Effect of Heat Treatment on Microstructure,
Mechanical Properties and Erosion Behaviour of Cast
21-4-N Nitronic Steel
Avnish Kumar1*, Gaurav Tripathi1,2, Anurag Hamilton1, Ashok Sharma1
1Department of Metallurgical and Materials Engineering, Malaviya National Institute of Technology Jaipur, Rajasthan 302017, India
2Department of Mechanical Engineering, Ajay Kumar Garg Engineering College Ghaziabad, Uttar Pradesh 201009, India
Avnish Kumar, Department of Metallurgical and Materials Engineering, Malaviya National Institute of Technology Jaipur,
Rajasthan 302017, India, Tel/ Fax: +91 141 2713343; E-mail: firstname.lastname@example.org
Received: May 02, 2016; Accepted: June 19, 2016; Published:July 30, 2016
Citation: Kumar A, Tripathi G, Hamilton A, Sharma A (2016) Effect of Heat Treatment on Microstructure, Mechanical Properties and Erosion Behaviour of Cast 21-4-N Nitronic Steel. SOJ Mater Sci Eng 4(2): 1-5.
Microstructure and mechanical properties are closely related to
the erosion behavior of the material. Modification of microstructure
and mechanical properties through heat treatment can significantly
enhance the erosion resistance of steel. Therefore, cast 21-4-N
nitronic steel was solution treated at two different heat treatment
cycle to observe the effect on microstructure, mechanical properties,
and erosion behavior. The microstructures, phases and eroded
surfaces were studied by optical microscopy, X-Ray Diffraction (XRD)
analysis and Scanning Electron Microscopy (SEM) respectively. It has
been observed that solution treatment leads to the dissolution of
carbides, which improved the tensile strength, ductility, and impact
energy with a minor reduction in hardness of cast 21-4-N nitronic
steel. It resulted in enhancement of the erosion resistance of cast 21-
4-N nitronic steel.
Keywords: Erosion; Heat Treatment; Nitronic Steel; Microstructure;
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 .
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 .
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.  observed that heat treatment increased
the cavitation resistance due to morphological change of carbide
in 316 and 304 steel. Salik and Buckley  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.
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
Table 1: Chemical composition of 21-4-N nitronic steel (wt. %).
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: HNO3). 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 V-notch 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
Figure 1: Schematic view of slurry pot tester.
Table 2: Parameters used in slurry erosion testing.
Specific gravity of sand particle
Sand concentration in water
15% sand in 17-liter water
Average size of sand particle
Figure 2: Morphology of silica sand particles used in slurry erosion test.
The eroded samples were examined under the scanning electron
microscope (FEI Netherlands, Quanta 2000).
Results and Discussions
Figure 3 shows the X-ray diffraction patterns of the cast and
solution treated 21-4-N nitronic steel at 1030οC and 1080οC 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 Cr7C3
and Cr23C6 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 1030οC and 1080οC 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 (1080οC) dissolved the most of the
carbides and resulted in an increase in volume fraction of the
austenite phase. Therefore, austenite peaks become sharper and
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 M7C3 (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 [3-5]. This type
of carbide forms a continuous network of lamellar morphology
. Higher magnification microstructure shows the lamellar
morphology of M7C3 carbide (Figure 4 (b)). M23C6 carbide was also
observed to precipitate at the grain boundary because it is the
most favorable place for such types of carbide.
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 Figure 6 (a, b). Solution
treated (1080οC/150min) 21-4-N nitronic steel exhibits a better
dispersion of carbides in the austenitic matrix than cast structure.
Table 3 shows the mechanical properties of cast and solution
treated 21-4-N nitronic steel at 1030οC and 1080οC 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
Figure 3: X-ray diffraction pattern of cast and solution treated 21-4-N
nitronic steel at (1030°C/150 min) and (1080°C/150 min).
Figure 4: Optical microstructure of cast 21-4-N nitronic steel: (a) lower
magnification; (b) higher magnification.
Figure 5: Optical microstructure of solution treated 21-4-N nitronic
steel at 1030°C for 150 min: (a) lower magnification; (b) higher magnification.
Figure 6: Optical microstructure of solution treated 21-4-N nitronic
steel at 1080°C for 150 min: (a) lower magnification; (b) higher magnification.
Table 3: Mechanical properties of cast and solution treated (ST) 21-4-N
nitronic steel at (1030°C/150 min) and (1080°C/150 min).
Impact energy (J)
Cast 21-4-N nitronic steel
ST (1030°C/150 min) 21-4-N nitronic steel
ST (1080°C/150 min) 21-4-N nitronic steel
austenite has the tendency of good ductility, strength and impact
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 7: Cumulative weight loss of cast and solution treated 21-4-N
nitronic steel at (1030°C/150 min) and (1080°C/150 min) with test
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 (1080οC/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  proposed that ductility of the steels have a significant
effect on erosion resistance, which increases with increasing
in ductility. Hutching  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
of cast 21-4-N nitronic steel.
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
(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 (1080οC/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
Figure 8: SEM micrographs of eroded surfaces of (a) cast 21-4-N nitronic
steel; (b) solution treated (1030°C/150 min); and (c) solution
treated (1080°C/150 min) 21-4-N nitronic steel after 24 h of erosion
The work is carried out under R&D project entitled “Study
of Metallurgical Aspects of Nitronic Steel for Underwater Part
Applications” sponsored by Central Power Research Institute
(CPRI), Bangalore (R&D management code no.: 12/RSOP-GHMNIT-
2011). Authors greatly acknowledge the funding and
cooperation received from CPRI, Bangalore (India).
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