Process for the production of a composite metal part and products thus obtained

The present invention relates to the production of metal parts whose core and surface must have different characteristics. Its subject comprises firstly a process for the production of a composite metal part by coating or hard-surfacing a metal core of low-alloy structural steel with a harder metal layer composed of a high-speed chromium-tungsten or chromium-molybdenum steel or a high-speed steel containing chromium associated at the same time with a plurality of elements such as tungsten, molybdenum, vanadium, and cobalt, this process being at the same time characterized in that this high-speed steel has a hardness higher than 57 Rockwell C, in that the low-alloy steel which constitutes the metal core is selected to be compatible with the said high-speed steel, in that the said high-speed steel is applied exclusively in the form of a prealloyed powder, and in that the coating or hard-surfacing of the core with this powder is effected with the aid of one of the welding processes of the follow-group: welding by means of a transferred or semi-transferred arc plasma torch, or welding by means of a laser torch, the operation of coating or hard-surfacing by welding being followed by heat treatment. Another subject of the invention comprises the products obtained by the abovedescribed process. It is particularly applicable to the production of rolls for cold rolling mills, rolling or forming rollers, shear blades, wear plates, and armor plates.

FIELD OF THE INVENTION 
The present invention relates to the production of metal parts whose core 
and surface must have different, and often even contradictory 
characteristics, since they are subjected to stresses which are quite 
different in the core and on the surface. 
This is the case, for example, with the rolls of cold rolling mills or with 
certain rolling or forming rollers, or again with blades of shearing 
machines, wear plates or armour plates. 
For all these parts the core must have low fragility, and the surface must 
on the contrary have high tensile strength, fatigue strength, abrasion 
resistance, and great hardness. 
PRIOR ART 
These parts are traditionally made from a homogeneous material, their 
surface being subjected to a special heat treatment. 
More recently, use has sometimes been made of composite metal parts. 
In all cases, the production of these metal parts, in which the core and 
the surface undergo different stressing, entails difficult problems. 
The difficulties encountered in the production of rolls for cold rolling 
mills will now be described by way of example. 
Rolling is a metallurgical operation which consists in reducing the section 
of a solid hot or cold product by passing it between two bodies of 
revolution, known as rolling mill rolls. The rotation causes the product 
to pass between the two rolls, and the thickness of the product is reduced 
to the gap between the two generatrices of the rolls. 
The products are rolled either in the hot state (up to 1200.degree. C.), or 
in the cold state. In the first case, the stresses are above all of 
thermal origin, while they are essentially of mechanical origin in the 
second case. 
The rolls of cold rolling mills must withstand three types of mechanical 
stressing: 
(1) The cyclic deformations to which the entire roll is subjected, firstly 
through the bending stresses resulting on the one hand from the pressures 
applied to the journals and on the other hand from the reaction of the 
product rolled, and secondly through the torsional stresses resulting from 
the driving torque; 
(2) The cyclic rolling stresses which are deduced from the Hertz contact 
theories. The compressive stresses attain very high values at the surface, 
while in the underlying layer the main shearing tension, acting in a plane 
oriented at 45.degree. in relation to the direction of application of the 
load, has a maximum at a point called the "Hertz point". Compared with hot 
rolling, these Hertz stresses are considerable in the rolls of cold 
rolling mills, because the flow stresses of the cold metal are very high, 
particularly in the case of hard, strain hardened metal. 
As an indication, evaluation of the stesses in a roll of a diameter equal 
to 600 mm leads to maximum compressive stresses of the order of 1000 to 
1500 N/mm.sup.2, and the maximum shearing tension at a depth of between 3 
and 5 mm under the skin is of the order of 300 to 600 N/mm.sup.2. 
During the rotation of a roll, each fiber of this roll is cyclically 
stressed. 
(3) The wear caused by the sliding of the product being rolled in the nip 
of the rolls. This wear results in deterioration of the surface of the 
rolls, which in turn impairs the appearance of the surface of the rolled 
product. The worn cylinder must be reconditioned by the removal of 
material. 
These three categories of stresses impose contradictory properties on the 
roll: 
The core must have moderate tensile strength of the order of 1000 MPa, 
accompanied by good resilience; 
The working layer must have great resistance to cracking through fatigue, 
great hardness of 85 to 100 Shore (60 to 66 HRC*), a low coefficient of 
friction and high abrasion resistance. 
FNT (*) HRC=Hardness Rockwell C according to the Standard AFNOR No. NF A 03153. 
Traditionally, the rolls of cold rolling mills are made of a homogeneous 
material. This material can only be a compromise which must provide at one 
and the same time the tenacity of the core and the strength of the working 
layer; the entire roll is treated for the level of strength required in 
the core, while the properties of wear resistance and fatigue strength are 
imparted to the working layer by a hardening treatment. 
The great majority of rolls of cold rolling mills are thus made of grades 
of steel derived from 100 C 6, a steel containing 1% C and 1.5% Cr, or 85 
CDV 7, a steel containing 0.85% C, 1.75% Cr, with a little molybdenum and 
a little vanadium, although these steels do not represent the optimum for 
the working layer. 
For rolls of small diameters, and more particularly for those intended for 
multiroll stands, use may also be made of more highly carburized and 
higher-alloy steel grades, whose behavior in respect of wear is better 
than that of steels derived from 100 C6. These are chromium steels of the 
Z 150 CDV 12 type, a steel containing 1.5% C, 12% Cr, with a little 
molybdenum and a little vanadium, or steels derived therefrom. 
For these last-mentioned rolls, intended for multiroll stands, important 
progress has been made by the use of high-speed vanadium steels. Their 
high content of vanadium carbides V.sub.4 C.sub.3 very substantially 
increases wear resistance and the efficiency and life of the rolls. 
However, it is difficult to envisage the production of large rolls with 
these high-speed types of steel, because of the technological difficulties 
and the prohibitive cost. 
Nevertheless, it is possible to extend to rolls of all diameters the 
advantage of the use of high-speed steels, if bimetallic rolls are made, 
in which only the working layer is composed of high-speed steel. 
The composite construction principle is already used for the repair or 
manufacture of certain rolls for hot rolling. The working layer is 
restored to size, or formed, by depositing on the core of the roll a 
material suitable for use in hot rolling; this material is usually a 
low-alloy steel a little harder than the core of the roll and is selected 
for its good resistance to heat fatigue, and therefore of moderate 
hardness. This hard-surfacing is effected by means of a customary welding 
process, the most usual being the submerged-arc process using a weldable 
steel wire or strip. These hard-surfacing processes are suitable for rolls 
for hot rolling mills because the mechanical stresses to which these rolls 
are subjected are relatively moderate; they perish essentially through 
surface cracking of thermal origin and through hot wear, while rolling 
pressures entail only very low mechanical stresses in comparison with 
those undergone by a roll in a cold rolling mill. 
By way of comparison, the working layer of cold rolling rolls requires more 
resistant materials and, above all, a very high metallurgical quality 
which is not obtained by the processes used for hard-surfacing hot rolling 
rolls; thus, in the working layer of cold rolls the inclusions or defects, 
beyond a critical size, may become sites for the commencement of fatigue 
cracking.

SUMMARY OF THE INVENTION 
The aim of the invention is to provide composite metal parts which have 
good resistance to stresses of high intensity, which are different in the 
core and on the surface, to an extent far greater than in known solutions, 
by simultaneously making use of the excellent properties of high-speed 
steels on the one hand and of the most modern welding processes on the 
other hand. 
To this end, the subject of the present invention is a process for the 
production of a composite metal part by coating or hard-surfacing a metal 
core of low-alloy structural steel with a harder metal layer composed of a 
high-speed chromium-tungsten or chromium-molybdenum steel or a high-speed 
steel containing chromium associated at the same time with a plurality of 
elements such as tungsten, molybdenum, vanadium, and cobalt, this process 
being at the same time characterized in that this high-speed steel is so 
selected as to exhibit a hardness higher than 57 Rockwell C, in that the 
low-alloy steel forming the metal core is so selected as to ensure a 
non-fragile connection to the high-speed steel and to be compatible with 
the utilisation stresses of the part in question, in that the said 
high-speed steel is added solely in the form of a prealloyed powder, and 
in that the coating or hard-surfacing of the core with this powder is 
effected with the aid of one of the welding processes of the following 
group: welding by means of a transferred or semitransferred arc plasma 
torch or welding by means of a laser torch, the operation of coating or 
hard-surfacing by welding being followed by tempering heat treatment or by 
hardening and tempering heat treatment adapted to the nature of the powder 
used. 
According to one particular characteristic of the present invention, the 
low-alloy structural steel which constitutes the metal core may 
advantageously have an analysis within the following range: 
______________________________________ 
C % = 0.20 to 1. Ni % = 0 to 2.5 
Mn % = 0.2 to 1.5 Cr % = 0.50 to 6 
Si % = 0.20 to 1 Mo % = 0 to 2 
S % = 0.005 to 0.200 
V % = 0 to 0.50 
______________________________________ 
A low-alloy structural steel of this kind is compatible with the welding of 
high-speed steels, in the sense that, taking into account the heat balance 
imposed by the welding and after suitable heat treatment, a hardness curve 
having no abrupt discontinuity at the weld, between the core and the 
welding metal, is obtained. In other words, a steel of this kind must make 
a tenacious joint with the high-speed steel to be welded on the surface, 
that is to say there must not be in the joint or in the underlying layer 
either a region of excessively weakened mechanical strength (which would 
entail the risk of the driving-in of the layer of high-speed steel) or a 
fragile region (which would entail the risk of the peeling-off of the 
layer of high-speed steel). 
The heat treatment in the zone affected by the surfacing operation is 
imposed by the surfacing itself, and therefore the steel selected for the 
core must be one whose transformation points and softening curves are well 
suited to the welding heat balance, so that a joint and an underlying 
layer are obtained which are both tenacious. 
According to another particular characteristic of the present invention, 
the high-speed steel powder, which after the welding forms the hard metal 
layer, has a composition within the range defined below: 
______________________________________ 
C % = 0.5 to 2.6 Mo % = at most equal to 12 
Mn % = 0.2 to 1.7 W % = at most equal to 20 
Si % = 0.2 to 1.4 V % = at most equal to 10 
S % = at most equal to 0.2 
Co % = at most equal to 16 
Cr % = 2 to 14 with: W % + V % + Mo % + 
Co % at least equal to 3. 
______________________________________ 
This high-speed steel powder may usefully contain in addition a content of 
boron at most equal to 2% and a content of silicon at most equal to 3%. 
Finally, it may usefully have an aluminum content at most equal to 1.2%. 
The invention also relates to all the composite metal products obtained by 
the previously mentioned processes, and in particular: 
Rolls for cold rolling mills, whose core of low-alloy structural steel 
constitutes the central portion, while the harder metal layer of 
high-speed steel constitutes the surface layer which alone comes into 
contact with the products which are to be rolled. 
Rollers for rolling or forming solid and hollow sections, whose core of 
low-alloy structural steel constitutes the central portion, while the 
harder metal layer of high-speed steel constitutes the surface layer which 
alone comes into contact with the products which are to be rolled or 
formed. 
Circular or straight shear blades, whose core of low-alloy structural steel 
constitutes the actual shear blade, including the edges, these edges being 
coated or surfaced with a harder metal layer of high-speed steel, which 
alone comes into contact with the product which is to be sheared. 
Wear plates and armour plates, whose core of low-alloy structural steel 
constitutes the thicker part and whose surface layer, which is harder and 
thinner, of high-speed steel constitutes the layer subjected to wear or to 
the shock of projectiles. 
Depending on the nature of the products to be obtained, the following must 
naturally be judiciously selected: 
the grade of low-alloy structural steel which constitutes the core of the 
composite part which is to be produced, which steel must make a 
non-fragile joint with the high-speed steel to be welded on the surface; 
the grade of high-speed steel constituting the powder to be welded, in such 
a manner that the hardness of the coating will exceed a hardness of 57 
Rockwell C, 
the welding process: transferred or semitransferred arc plasma torch, or 
laser torch; 
the heat treatment: for example tempering or a series of successive 
temperings, or hardening followed by one or more temperings, for the 
purpose of transforming the residual austenite and precipitating the 
carbides in the hard layer. 
Six examples of such choices are given later on. 
The grades of high-speed steel which can be used for the powder coating or 
surfacing of the core are within the following ranges of composition: 
______________________________________ 
(a) Chromium-tungsten steels: 
C %: 0.6 to 1.5 W %: 10 to 20 
Cr %: 2.5 to 7 V %: 0 to 6 
Mo %: 0 to 3 Co %: 0 to 2 
(b) Chromium-molybdenum steels: 
C %: 0.6 to 1.5 W %: 0 to 5 
Cr %: 3.5 to 5 V %: 0 to 4 
Mo %: 3 to 12 Co %: 0 to 2 
(c) Chromium-tungsten-molybdenum steels: 
C %: 0.6 to 1.8 W %: 5 to 12 
Cr %: 3.5 to 5 V %: 0 to 4 
Mo %: 3 to 12 Co %: 0 to 2 
(d) Chromium-tungsten-cobalt steels: 
C %: 0.6 to 1.8 W %: 10 to 20 
Cr %: 3.5 to 5 V %: 0 to 7 
Mo %: 0 to 3 Co %: 2 to 14 
(e) Chromium-molybdenum-cobalt steels: 
C %: 0.5 to 1.4 W %: 0 to 5 
Cr %: 3.5 to 5 V %: 0 to 5 
Mo %: 3 to 12 Co %: 0.3 to 12 
(f) Chromium-tungsten-molybdenum-cobalt steels: 
C %: 0.7 to 1.9 W %: 5 to 12 
Cr %: 3.5 to 5 V %: 0 to 7 
Mo %: 3 to 12 Co %: 2 to 15 
(g) Supercarburized chromium-tungsten-molybdenum- 
vanadium-cobalt steels: 
C %: 1.1 to 2.6 W %: 4 to 12 
Cr %: 3.5 to 7 V %: 2 to 10 
Mo %: 3.3 to 7 Co %: 8 to 16 
(h) Steels containing 12% of chromium: 
C %: 1.4 to 2 W %: traces 
Cr %: 11 to 14 V %: 0.4 to 1 
Mo %: 0.5 to 1.5 Co % 2.5 to 3.5 
______________________________________ 
The advantages of the products obtained by the processes according to the 
invention are due on the one hand to their composite construction, and on 
the other hand to the coating or surfacing by the welding of a high-speed 
steel of great hardness, exclusively in the powder state, followed by heat 
treatment well suited to the grade of this steel. 
The composite construction makes it possible to adapt the selected steels 
to their respective functions: mechanical durability for the core, 
resistance to fatigue and wear for the working layer, more effectively 
than in the case of homogeneous material. 
The composite construction represents an obvious saving of precious 
materials, because only the working layer is made of expensive materials. 
However, the invention offers the following additional advantages: 
Because of the welding methods used, and because of the use of a high-speed 
steel powder of great hardness, the invention makes it possible to obtain 
coatings in which metallurgical defects such as porosities, inclusions, or 
segregations are absent, or are sufficiently slight not to become sites 
for the commencement of fatigue cracks in the parts under the action of 
cyclic stresses during operation (example: cyclic Hertz stresses in the 
case of rolls for cold rolling). 
Moreover, the metal constituting the core of the composite part is selected 
from the composition range indicated above in such a manner that: 
(1) It ensures a tenacious joint between the coating and the core, so that 
there is no risk of decohesion between the coating and the core through 
the action of operating stresses on the surfaced part; 
(2) It provides the core with the mechanical strength properties desired 
for the specific use in question, after the heat cycles imposed on the 
core by the welding operation. 
In the case of rolls for cold rolling mills, the manufacturing processes 
claimed also constitute a saving of energy and a saving in general in 
comparison with the hypothetical construction of rolls made entirely of 
high-speed steel. A roll made of solid high-speed steel would in fact 
require, in the hot transformation and heat treatment stages, a greater 
consumption of energy than in the case of manufacture by the processes 
claimed. 
The composite construction makes it possible to extend to rolls of large 
diameter the benefits of a working layer of high-speed steel, whereas it 
would be difficult to contemplate the manufacture of these same rolls in 
solid high-speed steel, because of the technological and metallurgical 
difficulties that this would cause. 
The manufacturing processes claimed make it possible to obtain a 
homogeneous structure in the hard layer deposited, with a fine 
distribution of carbides and without metallurgical defects of the 
inclusion or porosity type, which would be capable of impairing the 
endurance properties of the material when subjected to mechanical fatigue 
stresses. 
The metal constituting the hard layer is applied in the form of powder in 
the processes claimed. This permits the use of supercarburized steels such 
as those mentioned above in (g), with a high proportion of V.sub.4 C.sub.3 
carbides, which are very efficient, even if these compositions are such 
that these steels could not be used by traditional welding means. 
The performance of the rolls produced according to the invention (expressed 
for example as the tonnage rolled before deterioration of the surface 
necessitates reconditioning) is improved in comparison with traditional 
rolls because of the gains in respect of coefficient of friction and wear 
resistance. The performance of these rolls may be twice or three times as 
high as that of traditional rolls. 
The surface quality of the products rolled with the rolls produced in 
accordance with the invention is greatly improved in comparison with 
products rolled with traditional rolls. 
EXAMPLES 
In order to enable the invention to be well understood, six forms of 
production of products according to the invention are described below as 
non-limitative examples. 
FIRST EXAMPLE 
Roll for the cold rolling of small strip. 
The starting blank is taken from a rolled bar of low-alloy structural steel 
of the following composition: 
______________________________________ 
C % Mn % Si % S % Ni % Cr % Mo % V % 
______________________________________ 
0.49 0.88 0.27 0.009 0.18 0.98 0.08 0.13 
______________________________________ 
Its heat treatment comprised hardening and tempering for a hardness of 320 
Brinell. The diameter of the blank is 170 mm. This blank is preheated to a 
temperature of 450.degree. C. The steel constituting the hard layer is 
deposited by welding with the aid of a semitransferred arc plasma torch 
fed by two generators, supplying the blown arc and the transferred arc. 
The deposition is effected in juxtaposed beads with slight overlapping, in 
such a manner as to obtain a uniform surface, and in a plurality of layers 
in order to obtain the desired thickness. The steel constituting the hard 
layer is applied in the form of a powder of a particle size between 60 and 
180 microns, with the following composition: 
______________________________________ 
Ni Cr Mo Co 
C % Mn % Si % S % % % % W % V % % 
______________________________________ 
0.86 0.22 0.23 0.010 
0.22 4.35 5.20 6.15 1.95 0.35 
______________________________________ 
The blown or pilot arc current is 85 amperes. The transferred arc current 
is 195 amperes. The arc voltage is 32 volts. At the end of the hard 
surfacing operation the part has a diameter greater than or equal to 203 
mm. 
The part is cooled in still air to ambient temperature, thus ensuring 
natural hardening of the hard layer. The part then undergoes a double 
tempering heat treatment at 550.degree. C., thus imparting to the 
deposited layer its optimum hardness, without affecting the hardness of 
the core. The working layer is then ground to the nominal diameter of the 
roll. Its surface hardness verified is 64 HRC (64 Hardness Rockwell C). 
FIG. 1 is a vertical section of the roll obtained after grinding machining. 
The core 1 of low-alloy steel is thus coated on its working surface 2 with 
a layer 3 of high-speed steel of a thickness of about 15 mm. 
After this layer 3 has worn in service, it is possible to resurface it by 
the process according to the invention. 
SECOND EXAMPLE 
Wire rolling roller. 
FIG. 2a is a vertical half-section of the blank for the roller, with its 
two circular grooves. 
FIG. 2b is a vertical half-section of the same roller after application of 
the invention and machining. 
The starting blank is taken from a rolled bar of low-alloy structural steel 
of the following composition: 
______________________________________ 
C % Mn % Si % S % Ni % Cr % Mo % V % 
______________________________________ 
0.40 0.70 0.73 0.005 0.095 3.18 0.81 0.29 
______________________________________ 
so that there is a higher content of chromium and molybdenum than in the 
preceding example. 
Its heat treatment comprised hardening and tempering for a hardness of 360 
Brinell.(*) The blank has a diameter of 192 mm and has two circular 
grooves. 
FNT (*) Brinell hardness according to AFNOR Standard NF A 03-152. 
The blank is preheated to a temperature of 400.degree. C. The steel 
constituting the hard layer is deposited by welding with the aid of a 
semitransferred arc plasma torch fed by two generators supplying the blown 
arc and the transferred arc. Hard-surfacing is effected in superposed 
beads. The first bead has a width of 10 mm. The superposed beads widen 
progressively to fill the entire width of the groove. The steel 
constituting the hard layer is applied in the form of powder of a particle 
size between 60 and 180 microns, with the following composition: 
______________________________________ 
Ni Cr Mo Co 
C % Mn % Si % S % % % % W % V % % 
______________________________________ 
1.95 0.35 0.30 0.005 
less 3.47 3.02 9.18 5.02 14.80 
than 
0.10 
______________________________________ 
The blown or pilot arc current is 95 amperes. The transferred arc current 
is 190 amperes. The arc voltage is 30 volts. 
At the end of the hard-surfacing operation the part is cooled naturally in 
still air to ambient temperature, thus ensuring the hardening of the 
deposited layer, and then undergoes five tempering heat treatments at 
550.degree. C. The hardness is 67 Rockwell C. 
The part is machined to the nominal diameter and the grooves are ground to 
the desired profile, as shown in FIG. 2b, in which the core is designated 
4 and the hard layer 5. 
THIRD EXAMPLE 
Roll for the cold rolling of wide strip. 
FIG. 3 is a vertical section of the roll after the final machining 
operation. 
The starting blank is taken from a rolled bar of low-alloy structural steel 
of the following composition: 
______________________________________ 
C % Mn % Si % S % Ni % Cr % Mo % V % 
______________________________________ 
0.47 0.90 0.24 0.012 0.16 1.02 0.07 0.14 
______________________________________ 
This is practically the same steel as in the first example. 
Its heat treatment comprised hardening and tempering to a hardness of 340 
Brinell. In the zone intended to receive the hard layer, the diameter of 
the blank is 174 mm. This blank is preheated to a temperature of 
500.degree. C. The steel constituting the hard layer is deposited by 
welding with the aid of a semitransferred arc plasma torch fed by two 
generators supplying the blown arc and the transferred arc. 
The material is deposited in juxtaposed beads with a slight overlap, in 
such a manner as to obtain a uniform surface, and in a plurality of 
successive layers in order to obtain the desired thickness. The steel 
constituting the hard layer is applied in the form of a prealloyed powder 
of a particle size between 60 and 180 microns, with the following 
composition: 
______________________________________ 
Ni Cr Mo Co 
C % Mn % Si % S % % % % W % V % % 
______________________________________ 
0.85 0.27 0.22 0.007 
0.15 4.60 5.15 6.2 2.05 0.4 
______________________________________ 
The blown or pilot arc current is 50 amperes. The transferred arc current 
is 215 amperes. The arc voltage is 33 volts. 
In the course of the hard surfacing, the temperature of the part is kept 
above 360.degree. C. by means of rows of heaters. At the end of the 
hard-surfacing operation the part must have a diameter greater than or 
equal to 202 mm. The heat balance of the operation makes it possible to 
effect the natural hardening of the steel deposited by simple natural 
cooling of the part in still air when the deposition is completed. 
The part is thereupon subjected to double tempering heat treatment at 
550.degree. C., which imparts its optimum hardness to the deposited layer 
without affecting the hardness of the core. 
The working layer is then ground to the nominal diameter of the roll. Its 
hardness is from 63 to 65 Rockwell C. The machining of the roll is 
completed in accordance with FIG. 3, in which the working surface 7 of the 
core 6 is covered with the hard layer 8. 
FOURTH EXAMPLE 
Circular shear blade. 
Known types of manufacture lead either to solid, homogeneous shears of 
steel or to shears having a steel core and an attached crown of carbide. 
The present embodiment of the invention may be applied in two possible 
ways: 
The first solution is illustrated in FIG. 4. 
It consists in circularly hard-surfacing, in accordance with the invention, 
ech of the two flanks 9 and 10 of the peripheral portion 11 of the core 
12. 
The second solution is shown in FIG. 5. 
A bar is cut into slices. One of the slices 13 is hard-surfaced in 
accordance with the invention at 14 by plasma torch welding. 
FIFTH EXAMPLE 
Straight shear blade. 
FIG. 6 is a vertical section of a straight shear blade according to the 
invention. 
The core 15 of low-alloy steel is hard-surfaced at 16 with high-speed steel 
by plasma torch welding. 
SIXTH EXAMPLE 
Wear plate (or armour plate). 
By plasma torch welding it is possible to deposit a layer of 6 to 12 
millimeters of high-speed steel on a plate of low-alloy structural steel. 
It is clearly understood that without departing from the scope of the 
invention it is possible to conceive variants and improvements of details, 
and also to contemplate the use of equivalent means.