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Sintered Fe-Ni-Si-C alloys

The aim of this research is to explore the effect of nickel content on microstructural development and mechanical properties of sintered Fe-Ni-Si-C alloys. The sintered alloys were prepared from powder mixtures of pre-alloyed Fe-Ni-based powders (nickel contents varied as 0.45, 0.90, 1.80, and 4.00 wt.%) and fixed 4 wt.% silicon carbide powder by using a powder metallurgical process. Sintering was performed in a vacuum furnace at 1250°C for 45 minutes and slow cooling in the furnace. Microstructures of sintered alloys varied with nickel contents. Nickel showed a strong influence on promoting bainitic ferrite/martensite-austenite (BF/M-A) structure formation. Retained austenite in BF/M-A structure was found to increase with increasing nickel content. Tensile properties (strength and elongation value) of sintered alloys increased with increasing nickel content.


Introduction
Thailand has a geographic location suitable to become a primary regional economic hub. Not only in trade, investment, and tourism but also in communication and transportation networks, that connect to other regions around the world. The Thai government has, in recent years, made investment in domestic infrastructure, a top priority. Railway transportation is increasingly important as a mass transit, to link destinations with manageable travelling time. With the project of high-speed railway development, sintered friction materials (SFMs) are expected to be widely used. These materials are used in train brake system and have many advantages, such as high performance, wear resistance, improved fade resistance, enhanced wet-effectiveness, and extended life SFMs all important components of rail brakes to ensure safety and comfort expectation [1]. SFMs are composites commonly consisting of graphite particles embedded in metallic matrix [2], [3]. The graphite particles act as solid lubricant for anti-wear purpose. According to the recent study [4], the SFM matrix has important role on braking performance such as IOP Publishing doi:10.1088/1757-899X/1137/1/012035 2 stable coefficient of friction (COF) and wear rate. This means the composite matrix integrity is important for high-speed train SFM brake pads. Previous works have shown that sintered composites having graphite particles embedded in iron-base matrices could be produced by using the traditional 'press and sinter process' [5][6][7][8]. These sintered composites have different matrices depending on the compositions of iron-base powders employed. The matrix microstructure has prime contribution to mechanical properties of a sintered composites.
Research works have shown that nickel addition to steels is able to modify microstructure hence improve mechanical property. The increase in Ni contents (0-6%) causes microstructural change from dominant granular bainite to lath bainite and/or lath martensite [9]. The roles of nickel addition include (i) lowering the pearlite start (Ps) and the bainite start (Bs) temperatures and shifting both pearlites, (ii) and bainite transformation curves to the right-hand side of continuous cooling transformation (CCT) diagram and (iii) retardation of isothermal bainite transformation kinetics by shift the bainite transformation curve to the right-hand side of time-temperature-transformation (TTT) diagram [10]. The decrease of Bs and martensite start (Ms) temperature with nickel addition [11] or increasing nickel content [12] were previously reported.
The shifting of the pearlite transformation curve to the right-hand side of CCT diagram opens the opportunity for a steel produced under a considerably slow cooling rate to avoid pearlite transformation at a high temperature thus undergoes other lower temperature phase transformations. The additions of 1-3 wt.% nickel powder to sintered Fe-Cr-Mo base steels, processed under a slow or furnace cooling, seem to prove that concept [13]. The retardation of isothermal bainite transformation kinetics due to nickel addition is reported in [10,14].
Due to the important roles of nickel addition as given above, it was hypothesized that by using prealloyed nickel-containing powders, the matrices of sintered SFMs would be developed under the influence of nickel content. Due to the delay of pearlite transformation, other transformations occurring below the pearlite field are expected. One of low temperature transformation products of ductile cast ions known as ausferrite consisting of ferrite and carbon-rich austenite is expected to form by the nickel influence in sintered nickel-containing SFMs. The ausferrite is one type of microstructural features important for mechanical property improvement in cast ductile irons [15][16][17][18][19]. The effects of microstructural components, varied with nickel content, on mechanical properties of sintered nickelcontaining SFMs had been investigated in this work.

Materials preparation
The sintered alloys were prepared from powder mixtures of pre-alloyed Fe-Ni based powders (nickel contents varied as 0.45, 0.90, 1.80, and 4.00 wt.%) and fixed 4 wt.% silicon carbide powder (shown in Table 1) by using a powder metallurgical process. The powder mixtures were compacted into tensile test bars with a green density of 6.50 ± 0.05 g/cm3. For each steel alloys, sintering of green compacts were performed in a vacuum furnace at 1250°C for 45 minutes and slowly cooled in the furnace. The density of green and sintered specimens was determined in accordance with MPIF Standard 10, ASTM B783).

Microstructure investigation
Specimens for metallographic examination were ground on silicon carbide papers from 80 to 4000 grits and then progressively polished with 6, 3, and 1μm diamond pastes, respectively. The polished samples were etched with 2% Nital. Microstructures of the sintered specimens were observed by using optical microscopy (OM) and scanning electron microscopy (SEM) equipped with an EDS.

Hardness and tensile test
Rockwell scale B hardness test (HRB) was carried out on un-etched specimens using 100 kg-f loads with 15s indentation time (Standard ISO 6508-1). The mean values were based on ten different areas.
The tensile property of the sintered materials was carried out at room temperature using Instron Universal Instrument with speed 1 mm/min. Three samples were tested for each sintered steel and the average value was reported.

Microstructure and phase characterization
Sintered Fe-Ni-Si-C alloys showed microstructures resembling those of ductile cast irons ( Figure 1). The sintered 0.45Ni ( Figure 1a) and 1.80Ni (Figure 1c) alloys showed common microstructural feature consisting of black particle surrounded with ferrite and striated structure, whereas sintered 0.90Ni ( Figure 1b) Figure 3 show peaks corresponding to two different crystal structures, i.e., body-centered cubic (bcc) crystal structure belonging to ferrite (α) phase and facecentered cubic (fcc) crystal structure belonging to austenite (γ) phase. The formation of microstructure consisting of black particle surrounded by ferrite and striated structure in sintered Fe-Ni-Si-C alloys is already explained and given in [6]. However, the striated patterns in the sintered nickel-containing alloys are quite different from those observed in sintered Fe-Ni-Si-C alloys. Thus, additional discussion in this work is focused on the formation of striated structure. When the striated structures in sintered 0.45Ni alloy (Figure 2a), 1.80Ni alloy (Figure 2c) and 4.00Ni alloy (Figure 2d) are compared, the effect of nickel content on phase transformations is remarkable. The proportion of pearlite decreases but that of BF/M-A structure increases with increasing nickel content. Taking the sintered Fe-Mo-Si-C alloys [6,7]as references, all sintered nickel-containing alloys (in this work) have higher proportions of BF/M-A structure. The reduction or disappearance of ferrite and pearlite in sintered nickel-containing alloys confirms the effect of nickel on shifting of high-temperature transformation curves to the right-hand side of CCT diagram [10]. Experimental results suggest that the higher nickel content the farther high-temperature transformation curves are shifted and the wider window is opened for the low-temperature transformation products (BF/M-A structure and martensite) to form.

Mechanical property
The sintered Fe-Ni-Si-C alloys showed appreciably high tensile strengths (ultimate tensile and yield strengths) as given in Figure 4. Ductility values were quite low, no higher than 3.0%. Both strength and elongation values increased with increasing nickel content. Rockwell scale B hardness value showed relationship with nickel content for most sintered alloys, except the value drop in the sintered 1.80Ni alloy. Note that the prominent strengths and hardness of the sintered 0.90Ni alloy is also attributed to high Cr and Mo contents.  (Figure 4c) but is not directly related to retained austenite volume indicated by the γ (111) peak intensities shown by XRD patterns in Figure 3. The ductility of the sintered alloys is partly influenced by retained austenite. The hardness (HRB) value has no direct relationship with nickel content (Figure 4d). The hardness value drops in the sintered 1.80Ni alloy is attributed to pearlite structures distributed in BF/M-A matrix.

Fracture surface
Fracture surface of the sintered 0.45Ni alloy with the lowest elongation value showed cleavage with river pattern (Figure 5a) whereas that of the sintered 4.00Ni alloy with higher elongation value showed mixture of cleavage (on the right) and quasi-cleavage (on the left) (Figure 5b). Cleavage, as a typical sign of brittle fracture, found in pearlite-containing matrix of sintered 0.45Ni alloy, may not be uncommon as it was found on a fracture surface of a fully pearlitic eutectoid steel [21,22]. Quasicleavage, as one form of cleavage, exhibits both cleavage and plastic deformation [23]. This fracture mode found in the sintered 4.00Ni alloy reflects small plastic deformation as this alloy has high strength but small elongation value.

Conclusion
Sintered Fe-Ni-Si-C alloys, designed and fabricated for application as SFMs, could be produced from mixtures of pre-alloyed Fe-Ni and fixed 4 wt.% silicon carbide powders by using traditional press and sinter process. Nickel content in pre-alloyed showed influence on microstructures and mechanical properties of sintered alloys. High nickel content could suppress ferrite and pearlite transformations and open a window for low-temperature transformations of BF/M-A and martensite. The low-temperature transformation products showed strong influence on mechanical properties of sintered alloys.