery.

Fig. 11

Tensile stress TS and TS ratio in δ ferrite and austenite single phase steels at 1050°C with extension speed of 0.1 to 100.0 mm/s.

Next, the effect of microstructural change was investigated. Figure 12 shows the austenite phase fraction measured by EBSD for all extension speeds. The horizontal axis indicates the time to fracture, which was calculated by dividing the amount of elongation to fracture by the elongation speed. In order to investigate the effect of strain on microstructural change, experiments were carried out with isothermal holding for 0, 0.2, 2.0, or 200.0 s without hot working after cooling at the cooling rates of (a) 0.3°C/s or (b) 10.0°C/s before hot working, and the austenite phase fractions were measured immediately after isothermal holding. From these results, under the slow cooling condition (a), there was no change in the austenite phase fraction regardless of the extension speed during working or the isothermal time without working. In short, no microstructural changes occurred during hot working. This is thought to be because the change in the phase fraction became saturated under the slow cooling condition (a) before hot working, and the microstructure was simply elongated by hot working. In the case of hot working after rapid cooling, i.e., condition (b), the austenite phase fraction after hot working increased as the extension speed decreased, indicating that microstructural change occurred under condition (b). In addition, at the slowest extension speed of 0.1 mm/s, the phase fraction after hot working was equivalent to that under condition (a), in which slow cooling was conducted before hot working. Therefore, it is considered that the equilibrium state was achieved during hot working after rapid cooling under condition (b). Under condition (b) without hot working, the austenite phase fraction gradually increased as the isothermal holding time increased, and it is thought that it gradually approached the equilibrium state. However, the austenite phase fraction was substantially lower than that under the condition of hot working at 0.1 mm/s. That is, the non-equilibrium state was maintained even after isothermal holding. From this result, the conditions for progress of the phase transformation to a phase fraction approaching the equilibrium state after low-speed hot working at 0.1 mm/s under condition (b) are not only a sufficiently long time for the element diffusion necessary for transformation, but also promotion of nucleation of the austenite phase and element diffusion by strain. In other words, it can be concluded that the effect of strain-induced transformation is a factor in this case.

Fig. 12

Austenite phase ratios for different holding and extension times at 1050°C. (a) Cooling rate: 0.3°C/s, (b) Cooling rate: 10.0°C/s.

Summarizing the above results, it can be concluded that the large decrease in hot workability accompanying the reduction in the extension speed under condition (b) shown in Fig. 8 was caused by an increase in the isothermal holding time and the strain-induced austenite transformation by low-speed hot working, resulting in a decrease in the phase fraction of the δ ferrite phase and concentration of strain on the relatively soft δ ferrite phase. However, under the rapid cooling condition (b), the phase fraction of the relatively soft phase could largely be maintained in the non-equilibrium state in the initial stage of hot working, and the hard phase generated by strain-induced transformation also contained no accumulated strain immediately after transformation. Therefore, it is considered that hot workability will always be better under the rapid cooling condition (b) than that under the slow cooling condition (a), assuming the same hot working temperature and deformation rate conditions.

5. Conclusion

In a study of the effect of the thermal history before working on the hot workability of 25Cr steel, which is a typical material with a dual-phase microstructure in the hot working temperature range, the cooling rate immediately before hot working was changed to slow cooling at 0.3°C/s or rapid cooling at 10.0°C/s, and a hot uniaxial tensile test was then conducted at temperatures in the range from 650°C to 1150°C. In addition, for the hot working temperature of 1050°C, the effect of the extension speed was also examined by changing the speed in the range from 0.1 to 100.0 mm/s. The following conclusions were obtained as a result of comparing the hot workability at each cooling rate before hot working and the microstructure after hot working.

  1. (1)    Comparing hot workability under conditions of rapid cooling and slow cooling immediately before hot working, hot workability improved at temperatures of 850°C or higher, even at the same hot working temperature, and was the same or decreased at temperatures below 850°C.
  2. (2)    Under condition (a), in which slow cooling was conducted immediately before hot working, the austenite phase transformation proceeded during slow cooling, and the phase transformation was near the equilibrium state at the time of hot working. Under condition (b), i.e., rapid cooling, a non-equilibrium state with a high fraction of δ ferrite phase was maintained until immediately before hot working.
  3. (3)    The strength of the austenite single phase was approximately double that of the δ ferrite single phase at hot working temperatures of 850°C or higher. However, the strength difference decreased at 750°C or lower, and the strength of the two phases was the same or the strength relationship was reversed at 650°C.
  4. (4)    Hot workability decreased when the single-phase strength difference between the δ ferrite phase and the austenite phase immediately before hot working was large and the phase fraction of the soft phase was low.
  5. (5)    When the extension speed was changed at the hot working temperature of 1050°C, hot workability was always better under the condition of rapid cooling immediately before hot working than under the condition of slow cooling, even at the same hot working temperature. However, the difference in hot workability became smaller as the extension speed decreased. This is because, in low-speed hot working, the strain-induced austenite phase transformation proceeds during hot working, the phase fraction of the relatively hard phase increases, and strain concentrates in the soft phase.

This paper has reported the results of research on the hot workability of duplex stainless steel. However, it is important to evaluate the hot workability of materials in which a non-equilibrium state occurs easily due to changes in the temperature history immediately before hot working, considering the influence of the temperature history immediately before hot working on the phase balance, which changes depending on the cooling rate, and the strength of the single phases before hot working, as these factors have a critical effect on hot workability.

REFERENCES
 
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