SURFACE PROPERTIES ENHANCEMENT BY THE NITROTEC® PROCESS

S.GOWRI1, PRATAP GHORPADE1 and STEPHEN PLUMB2

ABSTRACT:

Nitrotec® belongs to the family of Surface Engineering processes. The name Nitrotec is derived from the three important stages of the process namely NITRiding, Oxidising and ProTECtion. This process is now available in India. This enhanced nitro-carburizing treatment gives finished ferrous parts an exceptional corrosion and wear resistance, bearing characteristics, high indentation, and scuffing, scoring and galling resistance and a very smooth aesthetically pleasing black coloured surface. In addition to these beneficial properties, some steels show great improvements in yield and tensile strength. Nitrotec® allows for treatment with far greater dimensional control compared to traditional hardening practices where distortion occurs due to metallurgical phase changes and high process temperatures. Nitrotec® allows treatment of close tolerance precision parts especially thin sections. Nitrotec® is a worldwide trademark protected process and can be applied to both steels and cast irons. This paper discusses the process, properties and applications of Nitrotec®. 1 Hightemp Furnaces Limited, Bangalore 2 Nitrotec, Birmingham, UK

INTRODUCTION

Nitrotec process is an enhancement of

nitrocarburising treatment. Nitrocarburising

is an old process and has been in existence

for over fifty years. In nitrocarburising, a thin

hard compound layer is formed on the

surface by the simultaneous diffusion of

nitrogen and carbon in steel. Primary

objective of nitrocarburing is to improve

wear properties. It is a low temperature

process and therefore is a low distortion

process. Initially this process was developed

with salt bath as the medium for enriching

the surface with nitrogen and carbon. Salt

baths used were based on alkali metal

cyanides and cyanates. This liquid

nitrocarburising goes by several commercial

names such as sulfinuz, tufftride, sursulf.

Because of the strict environmental laws and

high cost of disposing the toxic sludge, these

liquid carburizing processes based on

cyanides are slowly phasing out.

In early sixties, Lucas Industries in UK did

considerable amount of research in

developing a gaseous medium for

nitrocarburising. Their patented process used

a mixture of ammonia and endothermic gas

mixture. Over the next several years through

their pioneering work, they perfected the

gaseous nitrocarburising process and

technology.

Nitrocarburising has several advantages. The

process produces a very hard compound

layer with good wear resistance, galling and

scuffing resistance and bearing

characteristics. However, the treated parts

lacked good corrosion resistance and surface

finish, limiting their application only to wear

parts.

Lucas Industries extended their research

focus to develop a technique to improve

these two aspects of nitrocarburisng, namely,

corrosion and appearance. They came up

with a very simple and innovative but unique

technique to achieve this. They named the

new process “Nitrotec”. The name is based

on the three important processing

requirements namely; NITRiding, Oxidising

and proTECtion.

This world wide patented technology is now

licensed through TTI group of UK.

NITROTEC PROCESS

Sequence of events in Nitrotec process is,

therefore, Nitriding (nitrocarburising) the

parts, immediately subjecting them to flash

Oxidation before quenching followed by the

application of an additional protection

sealant. Oxidation gives enhanced corrosion

resistance and quenchant and sealant give

additional protection to the surface.

Nitrocarburising

Nitrocarburising is a thermochemical process

which produces a very shallow surface (case)

hardening. It is done at a lower temperature

of 530 ºC-720 ºC and for a short duration.

The process is further classified as ferritic

when the processing temperature is 530 ºC -

590 ºC and austenitic when the processing

temperature is 590 ºC -720 ºC.

Thermochemical enrichment of surface is

provided by a controlled gaseous mixture.

Ammonia is the main “active” gas in the

mixture for nitrogen enrichment. At the

operating temperature ammonia dissociates

as follows: 2NH3 → N2 (gas) + 3H2 (gas).

But this gaseous nitrogen does not contribute

to nitrogen enrichment. It is the catalytically

dissociated ammonia on the steel surface that

releases nascent nitrogen which contributes

to nitriding as follows: 2NH3 → N (in iron) +

3H2 (gas). So the nitriding potential inside

the furnace is controlled by the residual

ammonia residing in the furnace. Too much

or too little residual ammonia in the furnace

will affect the process and properties. Carbon

is enriched from carburizing agents such as

endothermic or methane or propane gas in

the “gaseous mixture”.

Compound Layer Formation

When residual ammonia in the furnace

comes into contact with steel surface nascent

nitrogen is released. This nascent nitrogen

diffuses into the steel and forms a solid

solution. Nitrogen readily diffused in ferrite

and austenite phase and the solubility

increases with increase in temperature.

Nitrogen continues to diffuse into steel until

the solubility limit is reached. At operating

conditions, nitrogen diffusion is observed to

a depth of 100 µm towards the core marked

by a clear “diffusion zone” also known as

“nitrogen enriched zone”.

When the limit is exceeded, if the gas

mixture is right, excess nitrogen starts

forming € Fe3N iron nitride ( iron carbo

nitride) non metallic compound referred as

“compound layer”. This compound layer is

very uniform and hard and can grow to a

depth of 5-40 µm. Hardness up to 1000VPN

can be easily achieved. Steels containing

alloying elements like Al, Mo, Cr, Mo can

attain even higher surface hardness.

Compound layer is micro porous at the

exterior surface and non porous in the

interior.

Picture (Fig 1) shows the microstructure of

the steel after nitrocarburising treatment.

Nitrocarburised surface consists of two

layers: first compound layer, (here about 20

µm thick) and beneath it is the nitrogen

enriched “diffusion zone”.

As indicated on the picture, compound layer

imparts wear and corrosion resistance and the

diffusion zone enhances yield, tensile and

fatigue strength. Enhancement in strength

properties are more pronounced in thin

sections.

Thus by proper control of nitriding potential

(residual ammonia) and temperature

(diffusion rates) and time (depth), hardness

of the compound layer and properties of

diffusion zone can be controlled. Success of

nitrocarburissing treatment depends on these

three important process controls.

Figure 1 – Microstructure after

Nitrocarburising

Oxidation

In regular nitrocarburising process, parts are

quenched or cooled after the nitriding

treatment.

In Nitrotec, parts are subjected to oxidation

step before quenching. In this post

nitrocarburising operation, parts are

deliberately exposed to oxygen atmosphere

for a controlled period of time and then

quenched. Oxidation step produces a thin

layer of oxide on the surface typically 1-2µm

in thickness. This oxide layer is mainly

magnetite Fe3O4 which is black in color and

corrosion resistant.

Oxidation step gives (a) additional protection

against corrosion (b) imparts an aesthetic

appealing black color to the surface (b) acts

as a carrier for subsequent organic sealant.

Quenching:

Quenching is done is a specially formulated

oil in water emulsion. Quenchant

temperature is carefully selected and

controlled to give an optimum cooling rate.

In Nitrotec cooling rate is optimized to (a) to

eliminate distortion (b) retain high temp

nitrogen enriched solid solution for

strengthening.

Sealant

After quenching work load is degreased and

immersed in specially formulated organic

sealant. Sealant covers the porous outer

layer. Sealant gives a very hard dry invisible

film on the surface.

Figure 2 – Microstructure after Nitrotec

treatment

Figure 2 shows the microstructure after

Nitrotec treatment. With the two additional

treatment steps, micro now shows

- a thin Layer of sealant - withstands

corrosion in excess of 250 hours

- a layer of iron oxide 1-2 µm – black

corrosion resistant

- compound layer of Fe3N – first half with

controlled porosity and second half with no

porosity

- Nitrogen enriched “diffusion zone” about

100 µm thick.

Figure 3 shows a picture of a Nitrotec treated

part. This part is one of the many Nitrotec

parts that are being processed in our facility.

Figure 3 - Photo of a part after Nitrotec

treatment

NITROTEC FURNACE:

Time, temperature and atmosphere are the

three most important parameters which

influence the Nitrotec process. By

controlling time, temperature and nitriding

potential, structure, composition and

hardness of the compound layer and

subsurface diffusion zone can be controlled.

Nitrocarburising parameters are chosen to

obtain optimum oxidation layer for

maximum corrosion protection.

Figure 4 shows the photo of a Nitrotec

furnace. Clean degreased parts are loaded on

the right side into the preheating vestibule.

Quenched parts that come out in the left are

again degreased and transferred to sealant

application tank. Process is controlled at

each step by programmed automatic

controllers.

Figure 4 – Photo of a Nitrotec treatment

furnace

BENEFITS OF NITROTEC TREATMENT

Property Improvements:

There are several properties that are

enhanced by the Nitrotec process. The hard

compound layer is resistant to wear, galling,

scoring, scuffing, has good bearing

characteristics, black oxide layer is resistant

to corrosion and nitrogen enriched diffusion

zone is strengthener. In addition to these

properties, Nitrotec process gives a

universally appealing black color to the parts.

Hardness:

Nitrotec process generates a hard nitride

layer 700-900 HV. The diffusion zone of

slightly lower hardness acts as a gradual

buffer between the hard surface and soft

core. Hardness developed is a

function of base material, alloying elements

and process temperature and time.

Figure shows a typical hardness profile after

Nitrotec treatment of different steels.

Figure 5 – Hardness profile of various steels

after Nitrotec treatment

Corrosion:

Nitrotec process improves significantly

corrosion resistance nitrocarburised parts.

Enhanced corrosion resistance is the major

benefit of Nitrotec treatment.

There are several methods to evaluate

corrosion of metals. Generally in surface

0

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200

300

400

500

600

700

800

900

0.0 0.1 0.2 0.3 0.4 0.5

Depth from Surface mm.

Ha

rdn

ess (

0.5

kg

)

817M40

709M40

605M36

080M40

Mild Steel

coating industries, accelerated corrosion

testing is carried out to check suitability

of the coating as a protective surface against

corrosion (b) to evaluate relative

(approximate) life of the coating with respect

to uncoated surface.

ASTM B117 is the common accelerated test

method which uses salt spray as the medium.

In this method parts to be tested are placed in

a closed chamber where salt solution is

sprayed continuously though nozzles. This

spray produces a severe corroding

atmosphere. Parts are periodically observed

for the start of visible corrosion product

(rust) on the surface of the part. More

resistant the coating is, more time for the

appearance of the corrosion products. This

method of testing is easy, simple and proven.

Figure 6 shows a comparison of corrosion

rates of EN 8 steel processed by different

treatments.

It is seen that nitrocarburised treatment by

itself does not give a good protection.

Incorporating oxidation and organic sealant

to nitrocarburised surfaces provide additional

corrosion resistance making it comparable to

that of hard chrome plating and medium

grade stainless steel.

In Nitrotec process, the layer formed is an

integral part of the surface. There is no

cracking or flaking. Whereas in chrome

plating the layer is deposited (built) on the

surface of the part and it has tendency to

flake off or blister or micro crack and expose

the bare metal surface to corrosion. Thus

Nitrotec is far superior to chrome plating and

is also much cheaper and cleaner than

chrome plating.

COMPARISON OF CR OF DIFFERENT TREATMENTS

0

50

100

150

200

UNTREATED NITROCARBURISED HARD CHROME NITROTEC NITROTEC S 18/8 STAINLESS

SURFACE COATING PROCESS

CO

RR

OS

IO

N R

AT

E, µ

m P

ER

Y

EA

R

Figure 6 – Comparison of corrosion rate of

EN8 steel processed by different treatments

A slightly modified version of Nitrotec

process called “S” process gives corrosion

resistance comparable to that of 18/8

stainless steel.

Figures 7 (a) and (b) show the result of salt

spray test on chrome plated and Nitrotec

treated part.

Corrosion spots started appearing in chrome

plated parts very early. Picture shows the

progress of corrosion after 24 hours of

exposure.

Figure 7(a) – Chrome plated parts exposed to

salt spray – after 24 hours

In contrast to this, Nitrotec parts (Figure

7(b)) showed no sign of corrosion spots even

after 144 hours of exposure. Our regular

production parts are tested and certified for

minimum 240 hours of salt spray exposure.

Figure 7(b) – Nitrotec part exposed to salt

spray – after144 hours

Excellent Dimensional Stability:

Traditional hardening processes where parts

are heated to high temperature and quenched

to form high strength martensite are prone to

distortion, warping and cracking due to phase

transformations and accompanying volume

change. In high temperature hardening

process retaining the parts to the original

dimension is a challenging problem.

Because Nitrotec is a low temperature

process and carried out below the

transformation temperature distortion or

dimensional changes are minimal to nil.

Therefore finished parts can be treated and

used as is in applications. Even close

tolerance precision parts can be treated

without any problem.

Wear Resistance:

High surface hardness of the compound layer

gives Nitrotec parts superior wear resistance.

Iron nitride makes the parts more resistant to

galling, scuffing and scratching. Presence of

surface micro porosity also helps wear by

retaining lubricants.

Wear resistance of nitrotec steel is well

studied. Similar improvement in wear

resistance is also observed in Cast Iron.

Figure 8 shows % weight loss vs. time of SG

iron and Gary Iron test pins with and without

Nitrotec treatment. Wear testing was carried

out in dry wear conditions in a pin on disc

equipment. Several of 6 mm x 30 mm pins

(dia x length) were processed in Nitrotec

furnace at two different processing

conditions. Treated and untreated pins were

subjected to wear test under four different

loads and four different speeds. Wear was

measured as weight loss as a function of

time. As expected, wear increased with

increasing load and rpm. Only results of high

load (2.5kg) high speed (650 rpm) is given

here.

Matrix of SG iron was ferritic with a as cast

hardness of 160 BHN while that of GI was

pearlitic with a as cast hardness of 212 BHN.

After Nitrotec, surface hardness was found to

be 684-730 and 764-798 HV in SG and GI

respectively. This resulted in better wear as

seen from the figures.

Nitrotec is a viable option for increasing

surface properties of cast iron. Added

advantage is additional corrosion protection

from oxidation treatment.

% WEIGHT LOSS vs. TIME IN SG IRON

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0.7

0.8

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TIME IN MINUTES

% W

EIG

HT

LO

SS

NITROTEC 1 NITROTEC 2 AS CAST

% WEIGHT LOSS vs. TIME - GRAY IRON

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 10 20 30 40 50 60 70

TIME IN MINUTES

% W

EIG

HT

LO

SS

NITROTEC 1 NITROTEC 2 AS CAST

Figure 8 – % weight loss as a function of

time – load = 2.5 kg at 600 rpm

Yield Strength and Fatigue Strength:

Strengthening effect of nitrogen in the

diffusion zone gives nitrotec parts higher

yield, tensile and fatigue strengths. Fatigue

strength has been reported to increase four

times.

Figure 9 gives the yield strength of treated

and untreated unalloyed low carbon steel. It

is seen from the figure that yield strength

increases as section thickness decreases. This

increase is more than three times in thin

sections. That is for the same yield (or

tensile) strength, thickness of the original

part can be reduced by opting for nitrotec

treatments. By redesigning the components,

parts can be made thinner and less costly.

MATERIAL THICKNESS (mm)

0

100

200

300

400

500

600

700

YIE

LD

S

TR

EN

GT

H (M

.p

.a.)

Strength profiles of low carbon, non-alloy steel

after Optimised Nitrotec treatments.

Untreated

Nitrotec Treated

0.5 1.0 1.5 3.0

Figure 9 – Yield strength increase after

Nitrotec treatment

Lucas first exploited this increase in yield

strength in changing the wind screen wiper

linkage part into a lower thickness part and

thereby obtaining 66% cost savings.

Similar property increase is seen in tubular

products. Figure 9 gives compression and

bending strength of 1.6mm treated and

untreated mild steel tube. Our studies have

shown that bending and compression results

of 1.6 mm untreated tube can be matched by

1.2 mm Nitrotec treated tube. This when

applied to one of the commercial two

wheeler part translated to a weight savings of

30%. A quick comparison of processing cost

(Nitrotec vs. chromium plating) translated to

an additional savings of 40%. Overall cost of

the part was reduced by 80 rupees per piece.

Figure 10 – Compression and bending

strength of Nitrotec treated (blue) and

untreated (black) MS tubes

Aesthetic Finish:

Nitrotec treatment gives an aesthetically

pleasing black appearance to parts.

Surface Finish:

With Nitrotec treatment initial good surface

finish can be retained.

Surface finish can be further improved by

Nitrotec S process. S process is another

enhancement to Nitrotec process where the

Nitrotec surface is polished to smooth finish

Bending Strength Comparisons, (1.6mm)

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600

800

1000

1200

1400

1600

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2000

0 5 10 15 20 25 30

Displacement, (mm)

Kg F

orc

e

Crush Strength Comparisons, (1.6mm)

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0 1 2 3 4 5 6 7 8 9

Displacement, (mm)

Kg F

orc

e

and reoxidised to get an extremely smooth

shiny surface (low Ra of < 0.15). By S

process surface finish rivaling that of chrome

plating can be achieved.

Figure 11 –– surface finish improvement

With Nitrotec S process

APPLICATIONS OF NITROTEC:

Nitrotec parts are widely used by automobile

manufacturers, hydraulics and actuation

system providers, machinery builders and

producers of office supplies and consumer

goods.

CLUTCH LEVER

Material: - Low carbon Steel

This is a Clutch Lever used on a 4 x 4

vehicle, the specified surface protection

being a phosphate coating. Failure in service

occurred due to wear in the pressed dome of

the lever, this was caused by high pressure

metal to metal contact with the clutch

operating mechanism. Nitrotec treatment

gave increased strength together with the

required wear resistance in the dome and

eliminated the need for phosphate coating.

SEAT SLIDERS

This was a collaborative exercise between

Lucas and Rockwell to produce strength

properties by the required in seat slider

mechanisms fitted to an executive Renault

car. These components are normally

manufactured from low carbon steel and

painted. During final assembly, a quantity of

grease is applied to the channel and this

created problems when contacted by driver

or passenger. The hardened steel ball running

between the inner and outer slide during

transverse can cause indentation in the track.

Any indentations in the track affected the

smooth running of the seat adjustment. Also,

in this application, the seat slider was used as

an anchorage point for the safety belt. The

dimensional control of the profile of the slide

was critical to ensure the smooth running of

the roller track.

Several surface hardening techniques were

tried, but all of them produced unacceptable

levels of distortion and still required a

finishing operation. Nitrotec process

parameters were developed to produce a

specific hardness profile in the substrate

layer, together with the required wear

resistance, and an aesthetic black finish. In

addition the increased yield strength ensured

that the belt anchorage requirements were

also achieved.

WEAR LINKAGE

Material: - Mild Steel.

This linkage is used in the turbo unit in small

passenger cars with the requirement of the

following properties: - Wear resistance at

elevated temperatures (300-500 ºC),

corrosion resistance, dimensional control

and aesthetic finish.

Problem encountered with these linkages was

wear and corrosion experienced at the

elevated operating temperatures. Normal

process route to produce the linkages was to

case harden followed by zinc plating.

Traditional case hardened surface tends to

soften at around 200ºC. And zinc

electroplating loses its passivation and

corrosion resistance at that temperature.

Since the epsilon iron nitride layer of

Nitrotec is stable up to temperatures of 500

ºC, thus, maintaining the surface hardness.

The Nitrotec treatment provided an excellent

remedy and still having sufficient corrosion

resistance to maintain the product

requirements.

PIVOT PINS

Material: - Plain medium carbon steel

These Pins are used in the pivot hinges on

the arms of excavating vehicles, originally

manufactured from medium carbon and alloy

steels, some being induction hardened, but,

all sheradised (zinc hot dip coating). The pin

to the hinge is an interference fit. Above said

method of production resulted in poor

dimensional control and was creating

difficulties during assembly. Nitrotec

treatment produced good dimensional control

and improved corrosion resistance.

BALL STUD

Ball studs are the pivot points in automobile

steering mechanisms, and suffer from

corrosion when the sealing mechanism fails,

which allows the ingress of water. The ball

has to maintain a high surface finish to allow

it to operate within a lubricated plastic cap, to

ensure smooth operation of the steering

mechanism. Surface finish requirement is

0.5-1.5 microns RZ.

A controlled Nitrotec process with an

innovative polishing technique allowed us to

maintain the strength and the ball surface

finish to its required standard without

removing any of the surface oxide.

Improvements in the fatigue strength also

benefit the performance of the pin.

PISTON RODS

Material: - 0.1-0.4% Low Carbon Steel.

Traditional method for the manufacture of

Gas Piston Rods is to machine the Rod from

hard chrome plated low or medium carbon

steel bar. Nitrotec processing is capable of

producing all of the design requirements for

the application: - Wear resistance – Bearing

characteristics - Surface topography -

Corrosion resistance - Aesthetic finish. The

Nitrotec ‘S’ surface treatment provides a

minimum of 25 µm compound layer with a

surface finish of 0.15 µm maximum, which is

competitive with chrome plating.

STAPLER

Material: - Aluminium Low carbon steel.

The design requirements for the magazine for

a hand stapler machine are wear resistance,

dimensional control, corrosion resistance

with an aesthetic appearance. Low

temperature carbo-nitriding followed by

chrome plating had been the traditional route,

but, this gave excessive distortion leading

upwards of 40% rejection.

Aluminium stabilised plain low carbon steel

was used and the Nitrotec process produced a

20-35 µm thick epsilon iron nitride layer,

fulfilling all of the design requirements

OTHER APPLICATIONS:

Parts shown are

Dry cleaning Hanger and Rods,

ABS Ring

Swivel Hubs, Various steel Stampings

Door locks for passenger Cars

Caliper Pistons.

HIGHTEMP FURNACES Heat Treatment

NITROTEC APPLICATIONS

STEEL STAMPINGS

SUMMARY:

Nitrotec is a surface enhancement process.

Benefits of Nitrotec process are superior

wear and corrosion resistance, bearing

characteristics, excellent dimensional

stability, improved yield and fatigue strength.

Redesigning parts to take advantage of

increased strength can result in substantial

cost savings. Excellent properties combined

with the attractive black color make Nitrotec

a popular choice material in several fields.

Nitrotec is an environmentally clean process

and is fast emerging as an alternative to

chrome plating in many applications.

ACKNOWLEDGEMENTS:

We sincerely thank Nitrotec Division of TTI

Group for sharing valuable information on

Nitrotec process and applications.

LITERATURE:

Lucas Industries British Patent No: 1011580

Nitrotec Notes, TTI Group

ASM Handbook-Volume 4, Heat Treating

Project Report, Master of Technology, M S

Ramiah Institute of Technology, Bangalore.