Cavitation Erosion and Solid Particle Erosion Behaviour of a Nitrogen Alloyed Austenitic Stainless Steel

Abstract

The effect of grain size on solid particle erosion and cavitation erosion of a nitrogen alloyed austenitic stainless steel has been investigated. Heat treatment of the steel at elevated temperatures results in an increase in grain size and thus modification in mechanical properties. Particle erosion tests were performed using an air jet erosion tester. An ultrasonic processor with a stationary specimen was used to investigate the cavitation erosion performance. The cavitation erosion rates were found to increase with the increase in grain size. The particle erosion rate shows no significant change with increase in grain size. The worn surfaces were examined to study the characteristic damage features using scanning electron microscope (SEM). The nitrogen alloyed austenitic stainless steel exhibited superior resistance to cavitation erosion and particle erosion than a 316L stainless steel. The hardness, yield strength and ultimate tensile strength of the steels are related with the erosion resistance.

1. Introduction

Erosion of underwater parts is a serious problem encountered in hydro power plants.¹⁾ Similar problems are also encountered in hydraulic components, mining industries, dredging work, and waste disposals.²⁾ Underwater components of hydro turbines operating in silt (abrasive particles) laden water suffer from extensive erosion. Due to high amount of silt damage it becomes impossible to run the unit for longer time thus incurring heavy loss of power generation as well as replacement/repair cost.³⁾ Currently 13Cr-4Ni martensitic stainless steel is used in underwater parts in hydroelectric projects. It has limited wear life and frequent repair/maintenance is needed. Further, the repair is handicapped by poor weldability of the steel.⁴⁾ Austenitic stainless steels such as 304 and 316 have been in use in hydraulic machinery because of` their resistance to cavitation erosion.^5,6,7) These austenitic stainless steels exhibit better erosion resistance compared to the 13/4 martensitic stainless steel. Cr–Mn–N stainless steels have potential to substitute these Cr–Ni–Mo stainless steels because of their lower cost and higher cavitation erosion resistance.^1,8)

Damage concerning water turbines is caused mainly by cavitations problem, silt erosion and material defect. In presence of silt erosion, cavitation enhances the erosion damages. Erosion resistance increases with the increasing hardness at constant ductility.⁹⁾ When water bubbles collapse on a surface, the liquid adjacent to the bubbles is at first accelerated and then suddenly decelerated as it collides with the surface. The collision between liquid and solid generates large stresses which can damage the solid. Transient pressures as high as 1.5 GPa are possible.¹⁰⁾ This can cause cavitation erosion. The solid particle erosion involves impact of eroding particles on a surface. The process depends on the particle kinetic energy, impact angle, the particle shape, properties of target-material, and environmental conditions.¹¹⁾ At lower impact angles the mode of erosion is cutting and ploughing deformation. During ploughing lip formation occurs due to deformation in the localized regions near the surface of the target and lip is taken out either by inertial-stress-induced tensile fracture or by separation across adiabatic shear bands formed at the base of the lip. At high impact angles strain hardening and flaking/pitting of the surface takes place.¹²⁾

Traditionally Ni is the most common addition in austenitic stainless steels but it is very expensive. Economically viable austenite stabilizers are carbon, copper, manganese and nitrogen. Among these, carbon has a tendency to sensitize the steel, copper leads to hot shortness and manganese alone cannot totally replace Ni to form austenite phase.¹³⁾ Nitrogen-alloyed austenitic stainless steel exhibit attractive properties such as good corrosion resistance, high strength and reduced tendency to grain boundary sensitization.^14,15) Nitrogen addition causes more effective solid-solution strengthening than carbon and also increases grain size (Hall-Petch) strengthening. Nitrogen reduces the tendency to form ferrite and deformation-induced α’ and ε martensites. Nitrogen has greater solid-solubility than carbon, thus decreasing the tendency for precipitation at a given level of strengthening.¹⁶⁾

Austenitic stainless steels also exhibit resistance to solid particle erosion.¹⁷⁾ Similar surface morphologies for low and high impact angles could be observed when 304 and 316 stainless steel was subjected to the impact of sharp particles.^18,19) Lopez et al. reported 0.55% addition of nitrogen to 304L steel improves erosion resistance.²⁰⁾ The grain size and chemical composition of nitrogen alloyed steels is reported to influence mechanical properties, corrosion resistance and wear resistance.²¹⁾ In the present work, the grain size of the nitrogen containing austenitic steel is varied and its effect on the solid particle erosion and cavitation erosion behavior is studied. The erosion resistance of nitrogen steel is compared with that of 316L stainless steel.

2. Experimental

The material used in this study is a nitrogen-alloyed austenitic stainless steel. It was supplied by M/S Star Wire (India) Ltd., Ballabhgarh, Haryana, India. The steel was received in hot rolled condition. The chemical composition of as-received (AR) steel is reported in Table 1. The as-received steel was solutionized by heat treating at 1000°C (HT-1) and 1150°C (HT-2) for 3 hr followed by water quenching. For comparison a 316L stainless steel was also included in the present study. The steel samples were mechanically polished using diamond suspension. A surface roughness factor (Ra), 0.01 μm was obtained after polishing. The polished surfaces were subsequently etched using aquaregia (3HCl:1HNO₃) for observation in a Leica DMI5000M optical microscope. The grain size of the specimens was measured using the line intercept method in the optical microscope. Polished as well as etched samples were subjected to microstructural characterization using a Zeiss EVO18 scanning electron microscope (SEM) equipped with energy dispersive analysis (EDS). X-ray diffraction studies were carried out on polished samples in a Bruker AXS, D8 Advance diffractometer using a CuK_α radiation for phase identification. Hardness measurements were carried out on polished samples using a Vickers hardness testing machine at a load 10 kg for 15 s. Tensile tests were carried out as per ASTM E8M-09 ASTM²²⁾ standard at a strain rate of 10⁻³ s⁻¹ using a computer controlled Hounsfield H25K materials testing machine. Tensile toughness was determined by calculating the area below the engineering stress-strain curve between the YS and fracture stress. Tensile toughness measures the capacity of material to absorb energy in the plastic range. Charpy impact tests were carried out as per, ASTM E-23-07²³⁾ standard.

Table 1. Chemical composition of AR steels (wt%).

Steel	C	N	Cr	Ni	Mn	Si	Mo	Fe
AR	0.195	0.20	20.92	10.65	1.15	1.06	0.172	Bal.

The cavitation erosion (CE) tests for the steel samples were conducted using the vibratory test method, as per ASTM standard G-32-10.²⁴⁾ A piezoelectric ultrasonic transducer was used to produce oscillations at a frequency of 20±0.5 kHz in distilled water. Water temperature was maintained in the range of 25±2°C. The samples were placed in distilled water next to the transducer. A distance of 0.5 mm was maintained between the tip of the ultrasonic probe (horn) and the specimen during the test. The processor was stopped after every 3 h cycle. The specimen was cleaned by acetone using ultrasonic cleaner and subsequently weighed with an accuracy of 10⁻⁴ g. After weighing, surface roughness was measured by Mitutoyo SJ-400 surface profilometer. The cavitation erosion test cycle was repeated after every three hr up to 24 hr. Primary result of CE test was cumulative weight loss which was further converted into mean depth of erosion (MDE) using the following formula:   

$$\mathrm{M}\mathrm{D}\mathrm{E}\mathrm{\hspace{0.17em} }(\mathit{\mu }\mathrm{m})=\frac{10\hspace{0.17em}X\hspace{0.17em}\mathrm{\Delta }W}{\rho \hspace{0.17em}X\hspace{0.17em}A}$$(1)

Where, ΔW (mg) is the weight loss, ρ (g/cm³) is the density of steel and A (cm²) is the eroded area.

The solid particle erosion (SPE) was measured using an air jet erosion tester, as per ASTM G76-10 standard.²⁵⁾ A nozzle diameter of 3 mm was used. The test was performed using alumina erodent powders having particle size range between 53 and 75 μm. The parameters used in testing are shown on Table 2. Polished samples were ultrasonically cleaned in acetone, dried and weighed to an accuracy of 10⁻⁴ g using an electronic balance. Erosion test was conducted for 3 min and samples were then weighed again to determine the weight loss. The respective volume loss was calculated for 3 min erosion testing. The ratio of this volume loss to the weight of the eroding particles (i.e. testing time × particle feed rate) causing the loss was then computed as erosion rate in mm³/g. Erosion testing was repeated with each subsequent test of 3 min duration for nine cycles when a steady erosion rate was obtained. The experiments were conducted at 30°, 60° and 90° impingement angles at ambient temperature for all the samples. The worn surface morphology after cavitation as well as SPE tests was studied using SEM.

Table 2. Erosion parameters used for particle erosion testing.

Erodent Particle	Feed Rate	Pressure	Particle Velocity	Angle	Particle Size	Working Distance	Time of Testing
Alumina	3±0.3 g/min	0.5 kg/cm2	41 m/sec	30°, 60° and 90°	53–75 μm	10 mm	27 min (3 min cycle)

3. Results and Discussion

X-ray diffraction results confirmed the steel to be austenitic. Figures 1(a), 1(b) and 1(c) show the microstructures of AR, HT-1 and HT-2heat treated steels, respectively. AR steel exhibits equiaxed grains. Some fine (<1 μm) precipitates are observed along the grain boundaries. EDS confirmed these precipitates to be carbides (Fig. 1(d)). These are expected to be M₇C₃ and M₂₃C₆ types of carbides.²⁶⁾ The volume fractions of these carbides are very small (~3% as determined by Thermo-calc software) while no nitride precipitation is expected. Annealing twins are observed in some grains. The solutionising treatment also resulted in grain coarsening. The resulting average grain size is 15 μm (fine grain AR), 25 μm (medium grain HT-1) and 65 μm (coarse grain HT-2), respectively.

Fig. 1.

Secondary electron SEM micrographs (a to c) showing Microstructure of (a) AR, (b) HT-1, (c) HT-2 steels. EDS spectrum (d) of grain boundary precipitates of chromium carbide in AR steel.

Table 3 shows the mechanical properties of AR, HT-1 and HT-2 steel. Figure 2 Engineering stress strain curves of the same alloys. The heat treatment results in reduction in hardness, yield strength (YS), ultimate tensile strength (UTS) and strain hardening exponent. This is mainly due to increase in the grain size. After the heat treatment, the impact strength, tensile toughness and ductility show marginal improvement. The mechanical properties of 316L stainless steel were also determined and are listed in Table 3 for comparison.

Table 3. Mechanical properties of the AR, HT-1, HT-2 and 316L steel.

Properties of steel	AR	HT-1	HT-2	316L
Hardness (VHN)	250	235	218	215
Yield Strength (MPa)	480	445	382	270
Ultimate Tensile Strength (MPa)	763	732	716	608
Ductility (% Elongation)	45.3	46.5	50.5	50
Impact energy (J)	217	266	299	–
Tensile toughness MJ/m3	281	288	291	240
Strain Hardening exponent (n)	0.52	0.47	0.46	0.36

Fig. 2.

Engineering stress strain curves for AR, HT-1 and HT-2 steel. The tensile curve obtained for 316L steel is also included for comparison.

The cumulative weight loss (CWL) due to cavitation erosion is plotted as a function of time in Fig. 3(a), while Fig. 3(b) shows mean depth of erosion (MDE) versus time of erosion. Both CWL and MDE consistently increase with time for the AR steel. The erosion rate increases gradually up to 6 hr and then increases sharply for the HT-1 and HT-2 steel. The AR steel which had the finest grain size exhibited the best erosion resistance. Thus the erosion rate seems to increase with increase in grain size. Bregliozzi et al.²¹⁾ have reported that fine grains give rise to increase in the surface to volume ratio of grain boundary, which provides a dominant supporting action against cavitation. This was attributed to a restriction in the movement of the dislocation at the grain boundaries, which act as slip barriers. Coarse grains provide less grain boundary area for impeding dislocation movement. Thus coarse grain size after the heat treatment may cause an increase in the erosion rate. The surface roughness R_a values after cavitation erosion are shown in Fig. 3(c). R_a value of HT-1 and HT-2 steel increased sharply after 6 hr. The erosion rate decreased after 15 hr (Fig. 3(b)). R_a value of AR steel increased at a lower rate. The cavitation erosion behaviour for the nitrogen steel is better than that observed for the 316L stainless steel (Fig. 3).

Fig. 3.

Plot of cavitaion erosion of as received, HT-1, HT-2 and 316L steels (a) cumulative weight loss (CWL), (b) mean depth of erosion (MDE) and (c) surface roughness R_a as function of time.

Figures 4(a)–4(e) shows SEM images of eroded surfaces after cavitation testing. Figure 4(a) shows that damage mainly occurred at the austenite grain boundaries. The carbides are located at the grain boundaries and the interface regions between carbide and austenite serve as high stress regions.²⁷⁾ These regions served as the preferential sites for erosion. Preferential attack on the weakest phase of material is a characteristic of cavitation erosion.^28,29) A few holes are seen in the carbides region after erosion for 12 hr (Fig 4(b)). This is probably due to brittleness associated with the carbides. Thus, the carbide-austenite interfaces are largely attacked in comparison to austenite grains. After 24 hr of cavitation erosion, wide and deep cavities are formed due to removal of carbides and matrix (Figs. 4(c) and 4(f)).

Fig. 4.

Secondary electron SEM micrographs of AR steel (a) 6 hr, (b) 12 hr, (c) 24 hr), HT-1 steel (d) 6 hr, (e) 12 hr, (f) 24 hr and HT-2 steel (g) 6 hr, (h) 12 hr, (i) 24 hr after cavitation erosion.

Figure 5 shows the morphology of coarse grain heat treated steel after 12 hr of cavitation erosion. As can be seen in Fig. 5(a) cavitation testing leads to the formation of slip lines within the individual grains of the austenite that end at the boundaries. The formation of waviness on the surface corresponding to the presence of grain boundaries and slip bands can be observed (Fig. 5(a)). At longer times of exposure to cavitation, the number of slip lines increases. The slip lines and grain boundaries act as preferential sites of stress concentration. The continual impact of micro-jets produces these defects and later, material removal starts from these sites. Similar mechanism of material removal has been reported for other austenitic steels.²¹⁾ Figure 5(b) shows material removal from annealing twins in the same sample.

Fig. 5.

(a) and (b) Secondary electron SEM micrographs of heat treated (HT-2) steel after cavitation erosion for 12 hr.

Figure 6 XRD trace of relative intensity vs 2θ showing the absence of strain induced martensite in as received nitrogen steel samples subjected to cavitation erosion and solid particle erosion.

Fig. 6.

XRD trace of relative intensity vs 2θ showing the absence of strain induced martensite in samples subjected to cavitation erosion and solid particle erosion.

The XRD results show that strain induced martensite does not form after cavitation erosion. This may be because addition of nitrogen may prevent formation of strain induced martensite.²¹⁾ Similar results were obtained after particle erosion. The role of strain induced martensite may be different in these two modes of erosion. For cavitation erosion the formation of strain induced martensite has been reported to be detrimental.⁸⁾ On the other hand it is reported that strain induced martensite transformation may improve solid particle erosion resistance by absorbing impact energy.³⁰⁾

Figures 7(a) and 7(b) show the cumulative weight loss (CWL) curves in AR and heat treated steels at the impact angles of 30° and 90°. At both the angles, CWL in heat treated steels is almost same as that in the AR steel. Figure 7(c) gives the steady state of erosion rate at various angles (30°, 60° and 90°). At all impact angles, erosion rate in heat treated steel is slightly more than that of AR steel. Particle erosion resistance of the nitrogen steel was found to be better than that of 316L stainless steel (Fig. 7). Erosion behaviour is affected by the impact angle with erosion rate being higher at 90°. In general, the maximum erosion rate appears at a low angle (30°) for ductile materials and at a high angle (90°) for brittle materials.³¹⁾ The higher erosion rate observed at 30° in AR as well as heat treated steels in the present study indicates ductile nature.³²⁾ Figure 7(d) shows SEM micrograph of alumina particles used for solid particle erosion.

Fig. 7.

Cumulative weight loss (CWL) at (a) 30°, (b) 90°, (c) steady state erosion rate of AR, HT-1, HT-2 and 316L steels and (d) secondary electron SEM micrograph of alumina particles.

Figure 7 Cumulative weight loss (CWL) at (a) 30°, (b) 90°, (c) steady state erosion rate of AR, HT-1, HT-2 and 316L steels and (d) secondary electron SEM micrograph of alumina particles.

The depth of the wear scars after erosion test at 30° and 90° is shown in Table 4. The depth of erosion is more for samples tested at 90°. This is because the area of impact reduces with increase in impact angle from 30° to 90°. Thus variation in depth of wear is reverse of that for data for material removal (Fig. 7). Weight loss during particle erosion is a better measure of the erosion rate.

Table 4. Depth of wear scar (μm) after particle erosion.

No. of test	AR depth (μm)		HT-1 depth (μm)		HT-2 depth (μm)		316L depth (μm)
	30°	90°	30°	90°	30°	90°	30°	90°
1	70	105	79	117	80	135	165	198
2	72	115	76	119	81	127	169	205
3	75	108	71	111	75	125	163	196
Avg. value	72.33	109.33	75.33	115.67	78.67	129	165	199.67

Figures 8(a)–8(f) display the images of worn surfaces obtained from AR and heat treated steels at 30° and 90°. The erosion at 30° in AR and heat treated steels mainly occurred due to ploughing. Shear process was observed by the presence of ploughs with lip formation at sides. At impact angle of 30°, the erosion loss occurred mainly due to shear cutting of surface material. However, wider and deeper ploughs are observed in heat treated steel (Figs. 8(c) and 8(e)) than that in AR steel (Fig. 8(a)). The erosion occurred at 90° shows rougher surface in comparison to 30°. The removal of the material occurred by detachment. Some wear debris were moved into the cavities produced by abrasive particles. The same phenomenon was observed in the brittle fracture in ceramics and other hard materials.^33,34,35) The ploughing is significantly less in the surfaces eroded at 90° in AR and heat treated steels. Deeper cavities are observed in heat treated steel (Figs. 8(d) and 8(f)) as compared to AR steel (Fig. 8(b)). The erosion rate at any impact angle is slightly higher for heat treated steels as compared to AR steel.

Fig. 8.

Secondary electron SEM micrographs of eroded surfaces after solid particle erosion of as received steel (AR) at ((a) 30°, (b) 90°); HT-1 at ((c) 30°, (d) 90°) and HT-2 ((e) 30°, (f) 90°).

In solid particle erosion, the austenitic phase is considered to be a beneficial constituent because of its plasticity and toughness. The localized strength of austenitic phase increases with work hardening of matrix resulting in improvement in the erosion resistance.³⁶⁾ The target material deforms plastically due to the repeated attack by hard abrasive alumina particles. The tensile toughness also plays important role in erosion resistance as the tough material absorbs kinetic energy of impinging particles.³⁷⁾ The erosion rate decreases with increasing toughness and ductility.³¹⁾ In the present work there is no significant variation in ductility and tensile toughness of the nitrogen steel with heat treatment. During erosion, the high strain hardening exponent (n) of AR steel may lead to significant strain hardening and higher erosion resistance in comparison to HT1, HT2 and 316L steel. The lower hardness, YS, UTS, tensile toughness and strain hardening exponent of 316L results in a lower resistance to erosion when compared with the nitrogen steel samples. The grain size does not seem to significantly affect the particle erosion behaviour in the nitrogen steel tested. This is in agreement with the observation that grain boundaries did not act as preferential sites for solid particle erosion.

4. Conclusions

The nitrogen alloyed austenitic stainless was subjected to solutionizing heat treatment. This also resulted in an increase in grain size from 15 to 65 μm. The influence of grain size on cavitation erosion and particle erosion behaviour was studied. The following are the major conclusions:

(1) The nitrogen alloyed austenitic stainless steel exhibits superior resistance to cavitation erosion and particle erosion than 316L stainless steel. This is related to the superior tensile toughness of the nitrogen steel samples.

(2) The cavitation erosion damage started at grain boundaries. Because of this the nitrogen alloyed austenitic stainless steel with fine grain size exhibits superior resistance to cavitation erosion.

(3) No significant effect of grain size was observed on particle erosion behaviour.

(4) No formation of strain induced martensite in the nitrogen alloyed austenitic stainless steel during erosion studies.

Acknowledgments

The authors are thankful to M/S Star Wire (India) Ltd., Ballabhgarh, Haryana, India provide steel samples for current study.

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