**1. Introduction**

Almost all engine valves consist of bi-metallic welded materials, which are composed of hardened steel stem and a superalloy. Superalloys previously used for valve manufacturing include Inconel 751, Pyromet 31 and Nimonic 80A which are resistant to heat, oxidation/corrosion and wear at elevated temperatures [1]. Due to the higher cost and

**Citation:** Murali, A.P.; Ganesan, D.; Salunkhe, S.; Abouel Nasr, E.; Davim, J.P.; Hussein, H.M.A.

Characterization of Microstructure and High Temperature Compressive Strength of Austenitic Stainless Steel (21-4N) through Powder Metallurgy Route. *Crystals* **2022**, *12*, 923. https://doi.org/ 10.3390/cryst12070923

Academic Editor: Wojciech Polkowski

Received: 25 May 2022 Accepted: 23 June 2022 Published: 29 June 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

processing requirements of nickel-based superalloys, valve manufacturers initiated the development of an alternate material consisting of titanium and austenitic stainless steels (21-4N). Due to the low density of titanium alloys, it had a good strength to weight ratio when compared to existing materials consisting of nickel superalloys. However, titanium alloys had poor room temperature ductility, low toughness properties and a high cost of production [2]. An alternate suitable material to improve strength, resistance to creep, corrosion and oxidation resistance at elevated temperatures is 21-4N austenitic heat resistant stainless steel [3]. It consists of a high chromium and manganese content which improve the material properties and provide structural stability. The high pressure and high operating temperature of automotive valves ranges between 800–950 ◦C, where the instability in strength was noted [4].

Zhang et al. analyzed the failure of valve heads due to the non-uniform distribution of temperature in the valve region, which resulted in an increased stress concentration and led to a crack in valve heads [5]. Similarly, a review of the failure of the engine valve conducted by Naresh et al. suggested that the principal reason for the failure of the valve is excessive heating leading to the degradation of valve material, fretting, microstructural instability and impact load [6]. If the engine valve is deemed poor, the output power decreases or can even lead to seizure. Overall, analyses regarding automotive failure have been conducted predominantly in engines [7].

Several researchers carried out the failure analysis of engine valves and interpreted the cause for its failures. Yu et al. investigated the failure of an exhaust valve made of 5Cr21Mn9Ni4N steel. It was stated that the reason for the failure is the disintegration of the austenitic structure due to the depletion of chromium from the grain structure. This led to impaired mechanical properties and subsequently fatigue failure [8]. Kum et al. found that the thermal deformation of valve heads leads to valve fracture. Moreover, they found that fractures occurred as a result of non-uniform stress distribution near the valve region [9].

Overall, it can be seen that materials produced through the cast route showed inhomogeneous grain growth throughout the product due to the uneven solidification rate during the casting process. To overcome these defects, alloys were developed through directional solidification to uplift the mechanical properties along the longitudinal directions. This will eliminate the weakening of the grain boundary and enhance high temperature mechanical strength. One such method to obtain high strength with fine grain structure is the powder metallurgy route. Near net shapes are obtained using the powder metallurgy process which reduces secondary machining processes such as milling, turning and drilling [10,11]. A better surface finish, close tolerance and post or secondary processing are needed for many parts in the service industry. With the introduction of powder metallurgy components, market review shows that 60% of components will only require one machining processes [12]. Powder consolidation techniques using vacuum hot pressing will eliminate unwanted and undesirable reactions within metal matrices due to the lower processing temperature [13]. Great flexibility during material design and selection can be achieved using powder metallurgy technique. Due to these advantages, materials developed using the powder metallurgy route exhibit superior properties in comparison to their counterparts developed through traditional methods such as the melting and casting route [14–16]. Components produced through the powder metallurgy route present fine microstructure and a homogeneous distribution of particles [17]. To consolidate metal powder particles in order to attain superior strength, hot pressing under vacuum conditions was carried out. During this process, both temperature and pressure were applied to the powder compacts. Vacuum condition was used to provide control over the atmosphere to prevent oxidation at a high temperature. These powder particles consist of a single phase before consolidation. It is attained by the process of mechanical alloying (MA). Without melting, the solid solution can be attained by violently deforming the powder mixtures under controlled conditions. This non-equilibrium process works based on energizing and quenching, whereby the alloyed materials are brought into the metastable state with the application of pressure and temperature. Moreover, studies related to hot compression for 21-4N were carried out

previously using the casting route [18–20], whereas hot compression studies of 21-4N steel developed using the powder metallurgy route have yet to be reported.

The literature related to the comparison of mechanical and microstructural properties of similar materials with different processing routes are relatively scarce. Bartolomeu et al. studied the microstructure and mechanical properties of austenitic 316L stainless steel developed using both conventional casting and also the powder metallurgy route (hot pressing). It was reported that 316L stainless steel samples had a coarse grain structure in cast route whereas they possessed an equiaxed grain structure when developed using the hot pressing technique. A slight increase in hardness of 6% was reported for 316L developed through hot pressing. Yield and tensile strength increased by 33% and 50%, respectively, due to the equiaxed grain structure [14]. Garbacz et al. compared the mechanical and microstructural properties of platinum and rhodium (Pt-Rh) alloys manufactured using the isostatic pressing and casting route. When subjected to tensile testing, both the alloys exhibited an increase in tensile property by a factor of 2.5 when compared to alloys manufactured using the casting technique. The increase in tensile properties for Pt-Rh alloys developed using the powder metallurgy route can be attributed to grain size strengthening as a result of the fine grain size when compared with the coarser grain obtained with the casting route [15]. High entropy alloys consisting of Fe-Ni were developed by Larissa et al. using the cast and spark plasma sintering technique. Alloys developed using powder metallurgy possessed a nano-sized FCC metal matrix with a uniform and fine grained microstructure, whereas in contrast, cast alloys showed a coarse grained structure. Similarly, the mechanical properties were far superior when compared to its cast products [16]. Overall, the alloys developed through the powder metallurgy route showed improved mechanical properties when compared to cast products of a similar composition.

In the present work, we developed a suitable substitute for valve materials consisting of austenitic stainless steel (21-4N) using the powder metallurgy technique. It is very important that the microstructural stability of the material is evaluated using an optical micrograph (OM), scanning electron microscope (SEM) along with energy dispersive spectrum (EDS) and transmission electron microscopy (TEM) studies. The structural evolution of the powders was studied using SEM, and the X-ray diffraction (XRD) patterns were also evaluated for the hot pressed samples. The Vickers hardness (HV) for the consolidated samples was evaluated and compared with results obtained with various strengthening mechanisms.

#### **2. Experimental Procedures**

The chemical composition for the valve material of 21-4N austenitic stainless steel was developed with ferro-alloy powders of Fe-Ni, Fe-Mn and Fe-Cr which had a particle size of less than 25 μm. The desired chemical composition of 21-4N heat resistant steel is shown in Table 1.


**Table 1.** Individual elemental composition for 21-4N steel.

To convert the pre-alloyed mixtures into austenitic alloy, high energy planetary ball milling was used. MA was executed in a Fristch GmbH planetary ball mill with tungsten carbide balls and vials where the ball to powder ratio (BPR) was 10:1. Ball milling was carried out for 10 h at 300 rpm with toluene as the process control agent. The structural morphology of the milled powders at regular intervals were studied using SEM analysis. Milled powders were consolidated using vacuum hot pressing with a temperature, pressure and vacuum level of 1200 ◦C, 50 MPa and 10−<sup>3</sup> Torr, respectively, for a period of 2 h. A schematic representation is shown in Figure 1 for vacuum hot pressing. After vacuum hot

pressing, the dimensions of the consolidated sample obtained were 30 mm in diameter and 12 mm in height as shown in Figure 2.

**Figure 1.** Schematic representation of vacuum hot pressing.

**Figure 2.** Schematic representation of vacuum hot pressed sample.

Vacuum hot pressed samples were analyzed for densification studies, microstructural examination and phase determination. Metallographic techniques were used to prepare the sample with ferric chloride as the etchant to reveal the microstructure. The phase distribution, shape and morphology of both powder and vacuum hot pressed samples were studied using SEM-EDS. INCA Xsight JOEL–JEM–2100 was used to carry out the transmission electron microscopy (TEM) study operated at 200 kV. The preparation of samples for TEM studies was initially carried out with Gatan model 656 for dimpling the specimens, followed by ion milling with Gatan model 691. Measurement of hardness for the hot pressed samples was evaluated at room temperature in a Vickers micro hardness tester using a FIE Model OMEGA hardness tester (Deckenpfronn, Germany). Indentation was carried out using a diamond intender at a load of 1 kg for a dwell time of 15 s. The bottom of the test specimens were filed flat before each testing. From each of the three hot pressed samples, 10 indentations were carried out and each of the indents were spaced 3 mm from each other. Zwick/Roell Z100 was used to carry out compression testing to evaluate the compression strength of the hot pressed samples as per ASTM E209, with a strain rate of 0.001 s−<sup>1</sup> and at 650 ◦C. Specimens of cylindrical sizes with a height 10 mm and diameter 8 mm were subjected to uniaxial compression. During hot compression, the sintered samples were heated through resistance heating and the temperature measurements were conducted in thermocouples. To reduce the effect of friction during hot compression, the two ends of the sample were padded with a lubrication sheet consisting of graphite.

#### **3. Results and Discussion**
