WEAR STUDIES OF COMMERCIAL AND TI/NB
HIGH-SPEED STEELS

Dr. Janusz Richter

Department of Materials Science, Silesian University of Technology

ul. Krasinskiego 8,

PL-40-019 Katowice,

Poland

Abstract

Abrasion resistance is an important feature of high-speed steels’ tribology
profile. This characteristic depends on material bulk properties, mainly hard-
ness, as well as microstructure. Nonledeburitic high-speed steels consider-
ably differ from conventional grades in respect of carbide phase content and
proportions of MC, M

2

C and M

23

C

6

particles.

The micro-scale abrasion tests were performed using ball-cratering method.

It consists in rotating a firmly clamped steel ball against a specimen in the
presence of abrasive particles. Such a design, together with loading of a sam-
ple by a dead weight, provides accurate control of the normal load and sliding
distance. The diameters of resulting worn craters generated during the test
enable calculation of wear coefficients using formula equivalent to Archard
model of sliding wear.

Commercial HS 6-5-2 steel was compared with the nonledeburitic steels

containing high fraction of MC type carbides of titanium and/or niobium.
The worn scars were investigated using field emission SEM, which revealed
wear damages, mostly typical for three-body abrasive micro mechanisms.
The calculated wear coefficients of the nonledeburitic alloys proved to be
better in comparison with the investigated conventional steel grade.

Keywords:

high-speed steels, carbide phase, abrasion testing

INTRODUCTION

Numerous authors of HSS devoted scientific works and monographs cor-

relate wear resistance or even broader understood service properties, with

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6TH INTERNATIONAL TOOLING CONFERENCE

presence and features of carbide phase. They consider increasing fraction
of the hardest MC type carbide particles as factor contributing to better wear
dependent properties of tool steels e.g. [1, 2]. HSSs are employed also in
cold working applications (as bearings, rolls, wear resistant parts) and in-
tensively cooled cutting edges. Therefore it seems advisable to assess their
wear resistance with abrasive particles of specific hardness and grain size
comparable with that of the carbide particles.

MATERIAL

Nonledeburitic high-speed steels are materials with balanced content of

carbon and carbide formers. The chemical composition projected this way
increase fraction of homogeneously arranged large MC type carbide particles
while decreasing content of eutectic carbides forming deleterious net and
banding in conventional grades. Idea, microstructure and properties of these
steels as well as technology and service properties of both wrought and cast
tools are described in details elsewhere [3, 4, 5, 6]. Chemical composition
of the investigated nonledeburitic HSSs containing strong carbide forming
elements (Ti and/or Nb) are presented in Table 1 together with data for
classical HS 6-5-2 steel grade.

Table 1.

Chemical composition (carbon and carbide formers) of the investigated high-speed

steels (wt.%), sum of MC forming metals and wear coefficients

Steel
(W-Mo-V)

C

Cr

W

Mo

V

Nb

Ti

Zr

P

V,Nb,Ti

(MC formers)

k (wear coeff.)

[10

−

13

m

2

N

−

1

]

HS 6-5-2

0.8

4.0

6.5

5.0

1.9

—

—

—

1.9

9.79

0-5-1-3Ti-2Nb

1.9

4.2

0.1

5.2

1.1

2.1

3.1

0.1

6.3

7.20

0-5-1-2Ti

1.1

4.4

—

5.2

1.3

—

1.5

0.1

2.6

8.86

6-5-1-3Nb

1.7

4.4

6.2

5.1

1.3

3.1

—

—

4.4

8.04

METHODOLOGY

Micro-scale abrasion test consists in rotation of hard ball against a speci-

men in the presence of an abrasive particles slurry. Such a measuring system
produces a wear scar of an imposed spherical geometry. For homogenous,
not coated material the wear volume

V , sliding distance S and the normal

Wear Studies of Commercial and Ti/Nb High-Speed Steels

527

load

N can be interrelated by a simple model equivalent to Archard equation:

V = kSN

(1)

where

k is the wear coefficient. In case of worn crater of diameter a being

much smaller than R radius of the ball, the wear volume can be approximately
expressed as:

V ≈

πa

4

64

R

(2)

thus

SN ≈

1

k

πa

4

64

R

(3)

Therefore linear graph of

πa

4

/64R against SN can be plotted for series of

increasing sliding distances, the gradient of such a straight line being the
wear coefficient reverse [7].

Measurements were conducted with a commercial Plint TE-66 apparatus,

using a 25.4 mm diameter steel bearing ball (990 HV) clamped into the rotat-
ing shaft. The sample was mounted on the counterbalanced beam to provide
accurately defined load between the sphere and sample. The small silicon
carbide particles were grade F1200 (4.5 'µm mean particle size) applied in a
water slurry form (0.75 g + 1 cm

3

H

2

O). On the basis of earlier experience

[8] 5, 10 15, 20 and 25 meters sliding distances were chosen at sliding speed
equal 0.05 ms

−

1

and 0.25 N normal load.

RESULTS AND DISCUSSION

For the investigated steels wear coefficients k were calculated using the

formula (3) for series of craters, with correlation coefficient better than 0.98,
the results are included in Table 1.

The wear coefficient for classical HS 6-5-2 steel grade proved to be very

close to that obtained in other work employing the same method [7] - 9.79 and
9

.40 × 10

−

13

m

2

N

−

1

respectively. This finding corroborates good repeata-

bility of the method assuming identical test conditions. The best wear resis-
tance revealed titanium-niobium 0-5-1-3Ti-2Nb steel (7

.20×10

−

13

m

2

N

−

1

),

titanium 0-5-1-2Ti nonledeburitic grade 8

.86 × 10

−

13

m

2

N

−

1

and niobium

6-5-1-3Nb steel 8

.04 × 10

−

13

m

2

N

−

1

, all these results being significantly

better than that for the referring HS 6-5-2 steel.

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6TH INTERNATIONAL TOOLING CONFERENCE

Last but one column in Table 1 indicates that wear resistance ranking

correlates directly with total contents of elements forming the hardest MC
type carbide particles.

Field emission SEM observations were performed using secondary elec-

tron contrast with magnifications in 2500–10 000 times range. Worn surfaces
shown in Figs. 1, 2 indicate a three-body abrasive wear mechanism, practi-
cally with no directionality. The appearance of surface of both classical and
nonledeburitic steels resulted mainly from plastic, multiply indentation, mi-
cro cutting and ploughing being also involved. Large, primary MC carbides
in the nonledeburitic steels subjected to the SiC particles abrasion remain
embedded in the matrix and no signs of grooving or ploughing through the
MC carbides were found.

Figure 1.

Worn surface of steel matrix in HS 6-5-2 grade, FE SEM, ×10 000.

Wear Studies of Commercial and Ti/Nb High-Speed Steels

529

Figure 2.

Primary MC carbide particle and matrix in 0-5-1-2Ti grade sample after wear

test, FE SEM, ×2500.

ACKNOWLEDGMENTS

The author would like to thank prof. I. Hutchings and dr. D. Kelly

(University of Cambridge, UK) for valuable discussions and kind assistance
in the wear measurements.

REFERENCES

[1] G. ROBERTS et al., in "Tool Steels" (ASM International 1998).

[2] S. KARAG ¨

OZ et al., Metall. Trans. 20A (1989) p. 2695.

[3] J. CWAJNA et al., in Proceedings of the 5th Inernational Conference on Toolig, Leoben,

Sept- Oct 1999, edited by F. Jeglitsch, R. Ebner, H. Leitner (University of Leoben 1999)
p. 383.

[4] J. RICHTER et. al., J. Mater. Proc. Technol. 53 (1998) p. 341.

[5] J. RICHTER et. al., Pract. Metalogr. 9 (1996) p. 479.

[6] J. RICHTER et al., Mater. Char. 46 (2001) p. 137.

[7] K. L. RUTHERFORD and I. M. HUTCHINGS, J. Test. Eval. 25/2 (1997) p. 250.

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6TH INTERNATIONAL TOOLING CONFERENCE

[8] J. RICHTER et al., in Proceedings of the 5th Inernational Conference on Toolig, Leoben,

Sept 1999, edited by F. Jeglitsch, R. Ebner, H. Leitner (University of Leoben 1999) p.
348.