nitriding ppt

VACVACVACVACVACVACVACVAC AEROAEROAEROAEROAEROAEROAEROAEROI N T E R N A T I O N A L I N C.I N T E R N A T I O N A L I N C.

Gas Nitriding:An Industrial Perspective

By Jerome Darbellay

Supervisors: Drs D. Embury and H. Zurob

MSE 701 Seminar Department of Materials Science and Engineering

March 22, 2006 McMaster University

MSE 701 - March 22, 2006 Gas Nitriding: An Industrial Perspective 2

Outline

� Review of gas nitriding process

� Research objectives

� Fe-N diagram and process equilibrium

� Nitride layer growth model for pure iron

� Experimentation Equipment

� Nitrided Case Evaluation

� Future work


Fundamental of Nitriding

� Thermochemical treatment producing enhanced surface properties from the bulk properties

� Case hardening results from diffusion of N into substrate (solidsolution) and precipitation of nitrides (FeN and alloy elements nitrides) when holding the metal at suitable temperature (generally below 575°C)

� Ammonia (NH3) is nitrogenous gas typically used since it is metastable at nitriding temperature and decomposes on contact with iron

� Quenching is not required for production of a hard case

� Process methods include: gas, liquid (salt bath) or plasma (ion)

� low alloy steel (4140), depth of case [µm]

205

(575ºC, 2hrs)

210

(475ºC, 3hrs)

200(525ºC, 4hrs)

Salt BathPlasmaGas


Liberation of nascent N

2NH3 ——> 3H2 + 2N (dissolved

——> 3H2 + N2 (gas)

——> N2 + 6H (dissolved

Possible reactions taking place at the steel surface:

(catalyst)

(heat)

in α-Fe)

Diff

usio

n

N

H

H H

H

HN

N

N

N N

Adsorption

Recomb.

Ads

orpt

ion

laye

r

N

H

H H

Diff

usio

n

Recomb.

N

H

H H

Ste

elA

tmos

pher

e

H H

Diff

usio

n

N

H

H H

H

HN

N

N

N N

Adsorption

Recomb.

Ads

orpt

ion

laye

r

N

H

H H

Diff

usio

n

Recomb.

N

H

H H

Ste

elA

tmos

pher

e

H H

in α-Fe)


Advantages of Nitriding

� Relatively low-temperature process when compared to other case hardening process such as carburization (C diffusion), carbonitriding (C and N diffusion)

� No quench requirement, thus reducing distortion

� High hardness value obtained with this family of steels (above 1000HV, 700HV ≈ 2200 MPa UTS):

� low-alloyed steels 0.5C, Ni, Cr, Mo, Mn, Si, Al (Nitralloys, 4340,4130,…)

� die steels 0.4C-5Cr-Si-Mo (H11, H12 & H13)

� tool steels 0.5-1.5C-Cr-Mo-W-Va (A-2, D-2, Mo, S-7,…)

� Resistance to corrosion (for alloyed and tool steels). Corrosionresistance grows in the order of γ’-nitride, ε- nitride and ε-carbonitride

� No change to core properties


Cost of Nitriding

Approximate cost for low-alloyed steel family:� $2/lb for 0.005” case (~ 5hrs, no evaluation coupons) to $10+/lb for 0.025” case (~25hrs, acceptance coupons included)

(After J.R. Davis, Surface Engineering for Corrosion and Wear Resistance, ASM International, 2001)


Applications of Nitriding

� Principal reasons for nitriding:� High surface hardness� Wear / abrasive resistance and anti-galling properties� Improve fatigue life (compressive stress at surface)� Improve corrosion resistance (except for SS material)� Improve resistance of surface to softening effect of heat up to nitriding temperature

� Typical applications:


Limitations of Nitriding

� Distortion occurring in the form of growth (compound layer formation) and increase in volume in the case

� Nitriding can also act as stress relief/tempering operation if not previously performed, leading to unexpected distortion

� Uniform case hardening can only be obtained through continuous controlled circulation (preferably by use of internal fan), accurate control of process gas and temperature and proper part preparation (cleaning, blasting, handling)

� Ammonia is used in a very wasteful manner (typically up to 70% is vented) and large percentage of N atoms liberated will combine to form inert N2. This gas, as well as formed H2 are also vented.

� Nitriding of SS material requires removal of noble oxide layer (depassivation using ammonium chloride (NH4Cl) ). Nitrided SS components will exhibit a significantly reduced corrosion resistance due to precipitation of CrN


Objectives of Research� Problem

� Nitrided case microstructure and properties of industrial alloyed steels can not precisely be modeled to optimize the nitriding parameters

� Objectives� Compile database of N diffusion parameters and models for gas nitriding of pure iron and alloyed steels currently available inliterature

� Combined experimental and modeling activities to explore the various microstructure produced during gas nitriding of iron alloyed with C (and other selected alloying elements) to be compared to mechanical properties of hardened case

� Combined experimental and modeling studies of the detailed evolution of the nitride layer in alloyed steel and its subsequent relation to the process parameter

� Interaction of Industrial Project with McMaster University� Development of models related to carefully defined compositions� Range of experimental techniques available for characterisation of nitrided metals


Fe-N Phase Diagram

(Adapted from D.T. Hawkins, ASM Source Book on Nitriding, 1977)

11

35

ζζζζ


Phases in the Fe-N system

Orthorhombic5011.4 (33.3)Fe2Nζ

Hexagonal22 - 49.34.5-11.0 (18-32)Fe2N1-xε

Cubic255.9 (20)Fe4Nγ’

B.c. tetrag.11.12.6 (10)FeMartensite (α’)

F.c.c12.42.8 (11)FeAustenite (γ)

B.c.c.-0.10 (0.40)FeFerrite (α)

Bravais LatticeInterstitial

atoms per 100 Fe atoms

Wt. % (At. %) NCompositionPhases

(After D.H. Jack, Carbides and Nitrides in Steel, Materials Science and Engineering, 1973)

Fe4N crystal structure

Iron Atoms

Nitrogen Atom

X Unoccupied Interstices


Nitriding Gas – Pure Iron Equilibrium Diagram

(E. Lehrer, Z. Electrochemie, 1930)

(L. Maldzinski, New possibilities for controlling gas nitriding…, Surface Engineering, 1999)

-1 .5

-1

-0 .5

0

0 .5

1

1 .5

log

(K

N)

(atm

-1/2

)

KN (

atm

-1/2

)

3 1 .6

1 7 .8

1 0 .0

5 .6

3 .2

1 .8

1 .0

0 .3

0 .1

0 .0 3

εεεε

γγγγ '

γγγγαααα

0.001

0.005

0.01

0 .03% N

0.05

0 .0 7

7 0 0 6 0 0 5 5 0 5 0 0 4 5 0 4 0 0 o C

1 .0 1 .1 1 .2 1 .3 1 .4 1 .5 x 1 0 -3

5.8 5

5 .8 25 .78

5 .7 3

5 .87 % N

5.89

5 .89 7

8.5 08 .759 .009 .259 .50

9 .75

8 .2 5

10.00 % N %N

F e - N

1 /K

(Wt. %N)

Typical Nitriding Temperature Range


Monolayer growth in binary system (1)

� Diffusion of solute (N) governs the growth kinetics. Flux balance equation at the γ’/α interface is given by:

'/'/ / ' '

'( )

γ αγ α α γ γ α

γ − = −N N N NN

vu u J J

V

(H. Du, Gaseous Nitriding Iron - Evaluation…, Z. Metallkd., 1995)

'γNJ

ααααγγγγ’

uN,αααα/γγγγ’

Distance from surface z

αNJ

uN,γγγγ’/αααα

uN,s/γγγγ’

uN

'γl

is migration rate of γ’/α interface

is partial volume/mole of N atom of γ’ phasecontents of N on the γ’ and α side of interface

diffusion fluxes of N on the γ’ and α side of interface

'/γ αv'γ

NV'/ / ',γ α α γ

N Nu u',γ α

N NJ J

� The following assumptions are made: � Planar interface parallel to surface� Growth of new phase obeys parabolic growth (i.e. where l is thickness of

layer, t is time and k is called growth rate constant)� α substrate is saturated with N (i.e. )� Diffusion in γ’ is stationary due to narrow composition range in the phase� Mobility MN is independent of concentration

=l k t

0α =NJ


Monolayer growth in binary system (2)

''

'

γγ

γ∂=∂

ɶN N

NN

D uJ

V z

� The partial Gibbs free energy (=chemical potential) of N is the driving force

for diffusion. The chemical diffusivity can be related to the self diffusion

coefficient using the thermodynamic factor Ψ :NDɶ

*

ND

* * N NN N N

N

uD D D

RT u

µ∂= Ψ = ⋅∂

ɶ uN,µN N concentration and chemical potential

R gas constant

T absolute temperature

� Using Fick’s first law of diffusion for flux:

� Integrating over the thickness of layer and applying stationary diffusion

assumption (constant flux over system):'/

'/

' ''

0

1γ α

γ

γ γγ= −∫ ∫ ɶ

N

sN

ul

N N NN u

J dz D duV

N content at the surface'/γ s

Nu



Multiphase growth in binary system (1)

� The treatment of a diffusion-controlled growth of a bilayer in a binary metal-interstitial system is defined analogously as for the case of monolayer growth. Applying Flux balance equation at both the ε/γ’ and γ’/α interface yields:

ε/γ’:

γ’/α:

Where

� The following assumptions are made: � Stationary diffusion may not be satisfactory for ε phase due to it’s wide range of composition (-

> flux balance equation to be solved numerically)� Self-diffusivity of ε is likely to be concentration dependent for the same reason

/ '/ ' '/ '( )

ε γε γ γ ε ε γ

ε − = −N N N NN

vu u J J

V


'γNJ

ααααγγγγ’

uN,αααα/γγγγ’

Distance from surface z

αNJ

uN,γγγγ’/αααα

uN,γ,γ,γ,γ’/εεεε

uN

εl

εεεε

uN,ε/γ,ε/γ,ε/γ,ε/γ’

uN,ε/,ε/,ε/,ε/s

'γl

εNJ

'/'/ / ' '

'( )

γ αγ α α γ γ α

γ − = −N N N NN

vu u J J

V

( , 'or )φ

φφ φ ε γ α∂= =

∂

ɶN N

NN

D uJ

V z


Complications from pure Iron kinetic model

� Nitride layer nucleation� Most models assumes an equilibrium concentration of N with the

atmosphere, but the formation of a compact nitride layer is retarded by:a) Incubation time for nucleationb) Time for nuclei to coalesce into a compact layer

� Effect of C content� Affect N activity (coefficient of diffusion) in α-Fe and ε-nitride� Complex phases transformation (i.e. case of θ cementite -> ε

carbonitride -> γ’ in α at α/γ’ boundary)� Effect of alloying elements

� Both nitride (Cr, Mo, Al, V, Ti) and non-nitride (Ni) forming elements reduces N diffusion coefficient in a-Fe

� Reaction of nitride forming elements with N to form stoichiometricnitrides

� Do we have both external and internal nitriding?


Nitriding Furnace Schematic

(After M. Bever, Case Hardening of steel by nitriding, ASM Source Book on Nitriding, 1977)


Vac Aero Gas Nitrider


Vac Aero Nitriding Retort

0 0.25 0.5 m

Gas circulation path

� Retort, fixturing basket and internal liner all made of IN 600 material


Process Control� Good process control improves:

� Process repeatability and economics

� Metallurgical requirements

� Operator interfacing, data trending and archiving

� Key Control in nitriding

� Temperature (± 15ºF per AMS 2759/6 Gas Nitriding Specification) by means of T/C and SPP

� Process gas by means of flow meter. Anhydrous ammonia and nitrogen shall be of the high purity grade (99.98%) with dew point -54ºF (-48ºC) or lower.

� Atmosphere by means of a dissociation pipette (burette) (±15% dissociation per AMS 2759/6) or gas analyzer

� Retort pressure (slightly above atmosphere to prevent any O2 to enter vessel (risk of explosion) and maintain flow through pipette or gas analyzer


Nitriding Potential

� Nitrogen solubility at the surface of the iron is determined by equilibrium:

hence (for dilute solution of N in Fe, Henry’s law)

where k is an equilibrium constant at a given temperature and pNH3 and pH2 are the partial pressure in the gas

� Nitriding potential definition:

[ ] ( )Fein dissolved 2

323 NHNH +⇔

[ ] [ ] 33 / 2

2

% N

pNHa N k

pH= = ⋅

3

3 / 2

2

N

pNHK

pH=


Atmosphere Control - Burette

(D. Pye, Practical Nitriding and Ferritic Nitrocarburizing, ASM International, 2003)

� Gas dissociation is simply the amount of decomposition of the ammonia gas. The exhaust gas is a mixture of 2NH3 + H2 + N2 and ammonia is the only one soluble in water.

H2O

H2OH2O


Atmosphere Control – H2 Gas Analyzer

Inlet Atmosphere

NH3= A

H2 = h = 0.75d

N2 = n = 0.25d

NH3 Atmosphere

Dissociated

NH3 = αααα

r

d

Burette Logic

dNH3 expanded

= 2αααα

After Dissociation

A’ = A-αααα

B. THE METHOD:

1. Measure %H2 using the analyzer: h

2. Calculate Dissociation Rate d: d = h/0.75

3. Calculate αααα as a function of dissociation d: αααα = d/(2-d)

4. Calculate Residual NH3: r = 1 – d

5. Determine Nitriding Potential:

)75.0(

12/32/3 d

d

h

rKn

⋅−==

Normalized Outlet Atmosphere (volume =1)

r = A – d =

A-αααα/(1+αααα)

Dissociation Rate = d =

2αααα/(1+αααα)


Precision of Control

� Control System

� Precision of Gas Flow Regulation

1) AMS Standard 2759/10 for automatic control of gas nitriding by Kn requires precision of ± 10% at setpoint

2) An error of 2% in the analysis of the H2 content of the exhaust gas translates into an error of about 20% for KN for the low KN setpoints required to achieve zero compound layer (Fe-Nitride)

3) Original flow regulation (motor driven needle valve) supplied with the control panel was not suitable (precision of ~ 4%)

� New gas regulations relies on Mass Flow Controller with precision better than 1%


Precision of Control

� Error on H2 measurement – Effect on KN Control


Samples Evaluation Method

- Light Microscopic Analysis with positive phase contrast microscopy of the etched samples

- Scanning Electron Microscopic analysis

- X-Ray Diffraction (phase concentration)

- Transmission Electron Microscopy (TEM): Diffraction Pattern

Phase Identification

- Light Microscopic Analysis, by etching

- Scanning Electron Microscopic analysis

- Microhardness Profile

- Methods used for N concentration as a f(depth)

Depth of case

- Electron Probe X-ray Micro-Analysis (EPMA)

- Focused Ion Beam (FIB)

- Glow Discharge Spectroscopy (GDOES): qualitative only, quantitative possible using standard of known N content

- Transmission Electron Microscopy (TEM): Electron Energy-Loss Spectroscopy (EELS)

- X-Ray Diffraction (XRD): lattice parameters -> N concentration

- Chemical Analysis: total N concentration

N concentration as a f(depth) below surface

- Light Microscopic Analysis (LMA), by etching

- Scanning Electron Microscopic (SEM) analysis

Fe-Nitride Layer Thickness

MethodCase properties to

be evaluated


Samples Evaluation (1)

� Microhardness profile: Effective and Total Case Depth

250 µm100



� Microstructure of Nitralloy 135M (0.4C-0.6Mn-0.3Si-1.6Cr-0.35Mo-1.2Al), quenched and tempered, Nital etched

Fe-Nitride

Hardened Case

Substrate Core



� Microstructure of SAE 1006 (0.08C-0.3Mn): Automotive Seat Rails application

� Microstructure of SAE 1144 (0.45C-1.5Mn-0.25Si): Throttle Valves application

� Microstructure of SAE 4140 (0.40C-1.0Cr-0.85Mn-0.25Si-0.2Mo): Automotive Shafts application

(Adapted from www.nitrex.com website)

Fe-Nitride

Substrate

Fe-Nitride

Substrate

Fe-Nitride

Substrate



� Microstructure of H13 Hot-work Tool Steel (0.4C-5Cr-1.3Mo-1V), Quenched and Tempered, NitalEtched: Forging Dies application

� Microstructure of ASTM A1011 Commercial Steel Type B (Fe-0.04C-0.26Mn), Nital Etched: Part Manufacturing application

50 µµµµm

50 µµµµm

Fe-Nitride

Hardened Case

Fe-Nitride



� Phase Identification by means of LMA with positive phase contrast microscopy of an Fe sample etched with 1 pct Nital+ 0.1 vol pct HCl

(H. Du, Microstructural and Compositional Evolution…, Metall. Trans. A, 2000)

� Nitrogen content as a function of depth below the surface determined with EPMA in layers obtained on an Fe specimen

(H. Du, Microstructural and Compositional Evolution…, Metall. Trans. A, 2000)


Future Work

� Comprehensive literature review of detailed structures and properties for a variety of nitrided steels

� Review the formation of a compact Fe-nitride layer by nucleation and coalescence as well as the formation of multilayer Fe-nitride

� Study the effect of C content on the kinetic of nitride layer growth: experiments with carefully defined steel microstructures such as spheroidized carbides, samples with C gradient a surface (decarburized)

� Study the effect of alloying elements (such as Cr, Al) on the kinetic of Fe-nitride layer growth and mechanical properties of nitrided case

nitriding ppt

Documents