Introduction

Nitrogen hardened 17Cr/4Ni (Nitronic-60) is austenitic stainless steel (AS) known for its exceptional wear and abrasion resistance. Marine shafts, automobile valves, and fasteners are some of the typical applications for this grade of steel [1,2,3]. The mixed oxides film of Cr2O3 and SiO2 forms on the surface of this steel with the nominal composition of Fe-17Cr-8Mn-8.5Ni-4Si-0.14N-0.1C, which is stabilized by the nitrogen dissolved in the matrix, impart good high-temperature corrosion resistance [4, 5]. Due to the presence of high ferrite stabilizers like Cr and Si in the austenitic matrix, delta ferrite can sometimes form in the single-phase austenitic structure of this steel. The delta ferrite precipitates can inhibit grain growth and thus improves the alloy's strength [6,7,8]. The nitrogen dissolved in the austenitic matrix also dramatically increases the strength. The production of brittle stable nitrides and intermetallic precipitates due to sluggish cooling of ingot casting severely restricts the scope of nitrogen's use for strengthening.

Powder metallurgy (PM) processing is one of the effective ways of overcoming the limitations associated with high nitrogen steels [6, 9]. In nitrogen-alloyed steel, the PM method proved favorable due to its homogenous distribution, increased nitrogen content due to the quick solidification of powder particles during atomization, and nitrogen content control. By using the PM method, the addition of nitrogen up to 0.6% demonstrating good strength and ductility has been shown in this steel [4, 10]. Components with complex shapes, difficult to manufacture by casting, and thermo-mechanical processes can be produced using the various near-net shaping techniques. The dispersion of nano-sized oxides in the austenite grains can further enhance the Nitronic-60 steel's high-temperature strength, hardness, and wear resistance. Several researchers established the approach's effectiveness in ferritic stainless steels [4, 5]. The nano-size oxide particles prevent grain coarsening and arrest the movement of dislocations at elevated temperatures by the Zener pinning effect, which eventually increases the creep resistance [6]. Because of the enhanced particle strengthening effects resulting from the Orowan mechanism, these nano-sized oxide dispersion strengthened (ODS) alloys have superior mechanical properties to micron-sized ODS alloys with same volume content. [7]. Investigation of the effects of oxide nanoparticles dispersion in the austenitic stainless steel, in particular, 17Cr/4Si steel, is not yet reported.

The effect of nanoscale oxides on the microstructure and mechanical characteristics of the austenitic stainless steel 17Cr/4Si were examined. Mechanical milling and vacuum hot pressing are used to consolidate the pre-alloyed powders with (MM-N60-Y) and without the Y2O3 addition (MM-N60) into bulk samples under the same conditions. Evaluations and comparisons were made between the samples' densification, microstructure, and mechanical characteristics and those made by directly pressing the pre-alloyed powder.

Experimental Procedure

Powdered 17Cr/4 Si (Nitronic-60) austenitic stainless steel was employed in this investigation, and its chemical composition is listed in Table 1. The pre-alloyed austenitic stainless steel powder (referred to as N60) (Sandvik Pvt. Ltd, Sweden) was mechanically milled in a planetary ball mill (Fristch Pulverisette 7, Germany) with and without adding 0.3 wt.% Y2O3 (Alfa Aesar, USA). The powder was milled at a ball-to-powder ratio of 10:1 in a pair of WC–Co vials at 300 rpm for 2–10 h in a toluene medium. Particle size and its distribution were analyzed by using the laser diffraction technique (Malvern Instruments Limited, UK). Further, the powder morphologies were also assessed with scanning electron microscopy (Hitachi, Tokyo, Japan). The mechanically milled N60 powders without Y2O3 addition (MM-N60) and with 0.3 wt.% Y2O3 addition (MM-N60-Y) and un-milled starting powder (N60) were consolidated by hot pressing at 1250 °C and under 56 MPa pressure in a 10–3 mbar vacuum atmosphere for 90 min. After hot pressing, the samples were cooled at 30 degrees per minute. Archimedes' method was applied to determine the density of the consolidated samples. The relative density (ρr) was calculated using the theoretical density (ρth) given by the supplier for N60 and MM-N60 samples. The MM-N60-Y sample was estimated from the density of N60 and Y2O3 using the simple rule of mixtures. An optical microscope and SEM combined with energy dispersive spectroscopy were used to analyze the microstructure of hot-pressed materials (EDS). Transmission electron microscopy (TEM—INCA Sight- JEOL, JEM-2100) at 200 kV is used to examine the MM-N60-Y sample's finer details. Ion milling was done after dimpling grinding (Gatan Model 656) to prepare the TEM samples (Precision ion polishing system-Gatan Model 691). After polishing the samples to a mirror finish, the hardness of the hot-pressed samples was assessed using a Vickers micro-hardness tester with a 1 kg load and a 15 s dwell period. The hardness values reported are the average of ten readings taken at different locations in each sample. The Hounsfield Tensometer (Kundale India Ltd, India) was used to test the tensile qualities of the hot-pressed samples in accordance with the ASTM-E8 standard.

Table 1 Chemical composition of the three austenitic stainless steel powder
Full size table

Results and Discussion

Powder Preparation and Consolidation

SEM pictures of N60, Y2O3 powders and mechanically milled N60-0.3Y2O3 (MM-N60-Y) for 5 h and 10 h are shown in Fig. 1a–d. The N60 powder was spherical with a mean particle size of 3 µm, and the Y2O3 powder was irregular in shape with a mean particle size of 1 µm. Upon mechanical milling, the fracturing and welding between the particles resulted in spherical and irregular-shaped particles, as shown in Fig. 1c and d after 5 and 10 h milling. The particle size of the 10 h milled N60 and N60-Y powders are 18 nm and 12 nm, respectively. The Debye-Scherer formula has been used to determine the crystallite size of the milled powder. The structural evolution during mechanical milling of the N60 and N60 mixed with 0.3 wt.% Y2O3 powder was analyzed using XRD. Since the XRD patterns of MM-N60 powder and MM-N60-Y are identical, the XRD pattern of MM-N60-Y has only been presented in this discussion. Since the nitronic 60 steel powder is a special alloy powder, unmilled powder has shown dual peaks of austenite and ferrite. Even though N60 is monophasic stainless steel, the presence of ferrite stabilizer silicon and chromium in the steel could stabilize the alpha phase along with gamma phases.

Fig. 1
figure 1

SEM morphology of (a) Nitronic-60 (N60); (b) yttrium oxide powder; (c) MM-N60-Y milled for 5 h and (d) for 10 h

Full size image

When the milling time increases, dual peaks are diminished, and a single austenitic peak has been formed. The XRD pattern shows only peaks corresponding to the austenite (γ) phase though the presence of traces of ferrite (α) cannot be ruled out. Due to their reduced presence in the present composition, the XRD patterns could not identify the Y2O3 diffraction peaks [10]. The substantial broadening of the dhkl (2θ = 43°) in the 10 h milled powder confirms that the crystallite size of the MM-N60-Y powder was reduced to 12 nm. Similarly, a crystallite size of 18 nm was exhibited for 10 h of milled MM-N60 powder.

Table 2 gives the relative density of the hot press consolidated N60, 10 h mechanically milled N60 (MM-N60), and N60 mixed with 0.3 wt.% Y2O3 (MM-N60-Y) samples.

Table 2 Relative densities of the hot pressed N60, MM-N60, and MM-N60-Y compacts
Full size table

All the samples show density > 95%, indicating they reached the final sintering stage, having only isolated pores. The relative density differences between the samples are less than 2%. Compact prepared from MM-N60 has exhibited the highest density among them. Both the milled powders MM-N60 and MM-N60-Y showed relatively higher densities than unmilled base powder (N60), which is expected as the milled powders likely have more structural defects, and favor faster diffusion of atoms during hot pressing. Moreover, the finer crystallite and particle size in the milled powder, which will result in a very high volume of grain boundaries and specific surface area, can also increase the driving force for densification. The addition of Y2O3 tends to reduce the sintering kinetics marginally (Fig. 2).

Fig. 2
figure 2

XRD patterns of mechanically alloyed MM-N60-Y powder

Full size image

Microstructural Examination of Consolidated Samples

The optical micrographs of the hot-pressed N60, MM-N60, and MM-N60-Y samples are shown in Fig. 3. The microstructure of the N60 sample shows large equiaxed austenitic grains with an average size of 18 µm. However, in MM-N60 and MM-N60-Y samples, the austenite grain sizes are fine and not visible in a low magnification optical microscope. The 10 h ball milling carried out in the MM-N60 and MM-N60-Y powder substantially reduces the grain sizes of both AS powder and Y2O3. Apart from improving the densification, as explained in the previous section, such fine structures resulted in very fine austenitic grains in the consolidated samples. The ASTM (Jefrrie's Planimetric) method was used to measure the grain size of the hot-pressed AS samples, and the results are shown in Table 3. The grain sizes were measured from SEM micrographs taken at 2000 X magnification. The samples from mechanically milled MM-N60 and MM-N60-Y powder showed grain sizes one order lesser than the unmilled N60 sample. MM-N60-Y power has an average grain size of 2.8 µm, which is 1.43 times more refined than the MM-N60 powder (average grain size of 4 µm), suggesting that the Y2O3 in the milled powder act as a grain growth inhibitor during densification.

Fig. 3
figure 3

Microstructure of (a) N60 (b) MM-N60 and (c) MM-N60-Y AS hot-pressed compacts

Full size image
Table 3 Hot-pressed AS compacts' ASTM grain size and average grain size
Full size table

Figure 4 shows SEM and EDS investigation of the hot pressed MM-N60 sample on grain and at the austenitic grain boundary. Figure 4 shows that the grain mainly comprises austenitic stabilizing elements like nickel and manganese. This shows that the existing phase is austenite. Similarly, EDS analysis at the interfaces of austenitic grain (Fig. 4b) consists of higher chromium content and lower nickel content relatively within the grain structure, as is evident from Table 3. However, chromium concentration at the interface has not exceeded the critical limit, which means that small dark precipitates observed at the interfaces are delta ferrite. However, a good amount of Fe (approximately 63 percent) content was also observed at the interfaces, confirming ferrite stabilization.

Fig. 4
figure 4

SEM EDS analysis of MM N60 sample (a) on austenite grain and (b) along the grain boundary

Full size image

Many researchers reported that delta ferrite and chromium-rich precipitate formation depends on the cooling rate after the heat treated above 1000 °C. Nitronic-60 steel, heat treated at 1020 °C followed by water quenching, showed delta ferrite and austenite. However, the formation of chromium-rich precipitates was observed in the steel annealed between 400 and 850 °C [11,12,13]. The delta ferrite, sigma phase, austenitic phase, and amorphous silica were formed in the stainless steel heat treated in the CO/H2 mixture environment at 650 °C [14]. The stainless steel was processed at a temperature of 1250 °C with a cooling rate of 25–30 °C per minute, which is significantly over the critical cooling rate for the development of chromium-rich precipitates in powder metallurgy-prepared stainless steel [15, 16].

Table 4 shows the elemental composition of the hot pressed MM-N60 billets as obtained from the EDS analysis. From Table 2, it is clear that the grain and grain boundary contain appropriate composition of austenite and ferrite. On the other hand, the grains were found to contain an optimum content of chromium, nickel and manganese, which stabilized the austenitic phase.

Table 4 Chemical examination of hot-pressed stainless steel billet grains and boundaries
Full size table

Figure 5a, b shows the TEM micrograph of the hot pressed MM-N60-Y sample. The matrix consists of austenite grain, and fine dispersoids are seen in the micrograph. Oxide dispersed particle size, number density, uniform distribution, and structure are crucial to mechanical characteristics [17]. The average dispersoid size of approximately 30 nm is uniformly distributed along the austenitic matrix. Recent research shows that the fine spherical dispersoids are pinning the slipping dislocations, which arrest the dislocation movement [18] to improve mechanical properties. EDS analysis of fine dispersoids along with the matrix elements is shown in Fig. 5c.

Fig. 5
figure 5

TEM Micrograph of MM- N60-Y (a) austenite grain, (b) fine dispersoids along the Austenitic matrix and (c) EDS analysis of MM-N60-Y

Full size image

Mechanical Properties of Hot Pressed Samples

Microhardness, percent elongation, yield strength, and ultimate tensile strength of hot-pressed N60, MM-N60, and MM-N60-Y are shown in Figs. 6 and 7. Mechanically milled samples have higher hardness, yield strength, and tensile strength than the basic alloy. The Y2O3 dispersed MM-N60-Y sample showed the highest hardness and yield strength. The effect of the difference in densities in the samples over their mechanical properties is likely marginal. As mentioned earlier, the difference is less than 2%. The ultra-fine grain size of the austenitic phase was principally responsible for the greater hardness and strength in MM samples. The Y2O3 nano dispersoid in the austenitic grains also benefits the steel by improving the hardness and strength. This is demonstrated by the strength determined from the austenite grain size using the Hall–Petch relation: σy = σo + kd−1/2 where σy is yield strength in MPa, σo is intrinsic friction stress (Peierls stress) in MPa, k is a constant (Hall–Petch coefficient, MPa/m1/2) that represents the inhibition of the grain boundary to deformation, and d is the grain size [19]. The yield strength calculated for the MM-N60 sample of 4 µm grain size is 584 MPa which is around 12% lesser than the 666 MPa measured in the MM-N60-Y sample. Hence, the higher strength of MM-N60-Y is due to the contribution of fine austenitic grains and the nano Y2O3 dispersoid. The percentage elongation indicates the deformability of the alloy affected by the Y2O3 dispersion. Fracture morphologies of N60, MM-N60, and MM-N60-Y of the steels are shown in Fig. 8a, b, and c, respectively. Figure 8a is dimples of varying sizes from larger to smaller, in contradictory mechanically milled yttria added to steel (MM-N60-Y) very fine dimples along with micro-crack and micro-pores. Hence, MM-N60-Y steel has undergone the least ductile failure mode compared with the other two stainless steels. From these results, it is clear that yttria-added steel (MM-N60-Y) has high strength and less ductile nature due to the austenitic grain refinement along with second phase oxide dispersion strengthened particles [20, 21]. However, mechanically milled MM-N60 steel has a finer size of dimples compared with unmilled N60 steel. These failure modes of N60 and MM-N60 samples have undergone completely ductile fracture, which is evident from the percent elongation.

Fig. 6
figure 6

(a) Micro-hardness, (b) Elongation of hot pressed N60, MM-N60 and MM-N60-Ysteels

Full size image
Fig. 7
figure 7

Strength of hot pressed N60, MM-N60 and MM-N60-Y AS

Full size image
Fig. 8
figure 8

Fracture morphologies of (a) N60, (b) MM-N60 and (c) MM-N60-Y steels

Full size image

Conclusions

Yttria-added oxide dispersion strengthened NITRONIC-60 steel has been developed through mechanical milling followed by the hot pressing method.

The following results are

  • Mechanically milling of MM-N60 powder resulted in significant grain size reduction in the consolidated samples. The addition of Y2O3 (MM-N60-Y) also inhibits austenite grain growth by Zener pinning.

  • Y2O3 addition and mechanical milling improve the hardness, yield strength, and tensile strength of the N60 steel. The fine uniformly dispersed nano-sized Y2O3 in the equiaxed austenite grain resulted in maximum tensile strength of 758 MPa and a hardness of 484 VHN in this steel.