Effect of martensitic reversal and grain size on the corrosion and wear behaviour of Cr-Mn steel

In this study, the effect of corrosion and wear behaviour of Cr-Mn steel on fine grains were investigated. The sample were solution annealed (SA) for 1 h at 1050 °C and then cold rolled (CW) to 30%. Further the cold rolled sample were thermally aged (CW + TA) 900 °C for four hours. The findings showed that under the 10 N applied load, wear resistance increased with an increase in hardness and martensite fraction of the cold worked (CW) samples. However, the Cr-Mn steel had the superior wear resistance after thermal ageing (TA). In microstructural examination deformation bands can also be visible in cold work samples. The analysis implies that the γ-phase is apparent across all peaks within the spectra of SA samples. In instances involving 30% cold work, prominent α′ martensite peaks were observed, accompanied by minimal ε-martensite peaks. Electrochemical impedance spectroscopy (EIS) analysis discloses a reduction in impedance and a concurrent increase in the defect density of the passive film. The CW+TA structure with good inclusive performances created an early constant hardened layer, which didn’t delaminate and peel off prematurely, thereby effectively increasing the wear resistance, according to analysis of the wear mechanism. The results also concluded that the corrosion resistance of CW sample decreases due to SIM formation, however CW+TA sample provide better corrosion resistance due to smaller and refined grain size.


Introduction
Because of their excellent formability, minimum rate of corrosion, and acceptable weldability, austenitic stainless steels (ASS)-mainly 202 ASS-are well-known.They are frequently utilised in engineering applications, including the petrochemical, biomedical, and food sectors, as well as the oil and gas sector [1,2].They do not, however, sufficiently meet the requirements for corrosion and tribological applications due to their comparatively low hardness and yield strength [3].The enhancement of the mechanical characteristics of ASS has been the subject of extensive research in this area.Surface modification methods like laser cladding [4], nitriding [5], and surface mechanical attrition treatment (SMAT) [6] have been the main focus of the majority of experimental studies.However, very little research has been done on how to use grain refinement to improve the wear and corrosion behaviour of this steel [7].Though, this grade of stainless steel's high recrystallization temperature (over 900 °C) limits the ability to strengthen it using conventional grain refining techniques [8].Additionally, the resulting recrystallized grains are unable to reach the region of ultra-fine grains.According to reports, martensitic transformation may be able to drastically reduce the grain size of this grade of stainless steels, eliminating the aforementioned service shortcomings [9].As was already mentioned, certain studies have been done to determine how the steel's wear and corrosion resistance is affected by its ultra-fine grained structure.Smaller grain size has a favourable impact on the wear resistance of ultra-fine grained AISI 304 stainless steel, according to Valentini et al [10], mostly because microstructure changes occur throughout the wear test.Similar to this, Sourabh et al [11] investigated how grain refinement affected Cr-Mn steel's corrosion resistance and came to the conclusion that refinement increases corrosion resistance.Phase transition and grain size have been found to be the two microstructural factors that influence corrosion and wear resistance the most in this regard.First, it has been demonstrated that austenite to martensite transformation in AISI 202 results in surface hardening, which worsens corrosion resistance but improves wear resistance [12].In fact, it has been thought that the reason for such great wear resistance during the wear process is the steep hardness profile from the changed hard martensite on the surface toward the untransformed soft austenite inside the worn surface.Second, because of the smaller grains, it is well known that reducing grain size to the ultra-fine and nanocrystalline area can significantly increase wear and corrosion resistance [13,14].Different findings, though, have been reported by certain researchers.For instance, AISI 202 stainless steel surface nanocrystallization caused by surface mechanical attrition had no effect on wear behaviour in the unlubricated state [15].Additionally, Padap et al [16] shown that ferrite wear resistance decreased with decreasing ferrite grain size in an ultra-fine grained AISI 1016.On the other hand, some experts have identified steel's 'ductility' degree as the primary factor influencing steel's resistance to corrosion and wear.According to research by Kim et al [17] on the influence of ductility on the dry sliding wear behaviour of AISI 1045 steel, the sample with the higher ductility showed greater wear resistance despite having the same hardness values.The impact of grain size and phase transition on the wear and corrosion properties of stainless steels, then, appears to be the subject of contentious contention.In order to assess the impact of grain size and martensitic transformation on the wear and corrosion resistance of the steel, several grain sizes of stainless steel were tested for wear and corrosion in this study.The final goal of the study was to identify the ideal variables that would enhance the mechanical properties of AISI 202 stainless steel.

Experimentation
The chemical composition of the 6 mm sheet of Cr-Mn ASS is shown in table 1.The steel was procured from the market.The samples (50 × 10 × 6 mm 3 ) were solutionized for 1 h at 1050 °C with water quench and were labelled solution annealed (SA).The thickness of these samples was subsequently reduced by cold working to 30% (CW).The cold rolling was done using Manual Sheet Rolling Machine manufactured by Veerakumar Industrial Machineries.CW with thermal ageing (TA) at 900 °C for four hours, respectively, to become CW and CW+TA.For thermal ageing muffle furnace was used with digital controller to maintain the temperature in the furnace.The sample were cut in various dimensions (for test) using electric discharge machine (EDM).Samples (size 10 × 10 mm 2 ) for microstructure investigation were prepared by polishing via a series of emery papers up to 1500 grit, followed by cloth polishing with alumina powder and electro-etching in 10 weight percent oxalic acid at 1.0 V (90 s).A proportion of martensite was also discovered, and XRD (PANalytical) was used to analyse the various phases that were present in the samples on or before cold work.By employing a Vickers microhardness tester, hardness was measured (Fine Testing Instruments Model-HVD 1000).For electrochemical experiments, potentiostatic and potentiodynamic polarisation (PDP) and electrochemical impedance spectroscopy (EIS) were used (test solution is NaCl with 3.5%).
The VMP-300 potentiostate from Biologic was tested using EC lab software.The working electrode (WE) was given time to reach open circuit potential (OCP) in a steady state for 1.5 h before measurement of OCP was taken after the cell had been set up and filled with test solution.The EIS and PDP tests were then carried out in order.A sinusoidal AC signal with a frequency range of 10 6 to 10 −2 was employed in EIS test.At a scan rate of 0.018 mV s −1 , the PDP test was conducted between −0.5 V (versus OCP) and +0.5 V (versus OCP) [18].
The pin on disc set up (TR-20LE, Ducom make, India), was used to check tribological performance of sample, where samples of 10 mm square and 25 mm height were pressed against an abrading medium of SiC emery paper of 220 grade size, at 10 N applied load, speed of 0.838 m s −1 , sliding distance of 503 m and test duration of 5 min.To find the wear rate, following equation is used:  The XRD spectra of the SA, CW, and CW+TA samples are displayed in figure 1. Different peaks for ε and α′martensite with γ-phase are identified, along with the appropriate planes.The terms SIM will be used to refer to both ε-martensite and α′martensite.It suggests that the γ-phase can be seen in all of the peaks in the spectra of SA samples.Large peaks of α′ martensite with very minor peaks of ε-martensite were detected in the case of 30% cold work.Following TA, the intensity of the peaks at α′ and γ (220) rises while the intensity of γ (111) falls.This finding is in line with those of Jinlong et al [19].The change from austenite to martensite typically takes place in two stages: (i) γ-ε and (ii) ε-α′ [11].According to one piece of literature, ε-martensite platelets are created when two deformation bands intersect, and α′-martensite is produced when two ε-martensite platelets intersect [11].
Following TA, austenite peaks become more prominent whereas martensite peaks become less prominent.It has been noted that the peaks of α′-martensite have shrunk and the peaks of ε-martensite have vanished.It means that the TA at 900 °C for 4 h transforms ε-martensite and some α′-martensite into austenite.This decrease in martensite content on TA is a martensite reversal.
As a result, martensite reversal takes place with TA.These findings concur with the results that were previously reported [20][21][22].According to Padilha et al, martensite reversion in the 304 ASS takes place at temperatures lower than recrystallization temperature [20].According to Rathbun et al [23], 304SS undergoes phase change when the temperature exceeds 450 °C.Similar findings were made by S K Ghosh et al [24], who discovered that the fraction of martensite drops from 32% to 8% on 40% CW following TA at 800 °C.This means that all martensites of the cold-worked samples have undergone reverse transformation to austenite.

Microstructure 3.2.1. Effect of cold work
The microstructure of Cr-Mn ASS that has been cold worked (CW) and solution annealed (SA) is shown in figures 2(a)-(c).It is obvious that the SA microstructure consists of equiaxed grains with step structure and distinct grain borders.Due to grain elongation and the appearance of deformation bands in a few grains, the resolution of grain boundaries reduces as CW increases.The density of the deformation bands increases but their resolution falls as the cold work increases.

Effect of thermal ageing
Figure 3 depicts the optical microstructure of thermally aged SA and CW+TA.Both grain area and grain boundary (gb) attacks are prevalent.Grain boundary attack is visible in the microstructure of the solutionannealed, thermally aged sample in figure 3(a).The gb attack shows that carbide precipitation at gb and Crdepletion close to gbs occur during TA.The majority of the grain boundary attack zones are continuous.This causes the materials' rate of corrosion to rise. Figure 3(b) makes it clear that there are several black grains and dark GBs.It is well known that martensite exists in the CW + TA samples.Perhaps martensite may be the dark grains.It is also obvious that when CW increases, gb thickness and grain darkness decreases.In the CW sample, deformation bands can also be visible.Bright grains include subgrain borders, which show that the samples undergone a recovery process.It displays reduced attack as a result of recovery, which denotes a drop-in corrosion rate.This is purely result of martensitic reversal, which reveals that during deformation formation of martensite takes place from autensite, however after TA those transformed martensite started to release energy in form of recrystallized defect free austenite grains which is known as martensite reversal.

Hardness
Samples were put through a Vickers microhardness tester to quantify hardness in order to examine the effects of CW and TA on hardness and the fraction of strain induced martensite (SIM).Microhardness is tested using a 0.5 kg load and a dwell time of 15 s.The average of three readings for each sample has been shown in the graph.In figure 4, these SIM values and hardness values are also tabulated.The trend in hardness is similar to that in SIM volume fraction.It is obvious that as the amount of CW increases, so does the hardness.This is explained by an increase in the SIM volume fraction.However, due to martensitic reversal after TA, both hardness and SIM fraction drop.This martensitic reversal produced defect free refined austenitic grains which then reduced the hardness.

Passive film stability (EIS measurement)
EIS tests were run to look into the effects of CW and TA on the passive Cr-Mn ASS film's resistance.Figure 5 displays a Nyquist diagram for samples of SA, CW, and CW+TA.It is clear that the plots take the shape of an incomplete semicircle.The term 'unfinished capacitance arc' is frequently used to describe similar nyquist diagrams [25].This suggests that the resistance of the film will be better as the larger the semicircle's diameter.Additionally, there is evidence that film resistance is deteriorating with increased cold work.But after TA, passivity rises, suggesting that refining is crucial to improving the passivity of Cr-Mn ASS [26,27].The evolution of the film resistance is shown in figure 5 as a result of CW and TA.The diameter of the semicircle is found to decrease as the degree of CW rises.This is attributed to reduced impedence and increase in the defect density of the passive film.The resistivity of the film reduces with increasing CW, hence the film should have more defects.In comparison to SA and CW samples, which had impedence values close to 20000 Ω.cm 2 and 10000 Ω.cm 2 , respectively, after TA, the diameter of the semicircle increased above 40000 Ω.cm 2 .This indicates that the high passivity obtained after TA and lowest passivity observed for CW which is due to microstructural defect obtained through formation of SIM [11].

Potentiodyanamic analysis (PDP)
The PDP diagram of test steel samples in NaCl solution for SA, CW, and CW+TA is shown in figure 6. Evidently, the E corr switches to a more active (negative) potential as the degree of cold work is increased to CW, speeding up corrosion.This suggests that as the amount of cold work is increased, the steel's potential to corrode increases due to thermodynamics tendency.The curve in the cathodic portion of the plot indicates diffusion control of the cathodic reaction.These curves are typically attributed to the oxygen reduction process.The pH is 8 in NaCl solution and this value is too high for hydrogen reduction reactions to take place.In the anodic portion of the plots, 30% of the cold work sample exhibits active corrosion as compared to other circumstances, while the CW with TA exhibits somewhat higher passivating behaviour.It indicates that a passivating alloy has lost some of its passivity as a result of 30% cold work.With rising overpotential, the current density in the passive region rises.To produce E corr and I corr , Tafel extrapolation was used as the anodic and cathodic parts.Additionally, it produced cathodic (βc) and anodic tafel slopes (βa).After the passive range, the anodic portion of these graphs exhibit a sharp increase in anodic current density; the corresponding potential is regarded as the pitting potential (E pit ).All of the plots' anodic components are linear, which suggests that activation control is in control of the anodic reaction.The anodic portion of PDP plots indicates that the two primary anodic processes are film formation and metal dissolution [28,29].
Increasing CW increases I corr of SA sample suggests that the passivating film resistance is decreasing with increasing CW.However, the result of I corr for CW+TA indicates the development of passivation on the surface of Cr-Mn ASS.This finding is consistent with findings from Nyquist plot observations.It is attributed to martensitic reversal in CW+TA and rising SIM development in the material on CW.The increase in I corr can be attributed to the material's growing dislocation density because SIM has a dislocation density that is significantly higher than that of cold worked austenite [30].Additionally, it can be seen through EIS that defect density of the film to increase on CW and decrease on CW+TA through grain refinement.The cation migration from the M/F interface to the F/S interface is caused by defects in the film.Cation formation at the M/F interface, cation migration from M/F to F/S, and dissolving rate will all be higher the more defects there are in the film.This is what the current investigation has found.Thus, it can be said that cold work affects the dislocation density of the substrate as determined by SIM generation, which in turn affects the defect density of the passivating layer.As a result, the corrosion rate is higher in passive films with higher defect densities.The potentiodynamic polarisation results show that test steel exhibits passivity when it is solution annealed and 30% CW with TA.However, the test steel does not passivate when cold work is only 30%.This implies that the Cr-Mn ASS won't passivate when cold worked at 30%, leading to a higher rate of corrosion.This is something that practising engineers should keep in mind while fabricating with Cr-Mn ASS.

Wear behaviour
Under applied loads of 10 N, figure 7 shows the plot of coefficient of friction (COF) with wear time for the SA, CW, and CW+TA samples.It is noteworthy that the COF curves have two regimes, each of which corresponds to a different wear state.The portion of the system where COF fluctuates more, as seen in CW+TA and CW conditions, is the running-in period, and the portion where COF is stable, as seen in SA, is the steady-state [31,32].Because only the peaks of the irregularities in the interacting surfaces make touch with one another at the beginning of the slide, the actual contact area is minimal and the COF value is low.The actual contact area grows as the running-in stage advances, increasing COF [33].Along with elastic and plastic deformation, microstructural evolutions, temperature changes, and chemical reactions also take place on the contact surface during the running-in phase [34].When the ball and sample's contact surfaces are perfectly aligned, the steadystate is reached.A small portion of the wear process is caused by the running-in phase, therefore COF of the steady-state is typically used.Figure 7 shows that the SA Cr-Mn steel's COF value is marginally higher than that of the other two steels, with the CW+TA Cr-Mn steel's COF value being the lowest.Table 2 shows the weight loss for the wear test as a function of duration and load.Additionally, it shows that SA experiences greater weight loss during friction than CW and CW+TA due to the development of SIM after CW.Lower weight loss from CW+TA demonstrates how grain refining influence wear behaviour.It is widely known that a material's wear resistance is inversely correlated to its weight loss, meaning that the greater the weight loss, the lower the wear resistance.From this, it may be inferred that CW+TA had the highest wear resistance, followed by CW and then SA.It is seen that the weight loss lowers following the thermo-mechanical treatment (CW+TA) because the grain size has been finer.Also by restricting dislocation movement and hence preventing or limiting plastic deformation, the smaller grain size increases hardness [35].A material's toughness or resistance to impacts is also improved by its smaller grain size.Smaller granules and an orientation toward the direction of the applying stress or loading characterise coarse grain steels [36].As a result, under the impact abrasion condition, the grain bodies only produce a little amount of shear, while the grain boundaries mostly experience plastic deformation.As a result, the impact abrasion wear rate increases as fracturing begins early.However, for finer grains (higher number of grains), the pressure is distributed across the entire grain and the grain bodies also participate in the deformation, which lowers the impact abrasion wear rate.Therefore, it can be concluded that reducing the grain size will result in a noticeable increase in Cr-Mn steel's wear resistance.

Effect of SIM and Grain refinement on the corrosion and wear rate
The impact of CW and TA on the rate of corrosion and wear is depicted in figure 8.The graph shows that the creation of SIM causes the corrosion rate of CW to increase when wear rate decreases.However, due to grain refinement after TA, both the wear rate and the corrosion rate drop.It is well known that microstructure, wear and corrosion properties of austenitic steels depend on a heat treatment applied and plastic deformation.According to Ghayad et al [37], cold working causes SIM and deformation twins, which are generated during deformation, to accelerate the rate of corrosion.Due to the twins' representation of areas with different potential from the matrix, the corrosion current density increased.Additionally, they noticed that high-Mn steel corroded more severely when SIM fractions produced during ageing were present.The production of ε and α′ martensites might also be a result of the plastic deformation.According to reports [38,39], the corrosion process advances more quickly as martensite content rises.In this work, the steels undergo cold strain while maintaining their original austenitic microstructure.Due to the density of structural defects, cold-deformed steel specimens' reduced corrosion resistance must be attributed to other sources.The electrochemical results demonstrate that steel's corrosion resistance is significantly impacted by cold deformation in the 3.5% NaCl solution.The corrosion resistance of the cold deformed steel specimens is lower than that of the thermomechanically processed (CW+TA) specimens.After cold working, there are more twins and a higher dislocation density, which leads to SIM development (figures 1, 2, and 4).Pit nucleation is especially prone to occur at grain  boundaries [40].Additionally, because austenite and SIM have different potentials and hence produce local electrochemical corrosion cells, the high concentration of SIM seen in the microstructure of cold-deformed steel reduces its corrosion resistance [37,41].Grain size reduces after TA of the CW specimen, resulting in refined austenite with fewer defects and a slower rate of corrosion.The rate of corrosion decreases as grain size is reduced.The volume percentage of precipitated carbides per unit of grain boundary area influences the corrosion rate because inter-granular corrosion is brought on by the precipitation of carbides in grain boundaries.For a given C content, the degree of Cr depletion brought on by carbide precipitation will decrease as grain size is refined because of an increase in the grain border regions per unit volume.As a result, in finely grained materials, boundaries might not be sensitised [42].Moreover, the grain refinement improves the pitting corrosion resistance in both steels as opposed to reducing the overall corrosion resistance.In a few select locations, massive, deep individual pits form when coarsely grained steel begins to pit.In contrast, ultrafine grained steel pits in multiple locations, but only reveals small, discrete pits, which slows the process of corrosion.A decreased anodic current density can quantitatively explain this; in fact, the rise in pitting corrosion sites of the ultrafine grained steel causes a reduction in cathodic regions, which in turn causes a reduction in anodic current density.
It is well known that a material's hardness significantly affects its wear qualities [43].A material's capacity to harden is crucial since it affects the surface's hardness during wear [44].Figures 4 and 8 show that, under identical test conditions, the CW sample had a greater initial hardness and worn sub-surface hardness than the SA sample, demonstrating improved wear resistance.After TA, grain refining caused a decrease in wear rate [45].According to a report, following the wear tests, ε and α′ are produced at the subsurface of the CW+TA samples [46].Near the wear surface, martensite production is higher.However, we are unable to discriminate between the newly formed α′-martensite during wear in the CW+TA steels and the α′-martensite that was not reverted during the annealing process.Despite the CW+TA sample's greatly refined grain size, ε-martensite still forms following the wear test.Additionally, in micron-sized grains, ε-martensite is mostly found on the deformation bands; but, in nano-sized grains, there are no notable modifications.Under the influence of friction shear stress, the subsurface of steel experiences evident plastic deformation, refinement in grains, and SIM transition.The underlying deformed layer supports the hardened layer in this way.The CW+TA steel creates the continuous harden layer early because it has a good balance of hardness, strength, plasticity, and work hardening capacity.Additionally, the hardened layer can be effectively supported by the subsurface so that it can withstand damage during a wear test and does not prematurely delaminate or peel off.As a result of their superior work hardening properties, stainless steels with lower initial hardness exhibit greater wear resistance under high applied loads, according to Xu's research [47].

Conclusion
The corrosion and wear behaviour of the Cr-Mn austenitic structures produced by SIM reversion process was investigated and conclusion is mentioned below: • It was observed that the γ-phase is apparent across all peaks within the spectra of SA samples.With 30% cold work, prominent α′ martensite peaks on the sample as observed in the XRD analysis, accompanied by minimal ε-martensite peaks.Also, there was a decrease in martensite content on TA sample which is evident for martensite reversal.During CW, SIM formation takes place but when those CW samples TA at 900 °C for 4 h then those SIM reverted into refined austenite.
• In the microstructural examination, the solution-annealed sample shows equiaxed grains characterized by a step structure and well-defined grain boundaries.However, during cold work (CW), grain elongation occurs, accompanied by the emergence of deformation.
• The sample with cold work increases the hardness value as compared to the SA sample.Which is due to an increase in the SIM volume fraction.However, during the TA of the sample the martensitic reversal occur which reduces both hardness and SIM fraction drop.
• CW sample indicates more corrosion rate than SA sample due to formation of SIM and active sites which results in degradation of passivity.CW shows increase in the I corr value as compared to SA sample, the passivating film resistance is decreasing as the CW increases.However, the I corr result for CW+TA shows that passivation has developed on the surface of Cr-Mn ASS.This result is in line with observations made on the Nyquist plot.The rise in Icorr can be attributed to the material's increased dislocation density.
• CW sample shows less wear rate than SA samples due to increase in hardness which obtained because of formation of SIM.After TA, CW sample becomes more corrosion and wear resistance compare to SA and only CW, due to grain refinement which helps to maintain the passivity and also form the martensite during wear test which helps it to maintain the hard surface and increases the wear resistance.

Figure 2 .
Figure 2. Microstructure of (a) SA, (b) CW samples where blue arrow indicates α′ martensite and grey arrow indicates ε martensite, and (c) SEM Image of CW samples where red arrow indicates α′ martensite and blue arrow indicates ε martensite.

Figure 4 .
Figure 4. Graph showing effect of SA, CW and CW+TA on Hardness and SIM formation.

Figure 3 .
Figure 3. Microstructure of (a) SA, and (b) CW + TA samples with orange arrow indicates boundary attack and red area with arrow indicates grain refinement.

Figure 5 .
Figure 5. Nyquist plots of SA, CW and CW +TA samples of Cr-Mn ASS.

Figure 7 .
Figure 7. Coefficient of friction curves of SA, CW and CW+TA of Cr-Mn ASS.

Figure 8 .
Figure 8. Graph showing effect of SA, CW and CW+TA on Wear and Corrosion rate of Cr-Mn ASS.

Table 2 .
Weight Loss of samples during Wear Test.