Abstract
In this paper, the super-ferritic stainless steels (SFSSs) were simulating treated in a fire accident, and their mechanical properties and corrosion performance before/after fire exposure were studied. The experimental results demonstrated that SFSSs have a good combination property with the tensile strength, total elongation, and corrosion rate in FeCl3 solution of ∼600 MPa, ∼22.5%, and 0.03 mm/a. After fire exposure, the mechanical properties and corrosion performance were affected based on the exposure temperature due to the precipitation of intermetallic compounds. When the fire exposure temperature was 700–800°C, the plasticity was exasperated and the value decreased to only 1.5% at the condition of 800°C × 4 h due to the bulk σ-phase precipitation at grain boundaries. Meanwhile, the brittle fracture patterns emerged, and the corrosion rate in FeCl3 solution increased to 0.4 mm/a. When fire exposure takes place at 600°C for less than 4 h, the mechanical properties and the corrosion performance were little affected. Thus, it was possible to use SFSSs in marine environments.
1. Introduction
With superior mechanical performance, high corrosion resistance, good welding performance, and low maintenance costs throughout the life cycle, stainless steel structures were widely used in civil engineering [1–3]. Stainless steel has been used in architectural structures since the early 20th century. Initially, it was primarily used in decoration engineering, building envelopes, and roof construction. With the improvement of design level and the development of construction technology, stainless steel structures have become increasingly popular in the past two decades. Stainless steels are highly regarded in iconic buildings for their excellent performance in terms of structure, aesthetics, corrosion resistance, durability, and more [3–6]. Compared with austenitic stainless steels (ASS) and duplex stainless steels (DSS), super-ferritic stainless steels (SFSS), originally developed in 1970 for chloride-containing media, have a lower coefficient of thermal expansion, lower Ni element content, and lower production cost features [7–9]. Besides, the SFSS also showed a good corrosion resistance performance in an environment that contains chloride ions compared with the heat-resistant martensitic stainless steels (MSS) [2, 3, 6]. Thus, these high-grade SFSS have great application potential in the field of construction, especially in the field of marine engineering construction.
In general, marine engineering construction required a good combination of the performance of corrosion resistance and mechanical property, especially the residual structural properties after an accidental elevated temperature exposure, such as a fire accident. In recent years many studies have been carried out on the structural behavior, design methods, and postfire residual structural properties of ASS, DSS, and MSS structures at elevated temperatures, however only a few studies have referred to the SFSSs [10–17]. Wang et al. [18] used high-temperature compression experiments to study the thermal deformation behavior of 00Cr26Mo4 super-ferritic stainless steel at 1050–1250°C and simulated the thermal deformation parameters to establish the corresponding thermal deformation constitutive equation. Dou et al. [19] used electrochemical testing technology to study the corrosion and scaling behavior of two kinds of SFSSs in different temperature geothermal water environments. Won et al. [20] studied the effect of cooling rate on ZST (zero strength temperature) and ZDT (zero plasticity temperature). The most common failure causes of SFSSs were studied by Azevedo and Padilha [21]. Gardner [5] presented the revised values for the heat transfer coefficient and emissivity of structural stainless steel members exposed to fire and presented an overview and reappraisal of previous pertinent research on the material properties of stainless steel at elevated temperatures.
Despite an outbreak of fire in steel structures occasionally in recent years, only a few steel structures have been burned out entirely, and most were in use after necessary assessing, repairing, and reinforcing, which reduced the economic losses of fire accidents [17, 22]. Hence, it was necessary to study the postfire microstructure and performance of steel structures in building engineering [23]. SFSSs contained a high content of Cr and Mo elements with the PRE (pitting resistance equivalent) of more than 38 [24]. While some intermetallic phases were usually precipitated during thermomechanical treatment processes at relatively high temperatures, such as Laves phase, chi phase (χ), and sigma phase (σ)[25]. It was reported that the sigma phase (σ) was composed of Fe and Cr and/or Mo elements, and it formed easily at grain boundaries. Laves phase was usually short for Fe2Mo/Fe2Nb in Mo or Nb containing steels, and it has a hexagonal structure. Chi phase (χ) has a body-centered cubic crystal structure with the chemical formula of Fe36Cr12Mo10 (as seen in Table 1). In general, the formation of these phases in SFSSs was very harmful to the mechanical properties and corrosion resistance performance, especially the plasticity [23]. Thus, it was urgently needed to study and evaluate the postfire microstructure and performance of SFSSs and provided a design guide to the design and use of steel structures.
In this paper, the S44660 SFSS plates were exposed to an electric heating furnace at various times at 600–800°C to simulate the firing process. Meanwhile, the postfire microstructure, corrosion morphology, and mechanical properties were studied and discussed aiming to understand the relationship between the fire accident and microstructure and their effects on the morphology and mechanical properties of the SFSSs after fire exposure.
2. Experimental Procedures
2.1. Experimental Materials
An S44660 ferritic stainless steel which was produced by continuous casting, hot rolling, and solution treating was used for the present study. The final thickness of the solution-treated plate was 4.2 ± 0.1 mm with the chemical composition of Fe-27.2Cr-0.06Cu-0.25Mn-3.85Mo-0.33Nb-2.01Ni-0.5Si-0.16Ti-0.015C-0.015N-0.021P-0.0020S.
2.2. Immersion Corrosion in FeCl3 Solution
In order to study the corrosion resistance in the marine environment, a FeCl3 solution containing 68.72 g FeCl3•6H2O, 600 ml deionized water, and 16 ml HCl (concentration: 36.5–38.0mass%) was prepared according to the ASTM G48 standard. Before the immersion test, the samples were polished by using the 400 # silicon carbide sandpaper. During the immersion test, the soaking temperature was set at 65 ± 1°C, and the soaking time was set as 48–168 h. After soaking for a different time, the surface corrosion morphology of the sample was observed, and the corrosion rate (weight loss rate) was further calculated.
2.3. Simulated Fire Test
In order to study the effect of fire on the residual properties of SFSSs, a fire process simulation was carried out in an electric furnace. The solution-treated plates were placed in an electric furnace at 600°C, 700°C, and 800°C, respectively, and the holding time was set at 0.5-4 h. After fire exposure at various temperatures, the samples were cooled in water, which was designed to simulate the fire extinguishing with water.
2.4. Microscopic Morphology Observation
Both the surface morphology and the microstructure of the specimens before and after a corrosion test were observed using a scanning electron microscope (SEM) equipped with an energy dispersive spectroscopy (EDS). The samples after simulated fire tests were electrolytically polished in a 5% perchloric acid alcohol solution after sandpaper burnishing. During the electrolytic polishing process, the electrolytic polishing voltage was set at 25–30 V, the current was controlled at 0.3–0.8 A, and the electrolyte temperature was maintained at −30 ± 5°C.
2.5. Mechanical Property Test
Behalf of evaluating the mechanical properties of the samples before and/or after fire stimulation treatment, the uniaxial tensile tests were conducted by using an electronic material testing machine (CMT5250) with an extensometer at a rate of 0.5 mm/min. Subsized specimens were machined with a parallel section of 6 × 32 mm according to ASTM E8M standard. Three interchangeable specimens were tested to calculate the average value and standard deviation of the tensile data.
3. Results and Discussion
3.1. Microstructure and Properties before Fire Stimulation Treating
Figure 1 shows the microstructure and the EDS results of the precipitated phase for the solution-treated samples before a simulated fire treatment. It was observed that the solution-treated samples consisted of ferrite grains together with a few TiN and Nb (CN) particles in them. The grain size was measured as 69 ± 5 μm, and the EDS spectra of the typical precipitated phase were illustrated in Figures 1(c)–1(e), respectively.
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Figure 2 shows the surface morphologies after immersion corrosion for various times in a FeCl3 solution. It was obvious that after soaking for 48 h, nearly none of the change was observed. A few small pitting corrosion pits were found on the surface when the soaking time was prolonged. After longtime soaking for 168 h, the largest size of the pits was only ∼300 μm indicating an excellent resistance to chloride ion corrosion for SFSSs. It was calculated that the corrosion rate was ∼0.03 mm/a after soaking for 168 h. This was also the case that the SFSSs can be used in marine architecture.
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Figure 3 shows the engineering stress-strain curve of solutions treated samples. It was calculated that the tensile strength (TS) was ∼600 MPa, the yield strength (YS) was ∼475 MPa, and the total elongation (TE) was ∼22.5%. The calculated results show that the SFSSs for this study have a good comprehensive mechanical property.
3.2. Microstructure and Properties after Fire Stimulation Treating
Figure 4 shows the precipitate morphology of the solution-treated samples after fire simulation treatment at different temperatures. As seen in Figure 4, some intermediate phases were observed in the microstructure after fire stimulation treatment, and the EDS spectrum of typical precipitates was displayed in Figure 5. After fire simulation treatment for 1-4 h at 600°C, only TiN and Nb (CN) particles were observed, which was similar to that in solution-treated samples (Figure 1). However, large numbers of Laves phase particles and χ-phase were found at grain boundaries and in the grains after fire exposure treating for only 1 h at 700°C, and a small number of σ-phase were detected at grain boundaries when the soaking time was 4 h. Especially, the Laves phase particles χ-phase, and σ-phase were wholly observed when the soaking temperature was 800°C. Nearly all the grain boundaries were occupied by the bulk σ-phase particles after soaking for 4 h at 800°C.
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Figure 6 shows the surface morphologies of representative samples after immersion corrosion for various times in a FeCl3 solution. It was obvious that large numbers of pitting corrosion pits were observed on the surface of the sample, and some pits were as large as 2 mm. A high magnification image of one pit was illustrated in Figure 6(b). It was observed that there were many fine precipitation particles around the corrosion boundaries, then the EDS results in Figure 7 show that the fine particles were Laves phase and the χ-phase. This result also indicated that the areas around the Laves phase and the χ-phase particles were easily corroded. Figures 6(c) and 6(d) show that the samples were corroded heavily together with all the grain boundaries were interspaced. After soaking for 24 h in FeCl3 solution, the corrosion rate of the samples exposed for 4 h at 800°C was ∼0.4 mm/a, which was 10 times than that before fire exposure treatment.
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The tensile properties of the samples after fire stimulation treatment were tested, and the results were displayed in Table 2. It was obvious that the mechanical properties were varied when the different fire stimulation treating was carried out. Especially, the TE values of all samples were reduced after fire stimulation treatment, and the TE value was only 1.5% after treating for 4 h at 800°C.
3.3. Performance Deterioration Mechanism via Fire Stimulation Treating
The results in section 3.2 showed that the fire stimulation treatment introduced a deterioration in the corrosion resistance and the mechanical properties. It was well known that the SFSSs have a precipitation priority when they were soaked at/around 800°C [21]. In this study, the pits were observed when the soak time reached 72 h, and the pitting formation in the samples without fire treatment was due to the damage of passivation film, as seen in Figure 8. While many precipitation phase parties were formed in the microstructure after fire stimulation treatment at 700 and 800°C, the precipitation of Laves and χ-phase particles made the Mo elements be absorbed, thus the Mo elements in the matrix were reduced indicating the decrease of corrosion resistance [8, 26]. Then, the corrosion damage was preferentially activated in the area that has Laves and χ-phase particles because the electrode potential of Mo-rich phases was higher than that of the adjacent ferrite [27]. Especially, when the samples were fired stimulation treated at 800°C, the grain boundaries were covered by σ-phase and the poor chrome/molybdenum areas were generated along grain boundaries because high content of Cr and Mo elements were absorbed from the adjacent ferrite matrix to σ-phase. Thus, the grain boundaries were corroded due to a high electric potential difference between the Cr/Mo-rich phases (σ-phase and χ-phase) and the ferrite matrix (Figure 6(c)). The pitting formation mechanism was illustrated in Figure 9. The big space at grain boundaries also indicated that the bulk σ-phase particles were deciduous when the grain boundary poor chrome area was corroded (Figure 6(d)).
On the other side, the bulk grain boundary σ-phase made the samples embrittlement, and the brittle fracture happened during the tensile test, as seen in Figure 10. Because the size of the Laves and χ-phase particles was fine, the ductile fracture morphology characterized by dimples was kept when the fire stimulation treatment was operated at a low temperature of 600°C, as seen in Figures 10(a) and 10(b).
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In a word, the SFSS showed a good mechanical property and corrosion resistance in a Cl-containing environment, which provided a possibility for use in marine engineering construction. Although the simulated fire test results indicated that the SFSS could also resist a short time of fire exposure, the fatigue property should be further studied to evaluate the residual life which can provide raw data for the structure design and maintenance.
4. Conclusions
Isothermal heating and water quenching test were conducted on an S44660 super-ferritic stainless steel. The postfire properties and microstructure were studied, and some conclusions are as follows:(1)The S44660 super-ferritic stainless steel plate has better properties with a tensile strength (TS) of ∼600 MPa, yield strength (YS) of ∼475 MPa, and total elongation (TE) of ∼22.5%.(2)The fire exposure at 700–800°C showed a heavy negative influence on the mechanical properties and the corrosion resistance of SFSSs. However, there was little influence on these performances for 600°C exposure. After hot exposure at 700–800°C, the ductile-brittle transition phenomenon occurred, and this was due to the precipitation of σ-phase particles at grain boundaries.(3)The super-ferritic stainless steel can be used in a marine environment for its high tensile strength and plasticity, and low corrosion rate, and fire exposure at the temperature of < 600°C showed little influence on the mechanical properties and corrosion resistance.
Data Availability
The raw data related to this paper would be made available upon request.
Conflicts of Interest
The author declares that they have no known conflicts of interest or personal relationships that could have appeared to influence the work reported in this study.