Method for hardening by nitriding

Abstract

The invention relates to a method for hardening nitridable steel parts, in which a part, or a portion thereof, is subjected to a multi-stage nitriding treatment in order to obtain a predetermined target hardness HCBL at a predetermined target depth ZCBL. The nitridable steel part, or the portion thereof, respectively, is subjected to a series, referred to as a decreasing series, of successive stages carried out at decreasing temperatures, each stage of the decreasing series being determined by a time-temperature pair defined for a thermal activation parameter PAT that is constant between stages. The thermal activation parameter PAT is specific to the nitridable steel forming the part and is calculated for a given temperature Tref and a treatment duration Dref, the treatment duration Dref representing the duration of the nitriding treatment at the temperature Tref for reaching a nitrogen concentration Cref corresponding to the target hardness HCBL at the target depth ZCBL.

Description

[0001] P-CENM2P-012 / WO 1

[0002] Nitriding hardening process

[0003] technical field

[0004] The present invention generally relates to the field of hardening steel parts and more particularly to nitriding.

[0005] State of the art

[0006] Nitriding treatments are thermochemical processes that introduce nitrogen onto the surface of parts via diffusion to achieve surface mechanical strengthening, typically improving hardness and fatigue resistance. This ensures the mechanical durability of parts during service while maintaining core ductility.

[0007] Nitriding of low-alloy steels is a common solution for many components, particularly power transmission parts in aircraft turbomachinery (gear teeth, splined shafts, bearings, ring gears, etc.) where the operating temperature precludes the use of case-hardened steels. To ensure the required mechanical strength, these parts must exhibit very high hardness to depths of up to two or three times the depth of the stressed sub-layer. This quality and depth of hardening can be achieved with steels containing alloying elements that allow for nitriding hardening.

[0008] Nitriding treatments are conventionally carried out on alloy steels that have been previously treated by austenitizing, quenching, and tempering at temperatures above 600°C. This allows the steels to then be nitrided at a lower temperature (conventionally at least 30°C lower) without affecting the underlying structural condition.

[0009] In practice, in typical gaseous nitriding treatments, steel is nitrided at a temperature of approximately 480°C to 580°C in an atmosphere containing ammonia, which releases nitrogen onto the steel surface. The adsorbed nitrogen causes the formation of a surface layer called the combination layer, composed of iron nitrides. From this layer, nitrogen atoms diffuse towards the core of the part to form the P-CENM2P-012 / WO 2 precipitation diffusion layer of MN-type nitrides, where M is a metal. The kinetics of nitrogen adsorption onto the steel surface, and the thickness of the combination and diffusion layers, depend on the nitriding potential, denoted KN, applied during the nitriding treatment.

[0010] The nitriding processes used classically consist of a single time / temperature step, possibly two steps with a first step at low temperature followed by a second at higher temperature.

[0011] Nitriding is a lengthy process because the treatment temperatures are low and, since diffusion is a thermally activated process, the rate of nitrogen atom diffusion is slow. Furthermore, to avoid excessively softening the material during nitriding, it is necessary to maintain relatively low temperatures, especially for deep nitriding, so as not to compromise the function of the treated part. Consequently, nitriding treatments to achieve significant depths (penetration depth greater than 1 mm) are very long and can last several hundred hours (typically, more than 500 hours). These lengthy treatments are difficult to reconcile with industrial production rates and are expensive.

[0012] It would be desirable to have a nitriding hardening process that reduces the duration of the nitriding treatment without significantly altering the surface hardness of the steels thus treated, i.e., allowing the required resistance properties to be maintained.

[0013] General description of the invention

[0014] With this objective in mind, the present invention proposes a multi-step nitriding hardening process according to claim 1.

[0015] In the present nitriding hardening process, a nitridable steel part, or a portion thereof, is subjected to a multi-step nitriding treatment to achieve a predetermined hardness, referred to as the target hardness HCBL, at a predetermined depth, referred to as the target depth ZCBL; P-CENM2P-012 / WO 3 wherein the nitridable steel part, or a portion thereof, is subjected to a series, referred to as the decreasing series, of successive steps carried out at decreasing temperatures, each step in the decreasing series being determined by a time-temperature pair that satisfies (defined for) a constant thermal activation parameter PAT between steps; wherein the thermal activation parameter PAT is specific to the nitridable steel forming the part and is determined for a given temperature T re f and a treatment duration D re f, which represents the duration of the gas nitriding treatment at temperature T ref to achieve a CCBL nitrogen concentration corresponding to the target hardness HCBL at the target depth ZCBL.

[0016] The treatment according to the invention is based on the implementation of a series of steps carried out at decreasing temperatures. This series is called the "decreasing series." The decreasing series comprises at least two steps, and in practice can include two, three, or four steps, or even more depending on the application. In the decreasing series, the steps are performed sequentially, that is, one after the other, with step N+1 being carried out at a temperature lower than the preceding step N. The temperatures are therefore strictly decreasing; the temperature of each step is constant. Remarkably, the time and temperature parameters for carrying out each step of the decreasing series are determined based on the same value of the thermal activation parameter PAT (i.e., constant).

[0017] One of the merits of the invention is that it has identified that the decreasing series treatment allows for a high-temperature nitriding step (the first in the decreasing series), which accelerates the nitriding process while enabling deeper nitrogen diffusion into the material. Subsequently, the progressive reduction of temperature with each decreasing step allows diffusion to continue while preserving the core hardness of the material.In other words, the present hardening process is based on the inventors' discovery that it is possible to effectively harden the surface by nitriding by working at high temperatures (typically higher than the conventional temperatures of a single-stage P-CENM2P-012 / WO 4 nitriding treatment), without negatively affecting the mechanical properties of the core of the material, in particular the hardness, due to the progressive decrease in the nitriding temperature of successive stages in the decreasing series.

[0018] Advantageously, such a nitriding hardening process makes it possible to obtain a nitrided layer up to 1.5 mm deep, preferably at least 400 µm or at least 600 µm, and more preferably at least 700 µm or at least 800 µm, with a treatment time reduced by at least 30% compared to a conventional single-step process. The number and time-temperature parameters of the steps are therefore adapted to achieve these depths and the desired hardness ranges.

[0019] Surprisingly, the inventors discovered that the nitriding process carried out in decreasing temperature steps and with a constant thermal activation parameter made it possible to optimize the nitriding treatment and reduce its duration.

[0020] In other words, to reduce nitriding cycle times (i.e., nitriding treatment duration), the present process proposes a step-by-step nitriding treatment at strictly decreasing temperatures, from high to low, ensuring that the holding times at different temperatures do not compromise the properties of the base material (before nitriding). This assurance is achieved by defining the duration of each step so as to maintain a constant value for the thermal activation parameter (PAT) between steps. In other words, each step in the decreasing series is determined by a time-temperature pair defined for a constant PAT between steps.

[0021] Thus, for the steps in the decreasing series, this can translate into increasing holding times (i.e., an increase in the duration of the holding period) as the temperature decreases. Optimizing the holding times at the different temperature steps prevents the non-nitrided zone from softening. P-CENM2P-012 / WO 5

[0022] The implementation of the decreasing series of nitriding stages according to the invention has the following advantages: the possibility of achieving greater nitriding depths from the very first moments of the treatment; the possibility of guaranteeing the non-alteration (i.e. the maintenance) of the core mechanical strength properties of the nitridable steel part during the nitriding treatment; the obtaining of a significant hardening effect of the precipitates on the surface while avoiding excessive coalescence linked to high temperatures (because the temperature is reduced at the next stage).

[0023] Initial tests have shown that on the hardened tempered state of a CrMoV type steel part, nitriding times for depths greater than 0.4 mm can be reduced by 30% compared to conventional nitriding.

[0024] Another merit of the invention is that it has succeeded in linking the concentration (also called content, corresponding to the mass fraction - %m) of nitrogen at a given depth to the corresponding hardening of the nitridable steel forming the part or part thereof to be hardened.

[0025] Indeed, by establishing a relationship between nitrogen concentration and hardness, it is possible to precisely adjust the degree of nitriding to achieve the desired hardness at the surface or at a given depth, thus ensuring a better match with the specific requirements of the end application. Preferably, such a correlation also allows for the optimization of the treatment duration and conditions (including temperature, treatment time, and the composition of the nitriding atmosphere) to achieve the desired mechanical properties without unnecessarily prolonging the process. This avoids over-nitriding, which could compromise the mechanical properties of the material core, while maximizing process efficiency.

[0026] In addition, this approach facilitates the modulation of the process: by knowing the relationship between a given hardness and the corresponding nitrogen concentration, it becomes possible to define the experimental conditions of the nitriding process, in particular taking into account the diffusion equations, to reach the target nitrogen concentration at the desired depth.

[0027] The present process was developed for the treatment of so-called nitridable steels. As will be understood by those skilled in the art, a nitridable steel is generally a steel designed to be treated by nitriding. Nitridable steels (also nitriding steels) are formulated with alloying elements that promote the formation of nitrides (compounds of nitrogen and metal) during the nitriding process.

[0028] According to certain embodiments, the part (or part of a part) to be treated may be made of a nitridable steel such as structural steel, or in particular such as a low alloy steel, a high alloy steel or a high speed steel as defined in EN 10027-1:2016. In particular, the steel may be a steel for nitriding according to EN 10085:2001-07.

[0029] Advantageously, nitridable steel (or nitriding steel) comprises nitriding alloying elements that allow hardening by nitriding (precipitation of submicroscopic nitrides from these nitriding elements, present in solid solution in the treated steel, etc.) and a carbon content generally between 0.15% and 0.90%, preferably greater than 0.20% and / or less than 0.80%, enabling the base material to acquire its core mechanical properties after heat treatment. The nitriding alloying elements are chosen from a list including: chromium, molybdenum, vanadium, aluminum, silicon, and manganese.

[0030] According to certain embodiments, one or more of the said alloying elements are present in the following proportions: Chromium 1 to 4 wt%; Nickel 0.5 to 4 wt%; Molybdenum 0.2 to 4 wt%; Vanadium 0.10 to 2 wt%; Aluminum 0.5 to 3 wt%; Silicon 0.4 to 1.5 wt%; and / or Manganese 1 to 2 wt%. Preferably, each of the chemical elements other than iron is present in a mass content less than or equal to 5.00 wt%. P-CENM2P-012 / WO 7

[0031] Preferably, the part treated (or a part of which is treated) by this nitriding hardening process is made of a low-alloy nitriding steel with nitriding elements such as Cr, V, Mo, and Al. Such steels are, for example, CrMoV steel grades such as 41 CrAIMo7-10, 34CrAINi7-10, 31 CrMo12, 15CrMoV5-9, 31 CrMoV9, 33CrMoV12-9,

[0032] 40CrMoV13-9

[0033] The process of the invention is applicable to various nitriding techniques. In a preferred embodiment, the multi-step nitriding treatment is either a gas nitriding treatment (diffusion of nitrogen into a metal under a nitriding atmosphere, i.e., comprising ammonia NH3 and dissociated ammonia N2 and H2) or an ionic nitriding treatment (diffusion of nitrogen into a metal using an ionized plasma). The temperature of the steps is easier to control than in a molten salt nitriding process, allowing for more precise control of the composition of the nitrided layer and therefore of the surface hardening.

[0034] In general, the thermal activation parameter (TAP) can be any parameter that describes the combined effects of heating temperature and heating time on the mechanical properties of metallic materials. The TAP typically defines a relationship or equivalence between the heating temperature and the heating time at that temperature required to achieve the desired effect (i.e., increased nitrogen content and thus surface hardness) and is specific to the material / steel.

[0035] The thermal activation parameter PAT advantageously satisfies the Hollomon-Jaffe or Larson-Miller parameter for the torque (D re f, T re f) In practice, the thermal activation parameter PAT is obtained by calculating the Hollomon-Jaffe (or Larson-Miller) parameter for the couple (D re f, T re f) The Hollomon-Jaffe parameter is defined as follows:

[0036] P AT= (T + 273) x (C + log(t)) x 0.001 where t is the duration of the nitriding rest in hours and T is the temperature of the nitriding rest in degrees Celsius. C is a constant specific to the treated steel, and can generally be approximated to 20 for the steels considered in the context of this nitriding hardening process.

[0037] The Hollomon-Jaffe parameter is a well-known parameter in the field of heat treatments (i.e., modifying the mechanical properties or microstructure of a material by applying heat without changing its composition). Surprisingly, inventors discovered that such a parameter could be advantageously used to predict the behavior of a steel part subjected to thermochemical treatment (modifying the properties or structure of a material by applying heat combined with chemical reactions such as nitriding, where elements penetrate the heated material to change its composition). Indeed, initial tests validated that the depth of nitrogen diffusion during a nitriding plateau could be related to a given activation parameter.Without being bound by any particular theory, the authors assume that the use of a thermal activation parameter is applicable to a thermochemical process due to long-distance diffusion (over 100 pm) from the surface of nitrogen atoms in the material. Besides the Hollomon-Jaffe parameter, other thermal activation parameters could be used, particularly those based on time-temperature equivalences, such as the Larson-Miller parameter.

[0038] According to some embodiments, to determine the treatment time D re f of nitriding at temperature T re f, we determine the time required to reach a nitrogen concentration CCBL at the depth ZCBL, CCBL being determined according to a relationship established at constant temperature linking hardness to nitrogen concentration in steel.

[0039] The hardness-concentration relationship can therefore be advantageously obtained at constant temperature, for the grade of steel to be treated and at temperature T. re desired f. This relationship is typically obtained experimentally, although it is possible to proceed by simulation.

[0040] Determining the duration of treatment (D re f) Based on the target nitrogen concentration (CCBL) and target depth (ZCBL), P-CENM2P-012 / WO 9 allows for more rigorous control of the nitriding process. This ensures that the nitridable steel part, or portion thereof, subjected to this nitriding hardening process achieves the target hardness at the target depth. Furthermore, the relationship between hardness and nitrogen concentration at a constant temperature facilitates process start-up determination, as the nitrogen concentration required to achieve the target hardness is known, regardless of the depth.

[0041] According to the same or other embodiments, for the calculation of processing time D re f, the nitriding temperature T re f is chosen from a range of 500°C to 550°C. In this text, the reference nitriding treatment is a single-step nitriding treatment carried out at the reference temperature Tret. The reference temperature is preferably below 550°C to avoid a tempering effect that could occur due to the long time period during which the part is subjected to this temperature (D re f typically in the order of more than 150-200 h).

[0042] The duration D re f of conventional single-step nitriding treatment at reference temperature T ref can be determined by any means known to a person skilled in the art. In particular, it can be determined using diffusion and precipitation equations (in particular but not limited to numerical simulation or directly calculated) and / or determined experimentally.

[0043] In general, tests at different temperatures for different durations on given steel compositions allow the nitrogen content (or mass fraction or concentration) at a given depth to be determined. Thus, according to certain embodiments, the relationship established at constant temperature is determined experimentally by applying a conventional single-step nitriding treatment (preferably gaseous) at constant temperature.

[0044] Preferably, the first step in the decreasing series is carried out at a temperature above T ref. Starting the series of decreasing temperature steps at a temperature higher than the reference temperature advantageously accelerates nitrogen diffusion and thus reduces the treatment time. P-CENM2P-012 / WO 10

[0045] In some embodiments, the temperature of the first step in the decreasing series of steps in the nitriding treatment is within a range from 550 °C to the tempering temperature of the nitridable steel forming the part to be treated, preferably between 550 °C and 630 °C, more preferably between 560 °C and 610 °C, and particularly preferably between 570 and 590 °C. Remaining below the tempering temperature advantageously avoids softening the material (i.e., avoiding a reduction in its core hardness) and preserves its microstructural properties.

[0046] According to the same or other embodiments, a temperature difference of at least 5 °C between the nitriding temperatures of two successive stages in the decreasing series of stages is preferred to be at least 10 °C, 15 °C, or 20 °C. Lowering the temperature of successive stages in the nitriding treatment allows for a continued increase in the depth of the nitriding treatment and the nitrogen concentration (content, mass fraction) at the surface of the part (up to the target ZCBL depth) while preventing alterations to the core properties of the steel. To control the process, particularly nitrogen diffusion, it is beneficial to maintain a temperature difference of at least 5 °C between stages.Furthermore, the temperature difference between stages cannot experimentally be less than what is reasonably expected given the accuracy of the furnace used to perform the nitriding treatment, in particular the accuracy of its temperature control system.

[0047] The various stages of the nitriding treatment are advantageously carried out in the same furnace. Thus, the different stages can be efficiently linked together without intermediate treatment or handling of the steel part being treated. This is particularly advantageous from an implementation standpoint (no downtime between stages, no energy loss from having to cool and then reheat the part at each stage, etc.).

[0048] The number of steps in the decreasing series of steps (i.e., the number of successive nitriding steps carried out at decreasing temperatures) is not limited and depends on the part being treated and the expected properties, in particular according to the type of steel, the target HCBL hardness and the target depth P-CENM2P-012 / WO 11

[0049] ZCBL. Typically, in order to reduce the duration of the nitriding hardening process as much as possible without negatively impacting the core hardness of the part (or part of part) being treated, the series of steps includes at least two (and preferably at least three, four, five or more) successive steps at decreasing respective nitriding temperatures.

[0050] The number of steps therefore depends on the use case, starting data (steel, HCBL, ZCBL) but also on the value of the thermal activation parameter PAT (dependent on the reference treatment at T ref) and the temperature of the first plateau of the decreasing series.

[0051] Thus, in practice, we determine (by numerical simulation, calculation and / or experimentation) for each step of the decreasing series the nitrogen concentration profile; and we carry out the number of steps necessary to reach the objective: the HCBL hardness at the ZCBL depth.

[0052] In this context, it should be noted that in some applications, precise control of the HCBL hardness is desired. In this case, the last stop in the decreasing series can be interrupted / shortened if it has been determined that the nitrogen concentration, and therefore the hardness, will be reached at the target depth in less time than the stop duration calculated using PAT. However, the HCBL hardness is often considered a minimum at ZCBL, and the last stop can be completed.

[0053] The process according to the invention was developed as a self-contained nitriding process, meaning that the hardening process consists solely of the decreasing series of steps. However, it can be combined with heat treatments and / or other nitriding steps / steps implemented before and / or after the decreasing series of steps. For example, one or more steps can be performed at temperatures lower than the first step in the decreasing series, and / or one or more steps at temperatures higher than the last step in the decreasing series.In a gas nitriding treatment, the different nitriding stages of the multi-step nitriding process can be carried out at the same nitriding potential (a single nitriding potential for all stages) or at different nitriding potentials (P-CENM2P-012 / WO 12), depending, for example, on the desired properties of the nitrided layer and / or the equipment performing the nitriding treatment. In some embodiments, all stages are carried out at the same nitriding potential to simplify the implementation of the multi-step treatment, with only the temperature varying between successive stages. Alternatively, two successive stages can be carried out at different nitriding potentials, for example, to reduce cementite network growth at grain boundaries.

[0054] Whether the nitriding potential is constant between stages or not, it is chosen to ensure a sufficient quantity of nitrogen in the nitriding environment (nitriding atmosphere) to achieve the target nitrogen concentration at the target depth, to obtain the target hardness, and at a minimum, the nitriding potential (kN) must be sufficient to allow the precipitation of the white layer. Preferably, the nitriding potential is between 1 and 6, more preferably between 1 and 3.

[0055] In this text, the nitriding potential is defined by where PNH3 and pH2 represent the partial pressures of ammonia and dihydrogen applied during the nitriding treatment, respectively. The higher the nitriding potential KN, the greater the amount of nitrogen adsorbed onto the steel surface.

[0056] The invention is particularly applicable to the treatment of parts in the aeronautical and automotive industries, such as gears and pinions, splines, raceways, bushings, and rolling elements, among others, enabling them to withstand the mechanical stresses to which they are subjected, which are primarily concentrated on the surface (flexural fatigue, contact fatigue, fretting, wear, etc.). The process according to the invention is particularly well-suited for improving the rolling fatigue resistance of mechanical parts (gears, bearings). P-CENM2P-012 / WO 13

[0057] Brief description of the figures

[0058] Other features and characteristics of the invention will become apparent from the detailed description of at least one advantageous embodiment presented below by way of illustration, with reference to the accompanying drawings. These show:

[0059] [Fig. 1] a graph representing the evolution of hardening (HV measured with a weight of 0.2 kg) as a function of the mass fraction of nitrogen for a temperature;

[0060] [Fig. 2] A graph representing the nitrogen mass fraction as a function of depth at the end of each stage and for a part treated using a comparative conventional process, calculated by numerical simulation

[0061] [Fig. 3] a graph representing the multi-step thermal cycle (temperature as a function of time) of a first embodiment of a nitriding hardening process according to the present invention;

[0062] [Fig. 4] a graph representing the hardening as a function of depth relative to the target depth (Z / ZCBL) for a steel part treated using the process according to the invention or treated according to a comparative conventional process.

[0063] Detailed description with examples

[0064] An embodiment of the nitriding hardening process of the invention is described in detail below through an example (Example 1) for a CrMoV type nitriding steel. The objective is achieved solely through a decreasing series of gas nitriding stages.

[0065] In practice, the process can be implemented as follows: a) define the target hardening level to be achieved, i.e., the target hardness HCBL and the target depth ZCBL; b) determine the nitrogen concentration CCBL present at the depth ZCBL for the target hardening HCBL from a relationship giving the nitrogen concentration CN as a function of depth for a constant temperature T re f. P-CENM2P-012 / WO 14 c) determine the duration of treatment D re f to achieve the target CCBL concentration at the target depth ZCBL by nitriding at temperature T ref; d) calculate the value of the thermal activation parameter PAT for T re f and D re f) e) Define the temperature Ti of the first nitriding stage (N=1) and determine the duration Di of this stage to satisfy the PAT value; and determine the nitrogen concentration profile by simulation; f) Define the temperature T of the next nitriding stage (i) and determine the duration Di of this stage to satisfy the same PAT value; and determine the nitrogen concentration profile obtained at the end of the stage by simulation; g) Repeat step (f) for decreasing temperature stages, the respective duration Di of which is calculated for the same value of the thermal activation parameter PAT. Determine the nitrogen concentration profile obtained at the end of each stage by simulation. The repetition stops at the stage in which the target nitrogen concentration CCBL is reached at depth ZCBL.

[0066] Next, nitriding hardening is carried out in a furnace under a controlled nitriding atmosphere. The steel part is subjected to a series of decreasing stages with the time and temperature parameters thus defined.

[0067] The target property (HCBL to ZCBL) will be reached during the last step. Depending on the specific case, the last step may be achieved in a shorter time than the step duration Di determined based on PAT, if diffusion calculations show that CCBL is reached after a duration shorter than Di.

[0068] Example 1

[0069] CrMoV steel test specimens of the same dimensions are treated using a nitriding hardening process according to the present invention (one specimen) and using a conventional single-step nitriding process (one specimen).

[0070] All nitriding steps (of the process according to the invention and of the comparative conventional process) are gas nitridings carried out under a nitriding atmosphere, with a nitriding potential KN satisfying KN = 1 atm 1 / 2 .

[0071] A first particular and non-limiting embodiment of the present invention can be carried out as follows.

[0072] 1 - Definition of the specifications

[0073] Traditionally, the specifications for a nitriding treatment are defined based on surface hardness and the depth affected.

[0074] In the case of this multi-step nitriding hardening process, the specifications are defined differently. The target is a certain hardness value or hardening depth. In Example 1, this means achieving 200 HV (HCBL) of hardening at a depth of 400 pm (ZCBL).

[0075] Hardening refers to the increase in hardness obtained through treatment, i.e., the difference between the hardness after treatment and the initial (core) hardness.

[0076] In the following, the example is presented with reference to hardening, but this can obviously be done with reference to hardness (by adding the core hardness value).

[0077] 2 - Calculation of the duration of a traditional nitriding treatment to comply with the specifications

[0078] Experimental results have shown that when applying a conventional single-step nitriding hardening process at a given temperature, the nitrogen content is independent of the duration of the treatment, see for example Figure 1.

[0079] By selecting a processing temperature, e.g. 530°C, such a curve is therefore used to determine the nitrogen content (CCBL) which allows obtaining 200 HV of hardening.

[0080] We then determine the duration, denoted D re f, necessary to obtain this CCBL nitrogen concentration at the target depth of 400 pm. This calculation can be performed (and the duration determined) either via diffusion and precipitation equations P-CENM2P-012 / WO 16

[0081] (in particular by numerical simulation), or by experimentation. In the case of a single step at 530 °C, the calculated duration is 66 hours.

[0082] 3 - Calculation of the thermal activation parameter PAT

[0083] After determining the duration D re f of the reference treatment at temperature Tref, we calculate the corresponding Hollomon-Jaffe parameter PAT, using the following equation: 0.001

[0084] With T re f in Celsius and D re f in hours.

[0085] In the case of example 1 with T re f = 530 °C and D ref = 66h, we have PAT = 17.34. This (unique) value of PAT will therefore be used in the decreasing series of steps.

[0086] 4 - Temperature selection for multiple stages

[0087] In Example 1, the nitriding hardening process consists solely of stages carried out at strictly decreasing nitriding temperatures. Therefore, the temperature Ti of the first stage is the highest temperature.

[0088] This temperature is determined by a person skilled in the art to be as high as possible in order to minimize the duration of the nitriding rest at that temperature (increasing the rate of nitrogen diffusion in the steel) while remaining below the tempering temperature of the nitriding steel (or nitriding steel) forming the part being treated, thus ensuring a 100% ferritic matrix. Similarly, the temperature difference between rests cannot experimentally be less than what is reasonably expected given the precision of the furnace, particularly the precision of its temperature control system.

[0089] In this example, the first nitriding step was chosen to be carried out at a temperature of 570 °C (T1) (greater than T re f) and that the temperatures of two successive plateaus differ by at least 10 °C. P-CENM2P-012 / WO 17

[0090] The temperatures used for the plateaus are 570 °C (T 1 ), 560 °C (T2), 540 °C (T3) and 530 °C (T4).

[0091] 5 - Calculation of multi-i

[0092] Once the temperatures of each of the stages have been determined, their respective durations are calculated using equation (1) in order to satisfy the value of the thermal activation parameter PAT calculated in step 3.

[0093] In this example, the calculated durations are respectively 4h (Di - level at T1), 6h (D2 - level at T2), 21h (D3 - level at T3) and 39h (D4 - level at T4).

[0094] 6 - Simulation of fraction profiles in nitrogen

[0095] In this step, simulations of nitrogen concentration profiles are performed at the end of each stop. The aim is to identify when the nitrogen concentration reaches the target CCBL value at the target depth. This allows for the determination of the final stop, and optionally, its shortening once the target is reached. These calculations can be performed either by numerical simulation (particularly using diffusion and precipitation equations), or by nomograms or diffusion-precipitation calculations.

[0096] Calculating the simulated nitrogen mass fraction profiles at the end of each step for the multi-step nitriding hardening process of Example 1 shows that the final step can be reduced to 15 hours while still achieving the target depth. Figure 3 illustrates the complete multi-step thermal cycle of Example 1 according to the invention.

[0097] 7 - Comparison of the results obtained using the method according to the invention and conventional methods

[0098] The thermal cycle in Figure 3 is applied to a specimen made of the said CrMoV type nitridable steel under a nitriding atmosphere (kN = 1 atrrr 1 / 2 ).

[0099] Another test specimen is respectively subjected to a single-step nitriding treatment at the temperature of the last step of the process according to the invention (T4, the lowest temperature of the process according to the invention), for 66 hours, P-CENM2P-012 / WO 18, i.e. a duration 43% greater than the total duration of the treatment decreasing to 4 steps (which is 46 hours - after reduction of the last step).

[0100] Figure 2 shows the simulated nitrogen mass fraction profiles as a function of depth for the treatment according to the invention and for the comparative conventional nitriding treatment. It can be seen that, using the process according to the invention, it is possible to obtain the same nitrogen content profile as with a conventional process while reducing the treatment time by 30%.

[0101] Figure 4 compares the experimental hardness profiles obtained with the multi-step nitriding hardening process according to the present invention with those obtained with a conventional process (reference nitriding, i.e., single-step at TREF - here T4). This figure shows that by using the process according to the invention, it is possible to obtain greater treated depths while reducing the treatment time by 30% compared to the comparative conventional process (reference process).

[0102] Increasing the target depth at which the target hardening must be achieved, and therefore the depth at which the target nitrogen concentration must be achieved, further increases the time savings (i.e., reduces the total duration of the nitriding hardening process according to the invention compared to what is obtained for a conventional single-step process). For example, on the quenched and tempered condition as conventionally used, nitriding times for depths greater than 0.4 mm can be reduced by at least 30% compared to conventional nitriding.

Claims

P-CENM2P-012 / WO 19 Demands 1. A process for hardening nitridable steel parts, in which a nitridable steel part, or a part thereof, is subjected to a multi-step nitriding treatment to obtain a predetermined hardness, referred to as the target hardness HCBL, at a predetermined depth, referred to as the target depth ZCBL; in which the nitridable steel part, or a part thereof, is subjected to a series, referred to as the decreasing series, of successive steps carried out at decreasing temperatures, characterized in that each step of the decreasing series is determined by a time-temperature pair which satisfies a constant thermal activation parameter PAT between steps, the temperature of a step being constant over the step;in which the thermal activation parameter PAT is a time-temperature equivalence parameter specific to the nitridable steel forming the part and satisfies the Hollomon-Jaffe parameter or the Larson-Miller parameter for a given temperature T; re f and a treatment duration D re f, which represents the duration of the nitriding treatment at temperature T re f to achieve a CCBL nitrogen concentration corresponding to the target hardness HCBL at the target depth ZCBL.

2. A method according to any one of the preceding claims, wherein the thermal activation parameter PAT satisfies the Hollomon-Jaffe parameter for the torque (D re f, T re f).

3. A method according to any one of the preceding claims, wherein to determine the treatment time D ref of nitriding at temperature Tref, we determine the time required to reach a nitrogen concentration CCBL at depth ZCBL, CCBL being determined according to an experimental or simulated relationship established at constant temperature linking hardness to nitrogen concentration in steel.

4. A method according to the preceding claim, wherein the relationship established at constant temperature is determined experimentally by nitriding at constant temperature, preferably by gas nitriding. P-CENM2P-012 / WO 20 5. A method according to any one of the preceding claims, wherein the first step of the decreasing series is carried out at a temperature above T re t 6. A method according to any one of the preceding claims, wherein for the calculation of processing time D re f, the nitriding temperature T ref is chosen in a range from 500°C to 550°C, and the corresponding nitriding time is determined by simulation, calculated using diffusion equation(s) and / or determined experimentally.

7. A process according to any one of the preceding claims, wherein the temperature of the first step in the decreasing series of the nitriding treatment is within a range from 550 °C to the tempering temperature of the nitridable steel forming the part to be treated, preferably between 550 °C and 630 °C, more preferably between 560 °C and 610 °C, particularly preferably between 570 and 590 °C.

8. A method according to any one of the preceding claims, wherein the hardness, respectively nitrogen concentration, is determined for each step in the decreasing series at the target depth ZCBL, and the last step is that at which the target hardness HCBL at ZCBL is reached.

9. A method according to claim 8, wherein the duration of the last plateau is limited to the time calculated to reach the HCBL hardness at the ZCBL depth.

10. A process according to any one of the preceding claims, wherein the different stages of the nitriding treatment take place in the same furnace, preferably in which the stages are carried out one after the other without intermediate treatment or handling of the steel part subjected to said hardening process.

11. A method according to any one of the preceding claims, wherein a temperature difference between the nitriding temperatures of two successive stages in the decreasing series of stages is at least 5 °C, preferably at least 10 °C, 15 °C or 20 °C. P-CENM2P-012 / WO 21 12. A process according to any one of the preceding claims, wherein the nitriding treatment is a gas nitriding treatment or an ion nitriding treatment.

13. A method according to any one of the preceding claims, wherein the part, or part of a part, is made of low-alloy steel, high-alloy steel or high-speed steel as defined in EN 10027-1:2016; or wherein the part, or part of a part, is nitriding steel as defined in EN 10085:2001-07.

14. A method according to any one of the preceding claims, wherein the part is made of CrMoV type steel, in particular 41 CrAIMo7-10, 34CrAINi7- 10, 31 CrMo12, 15CrMoV5-9, 31 CrMoV9, 33CrMoV12-9, 40CrMoV13-9.

15. A method according to any one of the preceding claims, wherein the nitridable steel part comprises one or more alloying elements in the following proportions: Chromium 1 to 4 wt%; Nickel 0.5 to 4 wt%; Molybdenum 0.2 to 4 wt%; Vanadium 0.10 to 2 wt%; Aluminum 0.5 to 3% w; Silicon 0.4 to 1.5% w; and / or Manganese 1 to 2% w.

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