Module 2 Nitriding

Nitriding

UN

IVE

RS

ITY

OF T

RE

NT

O

Prof. M. Pellizzari

Course

Metallic Materials Engineering

Module 2

Nitriding M. Pellizzari Metallic Materials Engineering, - Module II

Summary

Fe-N equilibrium diagram

Nitriding processes • Gas nitriding

Lehrer diagram, Nitrogen potential KN

N. paramaters: time, temperature, KN

N. atmosphere

• Plasma nitriding

Microstrucure of nitrided steel Diffusion layer, compound layer

Nitridability

Nitriding steels

Nitriding


Fe-N equilibrium diagram


Lehrer diagram

2/33

2/13333

2/32

3

)(5.1

)()()()(

)(

)(

disso

dissoodissoo

R

RN

NH

NHNHNHNH

H

NHK

Nitrogen potential

Ammonia dissociation

2NH3(gas) 2N + 3H2(gas)

Gas nitriding


Compound layer

(white layer)

Diffusion layer

Thickness

Composition

Constitution

Porosity

Depth

(Deff., Dtot)

Hardness

(Microh. profile)

Internal stresses

Performance!

Processing parameters

Unnitrided core

Material

Nitriding Base material – processing parameters - performance


depth from surface

MICROHARDNESS

PROFILE

Diffusion layer

HV0.1 550HV

S.E.

Stot

• Solid solution hardening(N)

• Precipitation hardening (nitrides)

Diffusion layer: hardening mechanisms


Nitrides precipitation

porosity

Fe2,3N

Grain boundary

carbonitrides

Fe4N


Residual stresses


10 m

porosity

%N

Compound layer

Diffusion layer

Fe 4 N

Fe 2,3 N

Grain boundary carbonitrides

7.8 - 11.3 %wt

5.7 - 6.1 %wt

Nitride N solub. limit

[%wt]

C solub. limit

[%wt]

Fe4N fcc 5.7- 6.1 <0.2

Fe2,3N hcp 7.8-11.3 <4

Compound layer

Fe4N


223

2

3

2

1HNNH

NN 2

2

1

23

2

3HNNH

aN = K(T) · KN

Gas nitriding Nitrogen activity aN and nitriding potential KN

2/33

2/13333

2/32

3

)(5.1

)()()()(

)(

)(

disso

dissoodissoo

R

RN

NH

NHNHNHNH

H

NHK

Nitriding potential: KN


Lehrer Diagram

Lehrer diagram

Local equilibrium at

worpiece surface !

No information about

process kinetiks !

No information about

The effect of C !


KN = 0.3

KN = 1.0

KN = 2.5

KN = 0.5

Nitriding Potential KN

0.5h 1h 2h 5h 10h 20h 30h 40h

Nitriding time

Nitriding

potential Normalized C45


Compound layer thickness = f (T, t, KN)

Nitriding time, t Co

mp

ou

nd

la

ye

r t

hic

kn

ess

, S

c.l

.

])(

1[02

401..

NN KKaettaS lc


wt% N wt% N

wt% C wt% C

Depth from surface Depth from surface

Compound layer composition


Diffusion layer thickness = f(T,t)

c = 0.2 [mm/h1/3]

t = nitriding time [h]

Nitriding time, t

Diffu

sio

n la

ye

r t

hic

kn

ess

, D

N

tcDN 3


DN = k • t1/2

DN = case depth

k = k(T)

t = time [h]

Effect of nitrding time and temperature


Nitridability


Oxigen probe

Relationship between the reactions of O2 and the nitriding reactions.

Possibilty to control the nitriding process through pO2 (O2 partial pressure)

)2

31log(0992.0 ETUE E

Fresh inlet gas Composition

2/32

3

)(

)(

H

NHK

R

RN

)(

)(

2

2

H

OHK

R

RO

Nitriding atmosphere Catalytic dissociator

Zirconia element


Nitriding by variable KN

Nitriding time

Co

mp

ou

nd

la

ye

r t

hic

kn

ess

A certain compound layer

thickness can be obtained

independently from diffusion

layer thickness (nitrided case

depth)!


Nitrding steels UNI EN 10085 2003

Quench and tempering steels

Low alloyed tool steels: (NiCrMo)

High alloyed tool steels

Hot work tool steels

High speed steels






Gas nitriding furnace


Plasma nitriding


Fe4N

Fe4N

Fe2N

FeN

FeN

Fe

Cathode (-)

STEEL

Anode (+)

N°

N

N

N

Plasma nitriding


Advantages of plasma nitriding

- Optimum control of N transfer at the workpiece surface

- Control of compound layer constitution (, ’)

- Possibility of avoiding compound layer formation

- Control/Elimination of porosity in the compound layer

- Better control of nitrided case depth

- Lower amount of grain boundary carbonitrides

- Lower process temperatures

- Lower distorsions

- Minimal wastes (no ammonia)

- Low energy consumption

- Automatic process

- Selective nitriding trough electrical masking

- Materials: PM materials, stainless steel…

Disvantages of plasma nitriding

- Limited size of treated parts (cold wall furnaces)

- Limited productivity

- Difficult by components with high volume/surface area ratio

- Edge effect

- Low penetrating power in cavities, holes…

Plasma nitriding


• Increased surface hardness

• Increased wear resistance

• Improved fatigue resistance

• Improved thermal stability

Benefits of nitriding


0 200 400 600 800 1000

200

400

600

800

1000

1200

GNH

GNL

GNC

IN

INCL

INCH

HV

0.2

distanza dalla superficie [m]

10 m GNH 20 m

20 m 10 m

10 m 10 m

GNL

GNC IN

INCH INCL

Nitrided steel - microstructure


A7 A6 A5 A4 A3 A2 A10

2

4

6

8

10

G

GGGI

I

I

--------- % Carbonitruri a bordograno -------->

Tasso d

i usura

(g

/rev x

E-6

)

I = plasma

G = gas

500 N

1000 N

2000 N

50m 50m

Molinari, G. Straffelini, M. Pellizzari, M. Pirovano: Surface Engineering, 1998 Vol.14 No.6., 489

Nitrided steel – extrusion screw


Curve di usura per l'acciaio EN40B nitrurato, rivestito con

TiN e nitrurato+TiN (trattamento duplex)

“Load bearing capability”

Duplex treatment es: Nitriding+PVD

Nitrided – duplex PVD coated steel


Die casting dies


0 100 200 300 400 500 600

0

100

200

300

400

500

600

N

NCZ

NC

NZ

CZ NO

P max =

d tot

100 cicli a Tmax

= 900°C

120 cicli a Tmax

= 700°C

Pm

ax [

m]

dtot

[m]

50 m

Pmax dtot

nitriding vs. duplex-PVD coatings

Die casting dies

Thermal fatigue resistance

Maximum crack crack length vs. total case depth


10 m

10 m 10 m

10 m 20 m

10 m

Inductor

Pyrometer

Cooling water

Sample

Water nozzle

50 m

M. Pellizzari, A. Molinari, G. Straffelini: To be published in Mat.Sci.Eng.

Die casting dies

Gas nitriding vs. plasma nitriding

Thermal fatigue resistance


Compound layer

INERT

Diffusion layer

REACTIVE

Unnitrided steel core

INERT

Chemical rectivity against Nital 2%

(2%vol Nitric acid + 98% Etilico alcool)

Plastic mould dies

Corrosion resistance


Nitrocarburizing

• As in nitriding (N) also in nitrocarburizing (NC) the main element transferred to

the steel is nitrogen but in addition also carbon.

• NC gives the principal surface microstructure and hardness of N

• Nitrocarburizing started to grow remarkably with the development of the salt bath

process Tenifer (Tufftride) and the gaseous process Nitemper developed in the

sixties.

• Compared to classical nitriding, nitrocarburizing is a short time process, typically

30 minutes to 4 hours, performed at higher temperature, about 570°C compared

to 500–510 °C for gas nitriding.

• Normally both phases coexist but γ' is dominating after nitriding and ε after

nitrocarburizing. The γ'-phase causes brittle compound layers whereas ε has a

higher ductility.

• The goal of NC is a monophase ε-compound layer. Good frictional porperties are

attributed to this iron nitride, having an HCP cristral structure, similar to graphite

(good solid lubricant).

• The outermost part of the compound layer normally has some porosity.

(beneficial to lubricant entrapment and deliver during poor lubricity service

conditions).


• A remarkable improvement in corrosion resistance is obtained if

nitrocarburizing is followed by a short oxidation at about 450 °C.

• A 1– 2 µm thick Fe3O4 (magnetite, HCP) layer is formed on top of the

compound layer.

• The oxidation treatment gives the processed parts an aesthetically

attractive black colour.

• The first gaseous process was developed by Lucas, England, and is

called Nitrotec = Nitemper process + oxidation treatment in air. Other

oxidation methods using for example water vapour or nitrous oxide

(N2O) have later on been used.

Nitrocarburizing + postoxidation


Nitriding vs. Nitrocarburizing

• Three groups related to the

process medium, which may be

salt, gas or plasma.

• The salt bath processes are

loosing market to the gas

processes due to the

environmental problems with

salts, containing cyanide.

• Plasma processes are still used

in a limited number of cases but

the use increases because of

specific advantages such as the

possibility to use very low

process temperature and to

nitride e.g. stainless steel, which

is difficult by other methods.

FURNACE ATMOSPHERES 3 Nitriding and Nitrocarburizing by Torsten Holm, Lars Sproge

https://www.ferronova.com/international/web/lg/fer/likelgfer.nsf/repositorybyalias/training_nitr/$file/Nitr3.pdf












Nitriding vs. carburizing

from “Surface Hardening of Steel – Understanding the basics”, Edited by J.R. Davis, ASM International, 2002, p.3

Documents

Module 2 Nitriding