Microstructure, macro-, and nano-hardness assessment of AISI 302 steel aged at 1000°C

. Changes in the microstructure of the AISI 302 steel must be analyzed in order to optimize its mechanical performance. In this paper, we studied the thermal aging behavior of the AISI 302 steel. The aging treatments of the steel were carried out at a fixed temperature of 1000°C and for durations between 0 and 6000 minutes. A metallographic microscope was used to see changes in the AISI 302 steel microstructure. In addition, the steel was mechanically characterized using the nano-indentation technique and macro-hardness tests. At the temperature investigated, the aging time increase caused a microstructure composed of large austenitic grains and a small area of grain boundaries per unit volume. For the mechanical characterization, the nano-indentation responses also showed a decrease in the nano-hardness and plasticity of the austenite as the aging time increased. The steel macro-hardness decreased significantly by incrementing the aging time.


Introduction
The most important group of stainless steels is that of austenitic stainless steels.Indeed, the Fe-18Cr-8Ni austenitic stainless steel remains the most widely used in the world.Moreover, stainless steels are among of the fastest-growing materials and their global production continues to expand in recent years [1,2].Due to their strength at high temperatures, great toughness, and formability, among other characteristics, they are utilized in a wide variety of applications [3,4].
At the level of scientific research, an attention has been paid to the assessment of the metallurgical behavior of the stainless steels, including their aging behavior [5][6][7][8][9].At hightemperatures and for prolonged holding times, several studies demonstrated that the microstructure of the austenitic stainless steels undergoes changes as the increase of the grain size and the decrease in the area of grain boundaries per unit volume [10][11][12][13].This phenomenon occurs by the grain boundaries migrating and the merging between the large and small austenitic grains [14,15].According to the findings of several research publications [16][17][18], the nano-hardness of the aged austenite decreased slightly at low and hightemperatures treatments and for different durations.However, as mentioned in the literature, the grain growth caused a reduction in the macro-mechanical properties of the stainless steels such as the ultimate tensile strength, yield strength, and macro-hardness [19][20][21].
The thermal aging behavior of the austenitic steels has been studied in several scientific papers, but limited works have reported the nanoindentation responses of the austenite aged at elevated temperatures.At a fixed temperature of 1000°C and in the time interval of 0 -6000 minutes, the aging behavior of the AISI 302 steel is investigated in order to optimize its mechanical properties.In this paper, therefore, we discussed the aging time effect on the microstructure variations of the steel under study.The links between the aging mechanism and microstructure variations of the steel were examined by the nano-indentation technique and macro-hardness test.

Chemical composition, and aging treatments
The nominal chemical composition (in weight percentage) of the AISI 302 steel is 18Cr, 0.15C, 8Ni, 0.4Si, 0.61Mn, 0.03P, and Fe in balance [3].Manufacturing processes of the AISI 302 steel were carried out industrially by a local plant (Casablanca, Morocco).The bars received of the AISI 302 steel were cut to form samples of 25 × 25 × 10 mm 3 in size.
In this study, the sample of the as-received AISI 302 steel referred to that aged for 0 minutes.The AISI 302 steel samples were thermally aged at 1000°C for the following times: 60, 600, 1000, and 6000 minutes.For each aging process, the steel samples were water-quenched to room temperature (~25°C).The heat treatments were performed using an air oven of SELECTA brand operating with a temperature control of ± 1°C and a heating rate of 20°C per minute.

Metallographic microscope
In order to remove the oxide layers formed during heat treatments and to achieve a mirror-polished surface, the AISI 302 steel samples were mechanically polished by abrasive papers (320, 600, 1200, and 4000 grits) and diamond pastes (1, 3, and 6 μm) suspended in distilled water.The solution used in chemical etching was aqua regia aqueous solution (mixture of 5 mL HNO3, 15 mL HCl, and 100 mL H2O) and the rinsing was done with bi-distilled water [22].Morphological changes in the AISI 302 steel samples were visualized using an OPTIKA metallographic microscope attached to AIPTEK high-resolution camera.

Nano-indentation technique
The nano-indentation experiments were carried out using Nano-indenter XP (CSM Instruments) tester fitted with Berkovich indenter of B-O 45 series (pyramidal diamond tip) [23].An OPTIKA metallographic microscope was used to perform the nano-indentations in the grain interior.
Figure 1 shows a typical load (P) versus depth (h) curve of nano-indentation test.Using the Oliver and Pharr method, the loading-unloading curves obtained were analysed and the steel nano-hardness values were calculated and reported [24,25].The parameters used in the calculation were illustrated in Figure 1.The contact depth (hc) can be calculated as follows: With Pmax is the peak load and equal to 50000 µN, hmax is the maximum penetration depth of the indenter reached after a dwell time of 5 seconds and at Pmax, the constant ɛ is equal to 0.75 for the Berkovich tip and S is the unloading curve tangent slope at the point of maximum load.
The contact projection area (Ac) can be determined as follows: Where C1 through C7 are constant.The first term describes a perfect pyramid-shaped tip and the other term describes deviations from perfect geometry due to the blunting tip.
The steel nano-hardness (HIT) is expressed as follows: 3) The nano-indentation curves obtained can be interpreted also based on the parameter hr which is the residual penetration depth of the indenter after releasing the load applied on the steel samples.As indicated in Figure 1, The hr value is the intersection of the unloading curve with the x-axis.

Macro-hardness test
The macro-hardness measurements of the steel under different aging conditions were effectuated by a durometer of TESTWELL brand and according to Rockwell C (HRC) method.In this method, a diamond indenter and a total load of 150 Kgf were used [26].At least, a total of 10 macro-indentations were made on the flat surface of each sample and the macro-hardness values average were reported.Figure 3 highlights the morphological characteristics of the AISI 302 steel aged for 0 and 6000 minutes.The M23C6 carbides were not detected by the metallographic examination due to their low fraction, as seen in Figure 1.The carbides M23C6 dissolution after high-temperature heat treatments can also explain their absence [27,28].A microstructure free of intragranular precipitates facilitates grain growth, as the carbides exert pinning forces on the grain boundaries [29][30][31].

Metallurgical study
A difference in the grain size and grain boundary area was observed between the steel aged for 0 minutes (Figure 3(a)), and 6000 minutes (Figure 3(b)).In these images, it was observed the increase in the austenite grain size by incrementing the aging time.As a result, a reduction in the area of grain boundaries per unit volume was also visible by increasing the aging time.Due to the grains merging and grain boundaries migrating phenomena, these microstructural changes take place [32,33].

Nano-indentation technique
Figure 4 shows the P-h curves of the austenite aged for different durations.As can be seen from the presented curves, the pop-in phenomenon [34][35][36] did not occur during the complete loading-unloading cycles.We also note that the unloading part of the different curves of the aged austenite was linear with a low slope.As shown in Figure 4, the P-h curves plotted showed a displacement to the right side with incrementing the thermal aging time.For times between 60 and 6000 minutes of aging, the displacement difference between the P-h curves is small.The difference between the nano-indentation curves obtained can also be discussed according to the parameters indicated in Figure 4 and which are hmax and hr [37].The hmax and hr values of the steel samples under conditions were reported in Table 1.By incrementing aging time, we can see that the hmax and hr values increased.The area beneath a P-h curve unloading part is known as the elastic deformation work (We) done by the indenter.The area between the loading-unloading curve and displacement axis corresponds to the plastic deformation work (Wp) done by the indenter.The sum of We and Wp is equal to the total work (Wt) done by the indenter [38,39].The We, Wp, and Wt values for the aged austenite are tabulated in Table 2.It was observed that by increasing the aging time, the We values declined and the Wp and Wt values increased.Based on the data in Tables 1 and 2, the increment of aging time softened and increased the austenite plasticity.Nano-hardness values of the aged austenite are summarized in Table 3.For the temperature and durations investigated, the steel nano-hardness decreased for aging times under 60 minutes.By contrast, the nano-hardness steel remained almost constant for times over 60 minutes.These results coincided with the findings of several papers dealing with the aging behavior of the austenite at temperatures below 800°C and for holding times up to 20000 h [16,[40][41][42].

Macro-hardness test
The macro-hardness measurements of the AISI 302 steel as a function of aging time are shown in Figure 5.At the macro-scale, the aging behavior of the steel exhibited two-softening stages; in the first stage, the macro-hardness decreased significantly in the time range of 0 -60 minutes.At this stage, the drop in the macro-hardness represented 85% of the AISI 302 steel total softening.In the second stage, the macro-hardness was characterized by a small decrease in the interval of 60 -6000 minutes.In stage II, the variation of the macro-hardness standard deviation observed was modest compared to that of stage I. Based on our previous work [16], the AISI 302 steel had almost the same mechanical behavior at 800°C and 1000°C.Nonetheless, the aging temperature parameter reduced this mechanical property; in fact, for the same duration range (between 0 and 6000 minutes), the steel macro-hardness decreased from 54.5 ± 0.6 to 44.25 ± 0.15 HRC and from 52 ± 0.55 to 41.25 ± 0.15 HRC at temperatures 800°C and 1000°C, respectively.
On the one hand, the significant loss of the steel macro-hardness can be explained mainly by macroscopic factors, such as the austenitic grains growth, very low fraction of precipitates, and the reduction in the area of grain boundaries per unit volume [13,20,43,44].These changes in microstructure reduce the dislocation motion barriers [45,46].On the other hand, E3S Web of Conferences 469, 00094 (2023) ICEGC'2023 https://doi.org/10.1051/e3sconf/202346900094 the two stages of the macro-hardness variation as a function of time are due to the decrease of the grain growth rate by incrementing the treatment time [14,15].

Conclusions
The present work explored the effects of the thermal aging time on the microstructure changes, nano-indentation responses, and macro-hardness variation of the AISI 302 steel.The conclusions were made as follows: 1.The increasing of the aging time changed the microstructural characteristics of the AISI 302 steel.In fact, the parameter of aging time led to an increase in the austenitic grains size and also a noticeable decrease in the grain boundaries area per unit volume.2. The incrementing of the aging time increased the austenite plasticity.The elastic work deformation carried out by the indenter was equal to 12454.95PJ for 0 minutes and it changed to 17287.02PJ after 6000 minutes.3.At the aging temperature, the nano-hardness of the AISI 302 steel decreased for times of less than 60 minutes; in fact, it was 3.69 ± 0.30 GPa for 0 minutes and it changed to 2.78 ± 0.30 GPa after 60 minutes.However, the nano-hardness of the AISI 302 steel remained nearly constant and it reached 2.41 ± 0.10 GPa for 6000 minutes.4. At the aging temperature, the macro-hardness of the AISI 302 steel reduced continuously by incrementing the aging time from 0 to 6000 minutes and its evolution as a function of the aging time consisted of two stages.In stage I, the AISI 302 steel macro-hardness decreased considerably from 52 ± 0.55 to 43 ± 0.8 HRC in the interval of 0 -60 minutes.
In stage II, the macro-hardness of the steel continued to decrease slightly from 43 ± 0.8 to 41.25 ± 0.15 HRC in the interval of 60 -6000 minutes.

Figure 2 Fig. 2 .
Figure2shows the thermodynamic computations of the AISI 302 steel using the JMatPro software (version 7.0) for the AISI 302 steel composition and the temperature range of 900 -1500°C.At 1000°C, the microstructure of the steel was biphasic composed of a fraction

Fig. 4 .
Fig. 4. Load versus depth curves of the AISI 302 steel aged at 1000°C up to 6000 minutes.

Fig. 5 .
Fig. 5. Aging curve of the AISI 302 steel aged at 1000°C up to 6000 minutes.

Table 1 .
Parameters (Maximum indenter penetration depth (hmax) and residual indenter penetration depth (hr)) determined from the load versus displacement curves of the AISI 302 steel aged at 1000°C up to 6000 minutes.

Table 2 .
The elastic deformation work, plastic deformation work, and total deformation work done by the Berkovich indenter at 1000°C up to 6000 minutes. https://doi.org/10.1051/e3sconf/202346900094

Table 3 .
Nano-hardness measurements of the austenite aged at 1000°C up to 6000 minutes.