Paramagnetic hard stainless steel and manufacturing method thereof

Abstract

One aspect of the invention relates to a paramagnetic stainless steel component (1) comprising a core (2) surrounded by a surface comprising at least a first region (3a) and at least a second region (3b), said core (2) and said second region (3b) having a hardness HV1 of 500 to 900, the invention relates to a component (1) comprising a core (2) and a second region (3b), the microstructure of which is formed from 40% to 80% by mass of a sigma phase and from 20% to 60% by mass of an austenite phase, the component (1) being characterized in that the first region (3a) is rich in Ni with respect to the core (2) and the second region (3b), the first region (3a) forming a layer consisting entirely of an austenite phase, referred to as an austenite layer (4), the hardness of the austenite layer (4) being less than 400 HV1. Another aspect of the invention relates to a method of manufacturing the component.

Description

Technical Field

[0001] The invention relates to a component, in particular a watch component, made of paramagnetic stainless steel having a core hardness of 500 to 900 HV1 and a surface hardness of less than 400 HV1 in its surface parts. The invention also relates to a method for manufacturing such a stainless steel component. Background Art

[0002] Hard non-ferromagnetic metal alloys are used in many fields, mainly for components that are subjected to high mechanical and / or frictional stresses and must be insensitive to magnetic fields. This is especially true for many watch components, such as wheels, pinions, shafts or hairsprings in the movement. Obtaining high hardness is also important for external parts, such as the middle, outer ring, back cover, clasp or crown. This is because high hardness (i.e. a hardness of more than 500 HV) generally produces better scratch resistance and wear resistance, thereby providing good durability for these components exposed to the external environment. Alloys with this hardness are usually ferromagnetic and are therefore not suitable for watch components.

[0003] Recently, a paramagnetic stainless steel with a hardness of 500 to 900 HV10 has been developed, the composition and microstructure of which are disclosed in patent document EP3835438. This steel contains by weight:

[0004] -26≤Cr≤40%.

[0005] -0≤Mn≤5%.

[0006] -5≤Ni≤20%.

[0007] -0≤Mo≤3%.

[0008] -0≤A1≤5%.

[0009] -0≤Cu≤2%.

[0010] -0≤Si≤5%.

[0011] -0≤Ti≤1%,

[0012] -0≤Nb≤1%,

[0013] -0≤C≤0.1%,

[0014] -0≤N≤0.1%,

[0015] -0≤S≤0.5%.

[0016] -0≤P≤0.1%,

[0017] The remainder is composed of iron and any impurities, each with a concentration less than or equal to 0.5%. Its microstructure is formed by 40% to 80% by mass of sigma phase and 20% to 60% by mass of austenite phase.

[0018] It is produced using a special method, which includes the following steps:

[0019] - providing or producing billets having the above chemical composition and a predominantly or entirely ferritic structure,

[0020] The blank is subjected to a heat treatment (called hardening treatment) to obtain the component, the hardening treatment being carried out at a temperature ranging from 650 to 900° C. for 30 minutes to 24 hours in order to transform the ferrite of the structure into an austenitic phase and an intermetallic sigma phase, followed by cooling to ambient temperature.

[0021] This special microstructure consists of two non-ferromagnetic phases and offers a very good compromise between hardness and toughness, good corrosion resistance and excellent polishability.

[0022] However, the microstructure and composition of this steel can be optimized to improve the component's ability to withstand impacts at specific points that are subject to greater stress. Summary of the invention

[0023] The present invention involves optimizing the composition and microstructure of specific points on the surface of prior art components that are subject to greater stress.

[0024] To this end, the above steel manufacturing method is improved by carrying out a local surface treatment step before the hardening heat treatment. This step consists in selectively depositing an element that produces a gamma phase, namely nickel, at these specific points before the hardening heat treatment and carrying out a diffusion heat treatment. This diffusion heat treatment diffuses the nickel to a given depth, transforming the ferrite into a 100% austenite layer, which is ductile and thus improves impact resistance.

[0025] The result is a paramagnetic steel component whose core and a surface portion surrounding the core have a high hardness of 500 to 900 HV1, while the rest of the surface has a hardness of less than 400 HV1. The core and the surface comprise a microstructure formed of a sigma phase and an austenite phase, while the rest of the surface is formed of austenite without a sigma phase, which makes it possible to reduce the hardness while maintaining the paramagnetic properties of the component.

[0026] More specifically, this is a paramagnetic stainless steel component comprising a core surrounded by a surface comprising at least a first region and at least a second region,

[0027] The core and the second region have a chemical composition comprising the following elements by weight:

[0028] ·26≤Cr≤40%.

[0029] 0≤Mn≤5%,

[0030] 5≤Ni≤20%.

[0031] 0≤Mo≤3%,

[0032] 0≤A1≤5%.

[0033] 0≤Cu≤2%.

[0034] 0≤Si≤5%,

[0035] 0≤Ti≤1%.

[0036] 0≤Nb≤1%,

[0037] 0≤C≤0.1%.

[0038] 0≤N≤0.1%,

[0039] 0≤S≤0.5%.

[0040] 0≤P≤0.1%,

[0041] The remainder consists of iron and any impurities, each in a concentration less than or equal to 0.5%,

[0042] The core and the second region have a hardness HV1 of 500 to 900, and a microstructure thereof consisting of 40% to 80% by mass of a σ phase and 20% to 60% by mass of an austenite phase.

[0043] The component is characterized in that the first region is rich in Ni relative to the core and the second region, the first region forming a layer consisting entirely of an austenite phase, which is called an austenite layer, the hardness of the austenite layer being less than 400 HV1.

[0044] More specifically, the method for manufacturing a paramagnetic stainless steel component comprises the following steps:

[0045] a) providing or producing a blank having essentially the shape of the part to be manufactured or a shape different therefrom, the blank having the above-mentioned chemical composition and having a predominantly ferrite or completely ferrite structure,

[0046] b) depositing a Ni layer over the entire surface or only over a first region of the surface, and if the deposit is applied over the entire surface, carrying out a step b′) of locally dissolving the Ni layer over a second region, or a step of locally machining the second region,

[0047] c) heat treating the blank at a temperature of 1050° C. to 1400° C. (referred to as diffusion treatment) to diffuse Ni to a given depth of the blank below the first region and transform the ferrite in the given depth into a complete austenite phase forming an austenite layer,

[0048] d) subjecting the blank to a heat treatment (referred to as hardening treatment) to obtain a component, the hardening treatment being carried out at a temperature of 650 to 900°C for 30 minutes to 24 hours to transform the ferrite in the core and the second region into an austenite phase and an intermetallic σ phase, followed by cooling to ambient temperature. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 A cross-sectional view of a component according to the invention having a ductile austenite layer on the surface, observed by optical microscopy under polarized light, is shown.

[0050] Figure 2 A cross-sectional view of the same component observed by optical microscopy under polarized light is shown, which shows a surface portion without a ductile austenite layer, this portion of the surface and the core having the same microstructure formed by sigma phase and austenite phase.

[0051] Figure 3 A cross-sectional view through an electron microscope of a component according to the invention is shown, which has a ductile austenite layer on the surface, thereby preventing crack propagation when stressed.

[0052] Figure 4 is a schematic cross-sectional view of a component according to the invention.

[0053] Figure 5 A schematic diagram of the crown cover is shown, with arrows indicating the areas that are stressed during an impact.

[0054] Figure 6 The work of fracture (in Nmm) is shown for samples of different thicknesses, some of which comprise a ductile austenite layer at the surface according to the invention and some of which comprise no ductile austenite layer at the surface according to the prior art. DETAILED DESCRIPTION

[0055] The present invention relates to components made of paramagnetic stainless steels having a hardness mainly ranging from 500 to 900 HV1, and methods for manufacturing components made of these steels. HV1 hardness is understood to be the Vickers hardness measured according to standard ISO6507-1:2018. For example, the component can be a watch component. It can be an external component selected from a non-exhaustive list, which includes a middle, a back cover, a bezel, a crown, a button, a bracelet link, a bracelet, a tongue, a clasp (e.g. a folding clasp type), a dial, a hand and a dial scale. It can also be a movement component selected from a non-exhaustive list, which includes gears, shafts, pinions, hairsprings, bridges, splints, screws and a balance wheel.

[0056] The component 1 has different chemical compositions and microstructures in the core 2 of the component and in the surface portion 3a surrounding the core 2 ( Figure 4 ). Compared to the core 2 of the component and the rest of the surface 3b (also called second area), this part 3a of the surface (also called first area) is rich in Ni, so that after heat treatment an austenite layer 4 is formed. According to the invention, only certain areas of the surface are targeted. These areas are subject to the greatest stress during impact. For example, Figure 5 The arrows show the inner areas of the crown cap that are subject to stress and require a more ductile layer to improve impact resistance. The surface is not completely covered with a ductile layer, because having a ductile layer on the outside of the cap would be detrimental, as the outer part of the component requires a higher hardness. Another example is the middle section. Some parts of the middle section are used to close the back cover and are characterized by being thin. It would be very beneficial to increase the ductility of these parts with an austenitic layer.

[0057] The nickel-rich austenite layer is typically less than 500 μm thick, with a more typical thickness of about 10-20 μm. Note that the choice of layer thickness depends on the size of the component, as the ductile layer must occupy a relatively small volume compared to the total volume to retain the advantage of an overall hard component.

[0058] In the core and in the surface parts not rich in nickel, the component is made of stainless steel with the following weight composition:

[0059] -26≤Cr≤40%,

[0060] -0≤Mn≤5%.

[0061] -5≤Ni≤20%,

[0062] -0≤Mo≤3%.

[0063] -0≤A1≤5%.

[0064] -0≤Cu≤2%.

[0065] -0≤Si≤5%.

[0066] -0≤Ti≤1%,

[0067] -0≤Nb≤1%,

[0068] -0≤C≤0.1%,

[0069] -0≤N≤0.1%,

[0070] -0≤S≤0.5%.

[0071] -0≤P≤0.1%,

[0072] The remainder consists of iron and any impurities, each in a concentration less than or equal to 0.5%.

[0073] Preferably, the weight composition of the stainless steel is as follows:

[0074] -28≤Cr≤38%,

[0075] -0≤Mn≤3%.

[0076] -5≤Ni≤15%,

[0077] -0≤Mo≤3%.

[0078] -0≤A1≤3%.

[0079] -0≤Cu≤2%.

[0080] -0≤Si≤5%,

[0081] -0≤Ti≤1%,

[0082] -0≤Nb≤1%,

[0083] -0≤C≤0.05%,

[0084] -0≤N≤0.05%,

[0085] -0≤S≤0.5%,

[0086] -0≤P≤0.1%,

[0087] The remainder still consists of iron and any impurities, each in a concentration less than or equal to 0.5%.

[0088] More preferably, the weight composition of the stainless steel is as follows:

[0089] -30≤Cr≤36%.

[0090] -0≤Mn≤3%.

[0091] -5≤Ni≤10%,

[0092] -0≤Mo≤1%.

[0093] -0≤Al≤1%,

[0094] -0≤Cu≤1%.

[0095] -0≤Si≤3%.

[0096] -0≤Ti≤1%,

[0097] -0≤Nb≤1%,

[0098] -0≤C≤0.05%,

[0099] -0≤N≤0.05%,

[0100] -0≤S≤0.5%,

[0101] -0≤P≤0.1%,

[0102] The remainder still consists of iron and any impurities, each in a concentration less than or equal to 0.5%.

[0103] The core and the surface portion not rich in nickel have a hardness HV1 of 500 to 900, and a microstructure thereof is formed of 40 to 80% by mass of a sigma phase and 20 to 60% by mass of an austenite phase.

[0104] The composition of the rest of the surface is close to that of the core, but rich in nickel. Its hardness is less than 400 HV1, preferably 150 to 350 HV1. It is completely composed of the austenite phase.

[0105] According to the invention, the method for manufacturing a stainless steel component comprises a step a) of providing or producing a blank having a composition within the above range. The blank has a structure that is mainly ferrite or preferably 100% ferrite. The blank is obtained from a substrate that is heat treated or thermomechanically treated at a temperature of 950° C. to 1450° C. and then quenched. The substrate can be in powder form or in the form of a consolidated material. It can be produced by casting, pressing, metal injection molding (MIM), additive manufacturing and more generally by powder metallurgy. The substrate and the heat treatment can conceivably be carried out in a single step, for example by selective laser melting (SLM) technology. These various technologies allow the production of a blank having substantially the same dimensions as the component to be produced with a substrate, in which case no subsequent forming step is required.

[0106] The composition of the substrate is optimized to obtain a predominantly or completely ferrite structure when maintained at a temperature of 950°C to 1450°C for 1 minute to 24 hours. The temperature is selected 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 presence of austenite allows for minimum hardness and maximum ductility, allowing for ease of forming, such as by forging, cutting or machining.

[0107] Heat treatment or thermomechanical treatment in the range of 950°C to 1450°C can be used for homogenization, recrystallization or stress relief of the substrate obtained by casting, or for sintering the substrate in powder form. Treatment in the ferritic or ferritic-austenitic range can be carried out in a single cycle or can involve multiple heat treatment or thermomechanical treatment cycles. It can also be carried out before or after other heat treatment or thermomechanical treatment.

[0108] After being held in the ferrite or ferrite-austenite range, the billet is rapidly cooled (also referred to as quenched) to a temperature below 500°C to prevent the formation of new phases during the cooling process. Thus, the ferrite or ferrite-austenite structure is maintained at ambient temperature. Due to the composition of the present invention, the ferrite structure is stable enough to be maintained at ambient temperature after rapid cooling, but metastable enough to be easily and quickly transformed into the sigma phase and the austenite phase when subsequently heat treated at an intermediate temperature of 650°C to 900°C.

[0109] At the end of step a), the alloy has a low hardness and a high ductility and can be easily shaped, for example by forging, blanking or machining.

[0110] After step a), the method includes an optional step of shaping the blank by machining, blanking or any operation involving deformation (e.g., forging). This step can be performed in a number of sequences. If the blank already has the final shape of the part to be manufactured at the end of step a), this step is not required. This step can also be performed after the diffusion heat treatment step below. As described below, this step can also be used to selectively and mechanically remove the nickel-rich layer.

[0111] In addition to forming, a plastic deformation operation may be performed, in particular to increase the ferrite transformation rate in a subsequent step of transforming ferrite into austenite and sigma phase. Moreover, since strain hardening has a low hardening effect on the ferrite structure and the alloy according to the invention is mainly or completely ferrite before the hardening treatment, this plastic deformation step does not cause any hardening that is problematic for the optional forming operation by machining or blanking. This plastic deformation can be carried out in one or more sequences at a temperature below 650°C.

[0112] The method then comprises steps b) and c), which are more particularly the subject of the present invention and which aim to selectively deposit nickel on the surface of the blank and diffuse this nickel to a given depth in the part. It should be noted again that these steps can optionally be carried out before the shaping step, if there is one.

[0113] In step b), nickel is deposited on the entire surface of the blank or on a portion of the blank surface. Typically, the layer is deposited by electroplating or PVD. The thickness of the deposited layer is 1 to 20 μm, preferably 3 to 10 μm, and more preferably 4 to 10 μm. When nickel is deposited only on a portion of the blank surface, the surface is partially masked in order to aim at the nickel deposition. For example, a paint that is subsequently dissolved can be used for masking. If the nickel deposit is deposited on the entire surface, step b') is performed before or after the diffusion step, or even after the hardening heat treatment step to dissolve the unwanted deposits, or a machining step is performed to selectively remove the nickel layer. One method of locally dissolving the nickel layer can be to mask the area where the layer is needed, such as using a paint, or simply by placing an element such as a plug in the case of a hollow area that needs to be covered with an austenite layer, and then soaking the part in an acid bath (e.g., dilute HNO3) for several hours. Then rinse the part. At the end of the step of local deposition of the nickel layer, a step c) is performed to heat treat the component at 1050° C. to 1400° C., preferably 1200° C. to 1300° C., for a period of 5 minutes to 5 hours, preferably 5 minutes to 1 hour, in order to diffuse the nickel into the alloy and transform the areas previously formed mainly of ferrite into austenite. This results in the component having a ductile austenite layer in certain areas of the surface. This layer is 100% austenite and its thickness depends mainly on two factors related to the diffusion of nickel in the alloy, namely the thickness of the Ni deposit and the high-temperature diffusion treatment time.

[0114] At this stage of the process, the core and the surface parts not rich in nickel are still mainly or completely formed by ferrite. In step d), the blank is subjected to a hardening heat treatment at 650° C. to 900° C., preferably 700° C. to 800° C., to obtain the final properties. The duration of the heat treatment at 650° C. to 900° C. is set to ensure complete transformation of the ferrite, thereby obtaining a microstructure formed by σ phase and austenite phases on the core of the component and the untreated surface parts, and the nickel-rich layer formed by austenite remains stable and has not transformed.

[0115] The rate at which ferrite transforms into austenite + σ phase depends, among other things, on the composition of the alloy and its thermomechanical history, as described above. Typically, the treatment lasts from 30 minutes to 24 hours. After the hardening treatment, the steel has a mass fraction of σ phase of 40% to 80% and a mass fraction of austenite of 20% to 60%, the percentages depending on the chemical composition and the heat treatment performed. As a result of the hardening heat treatment, the core thus obtained and the surface of the parts not rich in Ni have a high hardness of 500 to 900 HV1.

[0116] As with all stainless steels, small amounts of non-metallic inclusions may be present but do not affect the mechanical and magnetic properties. In addition, small amounts of inclusions that enhance machinability, such as manganese sulfides, may be present in the alloy.

[0117] This hardening heat treatment step may be followed by an optional surface finishing step e), such as polishing.

[0118] Alternatively, in case a blank having an austenite + ferrite structure is present in step a), the manufacturing method may include an additional step, prior to the Ni diffusion heat treatment of step c), of transforming the austenite + ferrite structure into a 100% ferrite structure at a temperature in the range of 950° C. to 1450° C. Alternatively, this step may form a single step with the diffusion step.

[0119] In summary, after high temperature heat treatment (950℃-1450℃) followed by quenching, the steel has the following properties in particular:

[0120] -Hardness 150 to 400 HV1

[0121] -Good ductility, plastic deformation under compression at ambient temperature greater than 50% without cracking -Ferromagnetic behavior due to the presence of ferrite.

[0122] After local nickel deposition and diffusion and hardening heat treatment, the steel of the invention has in particular the following properties:

[0123] - The hardness of the core and the surface part not rich in nickel is 500 to 900 HV1

[0124] - Hardness of some areas of the surface is less than 400 HV1

[0125] - Non-ferromagnetic behavior

[0126] -Excellent polishability due to very fine microstructure

[0127] -Good wear resistance

[0128] - Improved impact resistance in high stress areas

[0129] -Good corrosion resistance.

[0130] The tests were carried out with a nickel deposit of 5 μm thick electroplated over the entire surface of the component. After masking the areas where this layer was needed, the nickel layer was selectively dissolved. The dissolution was carried out by immersion in a dilute HNO3 bath (where 20 ml HNO3 was dissolved in 100 ml H2O) for 19 hours. The nickel diffusion heat treatment was carried out at 1250°C for several tens of minutes. This was followed by a hardening heat treatment at 750°C for 12 hours. Figure 1As shown, the thickness of the austenite layer 4 is 10-20 μm. Figure 2 The austenite layer 4 helps to prevent cracks 5 from developing towards the surface of the sample when the load is applied, thus delaying fracture ( Figure 3 ).

[0131] Bend tests were also performed on specimens with an austenite layer over the entire surface, as opposed to specimens without an austenite layer. To simplify the tests, the specimens were completely coated with an austenite layer. Figure 6 It is shown that the presence of the austenite layer leads to a greater energy absorption during the bending test.

Claims

1. A paramagnetic stainless steel component (1) comprising a core (2) surrounded by a surface, said surface comprising at least a first region (3a) and at least a second region (3b), The core (2) and the second region (3b) have a chemical composition comprising the following elements 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 consists of iron and any impurities, each in a concentration less than or equal to 0.5%, The core (2) and the second region (3b) have a hardness HV1 of 500 to 900, and a microstructure thereof consisting of 40% to 80% by mass of a σ phase and 20% to 60% by mass of an austenite phase. The component (1) is characterized in that the first region (3a) is rich in Ni relative to the core (2) and the second region (3b), the first region (3a) forming a layer consisting entirely of an austenite phase, which is referred to as the austenite layer (4), and the hardness of the austenite layer (4) is less than 400 HV1.

2. The component (1) according to claim 1, characterized in that The hardness of the austenite layer (4) is 150 to 350 HV1.

3. Component (1) according to one of the preceding claims, characterized in that The component (1) is an external component or a component of a watch movement.

4. Method for producing a component (1) made of paramagnetic stainless steel according to one of the preceding claims, comprising the following steps: a) providing or producing a blank having essentially the shape of the component (1) to be manufactured or a shape which varies, said blank having a chemical composition according to claim 1 and having a predominantly ferrite or completely ferrite structure, b) depositing a Ni layer over the entire surface or only over a first region (3a) of the surface, and if the deposit is applied over the entire surface, carrying out a step b′) of locally dissolving the Ni layer over a second region (3b), or carrying out a step of locally machining the second region (3b), c) subjecting the blank to a heat treatment at a temperature of 1050° C. to 1400° C., called diffusion treatment, so as to diffuse Ni to a given depth of the blank below the first region (3a) and transform the ferrite in said given depth into a completely austenitic phase, thereby forming an austenitic layer (4), d) subjecting the blank to a heat treatment, called hardening treatment, at a temperature of 650 to 900° C. for a period of 30 minutes to 24 hours, in order to transform the ferrite in the core (2) and the second region (3b) into an austenite phase and an intermetallic sigma phase, to obtain the component (1), followed by cooling to ambient temperature.

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

6. The method according to claim 4, characterized in that The thickness of the Ni layer is 4 to 10 μm.

7. The method according to claim 4 or 5, characterized in that: When Ni is deposited only on the first region (3a) of the surface, the surface is partially shielded to target Ni deposition.

8. The method according to claim 4 or 5, characterized in that: The local dissolution step b') is carried out after masking the first area (3a).

9. The method according to claim 4 or 5, characterized in that: The local dissolution step b') is carried out in an acid bath.

10. The method according to claim 4 or 5, characterized in that: If the blank in step a) has a shape different from the component (1) to be manufactured, the method comprises a step of shaping the blank, which is carried out between steps a) and b) or between steps b) and c).

11. The method according to claim 4 or 5, characterized in that: The structure of the blank in step a) comprises a mass fraction of austenite less than or equal to 40% and a mass fraction of ferrite greater than or equal to 60%.

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

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