EP2449143B1 - Cryogenic treatment of martensitic steel with mixed hardening - Google Patents

Description

1. (a) on chauffe la totalité de l'acier au dessus de sa température d'austénisation AC3,
2. (b) on refroidit ledit acier jusqu'à environ la température ambiante,
3. (c) on place ledit acier dans une ambiance cryogénique.

The present invention relates to a process for treating a martensitic steel which comprises contents of other metals such that it is capable of being hardened by a precipitation of intermetallic compounds and carbides, with an Al content of between 0.4% and 3%, and which has a martensitic transformation end temperature Mf lower than 0 ° C, this heat treatment process comprising the following steps:

1. (a) the whole of the steel is heated above its austenisation temperature AC3,
2. (b) cooling said steel to about ambient temperature,
3. (c) placing said steel in a cryogenic environment.

For certain applications, in particular for turbomachine transmission shafts, it is necessary to use such steels, which have a very high mechanical strength (elastic limit and breaking load) up to 400 ° C. and at the same time a good resistance to brittle fracture (high toughness and ductility). These steels have good fatigue resistance.

The composition of such a steel is given in the document FR 2,885,142 as follows (percentages by weight): 0.18 to 0.3% C, 5 to 7% Co, 2 to 5% Cr, 1 to 2% AI, 1 to 4% Mo + W / 2, traces at 0.3% of V, traces at 0.1% of Nb, traces at 50 ppm of B, 10.5 at 15% of Ni with Ni ≥ 7 + 3.5 Al, traces at 0.4 % Si, traces at 0.4% Mn, traces at 500 ppm Ca, traces at 500 ppm rare earth, traces at 500 ppm Ti, traces at 50 ppm O (elaboration from liquid metal) or at 200 ppm O (elaboration by powder metallurgy), traces at 100 ppm N, traces at 50 ppm S, traces at 1% Cu, traces at 200 ppm P, the rest being Fe.

), et une bonne tenue en fatigue.This steel has a very high mechanical strength (breaking load ranging from 2000 to 2500 MPa) and at the same time a very good resilience (180 · 10 ³ J / m ² ) and toughness (40 to 60 $\left(40\mathrm{at}60\mathrm{MPa}\cdot \sqrt{\mathrm{m}}\right),$

), and good fatigue resistance.

These mechanical properties are obtained thanks to the heat treatments to which this steel is subjected. In particular, steel is subject to the following treatment: the steel is heated and maintained above its austenization temperature AC3 until its temperature is substantially homogeneous, the steel is then cooled to about ambient temperature and the steel is placed and maintained in an enclosure where a cryogenic temperature prevails. "Cryogenic" means temperatures below 0 ° C.

The purpose of placing such steels in a cryogenic enclosure is to minimize the remaining austenite content in the steel, that is to say to optimize the transformation of austenite to martensite in steel. Indeed, the strength properties of steel increase inversely to its austenite content. For the steels that are the subject of the present application, the temperature Mf at the end of the martensitic transformation is between -30 ° C. and -40 ° C., estimated under conditions of thermodynamic equilibrium. To ensure optimal transformation of the austenite martensite, it is generally considered that the temperature in the cryogenic chamber must be slightly below the temperature Mf. Thus, given the athermal character of the transformation from austenite to martensite, it is accepted that the temperature in the cryogenic chamber must be less than -40 ° C, and that the optimum martensite transformation is performed when the hottest steel have reached this temperature. The steel is then removed from the cryogenic enclosure.

However, the results of mechanical hardness and tensile tests carried out on this steel after such a cryogenic treatment show a great dispersion in the mechanical properties of the steel, which is undesirable. In addition, these results do not follow a normal statistical law with respect to the parameters of the cryogenic treatment, conversely the results are distributed according to a sum of a multitude of normal laws depending on the heat treatment conditions and in particular of passage in cryogenic media. This multimodal behavior accentuates the calculated dispersion even more (when all these results are included in the same family) and lowers the value of the average. The minima (calculated at three standard deviations below the mean) of the design curves are then further lowered.

The present invention aims to remedy these disadvantages.

The aim of the invention is to propose a process for treating this type of steel which makes it possible to reduce the dispersions in its mechanical properties, which gives dispersions which follow normal statistical laws, and which on average increases these mechanical properties.

This object is achieved by a process according to claim 1, in which, in particular, the temperature T ₁ is substantially lower than the martensitic transformation temperature M f, and the holding time t of said steel in said cryogenic environment at a temperature T ₁ from the moment when the hottest part of the steel reaches a temperature below the martensitic transformation temperature Mf, is at least equal to a non-zero time t ₁ .

Thanks to these provisions, all the austenite that can potentially turn into martensite in steel as it is introduced into the cryogenic environment, is transformed optimally. Optimum transformation means that the remaining austenite content in steel is minimal in all steel. The dispersion in the values of the mechanical properties is therefore reduced, since the austenite content is homogeneous throughout the steel. In addition, these values are on average increased, since the austenite content in the steel is minimized.

For example, the temperature T ₁ (in ° C with a tolerance of +/- 5 ° C) and the time t ₁ (in hours with a tolerance of +/- 5%) are related substantially by the relation $${\mathrm{T}}_1=f\left({\mathrm{t}}_1\right)\mathrm{with}f\left(\mathrm{t}\right)=57.666\times \left(1-1/{\left({\mathrm{t}}^{0.3}-0.14\right)}^{1.5}\right)-\mathrm{97.389.}$$

Advantageously, the steel is placed in the cryogenic environment less than 70 hours after the temperature at the surface of the part during its cooling in step (b) reaches the temperature of 80 ° C.

Thus, the maximum conversion rate of austenite to martensite that can be achieved in steel by placing it in a cryogenic environment is as high as possible.

• la figure 1 montre la relation T₁ = f(t₁) entre le temps t₁ pendant lequel l'acier est maintenu dans l'enceinte cryogénique après que la partie la plus chaude de l'acier atteint une température
• inférieure à la température de transformation martensitique Mf, et la température T₁ dans l'enceinte, dans le procédé selon l'invention,
• la figure 2 montre la variation du taux d'austénite restante dans un acier en fonction de la température T₁ dans l'enceinte cryogénique pour différents temps t₁ pendant lequel l'acier est maintenu dans cette enceinte après que la partie la plus chaude de l'acier atteint une température inférieure à la température de transformation martensitique Mf,
• la figure 3 montre la variation de la dureté dans un acier en fonction de la température T₁ dans l'enceinte cryogénique pour différents temps t₁ pendant lequel l'acier est maintenu dans cette enceinte après que la partie la plus chaude de l'acier atteint une température inférieure à la température de transformation martensitique Mf,
• la figure 4 montre la variation du taux d'austénite restante dans un acier en fonction de la durée séparant la fin du refroidissement de cet acier depuis sa température d'austénisation, et le placement de cet acier dans l'enceinte cryogénique, pour différents temps t₁ pendant lequel l'acier est maintenu dans cette enceinte après que la partie la plus chaude de l'acier atteint une température inférieure à la température de transformation martensitique Mf.

The invention will be better understood and its advantages will appear better on reading the detailed description which follows, of an embodiment shown by way of non-limiting example. The description refers to the accompanying drawings in which:

• the figure 1 shows the relation T ₁ = f (t ₁ ) between the time t ₁ during which the steel is kept in the cryogenic chamber after the hottest part of the steel reaches a temperature
• less than the martensitic transformation temperature Mf, and the temperature T ₁ in the chamber, in the process according to the invention,
• the figure 2 shows the variation of the austenite rate remaining in a steel as a function of the temperature T ₁ in the cryogenic chamber for different times t ₁ during which the steel is maintained in this chamber after the hottest part of the steel reaches a temperature below the martensitic transformation temperature Mf,
• the figure 3 shows the variation of the hardness in a steel as a function of the temperature T ₁ in the cryogenic chamber for different times t ₁ during which the steel is maintained in this chamber after the hottest part of the steel reaches a temperature less than the martensitic transformation temperature Mf,
• the figure 4 shows the variation of the remaining austenite ratio in a steel as a function of the time separating the end of the cooling of this steel from its austenization temperature, and the placement of this steel in the cryogenic chamber, for different times t ₁ during which steel is maintained in this chamber after the hottest part of the steel reaches a temperature below the martensitic transformation temperature Mf.

As indicated in the preamble, a steel object of the present application is subjected to the following treatment with the aim of minimizing its residual austenite content: the steel is heated and kept above its austenization temperature until its temperature is substantially homogeneous, the steel is then cooled to about room temperature, then the steel is placed and maintained in a chamber where there is a cryogenic temperature.

The inventors have carried out tests on such steels having undergone the above treatment. These steels have the following composition: 0.18 to 0.3% C, 5 to 7% Co, 2 to 5% Cr, 1 to 2% Al, 1 to 4% Mo + W / 2 , traces at 0.3% of V, traces at 0.1% of Nb, traces at 50 ppm of B, 10.5 at 15% of Ni with Ni ≥ 7 + 3.5 Al, traces at 0.4% of Si, traces at 0.4% Mn, traces at 500 ppm of Ca, traces at 500 ppm of rare earths, traces at 500 ppm of Ti, traces at 50 ppm O (elaboration from liquid metal) or at 200 ppm O (elaboration by metallurgy of the powders), traces at 100 ppm of N, traces at 50 ppm of S, traces at 1% Cu, traces at 200 ppm P, the remainder being Fe. These steels more particularly have the following composition: 0.200% to 0.250% C, 12.00% to 14.00% Ni, 5.00% to 7.00% in Co, 2.5% to 4.00% in Cr, 1.30 to 1.70% in Al, 1.00 to 2.00% in Mo.

The figure 2 shows, according to the results of these tests, the variation of the austenite rate remaining in a steel as a function of the temperature T ₁ in the cryogenic enclosure for different durations t ₁ , where t ₁ is the duration during which this steel is maintained in this cryogenic chamber after the hottest part of the steel reaches a temperature below the martensitic transformation temperature Mf.

These results show that if the steel is kept in the chamber for 2 hours after the hottest part of the steel reaches a temperature below the martensitic transformation temperature Mf, it is necessary that the temperature of the enclosure is less than or equal to -90 ° C so that the residual austenite level is minimal. Above this temperature, the residual austenite rate is higher. Below -90 ° C, the residual austenite rate remains substantially constant and equal to its minimum value, in this case approximately 2.5% (a measure taking into account the natural dispersion of the measurement).

Similarly, if the steel is kept in the chamber for 5 hours or 8 hours after the hottest part of the steel reaches a temperature below the martensitic transformation temperature Mf, it is necessary that the temperature of the enclosure is equal to or lower than about -71 ° C and -67 ° C, respectively, so that the residual austenite level is minimal.

The results show that in all cases, the residual austenite level is substantially equal.

More generally, the residual austenite content is minimal and substantially constant when the time t ₁ and the temperature T ₁ lie below the curve T ₁ = f (t ₁ ) given in FIG. figure 1 .

This curve has for equation: $$f\left(\mathrm{t}\right)=57.666\times \left(1-\frac{1}{{\left({\mathrm{t}}^{0.3}-0.14\right)}^{1.5}}\right)-97.389$$

The curve T ₁ = f (t ₁ ) gives the temperature T ₁ (expressed in ° C) in the cryogenic chamber where the steel must be maintained for a time t ₁ (expressed in hours) after the hottest part of the steel reaches a temperature below the martensitic transformation temperature Mf so that all the regions of the steel are converted to a maximum of martensite, and therefore have a minimum and homogeneous residual austenite content.

The curve T ₁ = f (t ₁ ) is obtained by statistical approximation of the experimental results given in Table 1 below. It is therefore understood that for a given time t ₁ of maintaining the steel in the cryogenic chamber after the hottest part of the steel reaches a temperature below the martensitic transformation temperature Mf, the temperature in this chamber must be approximately equal or less than that given by the curve T ₁ = f (t ₁ ). The first derivative of the function f with respect to t, f '(t), is positive, and the second derivative of f with respect to t, f "(t), is negative.

The shape of this curve is valid for all the steels of this family and is translated in the vertical direction (variation in temperature) according to the chemical composition of the steel. The horizontal asymptote of this equation (the temperature T ₁ for which an infinite holding time t ₁ is necessary, that is to say the highest possible temperature for the enclosure) is a function of the chemical composition of the equation. This composition has a direct influence on the martensitic transformation end point Ms and the end time Mf. For the present steel, this temperature is approximately equal to -40 ° C. The minimum holding time t ₁ necessary is approximately equal to 1 hour, and is substantially constant for all the steels of this family. Table 1 Time t ₁ (hours) Temperature T ₁ (° C) 2 -90 5 -70 8 -68

It is unexpectedly noted that these temperatures T ₁ are well below the temperature of -40 ° C, which is commonly accepted as permitting an optimal transformation of the austenite to martensite, and that the holding time t ₁ is not zero. . Thus, the inventors show that it is not sufficient that the hottest parts of the steel have reached the temperature Mf (or a slightly lower temperature) so that the transformation into martensite in these parts is optimal, but it is necessary in addition that these hottest parts are maintained in the cryogenic chamber (where a temperature T ₁ prevails) after they reach a temperature below the Martensitic transformation temperature Mf for a period at least equal to t ₁ .

The figure 3 shows, according to other results of tests carried out by the inventors, the variation of the hardness in such a steel as a function of the temperature T ₁ in the cryogenic chamber for different durations t ₁ , where t ₁ is the duration during which this steel is maintained in this cryogenic chamber after the hottest part of the steel reaches a temperature below the martensitic transformation temperature Mf.

These results show that the hardness is maximum and substantially constant when the time t ₁ and the temperature T ₁ lie under the curve T ₁ = f (t ₁ ) given in figure 1 .

Comparing the curves of figures 2 and 3 a correlation can thus be established between the residual austenite content in steel and the hardness of this steel. It is concluded that the lower the austenite content in steel, the higher the hardness of the steel. The results of tests carried out by the inventors on other mechanical properties show a similar tendency, namely that the mechanical properties increase when the austenite rate decreases.

Thanks to the process according to the invention, the austenite content in the steel is minimized, and consequently the mechanical properties of the steel are increased on average.

Furthermore, the minimum austenite content in a region of a steel workpiece is reached only when this region has reached a temperature below the temperature Mf and is maintained there long enough, as shown by the curve of the figure 1 .

In the case where, after the hottest part of the steel reaches a temperature below the martensitic transformation temperature Mf, the part is held in the cryogenic chamber where there is a temperature T ₁ for a time t less than the time t ₁ satisfying the relation T ₁ = f (t ₁ ), then some more central regions of the room did not stay long enough below the temperature Mf, while some regions located on the surface of the room remained long enough at the temperature Mf. The residual rate of austenite therefore increases from these surface regions to these central regions. This spatial variation of the residual austenite rate results in a dispersion of the values of the mechanical properties obtained during the tests.

However, in the process according to the invention, the steel is kept in the cryogenic enclosure long enough after the hottest part of the steel reaches a temperature below the martensitic transformation temperature Mf, which ensures a transformation. optimal martensite of this part. It is thus clear why, thanks to the method according to the invention which makes it possible to obtain a residual level of austenite in steel which is homogeneous and minimum, the dispersion of the values of the mechanical properties is minimized, as found by the inventors . For example, applying a treatment method according to the prior art, the average hardness of the treated steel is 560 Hv with statistically a minimum at 535 Hv and a maximum at 579 Hv. Using the method according to the invention, the average hardness of the treated steel is 575 Hv with statistically a minimum at 570Hv and a maximum at 579 Hv.

Before placing it in the cryogenic enclosure, the steel undergoes, in step (b), a quenching in a fluid (a medium) in order to cool the steel to ambient temperature. Ideally, this fluid has a drasticity at least equal to that of air. For example, this fluid is air.

The drasticity of a quenching medium is understood to mean the capacity of this medium to absorb the calories in the layers closest to the part which is immersed therein and to diffuse them into the rest of the medium. This capacity conditions the rate of cooling of the surface of the room immersed in this medium.

The tests carried out by the inventors show that the steel must ideally be placed in the cryogenic environment less than 70 hours after the moment when the temperature at the surface of the part during its cooling in step (b) reaches the temperature of 80 ° C.

The figure 4 shows the results of these tests. When the steel is placed in the cryogenic environment (enclosure) 70 hours or less after the temperature at the surface of the workpiece during its cooling in step (b) reaches the temperature of 80 ° C, then the residual austenite content in the steel can reach its minimum after maintenance in the cryogenic chamber according to the conditions of the invention. On the other hand, when the steel is placed in the cryogenic environment more than 70 hours after this moment, then the residual content in austenite can not reach its minimum, regardless of the subsequent duration of maintenance and the temperature in the cryogenic chamber.

The minimum of the residual austenite content is of the order of 2.5% for the grade of steel tested during the tests. More generally, for the type of steel according to the invention, the minimum of the residual austenite content is less than 3%.

For other families of steel, the minimum values of time t ₁ vary. For example, the time t ₁ may be greater than 2 hours, or greater than 3 hours, or greater than 4 hours.

For each of these times t ₁ , the temperature T ₁ below which the temperature of the chamber must be is, for example, equal to -50 ° C., or -60 ° C., or -70 ° C.

The invention also relates to a piece made of a steel obtained according to a process according to the invention, the residual austenite content in this steel being less than 3%.

For example, this part is a turbomachine shaft.

Claims (6)

1. A method for producing martensitic steel that comprises a content of other metals such that the steel can be hardened by an intermetallic compound and carbide precipitation, with a martensitic transformation temperature Mf below 0°C, this thermal treatment method comprising the following steps:

   (a) heating the entirety of the steel above the austenizing temperature thereof,

   (b) cooling said steel to around the ambient temperature,

   (c) placing said steel into a cryogenic medium at a temperature T₁,

   the method being characterized in that the temperature T₁ is substantially less than the martensitic transformation temperature Mf, and the time for keeping said steel in said cryogenic medium from the moment when the hottest portion of the steel reaches a temperature lower than the martensitic transformation temperature Mf, is at least equal to a time t₁ longer than 1 hour, the temperature T₁ (in °C) and the time t₁ (in hours) being substantially linked by the equation T₁ = f(t₁), the function f being substantially given by $$f\left(\mathrm{t}\right)=57.666\times \left(1-1/{\left({\mathrm{t}}^{0.3}-0.14\right)}^{1.5}\right)-\mathrm{97.389.}$$

   

   or by a temperature-translated curve relative to f(t), wherein
   said steel has for composition: 0.18 to 0.3% of C, 5 to 7% of Co, 2 to 5% of Cr, 1 to 2% of Al, 1 to 4% of Mo+W/2, traces to 0.3% of V, traces to 0.1% of Nb, traces to 50 ppm of B, 10.5 to 15% of Ni with Ni ≥ 7+3.5 Al, traces to 0.4% of Si, traces to 0.4% of Mn, traces to 500 ppm of Ca, traces to 500 ppm of rare earths, traces to 500 ppm of Ti, traces to 50 ppm of O if developed from molten metal or to 200 ppm of O if developed through powder metallurgy, traces to 100 ppm of N, traces to 50 ppm of S, traces to 1% of Cu, traces to 200 ppm of P, the rest being Fe.
2. The method according to claim 1, characterized in that said steel has for composition: 0.200% to 0.250% in C, 12.00% to 14.00% in Ni, 5.00% to 7.00% in Co, 2.5% to 4.00% in Cr, 1.30 to 1.70% in Al, 1.00% to 2.00% in Mo.
3. The method according to claim 1 or 2, characterized in that in step (b), said steel is cooled to approximately ambient temperature by quenching in a medium with a drasticity at least equal to that of the air.
4. The method according to any one of claims 1 to 3, characterized in that said steel is placed in said cryogenic medium less than 70 hours after the moment when the surface temperature of the piece during cooling thereof in step (b) reaches the temperature of 80°C.
5. A piece made from a steel obtained using a method according to any one of the preceding claims, characterized in that the residual austenite level in said steel is less than 3%.
6. A turbomachine transmission shaft made from a steel obtained according to a method according to any one of claims 1 to 4, characterized in that the residual austenite level in said steel is less than 3%.

Images