|Title of Invention||
METHOD FOR HEAT TREATING CAST PARTS PRODUCED FROM A LIGHT METAL MELT, IN PARTICULAR AN ALUMINIUM MELT
|Abstract||The present invention relates to a method for heat treating cast parts produced from a light metal, in particular an aluminium melt, wherein the cast part is quenched after an annealing treatment or is quenched from the casting heat, after quenching is cooled to a low temperature and following low temperature cooling is suddenly heated to a high temperature in that it is immersed in a salt melt, of which the temperature is above the boiling temperature of water at normal pressure.|
|Full Text||Method for heat treating cast parts produced from a light metal melt, in
particular an aluminium melt
The invention relates to a method for heat treating cast parts produced from a light
metal melt, in particular an aluminium melt.
Maximum strengths are demanded of cast parts produced from light metals and which
are highly stressed in practical use. This applies in particular to cylinder heads,
manufactured from aluminium, of internal-combustion engines, which are exposed, in
particular in the case of diescl engines, to considerable loads owing to constantly
increasing ignition pressures in practical operation.
High strengths of light metal materials can be obtained by selecting a suitable alloy
which is subjected to a heat treatment after casting. Thus, for example in heat-
treatable aluminium alloys it is possible to considerably increase the strength level bv
applying what is referred to as a T6/T7 heat treatment. The maximum increase
potential then exists if. after solution treatment, the castings are quenched in a water
bath and then stored under heat.
However, the advantages of water quenching are confronted by the decisive
disadvantage that high inherent stresses can be established, in particular in the
quenching of components with a complex form. These inherent stresses lead to the
quenched component indeed having increased strengths but these strengths are
cancelled by the drawbacks caused by the inherent stresses or are even exceeded by
To remedy the drawbacks associated with quenching a thermal post-treatment has
been proposed in the article ' Uphill quenching of aluminium: rebirth of a little-known
process' by T. Coucher. published in Heat Treating/October 1983, page 30 IT., in
which, after quenching, the east parts are immersed in a bath of liquid nitrogen, of
which ,the temperature is approximately -196°C. They are kept at this low temperature
until a homogenous temperature distribution has been established in the cast part.
As soon as the homogeneous temperature distribution is achieved, according to the
known method the component cooled to a low temperature is suddenly uphill
quenched to a high temperature. The cold component is subjected to a hot jet of steam
or immersed in a hot water bath for this uphill quenching.
Uphill quenching to the high temperature brings about equilibrium in the stress profile
which has been established during quenching of the cast part. This effect may be
explained in that during quenching a decreasing temperature gradient builds up in the
direction of the outer regions, starting from the core of the cast part. As a result of the
edge layer's attempt to contract as a consequence of cooling, considerable
compressive stresses occur between the edge layers and the core zone of the casting.
If the cast part is cooled to a low temperature and then suddenly reheated the edge
layer heats up while the core region continues to remain cold. As the core hinders
their expansion the outer regions are subject to compressive stresses as a consequence
of heating which are the exact reverse of the compressive stresses which are
established as a result of quenching. The sudden reheating after the low temperature
cooling as such consequently constitutes a reversal of the quenching process both with
respect to the measures carried out and with respect to the stresses that occur in the
Practical tests have shown that while cast parts may be produced using the known
uphill quenching method, of which the inherent stresses are improved compared with
cast parts quenched only conventionally, however this mode of operation has proven
to be insufficiently effective, in particular for large-scale series production.
Starting from the prior art described above, the object was therefore to create a
method with which inherent stresses of castings with a complex form may be
eliminated particularly effectively and which can simultaneously be used
inexpensively and efficiently in the framework of series production.
Starting from the prior art described above, this object has been achieved according to
the invention by a method for heat treating cast parts produced from a light metal
melt, in particular an aluminium melt, wherein the cast part is quenched after an
annealing treatment or is quenched from the casting heat, after quenching is cooled to
a low temperature and following low temperature cooling is suddenly heated to a high
temperature in that it is immersed in a salt melt, of which the temperature is above the
boiling temperature of water at normal pressure.
The invention is based on the idea of improving the insufficiently effective uphill
quenching of the water steam used in the prior art for this purpose by using a salt melt
for rapid heating. The advantage of the use according to the invention of a salt melt
consists in that a melt of this type can be heated to temperatures which are clearly
above the boiling point of water at normal pressure. Salt melts may therefore be
heated to temperatures of 150°C and above without difficulty. Salt melt temperatures
of 250°C and above may therefore be established in order to achieve the largest
possible temperature difference between the low temperature and the salt melt
A further advantage of using salt melts as the medium for transferring heat during the
suddenly occurring uphill quenching consists in that the heat transfer between a salt
melt and the respective casting is much better than in the prior art, wherein the cast
part was merely subjected to water steam. In addition there is the fact that salt melts
may be controlled substantially better than water steam despite their much higher
As a result, the temperature gradient fundamental to the success of the entire heat
treatment process may be much improved compared with that in the prior art by uphill
quenching, which according to the invention takes place in the salt melt, of the cast
parts previously cooled to a low temperature. A high temperature gradient produces a
fundamentally higher compensation of the inherent stresses that exist in the respective
cast part after quenching.
The invention therefore makes available an inexpensive and reliably executable
method for heat treating castings, and in addition this method also leads to cast parts
which, compared with the parts produced according to the prior art. have improved
properties. High-quality components made of light metal, in particular aluminium,
may therefore also be produced using the method according to the invention if these
cast parts have a particularly complex and llnely structured form, such as is the case
in cylinder heads for internal-combustion engines for example.
In principle, the invention can be applied irrespective of how the respective cast part
has been quenched. However, it has proven to be particularly effective if the cast parts
have been quenched in a manner known per se with the aid of water or a comparably
intensively acting quenching agent.
In view of the fact that the greatest possible differences between the low temperature
to which the cast parts are cooled after quenching and the uphill quenching
temperature of the salt melt, in which the cast parts subjected to intense cooling are
uphill quenched, are sought, it is advantageous ifthc low temperature is less than -
180°C. This may be achieved in that, for cooling to the low temperature, the quenched
cast part is immersed in liquid nitrogen which at normal pressure has a temperature of
A further configuration of the invention already indicated above provides that the salt
melt is heated to at least I50°C, in particular to al least 250°C, in order to boost the
effect of the temperature differences existing between the intensely cooling and uphill
The salt concentration of the salt melt used according to the invention is preferably at
least 98% by weight, so that high bath temperatures are reliably achieved and an
equally high thermal conductivity of the melt is ensured with regard to the
respectively treated cast part. In this case nitrates and/or chromates. in particular alkali
metal nitrates and chromates or alkaline earth metal nitrates and chromates. such as
NaNO3. KNO3 or Na2CrO4, are used as the salts.
Practical tests on cylinder heads cast from an AlSi7MgCu0.5 aluminium alloy have
impressively confirmed the effectiveness of the method according to the invention.
After n preceding annealing treatment at a temperature of 520°C the cylinder heads
were cooled in water to approximately 60°C. After a short rest phase in air low
temperature cooling took place in liquid nitrogen at a low temperature of-I96°C. The
cooling time lasted until there was a homogeneously uniform temperature distribution
in the respective cylinder head. As soon as this state was achieved sudden heating to
approximately 240°C took place.
For this purpose, the cylinder heads were immersed in a salt melt at a temperature of
more than 250°C and which consisted of 52% by weight NaNO;,. 46.4% by weight
KNOi. 1.3% by weight N()2 and 0.24% by weight Na2Cr04 In a first test run the
cylinder heads uphill quenched in this manner from the low temperature to the high
temperature were cooled to room temperature in that water was washed over them.
This served to reliably rinse off the salt residues adhering to the cylinder heads. In a
second test run the uphill quenched cylinder heads were cooled to room temperature
over a much longer time in still air.
Diag. 1 shows the course of the temperature T of the castings during the heat
treatment according to the invention plotted over time. In this ease the test run in
which cooling of the uphill quenched cylinder heads took place with water is
designated 'uphill 1 water', while the test run in which the subsequent cooling took
place with air is labelled "uphill + air".
By applying the mode of operation according to the invention the inherent stresses of
the cylinder heads could be reduced to an extent which approximates that which is
only achieved in a conventional procedure if, following the annealing treatment, the
components are quenched not in water but relatively slowly in air.
Diag. 2 shows the success of the method according to the invention. During cooling in
air after the annealing treatment inherent stresses are achieved in the region of only
21 MPa. This value is illustrated in Diag. 2 by the column 'air'. However, the
components cooled slowly in air after the annealing treatment have only low
strengths. If after the annealing treatment quenching in water takes place, the level of
inherent stresses in cylinder heads, which arc conventionally cooled in water to 60°C
without subsequent low-temperature uphill quenching heating, is 103 MPa. The
relevant value is represented in Diag. 2 by the column 'water60'. If, on the other hand,
the above described heat treatment according to the invention takes place after
quenching in water, in the event that after uphill quenching to 240°C cooling with
water is carried out. there are inherent stresses in the region of 42 MPa (column
'uphill + water' in Diag. 2). An even better reduction in the inherent stresses is
achieved if the cylinder heads are cooled slowly in air to room temperature after
uphill quenching (column 'uphill -1- water' in Diag. 2). The cylinder heads heat treated
in this manner according to the invention have inherent stresses of 27 MPa and are
thus, with substantially greater strengths of the casting, greater by only 6 MPa than
the inherent stresses which are achieved purely by cooling in air.
The invention thus allows the advantages of quenching that takes place with a high
cooling rate, namely increasing the strength of the respective cast part, to be used
without large inherent stresses having to be accepted in the process. The components
obtained have high strength and minimised inherent stresses and therefore can cope
with even the highest loads in practical operation.
1. Method for heat treating cast parts produced from a light metal, in
particular an aluminium melt, wherein the cast part is quenched after an
annealing treatment or is quenched from the casting heat, after
quenching is cooled to a low temperature and following low temperature
cooling is suddenly heated to a high temperature, and wherein, it is
immersed in a salt melt, of which the temperature is above the boiling
temperature of water at normal pressure.
2. Method as claimed in Claim 1, wherein the cast part is quenched with the
aid of water.
3. Method as claimed in Claim 1 or 2, wherein the low temperature is less
than - 180°C.
4. Method as claimed in any one of the preceding claims, wherein, for
cooling to the low temperature, the cast part is immersed in liquid
5. Method as claimed in any one of the preceding claims, wherein the cast
part is cooled to the low temperature until its core temperature is
substantially equal to the low temperature.
6. Method as claimed in any one of the preceding claims, wherein the salt
melt is heated to at least 150°C.
7. Method as claimed claims 6, wherein the salt melt is heated to at least
8. Method as claimed in any one of the preceding claims, wherein the salt
melt has a salt concentration of at least 98% by weight.
9. Method as claimed in any one of the preceding claims, wherein the
nitrate and/or chromates, in particular alkali metal nitrates and chromates
or alkaline earth metal nitrates and chromates, such as NaNO3, KN03 or
Na2CrO4 are used as the salts.
10. Method as claimed in any one of the preceding claims wherein the cast
parts are cylinder heads.
11. Method as claimed in any one of Claims 1 to 9, wherein the cast parts are
The present invention relates to a method for heat treating cast parts produced
from a light metal, in particular an aluminium melt, wherein the cast part is
quenched after an annealing treatment or is quenched from the casting heat,
after quenching is cooled to a low temperature and following low temperature
cooling is suddenly heated to a high temperature in that it is immersed in a salt
melt, of which the temperature is above the boiling temperature of water at
|Indian Patent Application Number||2601/KOLNP/2005|
|PG Journal Number||25/2010|
|Date of Filing||14-Dec-2005|
|Name of Patentee||HYDRO ALUMINIUM DEUTSCHLAND GMBH|
|Applicant Address||ETTORE-BUGATTI-STRASSE 6-14, 51149 KOLN|
|PCT International Classification Number||C22F 1/00|
|PCT International Application Number||PCT/EP2005/003023|
|PCT International Filing date||2005-03-22|