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Journal of Materials Engineering and Performance

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Open Access 15-02-2023 | Technical Article

Influence of Various Heat Treatment Cycles on the Phase Transformation and Microstructure Evolution of AISI 329 Super-Duplex Stainless Steel

Authors: Nader El-Bagoury, Hossam Halfa, M. E. Moussa

Published in: Journal of Materials Engineering and Performance | Issue 19/2023

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Abstract

The importance of duplex stainless steel in the global steel industries takes from its application because of its utilization in the environment, the paper industries, and pipelines and reactions champers found in the gas, oil, and chemicals industries. This research investigates the effect of various heat treatment cycles on the intermetallic compound precipitation and different constituents' evolution of the AISI 329 steel. Also, this investigation aims at exploring the relationship of the microstructure evolution at various heat treatment cycles and its effect on the tensile properties of selected steel. The phase transformation studies were conducted theoretically utilizing the JMatPro program and experimentally employing differential scanning calorimetry tests and dilatation; the obtained microstructure was inspected by exploiting a light metallographic microscope, a field-emission scanning electron microscope, and x-ray diffraction. The results illustrated that the microstructure includes ferrite, austenite phases, and dispersive particles. The dispersive particles are carbide particles and intermetallic compounds. Furthermore, the sigma phase settled at the boundaries of the ferrite grains. Intermetallic compounds were observed in both cast and heat-treated steel microstructures. Observed intermetallic compounds have no specific engineering geometry with various sizes, and the main alloying elements are Cr, Fe, and Mn. Special heat treatment, including solid solution and aging at high temperatures, leads to thinning of the sigma phase while diminishing the size of the intermetallic compound. Tensile properties were enhanced by controlling the volume fraction of different constituents that are adjusted by heat treatment. The fracture morphology is consistent with the microstructure and tensile properties of the investigated steel.

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1 Introduction

Super-duplex stainless steel has widespread applications in the as-cast and as-rolled forms, remarkably proposed for the paper industry, pulp, and oil-pumping equipment parts (Ref 1-3). The austenitic–ferritic stainless steel provides an appreciated percentage to performance applications, such as reaction vessels and production pipelines used in gas, oil, and chemical industries.

The austenitic–ferritic stainless steel is characterized by better resistance to corrosion which is a cause of replacing this steel the traditional austenitic steel in the environmental industry, especially for parts working at temperature ranges from 50 to 300 °C. Moreover, this steel is characterized by a low-yield strength-weight ratio that permits a thickness reduction of many final products in structural applications (Ref 4, 5).

Duplex stainless steel microstructure is composed of ferrite and austenite phases with a volumetric percentage of about 50 ± 5 (Ref 6). Duplex stainless steel technological properties are a combination of characteristics and the beneficial effects of the two-phase structure of BCC-phase (ferrite) and FCC-phase (austenite). The mentioned duplex microstructure permitting the steel to have high toughness from the austenite phase. In contrast, this steel is strengthened by austenite and ferrite phases for parts exposed to low or subzero temperatures.

Super duplex and duplex stainless steel is regarded as Fe-Ni-Cr base steel alloys containing 0.1-0.3 wt.% of molybdenum, 0.10-0.22 wt.% of nitrogen, 1-5 wt.% of nickel, and 20-24 wt.% of chromium. SDSS has superior mechanical performance and similar corrosion resistance, comparable to copper alloys, as documented in many publications (Ref 7, 8). Besides the numerous advantages of this steel, it keeps its mechanical properties intense when working in acidic media (chloride ions), generally because it delivers crevice corrosion, stress corrosion cracking, and high resistance to pitting corrosion (Ref 9, 10). On the other hand, all mentioned properties deteriorated due to the existence of secondary phases, for example, secondary austenite (γ), carbide (M₇C₃, M₂₃C₆), and nitrides (Cr₂N, CrN) particles, as well as intermetallic compounds (χ-phase, π-phase, R-phase, and σ-phase) (Ref 11-13). Furthermore, the mentioned phases principally diminish the resistance to corrosion and the impact toughness of steel.

Many investigators reported the starting temperature for martensite transformation, precipitation of carbide, and intermetallic compounds (Ref 14-20). These studies stated that during aging isothermally or continuous cooling, the precipitation and transformation reactions occur at a temperature varying from 300 to 1000 °C. Spinodal decomposition of α-phase has happened at a temperature ranging from 300 to 500 °C, known as the embrittlement phenomenon. The sigma phase precipitated at a temperature ranging from 850 to 900 °C with an incubation time of about 5 min (Ref 20). Sigma-phase precipitation has a decisively negative effect on the toughness of the materials.

It was noteworthy that the heat treatment of investigated steel is a milestone technology for conferring the required technological properties of the product. Rapid cooling after isothermal solid solution annealing at temperatures ranging from 1000 to 1100 °C is the most ordinary sort of heat treatment documented elsewhere (Ref 21). The main aims of the applied heat treatment are to reduce the micro-heterogeneity (Ref 22, 23), dissociate the secondary phases (Ref 24), as well as to attain an optimal phase balance (Ref 25).

The phase balance in SDSS achieved by precise heat treatment is essential for improving the technological properties. The literature reveals a research gap in studies focused on the influence of heat treatment on the microstructure of this grade of super-duplex stainless steel AISI 329. This investigation evaluates the constituent's transformations and the precipitation of intermetallic phases of AISI 329 SDSS subjected to various heat treatment cycles. Also, this investigation aims at studying the relationship between the microstructure evolution at different heat-treatment processes and the tensile properties of examined steel.

2 Experimental Procedures

The super-duplex stainless steel AISI 329 studied was produced by melting crude materials in an INDUCTOTHERM furnace with a 30-kg capacity under an argon-controlled atmosphere. The super-heated liquid metal was teemed into a sand mold to attain the final cast solid ingot. Optical emission spectrometry was utilized to analyze the chemical composition of the examined steel alloy.

Dilatometry studies were conducted employing a Quenching LINSIES L87 dilatometer. Round samples of 4 mm radius and 20 mm height were heated gradually in a vacuum chamber by induction at 0.1 °C/s-1050 °C and kept for 0.5 h, then rapidly cooled at several rates of 30 and 40 °C/min. The difference in length was documented with temperature and time using a computer-controlled test process.

Phase changes during continuous heating of examined steels were studied using a differential scanning calorimetric (DSC) with a heating rate of 0.5 °C/s. SETARAM Labsys™ TG-DSC16 (SETARAM Instrumentation, Caluire, France) was used to perform the differential scanning calorimetric (DSC) measurements.

During this work, heat treatment cycles were designed to investigate their impact on the different constituents and precipitation of the intermetallic compound, as shown in Fig. 1. Samples of the cast alloy were homogenized for 1050 °C/1 h, followed by water quenching, then aged under various conditions. The homogenized specimens were separated into three main groups. In the first group, the homogenized steel specimens were aged at 475 °C for various time intervals ranging from 3 to 9 h. In the second group, homogenized steel samples were aged at 850 °C for an hour, followed by quenching in different media, once in the water (WQ) and another in the oil (OQ). In the third group, the as-cast samples were heat treated through the solid solution annealing at 1100 °C and maintained for 0.5 h, followed by furnace cooling (FC) and holding for an hour at 900 °C, then water quenched (WQ).

The microstructures of the investigated steel were distinguished utilizing a PANalytical Empyrean x-ray diffractometer (XRD). This XRD is equipped with a Ni-monochromatic and a copper target that radiates Cu-Kα radiation (λ = 1.540 Å). The x-ray diffraction scans were achieved continuously for diffraction angle (2-theta) values running from 30° to 100° at ambient temperature. XRD machine operated in working current 40*10⁻³ A and working voltage 45*10³ V, with the scan size step and time being 0.02° and 1 s, respectively. After the complete scanning, the XRD measurements were compared by Xpert PRO software with the related databases-34.

The austenite-phase volume fraction (Vγ) was identified by applying the well-established experimented relation clarified in Eq 1 (Ref 26-28):

$$V\gamma \, = {{1.4 \, *I\gamma } \mathord{\left/ {\vphantom {{1.4 \, *I\gamma } {\left[ {1.4*I\gamma \, + I\alpha } \right]}}} \right. \kern-0pt} {\left[ {1.4*I\gamma \, + I\alpha } \right]}}$$

(1)

where the integrated intensity "Iγ" is the mean intensity for the diffraction plane described in the austenite phase for characteristic miller indices (111) γ and (200) γ. “Iα” is characterized by ferrite phase intergraded intensity diffracted from the plane miller indices, like (110) α plane. The correction factor with the value of 1.4 in Eq 1 was verified experimentally, as reported by numerous researchers (Ref 26-28).

Light and field-emission scanning electron microscope (FESEM) were employed to explore the microstructure of the steel under investigation. The declared steel samples were prepared by grinding on consecutive grades of silicon emery paper (120-2000) and finally polished utilizing fine alumina and submicron-diamond paste. QUANTA FEG 250 instrument used to carry out the FESEM analysis. The etching solution used for the microstructure characterization consists of 10 g of FeCl₃ + 15 ml HCl + 10 ml HNO₃ + 20 ml H₂O. The amount and micro-segregation of alloying elements found in various microstructures were performed using EDS in FESEM.

The mechanical properties of investigated steel samples were machined to standard tensile dimensions and tested according to ASTM specification E-8. The tensile test results notified in this investigation are the mean value of three examinations for the condition.

3 Results and Discussion

Table 1 illustrates the chemical composition of melted AISI 329 duplex stainless steel.

Table 1

Chemical composition of AISI 329 super-duplex stainless steel, (Wt.%)

Alloy	Chemical composition, wt.%
Alloy	Cr	Ni	Mo	Mn	Si	N	C	P	S	Fe
AISI 329	21.4	5.14	1.16	0.755	0.254	0.01	0.073	0.005	0.0005	Bal

3.1 Transformation Temperature Determination

The temperatures of phase transformation of the existing constituents were studied theoretically using the JMatPro program. The temperatures of phase transformation of the existing constituents were studied theoretically using the JMatPro program and, experimentally, a dilatometer and diffraction scanning calorimetry, DSC.

3.1.1 JMatPro- Program Studies (As-Cast Structure)

Figure 2 presents the different constituents precipitated during the solidification of AISI 329 SDSS from the melt (1600ºC) to room temperature. Figure 2 shows that the molten steel begins to solidify through the ferrite region until solidus temperature. At a high temperature (≈1350 °C), solidus temperature reached, and the solid–solid reaction (ferrite transformation into austenite) proceeded. This transformation reaction consumes the BCC phase(ferrite) and precipitates the FCC phase (austenite). Continue decreasing the cooling temperature to 900 °C, which derives a growth in the amount of the austenite phase and encourages precipitation of many compounds, as listed in Table 2.

Table 2

Thermo-dynamical-equilibria calculation of investigated AISI 329 SDSS

Constituents	Temperature, °C		Weight fraction at R.T	Alloying elements, mole percent
Constituents	Start	Finish	Weight fraction at R.T	Fe	Cr	Mn	Mo	Ni	Si	C	S	P	N
Liquidus	> 1490
Solidus	< 1472
Liquid	> 1490	< 1472
Ferrite	1490	RT	60.54	99.1	…	…	…	…	…	…	…	…	…
Austenite	1206	RT	4.75	17.28	8.8	8.35	…	65.6	…	…		…	…
MnS	1191	RT	0.0014	…	…	60	…	…	…	…	36.86	…	…
Sigma-phase	838	RT	30.81	31.65	65.83	…	0.65	1.88	…	…	…	…	…
M₃P	758	RT	0.03		83.41	…	…	…	…	…	…	16.65	…
M₂(C, N)	670	570	XXX	0.09	86.78	…	0.82	0.77	…	…	…	…	11.54
PI-phase	655	RT	0.20	…	58.17	…	…	36.93	…	…	…	…	4.9
Laves-phase	593.1	RT	2.00	34.3	16.47	…	47.55	…	1.7	…	…	…	…
G-phase	497	RT	1.64	…	…	22.49	…	64	13.42	…	…	…	…

Beyond cooling temperature equal to 900 °C sequence of solidification behavior was inversed. In other words, several compounds were precipitated from the austenite phase leading to an increase in ferrite phase volume fraction with the decrease in cooling temperature until room temperature. The results of the JMatPro program suggested that solid solution temperature (for dissolving all compounds and precipitating in the as-cast structure) equals 1100 °C. The previous treatment produces a microstructure that includes ~ 50% wt.% ferrite phase and ~ 50 wt.% austenite phase. In addition, Fig. 2 emphasizes the presence of traces from intermetallic compounds.

3.1.2 Dilation Studies (Starting As Cast Steel)

JMatPro software suggested the amount of each phase and the phase transformation temperature when the molten metal solidified from the melt to room temperatures under equilibrium conditions. On the other hand, predicting the actual-phase transformation temperature of investigated steel dilation curve was accomplished for as-cast steel.

Figure 3 and 4 represents the change in the length of investigated steel with a temperature change (dilation curve). Parts (b) and (c) in Fig. 3 and 4 display the first derivative with temperature separately for the heating and cooling periods. Figure 3(b) shows the changes in the dilation function during heating. The deep study of this curve shows many precipitations or dissociation reactions occur during heating from room temperature up to 428 °C. The results extracted from Fig. 3 and 4 confirm the theoretical result obtained by JMatPro software and suggest that the microstructure of investigated steel in cast condition is very complicated. Therefore, the dilation diagram peaks are described by utilizing the results of the JMatPro software. Figure 3(c) represents the phase transformation during the cooling of investigated steel and shows that just two peak groups appeared during cooling. The first group peak at high temperature displays the ferrite–austenite transformation reaction. Another peak group for precipitation at low temperatures is for intermetallic compound particle precipitation and the secondary transformation of austenite (FCC-phase) to ferrite (BCC-phase).

The second phase precipitated during cooling after aging at temperatures ranging from 500 to 600 °C. Many investigators reported that the sigma phase was precipitated in a temperature ranging from 650 to 900 °C depending mainly on the chemical composition and cooling rate (Ref 29).

The extracted results from the dilation test confirmed the data attained from JMatPro. The final microstructure of investigated steel consists of ferrite, austenite, and secondary particles. Dilation data emphasized the importance and positive effect of the heat treatment process on this steel. During heating and holding, the presence of the intermetallic compound and other constituents dissolved in the matrix.

The previous section suggested that the microstructure of heat-treated samples consists of ferrite-phase and austenite-phase, in addition to intermetallic compounds particles (e.g., sigma phase).

3.1.3 Differential Thermal Analysis of the Cast-Specimen of AISI 329 SDSS

Results of DSC measurements on the cast AISI 329 super-duplex stainless steel are shown in Fig. 5. Two exothermic reactions were identified by this measurement where are exothermic. The first exothermic reaction peak was displayed at a temperature of ~ 480 °C. The second peak appeared at a temperature of ~ 660 °C. Since these temperatures peak are located in the transformation temperature ranges of the α-phases and α′, it is suggested that the first peak on the DSC curve reveals the formation of the α-phase, and the second peak is because of the precipitation of the σ- phase.

3.2 X-ray Diffraction Studies

Figure 5 displays the x-ray diffraction pattern obtained for cast and solution-treated specimens from AISI 329 SDSS steel. The appearances of (111), (200), (220), (311), and (222) super-structural peaks unambiguously indicate the FCC phase (austenite) existence. On the other hand, the presence of the (110), (200), (211), and (220) planes is emphatic proof of the ferrite phase existence in the microstructure of the investigated steel in both solution-treated and as-casted cases. There is no evidence for the σ-phase existence in the microstructure of both cast and solution-treated steel specimens, where no separate peaks belonging to the latter constituents appeared in the spectrum of both cases. However, it should be noted that the height of the (330) plane belongs to the sigma phase and overlaps with plane (111) belongs to the austenite phase. Moreover, another sigma phase peak has the miller indices of the plane (202) coincide with the plane (110) of ferrite. The previous result confirmed the result obtained by Elmer et al. (Ref 30).

Therefore, it could not notify the peaks related to the sigma phase because it superimposed the principal peaks of austenite and ferrite phases in the XRD spectra of the as-casted and annealing-treated specimens. The direct method was used to calculate the fraction of each constituent utilizing the result of the XRD pattern, as presented in Table 3. Figure 6 emphasizes the suggested microstructure by dilation test. The microstructure of investigated steel consists of austenite, ferrite, and the second phase. The XRD data supposed that the second phase is probably the sigma phase.

Table 3

The amount fraction of each constituent utilizing XRD data

Material	Condition	Phase, %
Material	Condition	Austenite, %	Ferrite, %
AISI 329 SDSS	As-cast	78	22
AISI 329 SDSS	Solid solution annealing	56	44

3.3 Microstructure of the Cast Steel

The cast microstructure of the prepared sample is displayed in Fig. 7. Photos show that it consists of the austenite phase (the white area) and the ferrite phase (the gray area) plus the sigma (σ) phase (black area). The σ-phase located at the grain boundaries between the austenite and ferrite phases. The formation of the sigma phase consumes ferrite-forming elements, for example, Mo and Cr, on the expenditure of the ferrite phase (Ref 31-33).

The photo-analysis technique was utilized to compute the amount of the sigma phase calculating volume fraction (Vf) existing in the as-cast microstructure with the assistance of optical micrographs, as per ASTM 562 standard. The percentage of the sigma phase in that microstructure was calculated utilizing the data from Fig. 7 and 8. The sigma phase volume fraction was about 10%. The Vf of the other constituents of the austenite and ferrite phases in the cast microstructure exhibited in Fig. 7 is almost 43 and 47%, respectively.

Some precipitate particles were noticed in the as-cast microstructure sample, as shown in Fig. 7. The precipitated particles are marked with a black circle to differentiate them from other phases. These precipitated particles are formed in triangle, square, rhombus, or sometimes non-geometrical shapes, as displayed in Fig. 8.

Figure 9 illustrates the mapping of different alloying elements presented in the as-cast microstructure of investigated steel. It emphasized that the chromium base phase was precipitated at the grain boundary of the primary constituents (austenite and ferrite). The other alloying elements are well distributed in the matrix. On the other hand, the variation in the chemical composition of the different constituents was known by utilizing EDS line analysis, as shown in Fig. 10. The average EDS analysis data are collected in Table 4. The EDS result emphasized the segregation of chromium and iron as evidence of the existence of the σ-phase. In addition, the EDS result confirmed its location at the grain boundaries of both the BCC and FCC phases.

Table 4

Average EDS analysis for α, γ, and sigma phases in as-cast steel samples

Conditions	Phases	Chemical composition, wt.%
Conditions	Phases	Cr	Si	Mo	Ni	Mn	Fe
As-cast	α	22.72	1.95	2.28	3.77	1.72	67.56
	γ	20.05	1.25	1.98	5.38	1.43	69.91
	σ	50.50	0.21	0.92	0.93	23.45	20.51

3.3.1 Theoretical Prediction of Sigma Phase

According to Eq 2 and 3, the Ni equivalent (Ni_eq) and Cr equivalent (Cr_eq) calculated for the alloy used in this investigation are equal to 18.445 and 10.72, respectively (Ref 21). Therefore, the ratio between Ni_eq and Cr_eq is about 1.72. This ratio indicates a significant probability of formation of the σ-phase in this type of steel, based on Martins and Casteletti (Ref 21).

$${\text{Ni}}_{{{\text{eq}}}} = \, \left( \% \right){\text{ Ni }} + \, \left[ {\left( {{3}0} \right) \, \left( \% \right){\text{ C}}} \right] \, + \, \left[ {\left( {0.{5}} \right) \, \left( \% \right){\text{ Mn}}} \right] \, + \, \left[ {{26 }\left( \% \right){\text{ N }}{-} \, 0.0{2}} \right)] \, + { 2}.{77}$$

(2)

$${\text{Cr}}_{{{\text{eq}}}} = \, \left( \% \right){\text{ Cr }} + \, \left[ {\left( {{1}.{5}} \right) \, \left( \% \right){\text{ Si}}} \right] \, + \, \left[ {\left( {{1}.{4}} \right) \, \left( \% \right){\text{ Mo}}} \right] \, + \, \left( \% \right){\text{ Nb }}{-}{ 4}.{99}$$

(3)

Moreover, according to Gow and Harder (Ref 34), the empirical formula (known as ratio factor) was employed to predict the precipitation tendency of σ-phase (intermetallic compounds) in the stainless steel alloys, as expressed in Eq 4 (Ref 34):

$${\text{Ratio factor }} = {{\left( {{\text{Wt}}. \, \% {\text{Cr }}{-}{ 16 }*{\text{Wt}}. \, \% {\text{ C}}} \right)} \mathord{\left/ {\vphantom {{\left( {{\text{Wt}}. \, \% {\text{Cr }}{-}{ 16 }*{\text{Wt}}. \, \% {\text{ C}}} \right)} {{\text{Wt}}. \, \% {\text{ Ni}}}}} \right. \kern-0pt} {{\text{Wt}}. \, \% {\text{ Ni}}}}$$

(4)

Moreover, Gow and Harder (Ref 34) mentioned that if the ratio factor, using Eq 4, is higher than 1.7, the tendency of the formation of the σ-phase increases. However, the calculated ratio factor is approximately 1.62 for investigated steel which has 0.073 wt.% of carbon in the chemical composition, and the C_req = 18.445.

Depending on the mentioned calculations, the result obtained utilizing this equation is not coincide with that of the current work, where it appears at the ratio factor of 1.62, as mentioned by Gow and Harder (Ref 34). This result takes into consideration the effects of dilation measurements.

3.4 Microstructure of Heat-Treated Specimens

3.4.1 Solution Treatment at 1050 °C/1 h/WQ

Figure 1 represents the solid solution heat treatment cycle (1) diagram of the examined steel. The microstructure of homogenized samples shows that it contained some areas of the austenite phase with a small structure. It assembled a group of small grains-like islands, as shown in Fig. 11. The abnormal morphologies of these austenite islands are typically incomparable with other areas observed in the whole microstructure of the cast specimen may be due to the local cooling and solidification conditions.

After solid solution treatment, the sigma phase revealed in the as-cast microstructure in Fig. 6 was dissolved and vanished, as demonstrated in Fig. 12.

The microstructure of solution-treated samples contains a dual phase, the austenite, and the ferrite phases, without precipitation of secondary phases. The Vf (volume fraction) of the gamma-phase and alpha-phase is 63 and 37%, respectively, as displayed in Fig. 12.

From the microstructure point of view, the microstructure of the solution-treated sample possessed a high amount of austenite and lower ferrite volume fraction percentages compared with the composition of the as-cast.

Figure 12 shows some small ferrite grains inside the austenite phase. The presence of ferrite grains could be related to the solidification process and phase transformation during the cooling of the specimen material; according to the results from the JMatPro diagram in Fig. 1, the phase transformation process of AISI 329 SDSS has unique solidification behavior, which needs more attention. Cooling from solid solution (homogenization) temperature around 1100 °C splits into two main stages, the first stage from 1100 to 700 °C and the second stage from 700 °C to room temperature. In the first stage, the ferrite decomposes, resulting in austenite and a set of precipitated compounds containing high-chromium, as shown in Table 2. Precipitation of high chromium intermetallic compounds increases the ability of the material to stop ferrite transformation and reverse the direction of the transformation reaction. So, the second stage of solidification dominated. In the second solidification stage, the austenite phase transforms into ferrite. This reaction seems to be encouraged by rich-nickel intermetallic compound precipitation. Therefore, it leads to the forming or nucleating ferrite phase, forming small ferrite grains inside the austenite phase.

The results of Fig. 13 show that the sigma phase is still present in the obtained microstructure after solid solution treatment (heating in a temperature range beyond the dissolution of the sigma phase). With higher magnification for the microstructure of the solution-treated specimen, precipitation of a thin layer of sigma phase can be observed in the ferrite phase grain boundaries. The mentioned result confirms the results stated by many published works (Ref 13, 35, 36). Table 5 lists homogenized specimens’ average micro-chemical EDS analysis for ferrite, austenite, and sigma phases.

Table 5

Average of micro-chemical EDS analysis for the α, γ, and sigma phases in homogenized specimens

Conditions	Phases	Chemical composition, wt.%
Conditions	Phases	Cr	Si	Mo	Ni	Mn	Fe
Solution at 1050 °C/1 h	α	23.02	0.61	2.42	4.24	0.70	69.01
	γ	20.42	0.33	1.70	5.15	1.11	71.29
	σ	45.83	1	0.65	1.88	20.5	30.14

On the other hand, the chemical composition of the different constituents was determined by utilizing EDS line analysis, as shown in Fig. 14. The average results of the EDS micro-chemical analysis of different phases are collected in Table 5. The EDS result emphasized the presence of the sigma phase and confirmed its location at the grain boundaries of both the austenite and ferrite phases.

3.4.2 Heat Treatment Cycle (1)

Figure 1b shows investigated steel's schematic heat treatment cycle (1). Figure 15 shows the microstructures of the samples aged at 475 °C for 3, 6, and 9 h.

The microstructures of the aged specimens consist of the matrix ferrite phase (light phase) and mottled austenite phase (gray phase), in addition to the intermetallic compound, e.g., the G phase (the dark area). The aged specimens for 3, 6, and 9 h have a marked dissimilarity in the ferrite, most likely combined with spinodal dissociation products. The significant discrepancy interacts with compositional variations between iron-rich-martensite and chromium-enriched-ferrite. Weng et al. confirmed the results and reported that the 1-2 nm particles of spinodal structures in the ferrite of 22Cr-5Ni DSS after aging for 2 h at 475 °C (Ref 37). By prolonging the duration of the aging process from 3 to 9 h, the Vf and thickness of the dark phase (c) are increased, as shown in Fig. 16.

Figure 16 displays the microstructure metallography for the 475 °C/9 h aged specimen. It demonstrates that the precipitated blocky carbides have no specific engineering geometry. These carbides were found in other aged steel samples, which were heat-treated under different conditions (475 °C/3 h and 475 °C/6 h). Table 6 documents the micro-chemical analysis of the carbides containing higher alloying elements such as Cr, Fe, and Mn.

Table 6

EDS analysis for α and γ phases in various specimens

Conditions	Phases	Chemical composition, wt.%
Conditions	Phases	Cr	Si	Mo	Ni	Mn	Fe
Cycle 2 (Aged at 850 °C/1 h + WQ)	α	22.26	0.24	1.61	3.51	0.95	71.43
	γ	21.09	0.15	0.86	4.73	1.17	72.00
	σ	52.50	0.21	0.20	1.03	25.55	20.51
Cycle 2 (Aged at 850 °C/1 h + OQ)	α	23.04	1.28	2.65	4.27	1.02	67.74
	γ	20.02	0.80	1.87	5.29	1.26	70.76
	σ	30.84	7.84	1.38	0.73	28.20	29.69

3.4.3 Heat Treatment Cycle (2)

This cycle (2) deals with specimens aged after homogenization and quenching at 850C for 1 h, followed by quenching in water or oil. The microstructure of the water-quenched sample includes austenite, ferrite, and sigma (σ) phases. The σ-phase positioned on the ferrite grain boundaries. Furthermore, the austenite grains are decorated and surrounded by the heavy and thicker sigma phase, as shown in Fig. 17.

A rapid cooling technique for water from high aging temperatures (850 °C) captures and preserves the microstructure and volume fraction of different phases at this high temperature. A large amount of the sigma phase (thick film decorating the ferrite and austenite phases) may be due to heating the homogenized specimen. The sigma particles precipitated at a temperature ranging from 500 to 600 °C during heating, and a temperature of 850 °C is not enough for the complete dissolution of the sigma phase, as confirmed by the dilation test. Therefore, the rapid cooling conditions do not allow the redistribution of alloying elements between ferrite and austenite phases. However, the cooling rate of oil quenching is slower than water quenching. Slow cooling provides the various alloying elements chance to segregate among different constituents, as shown in Fig. 18. However, the water-quenching specimen was differentiated by a larger quantity of white area (austenite phase) and a smaller grain-size structure than the oil-quenched specimens due to the thicker sigma phase at the ferrite and austenite grain boundaries. It is easy to be recognized in the optical micrograph.

On the other hand, the chemical composition variation of the different constituents was verified by utilizing EDS line analysis, as shown in Fig. 19. The chemical composition analysis shows the distribution of alloying elements among ferrite, austenite, and secondary phases for steel specimens exposed to various heat treatment conditions. The average result of the EDS micro-chemical analysis of constituents is collected in Table 7. The EDS result emphasized the presence of the sigma phase and confirmed its location at the grain boundaries of both the austenite and ferrite phases.

Table 7

EDS analysis for α-phase, γ-phase, and blocky carbides precipitates found in different specimens heat-treat under cycle (1) conditions

Aging condition	Phase	Chemical composition, wt.%
Aging condition	Phase	Cr	Si	Mo	Ni	Mn	Fe
475 °C/3 h	α	23.03	1.12	2.82	4.07	0.77	68.19
	γ	20.50	1.10	2.18	4.91	1.31	70.00
	Blocky carbides	46.70	0.70	1.41	1.77	17.41	32.02
475 °C/6 h	α	23.04	1.33	3.22	4.41	0.47	67.53
	γ	20.62	0.98	2.24	3.91	1.11	71.14
	Blocky carbides	52.50	0.21	0.20	1.03	25.55	20.51
475 °C/9 h	α	22.71	0.66	2.44	3.65	1.13	69.41
	γ	20.40	0.65	2.05	5.42	1.18	70.30
	Blocky carbides	52.56	0.49	1.30	2.23	19.56	23.78

3.4.4 Heat Treatment Cycle (3)

In this heat treatment cycle (3), the as-cast specimen was exposed to solution treatment at 1100 °C/1 h, followed by step cooling in the furnace to 900 °C for 1 h, then water quenching at room temperature. This heat treatment scheme combines the solution treatment and aged one in just one step to save time and power. Its microstructure consists of an austenite phase, ferrite-matrix, and the σ-phase found in the cast microstructure after the complex heat treatment process almost vanished, as indicated in Fig. 20. While, Fig. 21 displays that the chemical composition variation of the different constituents was verified by utilizing EDS line analysis. The average results of EDS micro-chemical analysis of various phases are collected in Table 8. The EDS result emphasized that the sigma phase’s thin layer presents lower alloying elements content.

Table 8

EDS analysis for α -phase and γ-phase in various specimens

Conditions	Phases	Chemical composition, wt. %
Conditions	Phases	Cr	Si	Mo	Ni	Mn	Fe
Heat treatment cycle 3	α	25.62	0.07	0.95	2.74	0.06	70.56
	γ	21.75	0.00	0.61	4.85	0.19	72.60
	Blocky carbides	35.84	0.084	1.38	2.73	0.20	59.69

3.5 Tensile Properties of Investigated Steel

3.5.1 Tensile Properties of As-Cast AISI 329 Duplex Stainless Steel

Mechanical tensile properties of investigated steel are presented in Fig. 22, and the collected data are listed in Table 9. Figure 22 and Table 9 show that the tensile properties results were affected by heat treatment, where the as-cast AISI 329 duplex stainless steel showed the minimum value of all tensile properties. The tensile properties test result can be explained from the microstructure point of view. The microstructure of cast steel contained blocky precipitate with sharp edges, while the sigma phase is a thick film at grain boundaries. The Vf (volume fraction) of the σ-phase in the as-cast specimen was 10%. The presence of blocky precipitate and sigma phase increases the sites of crack formation, initiation, and propagation, which deteriorates the tensile properties of the cast specimen, as shown in Table 9. It is noteworthy that all heat-treated samples show higher ultimate strength and elongation than cast specimens.

Table 9

Mechanical tensile properties of investigated steel

Heat treatment		Engineering tensile properties			Average thickness of sigma phase, µm (Ten -reading)	The average area of the intermetallic compound, µm² (Ten-reading)	Area of intermetallic compound particles, µm²
Cycle no	Condition	YTS, MPa	UTS, MPa	Elong, %	Average thickness of sigma phase, µm (Ten -reading)		Min	Max
As-cast		496	526	11	3.65	153	42	255
Heat treatment
1	Solid solution annealing at 1050 °C/0.5 h and aged at 475 °C/3 h/WQ	580	710	10	3.27	19	8	48
	Solid solution annealing at 1050 °C/0.5 h and aged at 475 °C/6 h/WQ	520	670	13	3.11	27	6	90
	Solid solution annealing at 1050 °C/0.5 h and aged at 475 °C/9 h/WQ	500	620	18	3.3	36	7	135
2	Solid solution annealing at 1050 °C/30 min and aged at 850 °C/1 h/OQ	667	750	15	1.15	22	7	47
2	Solid solution annealing at 1050 °C/30 min and aged at 850 °C/1 h/WQ	608	740	12	1.5	29	12	73
3	Solid solution annealing at 1100 °C/0.5 h/AC/900 °C/1 h/WQ	678	800	37.6	1.17	18	10	40

3.5.2 Tensile Properties of Heat-Treated AISI 329 Duplex Stainless Steel

Heat-Treated Steel Following Cycle (1)

Table 9 shows the higher tensile properties of this specimen treated by cycle (1) than the cast specimen. Figure 22 presents the tensile properties results. The increased aging time improves the elongation of the tested steel specimen and vice versa with the tensile strength. These results may increase aging time, improving alloying elements’ transfer from one place to another, reducing the segregation of alloying elements, and facilitating the transformation reaction (α = γ).

Heat-Treated Steel Following Cycle (2)

Specimens of this group treated by cycle (2) show higher tensile properties than cast specimens. Partial dissociation helps to increase the alloying homogeneity and decrease the thickness of the sigma phase leading to an increase in the tensile properties, especially the ultimate tensile strength and elongation.

Table 9 shows the effect of cooling media on the final result of tensile properties. It is clear from Table 9 that oil quenching after aging at 850 °C shows higher tensile properties concerning the water-quenching specimen. The previous result can be referred to as the rate of cooling, enhancing the alloying element diffusivity from one to another phase, and the time for phase transformation (α = γ).

Heat-Treated Steel Following Cycle (3)

The result of tensile test specimens that were treated by cycle (3) is given in Table 9. It is clear from these results that specimens have the highest value compared with both cast and other heat-treated ones. The high values of tensile tests are a result of the changes that occurred in the microstructure. In this heat treatment cycle, the precipitated particles, sigma phase, and other carbide particles were dissociated in the matrix (ferrite and austenite phases). Therefore, the sigma phase under cooling conditions precipitated in thin film thickness, as indicated in Fig. 22. The thin-thickness of the σ-phase enhances both the tensile ductility properties, as shown in Fig. 22.

3.6 Fracture Surface of Investigated Steel

Origins of the fracture of investigated steel were studied utilizing SEM that worked at low magnification to scan the fracture surface of the examined steel, as presented in Fig. 23. SEM snapshots of the fracture surface of tension samples show cleavage fracture and dominated dimples. The dimple’s presence is evidence of the tensile ductility of the examined steel alloy. Many issues affect the features of the dimples, for example, the second-phase particle size and shape, the material plasticity, the index of deformation hardening, applied stress, and working temperature. The average thickness of the sigma phase and the range of area of the intermetallic compound are listed in Table 9.

3.6.1 As-cast AISI 329 Duplex Stainless Steel

Figure 23(a) represents the tensile fracture morphology of the cast steel specimen. The fracture surface of the cast steel specimen includes cleavage fracture and a small number of dimples. It is noteworthy that the microstructure of cast steel contained a thick film of sigma phase and intermetallic compounds (IMC) with a polygon shape. These intermetallic compounds have areas ranging from 45 to 255 μm² with a sharp edge, while the average thickness of the sigma phase is about 3.65 μm, as documented in Table 9. This kind of intermetallic compound was considered the site of stress concentration, formation, initiation, and propagation of cracks that deteriorate all mechanical tensile properties, as displayed in Table 9. In addition, a thick film of the σ-phase located at the boundaries of the grains prevents the dislocation movement from one grain to another. Also, dislocations pile up at grain boundaries encouraging cracks to form and propagate at a lower strength. The sigma phase presence at the grain boundaries changes the chemistry of adjacent grains from both sides (depletion of specified alloying elements like Cr) which changes the strength of the circumstance.

3.6.2 Heat-Treated AISI 329 Duplex Stainless Steel

From Fig. 23, it is also shown clearly that the heat-treated steel alloy samples are characterized generally by a small number of cleavage fractures and different numbers of dimples. The dimples number mainly depends on the microstructure controlled by the applied heat treatment cycle.

Heat-Treated Steel Following Cycle (1)

Figure 23 reveals a small amount of cleavage fracture and a high count of dimples for heat-treated steel samples by cycle 1, incomparable with cast steel samples. On the other hand, Fig. 23b demonstrated that the heat-treated samples show higher elongation (ductility) than the cast steel sample due to the higher dimple numbers.

The influence of aging time on the mechanical tensile properties of examined steel alloy was demonstrated by the accompanying fracture surface of the heat-treated specimen. Figure 22 shows that the tensile properties of a set of this steel deteriorated with a rise in the aging time.

The previous result explained by the aged time raising leads to a growth in the thickness of the σ-phase located at grain boundaries. Also, increasing aging time enlarges the area of intermetallic compounds. Intermetallic compounds and σ-phase are the sites of crack formation, initiation, and propagation. Table 9 listed the thickness of the sigma phase and area of the minimum/maximum intermetallic compound of investigated steel with a rise in the aging time. The presence of a thick sigma phase and the intermetallic compound particles with a large area decreases both the strength and elongation, as shown in Table 9.

Heat-Treated Steel Following Cycle (2)

The fracture surface of steel samples heat-treated by cycle (2), shown in Fig. 23, clarifies a lower amount of cleavage fracture and a higher amount of dimple fracture than cast steel alloy. The presence of dimples is evidence of the tensile ductility of the steel alloy. A higher number of dimples and a lower amount of cleavage fracture suggested an enhancement in mechanical tensile properties after heat treatment of this examined steel. The results extracted from Fig. 23c coincide with the data gathered in Table 9.

Heat-treated steel samples by cycle (2), the influence of the quenching media on the mechanical tensile properties was explained by the accompanying fracture surface of the heat-treated samples. Figure 23 shows that the oil-quenching specimen represents higher tensile properties than the water-quenched steel specimen. The previous result is attributed to the precipitation of a lower polygon intermetallic compound area and sigma phase thickness. Oil-quenched sample shows lower sigma phase thickness than the water-quenched (for oil-quenched = 1.15 µm, and water-quenched = 1.5 µm). Table 9 reveals that water-quenched steel samples contained intermetallic compounds with an area ranging from 12 to 73 µm². On the other hand, oil-quenched specimens comprised intermetallic compound particles with 7-47 µm².

Heat-Treated Steel Following Cycle (3)

The steel sample heat treated by cycle (3) represents dimple fracture mode (a ductile fracture mode), as shown in Fig. 23d. This figure revealed that this heat-treated steel sample shows a dimpled fracture incomparable with the cast steel sample, which exhibits the quasi-cleavage fracture with a river pattern. The dimpled fracture signifies an increase in elongation. On the contrary, the depth and size of dimples in the studied steel sample after heat treatment by cycle (3) are more influential than in the other heat-treated steel samples.

However, steel specimen heat treated by cycle (3) has the highest tensile property compared with other steel specimens. The previous result was due to the lower thickness of the sigma phase and the area of intermetallic compounds particle, as given in Table 9. Lower sigma phase and intermetallic compounds decrease locations of crack initiation and formation.

Also, this table shows that the tensile properties of this steel were enhanced by following the heat treatment cycle (3). The thin σ-phase and lower area with the spherical shape of intermetallic compound particles increase the elongation and strength of the investigated steel. The metallography of this steel specimen shows that the polygon intermetallic compounds disappear. On the other hand, new intermetallic compounds with spherical shapes appear. The EDS line analysis of this steel specimen shows that the thinning of the sigma phase film decreases the depletion of the alloying elements along the grain boundaries. The previous observation is the reason for increasing the strength and elongation of this steel sample.

It is concluded that the fracture morphology is reliable with the mechanical tensile properties and microstructure.

4 Conclusions

JMatPro program and dilation test emphasized that 1000ºC was enough for the complete dissolution of the sigma phase and intermetallic compound, but 1050ºC is preferred.

Dilation tests found that the sigma phase precipitation temperature ranges from 500 to 650 ºC according to the cooling rate.

The as-cast structure of AISI 329 SDSS shows a large amount of σ-phase located at the grain boundaries and a non-uniform intermetallic compound shape dispersed in the matrix.

After solution treatment at 1050 °C/1 h, the σ-phase was partially dissolved and formed a thin film positioned along the grain boundaries of austenite and ferrite phases. The microstructure of the solution-treated steel sample possessed a high amount of austenite and a lower ferrite volume fraction than the as-casted steel.

Two-step heat treatment (solid solution annealing and aging in separate steps) is unsuitable for removing the sigma phase and other intermetallic compounds. A two-step heat treatment strategy forms the thick sigma phase and intermetallic compounds found at the ferrite grain boundaries. A one-step heat treatment strategy builds a thin film of sigma phase and intermetallic compounds with lower contents of alloying elements.

Theoretical and experimental studies show the obligatory presence of the sigma phase. But the heat treatment cycles suggested that it is possible to diminish the volume fraction of the sigma phase by choosing heat treatment conditions such as heating rate, starting microstructure, solid solution temperature, holding time, interpreted cooling, and cooling rate.

The steel sample produced according to the condition of the heat treatment cycle (3) shows an excellent combination of elongation % and tensile strength among other investigated AISI 329 duplex stainless steel samples heat treated under different heat treatment cycles.

The fracture morphology is reliable with the mechanical tensile properties and microstructure. In addition, the tensile properties increase in the order of cast, then heat-treated steel sample in cycle 1, cycle 2, and cycle 3, respectively.

Acknowledgments

This work was supported by the Central Metallurgical Research and Development Institute, CMRDI, Egypt (research project number 318-DT-2021/2022).

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Title: Influence of Various Heat Treatment Cycles on the Phase Transformation and Microstructure Evolution of AISI 329 Super-Duplex Stainless Steel
Authors: Nader El-Bagoury
Hossam Halfa
M. E. Moussa
Publication date: 15-02-2023
Publisher: Springer US
Published in: Journal of Materials Engineering and Performance / Issue 19/2023
Print ISSN: 1059-9495
Electronic ISSN: 1544-1024
DOI: https://doi.org/10.1007/s11665-023-07893-7

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Influence of Various Heat Treatment Cycles on the Phase Transformation and Microstructure Evolution of AISI 329 Super-Duplex Stainless Steel

Abstract

Publisher's Note

1 Introduction

2 Experimental Procedures

3 Results and Discussion

3.1 Transformation Temperature Determination

3.1.1 JMatPro- Program Studies (As-Cast Structure)

3.1.2 Dilation Studies (Starting As Cast Steel)

3.1.3 Differential Thermal Analysis of the Cast-Specimen of AISI 329 SDSS

3.2 X-ray Diffraction Studies

3.3 Microstructure of the Cast Steel

3.3.1 Theoretical Prediction of Sigma Phase

3.4 Microstructure of Heat-Treated Specimens

3.4.1 Solution Treatment at 1050 °C/1 h/WQ

3.4.2 Heat Treatment Cycle (1)

3.4.3 Heat Treatment Cycle (2)

3.4.4 Heat Treatment Cycle (3)

3.5 Tensile Properties of Investigated Steel

3.5.1 Tensile Properties of As-Cast AISI 329 Duplex Stainless Steel

3.5.2 Tensile Properties of Heat-Treated AISI 329 Duplex Stainless Steel

3.6 Fracture Surface of Investigated Steel

3.6.1 As-cast AISI 329 Duplex Stainless Steel

3.6.2 Heat-Treated AISI 329 Duplex Stainless Steel

4 Conclusions

Acknowledgments

Publisher's Note

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