METHOD FOR HEAT TREATMENT OF AUSTENITIC STEELS AND AUSTENITIC STEELS OBTAINED THEREBY

Abstract

The invention concerns a method for heat treatment of an austenitic steel of the High Nitrogen Steel or austenitic HNS type, or of an austenitic steel of the High Interstitial Steel or austenitic HIS type, said austenitic HNS or austenitic HIS containing precipitates of nitrides, carbides or carbonitrides of chromium and/or of molybdenum, this method comprising the step which consists, after machining the austenitic HNS or austenitic HIS containing the precipitates, in redissolving the precipitates by bringing the austenitic HNS or austenitic HIS to its austenitizing temperature, then cooling the austenitic HNS or austenitic HIS sufficiently rapidly to avoid the re-formation of precipitates.

The invention also concerns different heat treatment methods allowing chromium and/or molybdenum nitride, carbide or carbonitride type precipitates to appear in an austenitic HNS or austenitic HIS. Indeed, the presence of these precipitates in the matrix of the austenitic HNS or austenitic HIS makes machining operations easier by promoting the formation and removal of chips during machining of the components.

Claims

1. A method for heat treatment of an austenitic steel of the High Nitrogen Steel or austenitic HNS type, or of an austenitic steel of the High Interstitial Steel or austenitic HIS type, said austenitic HNS or austenitic HIS containing precipitates of nitrides, carbides or carbonitrides of chromium and/or of molybdenum, this method comprising the step which consists, after machining the austenitic HNS or austenitic HIS containing the precipitates, in putting again the precipitates in solution by bringing the austenitic HNS or austenitic HIS to its austenitizing temperature, then cooling the austenitic HNS or austenitic HIS sufficiently rapidly to avoid the re-formation of precipitates.

2. The method according claim 1, wherein, in order to make chromium and/or molybdenum nitride, carbide or carbonitride type precipitates appear in the austenitic HNS or austenitic HIS before machining, there is provided an austenitic HNS or austenitic HIS alloy which is brought to its austenitizing temperature or sintered at the austenitizing temperature, then, immediately from the austenitizing temperature, the temperature of the austenitic HNS or austenitic HIS alloy is lowered sufficiently slowly for the precipitates to appear in the resulting austenitic HNS or HIS structure, then finally the austenitic HNS or austenitic HIS is returned to ambient temperature.

3. The method according claim 1, wherein, in order to make chromium and/or molybdenum nitride, carbide or carbonitride type precipitates appear in the austenitic HNS or austenitic HIS before machining, there is provided an austenitic HNS or austenitic HIS alloy which is brought to its austenitizing temperature or sintered at the austenitizing temperature, then this austenitic HNS or austenitic HIS alloy is subjected to a cooling heat treatment immediately from the austenitizing temperature, and the cooling of the resulting austenitic HNS or austenitic HIS is interrupted when the temperature reaches a value at which the precipitates appear, this austenitic HNS or austenitic HIS being maintained at this temperature and for a duration such that the precipitates appear, and then finally the austenitic HNS or austenitic HIS is returned to ambient temperature.

4. The method according claim 1, wherein, in order to make chromium and/or molybdenum nitride, carbide or carbonitride type precipitates appear in the austenitic HNS or austenitic HIS before machining, an austenitic HNS or austenitic HIS alloy is subjected to an austenitizing heat treatment or to a sintering heat treatment at the austenitizing temperature, then the austenitic HNS or austenitic HIS alloy is quenched and reheated to a temperature and for a duration such that chromium and/or molybdenum nitride, carbide or carbonitride type precipitates appear.

5. The method according to claim 4, wherein, after quenching and before bringing the austenitic HNS or austenitic HIS to a temperature and for a duration such that chromium and/or molybdenum nitride, carbide or even carbonitride type precipitates appear, the austenitic HNS or austenitic HIS is cold deformed.

6. An element of a timepiece or piece of jewellery obtained from an austenitic HNS or austenitic HIS obtained by implementing the heat treatment method according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Other features and advantages of the present invention will appear more clearly from the following detailed description of an example of implementation of the method for heat treatment of austenitic HNS and austenitic HIS according to the present invention, this example being given purely by way of non-limiting illustration with reference to the annexed drawing, in which:

[0024] FIG. 1 is a schematic time-temperature-transformation diagram which illustrates the heat treatment of an austenitic HNS or austenitic HIS according to the first implementation variant of the method of the invention.

[0025] FIG. 2 is a schematic time-temperature-transformation diagram which illustrates the heat treatment of an austenitic HNS or austenitic HIS according to the second implementation variant of the method of the invention.

[0026] FIG. 3 is a schematic time-temperature-transformation diagram which illustrates the heat treatment of an austenitic HNS or austenitic HIS according to the third implementation variant of the method of the invention.

[0027] FIG. 4 is a metallographic cross-section of a sample of X20CrMnMoN17-11-3 HIS which was annealed at its austenitizing temperature and then quenched and which has no precipitates.

[0028] FIG. 5 is a metallographic cross-section of a sample of X20CrMnMoN17-11-3 austenitic HIS that has been subjected to a heat treatment according to the third implementation variant of the method of the invention.

[0029] FIG. 6 is a metallographic cross-section of a sample of X20CrMnMoN17-11-3 austenitic HIS that has been subjected to a heat treatment according to the fourth implementation variant of the method of the invention.

[0030] FIG. 7 is a graph that shows the evolution of the hardness of the sample of X20CrMnMoN17-11-3 austenitic HIS of FIG. 6 according to the temperature to which the steel is brought to form the precipitates.

[0031] FIG. 8 is a metallographic cross-section of a sample of X20CrMnMoN17-11-3 austenitic HIS that has been subjected to higher cold working than the austenitic steel sample of FIG. 6 before a heat treatment according to the fourth implementation variant of the method of the invention.

[0032] FIG. 9 is a graph that shows the evolution of the hardness of the sample of X20CrMnMoN17-11-3 austenitic HIS of FIG. 8 according to the temperature to which the steel is brought to form the precipitates.

DETAILED DESCRIPTION OF ONE EMBODIMENT OF THE INVENTION

[0033] The present invention proceeds from the general inventive idea which consists in subjecting austenitic HNS and austenitic HIS to a heat treatment intended to put in solution precipitates made to appear in such austenitic HNS or austenitic HIS, for example during a prior precipitation treatment. “Precipitation heat treatment” means a treatment that intends to place these austenitic HNS and austenitic HIS for a certain duration in temperature conditions that allow precipitates to appear, such as nitrides, carbides or carbonitrides, particularly of molybdenum and/or of chromium. Indeed, it was observed that these precipitates are generally only bound weakly to the matrix of the material, so that they enhance the formation and removal of chips during machining of the components. Thus, according to the invention, after machining components made of an austenitic HNS or austenitic HIS containing precipitates, it is possible to subject these components to a second austenitizing treatment, which consists in returning the components to their annealing temperature and then quenching them to return the precipitates to a solid solution. Since bringing the austenitic HNS and austenitic HIS to their annealing temperature a second time after machining eliminates internal stresses in the material and thus decreases hardness, this annealing treatment will preferably, but in a non-limiting manner, be reserved for external elements for watches or pieces of jewellery for which corrosion resistance and polishability are more important properties than hardness.

[0034] It will be understood that the diagrams illustrated in FIGS. 1 to 3 are simplified schematic representations. Indeed, each austenitic HNS or austenitic HIS composition has its own time-temperature-transformation diagram which also depends upon the nature of the precipitate concerned.

[0035] FIG. 1 is a time (t)-temperature (T)-transformation diagram which illustrates the heat treatment of an austenitic HNS or austenitic HIS according to the first implementation variant of the method of the invention. Tr1 is the austenitizing or annealing temperature of an austenitic steel of the HNS or HIS type and a is the curve which, in the time-temperature-transformation diagram of FIG. 1, delimits an area that corresponds to time and temperature conditions allowing the formation of precipitates. 1 designates the rapid cooling curve which returns the austenitic HNS or austenitic HIS from its annealing temperature to ambient temperature avoiding the formation of precipitates, and 2 the cooling curve according to the invention which combines the time and temperature parameters such that, by lowering the temperature of the austenitic HNS or austenitic HIS following this curve 2, precipitates are allowed to appear in said steel.

[0036] FIG. 2 is a time (t)-temperature (T)-transformation diagram which illustrates the heat treatment of an austenitic HNS or austenitic HIS according to the second variant implementation of the method of the invention. Tr2 is the austenitizing or annealing temperature of an austenitic steel of the HNS or HIS type and b is the curve which, in the time-temperature-transformation diagram of FIG. 2, delimits an area that corresponds to time and temperature conditions allowing the formation of precipitates. The treatment starts with rapid cooling of the austenitic HNS or austenitic HIS from its annealing temperature Tr2 according to the curve 4, then the cooling of the austenitic HNS or austenitic HIS is interrupted when the temperature reaches a value Tp2 at which precipitates can appear, and the steel is maintained at temperature Tp2 for a duration such that precipitates appear (curve 6). Finally, the steel is returned to ambient temperature (curve 8).

[0037] FIG. 3 is a time (t)-temperature (T)-transformation diagram which illustrates the heat treatment of an austenitic HNS or austenitic HIS according to the third variant implementation of the method of the invention. Tr3 is the austenitizing or annealing temperature of an austenitic steel of the HNS or HIS type and c is the curve which, in the time-temperature-transformation diagram of FIG. 3, delimits an area that corresponds to time and temperature conditions allowing the formation of precipitates. The steel in question here is an austenitic HNS or austenitic HIS that has been cooled sufficiently rapidly from its annealing temperature Tr3 to ambient temperature to avoid the formation of precipitates. According to the third implementation variant of the method of the invention, such an austenitic HNS or austenitic HIS is heated according to curve 10 and maintained at a temperature and for a duration such that precipitates appear (curve 12), and is then cooled (curve 14).

[0038] The fourth implementation variant of the invention only differs from the third variant of the same method in that, after the annealing and quenching treatment and before the precipitation treatment, the austenitic HNS or austenitic HIS is cold worked, i.e. cold deformed. The heat treatment according to the invention which consists in bringing an austenitic steel to a temperature and for a duration such that precipitates form is thus applied, in this fourth variant, to a material that is pre-hardened by cold working.

[0039] Finally, the fifth and final implementation variant of the method of the invention consists in subjecting the austenitic steel to a cold deformation treatment after heat treatment according to any of the first three implementation variants.

[0040] Different tests were conducted on X20CrMnMoN17-11-3 austenitic HIS.

[0041] FIG. 4 is a metallographic cross-section of a sample of HIS X20CrMnMoN17-11-3 steel which was annealed at its austenitizing temperature and then quenched. From an examination of this Figure it is noted that the grain boundaries are barely visible, which indicates a lack of precipitates.

[0042] FIG. 5 is a metallographic cross-section of a sample of X20CrMnMoN17-11-3 austenitic HIS that has been subjected to a heat treatment according to the third implementation variant of the method of the invention. From an examination of FIG. 5 it can be seen that the grain boundaries are visible, which indicates the presence of large quantities of precipitates along these grain boundaries. It can even be seen (areas surrounded by a circle in FIG. 5) that some larger precipitates have grown inside the grains from the grain boundaries. It was possible to obtain such a concentration of precipitates by bringing the X20CrMnMoN17-11-3 austenitic HIS to a temperature of 800° C. for two hours, after rapid cooling from the annealing temperature.

[0043] For some applications, such as components for a timepiece movement, it is not possible to envisage annealing the components (after precipitation treatment) insofar as one wishes to maintain the hardness obtained after cold working. Samples of X20CrMnMoN17-11-3 austenitic HIS were thus subjected to a heat treatment method according to the fourth variant implementation of the invention, consisting, after an annealing, quenching and cold working treatment, in bringing the X20CrMnMoN17-11-3 austenitic HIS to a temperature and for a duration such that precipitates form. It was observed that the formation of precipitates is much quicker after cold deformation. Indeed, the dislocations and defects caused by cold deformation create diffusion paths promoting germination and the growth of precipitates.

[0044] FIG. 6 is a metallographic cross-section of a sample of X20CrMnMoN17-11-3 austenitic HIS which takes the form of a bar whose external diameter is reduced from 3 mm to 2.5 mm through cold deformation by drawing, namely a reduction in diameter of 16.6%. According to the fourth implementation variant of the method according to the invention, this sample was then brought to a temperature of 800° C. for two hours according to the temperature curve represented in FIG. 3. It is seen that the steel has numerous precipitates, both at the grain boundaries and inside the grains.

[0045] FIG. 7 is a graph that shows the evolution of the hardness of the X20CrMnMoN17-11-3 austenitic HIS of FIG. 6 according to the temperature to which the steel is brought to form the precipitates. It is observed that the hardness of the austenitic steel without the precipitation treatment according to the invention and after cold working is 450 HV10 (square symbol on the graph). The same austenitic steel is subjected, after cold working, to the heat treatment according to the fourth implementation variant of the method of the invention. Samples of this steel are respectively brought to temperatures of 750° C., 800° C., 850° C., 900° C. and 950° C. for a duration of two hours, then cooled (diamond-shaped symbol on the graph). It is observed that, for the samples heated between 700° C. and 900° C., the hardness is comprised between around 425 HV10 and 375 HV10. In other words, the hardness of these austenitic steel samples, which are heat treated according to the fourth variant of the invention, varies little with respect to the hardness of the cold worked austenitic steel that has not been subjected to a precipitation treatment. However, the machinability of the austenitic steel samples subjected to a precipitation heat treatment according to this fourth variant of the invention is markedly improved. Only the austenitic steel sample heated to 950° C. for two hours has a substantially lower hardness than that of the austenitic steel without precipitation treatment (less than 350 HV10). Finally, a sample of X20CrMnMoN17-11-3 austenitic HIS subjected only to an annealing treatment followed by quenching (triangular symbol on the graph) has a hardness of less than 250 HV10.

[0046] FIG. 8 is a metallographic cross-section of a sample of X20CrMnMoN17-11-3 austenitic HIS, which takes the form of a bar, whose external diameter is reduced from 3 mm to 2 mm through cold deformation by drawing, namely an even greater reduction in diameter of 33.3%. This steel sample is subjected to the same heat treatment as in FIG. 6, by being brought to a temperature of 800° C. for two hours according to the fourth implementation variant of the invention. It is seen that, compared to FIG. 6, the precipitation phenomenon is even more marked, since, in addition to the precipitates that form along the grain boundaries and from the grain boundaries towards the interior of the grains, there is a high concentration of precipitates actually inside the grains.

[0047] FIG. 9 is a graph that shows the evolution of the hardness of the steel of FIG. 8 according to the hardness and to the temperature to which the steel is brought, after cold working, to form the precipitates. It is observed that the hardness of the austenitic steel without the precipitation treatment according to the invention and after cold working is comprised between 550 HV10 and 560 HV10 (square-shaped symbol on the graph). This hardness is greater than that of FIG. 7, since the cold working rate is higher. The diamond-shaped symbols in FIG. 9 correspond to austenitic steel samples brought to respective temperatures of 700° C., 750° C., 800° C. and 850° C. for 45 minutes. The round-shaped symbols correspond to austenitic steel samples brought to respective temperatures of 700° C., 750° C., 800° C. and 850° C. for two hours. A comparison of the graphs of FIGS. 7 and 9 reveals that the higher the cold working rate, the easier it is for precipitates to form. Indeed, mechanical tensions within the steel make it possible for precipitates to nucleate and grow.

[0048] It is observed that, for the same precipitation treatment temperature, the hardness of the austenitic steel samples is lower when the duration of the precipitation treatment is longer. It is also observed that, for the same two-hour treatment duration, the higher the precipitation temperature, the lower the steel hardness. However, these graphs show that it is possible to obtain steels with many precipitates and with a hardness that is nonetheless close to the initial hardness.

[0049] It goes without saying that this invention is not limited to the embodiment that has just been described and that various simple modifications and variants can be envisaged by those skilled in the art without departing from the scope of the invention as defined by the annexed claims. A few non-limiting examples of HNS and HIS to which the precipitation method according to the invention can be applied are: X5CrMnN18-18, X8CrMnN19-19, X8CrMnMoN18-18-2, X13CrMnMoN18-14-3, X20CrMnMoN17-11-3 or even X5MnCrMoN23-21. Finally, a few examples of precipitates that may form during the precipitation method are: M23C, MC, M6C or even M2N, where M designates one or more of the metallic elements of the alloy able to combine with the carbon or with the nitrogen to form carbides or nitrides or carbonitrides. The invention applies especially to pieces of jewellery and to the external elements of timepieces.

[0050] It is understood from the foregoing that it is advantageous to machine an element, for example for a piece of jewellery or a wristwatch, using an austenitic steel of the HNS or HIS type containing precipitates. It may, however, also be advantageous, after machining, to make these precipitates disappear. Indeed, although the precipitates make machining operations easier by promoting the formation and removal of chips during machining of the components, it may be advantageous to eliminate these chips after machining to improve the ductility and corrosion resistance of these components. This is why the present invention teaches a method for heat treatment of an austenitic HNS or HIS containing precipitates, this method including the step that consists, after machining components, particularly for jewellery or horology, made using an austenitic HNS or austenitic HIS containing precipitates, in redissolving or putting the precipitates again in solution by bringing the austenitic HNS or austenitic HIS components to their austenitizing temperature, and then cooling the components sufficiently rapidly, typically by quenching, to prevent precipitates forming again. “Machining operations” mean in particular but not in a limiting manner, the operations of boring, milling, drilling, threading, tapping and cutting.