A NEW METALLURGICAL CONCEPTION

FOR WEAR-RESISTANT STEELS

J. Beguinot, J. G. Brisson, R. Blondeau, M. Junker

Creusot-Loire lndustrie • Div. Creusot-Marrel
Le Creusot, France

ABSTRACT domain, the production methods and the cor-

responding treatments, and the practical
In crder to improve the wear resistance of results thus obtained.
steels plates without detrimental consequences
on the ability of processing by the customer, APPROACH TO THE PROBLEM: THE NE\v
a ·new type of steels was developped, exhibi- METALLURGICAL CONCEPT EMPLOYED
tin& higher resistance to indentation and
micro-shearing at given initial hardness. The objective and the manner of approa-
Basically, this was obtained through higher ching the problem having been defined, we
work hardening ability during service under shall now describe the metallurgical elements
abrasive sollicitations , larger metastable which, following an analysis of the general
austenite content and optimal distribution of mechanism of abrasive attack, have led to the
fine microcarbides of Cr and Mo, as a result development of a metallurgical concept to be
of 'both appropriate contents of carbides-for- applied to new products.
ming alloying elements and a particular two- A BRIEF ANALYSIS OF THE RESPONSE OF A
stages cooling procedure after austenitisa- METAL SURFACE TO ABRASIVE ATTACK- Schemati-
tion. cally, keeping the essentials and setting
aside the complications introduced by dif-
ferent types of abrasive attack, local damage
THE GROWING DEMANDS OF THE MARKET for wear- of an exposed metallurgical surface can be
resistant materials, expressed in terms of split into two phases :
service life, have brought about, in recent - a process of indentation corresponding to a
years, a sustained imrpovement in the proper- local plastic deformation at the point of
ties of the steels employed in this region of impact, the severity of which is evidently
industry. This improvement has, however, come proportionate to the kinetic energy and the
abollt within the framework of an essentially weight of the abrasive, but is never zero,
constant metallurgical philosophy, which even in the case of a tangential attack by a
subordinates the wear resistance to the level fine-grained abrasive. This first stage does
of hardness at the time the products are not normally result in the removal of metal,
delivered. This continual growth in hardness except with very fraaile materials,
has, unfortunately been accompanied by in- -a process of micro tearing-off of fine metal
creasing difficulties for the customer, in particles by intense local shearing begiMing
preparing them for the final application, and at the indentation itself and/or lips formed
has faced him with a difficult choice between at its circumference, whick leads to a real
service performance and satisfactory prepara- damage, with loss of weight.
tion. The distinction between these two successive
It appears that one way of solving this pro- phases is in reality not so strict: local
blem requires a new metallur~ical approach, in damage develops continually from indentation
which the hardness at delivery is not the sole by compressive plastic deformation to local
contributor to wear resistance. We shall shearing of increasing intensity, leading in
describe in this paper the new conception wich the end to a general ductile tearing-away of
Creusot-Loire Industrie has brought to this fine metallic particles.

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 ;.:::;:-,;:..:..:.::.~::A:.. r ACTCRS P!:~:-::,r;:;7 TO THE sation, a quench is necessary, at a suffi-
REDUCTION OF DAMAGE BY ABRASION - In the cient speed through the ferritic and bainitic
attempt to increase wear resistance for a transform ation regions to avoid the occurence
given original hardness the following factors of these soft constituen ts, and then, below
have been taken into account, although in no this, in a temperatu re zone below 450°C, to
exhaustive fashion. They provide a series of provide a controlled cooling sufficien tly mild
additive barriers to the local deformatio n to produce an essentiall y martensiti c struc-
produced by abrasion, They are : ture which is :
-a taking account of the phenomenon of work - poorer in dislocatio ns than a sharply
hardening, starting with a structure in which water-quen ched martensite , and thus with a
dislocatio ns are infrequen t. This will benefit much lower level of residual constrain ts
from a work hardening in service much greater harmful in later operations of cutting and
than that undergone by one full of disloca- welding, a condition necessary for factor (a)
tions, such as a fresh martensit ic structure to be effective ,
obtained by a sharp water quench. - accompanied by a sufficien t quantity of
-b reinforcin g work hardening by retaining residual austenite to ensure that factor (b)
sufficien t austenite (of the order of 8 to makes an appreciab le contributi on,
15 %) capable of being transforme d to hard - complemented by the partial precipitat ion of
martensite as the intensity of deformatio n the carbon, in the form of carbides rich in Cr
increases . and Mo and sufficien tly fine and numerous
-c finally, replacing the braking of dislo- effective ly to impose the final influence of
cation movement by a swarm of dislocatio ns factor (c).
(process a), and by the islands of martensit e A water quench does not meet these require-
resulting from the transform ation of residual ments, nor does tempering after quenching , as
austenite process (b) by one ultimate source a series of prelimina ry experimen ts has shown.
of severe restraint: the presence of finely On the contrary, an oil quench may best
dispersed hard carbides. Indeed, without conform to the cooling kinetics required - see
undertakin g the long and careful studies Figure 2 for a comparison of the drasticiti es
necessary to establish the precise mechanisms of the oil and water quenches.
involved, it has been experimen tally confirmed The total cooling power of the oil, but also
that a ueat increase in abrasion resistance and dominantly its conformati on to the requi-
results from the addition of sufficien t red kinetics of cooling; is closely connected
carbide-fo rming elements (Cr at least 1.2 %, to its compositi on. The choice of one among
Mo at least 0.2 %) and a controlled cooling of the wide spread of commercial quenching oils
the material from 450°C downwards, after a was fixed by a series of comparativ e tests on
severe quench from 900° to 450°C to avoid a prototype quenching machine, in simulated
ferrite formation - see below. It seems very industria l condition s, on steel test coupons
likely that the fine carbide precipita tes from laboratory casts.
produced (see Figure 1) by the combinatio n of .The investmen t ot a specific oil-quench ing
the presence of Cr and Mo with the slow con- device was then decided at Le Creusot plant,
trolled cooling below 450°C is the ultimate the main character istics of which were des-
origin of the severe restraint of dislocatio ns cribed at a ATTT presentati on (1). The alloy
in those microreaio na subjected to shear. contents of different if'ades of steel have
The respective contribut ions ot these three been determine d, takina into account the
factors which have just been qualitativ ely minimum requireme nts in carbide-fo rmina ele-
described will be analysed later on, in the ments (C +Mo+ Si not less than 1.5 %), and
experimen tal part of this paper, after the the contents of Mn, Ni and B beina selected
metallura ical definitio n of the products and appropria tely for the quenchabi lity required
of their methods of treatment . in relation to the thickness.
One if'ade ie examined in the experimen tal part
GENERAL DEFINITION OF COMPOSITIONS ANO of the paper.
METHODS OF THERMAL TREATMENT
A BRIEF EXPERIMENTAL ASSESSMENT or THE
The delll&J'lds of an increasin a number of RESPECTIVE CONTRIBUTIONS OF THE MECHA-
users for higher wear resistanc e on one hand, NISMS INVOLVED IN nu; IMPROVEMENT or WEAR
and for good weldabili ty on the other leads to RESISTANCE
the adoption of an essential ly martensit ic
structure . If the three factors mentioned Abrasive tests were performed usin& three
above are taken into account to ensure an different laboratory device,, assumed to •
appreciab le work hardenina in service under include the main ranae of pratical wear aol-
the abraded surface layer throughou t the licitation s : Figures 3 to 5. In the•• teats,
wearing process, special restrictio ns on the th• aamplee are faced to quartz sand, quart-
thermal treatment to produce the desired zite aranules and small very hard steel balls
structure are encounter ed. After auateniti - respectiv ely. Computer monitored correction s
,,.

100
 accounting for the specific ~eometry of the Thus further experiment were conducted on a
test pieces are applied in order to convert 400 HS initial hardness steel designed accor-
the experimental results in terms of duration ding to the new concept. This grade proved to
of test for a given weight loss. be 45 % higher in wear resistance as compared
Measurements of hardness increase under the to conventionnal water quenched steel AC of
weared face, as a result of strain hardening, the same initial hardness.
were performed by HV 0.2 and converted in This results allows a crude partition of the
Brinell uni ts. supplementary contributions of indentation
INDENTATION RESISTANCE: SUPERFICIAL WORK resistance and tearing resistance responsible
HARDENING DURING SERVICE - Figure 6 shows the for the superior wear resistance of NC type
comparative evolutions of microhardness grades as follows:
(converted in Brinell) perpendicularily to the - • 13 % (l.45/1.28) resulting from higher
weared surface for AC (conventionnal steel indentation resistance,
water quenched) and NC (oil quenched according - • 28 % owing to higher microtearing resis-
to the new concept) after abrasion tests on tance.
the device of figure 4, representative of It should be kept in mind than these supple-
typical industrial wear sollicitations (AC mentary contributions remain active all along
14 MB6, NC: 14 CMDB6). The sub-surface the service life of the steel. In particular,
increase in hardness, resulting from the the strain hardening of the subsurface is
previously described contributions to strain- self-maintained by abrasive sollicitations.
hardening under impacts, is largely higher for
NC(• 65 HB) than for AC (•20 HB). CONCLUSIONS
This particular ability of the new concept
steels to experience high strain hardening As a consequence of extensive reflexions
would still hold at the highest values of as and experiments on wear mechanisms and invol-
delivered hardness with only a somewhat ved metallurgical parameters, a new concept
smaller amplitude (Fig. 7). for manufacturing anti-abrasive steel plates
Besides, the strengh of abrasive attack proves was designed and proper industrial investments
to affect very little the strain hardening decided. The new grades thus obtained exhibit
intensity but primarily the strain hardened a rough 45 % better wear resistance as compa-
depth. The latter appears to be linked direc- red to conventionnal water quenched abrasive
tly to and substantially larger than the mean steels of the same as-delivered hardness.
indentation depth so that the contribution of Oil quenching with a particular and severly
induced stain hardening is in practice fairly controlled oil composition monitoring a spe-
independent of the type of abrasive attack. cific evolution of cooling rate and adapted
1

MICRO-TEARING OFF RESISTANCE - Because of chemical analysis of steels, sufficiently rich

superior ability to experience strain harde- in Chromium and Molybdenum are the main means
nin&, the acquired sub-surface hardness of NC required.
steel (initial hardness 360 HB) is quite
similar to that of AC steel with initial REFERENCES
hardness 400 HB ( Fig. 6). Now, the wear re-
sistance of NC (through test of fii\,ll"e 4) is (1) F. Treillard, J. L~ger, J. Perriau et G.
28 % hi&her than that of AC. This shows that, Schweitzer, Revue Traitement Thermique ~ '
at &1ven acquired subsurface hardness (and 43 - 48 (1986).
thus given indentation sensitivity), the newly
designed steel NC profits by a supplementary
resistance to micro-tearing responsible for a
rough 30 % more wear resistance.
DISCUSSION - Thus, it proves possible
using the new metallurgical concept for abra-
sion uteels, to produce materials profiting
both by higher wear resistance and lower
hardness at delivery leading to an improved
easiness of processing by the customer, as
compared to conventionally manufactured
steels.
Now, leaving these practical considerations
for more theoretical ones, it may be of inte-
rest to tentatively assess the respective
contributions of the resistance to indentation
and the resistance to detachment of metal
particles to the improvement of wear resistance.
In this scope, it seems more convenient to
compare conventional and newly desianed steels
of the same initial hardness.

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