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449

RAMAX S - STEEL NITRIDING IN LOW-

TEMPERATURE PLASMA

Angel Petrov ZUMBILEV

Abstract: The aim of the present work is to investigate the

influence of the process of nitriding in low-temperature

plasma over the total thickness of the nitrided layer, the

thickness of the combined zone, the surface hardness, the

heat resistance and the impact strength of RAMAX S (420

F, modified – AISI ) tool steel.

Several modes of ion nitriding are considered at constant

pressure of the ammonia and at varying the time of

treatment as well as the temperature of nitriding.

The results show that after ion nitriding of RAMAX S steel

at temperature of nitriding 5000С , pressure of ammonia

in the chamber 400 Ра

and time of treatment 8h, thehighest level of micro hardness HV 0,1= 12000 M Р a is

obtained . It is also established that after nitriding in low-

temperature plasma the heat resistance increases, while

the impact strength of the steel reduces by 12%.

Key word : plasma, nitriding, RAMAX S tool steel

1. INTRODUCTION

The tools for material processing (prints, press moulds,

nozzles, squirts, swages etc.) work under very heavy

conditions and therefore the requirements towards the

choice of the material for a particular tool are very high

and they can be only satisfied by choosing steels with

heightened content of carbide forming elements. One of

these steels is RAMAX S (420 F ,modified – AISI), which is with

heightened content of chromium (17%) and this fact determines this

type of steel as corrossion-resistant.

In most cases steel is delivered in thermally treated condition of

hardness 32-35 HRC. In this condition it can be very well

mechanically processed because of the heightened content of

sulphur in its chemical composition. It is chiefly used for producing

squirts and details from polyvinylchloride. Data about plasma

nitriding of RAMAX S steel are not found [1, 2, 3 , 4, 5, 6, 7].

The aim of the present work is to investigate the influence of the

process of nitriding in low-temperature plasma over the total

thickness of the nitrided layer, the thickness of the combined

zone, the surface hardness, heat resistance and impact strengthof RAMAX S tool steel.

2. METHODOLOGY OF THE

INVESTIGATIONS

2.1. Materials under Investigation and Modes of

Thermal Treatment and Ion Nitriding

RAMAX S steel was used for the investigations. The chemical

composition of the steel was studied by means of equipment for

automatic analysis “Spectrotest” and the results are given in

table 1. Test samples from the steel with sizes 20 х 20 х 10 mm

and surface roughness of Ra= 0.63 µm were produced.

The test samples were thermally treated in a chamber

furnace in air medium according to the modes, given in

table 2.

The samples were ion nitrided afterwards in the installation

“ION – 20” according to the modes presented in table 3.

2.2. Metallographic Investigations

In order to establish the morphological characteristics of the

nitrided layers metallographic analysis was carried out.

For clearing out the structure and the thickness of the nitrided

layers a microscope "Axioskop" was used and metallographic

pictures were taken by means of it.

The thickness of the nitrided layer was defined through

the depth at which hardness equal to the core plus 500

MPa was obtained.

Measuring the micro hardness of the nitrided samples was

carried out by means of a micro-hardness meter

"Shimadzu" with a load of 0.98N following the “Vikers” method.

During the process of defining the heat resistance of the nitrided

layer, the level of hardness of the additionally tempered samples

was measured by the Vikers method with a load of 49.05N.

The impact strength of the thermally treated and ion nitrided

steel was defined by using standard samples with sizes of 55 х

55х10 mm and a V-shaped opening. The test on impact three-

point sagging was done with a fly hammer ИО 5003 with

maximum energy of 150 J.



450

3. EXPERIMENTAL RESULTS AND

DISCUSSION

3.1. Thermal Treatment of the Test Samples

The results from measuring the hardness of the test

samples after hardening and tempering are given in

table 4.

The micro structure of RAMAX S steel after hardening

and high temperature tempering is given by fig. 1 a. From

the figure it can be seen that the structure of the bettered

steel is fine-grained with grain rating 10-11, whichensures uniform distribution of nitrogen in depth and

favorable flowing of the process of diffusion.

3.2. Ion Nitrided Test Samples

The maximum surface hardness HV0.1 and the total

thickness of the nitrided layer - δtot were defined through

the in-depth measured micro hardness of the thermally

treated and nitrided samples. The in-depth measured

micro hardness remains nearly constant and sharply

changes at the moment of transition into the main material

(fig. 2a)

The results from the investigation of the nitrided steel are

given in table 5.

On the basis of the results (table 3) and the metallographic

analysis it becomes clear that under the first mode of

nitriding of RАМАX S steel (temperature of nitriding

5500С, ammonia pressure 400 Ра and time of treatment

Chemical elements, weight percentage

Steel

С Si Сг Ni S Р V Mn Мо

RAMAX S

4 20 F - AISI 0,34 0,35 17,1 0,55 0,12 0,02 0,47 1,37 0,17

SteelHardening

t ,0С

Cooling

medium

Tempering

t ,0С

Cooling medium

RAMAX S 1030 Oil 620 Air

Hardness , HRCSteel

Hardening Tempering

RAMAX S 52 35

№ of the mode

Tempersature of

nitriding

t nitrogen., °С

Ammonia pressure

- РNH3, Ра

Time of treatment - τ,

h

1 550 400 4

2 550 400 8

3 500 400 4

4 500 400 8

№ of the

mode from

table 3

t nitr оС

РNH3

Ра

τ

h

HV0.1

MPa

δtot

µm

1

2

3

4

550

550

500

500

400

400

400

400

4

8

4

8

10000

10100

9500

12000

60

100

50

70

Table 1. Chemical composition of the steel

Table 2. Modes of preliminary thermal treatment

Table 3. Modes of ion nitriding

Table 4. Hardness after preliminary thermal treatment

Table 5. Results from the ion nitriding of the test samples



451

4h) a layer with thickness δtot = 60 µm and maximum

micro hardness HV0.1 = 10000 MPa is obtained. It can be

noted that at the same temperature of treatment and

ammonia pressure but at prolonged time of detention 8h

(the second mode, table 3), the total thickness of the layer

increases δtot = 100µm and the micro hardness also

increases a bit HV0.1 = 10100 MPa. The combined zone of the nitrided layer is probably very thin, since it cannot be

seen by means of the metallographic microscope – fig.1.b.

After ion nitriding of RАМАX S steel following the third

mode (temperature of nitriding 5000С, gas pressure 400Ра

and time of treatment 4 h) the thickness of the nitrided

layer is δtot = 50µm and the maximum micro hardness is

HV0,1= 9500 MРa. Under this mode of treatment the total

thickness of the layer is smaller than the one, obtained

under the first mode of nitriding. This is due to the lower

temperature of saturation of the steel with nitrogen

(smaller coefficient of diffusion of the ammonia into the

steel), as well as to the heightened content of alloying

constituents (>19%) in the steel. Together with the

increase in the time of treatment to 8h (the fourth mode,

table 5) a nitrided layer with a higher total thickness δtot

= 70µm and the highest maximum micro hardness HV0,1 =

12000 MРa than in all the other modes of nitriding is

obtained. The high micro hardness is due to the lower

temperature of saturation (5000С), at which the process of

diffusion of nitrogen in depth is impeded. This leads to

satiating of the steel surface with nitrogen and formingthin lamellar nitrides (carbonitrides) of the alloying

constituents (Сr, Mo, V) which are coherently connected

to the matrix. At the higher temperatures (5500С) of

saturation with nitrogen this coherence is broken and

hardness decreases. Under these two modes of nitriding

(the third and the fourth) a very thin, broken and hardly

noticeable white zone is observed – fig. 1 b, c.

The metallographic analysis of the test samples shows

that for the chosen modes of ion nitriding the distribution

of micro hardness at a definite distance from the surface

remains approximately the same and suddenly drops indepth. This type of distribution of the micro hardness of

the nitrided layer leads to an increase in the wear

resistance and contact load resistance of the layer – fig.

2a. In the nitrided layer of the steel СrN is formed (Fig.4

), which leads to reducing the corrosion resistance of the

steel .

From the results and the metallographic analysis it could

be noticed that RАМАX S steel can be well nitrided in

low temperature plasma. Depending on the particular

mode of treatment a nitrided layer with high level of

hardness HV= 9500 - 12000 МPа, small total thickness

δtot = 60 -100µm and not formed combined zone is

obtained.

3.2.1. Heat resistance of the nitrided layer

The heat resistance of the nitrided layer was defined after

measuring the hardness (HV5) of the additionally

tempered at different temperatures (5500, 6000, 6500,

7000, 7500С) nitrided test samples. The four modes of

additional tempering of the nitrided steel were with 4hduration of detention period. The heat resistance was

found through the limiting temperature of additional

tempering, at which within the period of 4h the same

hardness of 5500 HV5 was retained.

On the basis of the obtained results curves were drawn up

in order to define the heat resistance of the nitrided layer

through the hardness of the test samples (measured at

room temperature) in dependence on the temperature of

additional tempering – fig. 2b.

The results for the heat resistance of the nitrided test

samples are given in table 6.

From table 6 it can be seen that under the first and second

modes of nitriding, conducted at the higher temperature of

treatment, better results for the heat resistance of the

nitrided layer are obtained. The highest heat resistance is

observed at: Tnitr = 5500С, Р = 400 Ра, τ = 8 hours. This

is due to the heightened content of alloying constituents

(Сr, V, Mo) in the steel, which increase the solvability of

the nitrogen in the hard solution and form at the higher

temperature of nitriding a bigger amount of special

nitrides with higher temperature of dissociation. It can be

noted that the positive influence of the process of ion

nitriding on the heat resistance of RAMAX S steel is

related to the considerable amount of alloying

constituents in the hard solution, the heightened resistance

of the solution containing nitrogen, and the slow

coagulation of the nitride and carbide particles, containing

vanadium, molybdenum, chromium or manganese. It

slows down the processes of softening the nitrided layer

and determines the high value of its heat resistance.

3.2.2. Impact strength of ion nitrided samples

The results obtained from testing the thermally treated

and ion nitrided test samples to impact three-point

sagging are given in table 7.

From table 7 it can be noticed that after ion nitriding of

RAMAX S steel, the impact strength of the nitrided

samples reduces to 12%. It shows that the nitrided layer

with its parameters (micro hardness, total thickness,

thickness of the combined zone) formed this way, does

not influence significantly the impact strength. A bigger

reduction of the impact strength (12%) is observed



452

under the mode 2 (temperature of nitriding 550 0С,

ammonia pressure 400 Ра and time of treatment 8h),

which is due to the bigger thickness of the nitrided layer

δtot = 100µm.

The nitrided layer formed on the surface is with higher

brittleness and higher module of elasticity than the basic

material [2,3,8]. The nitride (carbon nitride)

precipitations in the diffusion zone of the nitrided layer

are the concentrator of the pressures, which influences

favorably on the formation and the propagation of the

crack.

Table 6. Results for the heat resistance of the nitrided samples

№ of the

mode from

table 3

Tnitr,

оС

Р,

Ра

τ,

h

HV0,1*

MРa

δtot *

µm

Heat resistance at 5500

HV5/ 4 hоС

1 550 400 4 10000 60 715

2 550 400 8 10100 100 730

3 500 400 4 9500 50 650

4 500 400 8 12000 70 720

Note: * - hardness and thickness of the layer before additional tempering

Table 7. Results for the impact strength of nitrided

№ of the mode

from table 3

tnitr,оС

Р,

Ра

τ,

h

HV0,1

MРa

δtot

µm

KCV,

MJ/m2

1. 550 400 4 10018 60 0.28

2. 550 400 8 10097 100 0.26

3. 500 400 4 9460 50 0.29

4. 500 400 8 12006 70 0.28

Not nitrided

sample- - - 35HRC - 0.30

Fig.1 Microstructure of RAMAX S steel after hardening and tempering - а and after ion nitriding at:

b - t = 5000С , Р NH3 = 400 Ра , τ = 4h; c- t = 550

0С , Р NH3 = 400 Ра , τ = 8h



453

0

100

200

300

400500

600

700

800

900

1000

1100

1200

0 15 30 45 60 75 90 105 120 135

Distance from the surface, µ m

M i c r o h a r d n

e s s Х 1 0 Н V 0 , 1 , М Р а

RAMAX S

t = 5500С

Р = 400 Ра

t = 8 h

a b

Fig.2. Distribution of hardness in depth of the nitrided layer - а ,

b – Dependence between hardness and temperature of additional tempering

Fig.3. XRD analysis of nitriding steel 420F

4. CONCLUSIONS

4.1. It is proved that RАМАX S steel can be nitrided in

low-temperature plasma. Depending on the mode of

treatment a nitrided layer with high micro hardness

HV0,1= 9500 - 12000 МPа, low total thickness δtot = 60 -

100µm and not formed combined zone is obtained.

4.2. It is established that after ion nitriding of RAMAX S

steel at temperature of nitriding 500 0С, ammonia pressure

400 Ра and time of treatment 8h, the highest micro

hardness HV0,1= 12000 MРa is obtained.

4.3. It is proved that the highest heat resistance of the

nitrided layer (7300С) is obtained at temperature of

nitriding 550 0С, ammonia pressure 400 Ра and time of

treatment 8h.

4.4. It is established that after nitriding of RAMAX S

steel in low-temperature plasma the impact strength

reduces and, depending on the mode of treatment, it can

reach 12%.

0

200

400

600

800

1000

1200

550 650 750

Temperature of additional tempering, oC

H a r d n e s s x

1 0 - H V 5 , М

P a

730

RAMAX S

t = 550°С Р = 400 Ра

τ = 8h



454

REFERENCES:

1. RAMAX S, Prehardened stainless holder steel, Tool

Steel Facts, Uddeholm, 2001.

2. Buchkov D, Toshkov V., Ion nitriding, S., Technique,

1990.3.Toshkov V., Theoretical and practical aspects of the

process of nitriding iron and iron-carbon alloys in lowtemperature plasma, Thesis, Sofia, 1997.

4. Toshkov V., Nitriding in low temperature plasma,

King, 2004.

5. Fisher-ChatterjeeP.,W.Eysell,u.a.,Nitrieren und

Nitrocarburieren, Sindeifingen, Expert Verbag, 1994

6. Hocman R.F., Effects of Nitrogen in Metal Surfaces,

Proceeding of an International Conference on

Ion Nitriding, Cleveland, Ohio, USA 15-17,

September 1989, pp 23-30.7. Nitriding and Nitrocarburising, Contract Heat

Treatment Association,Aston University, Birmingham,

1996.

8. Lozev M.,A.Zyumbyulev, V.Toshkov and L.Boev,Fracture mechanic of ion-nitrided steel. MaterialsScience,

Volume 28, Number 2 \ March ,1993, pp116-119.

CORRESPONDENCE

Angel ZUMBILEV, Assoc. prof. Ph.D.

Technical University of Sofia – Plovdiv

Branch, Tzanko Dustabanov Str. 25,

Plovdiv, Bulgaria

[email protected]

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