Roll having a hard envelope surface

To increase abrasion resistance, a cast-iron roll is given a carbidic running-surface layer which has a defined depth of at least 1 mm and a fine-grained structure whose cementite dendrites have a length of at most 100 .mu.m and a thickness of at most 20 .mu.m. The fine-grained structure is produced by remelt hardening of the outer surface of the roll.

This invention relates to a roll having a hard envelope surface and a 
process of making the roll. More particularly, this invention relates to a 
roll for a papermaking machine having a hard outside surface. 
As is known, various types of rolls have been used in environments which 
require the rolls to have a hard envelope surface. For example, the rolls 
of papermaking machines which are used for smoothing, drying and creping a 
web of paper require hard running surfaces since these rolls are usually 
subjected to large forces and experience heavy wear. 
In many cases, the rolls or roll shells of the above type are produced as 
clear chilled castings in molds in which a chill casting surface layer 
with a carbide structure produced by chilling of the mold arises on the 
outside of a lamellar graphite cast iron core which solidifies as gray 
cast iron because of a relatively slow cooling. 
Because cooling rates differ, three discrete zones having different 
structures arise: 
(a) a gray cast iron core zone in which the carbon has separated out mainly 
as graphite, namely lamellar graphite; the structure in this zone is 
perlitic; 
(b) a transitional or mottled zone of white and gray cast iron in which the 
proportion of white (carbidic) cast iron decreases continuously towards 
the inside and the separation as lamellar graphite increases 
correspondingly; the extent and nature of this transitional zone vary 
within wide limits and escape arbitrary control; 
(c) a chilled zone in which the carbon has been completely bound as 
carbide; this zone has a coarse structure containing cementite dendrites 
whose length is mainly 200 .mu.m and whose thickness is mainly more than 
50 .mu.m. Dendrite orientation is relatively random; the coarse structure 
impairs abrasion resistance. 
Since defined chill cast layers are difficult to achieve in clear chill 
casting, rolls of the kind requiring a hard envelope cannot be produced 
from spheroidal graphite or vermicular graphite cast iron. 
Accordingly, it is an object of the invention to provide a roll having a 
hard envelope with a relatively high degree of abrasion resistance and 
with reduced grain sizes in a carbidic structure. 
It is another object of the invention to provide an economical process of 
making rolls with running surface of high abrasion resistance. 
It is another object of the invention to provide a roll with a carbidic 
structure of defined depth at an outer surface. 
Briefly, the invention provides a roll made of gray cast iron with a hard 
outside running surface which has a carbidic structure having a defined 
reference depth of at least one millimeter and with cementite dendrites 
having a maximum length of 100 .mu.m and a maximum thickness of 20 .mu.m. 
The term "defined" reference depth is intended to denote such depths of the 
layer having a carbidic structure which vary by not more than .+-.10% from 
their reference value over the whole running surface of the roll. 
The term "rolls" is intended in the present case to denote solid rolls and 
hollow rolls and particularly rolls that can be used in papermaking 
machines, for example, for smoothing, drying and creping a web of paper. 
If necessary, the rolls can be heated or cooled for these uses. The linear 
force pattern along a gap between a roll and, for example, another roll 
can be controlled along the roll length by variable internal or external 
forces. 
The invention also provides a process for the production of a roll with a 
hard running surface. In accordance with the process, a cast iron melt is 
first teemed into a sand mold to form a roll. The roll is then preheated 
from the cast state and the preheated running surface is then rapidly 
heated to above the liquidus temperature to melt the running surface of 
the roll locally, for example in a stepwise progressive manner in the 
circumferential and axial directions, to the intended reference depth, 
i.e. of at least one millimeter. The remelted running surface is then 
solidified by an immediately subsequent rapid cooling below the Ar.sub.1 
point of the iron-carbon diagram, whereafter the roll is cooled to ambient 
temperature. 
The rapid heating of the preheated running surface to at least the liquidus 
temperature of the case iron used i.e., to at least approximately 
1200.degree. C.--and the subsequent rapid solidification and cooling to 
below the Ar.sub.1 point of the iron-carbon-diagram--i.e., to below 
730.degree. C.--produces on the running surface a fine carbidic and, more 
particularly, a ledeburitic structure whose cementite dentrites do not 
exceed the above noted maximum dimensions. The point is that the treatment 
dissolves the carbon in the melted zone, the carbon subsequently 
crystallizing as iron carbide because of the high solidification and 
cooling rates. 
Towards the interior of the roll body, the carbidic layer is followed by a 
heat-influenced zone which can be considered to have a fine-perlitic 
bainitic quenched and tempered structure. The thickness of this latter 
zone corresponds at least approximately to the thickness of the preceding 
carbidic layer. 
The required remelt depth can be achieved only if the wall thickness is at 
least 5 times the remelt depth, otherwise sufficiently rapid cooling and 
solidification of the melted depth zone becomes uncertain. 
Since the depth of the melted zone in the remelting of the surface can be 
determined and observed relatively accurately, the required reference 
depth can be ensured over the entire surface to the required accuracy and 
with the required uniformity. 
To reduce the risk of cracking, particularly in the transition zone between 
the layer of the running surface and the roll body or core unaffected by 
remelting, the maximum reference depth of the carbidic structure is, with 
advantage, 8 millimeters (mm). This maximum reference depth makes economic 
sense since it can be achieved with cooling conditions that are not 
particularly expensive. 
The rolls which are produced have a minimum diameter of 200 millimeters and 
a minimum wall thickness of 20 millimeters. 
Very advantageously, the roll is made of spheroidal graphite cast iron and 
has the following composition (in weight %): 
______________________________________ 
C 2.3-3.8 
Si 1-3 
Mn 0.1-1 
P (max) 
0.08 
S (max) 
0.01 
Mg 0.03-0.08 
Fe Remainder 
______________________________________ 
The point is that the modulus of elasticity important for rigidity is the 
same over the whole roll cross-section including the carbidic layer and 
is, for example, approximately 160 000 to 170 000 N/mm.sup.2. In contrast 
to this, in previously known rolls, the modulus of elasticity in the 
carbidic layer is equally high, but drops continuously in the mottled zone 
in dependence upon graphite content and is only about 100 000 to 120 000 
N/mm.sup.2 in the gray zone. Also, spheroidal graphite cast iron has the 
further advantage of increased fatigue strength over con-ventional gray 
cast iron. 
Of course, the rolls can be made of vermicular graphite cast iron or 
lamellar graphite cast iron. In this case, the following composition (in 
weight %) has been found advantageous for rolls having segregations of 
vermicular graphite: 
______________________________________ 
C 2.8-3.6 
P (max) 
0.06 
Si 1-3 
Mn 0.1-1 
S (max) 
0.06 
Mg 0.01-0.04 
Fe Remainder 
______________________________________ 
For roll bodies made of conventional cast iron--i.e., of lamellar graphite 
cast iron--the following composition (in weight %) has proved 
advantageous: 
______________________________________ 
C 2.8-3.6 
P 0.01-0.5 
S (max) 
0.1 
Si 0.5-3 
Mn 0.2-1 
Fe Remainder 
______________________________________ 
Other roll properties such as tensile strength and fatigue strength can be 
improved in all three kinds of graphite segregations if the cast iron also 
contains at least one of the following alloy elements (in weight %): 
______________________________________ 
Ni 0.1-3 
Cu 0.1-2 
Mo 0.1-1 
Sn 0.01-0.2 
Cr 0.01-0.4 
B 0.01-0.1 
______________________________________ 
Since the roll surface requires machining after remelt hardening, the 
minimum depth of the melted and carbidically solidified running-surface 
zone must be--before such machining--approximately 1 millimeter (mm) more 
than the required reference depth. 
Advantageously, in the production process described, preheat temperatures 
of from 450.degree. to 600.degree. C. and heating rates of at most 
100.degree. C./hour are maintained and/or the cooling to ambient 
temperature after remelting proceeds at a maximum rate of 50.degree. 
C./hour. Conveniently, the latter cooling treatment can be performed in a 
furnace. 
The energy source for the melting operation can be provided by electric 
arcs. In this event, welding torches, particularly those which have a 
tungsten electrode and operate, for example, at an energy density of 2 to 
4 kW/cm.sup.2, are particularly suitable because of their relatively 
simple handling and for economic reasons. Since the melting proceeds 
ad-vantageously in a protective gas atmosphere, the energy source used is 
preferably in the form of TIG (tungsten inert gas) welding torches. 
However, the melting can be performed by means of laser or electron beams. 
The rapid cooling to temperatures below 730.degree. C. proceeds in still 
air. As previously stated, the minimum wall thickness specified ensures 
that heat is removed fast enough. The simplest way of achieving the 
stepwise advance of local melting is by a rotating and/or longitudinal 
relative movement between the torch and the preheated roll. It has been 
found convenient, in this case, to rotate the roll around the stationary 
torch while advancing axially in steps. 
Conveniently, to achieve a flaw-free running surface having a carbidically 
solidified structure, the roll is treated mechanically to remove any 
casting skin before treatment. After the treatment, the running surface is 
finish-machined conventionally, as previously stated, with carbidic 
running surfaces of from 1 to 8 millimeters (mm) then arising.

The following is one example of a roll shell made in accordance with the 
invention. 
A roll shell of 400 mm outer diameter and 70 mm wall thickness, made of 
spheroidal graphite cast having the following chemical composition (in 
weight %): 
______________________________________ 
C 3.4 
Si 2.4 
P 0.2 
S 0.1 
Mn 0.2 
Cu 1 
Mg 0.04 
Fe Remainder 
______________________________________ 
is cast as a sand casting in a sand mold. While in the cast state, the 
shell is initially preheated by a gas torch to a preheat temperature of 
500.degree. C. while slowly rotating. Next, the shell is rapidly heated by 
means of a TIG welding torch having a tungsten electrode of 3.2 mm 
diameter which is fixedly disposed opposite the outer periphery of the 
shell. The roll shell surface moves past the electrode at a speed of 
approximately 15 cm/min, a voltage U of 20.5 V between the electrode and 
the workpiece giving rise to an arc in which a current I of approximately 
200 A flows. A helium flow of 7 1/min from the torch maintains the melt 
zone of the torch arc in a protective gas atmosphere. 
Immediately after the torch, the molten zone cools immediately below the 
Ar.sub.1 point of the iron-carbon diagram. 
After completion of the remelting, the roll shell is 13 heated in a furnace 
preheated to 500.degree. C., then cooled slowly at a maximum cooling rate 
of 50.degree. C./hour. 
In the present example, this leads to a remelt depth of approximately 6 
millimeters (mm). 
Of note, all percentages indicating concentrations of compositions of 
material are to be understood as being in weight %. 
The invention thus provides a roll with a hard envelope surface which is 
characterized in having an improved abrasion resistance as well as a fine 
grained structure wherein the dendrites are of limited size. 
The invention also provides a relatively economical process for producing a 
hard layer on the running surface of a cast roll as well as a process 
which can use spheroidal graphite or vermicular graphite cast iron to make 
the roll.