WO2019243197A1

WO2019243197A1 - Method for hardening by nitriding

Info

Publication number: WO2019243197A1
Application number: PCT/EP2019/065732
Authority: WO
Inventors: Denis FLECHE; Simon Thibault; Grégory MICHEL
Original assignee: Institut De Recherche Technologique Materiaux, Metallurgie, Procedes
Priority date: 2018-06-18
Filing date: 2019-06-14
Publication date: 2019-12-26
Also published as: FR3082529B1; FR3082529A1

Abstract

A method for hardening parts made from nitridable steel is disclosed, in which a part made from nitridable steel, or a portion thereof, is subjected to initial hardening (I) by austenitisation and quenching; next, to low-temperature intermediate tempering (II), carried out at a temperature (TII) of less than 350°C; and to a nitriding treatment (III) carried out at a nitriding temperature (TIII) higher than the intermediate tempering temperature (TII) and lower than the austenitisation start temperature.

Description

Nitriding hardening process

DESCRIPTION

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

State of the art

Nitriding treatments are thermochemical treatments allowing the introduction by diffusion of nitrogen at the surface of the parts to achieve surface mechanical reinforcement, typically improving hardness and fatigue resistance.

The nitriding treatments are conventionally carried out on alloy steels previously treated by austenitization, quenching, and tempering at temperatures above 600 ° C. This then makes it possible to carry out nitriding at a lower temperature (conventionally at least 30 ° C. lower) without affecting the underlying structural state.

Concretely, in the usual nitriding treatments, the steel is nitrided at a temperature of the order of 500 ° C. to 550 ° C. in an atmosphere containing ammonia, which yields nitrogen to the surface of the steel. The nitrogen thus adsorbed causes the formation of a surface layer consisting of iron nitrides, called the combination layer, from which the nitrogen atoms diffuse towards the core of the part to form the diffusion layer.

The hardness resulting from the introduction of nitrogen to the surface is added to the basic hardness level of the steel. 11 shows a more or less high level of hardness, largely dependent on the chemical composition of the substrate.

In the case of parts requiring high treatment depths (typically 0.6 to 1 mm), the treatments must be carried out over periods of up to 120 or even 200 hours between 520 and 550 ° C (we will speak of deep nitriding )

In the current configuration of the production ranges (nitriding on quenched metallurgical state - tempering) these treatments cannot be shortened by temperature increase. This would lead to too much softening at heart through the income effect, as well as insufficient surface hardness. The processing time which can thus exceed 5 working days is an important problem for the manufacture of the parts concerned.

It would therefore be desirable to have a nitriding process with improved kinetics, thus allowing deep nitriding for a reduced treatment time.

The treatment times necessary for deep nitriding also have the consequence of obtaining a surface hardness which hardly exceeds 750 HV. Nitriding at low temperature (typically 480 ° C) would allow this level of hardness to be exceeded but would cause unacceptable treatment times.

It would therefore also be desirable to have a deep nitriding process provided with a higher surface hardness, typically up to 900 HV, without significantly increasing the treatment times.

General description of the invention

With these objectives in mind, the present invention provides a method of hardening by nitriding, in which a piece of nitriding steel, or a part thereof, is subjected to an initial hardening by austenitization and quenching, followed by an intermediate tempering. , then a nitriding treatment.

According to the invention, the intermediate income is a so-called "low temperature" income, which is produced at a temperature below that of the nitriding treatment, and precisely below 350 ° C. The nitriding treatment is carried out at a nitriding temperature above the intermediate tempering temperature and below the austenitization start temperature (Acl).

Compared to known methods, the method according to the invention is based on a structural nitriding treatment applied to a substrate whose tempering has been carried out at low temperature, in particular between 150 and 350 ° C. The process according to the invention therefore takes a position opposite to conventional methods, by carrying out nitriding at temperatures at which the structure will evolve. In the principle of the invention, the part, after the initial hardening (hardening austenitization), does not undergo any high temperature tempering, and is therefore not brought to temperatures above 590 - 600 ° C. Generally, the three phases of initial hardening, low temperature tempering and nitriding treatment are linked without any other treatment whatever its temperature.

Thus, the low temperature tempering is considered here as a relaxation, and less alters the hardness at the heart of the tempered part. At the start of nitriding, the hardness will therefore be greater than that of the conventional tempered state. The approach proposed by the present process will make it possible, depending on the variant embodiments, to reduce the duration of treatment or to increase the hardness. Processing depths greater than 0.5 mm are possible, in particular 0.6 to 1.2 mm. In general, a person skilled in the art will be able to adapt the time-temperature couple as a function of the target hardness and depth of treatment.

According to a first embodiment called "high temperature nitriding", the nitriding temperature is between 550 and 590 ° C, preferably between 570 and 590 ° C, especially 580 ° C. It has been observed, surprisingly, that the process makes it possible to reduce the duration of the nitriding step by up to 50% on conventional parts made of nitriding steel, in order to obtain comparable hardness over the same depth. The method according to the invention therefore saves substantial time. In addition, thermodynamic calculations carried out using the Thermocalc software make it possible to rule out the possibility of partial or local austenitization at the surface taking into account the temperature and the maximum surface nitrogen content.

According to a second embodiment called "low temperature" nitriding, the nitriding temperature is between 500 and 550 ° C., in particular between 500 and 520 ° C. The low temperature relaxation income makes it possible to maintain a high core hardness, and the low temperature nitriding carried out over periods of the order of 120 to 200 hours will make it possible to achieve hardnesses of the order of 800 at 950 HV on the surface. Thus, the process makes it possible, with conventional nitriding temperatures and durations, to obtain hardnesses greater than those obtained with conventional treatments.

The initial hardening, the first phase of the process, can be carried out in a conventional manner. The parts to be treated, generally in the form of a laminated bar, forged or stamped blank, of pre-machined parts, are heated to an austenitization temperature which is conventionally situated in the temperature range between 850 and 950 ° C. The pieces are then quenched and returned. The quenching is carried out from a temperature in the range from 920 to 940 ° C, for example for a steel of type 32CrMoV13. The fluid and the quenching method are adjusted, in a conventional manner, according to the section of the products.

The second phase of the process is the low temperature intermediate tempering, which forms a relaxation treatment. The temperature of the intermediate tempering is between 150 and 350 ° C, preferably between 180 and 250 ° C. It is adjusted according to the desired level of relaxation to ensure the shaping operations to be followed. The duration of the intermediate income is adapted according to the thickness of the part; preferably 1 hour per 20 mm section (thickness). In the present process, the tempering is always carried out at a temperature below the nitriding. Such a light income is suitable for machining, and therefore eliminates the risks of brittleness / deformation during machining.

The nitriding treatment, the third phase of the process according to the invention, is then carried out on a finished or almost finished machining part (the part is typically machined after intermediate tempering and before the nitriding treatment). The time and temperature parameters are to be set according to the desired compromise in surface hardness, depth and microstructure for the layer, as explained above. It is possible to carry out gaseous nitriding with ammonia, or plasma-assisted nitriding with nitrogen. The process used has little influence on the hardness gradient of the nitrided layer, which essentially depends on the chemical composition of the steel and its structural state. After nitriding, the part is not hardened, but a controlled cooling under neutral gas so as not to cause damaging thermal shocks.

According to the variants, it is possible to carry out induction hardening over an area limited to the depths desired for nitriding, which allows different hardness levels. The process then includes the initial hardening phases (quenching austenitization), followed by tempering carried out at a temperature above 600 ° C, then by induction quenching austenitization, followed by intermediate tempering, and finally by nitriding treatment.

The method according to the invention is suitable for the treatment of parts made of nitriding steel. These are typically low-alloy steels containing elements allowing the formation of nitrides, such as chromium, molybdenum, vanadium and aluminum. The carbon content is preferably from 0.15 to 0.9% m.

In particular, the part is made of nitriding steel comprising from 0.15 to 0.9% m of carbon and one or more alloying elements chosen from the list comprising: chromium, nickel, molybdenum, vanadium, aluminum, silicon and manganese. For example, the alloying elements are present in the following proportions: Chromium 1 to 4% m; Nickel 0.5 to 4% m; 0.2 to 4% m molybdenum; Vanadium 0.10 to 2% m; Aluminum 0.5 to 3% m; 0.4 to 1.5% m silicon; and / or Manganese 1 to 2% m.

The present process has been particularly developed for CrMoV steel grades (also denoted CDV), in particular 32CrMoV13.

The invention finds particular application for the treatment of parts in aeronautics and automobile, of the type of teeth and pinion, grooves, ring tracks and rolling rolling elements, among others, to allow them to resist the mechanical stresses to which they are subjected, which the particularity of being mainly concentrated on the surface (bending fatigue, contact fatigue, fretting, wear, ...). Detailed description using the figures

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

Fig.l: a time-temperature graph representing the range of heat treatment of a conventional nitriding process; and

Fig.2: a time-temperature graph representing the range of heat treatment according to an embodiment of the present method;

Fig.3: a graph representing the hardness profile (hardness-depth) for three samples A, B and C; and

Fig. 4: a graph representing the residual stress profile (stresses - depth) for samples A and B.

A. Conventional nitriding process

Figure 1 represents a principle time-temperature diagram which corresponds to the range of heat treatment of a conventional nitriding process in three phases noted (1), (2) and (3). The part to be treated first undergoes a preliminary treatment, here called initial hardening (1), by austenitization and quenching. The part is therefore brought to an austenitization temperature Ti for a predetermined period, then undergoes quenching (controlled rapid cooling) to induce a martensitic type microstructure.

There follows an annealing phase (2), in which the part is heated to a predetermined tempering temperature T ₂ , typically of the order of 550 to 640 ° C. This phase allows a softening of the part (lowering of hardness) and an adjustment of the traction and resilience characteristics, characteristics controlled according to the targeted applications and compatible with the machining operations.

This conventional approach then makes it possible to carry out gas or ionic nitriding, phase (3), between 450 and 550 ° C. without affecting the underlying structural state. It will be noted that in the conventional methods the tempering temperature T ₂ is higher than the nitriding temperature T ₃ , and this by at least 50 ° C.

The hardness resulting from the introduction of nitrogen to the surface is added to the basic hardness level of the steel. 11 shows a more or less high level of hardness, largely dependent on the chemical composition of the substrate. In the case of parts requiring high treatment depths (typically 0.6 to 1 mm), conventional treatments must be carried out over periods of up to 120 or even 200 hours between 520 and 550 ° C. This is represented by the long phase 111 in Fig.l.

In Fig. 1, the evolution of the hardness at the core (curve De) and at the surface (curve Ds) is also shown, as well as the evolution of the treatment depth (curve P _n ). As can be seen, the hardness at heart drops during the tempering phase but remains constant during nitriding. The surface hardness due to nitriding is added to the core hardness, and the thickness of the hard nitrided layer increases over time.

Thus, unlike the conventional process, a structural nitriding is carried out here, since the microstructure will evolve during phase 111, and will therefore modify the hardness at heart.

B. The present nitriding process

Two variants of the present process will be described below, which are both based on a three-phase process in which the tempering phase 11 consists of a low temperature tempering, and the nitriding phase 111 is carried out at a temperature higher than phase 11 .

B.l High temperature nitriding.

Fig.2 shows schematically the treatment range of the present process, called high temperature structural nitriding (NSHT).

The first phase, denoted 1, is a preliminary treatment phase, called initial hardening, similar to that (phase 1) of conventional treatments. 11 this is to give an initial hardness to the part by heating to a temperature Ti in the austenitization zone, followed by quenching.

Note that the part is then subjected to an intermediate tempering (phase 11) called low temperature. The intermediate tempering temperature Tu is here lower than the nitriding temperature Tm. Typically the intermediate tempering temperature Tu is between 150 and 350 ° C.

A nitriding treatment, phase 111, said to be high temperature, follows. It will be noted that the nitriding temperature Tm is much higher than the intermediate tempering temperature Tu. In high temperature nitriding, the treatment treatment temperature Tm is between 550 and 590 ° C, more particularly between 570 and 590 ° C.

The principle of the technical solution therefore here consists in applying a nitriding treatment at a temperature higher than conventional nitriding temperatures, on a substrate whose tempering has been carried out at a lower temperature (between 150 and 350 ° C.).

Such a range modification has the following effects:

The material in the low temperature returned state exhibits, at the start of nitriding, a higher hardness than the conventional pretreated state.

This higher core hardness allows nitriding at higher temperature while retaining a surface hardness and core equivalent to that of conventional processes at the end of treatment (hardness additivity).

The high nitriding temperature allows the nitriding depth to be obtained in a shorter time than usual, allowing up to 50% time savings for a depth of 0.7 mm.

The prolonged maintenance at the nitriding temperature allows the final adjustment of the properties at heart.

A decrease in the amount of angel hair on the surface due to a lower content of carbides in the substrate. Example 1:

Two same test pieces, referenced A and B, were respectively treated according to the conventional nitriding process, and according to a nitriding process in accordance with the process presented above for high temperature nitriding.

Test specimens A and B first underwent an initial austenitic hardening at 920 ° C for 1 hour, followed by oil quenching.

Test specimen A was then subjected to tempering at 620 ° C for about 2 hours, followed by nitriding at 550 ° C for 120 h, or 5 days.

For test piece B, in accordance with the present process, an annealing was carried out at 180 ° C. for approximately 2 h (hardness at the end of phase 11: 550 HV), followed by a nitriding treatment at 580 ° C. for 60 h.

The nitriding treatments were carried out in a horizontal gas nitriding oven with forced convection. The nitriding atmosphere (ammonia + dissociated ammonia) is controlled by regulating the nitriding potential, called KN, thanks to the measurement coming from a di-hydrogen probe. Cooling is ensured at the end of the cycle by a convection of nitrogen circulating in the oven and cooled on an internal exchanger.

The surface and core hardnesses obtained after nitriding for the two test pieces A and B, as well as the conventional nitriding depth (defined as the hardness increased by 100 Vickers) and the hardness to 0.75 mm are reported in Table 1. As can be seen in Figures 3 and 4, the profiles of hardness and residual stresses are similar. The process according to the invention therefore makes it possible to achieve the same surface hardness and depth as the conventional process, for a nitriding duration reduced by half.

B.2 Low temperature nitriding.

The procedure for the low temperature variant of the process is the same as in the high temperature variant (section B1, Fig. 2), but differs over the range of nitriding temperatures which is carried out at a lower temperature, but always higher than the tempering temperature.

The processing range of the low temperature version process can be represented in the same way as in Figure 2 with:

Phase 1: preliminary treatment phase (initial hardening) to give an initial hardness to the part by heating to a temperature Ti in the austenitization zone, followed by quenching.

Phase 11: intermediate income (phase 11) called "low temperature". The intermediate tempering temperature Tu is between 150 and 350 ° C, and is lower than the nitriding temperature Tm.

Phase 111: nitriding treatment at a temperature Tm between 500 and 550 ° C, more particularly between 500 and 520 ° C. Compared to variant B.l, nitriding is said to be low temperature, but remains much higher than the intermediate tempering temperature Tu.

The principle of low temperature structural nitriding (NSBT) consists in applying a nitriding treatment at a temperature lower than conventional deep nitriding temperatures, on a substrate whose tempering has been carried out at a lower temperature between 150 and 350 ° C. Such a range modification induces the following effects, at iso nitriding time compared to conventional nitriding:

- The material in the low temperature returned state exhibits, at the start of nitriding, a higher hardness than the conventional pretreated state

- The nitriding treatment, carried out at a lower temperature, induces softening lower than that generated by conventional tempering. The hardness at the heart at the end of treatment is therefore higher.

- The surface hardness at the end of the low temperature treatment is higher due to a more substantial hardening at low temperature added to a harder substrate (additivity of hardness) - The effective depth (by reasoning in absolute hardness and not relative to the core) obtained is greater or even infinite despite treatment at a lower temperature due to the higher core hardness.

Table 1

Example 2:

A test piece C, identical to test pieces A and B, was treated according to the low temperature variant of the present process. After initial hardening in the austenitic phase at 920 ° C for 1 hour, tempering was carried out at 180 ° C for about 1 hour. The test piece C was then nitrided at a temperature of 520 ° C for 120 h; this in the same oven as the test tube B.

The surface and core hardnesses obtained after nitriding for the test piece C, as well as the depth of nitriding are given in Table 1. A treatment of 120 hours at 520 ° C. on the test piece C at quenched-relaxed state, thus made it possible to obtain a surface hardness greater than more than 150 HV compared to conventional nitriding (reference A) and to obtain a hardness greater than 470 HV throughout the volume of the part (HV core + 100 HV = 470 HV on the reference nitriding A). Thus, despite a slightly lower conventional depth (HV core + 100), the hardness is greater than the core hardness obtained with the conventional process.

The hardness profile of sample C is shown in Fig. 3. The stress profiles, also measured, remain unchanged compared to the reference nitriding.

Claims

claims

1. Method for hardening nitriding steel parts, in which a nitriding steel part, or a part thereof, is subjected to an initial hardening by austenitization and quenching, followed by an intermediate tempering, then by a treatment nitriding, characterized in that:

- the intermediate income is a so-called low temperature income, produced at a temperature (Tu) below 350 ° C;

- the nitriding treatment is carried out at a nitriding temperature (Tm) above the intermediate tempering temperature (Tu) and below the austenitization start temperature.

2. Method according to claim 1, wherein the intermediate tempering is carried out at a temperature between 150 and 350 ° C, preferably between 180 and 250 ° C.

3. Method according to claim 1 or 2, wherein the nitriding temperature is between 570 and 590 ° C.

4. Method according to claim 1 or 2, wherein the nitriding temperature is between 500 and 520 ° C.

5. Method according to one of the preceding claims, in which the part, after said initial hardening, is subjected to tempering at a temperature above 600 ° C and then to induction hardening in an area limited to the depths desired for nitriding. , before said middle income.

6. Method according to any one of the preceding claims, in which the part is made of nitriding steel comprising from 0.15 to 0.9% m of carbon and one or more alloying elements chosen from the list comprising: chromium, nickel , molybdenum, vanadium, aluminum, silicon and manganese.

7. The method of claim 6, wherein the part is of CrMoV type steel, in particular 32CrMoV13.

8. The method of claim 6, wherein one or more of said alloying elements are present in the following proportions: Chromium 1 to 4% m; Nickel 0.5 to 4% m; 0.2 to 4% m molybdenum; Vanadium 0.10 to 2% m; Aluminum 0.5 to 3% m; 0.4 to 1.5% m silicon; and / or Manganese 1 to 2% m.

9. Method according to any one of the preceding claims, in which the part is machined after the intermediate tempering and before the nitriding treatment.

10. Method according to any one of the preceding claims, in which the intermediate income is conducted for a period corresponding to 1 hour per 20 mm of section.