INFLUENCE OF NITROGEN ALLOYING ON GALLING
PROPERTIES OF PM TOOL STEELS

I. Heikkil¨a and L. Slycke

Swedish Institute for Metals Research

Stockholm

Sweden

O. Sandberg

Uddeholm Tooling AB

Hagfors

Sweden

Abstract

The objective of this study is to investigate the effect of nitrogen alloying of
tool steels on their galling properties. Nitrogen is introduced to the steel com-
position by solid state nitriding with ammonia gas at 550–600

◦

C. The base

material composition is in powder form during nitriding. Different nitrogen
contents are experimented. The hard phases present in the steel are of type
M(C,N) and M

6

C. The materials are consolidated by HIP process. Galling

behaviour of the produced alloys is evaluated with slider-on-sheet tribological
test method. As a reference for nitrogen alloyed compositions various PM
tool steel grades and CVD coated tool steels are chosen. The ability of the
test materials to avoid formation of galling adherants is tested against AISI
304 stainless steel with a low viscous lubricant. The test results show that ni-
trogen alloying significantly increases the resistance against galling reaching
equal behaviour as for CVD coated tool steels.

Keywords:

carbonitride, PM tool steel, nitriding, galling, stainless steel

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6TH INTERNATIONAL TOOLING CONFERENCE

INTRODUCTION

Galling is one of the major causes for tool failure in sheet forming pro-

cesses for stainless steel. Galling develops gradually as accumulation of
sheet material pick-ups on the tool surface during succesive forming oper-
ations. The sheet material transfer process is considered to consist of an
initiation stage, a stable stage of accumulated growth, and a final stage when
the large pick-ups start to cause aesthetic problems at the workpiece and
establish unstable friction conditions for the forming process. [1].

The tribological phenomena taking place between the tool and sheet sur-

face is a complex process as surface interactions are affected by multiple
factors. The composition and surface characteristics of both the tool and
sheet material play an important role as well as lubrication. Also, geometry
of the tool, influencing stresses and temperature are of importance [2, 3, 4].

The focus on improving tribological properties of the sheet has been on

modifying surface topography for better lubricant retention and permeability
at the contact area. The material properties of austenitic stainless steel are
problematic with respect to galling. The thin surfaces oxides of austenitic
grades, low thermal conductivity, high rate for work-hardening and high
percentage elongatation after fracture increase the tendency for formation
of galling adherants. Nevertheless, occurrence of galling can successfully
be delayed by comprehensive optimatisation of the tool-sheet-lubricant in-
teraction phenomena [2, 5, 6, 7, 8].

Cold work tool steels are generally used as tool material in various forming

situations. However, tool life can be increased by applying a CVD or PVD
coating on the tool steel substrate. The good sliding properties of the coating
are lost as soon as the coatings is flaked away as a result of fatigue. The
coating cannot be refurnished. The anti-galling properties of plain tool steels
can be improved by increasing the content hard constituents, controlling their
composition, dimensions, shape and distribution [9, 10, 11].

PM technology offers possibilites to tailor unique microstructural fea-

tures. The objective of this study is to examine the effect of nitrogen alloying
of PM tool steel to resist galling against stainless steel. Nitrogen is intro-
duced into the alloy system by nitriding gas-atomised powder particles in
ammonia gas. Hard carbonitrides will be formed in the strucre during con-
solidation of the powder. Galling resistance is examined by slider-on-sheet
tribometer [12].

Influence of nitrogen alloying on galling properties of PM tool steels

257

TEST MATERIALS

The effect of nitrogen additions on the galling behaviour of tool steels

was investigated by producing alloy compositions with a varying content
of hard phases. The alloys were designed with the aid of thermodynamic
equilibrium calculations by Thermo Calc softwear. The constitution of the
alloys was adjusted such that two different hard phases will be formed on
martensitic matrix at the austenisation range. The hard phases are M(C,N)
carbonitride and M

6

C type of carbide. The letter M stands for the metallic

component of the carbide and it will constitute of several elements. Table 1
shows the chemical composition of the designed alloys and the content of
the hard phases according to Thermo Calc. The composed alloys were

Table 1.

Compostion of nitrogen alloyed test materials and their content of hard phases

according to Thermo Calc

Alloy

C

N

C

Mo

W

V

M(C,N)

M

6

C

Tot %

Vancron 1

1.28

1.34

4.37

2.80

4.10

7.84

18.3

4.0

22.3

Vancron 2

0.58

3.23

4.37

3.10

4.10

8.8

24.9

3.9

28.8

Vancron 3

1.96

2.82

4.18

3.20

4.80

15.20

33.1

3,9

37.0

designated as Vancron alloys.

The test alloys were manufactured by gas-atomising the base alloy com-

position and later nitriding the atomised powder. Nitriding was performed
as solid-state nitriding in ammonia gas at 550

◦

C. The nitriding equipment

consists of a chamber charged with powder material and a heating furnace
around. The chamber is furnished with in-let channel for ammonia feed at
the bottom of the chamber and out-let channel for the exhaust gases at the
top. Figure 1 presents a schematic picture of the nitriding equipment.

The transfer process of nitrogen from gas phase into the solid material

involves ammonia dissolution reactions on the particle surfaces and diffusion
controlled transfer phenomena inside the metal particles. The size of the
powder batch was 10 kg. Most of the ammonia dissolutes near the in-let at
bottom of chamber. Therfore, nitrogen content will be considerably higher
at the bottom of the chamber than in the top sections [13].

The nitrided powder was mixed uniformly in a rotary mill unit. The nitro-

gen content of the batch was analysed. In case the analysed nitrogen content
exceeded the aimed value, an appropriate amount of unnitreded powder was

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6TH INTERNATIONAL TOOLING CONFERENCE

Figure 1.

Nitriding equipment.

added in the mixture to achieve the desired content. In case nitrogen content
was below the aimed value, a second nitriding was acccomplished.

The consolidation of the test materials was done by HIP process. The

consolidated material billets were forged and soft-annealed to facilitate ma-
chining. Test materials were hardened at 950–1100

◦

Cand tempered 3 × 1

h at 560

◦

C. Figures 2 to 4 represent microstructures of Vancron 1 to 3 after

heat treatment.

The black phase in the structure is M(C,N) carbonitride and the white

phase is M

6

C carbide. The size of M(C,N) is small less than 1 µm. The

content of hard phases is clearly highest for Vancron 3.

The reference materials for the galling evaluation comprise of plain PM

tool steel grades and coatings on a tool steel substrate. Table 2 presents the
composition of the reference tool steels Vandis 23 and Vanadis 6. Table 2
also presents the calculated amounts of hard phases by Thermo Calc at the
austenitising temterature. Both the Vancron alloys and reference tool steels
were heat treated to 62 HRC. The coatings chosen for testing were TiCN-

Influence of nitrogen alloying on galling properties of PM tool steels

259

Figure 2.

Vancron 1 in heat treated con-

dition. 3000×.

Figure 3.

Vancron 2 in heat treated con-

dition. 3000×.

Figure 4.

Vancron 3 in heat treated con-

dition. 3000×.

Table 2.

Composition of reference PM tool steels

Alloy

C

N

Cr

Mo

W

V

MX

M

7

C

3

M

6

C

Tot %

Vanadis23

1.28

0.05

4.10

5.00

6.40

3.10

6.5

8.8

15.3

Vanadis6

2.07

0.04

6.80

1.50

0.10

5.35

12.4

1.2

13.6

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6TH INTERNATIONAL TOOLING CONFERENCE

coating produced by CVD and VC-coating produced by Toyota diffusion
method. The substrate material was Vanadis 23.

METHOD FOR GALLING EVALUATION

The tribological test methods should reproduce the existing conditions in

real industrial processes as well as possible. The slider-on-sheet tribometer
is developed to simulate the tool-lubricant-sheet interaction generally pre-
vailing in sheet forming operations. The test method is developed by TNO
Institute of Industrial Technology. The measurement principle is presented
in figure 5 [14].

Figure 5.

Schematic picture of slider-on-sheet tribometer

A slider made of tool material is pressed against a sheet with a normal

force F

n

. The slider is moved in the x-direction with a specific sliding speed

v. As the slider meets the end of the track the slider is lifted from the sheet
and replaced over a distance of 1 mm in the y-direction. The slider is returned
to the starting point in the x-direction. The normal force is addressed again
and a next track is made over the same track length l. By this practice,
sliding distance of 1 km can be accomplished on one square meter sheet
material. The test can be carried out as dry or lubricated tests. The test
conditions used in this investigation are presented in Table 3. The normal
force F

n

100 N will create a contact pressure of 900 MPa in the given slider

geometry. This is a typical pressure existing in the critical area of a drawing
die when pressing stainless steel.

Influence of nitrogen alloying on galling properties of PM tool steels

261

Table 3.

Test conditions

Test conditions

Value

Velocity v

0.5 m/s, sliding

Normal force F

N

100 N

Sliding length l

1700 m

Lubricant

4-5 ml/m

2

on the sheet

The normal force and friction force existing in the contact area are mea-

sured with a data collection frequence of 1000 Hz. An average value for
normal and friction force is calculated over each track. Since the track length
is known, development of friction coefficient as a function of sliding distance
can be followed.

The sheet used in testing is stainless steel grade AISI 304 with a thickness

of 0,8 mm. Sliding direction is parallel to rolling direction. The R

a

value of

the sheet in rolling direction 0,10 µm and in upright direction 0,10 µm. The
lubricant is a low viscose oil Quaker Coupex 046-EP.

Roughness data of the test sliders is represented in the Table 4. The data

comprises R

a

value parallel and perpendicular to sliding direction. The

surface roughness was measured by Mitutoyo Surftest 500.

Table 4.

Surface roughness R

a

of the sliders perpendicular and parallel to sliding direction

Vancron 1 Vancron 2 Vancron 3 Vanadis 23 Vanadis 6 V23+TiCN

V23+VC

perpendic.

0.09

0.06

0.08

0.08

0.07

0.15

0.07

parallel

0.02

0.04

0.04

0.02

0.03

0.12

0.06

RESULTS

Each slider was tested in two separate experiments. The development of

friction coefficient as a function of sliding distance is represented in Figs. 6,
7, and 8 for the Vancron alloys, reference tool steel grades and coated vari-
ants, respectively. The test was interrupted as soon as the friction coefficient
reached a value of 0.2. The maximum sliding length was 2000 m. Thus, test-
ing was interrupted at 2000 m eventhough friction coefficient stayed lower

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6TH INTERNATIONAL TOOLING CONFERENCE

than 0.2. The maximum sliding distance was applied to avoid long testing
times.

At the start point of the test friction coefficient was low for the uncoated

tool steel grades; in the range 0.05 to 0.07. The development of friction
coefficient with an increasing sliding length varied a lot depending on the
steel grade. For the PM tool steel grades f rose rapidly being up to 0.2 after
20-50 m running. For the nitrogen alloyed materials f rose slighly after start
up but remained relatively stable during the rest of the test. If the f suddenly
rose to 0.2 , the test was interrupted.

The start value for f varied from 0.07 to 0.17 for the coated sliders, thus

being higher than in plain tool steels. The friction coefficient stayed stable
during the whole test and never exceeded 0.2. Yet, the friction coefficent
stayed at high level near 0.2 for the TiCN coated variant.

During the course of testing, a transfer layer of stainless steel was devel-

oped on the slider surface. Figure 9 represents an example of such transfer
layer at the end of the test. The Fig. 10 includes contour profile of the
sheet surface at the point when testing was interrupted. The transfer of
sheet material to tool surface and scratching of the sheet surface are typical
characteristics for galling.

Figure 11 summarises the sliding lengths when f stayed below 0.2 for

each test material. If f was below 0.2 at the sliding lenght of 2000 m, the test
was interrupted. Thus, all the pillars in the figure 8 with L 2000 m are tests
that have been discontinued. The materials that achieved this highest sliding
length include Vancron 1 and Vancron 2, and TiCN and VC coated variants.
It can be expected that even higher sliding distances could be achieved for
these materials if the test was continued.

DISCUSSION

The results on slider-on-sheet tribometer suggest that various tool mate-

rials can be ranked with respect to their galling resistance. The differences
between various compositions are notably. The coated tool steels and ni-
trogen alloyed tool steels showed the best resistance against galling. The
reference PM tool steel grades without coating show inferior galling resis-
tance.

The content of hard phases in the reference tool steels Vanadis 23 and

Vanadis 6 is lower than in Vancron alloys. The Vancron alloys 2 and 3 had the
highest content of carbonitride phase. They also showed the best resistance

Influence of nitrogen alloying on galling properties of PM tool steels

263

Figure 6.

Vancron alloys: development of f as a fuction of sliding length.

Figure 7.

Reference uncoated PM tool steels: development of f as a fuction of sliding

length.

Figure 8.

Coated PM tool steels: development of f as a fuction of sliding length.

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6TH INTERNATIONAL TOOLING CONFERENCE

Figure 9.

Galling adherant on Vancron 3 slider at the end of the test. 100x.

Figure 10.

Wear track of the sheet at the end point of the test.

Influence of nitrogen alloying on galling properties of PM tool steels

265

against galling. The presence of carbonitride phase in high contents and in
small particle size in the tool steel structure has a distinctly positive effect
on galling resistance.

The used tribometer is developed to reproduce the tribological conditions

typically encountered in sheet metal forming. The contact pressure is in
the plastic range of the sheet. Sliding contact between the tool material
and sheet is accomplished in a such way that the slider continuously meets
a fresh sheet material. The method is flexible to adjust test parameters
to resemble inquired conditions. The experiments are conducted at room
temperature. However, this is seldom a situation in forming processes where
bulk temperture of the tool regularly increases to 60–70

◦

C [15].

The tests were performed with a low viscose lubricant. This will accen-

tuate formation of galling adherants, since low viscose lubricants cannot
sustain a separating lubricant film between the contacting metal surfaces at
high pressures. The surface roughness of sliders is finer than in forming
tools generally. A fine surface will reduce the effect of mechanical inter-
locking on galling and thus emphasize the sliding properties of tool material
[16, 17].

The surface roughness of the sliders lay typically between 0.02 to 0.09.

The only exception was the slider with TiCN coating which had R

a

0.12-

0.15. The friction coefficient of the slider with TiCN stayed near 0.2 during
the whole testing length. The rougher surface quality correlates well to the
higher friction level. An uneven surface produces locally high stesses at the
top of asperities thus giving higher resistance for sliding. Also, lubrication
retention at the contact area is inferior.

The results on sliding length of Vancron alloys showed some scatter.

Vancron 2 and 3 were able to reach sliding length of 2000 m without galling
but also some inferior results were detected. In tribological tests, the scatter
of the results is frequently high. This is due to the fact that tribological
phenomena are complex in nature and there are many influencing parameters
that can be changed during the course of testing. Only little changes in such
parameters as surface properties, load, lubrication, atmosphere, material
composition etc. can cause drastic changes to tribological process. It should
be emphasized that the nitrogen alloyed compositions were produced by
experimental procedures and therefore they are likely to have inhomogenites
such as inclusions in the structure [2].

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CONCLUSIONS

Nitrogen alloying has a positive effect on galling resistance of PM tool
steels.

A high content and fine distribution of carbonitride phase in the tool
steel structure can increase the galling resistance to same level as in
coated tool materials.

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Figure 11.

Ranking of the galling resistance of the test materials.