Nitriding hardening process

Abstract

The invention relates to a process for hardening nitridable steel parts, in which a part, or a portion thereof, is subjected to a multi-step nitriding treatment to obtain a predetermined target hardness HCBL at a predetermined target depth ZCBL. The nitridable steel part, or the portion thereof, is subjected to a series, referred to as a decreasing series, of successive steps carried out at decreasing temperatures. Each step in the decreasing series is determined by a defined time-temperature pair for a constant thermal activation parameter PAT between steps. 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 time Dref, which represents the nitriding treatment time at temperature Tref to reach a nitrogen concentration Cref corresponding to the target hardness HCBL at the target depth ZCBL.

Description

Title of the invention: Nitriding hardening process technical field

[0001] The present invention generally relates to the field of hardening steel parts and more particularly to that of nitriding. State of the art

[0002] Nitriding treatments are thermochemical treatments that introduce nitrogen by diffusion onto the surface of parts to achieve surface mechanical strengthening, typically improving hardness and fatigue resistance. They thus ensure the mechanical performance of the parts in service while maintaining core ductility.

[0003] Nitriding of low-alloy steels is a conventional solution for many components, particularly power transmission components in aircraft turbomachinery (gear teeth, splined shafts, bearings, ring gears, etc.) where the operating temperature does not allow the use of case-hardened steels. To ensure the required mechanical strength, these components 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 hardening by nitriding.

[0004] 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.

[0005] In practice, in conventional 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, on the surface, of a layer called the combination layer, composed of iron nitrides. From this layer, nitrogen atoms diffuse towards the core of the part to form the diffusion layer by precipitation of MN-type nitrides, where M is a metal. The kinetics of nitrogen adsorption on the steel surface, and the thickness of the combination and diffusion layers, depend on the nitriding potential, denoted KN, applied during the nitriding treatment.

[0006] The nitriding processes used conventionally consist of a single time / temperature step, possibly two steps with a first step at low temperature followed by a second at higher temperature.

[0007] Nitriding is a lengthy process because the treatment temperatures are low and, since diffusion is a thermally activated process, the diffusion rate of nitrogen atoms is slow. Furthermore, to avoid excessively softening the material during the nitriding treatment, 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 great 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.

[0008] 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. General description of the invention

[0009] With this objective in mind, the present invention proposes a multi-step nitriding hardening process.

[0010] In the present nitriding hardening process, a nitridable steel part, or a part thereof, is subjected to a multi-step nitriding treatment to obtain a predetermined hardness, referred to as target hardness HCbl, at a predetermined depth, referred to as target depth ZCBl;

[0011] in which the nitridable steel part, or a part thereof, is subjected to a series, called 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 constant between stages;

[0012] wherein the thermal activation parameter PATest specific to the nitridable steel forming the part is calculated for a given temperature Tref and a treatment time Dref, which represents the duration of gas nitriding treatment at temperature Tref to achieve a nitrogen concentration CCbl corresponding to the target hardness HCbl at the target depth ZCbl-

[0013] 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 may include 2, Three or four stages, or more depending on the application. In the decreasing series, the stages follow one another sequentially, meaning they are performed one after the other, with stage N+1 being performed at a lower temperature than the previous stage N. The temperatures are therefore strictly decreasing; the temperature of each stage is constant. Remarkably, the time and temperature parameters for each stage in the decreasing series are determined based on the same value of the thermal activation parameter PAT (i.e., constant).

[0014] 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 allowing 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-step nitriding treatment), without this 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 steps in the decreasing series.

[0015] Advantageously, such a nitriding hardening process makes it possible to obtain a nitriding layer with a depth of up to 1.5 mm, preferably at least 400 µm or at least 600 µm, 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.

[0016] Surprisingly, the inventors discovered that the nitriding process carried out by decreasing temperature steps and with a constant thermal activation parameter made it possible to optimize the nitriding treatment and reduce its duration.

[0017] In other words, in order to reduce nitriding cycle times (i.e., the duration of the nitriding treatment), the present process proposes to carry out a nitriding treatment in successive stages at strictly decreasing temperatures, from high temperatures to low temperatures, ensuring that the holding times at the different temperatures do not compromise the properties of the base material (before nitriding). This assurance is obtained by defining the duration of the different stages so as to keep the value of the thermal activation parameter PAT constant between stages.

[0018] 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 undergoing softening.

[0019] The implementation of the decreasing series of nitriding stages according to the invention has the following advantages:

[0020] - the possibility of reaching greater nitriding depths from the first moments of treatment;

[0021] - the possibility of guaranteeing the non-alteration (i.e., the maintenance) of the properties of core mechanical strength of the nitridable steel part during nitriding treatment;

[0022] - obtaining a significant hardening effect of the precipitates on the surface while avoiding excessive coalescence linked to high temperatures (because the temperature is reduced to 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 relating 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 obtain 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 makes it possible to optimize the duration and conditions of the treatment (in particular the temperature, treatment time, and composition of the nitriding atmosphere) so as 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 by 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 standard EN 10027-1:2016. In particular, the steel may be a steel for nitriding according to standard 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 wt% and 0.90 wt%, preferably greater than 0.20 wt% and / or less than 0.80 wt%, enabling the base material to acquire its core mechanical properties after heat treatment. The nitriding alloying elements are selected from a list including: chromium, molybdenum, vanadium, aluminum, silicon, and manganese.

[0030] According to certain embodiments, one or more of 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%.

[0031] Preferably, the part treated (or a part of which is treated) by the present 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 41CrAlMo7-10, 34CrAlNi7-10, 31CrMol2, 15CrMoV5-9, 31CrMoV9, 33CrMoV12-9, and 40CrMoV13-9.

[0032] The process of the invention is applicable to various nitriding techniques. According to 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, which allows for more precise control of the composition of the nitrided layer and therefore of the surface hardening.

[0033] 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 thermal activation parameter typically defines a relationship or equivalence between the heating temperature and the heating time at that temperature to obtain the desired effect (i.e., increased nitrogen content and thus surface hardness) and is specific to the material / steel.

[0034] Preferably, the thermal activation parameter PATest is obtained by calculating the Hollomon-Jaffe parameter for the pair (Dref, Tref). The Hollomon-Jaffe parameter is defined as follows:

[0035] P AT = (T+ 273) *(C + logl0 (t(h))} xO.001

[0036] with t the duration of the nitriding rest in hours and T 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., modification of the mechanical properties or microstructure of a material by applying heat without changing the material's composition). Surprisingly, the inventors discovered that such a parameter could be advantageously used to predict the behavior of a steel part subjected to thermochemical treatment (modification of 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 nitriding treatment time Dref at temperature Tref, the time required to reach a nitrogen concentration CCbl at depth ZCBl is determined, 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 the desired temperature Tref. This relationship is typically obtained experimentally, although it is possible to proceed by simulation.

[0040] Determining the treatment time (Dref) based on the target nitrogen concentration (CCbl) and the target depth (ZCBl) allows for more rigorous control of the nitriding process. This ensures that the nitridable steel part, or the portion thereof, subjected to this nitriding hardening process reaches the target hardness at the target depth. Furthermore, the relationship between hardness and nitrogen concentration at a constant temperature facilitates determination at the start of the process, 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 the treatment time Dref, the nitriding temperature Tref is chosen from a range of 500°C to 550°C. In the present text, the reference nitriding treatment is a single-step nitriding treatment carried out at the reference temperature Tref. The reference temperature is preferably below 550°C in order to avoid a tempering effect that could occur due to the long period of time during which the part is subjected to this temperature (Dref typically being on the order of more than 150-200 h).

[0042] The duration Dref of the conventional single-step nitriding treatment at the reference temperature Tref can be determined by any means known to those skilled in the art. In particular, it can be determined using diffusion and precipitation equations (in particular, but not limited to, by numerical simulation or direct calculation) and / or determined experimentally.

[0043] Generally, tests at different temperatures for different durations on given steel compositions make it possible to determine the nitrogen content (or mass fraction or concentration) at a given depth. 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 Tref. Starting the series of decreasing temperature steps at a temperature higher than the reference temperature advantageously accelerates nitrogen diffusion and thus reduces the time required for treatment.

[0045] According to certain 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 the depth of the nitriding treatment and the concentration (content, mass fraction) of nitrogen at the surface of the part to continue increasing (up to the target depth Zcbl) while preventing / preventing changes in the core properties of the steel. In order to control the process, in particular to control nitrogen diffusion, it is useful to ensure 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 precision of the furnace used to perform the nitriding treatment, in particular the precision 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 advantageously be carried out consecutively without intermediate treatment or handling of the steel part being treated. This is particularly advantageous from an implementation standpoint (no downtime between the different 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 to be treated and the expected properties, in particular depending on the nature of the steel, the target hardness HCbl and the target depth ZCbl- Typically, in order to reduce as much as possible the duration of the nitriding hardening process without negatively impacting the core hardness of the part (or part of the part) to be treated, the series of steps includes at least two (and preferably at least three, four, five or more) successive steps at respective decreasing nitriding temperatures.

[0049] The number of steps thus depends on the use case, starting data (steel, HCBl, Zcbl) but also on the value of the thermal activation parameter PAt (dependent of the reference treatment at Tref) and the temperature of the first step of the decreasing series.

[0050] Thus, in practice, the nitrogen concentration profile is determined (by numerical simulation, calculation and / or experimentation) for each step of the decreasing series; and the number of steps necessary to reach the objective is carried out: the hardness HCm at the depth ZCbl-

[0051] In this context, it should be noted that in some applications, it is desirable to precisely control the hardness HCBl- In this case, it will be possible to interrupt / shorten the last stop of the decreasing series if it has been determined that the nitrogen concentration, respectively the hardness, will be reached at the target depth in a time less than the duration of the stop calculated with PAt- However, often the hardness HCbl is considered as a minimum at ZCBl, and it will be possible to carry the last stop to its end.

[0052] 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 carried out before and / or after the decreasing series of steps. For example, one or more steps can be 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 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 to, for example, reduce cementite network growth at grain boundaries.

[0053] Whether the nitriding potential is constant or not between stages, it is chosen so as to ensure a sufficient quantity of nitrogen in the nitriding environment (nitriding atmosphere) to reach 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.

[0054] In the present text, the nitriding potential is defined by

[0055] _ PNH3 ^N~ n3 / 2

[0056] where p nh3 and p n2 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 on the surface of the steel.

[0057] 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, to enable them to withstand the mechanical stresses to which they are subjected, which are concentrated primarily 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). 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 shown 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 mass fraction of nitrogen as a function of the depth at the end of each step 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 in function of time) of a first embodiment of a hardening process by nitriding 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. Detailed description with examples

[0063] An embodiment of the nitriding hardening process of the invention is described in detail below by way of an example (example 1) for a CrMoV type nitriding steel. The objective is achieved solely through a decreasing series of gas nitriding stages.

[0064] In practice, the process can be implemented by proceeding as follows: a. Define the target hardening level to be achieved, i.e., target hardness HCbl and target depth ZCBl b. determine the nitrogen concentration CCbl present at depth ZCBl for target hardening HCbl from a relationship giving the nitrogen concentration CN as a function of depth for a constant temperature Tref. c. determine the treatment time Dref to reach the target concentration CCbl at the target depth ZCBl by nitriding at temperature Tref; d. calculate the value of the thermal activation parameter PAT for Tref and Dref; e. define the temperature Ti of the first nitriding step (N=l) and determine the duration Di of this step to satisfy the value of PAT; and determine by simulation the nitrogen concentration profile; f. define the temperature T; of the next (i) nitriding step and determine the duration D; of this step to satisfy the same value of PAT; and determine by simulation the nitrogen concentration profile obtained at the end of the step; g. Step (f) is repeated for decreasing temperature stages, the respective duration Di of which is calculated for the same value of the thermal activation parameter PAT. For each stage, the nitrogen concentration profile obtained at the end of the respective stage is determined by simulation. The repetition stops at the stage in which the target nitrogen concentration CCBl is reached at depth ZCBl-

[0065] 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.

[0066] 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 duration Di of this step determined on the basis of PAT, if diffusion calculations show that CCBl is reached after a duration shorter than Di. Example 1

[0067] CrMoV steel test pieces of the same dimensions are treated using a nitriding hardening process according to the present invention (one test piece) and using a conventional single-step nitriding process (one test piece).

[0068] All nitriding steps (of the process according to the invention and of the comparative conventional process) are gaseous nitridings carried out under a nitriding atmosphere, with a nitriding potential KN satisfying KN = 1 atm1 / 2.

[0069] A first particular and non-limiting embodiment of the present invention can be carried out as follows. 1 - Definition of the specifications

[0070] Traditionally, the specification for a nitriding treatment is defined based on surface hardness and affected depth.

[0071] In the case of the present multi-step nitriding hardening process, the specifications are defined differently. The target is a certain hardness value or hardening at a certain depth. In Example 1, this involves obtaining 200 HV (Hcbl) of hardening at a depth of 400 pm (ZCBl)-

[0072] By hardening, we mean the increase in hardness obtained by the treatment, i.e. the difference between the hardness after treatment and the initial (core) hardness.

[0073] In the following, the example is presented with reference to hardening, but this can obviously be done with reference to hardness (by adding the value of the core hardness).

[0074] 2 - Calculation of the duration of a traditional nitriding treatment to comply with the specifications

[0075] 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 [Fig.1].

[0076] By selecting a treatment temperature, e.g. 530°C, such a curve is therefore used to determine the nitrogen content (Ccbl) which allows obtaining 200 HV of hardening.

[0077] The time, denoted Dref, required to obtain this nitrogen concentration Ccbl at the target depth of 400 pm is then determined. This calculation can be performed (and the time determined) either via the diffusion and precipitation equations (in particular by numerical simulation) or by experimentation. In the case of a single step at 530 °C, the calculated time is 66 hours.

[0078] 3 - Calculation of the thermal activation parameter P AT

[0079] After determining the duration Dref of the reference treatment at temperature Tref, the corresponding Hollomon-Jaffe parameter PAt is calculated using the following equation:

[0080] pAT= (jref + 273) x(20 + logl0 (Dref)) xO.001

[0081] With Tref in Celsius and Dref in hours.

[0082] In the case of example 1 with Tref = 530 °C and Dref = 66h, we have PAT = 17.34. This (unique) value of PAT will therefore be used in the decreasing series of steps.

[0083] 4 - Temperature selection for multiple stages

[0084] In Example 1, the nitriding hardening process comprises only steps carried out at strictly decreasing respective nitriding temperatures. The temperature Ti of the first step is therefore the highest temperature.

[0085] This temperature is determined by a person skilled in the art so as to be as high as possible in order to reduce the duration of the nitriding rest period accordingly. This temperature (increasing the nitrogen diffusion rate in the steel) must remain below the tempering temperature of the nitriding steel (or nitriding steel) forming the workpiece, in order to ensure a 100% ferritic matrix. Similarly, the temperature difference between stages cannot experimentally be less than what is reasonably expected given the furnace's precision, particularly the precision of its temperature control system.

[0086] In the present example, it was chosen to carry out the first nitriding step at a temperature of 570 °C (Tl) (greater than Tref) and that the temperatures of two successive steps have a difference of at least 10 °C.

[0087] The temperatures selected for the plateaus are 570 °C (T1), 560 °C (T2), 540 °C (T3) and 530 °C (T4). 5 - Calculation of the multi-stage thermal cycle

[0088] 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.

[0089] In the present example, the calculated durations are respectively the following: 4h (Di - plateau at T1), 6h (D2 - plateau at T2), 21h (D3 - plateau at T3) and 39h (D4 - plateau at T4).

[0090] 6 - Simulation of nitrogen mass fraction profiles (%N) at each step

[0091] In this step, simulations of nitrogen concentration profiles at the end of Each stop is performed. The goal is to identify the point at which the nitrogen concentration reaches the target value Ccbl 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 through numerical simulation (particularly using diffusion and precipitation equations), or using nomograms or diffusion-precipitation calculations.

[0092] 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 last step can be reduced to 15 hours while still achieving the target depth. Figure 3 shows the complete multi-step thermal cycle of Example 1 according to the invention.

[0093] 7 - Comparison of the results obtained using the method according to the invention and of conventional methods

[0094] The thermal cycle of [Fig.3] is applied to a test specimen in said CrMoV type nitridable steel under nitriding atmosphere (KN = 1 atm 1 / 2).

[0095] 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 highest low temperature of the process according to the invention), for 66 hours, that is to say a duration 43% greater than the total duration of the treatment decreasing in 4 stages (which is 46 hours - after reduction of the last stage).

[0096] 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%.

[0097] 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 nitriding 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).

[0098] 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

Demands

1. A process for hardening nitridable steel parts, wherein a nitridable steel part, or a part thereof, is subjected to a multi-step nitriding treatment to obtain a predetermined hardness, referred to as target hardness HCBl, at a predetermined depth, referred to as target depth ZCBl; wherein the nitridable steel part, or a part thereof, is subjected to a series, referred to as a decreasing series, of successive steps carried out at decreasing temperatures, each step of the decreasing series being determined by a time-temperature pair defined for a constant thermal activation parameter PAt between steps;in which 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 time Dref, which represents the nitriding treatment time at temperature Tref to achieve a nitrogen concentration CCBl corresponding to the target hardness HCBL at the target depth ZCBl.;

2. A method according to any one of the preceding claims, wherein in order to determine the nitriding treatment time Dref at temperature Tref, the time required to reach a nitrogen concentration CCBl at depth ZCBL is determined, CCBl being determined as a function of a relationship established at constant temperature linking hardness to nitrogen concentration in steel.

3. 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.

4. 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 Tref.

5. A method according to any one of the preceding claims, wherein for the calculation of treatment time Dref, the nitriding temperature Tref 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.

6. 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 in 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.

7. A method according to any one of the preceding claims, wherein the thermal activation parameter PATest is obtained by calculating the Hollomon-Jaffe parameter for the couple (Dref, Tref).

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

9. A method according to claim 7, 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.

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 41CrAlMo7-10, 34CrAlNi7-10, 31CrMol2, 15CrMoV5-9, 31CrMoV9, 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 present in the following proportions: Chromium 1 to 4%m; Nickel 0.5 to 4%m; Molybdenum 0.2 to 4%m; Vanadium 0.10 to 2%m; Aluminium 0.5 to 3%m; Silicon 0.4 to 1.5%m; and / or Manganese 1 to 2%m.

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