Effect of Heat Treatment on Microstructure , Mechanical Properties and Erosion Behaviour of Cast 21-4N Nitronic Steel

Erosion in underwater parts of hydroturbine by hard silt particles becomes a serious issue in hydropower plants located in Himalayan region of India. Severe damages are reported in hydroturbine components due to silt erosion which leads to decrease in the efficiency of the power plant [1-3]. Therefore, it is necessary to pay special attention towards the development of better erosion resistant materials. Presently, 13/4 martensitic stainless steel (CA6NM) is being used as turbine blade material. However, this steel is severely affected by prolonged exposure to slurry erosion and also possesses poor weldability [4]. Extensive study has been carried out in the past to identify the factors responsible for severe erosive damages in hydroturbine components due to attack from silt-laden water. These factors can be broadly classified as impingement variables, erosive particle variables, and metallurgical variables. Due to practical limitations, it is possible only to regulate the metallurgical factors effectively. These metallurgical factors mainly include the microstructure and the mechanical properties of the material [2]. Nitronic steel is emerging as an alternative material to currently used martensitic stainless steel [3, 5-7] for hydroturbine applications. Nitronic steels are basically austenitic stainless steel containing nitrogen as an alloying element in the matrix. Nitrogen in austenitic stainless steels improves the strength, ductility, toughness, work hardening capacity, and resistance to erosion and corrosion [8, 9]. The initial structure of cast nitronic steels is usually a heterogeneous mixture of austenite and massive precipitates of carbides. Previous studies [10-12] showed that volume fraction, size, morphology, orientation and distribution of carbides affect the erosion resistance of material. Hard and disperse second phases in the microstructure can accelerate the hard particle release from the surface during the erosion and increase the total mass loss. Thus, appropriate solution treatment at high temperatures can play a major role in obtaining structures free from carbides for improving erosion resistance. Kim et al. [13] observed that heat treatment increased the cavitation resistance due to morphological change of carbide in 316 and 304 steel. Salik and Buckley [14] concluded that heat treatment and the resulting microstructure of plain carbon steel have an intense effect on the resistance to erosion.


Introduction
Erosion in underwater parts of hydroturbine by hard silt particles becomes a serious issue in hydropower plants located in Himalayan region of India.Severe damages are reported in hydroturbine components due to silt erosion which leads to decrease in the efficiency of the power plant [1][2][3].Therefore, it is necessary to pay special attention towards the development of better erosion resistant materials.Presently, 13/4 martensitic stainless steel (CA6NM) is being used as turbine blade material.However, this steel is severely affected by prolonged exposure to slurry erosion and also possesses poor weldability [4].
Extensive study has been carried out in the past to identify the factors responsible for severe erosive damages in hydroturbine components due to attack from silt-laden water.These factors can be broadly classified as impingement variables, erosive particle variables, and metallurgical variables.Due to practical limitations, it is possible only to regulate the metallurgical factors effectively.These metallurgical factors mainly include the microstructure and the mechanical properties of the material [2].
Nitronic steel is emerging as an alternative material to currently used martensitic stainless steel [3,[5][6][7] for hydroturbine applications.Nitronic steels are basically austenitic stainless steel containing nitrogen as an alloying element in the matrix.Nitrogen in austenitic stainless steels improves the strength, ductility, toughness, work hardening capacity, and resistance to erosion and corrosion [8,9].The initial structure of cast nitronic steels is usually a heterogeneous mixture of austenite and massive precipitates of carbides.Previous studies [10][11][12] showed that volume fraction, size, morphology, orientation and distribution of carbides affect the erosion resistance of material.Hard and disperse second phases in the microstructure can accelerate the hard particle release from the surface during the erosion and increase the total mass loss.Thus, appropriate solution treatment at high temperatures can play a major role in obtaining structures free from carbides for improving erosion resistance.Kim et al. [13] observed that heat treatment increased the cavitation resistance due to morphological change of carbide in 316 and 304 steel.Salik and Buckley [14] concluded that heat treatment and the resulting microstructure of plain carbon steel have an intense effect on the resistance to erosion.
In the present investigation, an attempt is made systematically to investigate the changes in the microstructure, mechanical properties and erosion behaviour of cast 21-4-N nitronic steel under different solution treatment condition.

Experimental Procedure
The cast 21-4-N nitronic steel was selected for present investigation.A rectangular bar of 40 mm x 40 mm crosssection was received from M/s Star Wire (India) Ltd., Ballabgarh (Haryana).The chemical composition of the 21-4-N nitronic steel is shown in Table 1.Specimens for metallographic examination, tensile, impact and erosion tests were prepared from this bar.In order to investigate the effect of heat treatment on microstructure, cubical specimens of 20 mm were solution treated at 1030°C and 1080°C for the holding time of 150 min.The specimens were then water quenched at room temperature to prevent the precipitation of carbides.After that, cast and solution treated specimens were ground and polished as per standard metallographic practice and etched with aquaragia (3:1-HCl: HNO 3 ).Etched samples were examined under Carl Zeiss Axiovert 200 MAT inverted optical microscope at different magnifications.The presence of phases in cast and solution treated samples was identified by X-ray diffractometer (XRD, D8-Bruker AXS, Germany with Cu Kα radiation).The Vickers hardness measurements were carried out using Matsuzawa hardness testing machine (Model No. DMN1).A load of 30 kgf was applied to the specimen for 10 sec for measuring the hardness.Tensile test measurements were carried using the Tinus Olsen tensometer testing machine.Standard tensile specimens with gauge length 20 mm and gauge diameter 4 mm were prepared as per ASTM E8M-08 to determine the values of Ultimate Tensile Strength (UTS) and ductility (% elongation).Standard Charpy impact specimens of dimension 10 mm × 10 mm × 55 mm were prepared as per ASTM E23-07 specification and the tests were performed at room temperature.The above results reflect the average value of five samples.Slurry erosion test was performed using slurry pot tester as shown in Figure 1.The erosion testing parameters are given in Table 2.The morphology of silica sand particle used in testing is shown in Figure 2. Two similar T headed specimens were inserted into the rectangular slots of the sample holding disc and tested for 24 hours.The weight loss of samples was measured after threehour interval in electronic balance having least count of 0.01 mg.
The eroded samples were examined under the scanning electron microscope (FEI Netherlands, Quanta 2000).

Effect of Heat Treatment on Microstructure, Mechanical Properties and Erosion Behaviour of Cast 21-4-N Nitronic Steel
Copyright: © 2016 Kumar et al.

Microstructure
The optical microstructure of cast 21-4-N nitronic steel is shown in Figure 4 (a, b).It possesses predominantly austenitic matrix (bright phase) along with the precipitates of carbides (dark phase).The M 7 C 3 (where M = Cr) carbide was found as the main precipitate in cast 21-4-N nitronic steel due to a higher concentration of N and C and also high C: Cr ratio [3][4][5].This type of carbide forms a continuous network of lamellar morphology [3].Higher magnification microstructure shows the lamellar morphology of M 7 C 3 carbide (Figure 4 (b)).M 23 C 6 carbide was also observed to precipitate at the grain boundary because it is the most favorable place for such types of carbide.The reduction in the volume fraction of carbide is attributed to diffusion of chromium and carbon elements into the matrix after gaining required energy.When the temperature was further increased to 1080°C dissolution of carbides in the austenitic matrix becomes more uniform.It can be clearly observed from the microstructures as shown in Figure 6 (a, b).Solution treated (1080 ο C/150min) 21-4-N nitronic steel exhibits a better dispersion of carbides in the austenitic matrix than cast structure.

Mechanical Properties
Table 3 shows the mechanical properties of cast and solution treated 21-4-N nitronic steel at 1030 ο C and 1080 ο C for 150 min.Cast 21-4-N nitronic steel exhibits higher hardness than both solution treated 21-4-N nitronic steel, which could be attributed to precipitation of chromium carbides.It is evident from tensile results that the Ultimate Tensile Strength (UTS) and ductility of solution annealed 21-4-N nitronic steel is better than that of cast 21-4-N nitronic steel.Impact strength was found to improve considerably after the solution treatment of steel.It is due to the dissolution of carbides, which enriched the austenite phase and    austenite has the tendency of good ductility, strength and impact energy.

Effect of Heat Treatment on Microstructure, Mechanical Properties and Erosion Behaviour of Cast 21-4-N Nitronic Steel
Copyright: © 2016 Kumar et al.
annealing heat treatment.The erosion resistance of solution treated 21-4-N nitronic steel at (1030°C /150min) is almost 20% higher than that of the cast condition.The erosion resistance of solution treated (1080 ο C/150min) 21-4-N nitronic steel was also found to be better about 34.8% than erosion resistance of cast steel.During the beginning stage of erosion, cast 21-4-N nitronic steel could provide the resistance to erosion to a certain time.It is due to the presence of hard carbide, which hinders the penetration of sharp-edged sand particle on the steel surface.Generally, hardness is considered the most important property that affects the erosion resistance of material [15,16].However, the presence of unevenly distributed carbides produce high-stress concentration at carbide-matrix interface, and continuous impingement of sharp-edged sand particle on the surface resulted in carbide cracking and de-cohesion at carbide/austenite interface, which leads to mass removal [3,7].The increase in hardness alone is not sufficient to improve the erosion resistance.Ductility and impact energy also play a major role in improving the erosion resistance.Poor ductility leads to a tendency for localization of strains and development of cracks.Foley [17] proposed that ductility of the steels have a significant effect on erosion resistance, which increases with increasing in ductility.Hutching [18] also suggested that high hardness with some amount of ductility offer better erosion resistance.The erosion resistance of solution treated 21-4-N nitronic steel was improved with an increase in ductility.During the erosion, the erodent particles impact on the target surface.Therefore, impact energy absorption should play a significant role in erosion resistance.In the present study, the erosion resistance of the solution treated 21-4-N nitronic steel was increased with increase in the impact energy.
From the above discussion, it can be inferred that optimum hardness and increased ductility and impact energy after solution treatment plays a crucial role in improving the erosion resistance

SEM Study of Eroded Samples
SEM micrographs of the eroded surface of cast and solution treated 21-4-N nitronic steel at (1030°C/150 min) and (1080°C/150 min) after 24 hours of erosion test are shown in Figure 8.It was observed that the craters formed during the erosion test were wider and deeper in the case of cast samples probably due to the removal of carbide particles.The plough was also seen in the eroded surface of cast 21-4-N nitronic steel.After the solution treatment of 21-4-N nitronic steel at 1030°C for 150 min, the craters and plough were formed smaller probably due to the dissolution of carbides resulting in enhancement of ductility and impact energy of steel.Eroded surface of solution treated (1080°C/150 min) 21-4-N nitronic steel shows that craters and plough were much smaller and less deep as compared to the cast and solution treated (1030°C/150 min) 21-4-N nitronic steel eroded sample.

Conclusions
(i) The XRD analysis exhibits that most of the carbides were dissolved after the solution treatment of samples.
(ii) Solution treatment of the cast 21-4-N nitronic steel shows the significant changes in microstructure.Solution treated (1080 ο C/150min) 21-4-N nitronic steel exhibits a better dispersion of carbides in the austenitic matrix.
(iii) Tensile strength, ductility and impact energy increased after solution treatment while hardness was decreased as compared to cast sample.
(iv) The better slurry erosion resistance of solution treated (1080°C/150min) could be attributed to the combination of optimum hardness coupled with high ductility and impact energy.

Figure 3
Figure3shows the X-ray diffraction patterns of the cast and solution treated 21-4-N nitronic steel at 1030 ο C and 1080 ο C for 150 min.The diffraction pattern of the cast 21-4-N nitronic steel showed austenite (γ) as major peak, which is overlapped with carbides peaks.It is confirmed from the JCPDF data that Cr 7 C 3 and Cr 23 C 6 carbides were the main precipitates.It is clearly visible from the XRD plot that the peak of carbides is much higher in cast condition than in solution treated condition.After the solution treatment at 1030 ο C and 1080 ο C for 150 min, carbides peaks drastically decrease due to the dissolution of carbides in austenite matrix.The relative intensity of the carbides peaks decreases as increasing the solution treatment temperature.Higher solution treatment temperature (1080 ο C) dissolved the most of the carbides and resulted in an increase in volume fraction of the austenite phase.Therefore, austenite peaks become sharper and more intense.

Figure 2 :
Figure 2: Morphology of silica sand particles used in slurry erosion test.

Figure 5 (
Figure 5 (a, b) shows the microstructure after solution treatment at 1030°C for 150 min.It was observed that volume fraction of the carbides decreases significantly as compared to cast structure due to its part dissolution into the matrix.The reduction in the volume fraction of carbide is attributed to diffusion of chromium and carbon elements into the matrix after gaining required energy.When the temperature was further increased to 1080°C dissolution of carbides in the austenitic matrix becomes more uniform.It can be clearly observed from the microstructures as shown in Figure6 (a, b).Solution treated (1080 ο C/150min) 21-4-N nitronic steel exhibits a better dispersion of carbides in the austenitic matrix than cast structure.

Figure 7 3 :
Figure 7 shows the cumulative weight loss of cast and solution treated 21-4-N nitronic steel at (1030°C/150 min) and (1080°C/150 min) for the total test duration of 24 hours.The experimental results indicate that the erosion resistance of 21-4-N nitronic steel has improved significantly after solution
Figure 1: Schematic view of slurry pot tester.

Table 2 :
Parameters used in slurry erosion testing.