Process for the heat-after treatment of a pig iron port

A iron alloy material for the manufacture of brake drums, specially of massive and ventilated brake discs and other braking bodies, which has a pearlitic structure with a 5% maximum portion of ferrite, and a tensile strength of at least 200N/mm.sup.2 and consists essentially of: PA0 carbon in an amount of 3.62 to 3.68 weight %; PA0 silicon in an amount not exceeding 2.10 weight %; PA0 manganese in an amount of 0.70 tp 0.85 weight %; PA0 phosphorus in an amount of less than 0.080 weight %; PA0 sulfur in an amount of less than 0.095 weight %; PA0 chromium in an amount of 0.18 to 0.25 weight %; PA0 molybdenum in an amount of 0.30 to 0.45 weight %; PA0 copper in an amount of 0.30 to 0.45 weight %; and PA0 iron in an amount of 92.045 to 94.9 weight %.

The invention concerns pig iron alloys for the manufacture of brake drums, 
massive and ventilated brake discs and other braking bodies having an 
alloy of 
over 3.6% carbon 
0.6 to 0.9% manganese 
1.8 to 2.5% silicon 
less than 0.1% phosphorus 
less than 0.12% sulfur 
and small component parts of chromium, molybdenum 
and copper 
the pig iron having a pearlitic structure. 
A pig iron alloy having this chemical composition has been described in 
DE-OS 33 05 184. Due to the development of new asbestos-free brake 
linings, it has become necessary also to use in brake drums, brake discs 
and the like, pig iron alloys that tolerate elevated temperatures. In this 
connection, the general tendency among the users has hitherto been toward 
employing iron sorts of high heat resistance and high carbon contents, but 
this has disadvantages, specially in relation to a coarse texture and to 
strength. 
DE-OS 33 05 184 proposed a material for braking bodies which was to have, 
on one hand, sufficient strength and on the other hand, a good heat 
conductivity and high damping property. In said publication it was said 
that even with low strength values the hot tensile strength at extremely 
high temperatures of GG 30 (GG is International Standard for Grey Iron) is 
only insignificantly higher or almost equal to that of GG 15. The grey 
cast iron of relatively low strength must in addition have less internal 
stresses, must become less heated during machining on account of the 
carbon content and must have under thermal load less warp phenomena than 
the grey cast iron of higher strength hitherto used. 
However, it has been shown in the practice that this grey cast iron has no 
small amounts of ferrite portions. But ferrite portions in the brake drum 
have the disadvantage that the friction match, that is, the friction 
coefficient between the brake linings and the brake drums or brake discs, 
changes. This means that the delay when braking is less. Thus, if 
possible, as high as 100% pearlitic structure would be optimal. 
Besides, it has been found that in many cases the tensile strength of this 
pig iron is not sufficient.

This invention is based on the problem of providing a pig iron of the type 
mentioned at the beginning which has the least possible portions of 
ferrite together with high tensile strength. 
According to the invention this problem is solved by an alloy having a 
combination of the following features, namely, a pearlitic structure with 
the maximum portion of ferrite of 5% and a tensile strength of at least 
200 N/mm.sup.2 and consisting essentially of: 
carbon in an amount of 3.62 to 3.68 weight %; 
silicon in an amount not exceeding 2.10 weight %; 
manganese in an amount of 0.70 to 0.85 weight %; 
phosphorus in an amount of less than 0.080 weight %; 
sulfur in an amount of less than 0.095 weight %; 
chromium in an amount of 0.18 to 0.25 weight %; 
molybdenum in an amount of 0.30 to 0.45 weight %; 
copper in an amount of 0.30 to 0.45 weight %; and 
iron in an amount of 92.045 to 94.9 weight %. 
Due to the alloy components such as chromium, molybdenum, manganese and 
copper, which were established in lengthy tests there is obtained a 
tensile strength of at least 200 N/mm.sup.2. 
It has additionally been found in a surprising manner that copper and also 
molybdenum have a stabilizing effect on the pearlitic, and this without 
leading to precipitations of carbide. It was found that with the pig iron 
according to the invention there can be obtained a 100% pearlitic 
structure. 
Molybdenum in addition produces, in combination with chromium, a high core 
strength of the structure and as alloy component gives good heat 
resistance under alternating thermal loads of the brake discs. The carbon 
content of up to a maximum of 3.68% is obtained by smelting in the cupola 
furnace at C (Carbon) level 0.34 to 3.45%. The remaining 0.25 to 0.30% is 
introduced by a special inoculation process when tapping the fluid iron in 
the casting ladle by means of electrode graphite. The resulting optimal 
inoculation allows A-graphite of the size 3-4 to generate. It was 
surprisingly found here that carbide precipitations do not occur in the 
pearlitic structure despite alloy elements thereof such as chromium and 
molybdenum. 
The high carbon content causes many graphite precipitations with their 
surprising properties for brakes of heat conductivity and high thermal 
resistance. This means that the accumulation of heat on the brake friction 
rings can be distributed in the shortest time over the whole disc whereby 
thermal stresses and cracks from overheating are clearly reduced. 
It is also an advantage that due to the absence of carbide precipitations 
in the pearlitic structure there result no roughened surfaces, cracks 
produced by the expansion flaws in the friction ring surfaces and the 
appearance of hotspots. The disadvantageous pulsing of the brake pedal due 
to hardness variations of the discs in the materials hitherto used, 
insofar as this is caused by the material itself, is eliminated by the pig 
iron according to the invention. 
Since silicon considerably reduces the heat conductivity, a component part 
of 2.1% must not be exceeded, for this property works against the desired 
quick heat distribution in the disc. The given value of 0.08% for 
phosphorus must not be exceeded in order to prevent steatite and therewith 
hard components in the structure. 
Sulfur is brought up to a maximum of 0.095% to obtain the manganese-sulfur 
ratio, but it should not exceed said value. 
In the pig iron according to the invention there results a fine texture and 
the graphite lamina become somewhat shorter whereby can be achieved the 
high resistance according to the invention. A high carbon portion by 
itself works against this, that is, produces a reduction of strength and a 
coarse texture. Besides, a high carbon portion represents a cost item. In 
lengthy tests it has now been found that, contrary to the general opinion, 
it is possible to make do with small carbon contents, specifically in the 
established range of from 3.62 to 3.68% when this is combined with the 
other alloy components. In this case, the desired high strength and heat 
resistance are achieved. 
To temper the cast parts (artificial aging) after having been produced, 
there is proposed according to the invention a heat after-treatment known 
per se which in an inventive manner has been adapted to the pig iron 
according to the invention. 
In this connection it is proposed according to the invention that the parts 
to be treated be heated over 180 minutes to a temperature of from 650 to 
720.degree. C. and then kept at this temperature for 30 minutes after 
which a slow cooling to 250.degree. C. takes place in the annealing 
furnace. 
Prior to annealing the parts must be pre-machined; namely scrubbed. The 
skin of the rubbing surfaces and of the inner bottom of the container must 
be removed (about 1.5 mm.) By the heat after-treatment that follows 
according to the invention, there are eliminated both internal stresses 
and stresses resulting from the machine. It is also of the essence here 
that the higher internal stresses that can be generated as result of the 
increased strength obtained in a certain area by the alloy components can 
be prevented by the heat after-treatment according to the invention. 
In this manner there are still needed after the heat treatment only small 
machine operations whereby a renewed appearance of stresses can be 
avoided. 
It has been found that by said longer heating time for the parts to be 
treated their warping can be prevented. The cooling after the indicated 
thermal retardation must in any case be slow so that no new stresses 
generate. This can be obtained in a simple manner, for instance, by 
disconnecting the annealing furnace, the parts remaining for a still 
longer time in the annealing furnace that is slowly cooling.