Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F10/00—Additive manufacturing of workpieces or articles from metallic powder
  • B22F10/20—Direct sintering or melting
  • B22F10/28—Powder bed fusion, e.g. selective laser melting [SLM] or electron beam melting [EBM]
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F10/00—Additive manufacturing of workpieces or articles from metallic powder
  • B22F10/60—Treatment of workpieces or articles after build-up
  • B22F10/64—Treatment of workpieces or articles after build-up by thermal means
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B33—ADDITIVE MANUFACTURING TECHNOLOGY
  • B33Y—ADDITIVE MANUFACTURING, i.e. MANUFACTURING OF THREE-DIMENSIONAL [3D] OBJECTS BY ADDITIVE DEPOSITION, ADDITIVE AGGLOMERATION OR ADDITIVE LAYERING, e.g. BY 3D PRINTING, STEREOLITHOGRAPHY OR SELECTIVE LASER SINTERING
  • B33Y10/00—Processes of additive manufacturing
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B33—ADDITIVE MANUFACTURING TECHNOLOGY
  • B33Y—ADDITIVE MANUFACTURING, i.e. MANUFACTURING OF THREE-DIMENSIONAL [3D] OBJECTS BY ADDITIVE DEPOSITION, ADDITIVE AGGLOMERATION OR ADDITIVE LAYERING, e.g. BY 3D PRINTING, STEREOLITHOGRAPHY OR SELECTIVE LASER SINTERING
  • B33Y40/00—Auxiliary operations or equipment, e.g. for material handling
  • B33Y40/20—Post-treatment, e.g. curing, coating or polishing
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B33—ADDITIVE MANUFACTURING TECHNOLOGY
  • B33Y—ADDITIVE MANUFACTURING, i.e. MANUFACTURING OF THREE-DIMENSIONAL [3D] OBJECTS BY ADDITIVE DEPOSITION, ADDITIVE AGGLOMERATION OR ADDITIVE LAYERING, e.g. BY 3D PRINTING, STEREOLITHOGRAPHY OR SELECTIVE LASER SINTERING
  • B33Y70/00—Materials specially adapted for additive manufacturing
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/02—Hardening by precipitation
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/0068—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for particular articles not mentioned below
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C33/00—Making ferrous alloys
  • C22C33/02—Making ferrous alloys by powder metallurgy
  • C22C33/0257—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements
  • C22C33/0278—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements with at least one alloying element having a minimum content above 5%
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/004—Very low carbon steels, i.e. having a carbon content of less than 0,01%
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F10/00—Additive manufacturing of workpieces or articles from metallic powder
  • B22F10/60—Treatment of workpieces or articles after build-up
  • B22F10/66—Treatment of workpieces or articles after build-up by mechanical means
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C2202/00—Physical properties
  • C22C2202/02—Magnetic
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P10/00—Technologies related to metal processing
  • Y02P10/25—Process efficiency

Definitions

• the invention relates to a paramagnetic stainless steel having a hardness greater than or equal to 575 HV and the part, in particular a watch component made of this steel. It also relates to the manufacturing process of this stainless steel part.
• Hard and non-ferromagnetic metal alloys find applications in many fields, mainly for components subjected to high mechanical and/or tribological stresses and which must remain insensitive to magnetic fields. This is particularly the case for many watch components, such as wheels, pinions, axles or springs in the movement.
• watch components such as wheels, pinions, axles or springs in the movement.
• high hardnesses for example for the middle, the bezel, the back or the crown. Indeed, a high hardness generally makes it possible to obtain a better resistance to scratching and wear and therefore a good durability of these components exposed to the external environment.
• any machining operations are very difficult and costly, since the alloy is already in the hardened state, at least partially.
• the process used does not involve plastic deformation, such as some powder metallurgy or additive manufacturing processes, it is simply not possible to harden these alloys.
• alloys having intrinsically a hardness greater than 500 HV such as certain high entropy alloys or certain intermetallics for example, but they would again be very difficult to machine and almost impossible to deform, due to their hardness. very high and their very low ductilities.
• the advantage of finding an alloy hardenable by heat treatment without the need for prior work hardening, while being non-ferromagnetic in the hardened state is thus understood.
• the shaping would thus be done in a soft and ductile state, and a hardening heat treatment would be carried out once the part is finished.
• This explains in particular the enormous success of carbon steels and martensitic stainless steels, but these are unfortunately ferromagnetic.
• the object of the present invention is to overcome the aforementioned drawbacks by proposing a stainless steel composition optimized to fulfill the following criteria:
• the stainless steel according to the invention has the following composition by weight:
• the balance being made up of iron and any impurities each having a content less than or equal to 0.5%.
• the process for manufacturing a stainless steel part consists in carrying out a first heat or thermomechanical treatment on a base material of the aforementioned composition in the ferritic or ferritic-austenitic range and then quenching the material in order to keep the ferritic or ferritic-austenitic structure at room temperature.
• This ferritic or ferritic-austenitic microstructure is soft and therefore ductile, which allows easy shaping if necessary.
• a hardening treatment is carried out in order to transform the ferrite into an austenitic phase and a sigma intermetallic phase.
• the originality of the present invention comes, on the one hand, from the use of the sigma phase as source of hardening and, on the other hand, from the molybdenum content greater than 3% in the composition.
• the sigma phase has always been considered harmful and therefore unwanted in stainless steels.
• the sigma phase is rich in chromium and usually forms at grain boundaries, it drastically decreases corrosion resistance by reducing the chromium concentration of other phases present in the alloy. Then, it very quickly and strongly weakens stainless steels, even in very small quantities. Indeed, this phase having a complex tetragonal structure, it is intrinsically very fragile and its presence at the grain boundaries creates a privileged path for the propagation of cracks.
• the composition of the stainless steel and the process are optimized in order to obtain a fine distribution both of the sigma phase and of the austenitic phase without preferential formation of the sigma phase at the grain boundaries.
• This particular microstructure consisting of two non-ferromagnetic phases makes it possible in particular to obtain a very good compromise between hardness and toughness, good corrosion resistance and excellent polishability.
• the Mo (molybdenum) content above 3% improves the corrosion resistance. Indeed, a high Mo content makes it possible to obtain a significant concentration of Mo in the austenite (> 1%), even if the Mo is always present in greater concentration in the sigma phase. By reinforcing the corrosion resistance of the austenite thanks to the presence of Mo, the overall corrosion resistance of the alloy is improved, and more specifically the resistance to pitting corrosion.
• a second advantage of a Mo concentration greater than 3% is that this makes it possible to carry out the hardening heat treatment in the y+o range at a higher temperature, which increases the Cr (chromium) concentration even more and of Mo in the austenite, thereby further improving the corrosion resistance.
• carrying out the heat treatment at a higher temperature reduces the risk of forming carbides. or nitrides harmful to corrosion resistance. Indeed, the higher the temperature, the greater the solubility of carbon and nitrogen in austenite.
• Figure 1 shows a phase diagram of an Fe-(32.5-x)%Cr-7%Ni-x%Mo alloy (% by mass) illustrating the effect of Mo on the Sigma + CFC (austenite) phase domain.
• Figure 2 shows the effect of the hardening heat treatment temperature on the PREN of austenite for an Fe-28.5%Cr-7%Ni-4%Mo alloy (% by mass).
• FIG. 3 represents a phase diagram of an alloy Fe-(29.5-x)%Cr-7%Ni-3%Mo-x%C (% by mass) according to the prior art, illustrating the effect of temperature on the solubility of carbon in the austenitic phase.
• FIG. 4 represents a phase diagram of an alloy Fe-(27.5-x)%Cr-7%Ni-5%Mo-x%C (% by weight) according to the invention, illustrating the effect of temperature on the solubility of carbon in the austenitic phase.
• FIG. 5 represents a phase diagram of an alloy Fe-(27.5-x)%Cr-7%Ni-5%Mo-x%N (% by weight) according to the invention, illustrating the effect of temperature on the solubility of nitrogen in the austenitic phase.
• FIG. 6 represents an image obtained by optical microscopy of an Fe-29.5%Cr-7%Ni-3.5%Mo (% by weight) steel according to the invention.
• FIG. 7 represents the curve of current as a function of potential in a potentiodynamic polarization test for an Fe-28.5%Cr-7%Ni-4%Mo (% by mass) steel according to the invention for three temperatures different heat treatment hardening in comparison with an austenitic stainless steel DIN 1.4435 according to the prior art.
• the invention relates to paramagnetic stainless steels having a hardness of between 575 and 900 HV10, preferably between 650 and 900 HV10, more preferably between 675 and 900 HV10.
• HV10 hardness means a Vickers hardness measured according to ISO 6507-1:2018.
• the invention also relates to a part and more specifically to a watch component made with this steel. It may be a casing component chosen from the non-exhaustive list comprising a middle part, a back, a bezel, a crown, a pusher, a bracelet link, a bracelet, a pin buckle, a dial, a hand and a dial index.
• the present invention also relates to the process for manufacturing the stainless steel part according to the invention.
• the stainless steels according to the invention have the following composition by weight:
• composition by weight Preferably, they have the following composition by weight:
• the percentage by weight of Mo for one of the ranges of compositions mentioned above is between 3.5 and 15% or between 3.5 and 10%. More preferably, it is between 3.5 and 6% or between 4 and 6%.
• the percentage by weight of W for one of the ranges of compositions above is between 0.2 and 5% and more advantageously between 0.5 and 5%.
• the Mo content greater than 3% by mass makes it possible to increase the Mo concentration in the austenitic phase with, as a corollary, an increase in the overall resistance of the steel to corrosion.
• the increase in the concentration of Mo makes it possible above all to carry out the hardening heat treatment (in the sigma + CFC range) at higher temperature as illustrated in figure 1 .
• the heat treatment can be carried out up to 950°C and for 5% Mo, it can be carried out up to 975°C.
• Performing the hardening heat treatment in the sigma + CFC range at a higher temperature further increases the concentration of Cr and Mo in the austenite, thereby further improving the corrosion resistance.
• the effect of processing temperature on the PREN (Pitting Resistance Equivalent Number based on the content of Cr, Mo and N such that PREN %Cr + 3.3 x %Mo + 16 x %N) of austenite is illustrated in Figure 2.
• the process for manufacturing a stainless steel part comprises a step a) of providing or producing a blank having a composition falling within the aforementioned ranges.
• This blank has a predominantly ferritic or, preferably, 100% ferritic structure.
• the blank is obtained from a base material subjected to a thermal or thermomechanical treatment at a temperature comprised in the range 1000° C.-1500° C. followed by quenching.
• the base material can be in the form of a powder or a consolidated material. It can be made by casting, by pressing, by injection molding (MIM: Metal Injection Moulding), by additive manufacturing, and more broadly by powder metallurgy.
• MIM Metal Injection Moulding
• the base material and the heat treatment in a single step, for example, by an additive manufacturing technique with laser (SLM: Selective Laser Melting). These different techniques make it possible to produce a blank with a base material having dimensions substantially equal to those of the part to be produced, in which case a subsequent shaping step is not required.
• the composition of the base material is optimized to obtain a predominantly or completely ferritic structure when maintained at a temperature of between 1000° C. and 1500° C. for a time of between 1 minute and 24 hours.
• the temperature is chosen to obtain a mass fraction of austenite less than or equal to 40% and a mass fraction of ferrite greater than or equal to 60%.
• the austenite formed during the maintenance between 1000° C. and 1500° C. is called primary as opposed to the so-called secondary austenite formed during the subsequent hardening heat treatment.
• the structure at the end of the maintenance between 1000° C. and 1500° C. is completely ferritic.
• Thermal or thermomechanical treatment in the 1000°C-1500°C range can be used to carry out homogenization, recrystallization or even stress relaxation treatments on base materials obtained by casting or to carry out sintering on base materials in powder form.
• the treatment in the ferritic or ferritic-austenitic range can be carried out in a single cycle or comprise several cycles of thermal or thermomechanical treatment. It can also be preceded or followed by other thermal or thermomechanical treatments.
• the blank After maintaining it in the ferritic or ferritic-austenitic range, the blank is subjected to rapid cooling, also referred to as quenching, to a temperature below 500° C. so as to avoid the formation of new phases during cooling.
• rapid cooling also referred to as quenching
• the ferritic or ferritic-austenitic structure is preserved at room temperature.
• the compositions according to the invention the ferritic structure is sufficiently stable to be preserved at room temperature after rapid cooling but sufficiently metastable to be easily and rapidly transformed into the sigma phase and into austenite during a subsequent heat treatment at intermediate temperatures. between 600°C and 1000°C (see step c) below).
• the alloy At the end of step a), the alloy has a low hardness and a high ductility allowing, where appropriate, easy shaping, for example by forging, by cutting or by machining.
• the method includes an optional step b) of shaping the blank by machining, cutting or by any operation involving deformation such as forging.
• This step can be carried out in several sequences. This step is not required if the tissue blank from step a) already has the final shape of the part to be manufactured.
• a plastic deformation operation can be used to increase in particular the rate of transformation of ferrite during the subsequent stage of transformation of ferrite into austenite and into the sigma phase.
• this plastic deformation step does not generate problematic hardening for a setting. possible shape by machining or cutting.
• This plastic deformation in one or more sequences can be carried out at a temperature below 650°C.
• the method includes a step c) of heat treatment for hardening the blank in one or more stages between 600° C. and 1000° C. to obtain the final properties.
• the duration of the heat treatment between 600° C. and 1000° C. is fixed so as to guarantee total transformation of the ferrite and therefore the obtaining of a microstructure formed of a sigma phase and an austenitic phase.
• total transformation is meant a transformation of more than 99% of the ferrite into an austenitic phase+a sigma phase.
• the final structure could contain traces of residual ferrite with a percentage lower than 1%.
• the rate of transformation of ferrite into an austenitic phase + a sigma phase depends in particular on the composition of the alloy and its thermomechanical history as mentioned above.
• the total duration of the heat treatment in one or more stages is between 30 minutes and 24 hours.
• the heat treatment is carried out in two stages with a first stage between 600°C and 850°C and a second stage between 850°C and 1000°C.
• the first stage at a lower temperature makes it possible to obtain a faster transformation rate and a finer microstructure, while the second stage at a higher temperature makes it possible to maximize corrosion resistance.
• the transformation is complete after the first level, the microstructure remains fine even after the second level at higher temperature.
• the steel After this hardening treatment, the steel has a mass fraction of sigma phase between 40% and 80% and a mass fraction of austenite between 20% and 60%, the percentages depending on the chemical composition and heat treatments. made. As mentioned above, the steel may contain traces of residual ferrite with a percentage lower than 1% by weight.
• the austenitic phase is formed of secondary austenite and possibly primary austenite.
• the primary austenite and the secondary austenite have a size of less than 10 ⁇ m and more advantageously than 5 ⁇ m.
• size is meant the most dimension of the phase in a cross-sectional view. It may thus be the thickness of the austenite lamellae when the austenite is in the form of lamellae or the diameter when the austenite is in the globular form. In the latter case, when the austenite is not perfectly spherical, the size is relative to the smallest dimension of the austenitic structure.
• the final structure is formed of secondary austenite and of the sigma phase without primary austenite.
• the microstructure thus obtained is a very fine and homogeneous eutectoid microstructure formed of the secondary austenite and the sigma phase.
• Secondary austenite has the characteristic of being finer than primary austenite. This coarser structure of the austenite formed before hardening is less favorable for obtaining mirror polishing.
• the differences in compositions between the austenites formed respectively before and after the hardening treatment are less favorable from the point of view of corrosion resistance.
• the part obtained has a high hardness between 575 and 900 HV10, and more specifically between 650 and 900 HV10 thanks to the hardening heat treatment.
• possible non-metallic inclusions may also be present in small quantities, without this affecting the mechanical and magnetic properties.
• inclusions to improve machinability such as manganese sulphides, may also be present in small quantities in the alloy.
• This hardening heat treatment step can be followed by a possible surface finishing step d) such as polishing.
• the manufacturing process may include an additional step b′) before the hardening heat treatment, in the temperature range 100°C- 1500°C to transform the austenite + ferrite structure into a 100% ferritic structure.
• steels after high temperature heat treatment (1000°C-1500°C) followed by quenching, steels have the following properties in particular:
• the steels according to the invention have in particular the following properties:
• the steel according to the invention is particularly efficient thanks to the concentration of molybdenum greater than 3%, which makes it possible to increase the resistance to corrosion of the austenitic phase and therefore of the alloy in its globality. These steels are therefore of particular interest for trim components.
• Fe29Cr8Ni5Mo contains in mass percentages 58% iron, 29% chromium, 8% nickel and 5% molybdenum. It was manufactured by arc fusion from high purity elements (> 99.9%) and cast in the form of a bar. It was then subjected to a homogenization heat treatment at 1300°C for 2 hours in an argon atmosphere followed by gas quenching (approx. 200K/min). The bar was then deformed at room temperature by compression with a reduction in thickness by a factor of 2 before being annealed in the ferritic range at 1080°C for 10 minutes in air and quenched in water.
• the Fe29Cr8Ni5Mo alloy has a Vickers hardness of 265 HV10. Then, a hardening heat treatment was carried out on one sample at 850° C. and on another at 900° C. for 6 hours. A fine, homogeneous and two-phase microstructure comprising the austenitic phase and the sigma phase is obtained. In this metallurgical state, the Fe29Cr8Ni5Mo alloy has a Vickers hardness of 705 and 675 HV10 respectively.
• Fe29Cr7Ni4Mo contains in mass percentages 60% iron, 29% chromium, 7% nickel and 4% molybdenum. It was also manufactured by arc fusion from high purity elements (> 99.9%), subjected to a homogenization heat treatment at 1300°C for 2 hours under argon followed by gas quenching, deformed at temperature by compression with a reduction in thickness by a factor of 2, subjected to an annealing heat treatment at 1100°C in air for 10 minutes followed by water quenching. After this annealing heat treatment, the Fe29Cr7Ni4Mo alloy has a single-phase ferritic microstructure. Then, a first sample was brought to 700° C.
• Fe29.5Cr7Ni3.5Mo contains in mass percentages 60% iron, 29.5% chromium, 7% nickel and 3.5% molybdenum. It was also manufactured by arc fusion from high purity elements (> 99.9%), subjected to a homogenization heat treatment at 1300°C for 2 hours under argon followed by gas quenching, deformed at room temperature by compression with a reduction in thickness by a factor of 2, subjected to an annealing heat treatment at 1100°C in air for 15 minutes followed by water quenching. After this annealing heat treatment, the Fe29.5Cr7Ni3.5Mo alloy has a single-phase ferritic microstructure. Then, a first sample was brought to 700° C.
• Example 4 The microstructure observed by optical microscopy under polarized light is represented in FIG. 6 for the first sample. A fine distribution of the two phases is observed with the austenitic phase in relief mainly in the form of lamellae and the sigma phase in the matrix.
• Example 4 The microstructure observed by optical microscopy under polarized light is represented in FIG. 6 for the first sample. A fine distribution of the two phases is observed with the austenitic phase in relief mainly in the form of lamellae and the sigma phase in the matrix.
• potentiodynamic polarization tests were carried out on a commercial reference stainless steel which is DIN 1.4435 steel and on an Fe28.5Cr7Ni4Mo grade according to the invention subjected to a hardening heat treatment at three distinct temperatures of 750, 800 and 850° C. respectively for 6 hours under vacuum.
• the steels were heat treated at 1300°C for 2 hours under argon then cooled by gas quenching to obtain a 100% ferritic structure at room temperature.
• the alloys After heat treatment at 750, 800 or 850°C, the alloys have a two-phase austenite + sigma phase structure.
• the potentiodynamic polarization measurements were carried out in an electrochemical cell comprising a calomel reference electrode saturated with potassium chloride and a platinum counter electrode.
• the sample to be analyzed i.e. the working electrode is in the form of a mirror-polished washer 8 mm in diameter.
• Potentiodynamic polarization tests make it possible to evaluate the resistance to corrosion in a chloride medium by comparing the pitting potentials. The latter correspond to the potentials for which a rapid increase in current is measured following local rupture of the passive film and corrosion by pitting. For the tests, the pitting potential is defined as the potential corresponding to a current of 0.25 mA.
• a 1 M NaCl solution is prepared, introduced into the cell and then purged with nitrogen for 1 hour.

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• 2020
  • 2020-11-09 EP EP20206523.1A patent/EP3995600A1/de not_active Withdrawn
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