FR2620735A1 - Process for the heat treatment of structural components made of nickel-based foundry alloys - Google Patents

Abstract

Process for the heat treatment of structural components made of nickel-based foundry alloys. The problem to be solved consists in removing the porosity regions by equaliser annealing. This process is characterised in that the component is heated to a temperature T1 from 0 to 10 DEG higher than the temperature of onset of melting Tm alpha / alpha ' of the phase containing carbide eutectics alpha / alpha ', the process is stopped at this temperature until the phase is completely homogenised. Cooling is carried out to a temperature T2 lower than Tm alpha / alpha ' but higher than the solubilisation temperature of the alpha ' phase, the process is stopped at T2 at a pressure of 900 to 2000 bars until the porosity is removed and cooling is carried out to produce the alpha ' phase. The invention can be applied especially to precision moulding.

Description

PROCESS FOR THERMAL TREATMENT
BUILDING ELEMENTS IN FOUNDRY ALLOYS
NICKEL BASE
The invention relates to a method for the heat treatment of structural elements of nickel-based foundry alloys.

 In the production of nickel-based casting alloys by the precision casting process, contradictory conditions are encountered with regard to the flowability of the material and its properties.

 In order for the molding shell of the building elements, which are often thin-walled and of complicated geometrical shape, to be completely filled with molten material, this molten material must not cool too quickly during casting. In other words, it must cool only slowly in the liquid melting zone. This slow solidification causes an excessive dendritic sedimentation or dendritic segregation of elements (for example Al, Ti, Nb) and constituents of the phases (for example the Si phase of carburized eutectic, generally phases with a low melting point). in the alloy.

Such a heterogeneous dendritic distribution greatly impairs the qualities of the building elements, especially the behavior, creep and relaxation of the material. A favorable point for the qualities of the material would be a uniform microstructure, in particular a uniform content, if possible high, in phase 2 soaking, with a defined shape of the particles. This uniform microstructure can only be obtained with the fastest possible solidification in precision casting. This results in an increased risk of filling defects and also a high foundry porosity value.

 In the first place, priority is usually given to a good flowability of the material, and during a heat treatment which follows, an attempt is made to improve the qualities of this material.

 Such a heat treatment process for superalloys is known from US Pat. No. 3,753,790. In this process, the low melting separated phases are homogenized by progressively raising the process temperature to a desired temperature. value lower than the melting start temperature. A disadvantage of this process is that the melting start temperature can not be exactly determined due to dispersions due to segregation. Thus, the residence temperature is kept too low below this temperature, which prevents a complete solution annealing and homogenization of the BJ phase, or the melting start temperature is exceeded and it occurs. then areas of unfavorable porosity by beginning of fusion.

 DE-OS 3415282 discloses a heat treatment process for monocrystalline objects, in which the heat is heated just above the melting start temperature to eliminate the areas of porosity by onset of melting, including areas from previous heat treatments.

 However, the process described in this document can only be used for monocrystalline objects since these only comprise a small amount of eutectic and a small amount of low melting point phase. They are also very homogeneous because of their solidification mode. This method is therefore not suitable for conventional polycrystalline molded elements.

The problem that arises at the base of the invention is to create a heat treatment process of polycrystalline materials allowing a
complete annealing annealing and equalizing the
phase γ separate curing, all zones of
porosity by beginning of fusion being eliminated.

The invention aims to solve this
problem and relates for this purpose to a process for the
heat treatment of building elements in
polycrystalline nickel-based foundry alloys,
characterized by the following steps in which
a) the building element is heated to a
temperature T1 greater than a value between 0
and at the first melting start temperature Tm 3 / ss of the carbureted E-eutectic phase.
b) we stop at this temperature T1 during
sufficient time to fully
solution and homogenize the phase separated \ partly
non-uniform due to casting,
c) cooled to a temperature T2
less than Tm wv />, but greater than the
temperature T of dissolution of the hard phase
separate N
d) we stop at this temperature T2 under
hydrostatic pressure between 900 and 2000
bars, until the porosity by beginning of fusion
be eliminated,
e) cooling at a rate of
cooling required to produce the desired phase
with separate particles.

Advantageous embodiments of
the invention are described below.

The process according to the invention presents the
particular advantages below.

Since the treatment temperature
thermal is high above the temperature of
start of merger Tm, we can get a
complete solution of the phase γ , especially in
the area of segregation. In addition, one can obtain
an even distribution of the separated elements, in particular a uniform distribution of the elements Al, Ti forming the phase Al and thus of the phase 8 '. An exaggerated proportion, due to the fusion, of the carburetted eutectics. and thus dissolved and homogenized. Additional forming elements are thus solubilized in the mass and can be used for the separate curing phase.

 At this temperature T1, small zones of fusion occur in the carburized phase of eutectics 8. These small isolated melting zones can form zones of porosity, called porosity zones by beginning of melting in a maximum volumetric proportion of approximately 0.5%. To eliminate this low porosity by onset of melting, the first heat treatment is followed by at least a second hydrostatic pressure shutdown period (HIP) at a temperature below Tm X / sec, but greater than the temperature T of solution of ss. During this second shutdown period, the material is isostatically post-shrunk under standard HIP conditions for nickel-based superalloys, ie at pressures between 900 and 2000 bar, the low porosity by beginning of melting being thus eliminated according to the process.

 This second quenching period is followed by cooling of the material to a sufficiently high cooling rate for the desired hardening of the particles separated from the phase. This speed is preferably in the range of speeds above 60 / min.

In the following, an example is described for the heat treatment according to the invention of a construction element made of a nickel-based alloy having the following composition
Mass Proportions in%
C, 0.15 to 0.20
If O to 0.2
Mn O to 0.2
S 0 to 0.015
Al 5.0 to 6.0
Ag 0 to 0.0005
Bi 0 to 0.00003
B 0.010 to 0.020
Co 13.0 to 17.0
Cr 8.0 + 11.0
Cu 0 to 0.2
Fe 0 to 1.0
Pb 0 to 0.0005
Se 0 to 0.0003
Mo 2.0 to 4.0
Ti 4.5 to 5.0
V 0.7 to 1.2
Zr 0.03 to 0.09
No remaining part
Sn 0 to 0.0025
Te 0 to 0.0001
Tl 0 to 0.0005
This material is known under the designation
IN 100. The thus treated building element was then subjected to element control in comparison with a conventionally processed IN 100 construction element.

 Example: An IN 100 molded series construction element was first heated in stages or continuously with a heating rate that could be well controlled at about 10 ° C per minute at a temperature T1 of 1231 ° C. to 125 ° C. When the set temperature was reached, the temperature overruns were avoided. Fig. 1 schematically shows the variations in temperature as a function of time of the heat treatment process according to the present invention. The T m \ / 2 2 melting point temperature is plotted on the ordinate, the solution solvation temperature domains T i 'solvus. Indeed, because of the dendritic segregations due to the fusion and the usual dispersion in the composition of the alloy, it is not generally possible to give an exact numerical value. The domain 1 of the graph then represents the heating process. The building element was maintained at this temperature T1 for one to two hours, as seen in area 2.

 At the temperature T1, a dissolution of the separated phase takes place and the material is homogenized. The $ /% eutectics domains are also partially resolved. This homogenization of the material causes the relatively broad domains of the Tm> /> i and T solvus temperatures to shrink and eventually fall within the bandwidths given by the dispersion of the alloy composition, as indicated. by reference numerals 6 and 7. It is disadvantageous, first of all, that due to abrupt variation of the compactness of the material at the beginning of melting, pores occur after cooling. produces stepped or continuous cooling, as shown in domain 3, at a temperature between 12100C and 12200C, as shown in domain 4. At the same time, hot isostatic pressing of 1000 bar is applied. The compression element is maintained at this temperature for at least one hour, but preferably for several hours.

 The early melting pores formed in the material are then closed. As a result of this, as shown in area 5, cooling is performed at 1000 ° C at a cooling rate of at least 6 ° C per minute. Then, in a usual manner, cooling is carried out at room temperature.

A number of structural elements thus treated were subjected to an examination in which tensile stress of 170 MPa was applied at 950μl. For comparative purposes, a number of structurally molded IN 100 components of the same shape were hot-isostatically compressed at 1200 ° C. for 4 hours at a pressure of 1000 bar and then subjected to the same conditions. review. Assuming a normal logarithmic distribution with a number of specimens n> 10, the average failure time (= 50% probability of occurrence) and the value at (= 2.3% probability of occurrence) were determined. The following break times were then obtained
Break time Value
medium to
Structural element treated according to the invention 275h 230h
Comparison building element 175h 95h
Fig. 2 is a micrograph, at 500X magnification, of an IN 100 construction element which has been solution annealed at 12200C for 30 minutes. A large number of non-uniform dendritic distributions are clearly visible.

 Fig. 3 is a micrographic structure image, with a magnification of 200 times, of an IN 100 construction element which has been subjected to the heat treatment process according to the invention. We see that, unlike the case of fig. 2, there are no more non-uniform dendritic distributions. On the contrary, we see a homogeneous texture with constitutive elements of 2 finely distributed. A part, which is seen in the form of white areas, X / El eutectics is also put in solution.

The bottom line is that there are no more pores in the structure due to early fusion.

 Fig. 4 is an extract, with a magnification of 400 times, of the same structure image.

Claims (2)

 1) Process for the heat treatment of structural elements made of nickel-based polycrystalline foundry alloys, characterized by the following steps in which  a) the building element is heated to a temperature T1 greater than a value between 0 and 10 at the first melting start temperature  Tm γ / γ of the carbureted eutectic phase Y / γ;  b) it stops at this temperature T1 for a time sufficient to completely dissolve and homogenize the separated phase%! partly non-uniform due to casting,  c) is cooled to a temperature T2 less than Tm 4 '/, but greater than the temperature T of dissolution of the separated hard phase \',  d) at this temperature T2 is stopped under a hydrostatic pressure of between 900 and 2000 bar, until the porosity by the beginning of melting is eliminated,  e) cooling to a cooling rate required to produce the desired separate particle phase.  2) Process according to claim 1, characterized in that for structural elements having the following composition (in percentages by mass) C from 0.15 to 0.2%, Si from 0 to 0.2%, Mn from 0 to 0.2%, S from 0 to 0.015%, Al from 5 to 6%, B from 0.01 to 0.02%, Co from 13 to 17%, Cr from 8 to 11%, Cu from 0 to 0.2%, Fe of 0 to 1%, Mo of 2 to 4%, Ti of 4.5 to 5%, V from 0.7 to 1.2%, Zr from 0.03 to 0.09%, and the remainder from nickel,  the temperature T1 is between 1231 ° C. and 2450 ° C.