Nanoindentation-Based Micro-Mechanical and Electrochemical Properties of Quench-Hardened, Tempered Low-Carbon Steel

The nanoindentation technique is widely used to measure the micro-scale mechanical properties of various materials. Herein, the nanoindentation-based micro-mechanical and electrochemical properties of low-carbon steel were investigated after quench hardening and tempering processes. The steel was produced on a laboratory scale and subjected to quench hardening separately in two different media-water and brine (10 wt% NaCl)-and subsequent moderate temperature tempering. Microstructure analysis revealed that the lath martensite phase formed after all heat treatments, having different carbon percentages ranging from 0.26% to 0.58%. A ferrite phase was also observed in the microstructure in three different morphologies, i.e., allotriomorphic ferrite, idiomorphic ferrite, and Widmanstätten ferrite. Nanoindentation analysis showed that the brine quench hardening process provided a maximum twofold improvement in indentation hardness and a 51% improvement in stiffness with a 30% reduction in reduced elastic modulus compared with as-received steel. Electrochemical performance was also evaluated in a 1% HNO3 solution. The water quench-hardened and tempered sample exhibited the highest corrosion resistance, whereas the brine quench-hardened sample exhibited the lowest corrosion resistance among all heat-treated samples.

Their mechanical properties can be improved by various methods, i.e., microalloying, controlling austenite recrystallization temperature and grain size, producing low-temperature austenitic transformation products, and quench hardening and tempering processes [11]. Quench hardening and tempering form one of the most cost-effective and commonly used heat treatment processes [12].

Quench Hardening and Tempering Process
The quench hardening and tempering process started with austenitizing at 950 • C for 30 min followed by water and brine (10 wt% NaCl) quench hardening in two separate baths maintained at room temperature. These samples were designated as water quench-hardened (WQ) and brine quench-hardened (BQ). For stress-relieving, tempering was performed at 400 • C for 120 min on quench-hardened samples, which were designated as water quench-hardened and tempered (WQT) and brine quench-hardened and tempered (BQT). The heat treatment cycles of the quench hardening and tempering processes are given in Table 2.

Microstructure Analysis
Before the microstructural examination, manual grinding of all steel samples of dimensions 1 × 1 × 1 cm 3 was performed on grinding papers graded upto P1000, and polishing on velvet and nylon cloths with 1 and 0.25 µm-sized diamond pastes was carried out using an automatic polisher (Struers Tegrapol-15 Grinder/Polisher, Westlake cleaveland, Ohio, USA). These samples were then etched in 2 vol% HNO 3 solution for 10 s. A scanning electron microscope (SEM) (Hitachi Model S. 37000 N, Tokyo, Japan) was used to examine and analyze the micrographs at 2500×. The chemical compositions of various phases of the microstructure were also studied using an energy-dispersive spectrometer (EDS) attached to the SEM through spot scanning techniques.

Micro Vickers Hardness Testing
A micro Vickers hardness tester (Shimadzu Model HMV, Kyoto, Japan) equipped with a diamond indenter, a 1000 g load cell, and a microscope was used for the hardness testing of the quench-hardened and tempered steel samples. The mean of five readings was used to find the final value for all samples.

Nanoindentation Analysis
Nanoindentation tests were performed on a nanoindentation tester (CSM-international NHTX S/N: 01-2569, Needham Heights, MA, USA) in load control mode at a uniform loading/unloading rate of 200 mN/min with a diamond tip (BerkovichBJ-48, USA), which had a three-sided pyramidal shape, a nominal angle of 65.3 • , and a diameter of 200 nm. A microscope (scanning probe) attached to a nanoindenter was used for the in situ examination of various phases in the microstructure during testing. For each sample, three tests were performed and the obtained values were averaged to obtain a final value.

Electrochemical Analysis
Single copper wire was used for soldering and epoxy resin for cold mounting, exposing one side of area 1 cm 2 to the electrolyte. The samples were then ground and polished using the aforementioned procedure. The corrosion kinetics and polarization behavior of all the samples were determined by the Tafel scan technique in 1 vol% HNO 3 solution at ambient temperature using a potentiostat (Gamry Interface 1000, Bucks County, Pennsylvania, USA). A three-electrode system comprising a reference electrode (saturated calomel electrode), an auxiliary electrode (platinum wire), and a working electrode (steel samples) was utilized for the electrochemical analysis. All the tests were repeated thrice for each sample to validate the results.

Microstructure
Quench hardening and tempering processes were found to have a considerable impact on microstructural transformation in low-carbon steel. SEM micrographs and the corresponding EDS spectra of the steel samples are illustrated in Figures 1 and 2. In as-received (AR) form, low-carbon steel exhibited a non-homogeneously distributed α phase in two morphologies, i.e., allotriomorphic (α al ) and idiomorphic (α id ), and a lamellar pearlite (P) phase ( Figure 1a). The EDS spectrum of this sample indicates 0.021 wt% C and 3.51 wt% Mn in the α phase as illustrated in Figure 1b. The water quench hardening and tempering process produced a lath α' phase as well as α al and α id phases ( Figure 1c). The EDS results verify the formation of the α' phase by indicating 0.45 wt% C in this phase as illustrated in Figure 1d. The γ phase to α' phase transformation is attributed to the fast cooling rate (130 • C/s) of water [30,31]. The fast cooling rate of water does not allow carbon to properly arrange for making pearlite or bainite phases, resulting in supersaturated α' phase formation [32][33][34][35].

Microstructure
Quench hardening and tempering processes were found to have a considerable impact on microstructural transformation in low-carbon steel. SEM micrographs and the corresponding EDS spectra of the steel samples are illustrated in Figures 1 and 2. In as-received (AR) form, low-carbon steel exhibited a non-homogeneously distributed α phase in two morphologies, i.e., allotriomorphic (αal) and idiomorphic (αid), and a lamellar pearlite (P) phase ( Figure 1a). The EDS spectrum of this sample indicates 0.021 wt% C and 3.51 wt% Mn in the α phase as illustrated in Figure 1b. The water quench hardening and tempering process produced a lath α' phase as well as αal and αid phases ( Figure 1c). The EDS results verify the formation of the α' phase by indicating 0.45 wt% C in this phase as illustrated in Figure 1d. The γ phase to α' phase transformation is attributed to the fast cooling rate (130 °C/s) of water [30,31]. The fast cooling rate of water does not allow carbon to properly arrange for making pearlite or bainite phases, resulting in supersaturated α' phase formation [32][33][34][35]. tempering process is mandatory after the quench hardening process to ensure lattice relaxation and α' phase stabilization [7,11,36]. In this work, the water quench-hardened and tempered sample demonstrated a relatively low-carbon α' phase along with αal and αid phases in its microstructure ( Figure 2a). The EDS spectrum validates the low carbon percentage of 0.26 wt% in the α' phase compared with the water quench-hardened sample (Figure 2b). The low carbon in the α' phase is attributed to the relaxation of the lattice in the α' phase, which occurs by the diffusion of excess carbon from the α' phase to the αal and αid phases [11]. The brine quench-hardened and tempered sample also demonstrates a low-carbon α' phase and an α phase in micrographs. However, a different morphology of the α phase, known as Widmanstätten ferrite (αw), was observed around grain boundaries with the αid phase as illustrated in Figure 2c. The carbon percentage (0.38 wt%) in the α' phase was lower than in the brine quench-hardened sample but greater than in the other samples ( Figure 2d).

Micro-Mechanical Properties
To evaluate the micro-scale mechanical properties of as-received and heat-treated low-carbon steel samples, nanoindentation analysis and micro Vickers hardness testing were performed. Normal force versus penetration depth curves are illustrated in Figure 3, while the corresponding Figure 2. SEM images and EDS spectra of (a,b) water quench-hardened and tempered and (c,d) brine (10 wt% NaCl) quench-hardened and tempered low-carbon steel.
On the other hand, the cooling rate of the brine (10 wt% NaCl) solution is much greater (220 • C/s) than that of water [23]. This high cooling rate caused the formation of an α' phase with a higher carbon percentage (0.58 wt%) than the water quench-hardened sample, as well as α al and α id phases (Figure 1e,f). In quench-hardened form, the microstructure of steel contains a carbon-supersaturated α' phase which makes the steel highly brittle. Therefore, performing the tempering process is mandatory after the quench hardening process to ensure lattice relaxation and α' phase stabilization [7,11,36]. In this work, the water quench-hardened and tempered sample demonstrated a relatively low-carbon α' phase along with α al and α id phases in its microstructure (Figure 2a). The EDS spectrum validates the low carbon percentage of 0.26 wt% in the α' phase compared with the water quench-hardened sample (Figure 2b). The low carbon in the α' phase is attributed to the relaxation of the lattice in the α' phase, which occurs by the diffusion of excess carbon from the α' phase to the α al and α id phases [11].
The brine quench-hardened and tempered sample also demonstrates a low-carbon α' phase and an α phase in micrographs. However, a different morphology of the α phase, known as Widmanstätten ferrite (α w ), was observed around grain boundaries with the α id phase as illustrated in Figure 2c. The carbon percentage (0.38 wt%) in the α' phase was lower than in the brine quench-hardened sample but greater than in the other samples (Figure 2d).

Micro-Mechanical Properties
To evaluate the micro-scale mechanical properties of as-received and heat-treated low-carbon steel samples, nanoindentation analysis and micro Vickers hardness testing were performed. Normal force versus penetration depth curves are illustrated in Figure 3, while the corresponding measured quantities, i.e., indentation hardness (H), stiffness (S), and reduced elastic modulus (E r ), are given in Table 3. Additionally, the Vickers hardness values obtained from micro Vickers hardness testing are plotted in Figure 4. H was calculated by relation (1) [37,38]: where A denotes projected contact area and P max maximum applied load. E r was calculated by relation (2) [37,38]: where A denotes projected contact area and S is stiffness.

√A
where A denotes projected contact area and S is stiffness. In the as-received form, low-carbon steel exhibited the lowest values of H, HV, and S and the highest value of Er among all samples. These values are attributed to the ductile and soft αal and αid phases and the pearlite phase in the microstructure. The water quench-hardening process transformed these phases into the α' phase and the αal and αid phases, respectively. The formation of the α' phase caused 80% improvement in H, twofold improvement in HV, 43% improvement in S and a 38% reduction in Er compared with as-received steel. Similarly, the brine quench hardening process produced an α' phase with relatively high carbon content (0.58%). This supersaturated α' phase offered the greatest improvements: twofold improved H, threefold improved HV, 51% improved S, and 29% reduced Er values compared with the as-received steel.   Table 3. Nanoindentation properties of as-received and heat-treated low-carbon steel. The tempering process reduced the carbon percentage in the α' phase formed after the quench hardening processes in both media, and relieved internal stresses. Therefore, the water quench-hardened and tempered sample demonstrated a 31% improvement in H, 55% improvement in HV, 36% improvement in S, and 43% reduction in Er compared with the as-received sample. The brine quench hardening and tempering process showed relatively better properties than the water quench hardening and tempering process. After the brine quench hardening and tempering process, there was a 36% improvement in H, twofold improvement in HV, 67% improvement in S, and 37% reduction in Er compared with the as-received samples.

Electrochemical Properties
Tafel scans of the as-received and quench-hardened, tempered low-carbon steel obtained after electrochemical analysis in 1 vol% HNO3 solution are illustrated in Figure 5. The polarization potential was compared with open circuit potential ranging from -0.25 to 0.25 V at a 1 mV/s scanning rate to evaluate the electrochemical response of the low-carbon steel samples. The kinetic  In the as-received form, low-carbon steel exhibited the lowest values of H, HV, and S and the highest value of E r among all samples. These values are attributed to the ductile and soft α al and α id phases and the pearlite phase in the microstructure. The water quench-hardening process transformed these phases into the α' phase and the α al and α id phases, respectively. The formation of the α' phase caused 80% improvement in H, twofold improvement in HV, 43% improvement in S and a 38% reduction in E r compared with as-received steel. Similarly, the brine quench hardening process produced an α' phase with relatively high carbon content (0.58%). This supersaturated α' phase offered the greatest improvements: twofold improved H, threefold improved HV, 51% improved S, and 29% reduced E r values compared with the as-received steel.
The tempering process reduced the carbon percentage in the α' phase formed after the quench hardening processes in both media, and relieved internal stresses. Therefore, the water quench-hardened and tempered sample demonstrated a 31% improvement in H, 55% improvement in HV, 36% improvement in S, and 43% reduction in E r compared with the as-received sample. The brine quench hardening and tempering process showed relatively better properties than the water quench hardening and tempering process. After the brine quench hardening and tempering process, there was a 36% improvement in H, twofold improvement in HV, 67% improvement in S, and 37% reduction in E r compared with the as-received samples.

Electrochemical Properties
Tafel scans of the as-received and quench-hardened, tempered low-carbon steel obtained after electrochemical analysis in 1 vol% HNO 3 solution are illustrated in Figure 5. The polarization potential was compared with open circuit potential ranging from -0.25 to 0.25 V at a 1 mV/s scanning rate to evaluate the electrochemical response of the low-carbon steel samples. The kinetic parameters calculated using NOVA 2.1 software by the method of Tafel extrapolation are given in Table 4, as per the Butler-Volmer relation [39]: where β a denotes anodic polarization slope, β c cathodic polarization slope, η a anodic polarization, and η c cathodic polarization of low-carbon steel surfaces in as-received and quench-hardened, tempered forms.
Crystals 2020, 10, x FOR PEER REVIEW 8 of 11 In the as-received form, low-carbon steel exhibited sufficient value of corrosion potential (Ecorr) and low values of βa (58 mVdec −1 ), corrosion current density (Icorr) (1.53 µA.cm −2 ), and corrosion rate (0.394 mpy). However, the water quench hardening process caused a 47% improvement in βa, 8% in Icorr, and 60% in the corrosion rate compared with steel in as-received form. The increased corrosion rate is attributed to two factors: the first is the increased number of grain boundaries, and the second is the homogeneous distribution of the αal and αid phases. The α phase has been reported to be the most highly susceptible to corrosion attack among all phases [40][41][42][43][44]. In as-received form, the steel possessed a non-homogenously distributed α phase and a low number of grain boundaries. Therefore, a low corrosion rate was achieved in the as-received sample, as HNO3 attacks at ground boundaries and with less intensity if phase distribution is non-homogeneous. The brine quench-hardened sample exhibited 60% increased βa, 17% increased Icorr, and a 70% increased corrosion rate compared with the as-received steel sample. The brine quench-hardened sample also exhibited a greater corrosion rate than the water quench-hardened sample. This is because high fractions of the α' phase were formed in the brine quenching process, resulting in an increased number of grain boundaries and corrosion rates. Tempering heat treatment reduced the corrosion rates of both the water and brine quench-hardened samples to some extent, but these were still greater than that of as-received steel. As discussed earlier, tempering heat treatment caused the relaxation of the lattice in the α' phase and  In the as-received form, low-carbon steel exhibited sufficient value of corrosion potential (E corr ) and low values of β a (58 mVdec −1 ), corrosion current density (I corr ) (1.53 µA.cm −2 ), and corrosion rate (0.394 mpy). However, the water quench hardening process caused a 47% improvement in β a , 8% in I corr , and 60% in the corrosion rate compared with steel in as-received form. The increased corrosion rate is attributed to two factors: the first is the increased number of grain boundaries, and the second is the homogeneous distribution of the α al and α id phases. The α phase has been reported to be the most highly susceptible to corrosion attack among all phases [40][41][42][43][44]. In as-received form, the steel possessed a non-homogenously distributed α phase and a low number of grain boundaries. Therefore, a low corrosion rate was achieved in the as-received sample, as HNO 3 attacks at ground boundaries and with less intensity if phase distribution is non-homogeneous. The brine quench-hardened sample exhibited 60% increased β a , 17% increased I corr , and a 70% increased corrosion rate compared with the as-received steel sample. The brine quench-hardened sample also exhibited a greater corrosion rate than the water quench-hardened sample. This is because high fractions of the α' phase were formed in the brine quenching process, resulting in an increased number of grain boundaries and corrosion rates.
Tempering heat treatment reduced the corrosion rates of both the water and brine quench-hardened samples to some extent, but these were still greater than that of as-received steel. As discussed earlier, tempering heat treatment caused the relaxation of the lattice in the α' phase and diffused the excess carbon of α' to the α phases [45,46]. In this way, the tempering heat treatment reduced the number of grain boundaries and reduced the corrosion rate compared with the quench-hardened samples. Overall, low-carbon steel exhibited good corrosion resistance against 1 vol% HNO 3 in both as-received and heat-treated forms.

Conclusions
The following conclusions can be extracted from this work: Microstructures comprising a lath α' phase with different carbon percentages ranging from 0.26% to 0.58% and an α phase of different morphologies were achieved after all quench hardening and tempering processes. α al and α id phases were observed in both water and brine quench-hardened samples, and α w and α id phases in brine quench-hardened and tempered samples. The lath α' phase was formed due to the high cooling rates provided by water and brine quench hardening processes, whereas the α phase was formed due to the presence of a low carbon percentage in the steel composition. The subsequent tempering process caused lattice relaxation in the lath α' phase due to carbon diffusion from the α' phase to the α phase. A maximum twofold improvement in H, threefold in HV, and 51% in S, and a 29% reduction in E r were achieved after the brine quench hardening process compared with the as-received steel. This is attributed to the formation of the lath α' phase in the microstructure due to the higher cooling rate of the brine solution. On the other hand, an 80% improvement in H, twofold improvement in HV, 43% improvement in S, and 38% reduction in E r were achieved after the water quench hardening process compared with the as-received steel, attributed to the formation of the lath α' phase. However, this α' phase has a lower carbon percentage than in the brine quench-hardened sample due to the relatively lower cooling rate than in brine solution.
Crystals 2020, 10, 508 9 of 11 An optimum amalgamation of micro-mechanical properties was obtained after the water and brine quench hardening processes, beyond that of all other heat treatments. The excellent corrosion resistance of low-carbon steel against the HNO 3 electrolyte was observed after all quench hardening and tempering processes, as evidenced by a very low corrosion rate. The water quench-hardened and tempered sample exhibited the highest corrosion resistance, whereas the brine quench-hardened sample displayed the lowest corrosion resistance among all the heat-treated samples. The high corrosion resistance of the water quench-hardened and tempered sample and the low corrosion resistance of the brine quench-hardened sample are attributed to the formation of a lath α' phase with different carbon percentages. The lath α' phase formed after the brine quench hardening process possessed a higher carbon percentage, whereas that formed after the water quench hardening process possessed a relatively lower carbon percentage. Nanoindentation was proved to be an accurate technique for measuring the indentation hardness, reduced elastic modulus, and stiffness of low-carbon steel at the micro-level. The nanoindentation technique cannot only measure the micro-mechanical properties but can also be implemented to determine the fracture toughness, creep, and yield stress of steels in the future. The nanoindentation technique also has some limitations. The most major limitation is associated with "pile-up" or "sink-in" of the material across the edges of the indent during indentation. Conventional calculation methods for the modulus of elasticity are useful only for linear and isotropic materials like metal and glass, which is another limitation of the nanoindentation technique. All these properties can also be explored using this technique in other steels after other novel heat treatment processes including austenite reverted transformation annealing, quenching-partitioning, and quenching-partitioning-tempering. The micro-level properties of inverse bainite and graphite phases can also be evaluated through this technique, allowing for engineering applications [47,48].

Conflicts of Interest:
The authors declare no conflict of interest.