|Fatayalkadri Citrawati||Research Center for Metallurgy and Materials, Indonesian Institute of Sciences, Kawasan PUSPIPTEK, Tangerang Selatan, 15314, Indonesia|
|Robby Dwiwandono||Sultan Ageng Tirtayasa University, Jl. Jenderal Sudirman KM 3, Cilegon, 42435, Indonesia|
|Leksono Firmansyah||Sultan Ageng Tirtayasa University, Jl. Jenderal Sudirman KM 3, Cilegon, 42435, Indonesia|
The formation of bainite in steel alloys is
affected not only by temperature, holding time, and cooling method but also by
steel alloy’s main alloying element. In iron-nickel (Fe-Ni) lateritic steels,
the steelmaking process from nickel pig iron (NPI) gives various Ni contents.
In this study, five Fe-Ni alloys with various Ni contents were subjected to
semi-continuous austempering. The holding temperatures were 535°C (T1) and
360°C (T2). All alloys were held for 1800 s. Both holding temperatures were
determined through the average temperatures of bainite start (Bs) and
martensite start (Ms) of each alloy. Observations of the microstructures using
an optical microscope and Scanning Electron Microscope (SEM) showed the
formation of one or more phases in the alloys. These phases are ferrite as the
matrix, degenerated pearlite (DP), lamellar pearlite (LP), and plate-like
bainite. In Fe-Ni alloys with Ni content of 0.01 to 2.2 wt% Ni, after holding
at either T1 and T2, a mixture of DP and LP in the ferrite matrix is more
pronounced. Some plate-like bainite is gradually formed in the 3.3% Ni alloy
after holding at T1 and T2. As the Ni content increases to as much as 4.5 wt%
Ni, pearlite is no longer visible and is replaced by plate-like bainite in the
ferrite matrix. These results indicate that the variation of Ni in Fe-Ni alloys
with Ni content less than and equal to 4.5 wt% results in different shapes of
bainite, which then affects the mechanical properties of the alloy.
Austempering; Bainite; Continuous cooling; Microstructure; Nickel laterite
The bainite phase in steel alloys is considered a favorable phase for several industrial applications due to its properties and its relatively low-cost production (Wang et al., 2016). In the automotive industry, bainite is understood to absorb energy well during crash trials (Tisza and Czinege, 2018). Meanwhile, in the railway industry, bainite is preferred due to its combination of good weldability (Hlavaty et al., 2009), fatigue resistance, and wear resistance (Vuorinen et al., 2016).
Currently, most developed bainitic steels use manganese (Mn) or chromium (Cr) as their main alloying element (Gong et al., 2015; Hofer et al., 2015; Meng et al., 2015; Toji et al., 2016; Zhou et al., 2017) with added silicon (Si), and almost none use nickel (Ni) as the main alloying element. This may be due to the relatively expensive cost of Ni. However, the lateritic steel alloys produced by the conversion process of nickel pig iron (NPI) from lateritic ores need a very minor to no addition of Ni during their steel-making process. Similar to Mn, as an alloying element, Ni is an austenite stabilizer. The addition of Ni provides several benefits, improving corrosion resistance in acidic environments (Cheng et al., 2017), enhancing mechanical properties (Far et al., 2019), and increasing resistance to hydrogen embrittlement (Shim et al., 2017). A steel alloy produced by processing NPI with Ni content less than 4 wt% will have approximately 3 wt% of Ni in the final steel product (Jamali et al., 2014), which is considered a significant amount of Ni in the alloy.
To obtain bainite in a steel alloy, the thermal treatment process includes an isothermal holding by quenching the sample in a salt bath after austenization. However, this method is rarely adopted by the industry (Takayama et al., 2018). In this study, a semi-continuous cooling method is used during thermal treatment. Instead of using a salt bath during the holding process at the bainite transformation temperature, air cooling and a furnace are used to hold the sample at the targeted temperature. It is expected that bainite will form uniformly throughout the sample.
Very few studies consider Fe-Ni steel as bainitic steel. Besides adding economic value to lateritic ore through its Fe-Ni lateritic steel, this study aims to observe further the effect of Ni as a main alloying element with a maximum content of 4.5 wt% on the formation of bainite at two different holding temperatures. These temperatures are considered to be in the temperature range of either lower bainite or upper bainite formation after semi-continuous cooling treatment.
The semi-continuous cooling heat treatment process with holding temperatures at T1 and T2 produced bainite in all samples, along with a ferrite matrix and pearlite with various morphologies and distributions.
The microstructures of samples held at 535°C (T1) and 360°C (T2) were observed to contain gradually less pearlite as the Ni content increased. In the alloy containing the highest concentration of Ni (4.5 wt% in alloy E), pearlite was not visible. Instead, the microstructure found in this alloy was plate-like bainite with blocky areas of ferrite as the matrix.
Subjecting samples to the lower holding temperature resulted in larger average sizes of the dark areas (cementite-containing phase) and higher volume fractions of dark areas and, thus, higher hardness. However, as the Ni content increases, holding at either 535°C or 360°C semi-continuously results in a smaller grain size of the dark areas. This decrease in the average grain size of dark areas was more pronounced in samples held at 535°C or at temperatures closer to Bs. In contrast, the hardness and the volume fraction of the cementite-containing phase increased as the Ni content increased.
With the same treatment
parameters, the variation of Ni in Fe-Ni alloys in the range of 0.01 to 4.5 wt%
affects the morphology of bainite formed in the as-treated samples, which then
affects their mechanical properties.
This study was funded by the Competence Research Program 2017–2019 under the Research Group for National Steel Based on Laterite at the Research Center for Metallurgy and Materials – Indonesian Institute of Sciences (P2MM LIPI).
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