Paramagnetic hard stainless steel and method for manufacturing the same

Abstract

To optimize the composition and microstructure of the prior art at specific points on a surface of a part that are subjected to greater stress.SOLUTION: One aspect of the present invention relates to a paramagnetic stainless steel part (1) including a core (2) surrounded by a surface including at least one first zone (3a) and at least one second zone (3b). The core (2) and the second zone (3b) have a hardness HV1 of 500-900, and a microstructure formed by a sigma phase at a mass percentage of 40-80% and an austenitic phase at a mass percentage of 20-60%. In the part (1), the first zone (3a) is richer in Ni than the core (2) and the second zone (3b), the first zone (3a) forms a layer entirely made of an austenitic phase, the layer is referred to as an austenitic layer (4), and the austenitic layer (4) has a hardness of less than 400 HV1. Another aspect of the disclosure relates to a method for manufacturing this part.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to a component made of a paramagnetic stainless steel having a core hardness of 500 to 900 HV1 and a surface hardness of less than 400 HV1 for a part of the surface, particularly a timepiece component. The present invention further relates to a method for manufacturing this stainless steel component. relates to.

Background Art

[0002] Hard non-ferromagnetic metal alloys are used in many fields, mainly for components that are subject to high mechanical and / or tribological stresses and need to remain non-responsive to magnetic fields. This is particularly true for a number of timepiece components, such as wheels, kana, shafts, springs, etc. within a movement. Having high hardness is also important for exterior components such as middle cases, bezels, back cases, clasps, or lugs. When the hardness is high, that is, when the hardness exceeds 500 HV, generally the scratch resistance and wear resistance are improved, so good durability is provided for these components exposed to the external environment. Usually, alloys having such a level of hardness are ferromagnetic and thus not suitable for timepiece components.

[0003] Recently, a paramagnetic stainless steel having a hardness of 500 to 900 HV10 with the composition and microstructure disclosed in Patent Document 1 has been developed. This steel contains the following by weight ratio, - 26 ≦ Cr ≦ 40% - 0 ≦ Mn ≦ 5% - 5 ≦ Ni ≦ 20% - 0 ≦ Mo ≦ 3% - 0 ≦ Al ≦ 5% - 0 ≦ Cu ≦ 2% - 0 ≦ Si ≦ 5% - 0 ≦ Ti ≦ 1% - 0 ≦ Nb ≦ 1% - 0 ≦ C ≦ 0.1% - 0 ≦ N ≦ 0.1% - 0 ≦ S ≦ 0.5% - 0 ≦ P ≦ 0.1% The remainder consists of iron and any impurities, each with a concentration of 0.5% or less. It has a microstructure formed of 40 - 80% sigma phase by mass percentage and 20 - 60% austenite phase by mass percentage.

[0004] It is manufactured using a special method that includes the following steps. - Providing or manufacturing a blank having the above chemical composition and mainly or completely having a ferrite structure. - A step of performing a heat treatment called hardening treatment on the blank to obtain a component. The hardening treatment is carried out at a temperature of 650 - 900°C for 30 minutes to 24 hours, transforming the ferrite of the structure into austenite phase and intermetallic sigma phase, and cooling to ambient temperature after the hardening treatment.

[0005] This special microstructure consisting of two non - ferromagnetic phases provides an extremely good compromise between hardness and toughness, good corrosion resistance, and excellent polishability.

[0006] However, by optimizing the microstructure and composition of this steel, the ability of components to withstand impact at specific locations under large stress can be improved.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0008] The present invention consists of optimizing the composition and microstructure of the prior art at specific locations on the surface of components that are subject to large stress.

Means for Solving the Problems

[0009] Therefore, the above-described steel manufacturing method is improved by adding a local surface treatment step before the hardening heat treatment. These steps all consist of selectively depositing a gamma-phase forming element, namely nickel, on these specific locations before the hardening heat treatment, and performing a diffusion heat treatment. By this diffusion heat treatment, nickel is diffused to a predetermined depth, and ferrite is transformed into a ductile 100% austenite layer, thereby improving the impact resistance.

[0010] As a result, a paramagnetic steel part is obtained in which a core and a part of the surface surrounding the core have a high hardness of 500 to 900 HV1, and the other part of the surface has a hardness of less than 400 HV1. The core and the said part of the surface contain a fine structure formed of a sigma phase and an austenite phase, and the other part of the surface is formed of austenite that does not contain a sigma phase, thereby making it possible to reduce the hardness while maintaining the paramagnetism of the part.

[0011] More specifically, this is a paramagnetic stainless steel part including a core surrounded by a surface including at least one first region and at least one second region. - The core and the second region have a chemical composition including the following by weight ratio. · 26 ≦ Cr ≦ 40% · 0 ≦ Mn ≦ 5% · 5 ≦ Ni ≦ 20% · 0 ≦ Mo ≦ 3% · 0 ≦ Al ≦ 5% · 0 ≦ Cu ≦ 2% · 0 ≦ Si ≦ 5% · 0 ≦ Ti ≦ 1% · 0 ≦ Nb ≦ 1% · 0 ≦ C ≦ 0.1% · 0 ≦ N ≦ 0.1% · 0 ≦ S ≦ 0.5% · 0 ≦ P ≦ 0.1% The balance consists of iron and any impurities, and the respective concentrations are 0.5% or less. The core and the second region have a hardness of HV1 of 500 to 900 and a microstructure formed of a sigma phase of 40 to 80% by mass and an austenite phase of 20 to 60% by mass. The component is characterized in that the first region is rich in Ni with respect to the core and the second region, the first region forms a layer consisting entirely of the austenite phase, the layer is called the austenite layer, and the austenite layer has a hardness of less than 400 HV1.

[0012] More specifically, the method for manufacturing a paramagnetic stainless steel component includes the following steps. a) Providing or manufacturing a blank having substantially the shape of the component to be manufactured, or a blank having a different shape, the blank having the aforementioned chemical composition and mainly or entirely having a ferrite structure. b) Depositing a Ni layer over the entire surface or only the first region of the surface, and when depositing over the entire surface, including step b') of locally dissolving the Ni layer over the second region, or a step of locally machining the second region. c) Performing a heat treatment called diffusion treatment on the blank at a temperature of 1050 to 1400 °C to diffuse Ni over a predetermined depth of the blank under the first region, and transforming the ferrite within the predetermined depth into a complete austenite phase forming an austenite layer. d) Performing a heat treatment called hardening treatment on the blank to obtain the component, the hardening treatment being performed at a temperature of 650 to 900 °C for 30 minutes to 24 hours, transforming the ferrite in the core and the second region into an austenite phase and an intermetallic sigma phase, and cooling to ambient temperature following the hardening treatment.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Mode for Carrying Out the Invention

[0014] The present invention relates mainly to parts made of paramagnetic stainless steel having a hardness of mainly 500 to 900 HV1, and a method for manufacturing parts made of these steels. The HV1 hardness is understood to mean the Vickers hardness measured according to the standard ISO 6507-1:2018. As an example, the part can be a timepiece part. It can be an external part selected from a non-exhaustive list including a middle case, a back case, a bezel, a screw, a push piece, a link of a bracelet, a bracelet, a pin buckle, a clasp such as a folding clasp, a dial, a hand, and an index. It can also be a component of a movement selected from a non-exhaustive list including gears, shafts, kana, springs, bridges, plates, screws, and templates.

[0015] Component 1 has different chemical compositions and microstructures in the core 2 of the component and the surface part 3a around the core 2 (Figure 4). This surface part 3a, also called the first region, is richer in Ni compared to the core 2 of the component and the remaining part 3b of the surface, also called the second region, and forms an austenite layer 4 after heat treatment. According to the present invention, only specific regions of the surface are targeted. These are the regions that receive the maximum stress during impact. As an example, in Figure 5, the inner region of the fuse cover, which is subjected to stress and requires a ductile layer to improve impact resistance, is indicated using an arrow. Since a higher hardness is desired on the outside of the component, having a ductile layer on the outside of the cover may disadvantageously affect it, so the entire surface is not covered with a ductile layer. Another example is the middle case. Specific parts of the middle case are used to close the back case and are characterized by being thin. Enhancing the ductility of these parts with an austenite layer is very beneficial.

[0016] The thickness of the nickel-rich austenite layer is generally less than 500 μm, more generally about 10 - 20 μm. It should be noted that the selection of the layer thickness varies depending on the size of the component because the ductile layer needs to occupy a relatively small volume with respect to the total volume in order to retain the advantages of the overall hard component.

[0017] For the core and the nickel-poor part of the surface, stainless steel with the following weight composition ratios is used. - 26 ≦ Cr ≦ 40% - 0 ≦ Mn ≦ 5% - 5 ≦ Ni ≦ 20% - 0 ≦ Mo ≦ 3% - 0 ≦ Al ≦ 5% - 0 ≦ Cu ≦ 2% - 0 ≦ Si ≦ 5% - 0 ≦ Ti ≦ 1% - 0 ≦ Nb ≦ 1% - 0 ≦ C ≦ 0.1% - 0 ≦ N ≦ 0.1% - 0 ≦ S ≦ 0.5% - 0 ≦ P ≦ 0.1% The remainder consists of iron and any impurities, and the concentration of each is 0.5% or less.

[0018] Preferably, the stainless steel has the following weight composition ratio. - 28 ≦ Cr ≦ 38% - 0 ≦ Mn ≦ 3% - 5 ≦ Ni ≦ 15% - 0 ≦ Mo ≦ 3% - 0 ≦ Al ≦ 3% - 0 ≦ Cu ≦ 2% - 0 ≦ Si ≦ 5% - 0 ≦ Ti ≦ 1% - 0 ≦ Nb ≦ 1% - 0 ≦ C ≦ 0.05% - 0 ≦ N ≦ 0.05% - 0 ≦ S ≦ 0.5% - 0 ≦ P ≦ 0.1% The remainder also consists of iron and any impurities, and the concentration of each is 0.5% or less.

[0019] More preferably, the stainless steel has the following weight composition ratio. - 30 ≦ Cr ≦ 36% - 0 ≦ Mn ≦ 3% - 5 ≦ Ni ≦ 10% - 0 ≦ Mo ≦ 1% - 0 ≦ Al ≦ 1% - 0 ≦ Cu ≦ 1% - 0 ≦ Si ≦ 3% - 0 ≦ Ti ≦ 1% - 0 ≦ Nb ≦ 1% - 0 ≦ C ≦ 0.05% - 0 ≦ N ≦ 0.05% - 0 ≦ S ≦ 0.5% - 0 ≦ P ≦ 0.1% The remainder also consists of iron and any impurities, and the concentration of each is 0.5% or less.

[0020] The nickel-poor parts of the core and the surface have a hardness of HV1 of 500 - 900 and a microstructure formed by 40 - 80% sigma phase and 20 - 60% austenite phase by mass percentage.

[0021] Other parts of the surface have a composition close to the core but are nickel-rich. The hardness is less than 400 HV1, preferably 150 - 350 HV1. The whole is composed of an austenite phase.

[0022] According to the present invention, a method for manufacturing a stainless steel component includes step a) of providing or manufacturing a blank having a composition falling within the aforementioned range. This blank mainly has a ferrite structure, or preferably a 100% ferrite structure. The blank is obtained from a base material that has been heat-treated or thermomechanically processed in a temperature range of 950 - 1450°C and then rapidly cooled. The base material may be in powder form or a consolidated body. Also, it can be manufactured by casting, pressing, metal injection molding (MIM), additive manufacturing, and more broadly by powder metallurgy. For example, it is also conceivable to perform the base material and heat treatment in a single step by selective laser melting (SLM) technology. With these various technologies, a blank can be manufactured with a base material having substantially the same dimensions as the component to be manufactured, in which case subsequent forming steps are not necessary.

[0023] The composition of the base material is optimized so that a mainly or completely ferrite structure is obtained when held at a temperature of 950 - 1450°C for 1 minute to 24 hours. The temperature is selected such that the mass fraction of austenite is 40% or less and the mass fraction of ferrite is 60% or more. The presence of austenite enables obtaining minimum hardness and maximum ductility, facilitating forming by, for example, forging, cutting, or machining.

[0024] Using a heat treatment or thermomechanical processing in the range of 950 - 1450°C, a homogenization, recrystallization, or stress relief treatment can be performed on the base material obtained by casting, or sintering can be performed on the powdered base material. The treatment in the ferrite region or ferrite-austenite region may be performed in one cycle or may include multiple heat treatment or thermomechanical processing cycles. Also, other heat treatments or thermomechanical processes may be performed before or after.

[0025] The blank is held in the ferrite or ferrite-austenite region and then quenched, also known as rapid cooling, to a temperature below 500 °C so that no new phase is formed during cooling. As a result, the ferrite or ferrite-austenite structure is maintained at ambient temperature. Thanks to the composition according to the invention, the ferrite structure is sufficiently stable after quenching and is maintained at ambient temperature, but is sufficiently metastable to transform easily and rapidly into the sigma phase and the austenite phase during subsequent heat treatment at intermediate temperatures between 650 and 900 °C.

[0026] After the end of step a), the alloy has a low hardness and a high ductility, which enables easy shaping, for example by forging, blanking, or machining.

[0027] After step a), the method includes any step of shaping the blank by any operation involving deformation, such as machining, blanking, or forging. This step can be carried out in several orders. If the blank already has the final shape of the part to be manufactured at the end of step a), this step is not necessary. This step may be carried out after the following diffusion heat treatment step. As will be described below, this step may also be used to selectively and mechanically remove the Ni-rich layer.

[0028] In addition to shaping, a plastic deformation operation can be carried out, in particular to increase the ferrite transformation rate in subsequent steps that transform ferrite into austenite and sigma phases. Furthermore, the degree of hardening by strain hardening is low in the ferrite structure, and since the alloy according to the invention has a mainly or completely ferrite structure before hardening treatment, this plastic deformation step does not cause hardening that could be a problem for any shaping operation by machining or blanking. This plastic deformation can be carried out continuously one or more times at a temperature below 650 °C.

[0029] The method then includes steps b) and c), which are more specifically the subject of the present invention and aim to selectively deposit nickel on the surface of the blank and diffuse this nickel over a predetermined depth of the part. It should also be noted that these steps can optionally be carried out before the forming step, if there is a forming step.

[0030] In step b), nickel is deposited on the entire surface of the blank or on a part of the surface of the blank. Usually, the layer is deposited by zinc plating or PVD. The thickness of the deposited layer is 1 - 20 μm, preferably 3 - 10 μm, more preferably 4 - 10 μm. When nickel is deposited only on a part of the surface of the blank, partial masking is applied to the surface for the purpose of nickel deposition. The masking can be carried out, for example, using a lacquer that will dissolve later. When depositing nickel on the entire surface, step b') is carried out to dissolve the deposits in the unnecessary parts, or a machining step is carried out before or after the diffusion step, or after the hardening heat treatment step, to selectively remove the nickel layer. One way to locally dissolve the nickel layer is, for example, to mask the desired areas of the layer using a lacquer or, if the hollow area is covered with an austenite layer, simply by placing an element like a plug, and then immersing the part in an acidic bath such as dilute HNO 3 etc. for several hours. Then the part is washed with water. At the end of the step of locally depositing the nickel layer, step c) is carried out in which the part is heat-treated at 1050 - 1400 °C, preferably 1200 - 1300 °C, for 5 minutes to 5 hours, preferably 5 minutes to 1 hour, to diffuse the nickel into the alloy and transform this area, which was previously mainly formed of ferrite, into austenite. As a result, a part with a ductile austenite layer in a specific area of the surface is obtained. This layer is 100% austenite and its thickness depends mainly on two factors related to nickel diffusion in the alloy, namely the thickness of the Ni deposition and the high-temperature diffusion treatment time.

[0031] At this stage of the process, the core and the Ni-poor parts of the surface are still mainly or completely formed of ferrite. In step d), the blank is subjected to a hardening heat treatment at 650 - 900 °C, preferably 700 - 800 °C, to obtain the final properties. The heat treatment time at 650 - 900 °C is set so as to ensure complete transformation of the ferrite. Thereby, a fine structure formed by the sigma phase and the austenite phase is obtained in the untreated parts of the core and the surface of the part, and the nickel-rich layer formed of austenite remains stable without transformation.

[0032] The transformation rate from ferrite to austenite + sigma phase depends, as described above, in particular on the composition of the alloy and its thermomechanical treatment history. Generally, the treatment time is between 30 minutes and 24 hours. After hardening treatment, the steel has a sigma phase mass fraction of 40 - 80% and an austenite mass fraction of 20 - 60%, and the ratio varies depending on the chemical composition and the heat treatment carried out. The untreated parts of the core and the surface thus obtained have a high hardness of 500 - 900 HV1 due to the hardening heat treatment.

[0033] As with any stainless steel, there may also be a small amount of non-metallic inclusions without affecting the mechanical and magnetic properties. Furthermore, inclusions that improve machinability, such as manganese sulfides, may be present in small amounts in the alloy.

[0034] Following this hardening heat treatment step, an optional surface finishing step e), such as polishing, may be carried out.

[0035] Alternatively, if a blank having an austenite + ferrite structure exists in step a), the manufacturing method may include an additional step of transforming the austenite + ferrite structure into a 100% ferrite structure in the temperature range of 950 - 1450 °C before the Ni diffusion heat treatment of step b). Alternatively, this step may form a single step including the diffusion step.

[0036] In summary, the steel quenched after high-temperature heat treatment (950 - 1450 °C) has the following characteristics in particular. - A hardness of 150 - 400 HV1 - Good ductility and plastic deformation with no cracks exceeding 50% in compression at ambient temperature - Ferromagnetic behavior due to the presence of ferrite

[0037] After local nickel deposition with diffusion and hardening heat treatment, the steel according to the invention has the following characteristics in particular. - A hardness of 500 - 900 HV1 in the core and in the nickel-poor parts of the surface - A hardness of less than 400 HV1 in specific areas of the surface - Non-ferromagnetic behavior - Excellent polishability due to a very dense microstructure - Good wear resistance - Improved impact resistance in high-stress areas - Good corrosion resistance

[0038] Tests were carried out using a 5-μm thick galvanic nickel deposit applied over the entire surface of the part. After masking the desired areas of the layer, the nickel layer was selectively dissolved. The dissolution was carried out by immersion in a dilute HNO 2 bath with 20 mL of HNO 3 added to 100 mL of H 3 O for 19 hours. The nickel diffusion heat treatment was carried out at 1250 °C for several tens of minutes. Then, the hardening heat treatment was carried out at 750 °C for 12 hours. As shown in Figure 1, the thickness of the austenite layer 4 is 10 - 20 μm. Figure 2 shows the areas of the surface without an austenite layer. The austenite layer 4 serves to stop the cracks 5 that occur towards the surface of the sample when stress is applied, thereby delaying fracture (Figure 3).

[0039] Bending tests were also carried out using a test piece with an austenite layer over the entire surface and, for comparison, a test piece without an austenite layer. To simplify the test, the test pieces were completely covered with the austenite layer. Figure 6 shows that the presence of the austenite layer results in a greater energy absorption in the bending test.

Description of Symbols

[0040] 1 part 2 core of parts 3a First area 3b Second area 4 Austenite layer 5 Crack

Claims

1. A paramagnetic stainless steel part (1) comprising a core (2) surrounded by a surface comprising at least one first region (3a) and at least one second region (3b), said core (2) and said second zone (3b) have a chemical composition comprising, in percentages by weight: 26≦Cr≦40% 0≦Mn≦5% 5≦Ni≦20% 0≦Mo≦3% 0≦Al≦5% 0≦Cu≦2% 0≦Si≦5% 0≦Ti≦1% 0≦Nb≦1% 0≦C≦0.1% 0≦N≦0.1% 0≦S≦0.5% 0≦P≦0.1% the remainder being iron and optional impurities, each at a concentration of 0.5% or less; the core (2) and the second region (3b) have a hardness HV1 of 500 to 900 and a microstructure formed of 40 to 80% by mass of sigma phase and 20 to 60% by mass of austenite phase; The component (1) is characterized in that the first zone (3a) is Ni-rich with respect to the core (2) and the second zone (3b), that the first zone (3a) forms a layer consisting entirely of austenite phase, said layer being called austenite zone (4), and that the austenite zone (4) has a hardness of less than 400 HV1.

2. Component (1) according to claim 1, characterized in that the austenitic layer (4) has a hardness of 150 to 350 HV1.

3. Component (1) according to claim 1, characterized in that the component (1) is an exterior component or a movement component of a watch.

4. A method for producing a part (1) of paramagnetic stainless steel according to claim 1, comprising the steps of: a) providing or producing a blank having substantially the shape of the part (1) to be produced or a blank having a different shape, said blank having a chemical composition according to claim 1 and having a predominantly or completely ferritic structure, b) depositing a layer of Ni over the entire surface or only over a first area (3a) of the surface, and, in the case of said deposition over the entire surface, locally dissolving said layer of Ni over said second area (3b) b') or locally machining said second area (3b), c) subjecting said blank to a heat treatment called diffusion treatment at a temperature between 1050 and 1400°C, diffusing Ni under said first zone (3a) and throughout a given depth of said blank, transforming the ferrite within said given depth into a fully austenitic phase forming an austenitic layer; d) subjecting said blank to a heat treatment called hardening, to obtain said part (1), said hardening being carried out at a temperature between 650 and 900°C for a period between 30 minutes and 24 hours, transforming the ferrite in said core and in said second zone (3b) into austenite and intermetallic sigma phases, followed by cooling to ambient temperature. The method includes:

5. The method according to claim 4, characterized in that the thickness of the Ni layer is 1-20 μm, preferably 3-10 μm, more preferably 4-10 μm.

6. 5. A method according to claim 4, characterized in that, if the Ni is deposited only on the first areas (3a) of the surface, a partial masking is applied to the surface for the purpose of the deposition of the Ni.

7. 5. A method according to claim 4, characterized in that the local dissolving step b') is carried out after masking the first areas (3a).

8. The locally dissolving step b') is carried out using HNO ３ 5. The method according to claim 4, characterized in that it is carried out in an acid bath, such as a bath.

9. 5. The method according to claim 4, further comprising a step of shaping the blank of step a) if the blank has a shape different from the part (1) to be manufactured, said shaping step being carried out between steps a) and b) or between steps b) and c).

10. 5. The method according to claim 4, characterized in that the structure of the blank of step a) contains an austenite mass fraction of less than or equal to 40% and a ferrite mass fraction of more than or equal to 60%.

11. 5. The method according to claim 4, characterized in that the structure of the blank of step a) contains 100% ferrite.

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