UDC 621.785.533

ADVANCEMENT OF THE PROCESS OF CARBONITRIDING

S. G. Tsikh,1 V. I. Grishin,2 A. V. Supov,2 V. N. Lisitskii,1 and Yu. A. Glebova1

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 9, pp. 7 12, September, 2010.

Low-temperature processes of surface hardening in molten salts (carbonitriding, Tenifer-QPQ, etc.) are con-

sidered. Results of tests of these processes are compared with those of galvanic (chromizing, nickelizing, etc.)

processes. The equipment for and examples of commercial use of processes of liquid carbonitriding are pre-

sented.

Key words: carbonitriding, oxidizing, grinding, polishing, wear resistance, corrosion resistance.

INTRODUCTION

The task of improving the operating properties of metal-

lic materials in modern mechanical engineering is closely re-

lated with development of new processes of surface harden-

ing of parts produced from them.

One such process has been designed by professor

D. A. Prokoshkin at the Bauman Moscow State Technical

University and is known as liquid carbonitriding. The method

of liquid carbonitriding essentially consists in subjecting ma-

chine parts from structural, stainless, heat-resistant tool, and

high-speed steels to heating in molten salts based on potas-

sium cyanate and carbonate at 540 600C with a hold for

5 40 min for cutting tools or for 1 6 h for machine parts

and dies depending on the required thickness of the hardened

layer.

Liquid carbonitriding is used for raising wear resistance

and fatigue resistance. In combination with oxidizing it is

used for raising corrosion strength. In many cases liquid

carbonitriding is an alternative to surface hardening, galvanic

chromizing, carburizing, nitrocarburizing, etc.

Prokoshkins composition of the melt for liquid carboni-

triding (75 85% KCNO + 15 25% K2CO3 ) [1] has been

accepted with some alterations by some foreign producers.

Specifically, the Durferrit Company (Germany), which is a

world leader in the field of liquid processes of surface engi-

neering, actively exploits the Tenifer technology (a virtu-

ally full counterpart of the Russian process of liquid carbo-

nitriding) all over the world [2].

RESULTS AND DISCUSSION

Liquid carbonitriding yields a hardened layer consisting

of several zones on the surface of the steel (Fig. 1). The ex-

ternal layer is represented by -carbonitride of type Fe3(N, C);

then goes a zone of -phase of type Fe4(N, C), under which

we find a diffusion (heterophase) layer consisting of a solid

solution of carbon and nitrogen in iron with inclusions of

carbonitride phases, the hardness of which exceeds consider-

ably the hardness of the core.

The concentration of nitrogen and carbon in the carbo-

nitride layer decreases markedly upon growth in the distance

from the surface. It should be noted that the process of

carbonitriding develops according to the laws of nitriding,

i.e., of the two elements (nitrogen and carbon) capable to sat-

urate the metal nitrogen diffuses into the surface layer domi-

nantly and forms nitrogen-base phases. In the thin surface

Metal Science and Heat Treatment Vol. 52, Nos. 9 10, 2010

408

0026-0673/10/0910-0408 2010 Springer Science + Business Media, Inc.

1NPO TsNIITMaSh Joint-Stock Company, Moscow, Russia

(e-mail: termohim@bk.ru).2

N. . Bauman Moscow State Technical University, Moscow,

Russia.

100

90

80

70

60

50

40

30

20

10

0 10 20 30 40 50

Porous

zone

h, m

Fe; C; N, wt.%

Fe

C

N

Fe

possible location

of N

Fig. 1. Structure of steel 40Kh and distribution of nitrogen, carbon

and iron in the hardened layer after liquid carbonitriding at 580C,

1.5 h (h is the distance from the surface).

layer (up to 15 m) iron is present primarily in a bound con-

dition in the form of carbonitrides distributed in the layer in a

manner typical for diffusion hardening.

It should be noted that in the process of liquid carbonitri-

ding carbonates are accumulated in the pool due to oxidation

of the cyanates, and this lowers considerably the reactivity

and the fluidity of the melt. Regeneration of the pool is a key

moment of the process of liquid carbonitriding at industrial

enterprises. The main components of regenerating composi-

tions are ammonocarbon compounds such as melamine,

melon, and dicyandiamide, which reduce the carbonates to

cyanates.

The reaction of regeneration can be represented schema-

tically as

carbonate (CO3

2) + regenerator cyanate (CNO

).

Control and timely regeneration of the melt ensure qual-

ity hardened layers.

In order to provide corrosion properties in carbonitrided

parts it is recommended to perform oxidizing in a nitrite-al-

kaline melt at 350 400C or in an aqueous solution at

130 150C.

The method of liquid carbonitriding followed by oxidiz-

ing has been called a NOK-process (low-temperature oxy-

carbonitriding) [3].

In addition to giving corrosion properties to the treated

parts oxycarbonitriding promotes complete removal of the

remnants of cyanides, which guarantees their absence in in-

dustrial waste and hence in the sewage system.

The main reactions that occur in the molten salts during

oxidizing can be represented in the following form:

(1) detoxication reactions

cyanide (CN ) + nitrate (NO3

) carbonate (CO3

2) +

nitrite (NO2

);

cyanate (CN ) + nitrate (NO3

) carbonate (CO3

2) +

nitrite (NO2

);

(2) oxidation of the carbonitride layer

iron nitride (FexN) + nitrate (NO

3

) magnetite (Fe3O4) +

nitrite (NO2

);

(3) regeneration of the pool

nitrite (NO2

) + oxygen (O2 ) nitrate (NO3

).

Thus, the reactions occurring during the hold of the parts

in a nitrite-alkaline melt ensure decomposition of the cya-

nides, cyanates, and nitrites into base nontoxic substances.

It should be noted that carbonitriding worsens the rough-

ness of the surface from Rz

= 1.6 to Rz

= 3.2 m on the aver-

age. The roughness of the surface can be restored after oxi-

dizing by various methods of polishing, namely,

rubbing with abrasive paper with grain 360 or finer;

polishing or fine grinding by special polishing discs in

a continuous manner similar to centerless grinding or grind-

ing in automatic lathes;

sliding grinding in a vibrating vessel (such treatment is

applied primarily to small and sheet parts);

blasting with glass balls 40 70 m in diameter.

Grinding and polishing may deteriorate the corrosive

properties of the parts and they are subjected to second oxi-

dizing. In Germany such mode of treatment is known as a

TENIFER-QPQ process, which includes carbonitriding, oxi-

dizing, polishing, and oxidizing and is represented schemati-

cally in Fig. 2 [2].

In Russia this process is known as NOK-PQ and is a

complete analog of TENIFER-QPQ.

All low-temperature processes of surface hardening de-

veloped and used in Russia and abroad are based on the main

principle of oxidation of cyanides and cyanates, which yields

active nitrogen and carbon atoms in the working melt. This

principle has first been implemented in the method of liquid

carbonitriding at the Bauman Moscow State Technical Uni-

versity and then was spread widely all over the world under

different names.

The equipment for conducting the process is presented in

Fig. 3.

Domestic equipment differs from foreign one, specifi-

cally, from that produced by the Durferrit Company (Ger-

many) in the degree of automation and in the cost.

In the absence of waste disposal plants the facilities may

be supplemented with an evaporator of industrial waste

(Fig. 3). In this case the process of surface hardening does

not yield liquid waste. The solid waste formed due to the

evaporation meets danger class 3 (for a five-point scale),

which is safe and can be utilized at any ground for industrial

waste.

The properties of parts after carbonitriding depend much

on the degree of alloying of the steel. The higher the content

of nitride-forming elements (Cr, V, Mo, Al, Ti, W, Mn) in the

composition the lower the thickness of the hardened layer

and the higher its hardness (Fig. 4).

Formation of the structure of the diffusion layer is af-

fected by the rate of cooling after the carbonitriding. After

rapid cooling in water nitrogen remains in the solid solution,

i.e., the layer is formed by a diffusion mechanism (formation

Advancement of the Process of Carbonitriding 409

m

pera

ture

Time

Preli

min

ary

heati

ng

Oxid

izin

g+

coo

lin

g

Repeate

d

ox

idiz

ing

Poli

sh

ing

Ten

ifer

Tenifer Q P Q

Air,

350

400

xide,

380

420

xide,

380

420

TF1,

580

Fig. 2. Scheme of the Tenifer-QPQ process.

of solid solutions and nitrides) and by a diffusionless (har-

dening) mechanism, which results in additional hardening of

the layer.

In the case of slow cooling or subsequent tempering a

part of nitrogen can be segregated in the form of veins of iron

nitride in the external part of the diffusion layer on plain

steels. This makes the surface layer virtually nonbrittle.

On high-alloy steels the hardened layer forms by only a

diffusion mechanism and the cooling rate does not virtually

affect the properties of the layer.

The results of a study of corrosion resistance of diffe-

rently treated test pieces of steels 45 and 38Kh2MYuA in a

5% solution of NaCl (a sea water type) show (Fig. 5) that the

operations of oxidizing and oiling increase the corrosion re-

sistance of plain steel 45 by up to a factor of 4, whereas for

alloy steel 38Kh2MYuA the effect is somewhat lower (growth

by a factor of 1.5 2).

According to the data of researchers of the Durferrit

Company the corrosion resistance of coatings deposited by

the Tenifer-QPQ process [2] is tens of times higher than after

various variants of chromizing (see Table 1).

The most rigid corrosion test of DIN 50021 is the CASS

one, where the test solution contains additional acetic acid

and copper chloride (5% NaCCl + 0.26 g Cu2Cl2, pH =

3.11 3.3), and the test temperature is 50C. The results of

comparative tests of parts treated by the method of QPQ and

of piston rods treated by solid chromizing with layer thick-

ness of 10 12 and 30 35 m, respectively, have shown

high efficiency of QPQ coatings in aggressive media.

In the case of long-term testing according to DIN 50905

(part 4) the corrosive medium is a 3% solution of common

salt + 0.1% solution of hydrogen peroxide (H2O2 ) (see Ta-

ble 1). None of the coatings participating in the test had cor-

410 S. G. Tsikh et al.

TABLE 1. Results of Testing of Steel S45 for Corrosion Resistance

in a Medium Containing 3% NaCl + 0.1% H2O2

Kinds of surface hardening

Decrease in the

mass, gm2,in 24 h

TENIFER-QPQ 0.34

Solid chromizing (hc

= 12 m) 7.10

Double chromizing: hc

= 20 m of soft chromium +

hc

= 25 m of hard chromium 7.20

Nickelizing (hc

= 20 m) 2.90

Triple coating: hc

= 37 m of copper + hc

= 45 m

of nickel + hc

= 1.3 m of chromium 1.45

Heating

furnace

Wet cleaning

of outgoing air

Vessel

for collecting

used water

Carbonitriding

tankOxidizing

tankCooling

tank Washing cascade

Fresh water

Evaporator

Exhaust

Fig. 3. Diagram of the facility for

processes of oxycarbonitriding and

TENIFER-QPQ.

1200

1000

800

600

400

200

0 100 200 300 400 500

h, m

HV0.12, kgf mm

12

3

45

6

Fig. 4. Distribution of hardness over the thickness of hardened

layer of steel and cast iron after carbonitriding at 580C, 3 h (h is the

distance from the surface): 1 ) Cr Ni iron; 2 ) 12Kh18N10T;

3 ) 30KhGSA; 4 ) 40Kh; 5 ) 3Kh2V8; 6 ) 20.

rosion characteristics equal to those of the test pieces treated

by the QPQ process.

We performed field tests of shock strut pistons subjected

to oxycarbonitriding (Fig. 6). The oxycarbonitrided coatings

had corrosion characteristics considerably superior to those

of coatings deposited by standard processes (galvanic chro-

mizing, nitriding); long-term field tests have shown full ab-

sence of leakage in the parts.

Oxycarbonitrided coatings are very effective for raising

antifriction properties and wear resistance. The nonmetallic

structure of the joining (oxycarbonitride) layer lowers the

friction factor and the susceptibility to fusion with the mating

nonmetallic surface.

Wear tests including field ones have confirmed high

wear resistance of parts and assemblies after liquid oxy-

carbonitriding as compared to hardened state and to chro-

mized surfaces. In many cases the wear resistance of the

joining (carbonitride) layer is increased additionally due to

an additional oxidizing treatment. For example, the carbo-

nitrided parts of gears, hydraulic units and press molds serve

for a longer time than those subjected to solid chromizing.

When studying wear resistance and antifriction proper-

ties it is interesting to determine the friction factor of the ex-

ternal layer. The interaction on the interface, which arises

due to sliding, depends more on the structure, on geometry of

the surface, and on the lubricant than on the hardness of the

rubbing pair.

The Durferrit Company has studied the antifriction pro-

perties of various kinds of coating [2]. The tests were per-

formed in an Amsler device for disc specimens. One disc

was immobile and the other rotated in contact with the for-

mer at a speed of 200 rpm. The load was 5 30 N in order to

avoid adhesive wear. Both discs had been treated similarly.

The specimen after chromizing, hardening + low-tempera-

ture tempering, and a Tenifer process with and without oxi-

dizing had a roughness Rz

= 3.2 m. After the treatment in

the Tenifer-QPQ mode the roughness of the surface of the

specimens was lowered by polishing to Rz

= 0.8 m. In the

mode of dry friction the specimens treated by the Tenifer

method had much lower friction factors than the specimens

with martensite structure or with chromized layer. Oxidizing

raised the friction factor (Fig. 7).

A study of the antifriction properties under conditions of

friction with a lubricant has shown that the specimens treated

by the Tenifer-QPQ method have the lowest friction factor.

Advancement of the Process of Carbonitriding 411

0.12

0.10

0.08

0.06

0.04

0.02

0.08

0.07

0.06

0.05

0.04

0.03

0.02

0.01

0 150 300 450

0 150 300 450

b

test , h

test , h

vcr2, g (m h)

vcr2, g (m h)

6

6

1

1

2

2

3

3

5

5

4

4

Fig. 5. Corrosion resistance of steels 45 (a) and 38Kh2MYuA (b )

after hardening by various methods: 1 ) heat treatment; 2 ) liquid

carbonitriding; 3 ) liquid carbonitriding + oxidizing; 4 ) liquid

carbonitriding + oxidizing + oiling; 5 ) NOK (carbonitriding + oxi-

dizing + polishing + oxidizing); 6 ) liquid carbonitriding + polishing.

100

90

80

70

60

50

40

30

20

10

024 48 72 96

S, %

test , h

1

1

1

1

2 2 2 23 3 3 3

Fig. 6. Results of corrosion tests of shock strut pistons subjected to

gas nitriding (1 ), galvanic chromizing (2 ), and oxycarbonitriding

(3 ) (S is the area stricken by corrosion).

0.4

0.3

0.2

0.1

0

ffr1

1

1

1

1

2

22

22

Galvanic

chromizing,

30 m

Hardening +

low-temperature

tempering

Without

oxidizing

With

oxidizing

QPQ

Tenifer process

Fig. 7. Friction factors ffr

of various coatings on a steel of type

40Kh: 1 ) without lubricant; 2 ) with lubricant (oil of type SAE 30).

It should be taken into account that oxycarbonitriding in-

creases substantially (almost doubles) the fatigue resistance

in contrast, for example, to galvanic chromizing and zincing.

For this reason the technology of liquid oxycarbonitri-

ding and its foreign counterparts find wide use in all

branches of industry, e.g., in power engineering, car making,

food industry, aviation, agricultural machines.

Figure 8 presents examples of successful application of

carbonitrided coatings in Russia and abroad.

CONCLUSIONS

Thus, the process of oxycarbonitriding is a universal

technology for any branch of industry. It is applicable for

hardening parts from any grade of steel, which operate under

conditions of wear including corrosive one. The costqualityratio of the oxycarbonitriding process is the best among the

hardening technologies.

REFERENCES

1. D. A. Prokoshkin, Thermochemical Treatment of Metals:

Carbonitriding [in Russian], Metallurgiya, Mashinostroenie,

Moscow (1984), 240 p.

2. U. Baudis und M. Kreutz, Technologie der Salzschmelzen:

Warmebehandlung, Hartetechnik, Warmeubertragung, Reinu-

gung, LandsbergLech: Verl. moderne Industrie (2001), 84 p.

3. A. V. Supov, Creation of the process of carbonitriding, in:

Metal Science. Heat and Thermochemical Treatment of Alloys

[in Russian], Izd. MGTU Im. Baumana, Moscow (2003),

pp. 142 151.

412 S. G. Tsikh et al.

b

Fig. 8. Examples of using oxycar-

bonitride coatings in car industry (a),

armature engineering upon manu-

facturing die tooling (b ) and press

mould (c).