Dynamic Precipitation Behavior of Secondary M₇C₃ Carbides in Ti-alloyed High Chromium Cast Iron

Abstract

In-situ observations on the dynamic precipitation behavior of secondary carbides in Ti-alloyed High Chromium Cast Iron (HCCI) were performed by using a Confocal Laser Scanning Microscope (CLSM). Moreover, the detailed characterization of the microstructure before and after heat treatment was performed by using scanning electron microscopy (SEM). The secondary carbides, which precipitate from the matrix during heat treatment, were identified as M₇C₃ type carbides by using transmission electron microscopy (TEM). The number, size and volume of secondary carbides during heating, holding and cooling process were quantitatively evaluated based on the in-situ observation and SEM results. It was found that ferrite (α) and secondary carbides start to precipitate from the matrix at around 575°C and 840°C, respectively, during the heating process. In addition, the in-situ results showed that the number of secondary carbides increase with an increased heating temperature and time. Moreover, it was found that the size of these secondary carbides increase at higher temperatures and longer holding times. However, the number of secondary carbides increased with a decreased temperature. Finally, it was found that the volume fraction (~5%) of secondary carbides was not changed to a large extent for the different heat treatment conditions being investigated.

1. Introduction

It is well known that the secondary carbide precipitation during heat treatment plays an important role in improving the final properties of High Chromium Cast Iron (HCCI).^1,2) Therefore, the research has focused on the type, size, volume, morphology and distribution of these secondary carbides. Many previous studies^{2,3,4,5,6,7,8,9,10,11,12,13,14,15,16)} have been carried out to determine the types of secondary carbides (M₇C₃ type or M₂₃C₆ type) after heat treatment by using Light Optical Microscopy (LOM), Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD) and Transmission Electron Microscopy (TEM), as shown in Table 1.

Table 1. A summary of studies on secondary carbides in HCCI.

Type	C (%)	Cr (%)	Cr/C ratio	Size (μm)	Volume (%)	Number	Holding temperature and time/cooling	When precipitations start	Identify methods	References No.
M7C3 type	3.1	17.7	5.71	N	N	N	1000°C × 0.25 h,0.75 h, 4 h/air	N	SEM	[2]
	4	17	4.25	N	N	N	900°C–1100°C × 2 h–5 h/air	N	SEM/EDS	[3]
	2.4	15	6.25	0.2–0.6 μm	N	N	950°C–1050°C × 1 h–4 h/air	Formed during destabilisation	TEM	[4]
	1.9	18	9.47	N	N	N	1130°C × 4 h/air	During heat treatment	TEM	[5]
	3	20	6.67	< 2 μm	7.5%	31/100 μm2	1000°C × 4 h/air	During destabilisation	TEM	[6]
M23C6 type	2.77	16.38	5.91	N	N	N	580°C × 10 h,/air	N	TEM	[7]
	3.1	17.7	5.71	N	N	N	1000°C × 0.25 h, 4 h/air	During cooling process	TEM	[8]
	2	26	13.00	< 1 μm	N	N	1040°C × 1 h/air	N	LOM	[9]
	2.91	26.96	9.26	2 μm	5%	N	1075°C × 2 h/air	Destabilisation heat treatment followed by air cooling	TEM	[10]
	2.91	26.96	9.26	N	N	N	1075°C × 2 h/air	Destabilisation heat treatment followed by air cooling	TEM	[11]
	2.5	29.3	11.72	N	N	N	1000°C × 0.25 h,0.75 h, 4 h/air	N	SEM	[2]
	2.4	30	12.5	N	N	N	1100°C × 1 h/air	N	TEM	[12]
	2.3	30	13.04	N	10–20%	30–200/100 μm2	900°C–1100°C × 2 h–8 h/air	The holding period	TEM	[13]
	2.3	30	13.04	N	10–20%	30–200/100 μm2	900°C–1100°C × 2 h–8 h/air	Formed during destabilisation	TEM	[14]
M7C3 type + M23C6 type	1.09	10.48	9.61	N	N	N	300°C–650°C × 2 h/air	N	TEM	[15]
	2.77	16.38	5.91	~0.1 μm	N	N	1000°C × 0.5 h/air	Maybe in the cooling process after a destabilization treatment	XRD/TEM	[16]
	2.72	26.6	9.78	< 1 μm	4.1%	58/100 μm2	1000°C × 4 h/air	During destabilisation	TEM	[6]

Note: N, Not discussed.

Based on SEM-Energy Dispersive X-ray Spectroscopy (EDS) investigations, Zhi et al.³⁾ reported that the secondary carbide was a M₇C₃ type, if the heat treatment temperature was higher than 900°C, in Fe-4.0 wt.%C-17 wt.%Cr HCCI alloys. Also, Pearce et al.^4,5) and Wiengmoon et al.^{6,7,8,9,10,11,12,13,14)} discussed the effect of the Cr content on the type of secondary carbides. Their reports show that in HCCI alloys containing 15–20%Cr, the secondary carbides are M₇C₃ type, and in HCCI alloys containing 25–30%Cr, they are M₂₃C₆ type. Wang et al.^15,16) and Wiengmoon et al.⁶⁾ reported that a mixture of M₇C₃ type and M₂₃C₆ type carbides exist after heat treatment in the case of Fe-2.7 wt.%C-16 wt.%Cr HCCI alloys and Fe-2.7 wt.%C-26.6 wt.%Cr HCCI alloys. In addition, Wiengmoon et al.^14,17) reported that the eutectic M₇C₃ carbide was transformed into M₂₃C₆ carbide during the heat treatment.

As a summary of previous studies, the type of secondary carbides is M₇C₃, M₂₃C₆ and M₇C₃ + M₂₃C₆. Moreover, it seems that the M₇C₃ type secondary carbides will form more easily in HCCI alloys with lower Cr content; and the M₂₃C₆ type will form more easily in HCCI alloys with higher Cr content.

Not only the secondary carbides type, but also their size, number and volume of secondary carbides are very important to determine. However, only a few reports discuss the size, number and volume of secondary carbides in HCCI, as shown in Table 1. Furthermore, most of the researchers reported that the secondary carbides precipitated during the holding process or cooling process of heat treatment, as shown in Table 1. More specifically, the discussion among previous researchers was limited to the microstructural observations before and after heat treatment. However, there are no reports that present information on the dynamic behavior of secondary carbides in HCCI alloys during the heat treatment process.

There is however in-situ studies of precipitation phenomena for inclusions in other types of materials. For instance, Hasegawa et al.^18,19,20,21) discussed the in-situ precipitation phenomena in Fe–Cu alloys and Fe–Si alloys by using the Confocal Laser Scanning Microscope (CLSM) technique. Moreover, they examined the effect of the holding time and holding temperature on the number of precipitates. However, since there are no in-situ studies on the carbide precipitation behavior in HCCI alloys during heat treatment, we pursued such investigations.

The focus of this study is on the dynamic behavior of secondary carbide precipitation in Ti-added HCCI alloys. The experiments are conducted in-situ observation by using the CLSM method. Furthermore, the detailed information (size, number and volume) of these secondary carbides before and after heat treatment was measured quantitatively by using a CLSM and a SEM in combination with an image analyzer. Also, the identification of these secondary carbides was carried out by using TEM. The effect of the holding time and temperature on the number, size and volume of secondary carbides is discussed based on size distribution results. Finally, plausible reasons of these experimentally observed phenomena are given.

2. Experimental Methods

A detailed description of the preparation of as-cast ingot samples containing Fe-4 mass%C-17 mass%Cr-1.5 mass% Ti-1.9 mass% Mn-0.8 mass% Mo-1.11 mass% Ni-0.95 mass% Si, including casting in a graphite mold, have previously been reported by Liu et al.²²⁾ The cast ingots were cut into small pieces (3 mm (L) × 2 mm (W) × 2 mm (H)). Each specimen was carefully polished following the preparation procedure that was described in our previous study.²³⁾ The polished specimens were set into a high purity alumina crucible (Φ5.5 mm O.D. × Φ4.5 mm I.D. × 5 mm height) by using a CLSM with an infrared heating furnace. The wavelength of 1.5 mW He–Ne laser, which is used in CLSM setup, is 632.8 nm. A detailed description of the CLSM setup can be found in previous works.^24,25) The specimens were heated to 890°C and 1026°C using a 10°C/min heating rate and it was held for 2 and 6 hours, respectively. Details of the CLSM experimental conditions are given in Table 2.

Table 2. Experimental conditions during heat treatment.

Sample No.	Holding temperature and time	Heating rate	Cooling rate
1	890°C × 2 hr	10°C/min	90°C/min
2	890°C × 6 hr	10°C/min	90°C/min
3	1026°C × 2 hr	10°C/min	90°C/min
4	1026°C × 6 hr	10°C/min	90°C/min

The atmosphere in the CLSM was pure Ar; moreover, a titanium foil was wound around the upper part of the crucible to avoid oxidation of the specimen surface. Once the desired temperature and holding time was reached, the specimen was cooled at a cooling rate of 90°C/min.

The furnace temperature was measured by a PtRh30%–PtRh6% thermocouple (type B), which was inserted at the bottom of the crucible. In this case, the actual specimen temperature was calibrated by using a Pt–PtRh13% thermocouple (type R) through welding of a R type thermocouple wire on the surface of the specimen. It was found that the temperature difference between the specimen surface and the furnace is 13°C and 31°C at 890°C and 1026°C, respectively. The temperature history of the specimen surface and furnace is illustrated in Fig. 1.

Fig. 1.

Temperature history of specimen surface and furnace during the heat treatment process.

The microstructure of as-cast and heat treated specimens were carefully characterized using field emission scanning electron microscopy (using FE-SEM, HITACHI, S-4800 type and a JEOL JSM-7401F type). The back scattered electron (BSE) mode and 10 k magnification micrographs were used to quantitatively study the size distribution and volume fraction of secondary carbides. The number of carbides per unit area, N_A, was calculated using Eq. (1).   

$${\text{N}}_{\text{A(i)}}={\text{n}}_{(\text{i})}/{\text{S}}_{\text{obs}}$$(1)

where n_(i) is the counted number of carbides for each selected step (i) of a size distribution. The parameter S_obs is the total area of the micrographs, which was 539.0 μm² for 5 typical SEM micrographs and 4856.0 μm² for the in-situ observations, respectively. The size of secondary carbides, d, and the volume fraction of carbides were calculated as the equivalent diameter of a circle by using WinROOF, which is a commercial image analysis software.

Thin-foil specimens (~40 nm) were prepared by focused ion beam (FIB)-SEM (FEI, Nova 600) technique. Transmission electron microscopy was conducted using a TEM, JEOL JEM-2100F at an operating voltage of 200 kV, and the secondary carbides type were evaluated.

3. Experimental Results

The general microstructure of as-cast and heat treated Ti-added hypereutectic HCCI were reported in our previous studies.^22,23) It consists of primary M₇C₃ carbides (> 11.2 μm) with a large hexagonal columnar structure, eutectic M₇C₃ carbides (< 11.2 μm) with an irregular shape, TiC carbides with a cubic structure, and a matrix mainly containing the austenite phase (γ) in the as-cast condition (before heat treatment). After heat treatment, the matrix changes from austenite (γ) into martensite (α′) and the secondary carbides have precipitated from the matrix. In the present study, the precipitation behavior of secondary carbides during heat treatment is the main focus.

3.1. In-situ Observations of Secondary Carbide Precipitation

The images were extracted from a series of CLSM video sequences at a rate of 30 frames per second during the whole heat treatment. Images were further quantitatively analyzed by using an image analyzer, WinROOF, to determine the number, size and volume of secondary carbides.

Figure 2 illustrates the in-situ observations of the microstructure evolution in the Ti-alloyed HCCI heated to 890°C and held there for 2 h with a heating rate 10°C/min. The corresponding temperature and time during the heat treatment are also displayed in the images. It was found that the ferrite (bcc, α) start to form from the matrix at approximately 575°C during the heating process, as is shown in Fig. 2(b). The ferrite (bcc, α) phase should be stable at low and intermediate temperatures as determined from the phase diagram for a Fe-17 mass% Cr-4 mass% C-1.5 mass% Ti alloy, which was calculated in a previous study.²³⁾ It is the retained austenite (fcc, γ) that starts to transforms into ferrite (bcc, α) at 575°C. The ferrite will disappear again when approaching the holding temperature where austenite (fcc, γ) is the stable phase. The temperature where ferrite → austenite transformation takes place was at approximate 703°C, which is estimated by the phase diagram of this alloy.²³⁾ In addition, the volume of the ferrite increased with an increased temperature, as shown in Fig. 2(c). Moreover, at 666°C, it seems that some precipitates start to appear from surface of primary M₇C₃ carbides, as shown in Fig. 2(c). These precipitates increased with an increased temperature. However, the precipitates inside the samples were polished and studied carefully by using SEM after the heat treatment in our previous study²³⁾ and none of precipitates were found on the surface of primary M₇C₃ carbides. Hence, this indicates that the observed feature on the primary M₇C₃ carbides is a surface phenomenon.

Fig. 2.

In-situ observations of secondary carbide precipitations during the heat treatment process by using a CLSM to study sample No. 1.

Furthermore, at 840°C, the precipitates start to appear from the matrix as shown in Fig. 2(d) and the matrix phase is austenite (fcc, γ) according to the phase diagram. From Figs. 2(e)–2(h), it can be seen that the number of precipitates increased and the sizes became larger with an increased time and temperature during the holding and cooling process.

The increasing number of precipitates during the heat treatment is illustrated in Fig. 3. The total area for the counted precipitate number is 4856.0 μm² (73.8 μm (L) × 65.8 μm (W)). It was found that the precipitation starts at approximately 840°C and 847°C for the 890°C × 2 hr and 1026°C × 2 hr conditions, respectively. Moreover, the number of precipitates increased sharply at these temperatures. It was also found that the precipitate number per unit area did not change too much when the temperature reached the holding temperatures. Comparing the two different temperatures, the number of secondary carbides is higher for 890°C than for 1026°C.

Fig. 3.

Change of number of secondary carbides with time and temperature during heat treatment for samples No. 1 and No. 3 based on in-situ observation results.

Figure 4 shows the evolution of the size distribution of secondary carbides during heat treatment with holding temperatures of 890°C and 1026°C. The size distributions are plotted using a logarithmic scale. The value of size d_(j), for the j-th step, Δ_(j), of a log-normal distribution can be determined as follows:   

$${\text{d}}_{(\text{j})}=\text{ 1}{0}^{\mathrm{\Delta }(\text{j})}$$(2)

where Δ_(j) is the width of the j-th step for log-normal distributions (Δ_(j) = Δ_(j–1) + 0.15). This range is from –2.25 to 1.05. In this case, the mean value of the secondary M₇C₃ carbides size for the j-th step of the size distribution, d_(j), varies from 0.007 μm to 9.582 μm. In this section, only the secondary carbides, which are larger than 1 μm, are discussed because of a limited resolution of CLSM. The smaller secondary carbides (< 1 μm) will be discussed in detail in section 3.2.2.

Fig. 4.

Size distribution of secondary carbides determined in-situ observation by using CLSM for the following conditions: (a) 890°C × 2 hr and (b) 1026°C × 2 hr.

From Fig. 4(a), it was found that the number of secondary carbides increased with an increased heating temperature. Moreover, the number of carbides did not change to a large extent during the holding and cooling stage. However, the number of secondary carbides decreased with increased holding and cooling times. It can also be seen that the size of the secondary carbides increases from the heating stage to the cooling stage. Compared to the 890°C × 2 hr condition, the same tendency was obtained for a 1026°C × 2 hr condition, as seen in Fig. 4(b). However, it was found that the number of secondary carbides is lower for 1026°C × 2 hr condition compared to 890°C × 2 hr condition.

Figure 5 shows the evolution of volume fraction of secondary carbides, which was obtained from the in-situ observation. The volume fractions of secondary carbides increase gradually with an increased heat treatment time. In addition, it was found that the volume fraction of secondary carbides at lower temperatures is a little bit higher than that at higher temperatures. However, the volume fraction will not change to a large extent (5% ± 2%) when the holding time and holding temperature change after holding for 2 hr according to results in section 3.2.2. A detailed explanation of the influence of holding time and temperature on the volume fraction of the secondary carbides will be given in section 3.2.2.

Fig. 5.

Volume fraction of secondary carbides as a function of time determined from in-situ observation by using CLSM.

In summary, the in-situ observation results showed that the precipitation of secondary carbides start at about 840°C during the heating stage. This indicates that the nucleation and growth of secondary carbides already happened during the heating process. Meanwhile, the number of these secondary carbides increased suddenly at temperatures higher than 840°C. Thereafter, it did not change too much when the temperature reached the holding temperatures. Moreover, both the size and volume fraction of secondary carbides increased with an increased heat treatment time.

3.2. Characteristic of Carbides after Heat Treatment

3.2.1. Primary and Eutectic M₇C₃ Carbides

Figure 6 shows SEM observations from the same position before and after heat treatment. The size of primary M₇C₃ carbides and eutectic M₇C₃ carbides increases at higher temperatures and longer holding times, as shown in Figs. 6(c)–6(f). The primary and eutectic M₇C₃ carbides grow and finally merge with each other, as marked by A and A′ in Figs. 6(c) and 6(d). This is also indicated by the length of mark B and B′ in Figs. 6(e) and 6(f), which shows that the distance decreases from 9.0 μm to 5.3 μm. These direct data from the SEM observations is in agreement with our previous conclusions regarding the M₇C₃ carbides growth phenomenon.²³⁾

Fig. 6.

SEM observations before heat treatment (a, c and e) and after heat treatment (b, d and f) at the same position.

Compared to the microstructure before heat treatment, it can be seen in Fig. 6(b) that the secondary carbides precipitate from the matrix during heat treatment. The following section will discuss the secondary carbides in detail after heat treatment.

3.2.2. Secondary Carbides

(1) Identification of Secondary Carbides:

As was illustrated in Table 1, many researchers have determined the type of secondary carbides (M₇C₃ type or M₂₃C₆ type or mixture of M₇C₃ type and M₂₃C₆ type) after heat treatment. The identification methods include LOM, SEM/EDS, XRD and TEM. However, it is not possible to classify the secondary carbides type by LOM. It is also hard to identify the secondary carbide types by SEM/EDS, because it is not possible to obtain accurate carbon contents from the EDS results. There are also no obvious peaks for secondary carbides during an XRD analysis if the volume fraction of secondary carbides is low (~5%). Thus, the characterization of the secondary carbides type was solely done by using the crystal structure information from the TEM examinations.

Figure 7 shows a TEM micrograph, the selected area diffraction pattern (SADP) and the EDS results for a secondary carbide in sample No. 2. It shows that the secondary carbides have a size of ~250 nm in diameter. The SADP in Fig. 7(b) form the particle, marked in Fig. 7(a), is indexed to be a M₇C₃ carbide in the Fe-4 mass%C-17 mass%Cr-1.5 mass% Ti hypereutectic HCCI alloy. In addition, the EDS result from the secondary M₇C₃ carbide also shows that the composition of this particle is rich in Cr, Fe and C, as is shown in Fig. 7(c). It is different from the matrix (Martensite, α′), which is only Fe rich, as is shown in Fig. 7(d).

Fig. 7.

TEM micrograph of secondary carbide in sample No. 2 (890°C × 6 hr): (a) a bright–field TEM micrograph and (b) a selected area diffraction pattern (SADP) of secondary M₇C₃ carbide and (c) an EDS analysis of a secondary M₇C₃ carbide and (d) an EDS analysis of matrix.

(2) Size Distribution and Volume Fraction of Secondary Carbides:

Figure 8 shows the secondary carbides for different heat treatment conditions observed in SEM. It can be seen that the number of secondary carbides at lower temperatures such as 890°C (Figs. 8(a) and 8(c)) is higher than that at higher temperatures such as 1026°C (Figs. 8(b) and 8(d)). In addition, the size of secondary carbides seems to increase with an increased holding temperature and time.

Fig. 8.

SEM observations of secondary M₇C₃ carbides for the following heat treatment conditions: (a) 890°C × 2 hr, (b) 1026°C × 2 hr, (c) 890°C × 6 hr and (d) 1026°C × 6 hr.

The size distribution of secondary carbides is shown in Fig. 9. The size distributions are plotted using a logarithmic scale. The value of size d_(j), for the j-th step, Δ_(j), of a log-normal distribution can be determined by using Eq. (2). The Δ_(j) range is from –2.25 to 1.05. In this case, the mean value of the secondary M₇C₃ carbides size for the j-th step of the size distribution, d_(j), varies from 0.007 μm to 9.582 μm.

Fig. 9.

Comparison of the size distribution of secondary M₇C₃ carbides. (a) Data from present work and (b) data analyzed based on SEM images from Wiengmoon et al.^13,14)

Figure 9(a) shows the combination results from the SEM study after heat treatment and in-situ observation by using CLSM in this study. It was found that the number of secondary M₇C₃ carbides per unit area increases with a decreased temperature (as 890°C × 2 hr). Meanwhile, compared to a lower holding temperature and a shorter holding time (as 890°C × 2 hr), the size of secondary M₇C₃ carbides increases with increased holding times and holding temperatures (as 1026°C × 6 hr). Furthermore, SEM micrographs from the literatures^13,14) were studied by using an image analyzer and the size distribution results are shown in Fig. 9(b). The total area, S_obs, which were taken based on the SEM images from literatures,^13,14) were 181.9 μm² and 81.2 μm² respectively. Compared to the data in Fig. 9(a), it can be seen that these previous results (order of size and number) agree well with the results of the present study. The number of secondary carbides is high at a lower temperature (as 900°C). Moreover, the size of secondary carbides is large at higher temperature (as 1025°C).

Figure 10 shows the volume fraction of secondary M₇C₃ carbides based on the previous studies and the present study. It can be seen that the mean volume fraction of secondary M₇C₃ carbides is almost 5% ± 2% at different heat treatment conditions based on SEM and in-situ observation. In addition, it was found that the volume fraction of secondary M₇C₃ carbides is not changed to a large extent when changing the heat treatment conditions. As a confirmation of the results in the present study, data from Wiengmoon et al.^6,11,13,14) were used as a comparison. The same tendency between the present results and Wiengmoon et al.^13,14) results can be found. However, compared to the present study, the volume fraction of secondary carbide is much higher in Wiengmoon et al. studies.^13,14) One possible reason for this difference is that the Cr content is higher in the Wiengmoon et al. study.^13,14) Furthermore, the volume fraction values (~5%) in this study agree with the Wiengmoon et al. results.^6,10)

Fig. 10.

Comparison of the volume fraction of secondary carbides between data from present work and data from Wiengmoon et al.^6,11,13,14)

In summary, the present results and the Wiengmoon et al. results on volume fraction of secondary carbides indicate that the volume fraction of secondary carbides is not obviously affected by the holding time and temperature when the holding temperature is reached.

4. Discussion

4.1. Secondary Carbides Formation

A precipitation process of carbides always includes three different stages: i) nucleation, ii) growth and iii) coarsening or Ostwald ripening.²⁶⁾ Some researchers gave the kinetics description of secondary carbides and discussed when the precipitation will occur.^2,27,28) Almost all of the literature reports concluded that precipitation occurred during the holding period, as is shown in Table 1. Powell et al.²⁾ reported that the precipitation occurs during the first 15 min and Kuwano et al.²⁷⁾ reported that precipitation occurs after 1 min when reaching a 1000°C holding temperature. Bedolla-Jacuinde et al.²⁸⁾ reported that the time of precipitation depends on the holding temperature. More specifically, it was started at about 10 min at 900°C and 5 min at 1000°C and 1150°C.²⁸⁾ However, from the present in-situ observation results, the precipitation of secondary carbides started at about 840°C in the γ phase region during the heating stage under 10°C/min heating rate before the holding temperature was reached, as shown in Fig. 11. It concludes that the precipitation kinetics should be influenced strongly by heating rate if the heating rate is slow, i.e. at 10°C/min.

Fig. 11.

Precipitation behavior of secondary M₇C₃ carbides that transformed during heat treatment.

The discussion in previous work has mainly been limited to when the precipitation behavior occur. Few reports can give the explanation on the selection of holding time and temperature based on the volume fraction of secondary carbides. In the present work, the secondary carbides suddenly occur at a critical temperature during the heating process; and the volume of secondary carbides gradually increases with an increased time; finally the precipitation was almost finished within 2 hr after reaching the holding temperature, as shown in Fig. 11. It indicates that the nucleation and growth of some secondary carbides already happened during the heating process. Thus, keeping the alloy for a longer holding time causes coarsening of secondary carbides. This is widely accepted that once secondary carbides have nucleated, the longer holding times at the target holding temperature lead to an increased size due to growth and coarsening. Furthermore, even longer holding times and higher holding temperatures cause a coarsening of the carbides and reduces the number of secondary carbides due to Ostwald ripening process.^27,28) Previous works^27,28) agree with the results of this study regarding the growth of secondary carbides.

4.2. Volume Fraction of Secondary Carbides

The secondary precipitations can increase the bulk hardness of the steel and the reason can be explained by the precipitation hardening mechanism.^29,30,31) In this case, the volume fraction of secondary carbides should be considered. As shown in Fig. 11, the volume fraction of secondary carbides are 5.7% ± 2.3%, 5.9% ± 1.8% for the conditions of 1026°C × 2 hr and 1026°C × 6 hr. Moreover, the volume fractions of secondary carbides are 5.5% ± 0.4%, 5.3% ± 2.5% for the conditions of 890°C × 2 hr and 890°C × 6 hr. It is indicate that the volume fraction of secondary carbides is not affected by the holding time and the holding temperature after holding for 2 hr. It should be noted that the total volume fraction of primary M₇C₃ carbides and eutectic M₇C₃ carbides did not reach the equilibrium fraction ~30% in Fe-4 mass%C-17 mass%Cr-1.5 mass% Ti HCCI alloys but instead the volume fraction was ~25% in the as-cast condition.²²⁾ Thus, the volume fraction of secondary M₇C₃ carbides of ~5% is reasonable.

Moreover, it can be seen from Table 1 that the volume fraction of secondary carbides seems to be related to the Cr/C ratio. More specially, a higher Cr/C ratio corresponds to a higher volume fraction, but the data are somewhat scattered. However, one agreement between the present work and other studies^13,14,29) is that the volume fraction of secondary carbides is not obviously affected by the holding time and the holding temperature when the holding temperature has been reached.

5. Concluding Remarks

In the present work, the secondary carbide precipitation behavior in Fe-4 mass%C-17 mass%Cr-1.5 mass% Ti HCCI alloys was directly observed by using an in-situ observation at 890°C, 1026°C for 2 hr and 6 hr, respectively. In addition, results on the number, size and volume of secondary carbides are presented and discussed based on CLSM, SEM and TEM analyses. The obtained conclusions can be summarized as follows:

(1) The secondary carbides were identified as a M₇C₃ type found in Fe-4 mass%C-17 mass%Cr-1.5 mass% Ti HCCI alloys.

(2) Precipitation of secondary M₇C₃ carbides started during the heating process (~840°C) and the number of these secondary M₇C₃ carbides increased sharply at temperatures higher than 840°C. Furthermore, it did not change much when the temperature reached the holding temperatures.

(3) Based on size distribution of secondary M₇C₃ carbides, it was found that the size of secondary carbides increases with an increased temperature and time; however, the number of secondary carbides increases with a decreased temperature.

(4) The precipitation of secondary carbides will almost finish within 2 hr after reaching the holding temperature since the volume fraction of secondary carbides appeared unaffected by the holding time and holding temperature after holding for 2 hr.

In summary, the results from this study show that a lower temperature and a shorter holding time, as 890°C × 2 hr, is preferred to precipitate secondary M₇C₃ carbides.

Acknowledgements

The authors would like to thank associate Prof. Lyubov Belova (KTH) for the help with TEM sample preparation using Focused Ion Beam (FIB) technique. Qiang Liu is grateful to Mr. Akifumi Harada for the help during SEM measurement in Tohoku University. He also acknowledges the China Scholarship Council for financial support of his research.

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