the microstructural analysis and tribological behavior of plasma nitrided 316ln stainless steel

Behavior of Plasma Nitrided 316Ln Stainless Steel

The Microstructural Analysis and Tribological

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Abstract

Abbreviations

Challenges of 316Ln Stainless Steel

Chemical Composition and Mechanical Properties of 316 Ln SS [5]

Plasma Nitriding Process

Tests Performed for this Study

Test ResultsTest Results

Micro Hardness Test

XRD Test Results

Wear Test Results

Scanning Electron Microscope (SEM)

EDAX Test Results

Inference

ConclusionConclusion

Reference

Author Info

3

3

4

4

4

5

66

6

7

8

8

9

9

1010

11

11

Table of Contents


316Ln stainless steel has wide engineering applications, particularly in nuclear and thermal power plants, to carry and control the flow of steam, which is prone to frequent leakage leading to major explosions. The surface of the mating parts in flow control valves stick together at a high steam temperature of 400°C to 500°C. This erodes the surface and leads to the leakage of steam. In order to avoid this, the 316Ln stainless steel surface needs to be treated and plasma nitriding is the preferred surface treating method compared to powder coating, carburizing and case hardening because the nitriding process is done at plasma state, which increasesincreases the diffusion rate of the chrome-nitride content at the surface of the substrate material to increase the hardness at surface and to increase the corrosion and wear resistance.

This whitepaper illustrates a microstructural analysis study that was conducted to understand the tribological behavior of plasma nitrided 316Ln stainless steel with the nitriding layer thickness of 30 – 40 microns. The plasma nitriding method is cost-effective as treatment times are reduced by a factor between 3 and 5; there is a 50% reduction in energy consumption, and a 50 to 100 times reduction in the use of other surface treat-ment methods [12]. The hardness and the deposits of the plasma nitrided 316Ln stainless steel were measured using a Vickers micro hardness test. The microstructure of the deposit was characterized by optical microscopemicroscope (OP), scanning electron microscope (SEM), X-ray diffraction (XRD), and energy dispersive X-ray analysis (EDAX). The Tribological behaviors were tested by wear, fatigue and crack tests. These real-time tests were conducted at IIT Madras, IIS Bangalore and Anna University, Chennai.

Sl.No

1

2

3

4

5

6

77

8

XRD

SEM

OP

MAD

FEG

LNG

EDAXEDAX

OM

X-Ray Diffraction

Scanning Electron Microscopy

Optical Profilometer

Multi-wavelength Anomalous Diffraction

Field Emission Guns

Liquid Natural Gas

Energy Disperse X-Ray AnalysisEnergy Disperse X-Ray Analysis

Optical Microscope

Full FormAcronyms

Abstract

Abbreviations

The Microstructural Analysis and Tribological Behavior of Plasma Nitrided 316Ln Stainless Steel | 3


While 316Ln stainless steels (SS) are widely used in the industry because of their distinct toughness and ductil-ity under cryogenic temperatures, they are prone to pitting corrosion especially in the presence of halide ions and acidic environments. It is well known that 300 series austenitic stainless steel provides high resistance to corrosion and oxidation, and retains high strength and excellent ductility over a wide temperature range [1,2]. This has limited their use in a wide range of engineering applications particularly in nuclear and thermal power plants, chemical tankers and chemical industries. In fact, corrosion depends strongly on the micro-structure and composition of the material at the near-surface region. The driving cause for pitting corrosion is the depassivation of a small area, which becomes anodic while an unknown but potentially vast area becomes cathodic, leading to very localized galvanic corrosion. The corrosion penetrates the mass of the metal, with limited diffusion of ions. The occurrence of such extremely localized corrosion leads to the creation of small holes in the metal and leads to the failure of components, which cause serious accidents in the industry.

Plasma nitriding is a heat treating process that diffuses nitrogen into the surface of a metal to create a case hardened surface. The intense electric fields are used to generate ionized molecules of the gas around the surface to be nitrided. Such highly active gas with ionized molecules is called plasma.

Surface hardness and corrosion resistance can be increased by the plasma nitriding process, by introducing CrN at the surface of the 316Ln stainless steel by the plasma diffusion method [3]. Before the plasma nitriding process, specimens were cleaned ultrasonically in acetone for 30 minutes and put into the deposition chamber. After the samples were placed on the cathode plate, the chamber was evacuated to 1x10-2 mbar.

Grade

316LnMin

Max

-

0.03

-

2.0

-

1.00

-

0.045

-

0.03

16.0

18.0

2.00

3.00

10.0

14.0

-

0.10

C Mn Si P S Cr Mo Ni N

GradeTensileStress

(MPa) min

Yield Stress 0.2% Proof (MPa) min

Elongation(% in50 mm)

minRockwell B(HRB) (*) max

Rockwell B(HRB) (*) max

Hardness

316Ln

(*) - Properties considered for Analysis.

485 170 40 95 228

Challenges of 316Ln Stainless Steel

Chemical Composition and Mechanical Properties of 316 Ln SS [5]

Plasma Nitriding Process



The glow discharge plasma was generated by a pulse dc power supply of 25 kW and frequency of 30 kHz. The gas flow rate was controlled by a mass flow controller. The samples were heated to the temperature of 570°C by ion bombardment. Plasma nitriding temperature was measured using a chromel–alumel thermocouple, placed at the bottom of the nitrided sample. The specimens were plasma nitrided at 570 °C for 24 hours in a gas mixture of 80% N2–20% H2 under a working pressure of 5 mbar[7].

Sl.No Test Name Purpose Pre-treatment or Pre-requisite

1 Micro Hardnesstest

To measure the surface hardness made with very low static loads, which is less than or equal to 1kgf.

The surface being tested was made to metallographic finish and it is achieved by diamond paste polishing followed by the etching process.

2 X-Ray Diffraction(XRD)

To determine the orientation and distribution of crystalline grains in 316Ln stainless steel. Also XRD was used to measure residual strain, crystallite size and micro strain. The index peak position states the distri-butionbution of different elements present on the sample

No pre-treatment was required.

3 Scanning ElectronMicroscope (SEM)

It is used to produce largely magnified images of the 316Ln stainless steel with plasma nitrided and non-nitrided samples, to analyze and compare the crystal structures of both the specimens.

The samples were well cleaned before the process by using chemical etching and were dried to avoid the evaporation of water inside the SEM chamber due to vacuum.

4 Energy DisperseX-Analysis (EDX)

It is performed with SEM to characterize the elemental composition of the analyzed volume for 316Ln stainless steel.

The samples were well cleaned by the chemical etching process and dried to avoid evaporation in a Vacuum envi-ronment.

5 Wear Test (Pin-on-disc tribometer)

To compare the wear rate and volume of metal removal between the coated and non-coated samples. Surface profile of the wear track on the ring was measured using Optical profilometer (OP).

The wear test analysis was done by using the Pin-On-Disc tribometer. In this setup the stationary pin was slid against the samples at the speed of 0.0753 m/s at 11.2 N noraml loads at room temperature.

Tests Performed for this Study



Micro hardness measurements showed a significant increase in hardness from 210.94 HV 0.02 (for untreated samples) up to 859.45 HV 0.02 (plasma nitrided samples). The super saturation of nitrogen that exists beneath the surface, leads to the formation of a fine dispersion of chromium-rich particles, giving rise to the peak hardness. The lower hardness nearer to the surface may be attributed to the presence of mainly iron nitrides of a relatively large particle size compared with the subsurface structure. The larger particles have probably resulted from the continuous sputtering, which is characteristic of plasma nitriding. However, the formation of nitride precipitates beneath the surface is essentially a diffusion controlled reaction.of nitride precipitates beneath the surface is essentially a diffusion controlled reaction.

The photomicrograph shows the white layer of the nitrided zone with the compounded zone, forming alloy nitride. The grains of austenite are behind the nitrided layer. The layer thickness is between 30 – 40 microns. The parent metal shows large equaled austenite grains. Some slip bands are noticed on scanning the matrix. Images in Figure 3 were taken by an Optical Microscope (OM).

Test ResultsMicro Hardness Test

Micro Hardness Test

Figure 3. Photomicrograph

DEPTH, MicronHARDNESS VALUE

Figure 2. Micro Hardness Graph Figure 1. Micro Hardness Surey

0 10 20 30 40 50 60 70 80

10009008007006005004004003002001000



1. At 0.010 kg load on the surface: 418.1, 420.2 &418.6 HV2. At 0.025 kg load on the surface: 398.6, 395.8 & 396.4 HV3. At 0.050 kg load on the surface: 346.3, 348.5 & 349.4 HV4. At 0.100 kg load on the surface: 320.5, 322.6 & 324.9 HV5. At 0.500 kg load on the surface: 290.6, 292.4 & 294.7 HV6. At 1.000 kg load on the surface: 280.6, 282.4 & 277.4 HV

Remarks: Parent metal hardness was reached at 0.5 kgf and 1.0 kgf loads. Therefore, the indenter has penetrated beyond the nitrided layer. The images in Figure 4 & 5 show the indentation marks on the surface of the nitrided layer at different loads. Note the indentation marks at higher load in Figure 5, which shows edges of the coating that has undergone fissures at the boundary.

XRD Analysis shows the size, shape and internal stress of small crystalline regions for coated and non-coated 316Ln samples. It also helps to find the crystal structure of an unknown material.

Hardness Values at Different Loads

XRD Test Results

Figure 6. Non-Coated Sample

X Axis Title

Fe Fe

Cr

NMgCrY

Axis Title

Y Axis Title

X Axis Title

Figure 7. Coated Sample

Figure 5. Indentation at Higher Loads Figure 4. Indentation at Lower Loads



The wear tests were conducted for both untreated (316Ln stainless steel/316Ln stainless steel) and plasma nitrided (CrN/CrN) samples, with a normal load of 11.2N, sliding speed of 0.0753 m/s and at 250°C for one hour.

The friction coefficient of untreated metal mating (0.81454) is higher than the plasma treated metal mating (0.69004).The cross-section length of the wear track and the worn depth of the wear track for untreated metals are also high. The volume of metal loss of untreated metal (7.56x10-10 m3) is one order of magnitude higher than the plasma treated metal (4.096x10-11 m3).

The wear of the untreated 316LN SS- 316Ln SS combination was severe, resulting in a very rough metallic surface, and micro-cracks and small wear debris distributed sporadically on the track. This is typical of the result of adhesive wear, which is established for austenitic stainless steels sliding against steels [12]. Due to its relatively low hardness, the untreated 316 stainless steel surface was severely deformed during the wear test. The role of nitriding in reducing wear is to produce a hard layer to resist plastic deformation and to change the surface chemistry so as to eliminate adhesion between the contact surfaces. Indeed, no adhesion and plastic were observed at the nitrided surfaces and subsurface.and plastic were observed at the nitrided surfaces and subsurface.

Wear Test Results

Scanning Electron Microscope (SEM)

Figure 8. Wear Test for Coated

X Profile

X ProfileX 2.450 mm

X 1.650 mm

1.8

1.5

1.2

0.9

0.6

0.3

0.00.0

0.0 0.5 1.0 1.5 2.0 2.4

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.9

3.7

3.0

2.0

1.0

0.0

mm

mm

mm

mm

Figure 9. Wear for Non-Coated



Plasma nitriding was done on 316Ln stainless steel to study the microstructure, hardness and tribological parameters of the friction coefficient, the wear mechanism and wear rate.

Microhardness measurements showed significant increase in hardness from 210.94 HV0.02 (for untreated samples) up to 1040.45 HV0.02 (plasma nitrided samples).

EDAX Test Results

Element

CK

OK

AlK

SiK

SK

CrK

FeKFeK

NiK

Matrix

26.16

08.43

01.40

01.07

00.40

10.18

46.2646.26

06.11

Correction

55.34

13.38

01.32

00.97

00.32

04.97

21.0521.05

02.65

ZAF

Wt% At% Element

CK

NK

OK

SiK

CrK

FeK

NiKNiK

Matrix

24.43

04.20

08.78

01.03

09.99

45.56

06.0106.01

Correction

50.48

07.43

13.62

00.91

04.77

20.25

02.5402.54

ZAF

Wt% At%

Inference

Figure 10. Non-Coated Sample Figure 11. Coated Sample

Figure 13. EDAX for coated Figure 12. EDAX for Non-coated



The plasma nitriding process has the ability to increase surface hardness up to 1040.45 HV by altering the microstructure at the surface level of the 316Ln stainless steel with a minimal layer thickness of 30-40 microns and increasing the corrosion and wear resistance at a lower cost and time when compared to the other surface treating processes.

PlasmaPlasma nitrided parts are usually ready to use straight after the process with no extra machining needed. This process can be performed at lower temperatures than other processes. It provides high surface hardness with low wear. High case depth supports the coating and it reduces the coating failure. It also provides high internal compressive stress – i.e. resistance to volume of contact fatigue. Temperature resistance allows hard coating up to 600°C.

In the EDX spectra of elements in the nitriding samples, a nitrogen peak was clearly visible, indicating the presence of nitrogen but no noticeable nitrogen peak was observed in the non-nitriding sample.

The volume of metal loss of untreated metal (7.56x10-10 m3) is one order of magnitude higher than the plasma treated metal (4.096x10-11 m3). Untreated 316Ln stainless steel suffered severe wear and was characterized by strong adhesions and abrasions whilst the wear of the plasma nitrided 316Ln stainless steel was mild and dominated by plastic deformation, slight abrasion and frictional polish-ing.

Conclusion


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Reference

Author Info

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