The Effects of Hot Rolling Process on Mechanical Properties, Corrosion Resistance, and Microstructures of Mo-Ni Alloyed Steels Produced by Powder Metallurgy

Abstract

This study examined the effects cfhot rolling on the microstructure, tensile strength, and corrosion behaviors cf three deferent alloy steels made by powder metallurgy: Fe-0.55C, Fe-0.55C-3Mo, and Fe-0.55C-3Mo-10Ni. 700 MPa pressure was applied to press the particles. The cold pressed samples were sintered in a mixed-gas atmosphere (90% nitrogen, 10% hydrogen) at 5°C/min up to 1400°C for 2 hours. Then, the produced steels were hot rolled with a deformation rate cf 80%. The microstructures show that deformed Mo and MoNi steels have finer microstructures, better mechanical properties than undeformed Mo and Mo-Ni steels, and M°C, MoN, or M°C(N) was formed in the Mo-Ni steels. The highest mechanical properties were obtained in rolled steel samples containing Mo-Ni, followed by rolled Mo steel and rolled carbon steel samples, and then unrolled samples. Additionally, Tcfel curve analysis demonstrated that alloy corrosion resistance rose as Ni concentration increased. It has also been observed that the hot rolling process improves corrosion resistance. The increase in the density value with the rolling process emerged as the best supporter cf corrosion resistance.

Headnote

Abstract

This study examined the effects cfhot rolling on the microstructure, tensile strength, and corrosion behaviors cf three deferent alloy steels made by powder metallurgy: Fe-0.55C, Fe-0.55C-3Mo, and Fe-0.55C-3Mo-10Ni. 700 MPa pressure was applied to press the particles. The cold pressed samples were sintered in a mixed-gas atmosphere (90% nitrogen, 10% hydrogen) at 5°C/min up to 1400°C for 2 hours. Then, the produced steels were hot rolled with a deformation rate cf 80%. The microstructures show that deformed Mo and MoNi steels have finer microstructures, better mechanical properties than undeformed Mo and Mo-Ni steels, and M°C, MoN, or M°C(N) was formed in the Mo-Ni steels. The highest mechanical properties were obtained in rolled steel samples containing Mo-Ni, followed by rolled Mo steel and rolled carbon steel samples, and then unrolled samples. Additionally, Tcfel curve analysis demonstrated that alloy corrosion resistance rose as Ni concentration increased. It has also been observed that the hot rolling process improves corrosion resistance. The increase in the density value with the rolling process emerged as the best supporter cf corrosion resistance.

Keywords: Powder Metallurgy; Hot rolling; Alloy steels; Characterization; Corrosion.

1. Introduction

In contrast to conventional production techniques, powder metallurgy (PM) generates small, useful, and challenging-to-manufacture items, including composite structures, with minimal tolerance and high strength. PM is used to make several industrial components, including those for the automotive, defense, health, and energy industries [1-3]. PM also minimizes material loss depending on the shape of the material to be produced with fewer production steps [4-6]. The finished product might occasionally require supplemental procedures like rolling, heat treatment, and forging [7]. Despite many benefits of powder metallurgy, since porosity cannot be eliminated during manufacturing, it can be controlled by a thermo-mechanical process applied to the steel after production. Hot deformation, an efficient initial step in decreasing porosity, takes place above the recrystallization temperature. High temperatures generally crystallize the grains that extend by changing shape during deformation, and tiny grains are created with high-strength structures. The material is readily distorted using the hot deformation technique without significant deformation force. In addition, the heat deformation process promotes ductility. This deformation does not result in the hardening of the material. Aside from achieving a homogeneous structure, hot deformation can also eliminate or reduce porosity because high temperatures increase diffusion [8].

Material selection for the fabrication of machine parts is essential in a working environment. The materials' lifespan should be as long as possible for components subjected to high stresses, such as severe tensile, compression, and shear stresses, to be applied singly or in multiples, together with environmental variables like corrosion and wear. High-stress materials are employed in various applications, including airplanes, automobiles, crankshafts, and gears. Steel may have its mechanical characteristics enhanced and new qualities added using heat treatments, adding elements and thermomechanical processes [9]. Alloyed steels are a class of substances with exceptional qualities such as great weldability, durability, toughness, and corrosion resistance attained by using various mechanisms for strengthening materials and suitable thermomechanical techniques [10]. Although steels contain carbon in their composition, it is important to understand that alloying elements besides carbon are also present when alloy steel is processed. Moreover, steel's mechanical qualities can be enhanced by adding alloying elements, including Ni, Mo, Nb, W, Cr, and Si [11,12].

Steel has improved passive film stability because of Mo and Molybdate compounds, regarded as anodic corrosion inhibitors shielding different metals and alloys. Additionally, they promote the growth of passive oxide coatings on metal surfaces. Because they are nontoxic and safe for the environment, these compounds prevent corrosion in steel reinforcing. It is typically chosen for use in airplanes, ground vehicles, and the steel manufacturing industry [13]. Molybdate chemicals work through three different mechanisms: adsorption, oxidation, and deposition. Molybdates and chloride ions compete for surface locations during adsorption. Mo can also be used to coat other metals with a flame-retardant finish [14,15]. According to Hossam et al. [16], when molybdenum is dissolved in austenite, the hardness of steel is increased by a small amount. According to Lee et al. [17], Mo enhanced the precipitation-hardening effect in HSLA steel with Nb addition. Junhua et al. [18] investigated how the Mo component affected the microstructure and mechanical properties of the X80 high-strength steel pipeline. The lifting of the ferrite's volume fractions was regarded as a sign of increased Mo content, which added yield strength and toughness. Steel (4%C) and sintered Fe-Cr-Mo alloy were both tested for dry rolling-sliding wear resistance by Straffelini et al. [19]. They claimed that the bainitic microstructure, when sintered, offers an excellent balance of ductility and strength.

The nickel acts as a steel stabilizer, causing the austenite zone to grow and the ferrite region to contract. As a result, Ni becomes more resistant to oxidation and corrosion at high temperatures. In addition, the strength and hardness of Ni are increased by reducing the grain size. Combined with chromium, it enhances the critical cooling rate, ductility, hardness, and fatigue resistance. Ni diffuses into iron more slowly than other elements due to its lower diffusion coefficient [20-23]. There are studies on the addition of Ni to different types of steel in the literature. For instance, according to Chandramouli and Shanmugasundaram [24], when alloying metals like Cr, Mo, and Ni were added to Fe-0.2%C, the presence of carbides and the formation of a ferritic-bainitic microstructure made steel stronger and tougher. A C-Nb-V and Fe-matrix material produced by powder metallurgy was studied by Ahssi et al. [25] to determine the impacts of varied Ni addition ratios on its microstructure, tensile strength, hardness, and corrosion behaviors. They concluded that Ni content enhanced yield and tensile strengths, which peaked at 13% Ni concentration. However, the strength decreased when additional Ni was added. Furthermore, Tafel curve analysis revealed that the alloys' corrosion resistance increased when Ni concentration increased [25-27]. According to Phillips et al. [28], the alloying element Ni helps sustain austenite even at high relative densities, and the strength of the sintered alloy can be comparable to that of wrought steels.

One of the more affordable methods of hot deformation is hot rolling, which is also easily adaptable to mass manufacturing. In the steel industry, hot rolling is a hot deformation technique for commercial products [29]. Hot deformation, a mechanical process earned out at temperatures higher than the recrystallization temperature, is used to machine various alloys and metals mechanically. The high temperature causes the grains to crystallize, producing little grains with a high-strength structure. Typically, during deformation, the grains elongate and change shape. During the hot deformation process, the material's ductility is increased by heating, allowing it to deform with less force. The rise in diffusivity with rising temperature also makes it possible to produce a homogenous structure [30]. In the literature, some studies show the effects of hot rolling on steels' mechanical properties and microstructures. For example, Xu et al. [31] reported that ferrite matrix, minor amounts of martensite and MC carbides, M7C3 carbides, and М2зСб carbides were made up after the rolling process of M390 HSS produced by powder metallurgy (PM). Chilton and Roberts [32] investigated the characteristics of carbon steels after hot-rolling. They mentioned that the combined effects of grain refining and decreased precipitation hardening dramatically improved toughness. In their study of TRIP (Transformation-induced plasticity ), Hashimoto et al. [33] added Nb and Mo in coiling conditions. The findings show that serial additions of 0.2%Mo with 0.05%Nb under hypothetical hot rolling conditions result in stronger TS (Tensile Strength). Additionally, preserved austenite that was finely disseminated contributed to the increase in ductility. According to Baharvand et al. [34], during hot rolling, the average ferrite grain size and band spacing reduced besides reduction in thickness, improved mechanical properties, and created a more uniform microhardness profile.

Little research has been done on the impact of different rates of deformation and cooling on the alloy steels' microstructures and mechanical properties used in powder metallurgy, according to a literature survey. This work examined the effects of hot rolling on the microstructures, mechanical properties, and corrosion properties of unalloyed Mo and Mo-Ni steels generated by powder metallurgy.

Three distinct steel compositions were created in the current investigation using the powder metallurgy process and cold pressing. The produced steels were hot rolled with a deformation rate of 80%. The microstructures, mechanical characteristics, and corrosion resistance of both deformed and undeformed samples were investigated.

2. Materials and Experimental Procedures

This study produced alloyed steel samples with Fe-0.55C, Fe-0.55C-3Mo, and 0.55C-3Mo-10Ni compositions using the powder metallurgy technique. The sizes and purities of powders are given in Table I. Fig. 1 shows the steps used in the production of of the samples. The sizes and purities of graphite (Höganas, USA), iron (Höganas, USA), Molybdenum (Aldrich, Germany), and nickel (Aldrich, Germany) powders used in the study are < 20 pm, <180 pm, < 150pm and 5 pm, and 96.5%, 99.9%, 99.9%, and 99.7%, respectively.

The powders were weighed using a precision scale marked RADWAG AS-60-220 C/2 at 0.0001 g. Each composition of Fe-0.55C-3Mo, Fe-0.55C-3Mo-10Ni, and Fe-0.55C powders was developed and mixed for a couple of hours with the help of a TUR-BULA T2F device (Willy A. Bachofen AG, Muttenz, Switzerland). It worked on the 3D motion concept. To form the tensile test specimen according to ASTM E8/E8M [35], a hydraulic press (Hidroliksan, Konya, Turkey) was used to compact the powders by applying 700 MPa pressure. Then, the samples were sintered in a mixed-gas atmosphere (90% nitrogen, 10% hydrogen) at 5°C/min up to 1400°C for 2 hours. It was then cooled down at room temperature at 5°C/min cooling rate.

The hot rolling process was applied to the powder metal steels produced after the sintering process at a rate of 80%. Before rolling, the samples were preheated at 1150°C for 30 minutes in a Protherm brand heat treatment furnace. At the end of the preheating, the hot rolling process was completed by reaching 80% deformation in 7 passes and 20% deformation in each pass in the rolling device. Preheating at 1150°C was applied each time between rolling passes. The roller diameter used in the rolling process is 200 mm. The number of rolling cycles is 20 cycles/minute (Table II).

The rolled samples were cut using wire cutting through wire erosion equipment (Sodick AQ600L, Düzce, Turkey) to create the desired shape of the tensile test specimen according to ASTM E8/E8M [35]. The sintered and rolled samples were then tested using a SHIMADZU tensile test instrument with a 50 KN capacity (Shimadzu, Tokyo, Japan) at 0.5 mm/min crosshead speed. All samples were pulled with the same parameter, and three samples of each composition were tested. Each test resulted in a stress-strain graph. Tensile strength and strain values of the samples were measured from these graphs, and it was possible to infer the variation in mechanical characteristics caused by the change in chemical composition and deformation rate. The picture of the hot-rolled tensile test samples is displayed (Fig. 2).

The specimen densities were ascertained with the help of a Radwag density kit (Bruker Alpha, Bursa, Turkey) based on the Archimedes principle (ASTM В 328-96) [36]. The surfaces of the samples were cleaned before using an optical microscope. Various mesh sizes of abrasive papers were used for cleaning: 400, 600, 800, 1000, 1500, 2000, 2500, 3000, 4000, 5000, and 7000. Then 0.3 /trn ALO3 suspension was used for polishing, and this process was followed by etching for 3 seconds in 2% nitric acid-98% ethyl alcohol for samples (Fe0.55C and Fe-0.55C-3Mo) and HNO3+3 HC1 for Fe-0.55C-3Mo-10Ni. Finally, all of them were cleaned with ethyl alcohol and distilled water, then dried with air.

Based on optical micrographs, the grain sizes of non-alloyed and alloyed PM steel specimens were calculated using the mean linear intercept method. On the micrographs, a line (at a 45° angle) was drawn for each specimen, cut by the intersecting line, and numbered [37,38].

An optical microscope, Nikon ECLIPSE L150 type (Melville, NY, USA), was used for the research. Microstructural and fracture surface analyses were performed using SEM and XRD using a Zeiss microscope and a Rigaku Ultima IV diffractometer, respectively. XRD was used for qualitative analysis of the structural changes in alloys after Mo and Ni were added.

Before the corrosion test, copper wire was soldered to the samples to ensure the conductivity of the samples with the corrosion unit. Then they were molded with epoxy resin to provide insulation. After cleaning the test surface, a thick adhesive tape was applied (with a 0.25 cm2 diameter hole) to the specimen surface. All specimens' corrosion tests were carried out in the same place to prevent any potential adverse effects. At room temperature in a 3.5% NaCl solution, the potentiodynamic polarization tests were carried out using a Gamry model PC4/300 mA potentiostat/galvanostat with computer controlled DC105 corrosion analysis. A saturated calomel electrode was the reference electrode, and graphite was selected as the counter electrode in a three-electrode electrochemical cell arrangement. Figure 3 shows the corrosion cell and corrosion specimen.

3. Results and Discussion

Fig. 4 shows alloys 1 and 2 with ferrite and perlite phases. In order to give alloyed steels, the desired mechanical qualities, alloying elements can be added in single, double or tiple combinations. This study observed that the amount of perlite in the microstructure increased by adding 3% molybdenum. In addition, it has been observed that microstructure occurs in bainitic structures. Moreover, a decrease in grain size was observed (Fig. 4).

Fig. 4 demonstrate how the ferrite and pearlite microstructure increased the perlite amount and created the bainitic structure when 3% Mo was added to unalloyed steel. In addition, it was observed that bainite, martensite, austenite, and residual austenite were formed in the microstructure with the addition of 10% Ni. In addition, when the grain sizes of the unalloyed sintered sample and the sintered sample containing 3% Mo were compared, it was observed that the average grain size of the unalloyed steel was 23 pm. At the same time, it decreased to 13 pm with the addition of Mo. It was found that the addition of Mo decreased the grain size. The production of M°C(N) precipitates in grains and grain borders might cause grain refining. Ferrite and perlite phases with equal-sized grain diameters are in the sintered unalloyed and Mo steel samples. The microstructure of the samples cooled in the air after 80% plastic deformation consists of fine ferrite and perlite grains (Fig. 4). The average grain size of the 80% deformed sample is smaller when compared to the unrolled samples and the 80% deformed unalloyed and alloyed steel samples after sintering [27,39]. For instance, when 80% deformation was applied, the average grain size of unalloyed steel, 23 pm after sintering, was reduced to 2 pm. When Mo-containing steels are compared, the sintered Mo PM steel's average grain size is 13 pm, but it is 1.5 pm when deformation is applied. Finally, while the average grain size of austenite grains in sintered Mo-Ni steel was 51 pm, it decreased to 7.3 pm after deformation. Many studies in the literature show that deformation refines the grain size [40-43]. For instance, Inagaki [42] showed that adding highly stressed regions at grain boundaries or near the boundaries of annealing twins increases ferrite nucleation and hence grain refinement. This indicates that the deformation in the non-recrystallizing region of austenite accelerates the nucleation rate and ferrite formation. As a result, small ferrite particles are formed, and the volume fraction of ferrite in the structure increases [43]. In addition, the results obtained from this study showed that 80% of deformed samples exhibited smaller grains than sintered materials. Li [41] investigated how the forging rate affected the microstructure of H13 steel and found that as the forging rate increases, grain size reduces, providing more nucleation points and storing energy for grain recovery and recrystallization. Therefore, it has been observed that it promotes grain thinning.

Fig. 5 depicts the SEM microstructure of the sintered and 80% deformed samples, when the microstructures of the materials were analyzed, it was discovered that the applied heat deformation reduced the number of pores while simultaneously reducing the grain size. The solution-based alloying of elements has minimal impact on austenite recrystallization. Precipitated particles have a substantially higher inhibitory influence on grain boundary movement than solute atoms [32]. The EDS values for sintered, and 80% deformed samples are shown in Figures 5 and 6. Examining the results reveals that precipitates of various sizes are produced. In addition, EDS results show that Fe3C, M°C, and M°C(N) precipitates are formed because of the inclusion of Fe, C, and Mo elements. Moreover, deformation accumulation can initiate the precipitation of carbonitrides in steels by alloy addition [44,45]. When the EDS analysis results from this study are compared to previous investigations, it is clear that alloyed PM steels can include precipitates like M°C(N). This could happen due to the creation of microscopic precipitates such as M°C, MoN, and M°CN either at the 1400°C sintering process or during the post-sintering cooling. According to the EDS examination, Mo element precipitated in the form of grains/ grain boundaries (Fig. 5 a,b). The EDS line analysis also revealed that Alloy 2 had a variety of elements, both in terms of kind and quantity, along the line that crossed the matrix and precipitates (Fig. 5). Mo is abundant in the spherical precipitate, whereas iron is abundant in the matrix phase. The concentration of Mo increases significantly when the analytical line and the precipitates converge. The PM steel samples have precipitates (confirmed by SEM and EDS studies) that are known to substantially affect austenite recrystallization and grain development [18]. Small ferrite grains are created when precipitates that do not dissolve at the sintering temperature stop the growth of austenite grains [1,46-48].

When the SEM microstructure and EDS results obtained from Mo-Ni steel after deformation (Fig. 5) were examined, it was observed that nanosized precipitates were formed on the grain and the grain boundary [30,49]. In addition, the microstructure of Mo-steel, which does not contain Ni, is composed of ferrite and perlite; however, if the Ni content is 0-2wt% in these alloys, harder phases are formed in the microstructure and specifically when the Ni content increases. Tracey [50] and Reyes et al. [51] observed the emergence of bainite and martensite phases. In the research by Alharthi [52], it reaches the bainitic structure, which cannot be reached by Ni atoms in the center of the Fe particle, through C and Mo. It was determined that Ni-rich austenitic areas are present even after 30 minutes of sintering, transforming into martensitic areas when a longer sintering time is provided. As the longer sintering time, the amount of Ni-rich martensite increases. When Fig. 4c and Fig. 6 are examined, it is observed that the martensite phase is formed in the microstructure. Moreover, it is considered that the alloying elements prevent the growth of austenite grains by forming precipitates such as M°C, MoN, and M°CN during sintering [48,53,54].

Depending on the alloy type, XRD patterns and predictions about which compounds may have formed in the pattern peaks are shown in Fig. 7. Considering the XRD pattern of Fe+0.55C+3Mo+10Ni alloy, where all elements and compounds are seen together, it is approximately Fe3Ni2 compound at 44°, Fe element at 45°, MoN compound at 48°, FeC compound at 68°, and 82° It can be concluded that the M°C compound with the element Fe.

Table III also shows the tensile test values of sintered and 80% deformed samples. It is obvious that deformed samples showed higher tensile strength and % elongation values compared to sintered samples. Grain refinement in steels is responsible for improving mechanical characteristics [55]. When the point EDS analysis results from this study are compared to previous investigations, it is clear that alloyed PM steels can include precipitates like M°C(N). This could happen because of the creation of microscopic precipitates like MoN, M°CN, and M°C during the 1400°C sintering or post-sintering cooling phase. According to the EDS analysis, Mo elements precipitated from the solution as grains and grain boundaries (Fig. 5 a,b). The EDS line analysis also showed that Alloy 2's elemental makeup varied along the line that crossed the matrix and precipitated (Fig. 5). The spherical precipitate is rich in Mo, while the matrix phase is rich in iron. When the analytical line meets the precipitates, Mo shows a noticeable increase. It is well known that the precipitates found in PM steel samples utilizing SEM and EDS studies have a considerable impact on austenite grain development and recrystallization [18]. Small ferrite grains are created when precipitates that do not dissolve at the sintering temperature stop the growth of austenite grains.

As indicated in Table III, the sintered and deformed samples' density (%) values increase with deformation. The structure of materials produced by PM is porous, and an increase in porosity may cause a decrease in mechanical properties of the materials. The porous structure limits the use of porous materials in applications that require high strength, as it causes increased stresses. [55]. Increasing the amount of alloy in powder metal steels generally increases the number of pores [48]. The pore ratio is connected to the mechanical characteristics of objects made using PM. While serving as stress concentration points, pores also aid in the spread of cracks [48]. Accordingly, it was determined that the number of pores in the material decreased significantly, and its density increased with hot rolling applied in this study. [54].

Additionally, it has been found that the reduction in grain size in the microstructure after rolling, the increase in the amount of precipitate in the grain and grain boundaries, the increase in density, and the decrease in % porosity significantly increase the % elongation values and tensile strength of all alloys [30]. For example, the % elongation value and tensile strength of unalloyed steel were 13.11% elongation and sintering are 278 MPa, respectively, while the tensile strength and % elongation value after hot rolling were 303 MPa and 33.33%. After hot rolling, unalloyed steel showed a 10% improvement in tensile strength and a 254% rise in elongation value. Similar to this, in steel containing Mo, after sintering, the tensile strength and % elongation values were 708 MPa and 8.15%, respectively, but after rolling, they were 801 MPa and 29.27%, respectively. After deformation, the increase in the tensile strength increased by approximately 14%, and the elongation value increased by approximately 260%. Likewise, while tensile strength and % elongation values of Mo-Ni steel were 1143MPa and 14.33%, respectively, after sintering, they increased to 1933 MPa and 38.41% after hot rolling.

After deformation, the increase in tensile strength increases by 70%, and the elongation value increases by approximately 170%. When evaluated in general, a significant increase in mechanical properties was observed in all compositions after hot rolling. The decrease in grain size, the rise in precipitate in the grain and grain boundaries, the increase in density, and the reduction in porosity percentage show the improvement in mechanical properties [30,39]. The highest mechanical properties of the deformed samples result from the increased nucleation zone. It should be noted that the most frequent nucleation sites differ depending on the amount of deformation. The у grain boundary is most important when the amount of deformation is small. With increasing deformation, the annealing twin boundary and the deformation band become more important factors for transformation [43]. This gives a finer structure and precipitate particles that improve mechanical properties. Also, formability and toughness, which are much more density-dependent, increase at higher density [55,56], as pores in fabricated steel samples cause the build-up of stresses that contribute to crack propagation [57-59]. Another reason why the percentage elongation values of the hot rolled samples are higher than the sintered samples is that the amount of pores is lower. As a result, the mechanical properties improved because of strength-enhancing mechanisms, including an increase in the amount of precipitate, inhibition of dislocation movement through precipitates and grain boundaries, and a reduction in grain size [1,30,46,48,49]. For example, when Gething et al. [49] evaluated how Ni addition affects the mechanical properties of Mo powder metal steels, the mechanical properties, such as hardness and tensile strength of the powder metal steels, improved with Ni additions in terms of weight.

After the PM steel sample underwent the tensile test, 1000X photos of the fracture surface were collected. Fig. 9, Fig. 10, and Fig. 11 show SEM images of unalloyed and alloyed steel samples. The cracked surfaces showed behavior that was a combination of partially brittle (separation planes) and partially ductile (honeycomb structure). All of the damaged surfaces had pores. This suggests that the microvoids' combining and spreading cause the fracture. Large gaps were also seen in the PM steel samples that were made. These voids show that during the tensile test, precipitates such as M°C(N) were detached from the surface [61,62]. Such substantial voids were found on the shattered surfaces of PM steel containing Cr, Mo, and Ni, according to Chandramouli and Shanmugasundaram. They explained this by separating carbonitride, carbide, and nitride from the surface during the tensile test. The tensile test revealed that when the amount of Mo, Cr, and Ni increased, tensile strength increased as well, and % elongation decreased. When the cracked surface pictures of the samples obtained after hot rolling were examined, it was observed that the large voids were significantly reduced, and the ultra-thin honeycomb structure increased. Fractured surface pictures show compatibility with the values obtained after the tensile test [24].

When Fig. 12 and Table IV are examined, the steel containing 3Mo+10Ni showed the best corrosion resistance by weight. In fact, an increase in corrosion resistance with Ni content is an expected result. Studies support these results in the literature [63]. In Pettibone's study, steel with high Ni content exposed to seawater for a long time showed an advantage in polarization resistance compared to mild steel. Alharthi et al. [52] investigated how much nickel affects an alloy's ability to resist corrosion in a IM hydrochloric acid solution for pickling. The polarization and surface resistances were substantially higher for the alloy with a high Ni concentration.

In 3.5% NaCl solutions at 25°C, Pavapootanont et al. [64] examined the corrosion characteristics of high Ni-containing steels. Although the passive current density of these three kinds of steels declined as the nickel content increased, both the pitting potentials and primary passive potentials improved. Nickel improved the steel's overall corrosion and pitting corrosion resistance in all three solutions. In another study, High-strength low alloy (HSLA) steel in the NaCl solution (3.5 wt%) was studied by Wang et al. [65] to determine the ideal Ni concentration for the steel's microstructure, phase, and electrochemical behaviors. A finer grain size of 10 m and a lower icon- of 2.169 pA cm2 were present in the sample with 3.55 wt% nickel. The nickel addition also improved the low alloy steel's charge transfer resistance, suggesting the sample had outstanding electrochemical reaction inhibition and corrosion resistance.

In addition, hot rolled powder metal steels exhibited lower polarization resistance than sintered powder metal steels. This effect may be due to the increased density and finer grain size obtained by rolling. Fines can form regions with different potentials and create cathodic as well as anodic areas. Solid particles deposited on the steel surface can also act as a cathodic site, increasing the corrosion of the steel and reducing the active area. The accumulation of solids on the steel surface should form aeration cells, where the region below the deposit is the anodic region (low oxygen concentration) [59].

The potentiostatic polarization curves of the corroded samples showed that the current densities of the samples initially decreased and then stabilized at a low positive current density due to passive film formation. The current densities of the hot-rolled samples were significantly lower than the current density of the sintered sample and did not show larger fluctuations.

The necessary formulas from ASTM-G102 were used to construct the Ecoit and Icoit values (Table IV) and Tafel curves (Fig. 12) obtained from corrosion rates and potentiodynamic corrosion tests. The effects of mass transportation are not seen in the figures. Apart from the anodic region of base steel, which is mainly outside the fitting region, a change in the reaction mechanism does not appear to have occurred for most scanning regions. The Tafel definition of generic corrosion dynamics is therefore thought to be plausible. Pure steel's corrosion resistance appears to increase significantly when Mo and Ni are added. The addition of Mo and Ni reduced the unalloyed steel's grain size.

In contrast to the coarse pearlite ferrite structure, the fine pearlite ferrite structure is more corrosion resistant. This happens because the formation of dense passive films benefits from particle size reduction. According to Ura-Bi'nczyk et al. [66], grain refining made the passive coating on the N80-1 steel surface denser and more uniform. As a result, it has been found that N80-1 steel is more corrosion-resistant than K55 steel. When Fig. 13 and Fig. 14 are examined, it is thought that a passive film is formed on the surface, especially with the addition of Mo and Ni elements, as can be observed from the point and regional EDS results. There are sources in the literature that this film layer is also beneficial on corrosion resistance [67] . In addition, it should not be forgotten that Mo and Ni elements are present in the Fe matrix, and as a result of the filling of the spaces formed during corrosion by these elements, it plays a core role in the diffusion of corrosion products, thus increasing the corrosion resistance [68].

4. Conclusion

Fe-C, Fe-C-Mo-Ni, and Fe-C-Mo compositions were successfully produced by powder metallurgy; all compositions were then subjected to 80% deformation by hot rolling. In addition, microstructural, mechanical, and corrosion properties of unrolled and hot-rolled steel samples were investigated. The present study has the following conclusions:

1. The microstructure of unalloyed steel consists of pearlite and ferrite phases. With the addition of Mo, the amount of perlite and bainitic phases have increased. The addition of Ni, bainite, martensite, austenite, and residual austenite phases have been found in the structure. The addition of nickel has also been attributed to the martensite and bainite phases formed in the microstructure.

2. EDS analysis of the PM steels revealed that Fe, C, N, and Mo elements exist in the iron matrix's precipitates, like Fe3C and M°C(N). Furthermore, XRD analyses have revealed that the Fe element, as well as Fe3C and Fe3Ni2 and its compounds, peaked at 65.06°C, followed by the Fe element and MoN compound at 82.42°C, and Fe element and M°C compound at 44.76°C in the alloy Fe+0.55C+3Mo+10Ni.

3. Deformed PM steels have a finer microstructure and better mechanical properties than undeformed PM steels after sintering. This happens because of decreasing the grain size, decreasing porosity, increasing density, and increasing the number of precipitates such as M°C, MoN, or M°C(N) formed in the Mo steels and Mo-Ni steels.

4т Hot rolled powder metal steels exhibited lower polarization resistance compared to sintered powder metal steels. This effect may be due to the increased density and finer grain size obtained by rolling. Moreover, fines can form in regions and create cathodic and anodic areas with different potentials. In addition, it is thought that the addition of Mo and Ni elements prevent corrosion by forming a passive film on the corrosion surface and by acting as nucleation in the formation of corrosion products by settling in the cavities during corrosion. It is understood that these mechanisms have positive effects on the fine-grained structure, increased density and corrosion resistance.

Acknowledgments

This study was carried out as a Ph.D. thesis by "Rajab Hussein Rajab Elkilani" in the Institute of Graduate Studies at Karabük University, Karabük, Turkey.

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