Comminuting media comprising a martensitic/austenitic steel which contains at least about 40 percent by volume retained austenite, a portion of which is work transformable to martensite. The steel contains sufficient alloy content such that the steel has a martensite start and finish temperature sufficiently low to allow partial transformation of austenite to martensite during quenching of the steel from the austenitic range, but leaving some retained transformable austenite. This steel is used as a comminuting media, the retained austenite transforming to martensite through working or abrasion of the comminuting media during use in a comminution process. The outermost volume of the comminuting media which forms the wear surface and which contains the retained austenite in an amount of at least 40 percent by volume comprises at least 25 percent of the total volume of the comminuting media.

FIELD OF THE INVENTION 
The present invention relates to comminuting media. More particularly, the 
invention is directed to comminuting members comprising a 
martensitic/austenitic steel containing retained, transformable austenite 
for improved wear characteristics. 
BACKGROUND OF THE INVENTION 
The cost of comminuting and processing ore in the mining industry is 
determined in part by the cost of the consumable wear surfaces and parts 
necessary to comminute the ore. To lower the operating costs associated 
with comminuting processes, it is desirable to increase the life of the 
comminuting media. 
In a typical ore processing arrangement, large pieces of rock or ore must 
be broken into smaller pieces to liberate the valuable mineral 
constituents. As a representative example of one method of comminuting 
ore, large pieces of ore are moved into an enclosed tubular housing known 
as a grinding mill which rotates the ore. The tubular housing typically 
includes a plurality of wear resistant plates or elements attached to the 
interior of the housing to form a liner therein. The rotation of the mill 
causes the ore to impact on itself and on the liner of the mill, causing 
break-up of the ore. 
In addition, loose comminuting elements are often added to the grinding 
mill to increase the rate of disintegration of the ore. These elements are 
steel spheres, rods, cones or the like which rotate within the mill with 
the ore, pounding the ore and increasing its rate of disintegration. 
The comminuting elements must be extremely durable so that when they impact 
one another, the mill liner and the ore, they do not themselves break 
apart or wear at an excessive rate. It is desirable for the comminuting 
elements to wear very slowly in order to increase their useful life. The 
slower the spheres, rods or other members wear, the less often they must 
be replaced, thus lowering the cost of the comminuting operation. 
The wear resistance of a steel is tied, at least in part, to its 
microstructure. It is known that martensitic steels exhibit low rates of 
abrasion wear, as compared to steels having another microstructure, such 
as pearlitic or stable austenitic steels. The microstructures of steels 
may be quite complex, but generally consist of one or more phases or phase 
mixtures, to wit, martensite, austenite, ferrite, carbide, pearlite, and 
bainite. 
As a result of the difficulties surrounding the obtaining and identifying 
of particular steel microstructures, however, the hardness of a steel has 
generally been used as the determinant for use of the steel as a 
comminuting media. In particular, it has generally been taken as a "rule 
of thumb" that the wear resistance of a steel increases with increasing 
hardness. It has, therefore, been the conventional wisdom that comminuting 
elements should be formed from high hardness steel. 
Typically, high hardness in steels is attained by increasing the carbon 
content and heat treating the steel, typically by using an austenitizing 
and quenching treatment, in such a manner as to form a high amount of 
martensite. Martensite is a very hard but very brittle phase. As a result, 
comminuting media comprising martensitic steel has the disadvantage that 
it may spall and chip. 
As one means for increasing the spalling resistance of the martensitic 
steel, the steel may be given a subsequent heat treatment called 
tempering. Tempering of a martensitic steel reduces its brittleness, 
increasing its "toughness" or ability to withstand impact loading without 
spalling and chipping. Tempering, of course, typically reduces the 
hardness of the steel, and presumably its abrasion wear resistance. 
Tempering also adds another step to the process of making the steel, 
increasing the cost of the end product. 
It is desirable to create a steel which is useful to form comminuting wear 
surfaces. The steel preferably has the wear resistance of high hardness 
steels such as high carbon martensitic steel, and yet is sufficiently 
ductile to minimize failure by cracking and spalling under impact loading. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, comminuting media is comprised of 
a martensitic/austenitic steel containing unstable or work transformable 
retained austenite. Preferably, the steel is a martensitic/austenitic 
steel containing at least about 40 percent by volume of retained 
austenite. 
In general, the carbon and alloy content of the steel of the present 
invention results in the steel having a martensite start temperature of 
between about 0 and 300 degrees Fahrenheit (-18.degree. C. to 149.degree. 
C.), and a martensite finish temperature below quenching and ambient room 
temperature. 
In accordance with the present invention, the composition of at least a 
steel having the above-stated properties typically includes about 0.4 to 
2.0 percent by weight carbon and an alloying element, preferably chromium 
and/or manganese, with the remainder iron and small amounts of other alloy 
elements such as nickel, silicon, molybdenum, vanadium, copper, and 
combinations thereof. Trace and residual impurities characteristically 
present in steel may likewise be present in the compositions useful in 
this invention. 
By a conventional forging or casting process, steel is formed into the 
shape desired for the comminuting member. For example, a steel grinding 
sphere for a grinding mill may be the desired form of the comminuting 
media. 
The microstructure of the steel forming the comminuting member is then 
changed by heating the steel to an austenitizing temperature at or above 
which substantially all of the carbides present in the steel go into 
solution. After heating, the steel has an austenitic structure. The steel 
is then quenched or cooled to below the martensite start, but not finish, 
temperature. Quenching transforms no more than about 60 percent by volume 
of the austenite into martensite, leaving a martensitic/austenitic steel 
with retained transformable austenite. 
Since it is the wear characteristics of the comminuting media sought to be 
improved, it is the outermost volumetric layer of the comminuting media 
which is desirably formed of the martensitic/austenitic steel containing 
at least about 40 percent by volume of retained austenite. Accordingly, 
the outermost volume of the comminuting member represents at least 25 
percent of the total volume of the comminuting member. 
Formed and processed as described, the comminuting member, such as a 
grinding sphere, is then ready for use in a comminution process. The 
impact loading or "working" of the comminuting member during normal 
operation of the comminution process (such as a grinding mill) has the 
effect of transforming some or all of the retained austenite at the wear 
surface into the more durable martensite. 
Advantageously, the resulting comminuting media in accordance with the 
present invention is extremely wear resistant, both as to abrasion and 
chipping and spalling. The comminuting member may have enhanced corrosion 
wear resistance as the result of the inclusion of sufficient levels of one 
or more alloys. It may be used in its as-quenched form, or it may be 
subjected to some tempering or other processing before use. 
Further objects, features, and advantages of the present invention over the 
prior art will become apparent from the detailed description which follows 
.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Unless otherwise expressly noted herein, all percent figures are to be 
construed on a weight basis, all temperatures are to be based on a 
Fahrenheit scale, and all chemical symbols are to be referenced to the 
periodic chart of elements. It is noted that the percent of retained 
austenite is consistently expressed herein on a volume basis. The portion 
of the comminuting media made up of retained austenite is also expressed 
as a volume percent of the total volume. 
The field of this invention relates generally to improved materials of 
construction and products for the wear surfaces of various equipment, 
parts and accessories utilized in material size reduction processes. 
Common processing terms associated with material size reduction include 
comminuting, grinding, crushing and pulverizing which may contemplate both 
wet and dry operations. Such processes may be carried out in equipment 
which may include, but not be limited to, jaw crushers, gyratory crushers, 
roll crushers, hammer mills, grinding mills, ball mills, vibratory mills, 
tower mills, verti-mills and the like. Accordingly, the term "comminuting" 
is used herein as a reference to any of the foregoing type material size 
reduction processes for various ores, rocks, aggregates and similar 
substances for which size reduction is necessary. The term comminuting 
"media", "member", or "element" is used herein as a reference to the 
generally consumable, wear surfaces of the foregoing equipment, parts and 
accessories which contact the ore, rock, aggregate or similar substance in 
carrying out a size reduction process 
More specifically, the present invention relates to comminuting media 
comprising a tough alloy steel. The steel is a martensitic/austenitic 
steel containing a large amount of unstable retained austenite. 
Preferably, the steel comprises at least approximately 40 percent by 
volume, with as much as 50 to 100 percent by volume, of retained 
austenite, the remainder of the steel preferably having a martensitic 
structure. Preferably, a portion of this retained austenite is of the 
unstable or "work transformable" type so that a portion thereof may be 
transformed to martensite under mechanical loading in accordance with the 
teachings of our invention. 
Inasmuch as it is an objective of our invention to provide comminuting 
media having improved wear characteristics, it should be understood that 
it is the outermost volumetric layer of the comminuting media which is 
desirably formed of martensitic/austenitic steel with a microstructure 
containing at least about 40 percent by volume of retained austenite. 
Stated differently, it is the consumable wear layer of the comminuting 
member which must contain the retained austenite. Accordingly, the 
outermost volume of the comminuting member, which contains the retained 
austenite of at least 40 percent by volume, represents at least 25 percent 
of the total volume of the comminuting member. For example, in a 6" (15 
cm.) diameter grinding sphere, an inner spherical core of roughly 5.5" (14 
cm.) diameter represents approximately 75 percent of the total volume such 
that the outer volumetric layer of a thickness slightly greater than 0.25" 
(0.6 cm.) represents about 25% of the total volume of the grinding sphere. 
It is this outer layer of at least 25 percent of the total volume of the 
grinding member which is to be formed of the martensitic/austenitic steel 
containing retained austenite of at least 40 percent by volume. Naturally, 
further benefits may be achieved as the percentage of the total volume of 
the grinding member made up of the retained austenite microstructure is 
increase from at least 25 percent up to 100 percent of the total volume of 
the grinding member. Moreover, the dimensional thickness which represents 
at least 25 percent of the total volume of the comminuting member will 
vary in accordance with the volumetric configuration of the comminuting 
member as determined by known mathematical relationships for calculating 
the volume of solids. 
As detailed below, steels in accordance with the present invention meeting 
this criteria have a composition which generally includes about 0.4 to 2.0 
percent by weight carbon and an alloying element, preferably chromium 
and/or manganese, with the remainder iron and small amounts of other alloy 
elements such as nickel, silicon, molybdenum, vanadium, copper, and 
combinations thereof. This steel is heated into the austenitic range until 
substantially all carbides are dissolved, at which point the steel is 
quenched or cooled, transforming some of the austenite to martensite. 
This as-quenched steel has a minimum unworked hardness of at least 20 HRC 
(Rockwell hardness). It has been observed that when worked, the hardness 
(at least on the worn surface) approaches a Rockwell hardness of 50 or 
more. It is believed that working the above-described steel has the effect 
of transforming the retained austenite at the wear surface into a 
martensitic structure. 
It has been found that the martensite start (Ms) and finish (Mf) 
temperature of the steel can be correlated to the desired retention of 
austenite in the as-quenched martensitic/austenitic steel. In particular, 
the Ms temperature of the steel of the present invention is preferably 
between about 0 and 300 degrees Fahrenheit (-18.degree. C. to 149.degree. 
C.), and most preferably between about 30 and 225 degrees Fahrenheit 
(-1.degree. C. to 107.degree. C.), and still more preferably between about 
50 and 150 degrees Fahrenheit (10.degree. C. to 66.degree. C.). The steel 
further preferably has a martensite finish temperature such that complete 
transformation to martensite does not occur during quenching or cooling to 
ambient temperature. Thus, as one aspect of the present invention, a steel 
having the above-stated level of retained austenite in martensite normally 
has an Ms temperature within the above-stated range. When the steel is 
quenched from the austenitic range, martensite transformation begins at 
Ms. However, because of the low martensite finish temperature, complete 
martensite transformation does not occur, with some austenite remaining 
untransformed. 
Those skilled in the art will recognize that several known computational 
procedures may be used to calculate martensite start temperature (Ms). As 
used in this invention, the Ms temperature may be preferably calculated 
using the Nehrenberg formula or equation as follows: 
EQU Ms (in .degree. 
F.)=930-540*(%carbon)-60*(%manganese)-40*(%chromium)-30*(%nickel)-20*(%sil 
icon)-20*(%molybdenum); or 
EQU Ms (in .degree. 
C.)=898-540*(%carbon)-60*(%manganese)-40*(%chromium)-30*(%nickel)-20*(%si 
licon)-20*(%molybdenum)!.div.1.8 
In accordance with this calculation, only the weight percentage of the 
elements which are in solution (i.e. in the martensite or austenite 
matrix) is utilized. 
It has been found that steels with certain specific compositions meet the 
above-described criteria. Preferably, the steel of the present invention 
has a carbon (C) content of between about 0.4 to 2 percent by weight, and 
most preferably between about 0.8 to 1.4 percent by weight, and still more 
preferably about 0.95 to 1.15 percent by weight. The steel preferably 
contains at least one alloying element. Preferably the alloying element 
includes either about 0 to 8 percent by weight of chromium (Cr) and/or 
between about 0 and 6 percent manganese (Mn) by weight. More preferably 
the steel includes either between 2 and 7 percent Cr or between about 1.5 
and 6 percent Mn. Most preferably, the steel includes about 3 to 6 percent 
Cr, about 3 to 6 percent by weight of Mn, or a combination of both Cr and 
Mn. 
The remainder of the steel comprises iron and small amounts of other 
elements. It is contemplated that a steel having the desired properties 
may contain, in addition or substitution of those elements (i.e., Cr and 
Mn) listed above, 0 to 4 percent by weight copper (Cu), 0 to 1 percent by 
weight vanadium (V), 0 to 2 percent by weight nickel (Ni), 0 to 2 percent 
by weight molybdenum (Mo) and 0 to 2 percent by weight silicon. 
Trace and residual impurities characteristically present in steel making 
may likewise be present in the compositions useful in this invention. 
Likewise, additives such as grain refiners to improve toughness of the 
steel may be included in amounts characteristically less than 0.10 percent 
by weight. Representative examples of suitable grain refiners include 
aluminum (Al), titanium (Ti), niobium (Nb) also known as columbium, and 
vanadium (V). 
Specifically, it has been found that the addition of alloys such as 
chromium and manganese has the effect of lowering the Ms and Mf 
temperatures. Other alloys, such as molybdenum, nickel and the like have 
an effect on Ms and Mf. These other alloying elements have been found less 
desirable because they do not affect Ms and Mf as greatly (when added in 
the same weight amounts). 
As a specific example of the present invention, it has been found that a 
steel comprising 1.05 percent carbon, 1.49 percent chromium, 0.26 percent 
molybdenum, 0.20 percent vanadium, 0.33 percent manganese, 0.25 percent 
silicon, 0.02 percent nickel and the remainder iron and other alloys in 
small amounts inherent in the steelmaking process has a retained 
austenitic content of about 47 percent by volume and an Ms temperature of 
approximately 271 degrees Fahrenheit (133.degree. C.) when manufactured in 
accordance with the techniques hereinafter to be described. 
As a second specific example of the present invention, it has been found 
that a steel comprising 0.99 percent carbon, 4.64 percent chromium, 0.87 
percent molybdenum, 0.96 percent vanadium, 0.92 percent manganese, 0.31 
percent silicon, 0.12 percent nickel and the remainder iron and other 
alloys in small amounts inherent in the steel making process has a 
retained austenitic content of about 80 percent by volume and an Ms 
temperature of approximately 125 degrees Fahrenheit (52.degree. C.) when 
manufactured in accordance with the techniques hereinafter to be 
described. 
As a third specific example of the present invention, it has been found 
that a steel comprising 0.97 percent carbon, 2.71 percent chromium, 0.03 
percent molybdenum, 0.60 percent manganese, 0.26 percent silicon, 0.12 
percent nickel and the remainder iron and other alloys in small amounts 
inherent in the steel making process has a retained austenitic content of 
about 50 percent by volume and an Ms temperature of approximately 252 
degrees Fahrenheit (122.degree. C.) when manufactured in accordance with 
the techniques hereinafter to be described. 
As a fourth specific example of the present invention, it has been found 
that a steel comprising 1.03 percent carbon, 5.17 percent chromium, 0.021 
percent molybdenum, 1.14 percent manganese, 0.27 percent silicon, 0.086 
percent nickel and the remainder iron and other alloys in small amounts 
inherent in the steel making process has a retained austenitic content of 
about 76 percent by volume and an Ms temperature of approximately 90 
degrees Fahrenheit (32.degree. C.) when manufactured in accordance with 
the techniques hereinafter to be described. 
As a fifth specific example of the present invention, it has been found 
that a steel comprising 1.02 percent carbon, 1.52 percent chromium, 0.03 
percent molybdenum, 1.52 percent manganese, 0.26 percent silicon, 0.09 
percent nickel and the remainder iron and other alloys in small amounts 
inherent in the steel making process has a retained austenitic content of 
about 66 percent by volume and an Ms temperature of approximately 219 
degrees Fahrenheit (104.degree. C.) when manufactured in accordance with 
the techniques hereinafter to be described. 
Various processing techniques may be utilized with the steel compositions 
selected in accordance with the foregoing principles of this invention in 
order to achieve comminuting media having a retained austenite in excess 
of 40 percent by volume. 
A comminuting member is first formed from a steel having a preselected 
composition as previously indicated. It may be formed into any desired 
shape. For the comminuting media of the present invention to be used in a 
grinding mill, for example, the steel may be formed into spheres to serve 
as loose comminuting members within the mill. Alternative shapes which may 
be used include, but are not limited to, rods, cylinders, cones, cylpebs, 
bullets and slugs. If attached grinding elements are needed for the mill 
liner, then the steel may be formed into any shape convenient for use as a 
liner plate. Likewise, various parts, accessories and wear surfaces which 
will be contacted by the ore, rock, or the like in a grinding or crushing 
process may be fabricated from the preselected steel composition as 
required. 
In short, the steel meeting the specifications of this invention is 
preferably manufactured by any of the known forging or casting processes 
into a comminuting member or element. In one preferred technique, the 
steel is heated to its forging temperature which is also above its 
critical temperature at which full austenitizing is achieved (i.e., the 
temperature at which all carbon and alloying elements have moved into 
solution). This temperature is alloy grade dependent and would typically 
range between 1650.degree. F. to 2050.degree. F. (899.degree. C. to 
1121.degree. C.). The steel is then cooled rapidly by water, oil, air 
quenching or the like. The quenching cools the steel to at or below the 
martensite start temperature, but not the martensite finish temperature. 
As an alternative technique, the comminuting member formed from a 
preselected steel composition may simply be allowed to cool to ambient 
conditions and then subsequently be reheated above its austenite start 
temperature. Similar to the technique previously described, the steel is 
then cooled by water, oil, air quenching to at or below the martensite 
start temperature, but not the martensite finish temperature. 
So formed, the microstructure of the steel in the comminuting media is 
altered to a martensite/austenite structure containing retained austenite. 
The steel at this point in time has a minimum unworked hardness of at 
least 20 HRC. 
The presence of the alloying elements in the steel, such as chromium or 
manganese, lowers the Mf temperature so that during quenching only a 
portion of the austenite transforms to martensite. Moreover, the 
preselected steel composition has the benefit that some of the austenite 
which is retained in the steel is transformable to martensite. Some 
portion of the austenite retained in the martensitic/austenitic structure 
must be transformable into martensite in order for the steel to exhibit 
the desired wear characteristics for a comminuting media. This form of 
austenite is distinguished from stable austenite which does not transform 
to martensite during subsequent working of the steel as now described. 
The steel of the comminuting member is next worked or deformed. Preferably, 
this is accomplished at the same time the comminuting member is used 
during the normal operation of the comminution process in which the 
comminuting member is present. In particular, the surface of the 
comminuting element is continually worked by the contact of the element 
against ore or against other loose or fixed comminuting elements. This 
working has the effect of transforming the retained austenite into 
martensite at the surface of the element. The resulting surface hardness 
of the martensitic surface structure of the element is greater than 50 HRC 
and may characteristically reach a hardness in excess of 60 HRC, although 
such measurements are difficult to make due to the thinness of the layer 
of martensite. 
Importantly, the comminuting media is durable even when subject to high 
impact loading. It is believed that the high percentage by volume of 
retained austenite in the microstructure of the element has the effect of 
bonding the areas of martensite together, minimizing the formation of 
cracks and other defects which would otherwise cause failure of the 
element during loading if the comminuting media were comprised solely of 
martensite. 
At the same time, however, the working of the comminuting media has the 
effect of transforming, especially at the surface where the loading is 
highest, the transformable retained austenite to durable, wear resistant 
martensite. As the martensite wears away at the surface, new martensite is 
continually created through the transformation from retained austenite. 
Notably, the above-referenced comminuting media does not necessarily have a 
microstructure before impact loading which provides for the highest 
hardness, contrary to the "rule of thumb" that for maximum wear resistance 
a steel should have the highest hardness possible. For example, for a one 
percent (1%) carbon steel, the steel microstructure which provides for the 
maximum hardness is likely to provide a hardness which may be 4 to 5 HRC 
or higher than comminuting media with the microstructure in accordance 
with the present invention. 
While the Ms temperature of a steel formed in accordance with the present 
invention is indicative of a steel having the desired level of retained 
austenite, such can be verified physically. In particular, x-ray 
diffraction techniques well known to those skilled in the art may be 
utilized to verify the level of retained austenite in the as-formed steel. 
As used herein, the method to determine the retained austenite level in the 
steel is an extension of ASTM (American Society for Testing Materials) 
Method E975. This extension makes use of three FCC (austenite) peaks 
(i.e., {111}, {200} and {220}) and three BCC/BCT (ferrite/martensite) 
peaks (i.e., {110}, {200} and {211}) rather than the two peaks for each 
specified in ASTM Method E975. This modification is to minimize the 
effects of preferred orientation in the determination of retained 
austenite. Also, chromium x-radiation is utilized to enhance the 
resolution. Typically, a one degree divergence slit collimator is used for 
limiting the amount of test area radiated. Such determinations are made in 
a manner well-known to those skilled in the art of using x-ray analysis 
for quantitative phase determinations. It is understood that the 
measurement yields the amount of retained austenite in the matrix of the 
material and not necessarily the amount of retained austenite in the total 
material which might typically include carbides and other nonmetallics. 
Those skilled in the art of x-ray analysis will appreciate the importance 
of further defining the sample site for the determination of retained 
austenite in the comminuting member to be tested. For the purposes of this 
invention, the sample site will naturally be selected near the surface of 
the comminuting member. When testing 6" (15 cm.) diameter grinding 
spheres, for example, we would consistently use a sample site beginning 
approximately 0.25" (0.6 cm.) beneath the outer surface of the sphere as 
manufactured and a one degree divergence slit collimator setting for the 
x-ray diffraction equipment. This procedure results in a sample site 
selection wherein the outermost layer of the grinding member represents 
roughly 25 percent of the total volume of the member. Other sample site 
locations may be selected in accordance with accepted standards of x-ray 
analysis and laboratory technique as may be required by the configuration 
of the comminuting member to be analyzed, or as may be suggested by the 
condition of wear if a used comminuting member is under consideration. 
As stated above, it has been determined that the Ms temperature is an 
indicator of the retained austenite in the steel's microstructure. As can 
be understood, calculating Ms from the Nehrenberg equation is often 
difficult because the weight percentages of each element in solution must 
somehow be known. 
In the prior art, when a steel is prepared to maximize hardness, there are 
carbides and alloying elements present in the structure. With these 
elements out of solution, Ms must be calculated from the Nehrenberg 
equation with either estimated "in solution" weight values or bulk weight 
values, both of which tend to provide rather inaccurate results. 
In accordance with the present invention, as stated above, the steel is 
heated to a sufficiently high temperature and for a sufficiently long time 
to dissolve the carbides and alloying elements so that use of the 
Nehrenberg relationship is effective in estimating Ms. At such elevated 
temperature, the weight values of the elements are their bulk values 
within the steel. 
As can be further understood, when the comminuting media is manufactured so 
as to provide the optimum structure as including retained transformable 
austenite as disclosed above, the Nehrenberg equation may be utilized to 
aid in customizing the steel. The proper Ms temperature may be, as set 
forth above, chosen by varying the weight amounts of various elements. 
Thus, the weight amounts of the elements may be chosen to provide the 
optimum structure as set forth herein, but at the same time, those 
elements which are most cost effective may be added in greater amounts to 
create a steel with the desired Ms. Also, in those instances where the 
inclusion of one element may be detrimental for reasons other than its 
effect on Ms, the amount of another element or elements may suitably be 
increased in the alternative to provide the desired Ms. For example, while 
carbon is relatively cheap and has a strong effect on Ms, inclusion of 
greater than about 1.2 percent by weight often has detrimental side 
effects. By simply increasing the amounts of one or more other elements 
such as chromium or manganese, however, the desired Ms and thus the 
desired microstructure in accordance with the present invention may still 
be obtained. 
In accordance with the present invention, it is believed that at least one 
entirely new class of products has been invented when considering the 
composition and processing of the steel. One well known low hardness, high 
alloy steel, known as Hadfield steel, utilizes typically over 6 percent Mn 
in order to stabilize the austenite and maintain a stable austenitic 
structure. On the other hand, Mn is utilized in amounts of less than 1.5 
percent in other steels in order to form compounds with sulfur in the 
steel so that the sulfur will not form low melting iron sulfides. In 
accordance with the present invention, however, it is proposed to utilize 
between about 1.5 percent and 6 percent Mn with proper amounts of other 
elements to provide a steel within the desired Ms temperature range and 
thus the desired retained transformable austenite in accordance with the 
present invention. 
So that our invention could be more readily understood, the detailed 
description to this point has been intentionally limited to 
microstructures having only two phases (i.e., martensite and retained 
austenite). However, the foregoing principles are equally adapted to the 
matrix present in nonhomogeneous microstructure systems. 
One such possibility is the matrix resulting from intercritical heat 
treatment. By the term matrix of a steel, we refer to that portion of the 
structure which is not carbides, nitrides, sulfides, oxides or other 
desired or attendant phases that may occur in steels either intentionally 
or because they cannot be avoided in the steelmaking process. The matrix 
then is that portion of the steel which contains or supports all other 
constituents. 
Consistent with the goal achieved in the two phase microstructure, the 
invention provides a matrix for nonhomogeneous microstructure systems 
wherein the matrix substantially comprises martensite and retained 
austenite. It is the composition of this matrix, and not necessarily the 
bulk or total composition of the alloy, which is the key to obtaining the 
proper Ms and proper amount of retained austenite. 
Intercritical heat treatment austenitizes a steel alloy in a two phase 
austenite and carbide region rather than in a single phase austenite 
region of the phase diagram. In this case, at equilibrium, the composition 
of the matrix would be determined from that portion of a tie line which 
intercepts the Acm (upper critical temperature line on the hypereutectoid 
side) and would have the same composition as an alloy with that same 
composition heated into the single phase austenite region. 
In any event, and regardless of whether intercritical heat treatment is 
employed or not, it is important and therefore a key feature of this 
invention that the matrix itself comprises at least approximately 40 
percent by volume of retained austenite. 
Another example of a nonhomogeneous microstructure system for which our 
invention may be adapted is a matrix resulting from a non-equilibrium heat 
treatment. It is possible to austenitize a steel at a temperature for 
which the phase diagram would indicate a single phase austenite region at 
equilibrium conditions, but because the system does not attain equilibrium 
or near equilibrium conditions (e.g., time at temperature is limited to a 
practical time from an engineering standpoint), not all carbides are taken 
into solution in the austenite at the austenitizing temperature. In such 
case, the steel when cooled to room temperature would consist of 
martensite, retained austenite, undissolved carbides and other 
constituents as described above. 
Therefore, it is possible to attain the minimum desired amount of retained 
austenite of at least 40 percent by volume even though all of the carbides 
and alloying elements are not in solution. The important feature is that 
the matrix itself resulting from the non-equilibrium heat treatment 
comprises at least approximately 40 percent by volume of retained 
austenite. 
From the foregoing it will be seen that this invention is one well adapted 
to attain all ends and objects hereinabove set forth together with the 
other advantages which are obvious and which are inherent to the 
structure. 
It will be understood that certain features and subcombinations are of 
utility and may be employed without reference to other features and 
subcombinations. This is contemplated by and is within the scope of the 
claims. 
Since many possible embodiments may be made of the invention without 
departing from the scope thereof, it is to be understood that all matter 
herein set forth is to be interpreted as illustrative, and not in a 
limiting sense.