Cermets having transformation-toughening properties and method of heat-treating to improve such properties

A powder metallurgy composite material comprising grains of a relatively hard material and a binder for binding the grains together, the binder being metastable and transformable at ambient temperature by the application of mechanical force from an initial state in which the major phase of the binder is austentic to a second state in which the major phase of the binder is martensitic, whereby the binder, while undergoing this transformation, absorbs mechanical energy applied to the composite material for increasing its fracture toughness and resistance to fatigue crack nucleation and propagation. Also disclosed is a method of heat-treating the composite materials to improve their transformation-toughening characteristics. A heat-treatable composite material having such improved transformation-toughening properties is also disclosed.

BACKGROUND OF THE INVENTION 
This invention relates to powder metallurgy composite materials of the type 
comprising grains of a relatively hard, abrasion resistant material, and a 
binder material for binding the grains together, and more particularly to 
a composite material having an improved binder exhibiting 
transformation-toughening properties. This invention also involves a 
method of heat-treating the composite material to improve its 
transformation-toughening properties, and a heat-treatable composite 
material having such improved transformation-toughening properties. 
This invention involves an improvement over powder metallurgy composite 
materials of the type, such as that disclosed for example in U.S. Pat. No. 
2,731,711, comprising grains of relatively hard, abrasion resistant 
material, such as tungsten carbide, and a binder material, such as a 
cobalt alloy, for binding the grains together. Such composite material, 
which is also referred to as a "cemented carbide" or more simply a 
"cermet", is widely used for cutting tools and as the cutting elements or 
so-called "inserts" for rolling cutter drill bits such as shown in U.S. 
Pat. No. 2,687,875, for use in drilling oil and gas wells. 
While tungsten carbide with a cobalt alloy binder has been the standard 
composite material for use in inserts in the drill bit manufacturing 
industry for the past 30 years, this material (and more particularly the 
cobalt binder therefor) presents several significant disadvantages. 
Because cobalt is principally mined outside of the United States and has 
significant strategic importance militarily, the supply of cobalt to the 
United States drill bit manufacturing industry is vulnerable to 
disruptions and shortages. Moreover, because of the relatively small 
quantity of cobalt mined each year, the price of cobalt is subject to 
dramatic escalations during periods of high demand, such as occurred 
during the late 1970's. In addition, excessive wear and breakage of 
inserts, which reduce the useful life of drill bits, is attributable to 
the failure of the cobalt binder to hold the tungsten carbide grains 
together under the relatively high compressive loads (which may exceed 
60,000 pounds) applied to the drill bit during drilling operations. More 
particularly, the limiting factor in the wear and life of drill bit 
inserts formed of conventional cermet material is the lack of adequate 
fracture toughness and resistance to fatigue crack growth, of the inserts 
made with a cobalt binder. 
SUMMARY OF THE INVENTION 
Among the several objects of this invention may be noted the provision of a 
powder metallurgy composite material having an improved binder material; 
the provision of such a composite material having a binder which is 
capable of transformation-toughening for improved mechanical properties; 
the provision of such a composite material having greater fracture 
toughness and resistance to fatigue crack nucleation and propagation than 
conventional composite materials; the provision of such a composite 
material, which as formed into an insert for a drill bit, offers greater 
resistance to wear and breakage than conventional cermet materials for 
extended insert life; the provision of such a composite material having a 
binder formed of materials which are relatively inexpensive and available 
domestically in abundant supply; and the provision of a method of 
heat-treating cermets to improve transformation-toughening properties. 
The powder metallurgy composite material of this invention comprises grains 
of a relatively hard, abrasion resistant material, and a binder material 
for binding said grains together, the binder material being metastable and 
transformable at ambient temperature by the application of mechanical 
force of at least a predetermined magnitude from an initial state in which 
the major phase of the binder material is austentic to a second state in 
which the major phase of the binder material is martensitic, whereby the 
binder material, while undergoing this transformation, absorbs mechanical 
energy applied to the composite material for increasing its fracture 
toughness and its resistance to fatigue crack nucleation and propagation. 
A heat-treatable powder metallurgy composite material of this invention 
comprises grains of a relatively hard, abrasion resistant material, and a 
binder material for binding the grains together, with the binder being 
metastable and transformable, upon being cooled to a transformation 
temperature below ambient temperature, from an initial state in which the 
major phase of the binder material is austenitic to a second state in 
which the major phase of the binder material is martensitic, the 
transformation causing deformation of the binder material due to volume 
changes and shear. The cooled binder material, upon being heated to a 
temperature above said transformation temperature reverting to a state in 
which the major phase of the binder material is austenitic, with the 
reverted austenite retaining at least some measure of said deformation and 
enhancing the stress-induced transformation characteristics of the binder 
and thereby the fracture toughness and resistance to fatigue crack 
nucleation and propagation of the composite material. 
The method of this invention of heat treating heat-treatable powder 
metallurgy composite material of the above-described type comprises the 
steps of cooling the composite material to a temperature at least as low 
as the transformation temperature to transform the binder material to its 
second state and to cause deformation of the binder material, thereafter 
heating the composite material to a temperature above ambient to cause the 
binder material to revert while retaining at least some measure of said 
deformation, and thereafter quenching the composite material to ambient 
temperature. 
Other objects and features will be in part apparent and in part pointed out 
hereinafter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The powder metallurgy composite material of this invention comprises grains 
of a relatively hard, abrasion resistant material, and a binder material 
for binding the grains together. The hard, abrasion resistant material is 
a transition metal compound, preferably a transition metal carbide. The 
composite material thus may be referred to as a "cemented carbide" or a 
so-called "cermet". The bindder material is an alloy of iron, and 
preferably an alloy of iron, carbon and at least one element from the 
group of elements comprising nickel, manganese, aluminum and chromium. Key 
to this invention is the fact that the binder material has so-called 
"transformation-toughening" properties, which will be described in detail 
hereinafter. 
While the cermet of this invention is contemplated to have numerous uses 
such as for cutting tools and bearing assemblies, including bearing 
assemblies for rotary drill bits, its present preferred use is as cutting 
elements or inserts for a roller cutter drill bit. Accordingly, to 
facilitate the discussion of the cermet of this invention and give the 
most complete and detailed description of the preferred embodiments of the 
cermet, the cermet will be considered hereinafter as it is used in the 
form of inserts for rotary drill bits. With the cermet of this invention 
so used, the hard, abrasion resistant material preferably comprises 
tungsten carbide, and the binder material preferably comprises an alloy of 
iron, carbon and nickel. Since iron is the principal constituent of the 
binder, the binder is commonly referred as an iron-based binder or more 
simply an iron binder to distinguish it from the cobalt alloy binder (or 
cobalt binder) of the conventional cermet material for inserts for drill 
bits. 
The concept of "transformation-toughening" has been used in the past with 
certain select steels (i.e., steels of certain compositions and subject to 
certain heat-treatment processes) to improve their fracture toughness. 
Basically, the concept involves the transformation of the steel from an 
initial austenitic state to a subsequent martensitic state at ambient 
temperature upon application of mechanical energy to the steel, with a 
resultant absorption of mechanical energy by the steel during its 
transformation. This property of steel to absorb mechanical energy at 
ambient temperature, such as the energy generated by normal working loads 
applied to steel, increases the steel's fracture toughness and resistance 
to fatigue crack nucleation and propagation, as described more fully 
hereinafter. 
In order to have "transformation-toughening" properties, a steel must have 
a martensite start temperature, Ms, (i.e., the temperature at which the 
steel when cooled begins to transform from its initial austenitic state to 
a subsequent martensitic state) below ambient temperature, so that, at 
ambient temperature, mechanical energy may be the driving force used to 
cause the steel to transform. However, the Ms of the steel should not be 
so far below ambient temperature that the steel will become stable in an 
austenitic state at ambient temperature regardless of the application of 
normal working loads to the steel (i.e., too stable to transform). In this 
later regard, the steel thus also should have a so-called 
deformation-induced transformation temperature Md (i.e., the temperature 
at which application of mechanical energy will cause the steel to begin to 
transform from austenite to martensite) above ambient temperature. 
In contrast to the above-described "transformation-toughening" steels, 
commonly used steels, such as AlSl 1020, 1040 and 1065 for example, do not 
have "transformation-toughening" properties. The Ms, as well as, the 
martensite finish temperature, Mf (i.e., the temperature at which the 
transformation to martensite is complete) of these steels is well above 
ambient temperature. Thus, at ambient temperature, these steels are 
already completely transformed to martensite and hence incapable of 
further transformation to martensite upon the application of mechanical 
force. 
The key to "transformation-toughening" in steel is then to suppress the 
temperature-induced martensitic transformation, thereby enabling the steel 
to undergo a deformation-induced martensitic transformation in response to 
mechanical loads applied to the steel at ambient temperature. Because the 
points of greatest stress or deformation in a steel body subject to a load 
are in the stress fields of nucleating and propagating cracks, the points 
at which the deformation-induced martensitic transformation will occur, 
will also be in the stress fields of the cracks. This positioning of the 
martensitic transformation in the crack stress fields has the highly 
desireable result that at least a portion of the mechanical energy causing 
the cracks to form and propagate is absorbed or dissipated in inducing the 
martensitic transformation, thereby slowing the formation and propagation 
of these cracks. Thus "transformation-toughening" steels, as compared to 
to similar steels without this property, have increased resistance to 
crack nucleation and propagation, as well as, increased fatigue strength 
and fracture toughness. 
Heretofore, the concept of "transformation-toughening" has not been used in 
powder metallurgy composite materials (e.g., cermets). In deed, it is 
believed that heretofore there has not even been a recognition that it 
would be possible to use this concept in such materials, much less that 
utilization of the concept could markedly improve the mechanical 
properties of such materials. This is likely due to the fact that the 
physical and metallurgical differences between steel and cermets are so 
great that these materials are studied more or less independently of each 
other. 
However, at the same time, many of the properties of the constituent 
materials of cermets observed when the materials are not used in a cermet 
cannot be applied to these same materials when they are used in a cermet. 
For example, certain observed properties of a binder material, per se, do 
not hold true for the binder material as used in a cermet. This is due, in 
part, to the constraint imposed on binder deformation by surrounding 
grains of hard material, such as tungsten carbide, and the elastic and 
thermal mismatch strains between the grains and the binder. 
In the development of the improved cermet of this invention for use as 
inserts for rotary drill bits, it was concluded that tungsten carbide was 
the best material to be used as the hard phase of the cermet because of 
its ready availability, high cleavage energy and high elastic modulus. It 
was further concluded that any significant improvements in the mechanical 
properties of cermets would be the result of improvements in the 
mechanical properties of the binder. More particularly, it was concluded 
that improvements in the cermet for inserts would be the result of 
improvements in the binder's role in controlling the fracture toughness 
and resistance to fatigue crack nucleation and propagation, of the insert. 
In addition to these mechanical strength properties, it was recognized 
that any new binder (like the conventional cobalt binder) must also be 
able to wet carbide, have solubility for both tungsten and carbon, form a 
liquid at a temperature which could be achieved in conventional sintering 
and hot isostatic press ovens, be a non-carbide former, and not permit 
intermetallic compounds to be formed at the carbide/binder interface. The 
metals from group VIII of the periodic table satisfy these above-described 
requirements to varying degrees. 
In attempting to identify an improved binder which could meet all these 
requirements, the idea was conceived that it may be possible to formulate 
a metal alloy binder having "transforming-toughening" properties. 
Extensive testing was then conducted to verify that this idea was in fact 
feasible and to identify specific compositions of group VIII metals having 
"transformation-toughening" properties and which would otherwise be 
suitable as a binder for cermets for inserts. As a result of the testing, 
improved binder were in fact found. As described previously, these binders 
preferably comprise alloys of iron, nickel and carbon. More particularly, 
the binder should comprise approximately 73 to 83 weight percent iron, 16 
to 26 weight percent nickel and 0.45 to 1.4 weight percent carbon. 
Among the tests conducted to identify improved binder materials (and more 
particularly improved cermets using such binders) having 
"transformation-toughening" properties, were tests to confirm that a given 
binder material had an initial state in which its major phase was 
austenitic and a second state in which its major phase was martensitic, 
and that the material had a martensite start temperature, Ms, below 
ambient temperature. FIG. 1 illustrates in graphical form the results of 
one such test conducted on a cylindrical sample of cermet having tungsten 
carbide grains and a binder comprising 77 weight percent iron, 22 weight 
percent nickel and 1.0 weight percent carbon. In this test, the thermal 
expansion of the sample was observed by measuring the change in length of 
the sample as a function of temperature. 
In FIG. 1, the thermal expansion of the sample is shown to follow a curve 
having a lower reach representing the change in length of the sample upon 
cooling the sample, and an upper reach representing the change in length 
upon reheating the cooled sample. As will be observed from FIG. 1, upon 
cooling the sample from approximately +25.degree. C., the sample contracts 
more or less linearly until the temperature drops to about -40.degree. C. 
Cooling the sample beyond -40.degree. C., however, resulted in expansion 
of the sample, inicating that the binder was undergoing a transformation 
from austenite to martensite, which is of lower density. The expansion due 
to transformation to martensite continued upon further cooling until the 
temperature dropped to approximately -130.degree. C., at which point the 
transformation was nearing completion. Cooling beyond this point, resulted 
in contraction of the sample. It was thus concluded that the martensite 
start temperature for the binder of this cermet designated Ms in FIG. 1, 
is approximately -40.degree. C., and thus is well below ambient 
temperature, as required for "transformation-toughening" . The upper reach 
of the curve in FIG. 1 shows that the sample expanded more or less 
linearly when reheated to ambient temperature. 
Tests were also conducted to confirm that the binder material would 
experience martensitic transformation at ambient temperature upon 
application of mechanical force. Referring to FIGS. 2A and 2B, there is 
illustrated the X-ray diffraction patterns taken from surfaces of first 
and second samples, respectively, of a cermet similar to that used in the 
FIG. 1 testing. The first sample had a polished surface from which the 
X-ray diffraction pattern was taken. The second sample had been subjected 
to fracture at ambient temperature and presented a fracture surface on the 
plane of fracture from which the X-ray diffraction pattern was taken. As 
illustrated in FIG. 2A, in the initial state of the binder of the cermet 
(i.e., its state prior to fracture), the major phase of the binder was 
austenitic, as is designated by "A" in FIG. 2A. As illustrated in FIG. 2B, 
in the second state of the binder material (i.e., its state upon fracture 
of the cermet and thus upon undergoing maximum deformation), the major 
phase of the binder was substantially martensitic, designated M. Thus, 
this cermet was found to have a deformation-induced temperature Md above 
ambient temperature, as is required for "transformation-toughening". 
Because of the range of ambient temperatures to which inserts and the 
drill bits to which they are secured, may be exposed during use, 
"transformation-toughening" cermets for inserts should have an Ms below 
0.degree. C. and an Md above 50.degree. C. 
Having once established that iron, nickel and carbon alloy binders have the 
properties necessary for "transformation-toughening", it was necessary to 
conduct tests to establish that these binders offer superior mechanical 
properties as compared to conventional cermets, and to find those binder 
compositions offering the best performance. In this latter regard, it was 
found the iron and nickel contents of the binder could be varied 
relatively widely (e.g., between 73 to 83 weight percent iron and between 
16 to 26 weight percent nickel) and a satisfactory binder could still be 
formed. However, it was also found that carbon content was critical and 
was a function of nickel content. 
FIG. 3 illustrates the results of laboratory tests conducted on a series of 
different cermets, each having a binder comprising approximately 77 weight 
percent iron and approximately 22 weight percent nickel but different 
carbon contents. In this test, the critical stress intensity factor 
K.sub.IC, which is a measure of fracture toughness, was measured for 
"short-rod" specimens of the different cermets. As shown in FIG. 3, the 
fracture toughness of these cermets reached a maximum when the carbon 
content was approximately 0.8 weight percent. At carbon contents less than 
0.8 weight percent, the binder, in its initial state, was not fully 
austenitic (i.e., some portion of the binder was martensitic) and thus not 
all of the binder could undergo martensitic transformation. At carbon 
contents in excess of 0.8 weight percent, the binder exhibited a tendency 
to form a stable phase of austenite incapable of transformation. Thus, 
upon increasing carbon content of the binder material beyond 0.8 weight 
percent, the amount of metastable austenitic binder available for 
transformation again decreased. Also shown in FIG. 3, is the maximum 
fracture toughness of conventional cermet having a cobalt binder, 
designated WC-Co in FIG. 3. As will be observed from FIG. 3, certain 
compositions the iron binder cermets of this invention have greater 
fracture toughness than the conventional cermet. 
Field tests were conducted to establish that the iron binder cermets of 
this invention which offered improved fracture toughness in laboratory 
tests, would also offer superior performance over conventional cermets in 
actual use. For these tests, inserts of longer than normal protrusion 
length (and thus more subject to fracture) were fabricated of four 
different cermets. One of the cermets was of conventional cermet 
composition having a cobalt binder, and the other three cermets were of 
cermet compositions of this invention having a binder comprising 
approximately 77 weight percent iron and 22 weight percent nickel but 
different carbon contents. One of these three cermets had 0.8 weight 
percent carbon, while the other two had 1.0 and 1.2 weight percent carbon, 
respectively. All four cermets, however, included the same amount and type 
of tungsten carbide material and were formed by the same conventional 
powder metallurgy manufacturing techniques such as those generally 
described in U.S. Pat. No. 2,781,711 involving sintering or hot isostatic 
pressing (i.e., hipping). 
Inserts of the four different cermets were secured in a equally distributed 
pattern in the insert bores or holes in two 121/4" diameter tri-cone 
rotary drill bits. The first of these bits was used to drill some 1256 
feet of soft rock and hard ore, and the second bit used to drill some 485 
feet of hard ore at the Griffith iron ore mine in Red Lake, Ontario. 
Drilling with both bits was done at 80 rpm with 100,000 pounds on the bit. 
After drilling, the bits were examined for insert fracture, with the 
inserts being given numerical ratings from 0 to 10 reflecting the volume 
of insert material lost due to fracture of the inserts. FIG. 4 illustrates 
the results of the test of the first bit. It shows that the conventional 
cermet, designated WC-Co, had the highest degree of fracture, 
(approximately 130) and the cermet comprising an iron binder having 0.8 
percent weight percent carbon had the lowest degree of fracture, 
(approximately 95) with the other two cermets (i.e., those comprising 
binders having 1.0 and 1.2 weight percent carbon) having intermediate 
degrees of fracture. FIG. 5, illustrating the results of the test of the 
second bit, shows the conventional cermet to have a relatively high degree 
of fracture (approximately 185), while all of the iron binder cermets of 
this invention had about same relatively low degree of fracture 
(approximately 80). The similarity in degrees of fracture of the three 
iron binder cermets seems to be inconsistent with the laboratory test 
results of these cermets shown in FIG. 3. This may be due to the fact that 
the loading rate during field testing was found to be four orders of 
magnitude higher than in the laboratory testing. 
As will be observed from the foregoing, through composition control it is 
thus possible to form improved cermets having "transformationtoughening" 
properties, which provides greater fracture toughness and resistance to 
fatigue crack nucleation and propagation than conventional cermets having 
a cobalt binder. However, to further enhance the 
"transformation-toughening" properties of these cermets "thermomechanical 
treatment" of iron binder cermets was investigated. "Thermomechanical 
treatment" practices have been used in the past with certain alloy systems 
to improve fracture toughness and, for such systems, involved a 
combination of mechanical working and suitable heat treatment of the alloy 
material. However, in cermets, mechanical working of a cermet to deform 
the binder cannot be effected without also cracking the tungsten carbide 
grains. Even if such cracking were permissible (which it is not), the 
level of strain which could be induced in the binder by mechanical working 
of the cermet would be insufficient to effect meaningful "thermomechanical 
treatment". Thus, initially, enhancement of iron binder cermets via 
conventional "thermomechanical treatment" practices did not appear 
possible. However, a modified treatment practice was conceived when it was 
recognized that the martensitic transformation of iron alloy binders 
occurring upon cooling the cermet to liquid nitrogen temperature involved 
large volume increases (approximately 3%), shear and shape strain. Under 
this treatment practice, the iron binder cermet is first cooled to liquid 
nitrogen temperature to effect significant deformation of the binder, and 
then heated above its austenite finish temperature to revert the binder to 
its original austenitic state. This practice was found to be highly 
effective in enhancing the binder's "transformation-toughening" 
properties, while having little or no effect on the carbide grains. 
More particularly, in the heat-treating method of this invention, an iron 
binder cermet, manufactured in accordance with known power metallurgy 
techniques such as disclosed for example in U.S. Pat. No. 2,731,711, is 
immersed in a bath of liquid nitrogen and allowed to cool until 
temperature equilibrium occurs at about -196.degree. C. As previously 
described, this cooling causes transformation of the binder to martensite, 
with the resultant deformation creating a large number of point defects, 
internal twins, faults and other dislocations in the binder. Thereafter, 
the cermet is removed, washed, dried and transferred to a molten salt 
bath, such as a chloride bath containing barium, sodium and potassium 
chlorides having an operating range of between 550.degree. and 900.degree. 
C. The salt bath provides rapid and uniform heating of the cermet. This 
rapid heating causes the binder rapidly to revert to its initial 
austenitic state, with insufficient time being available for the defects 
and dislocations previously formed to anneal out. After a suitable time in 
the salt bath, the cermet is removed and quenched in oil. 
It was found that the amount of time the cermet was allowed to soak in the 
salt bath, which is also referred to as "heat treatment time", had a 
significant affect on its "transformation-toughening" properties. 
Referring to FIG. 6, the fracture toughness K.sub.IC of a cermet 
comprising a binder having 77 weight percent iron, 22 weight percent 
nickel and 1.0 weight percent carbon is illustrated graphically as a 
function of "heat treatment time." As will be observed from FIG. 6, the 
fracture toughness of the cermet increased with heat treatment time, with 
the rate of increase in fracture toughness being relatively high for the 
first hour of treatment and decreasing thereafter. Also illustrated in 
FIG. 6 is the maximum fracture toughness of a similar iron binder cermet 
which was not subject to "thermomechanical treatment", designated Maximum 
K.sub.IC w/o "thermomechanical treatment." In addition, the maximum 
fracture toughness of a comparable conventional cobalt binder cermet 
designated "WC-Co.", is also shown in FIG. 6. A comparison of the fracture 
toughness of these three different cermets reveals that an iron binder 
cermet subject to heat treatment for one half hour or more has greater 
fracture toughness than the others. 
Other measures of the fracture behavior of iron binder cermets were found 
to be altered by "thermomechanicaltreatment." Referring to FIG. 7, there 
is illustrated the results of load tests conducted on two notched samples 
of an iron binder cermet having a composition similar to that of the 
cermet tested in FIG. 6. One sample was subject to thermomechanical 
treatment and the other was not (and thus remained in its as-sintered or 
as-hipped condition). In these tests, the notched samples were mounted in 
a testing fixture applying loads at opposite side of the mouths of the 
notches in the samples tending to cause the mouths to open. FIG. 7 
illustrates the load-displacement curves relating the loads, as measured 
in pounds, applied across the mouths of the samples and the extent of 
mouth or crack opening displacement. As will be observed from FIG. 7, the 
thermomechanically treated cermet not only could support a significantly 
greater load at any given crack opening displacement but also could 
support loads at significantly greater crack opening displacements than 
the same cermet which was not subject to thermomechanical treatment, 
designated "As-Hipped" in FIG. 7. Moreover, the areas under the 
load-displacement curves in FIG. 7, which are measures of the energy 
dissipated in the fracture of the cermet, further establish that treated 
iron binder cermets are able to withstand significantly higher levels of 
mechanical energy prior to catastrophic failure than the same cermets 
without treatment. 
The reasons for this dramatic improvement in the properties of iron binder 
cermets when thermomechanically treated are not fully understood. However, 
as indicated previously, it has been found that the steps of cooling the 
cermet to liquid nitrogen temperature and then reheating it to a 
temperature above the austenite finish temperature to revert the binder to 
austenite cause the formation of numerous dislocations in the binder of 
the cermet. Electron micrographs of iron binder cermets before and after 
treatment revealed that the discloations in the binder of treated cermets 
were not only more numerous but also more uniformly distributed and 
smaller than the dislocations in the binder of untreated cermet. In 
addition, it was found that during heat treatment of the cermet in the 
salt bath, excess carbon in the binder (i.e., carbon in excess of the 
minimum required to make the binder fully austenitic) tended to segregate 
to the dislocations, with the amount of carbon segregating being a 
function of time. Based on these observations, it was theorized that these 
small, uniformly distributed carbon-segregated regions restrict the size 
of deformation-induced martensite regions. This enables the martensitic 
transformation in the stress fields of nucleating and propagating cracks 
in a treated cermet to proceed earlier (under less stress), more gradually 
and more uniformly under a mechanical load than in untreated iron binder 
cermets, thereby enhancing the "transformation-toughening" properties of 
the cermet. 
While the concepts of "transformation-toughening" and "thermomechanical 
treatment" are disclosed as having been used with cermets having grains of 
tungsten carbide and and iron, nickel and carbon alloy binder, it is 
contemplated that these concepts could be used with other powder 
metallurgy composite materials having grains of a transition metal 
compound and a binder comprising an alloy of iron. Moreover, while the 
composite material of this invention has been described as being used to 
form inserts for rotary drill bits, it is contemplated that the material 
could also be used in other applications, such as for bearing members of a 
bearing assembly. 
In view of the above, it will be seen that the several objects of the 
invention are achieved and other advantageous results attained. 
As various changes could be made in the above constructions and methods 
without departing from the scope of the invention, it is intended that all 
matter contained in the above description or shown in the accompanying 
drawings shall be interpreted as illustrative and not in a limiting sense.