Method for low-pressure carburising of a workpiece comprising steel

Abstract

The invention relates to a method for carburising a steel workpiece, implementing between 1 and 30 consecutive cycles (C1, C2, C3, C4), each cycle (C1, C2, C3, C4) comprising a step (10) of injecting carburising gas in such a way as to increase the surface carbon rate until a predetermined higher rate (14) is reached, the carburising gas being injected at a flow rate of between 1000 Nl / h and 3000 Nl / h, at a temperature of between 950°C and 1050°C, for a period of time (t1), and a step (12) of injecting a zero gas so as to decrease the surface carbon rate of the workpiece until it reaches a predetermined lower rate (22), the step of injection of zero gas comprising a first phase of injection of zero gas, at a flow rate of between 1000 and 10000 Nl / h, for a period of time (t2), followed by a second phase of injection of zero gas, at a rate of between 500 Nl / h and 3000 Nl / h, for a period of time (t3).

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for low-pressure carburizing a part comprising steel. STATE OF THE ART

[0002] The term carburizing refers to a classic process in the field of metallurgy. It is a thermochemical treatment consisting of superficially penetrating carbon (C) into a steel part in order to transform it, on the surface, into a highly carburized steel. Steel is a metallic alloy consisting mainly of iron (Fe) and carbon (ranging, for carbon (C), from a mass content close to 0%, corresponding to minute traces, up to a content of 2%). The carbon content has a considerable influence on the properties of the steel. Below 0.008% carbon (C), for example, the steel is rather malleable and is referred to as "iron". The carbon (C) content notably profoundly modifies the melting temperature and the mechanical properties of the steel. Increasing the carbon content of a steel improves its hardness (resistance opposed by a surface to the penetration of a point) and reduces its elongation at break.Increasing the carbon content on the surface thus makes it possible to increase the surface mechanical properties of the part, and to increase its resistance to wear and its endurance.

[0003] Carburizing creates a decreasing carbon gradient towards the core of the part. The carbon concentration is thus enriched at the surface of the part. The carburizing process has been known for over a hundred years, its implementation has evolved considerably since its beginnings, and it has been practiced in gaseous form for over 20 years. In particular, today, the most frequently used method is low-pressure gas carburizing (LPG), which is a carburizing process carried out in a vacuum carburizing chamber, and which uses gaseous hydrocarbons at very low pressure and high temperature in order to obtain a carbon-enriched and hardened surface layer, as for example explained in documents - FR 2 678 287 A1 and WO 2016 / 160751 A1.

[0004] Low pressure carburizing is a carburizing process carried out in a cold-walled vacuum furnace, using gaseous hydrocarbons at low pressure: 2 to 20 mbar (200 to 2000 Pa) absolute, for example 10 mbar (1000 Pa), and at high temperature.

[0005] ECM Technologies' Infracarb ®< process is the patented process used in our Modular Carburizing and Treatment Facility. More specifically, Infracarb ®< consists of alternating injections of C2H2 hydrocarbons to create surface enrichment by cracking molecules at high temperature and a neutral N2 gas for diffusion.

[0006] The difficulty of CBP cementation lies in the limitation of the carbon diffusion depth, in particular when cementing materials very sensitive to the presence of carbon, such as Ferrium ®< C61 ™< and Ferrium ®< C64 ™< alloys marketed by the company Questek, and for which a rate of 0.5% carbon (C) on the surface must not be exceeded.

[0007] Indeed, Ferrium ®< C61 ™< and C64 ™< alloys differ from conventional compositions (for example compositions comprising 0.16% carbon (C), 3% nickel (Ni), less than 1% chromium (Cr) and traces of molybdenum (Mo) by higher levels of carbon (C) and gammagenic elements: for example levels of around 0.2% carbon (C), 9% nickel (Ni), 3.5% chromium (Cr) and 1% molybdenum (Mo).

[0008] Gammagenic elements, which include cobalt (Co), nickel (Ni), nitrogen (N), and copper (Cu), are addition elements that increase the stability range of a particular allotrope of iron: austenite. Austenite allows for high solubility of carbon. The vast majority of so-called stainless steels are austenitic and combine good corrosion resistance with high mechanical properties.

[0009] However, these Ferrium ®< C61 ™< and Ferrium ®< C64 ™< alloys also contain a higher level of alphagenic elements, such as molybdenum (Mo) and chromium (Cr). Alphagenic elements tend to destabilize austenite, in favor of ferrite. Ferrite is an allotrope of steel that dissolves carbon (C) poorly and has ferromagnetic properties at low temperatures. Thus, as soon as the carbon (C) concentration becomes too high, the precipitation of ferric complexes is observed, forming undesirable bodies with a catastrophic effect on the microstructure of the steel part. Indeed, these ferric precipitates take the form of carbides forming networks that locally reinforce the hardness of the part but weaken the part as a whole.

[0010] Therefore, it is necessary to be able to cement the part in the correct proportions of carbon (C) in order to reinforce the surface mechanical properties of the part, without altering its microstructure.

[0011] For Ferrium ®< C61 ™< and Ferrium ®< C64 ™< alloys, the desired properties are particularly dependent on variations in the metallographic structure and the surface carbon content (C). These two characteristics depend directly on the thermochemical treatments implemented during the carburizing processes. Obtaining a structure with the characteristics necessary for the proper use of the part therefore depends directly on the parameters defined during carburizing.

[0012] The invention aims to achieve this objective. It thus proposes a low-pressure carburizing process suitable for steel parts containing alphagenic and gammagenic elements. STATEMENT OF THE INVENTION

[0013] The invention thus relates to a method for low-pressure carburizing a part comprising, in particular on the surface, steel, according to independent claim 1. Preferred embodiments are present in the dependent claims.

[0014] This allows carburizing to be carried out under an inert atmosphere, in particular under partial pressure of neutral gas (for example nitrogen (N2)) and no longer under vacuum. Diffusion is thus carried out under partial pressure of neutral gas after a purging phase using a high flow rate of neutral gas). This results in healthy structures, free from intergranular precipitates forming networks and with an acceptable residual austenite level.

[0015] The multiplication of injection steps makes it possible not to oversaturate the austenite surface, and to provide the right quantity of carbon (C) within the maximum absorption limits by diffusion in the subsurface of the steel (this absorption being a function of the desired carburized depth). Consequently, this multiplication of injection steps makes it possible to avoid the formation of undesirable compounds, in particular in the sub-surface zone (for example, cementite precipitates at the grain boundaries).

[0016] Thus, the injection step allows the enrichment in carbon (C), and the diffusion step allows the dilution and diffusion of this enrichment of carbon (C) in the austenite so as to reach the desired depth. This diffusion thus avoids supersaturation on the surface and the precipitation of carbon (C) which can possibly lead to soot deposits harmful to the enrichment of the steel.

[0017] The method according to the invention may comprise one or more of the characteristics or steps below, taken in isolation from one another or in combination with one another: the predetermined upper surface carbon content is the same for all cycles, the cementing gas has a dilution rate of between 0 and 75%, a very low dilution rate being preferred for high cementation depths and while a very high dilution rate is preferred for (very) low cementation depths, after the last cementation cycle has been completed, the process comprises a final diffusion step followed by a cooling step during which the cooling rate is between 7°C / min and 200°C / min, the cooling step following the final diffusion step is carried out outside the cementation enclosure, in a dedicated cooling cell. the part is made of said steel. DESCRIPTION OF FIGURES

[0018] The invention will be better understood and other details, characteristics and advantages of the invention will appear on reading the following description given by way of non-limiting example and with reference to the appended drawings in which: there figure 1 is a diagram of a low-pressure cementation chamber in which a part intended to be cemented according to the method of the present invention is deposited; figure 2 is a schematic graph of the mass rate of carbon (C) present on the surface of a part subjected to a low pressure carburizing process according to the invention, as a function of time, the figure 3 is a graphical representation of the mass rate of carbon (C) present in the part as a function of the distance from the surface towards the core of said part, and at different stages of a cycle of the process according to the invention, for a sample of pure iron. DETAILED DESCRIPTION

[0019] The method described here advantageously applies to a part 1 comprising or consisting of a steel comprising alphagenic and gammagenic elements, said steel comprising, for example, in percentage by weight: from 0.10% to 0.20% in carbon (C), from 0.1% to 20% in cobalt (Co), from 2% to 15% in nickel (Ni), from 1% to 10% in chromium (Cr).

[0020] The method preferably applies to a part 1 comprising or consisting of an alloy of the Ferrium ®< C61 ™< and Ferrium ®< C64 ™< type. The Ferrium ®< C61 ™< alloy is a steel of composition: 0.15% carbon (C), 3.5% chromium (Cr) 9.5% nickel (Ni) 18% cobalt (Co), 1.1% molybdenum (Mo), 0.08% vanadium (V).

[0021] Ferrium ®< C64 ™< alloy is a steel of composition: 0.11% carbon (C), 3.5% chromium (Cr) 7.5% nickel (Ni) 16.3% cobalt (Co), 1.75% molybdenum (Mo), 0.2% tungsten (W), 0.02% vanadium (V).

[0022] In this example, the alloy tested is pure iron.

[0023] As visible on the figure 1 , the method according to the invention consists of one or more cycles which follow one another. In the example illustrated in figure 2 , it implements a succession of four cycles C1, C2, C3, C4. Each cycle C1, C2, C3, C4 comprises a step 10 of injecting a cementing gas, for example propane (C 3 H 8 ) or acetylene (C 2 H 2 ), followed by a step 12 of injecting a neutral gas, for example argon (Ar) or nitrogen (N 2 ). This is a step 12 of diffusion under partial pressure of a neutral gas. Preferably, each step 10 of injecting the cementing gas is directly followed by step 12 of injecting neutral gas.

[0024] The method firstly comprises a step of placing the part 1 in a cementation chamber 2, as visible in the figure 1This cementation enclosure 2 includes in particular a gas inlet 3 and a gas outlet 4 and can be closed in a sealed and isothermal manner.

[0025] Once part 1 is placed in the cementation chamber 2, it is closed and brought to temperature. Once part 1 has reached a targeted stabilized temperature, the succession of cycles C1, C2, C3, C4 begins.

[0026] As illustrated in figure 2, the first step of each cycle C1, C2, C3, C4 is a step 10 of injecting cementing gas, (also called carbon enrichment step) into the cementation chamber 2. This enrichment step 10 has a duration t 1 of between 30 and 250s, preferably between 50 and 150s. The duration t 1 of the enrichment step 10 is a function of a predetermined and ultimately targeted upper surface rate 14 of carbon (C) (i.e. it is a temperature intended to be reached at the end of the process) on the surface of the part 1. By "surface" is meant here a substantially zero depth, preferably zero. Under optimal conditions, this upper surface rate 14 can correspond to the solubility limit of carbon (C) in the austenite, at the given cementation temperature. This higher surface rate 14 is typically higher than the initial surface carbon (C) rate 16 of the part 1 to be treated.

[0027] During this enrichment step 10, the cementing gas is injected into the cementation chamber 2 at a flow rate of between 1000 NI / h and 3000 Nl / h, i.e. between 2.778 x 10 -4< and 8.333 x 10 -4< Nm 3< / s (preferably between 1300 and 1700 Nl / h, i.e. between 3.611 x 10 -4< and 4.722 x 10 -4< Nm 3< / s), the cementation temperature being between 950°C and 1050°C (and preferably equal to 1000°C ± 10°C).

[0028] The unit NI / h is one normo liter per hour. The normo liter is derived from the normo cubic meter. The normo cubic meter, symbol Nm3 or sometimes m3(n), is a unit of measurement of quantity of gas which corresponds to the content of a volume of one cubic meter, for a gas found in standard conditions of temperature and pressure (0 or 15 or more rarely 20 °C depending on the references and 1 atm, or 101,325 Pa).

[0029] This is a customary unit, however not recognized by the International Bureau of Weights and Measures. For a pure gas, one cubic normometer corresponds to approximately 44.6 moles of gas.

[0030] In particular, the dilution rate of the injected cementing gas is between 0 and 75%, preferably between 0 and 25%, and the cementing gas is injected at a pressure between 0.1 bar and 3 bar (between 10,000 Pa and 300,000 Pa), preferably equal to 230±50 mbar (23,000 ± 5000 Pa). By "dilution rate" we mean here a dilution of the cementing gas in a neutral gas, typically, the neutral gas argon (Ar) or nitrogen (N2).

[0031] During enrichment step 10, there is partial or total saturation of the austenite on the surface of the steel part. The maximum carbon (C) 14 content desired at the end of enrichment step 10 depends on the steel grade and the temperature at which the carburizing is carried out.

[0032] At the end of step 10 of injecting the cementing gas, the carbon molecules (C) which have enriched the carbon content (C) of the part 1 are diffused towards the heart of said part 1, in order to homogenize its composition, as visible on the figure 3 .

[0033] There figure 3 represents the carbon profile of a part 1 subjected to the process according to the invention, that is to say the mass rate of carbon present in the part 1 as a function of the distance from the surface towards the core of said part 1, that is to say the depth of the part 1. The figure 3illustrates both a carbon (C) 18 rate (dotted line) at the end of enrichment step 10, before the start of diffusion step 12, and a carbon (C) 20 rate (solid line) at the end of diffusion step 12. We can thus see that the carbon (C) 18 rate as a function of depth at the end of enrichment step 10 shows a substantially exponential decrease: at the surface of the part, the carbon (C) 18 rate is very high, whereas at a depth of 0.2 mm, this rate 18 is equal to a value close to 0 (in the case of the pure iron sample used for the measurements illustrated by figure 3 ).

[0034] The second step of each cycle C1, C2, C3, C4 is a diffusion step 12. This diffusion step 12 takes place in a neutral atmosphere and thus requires, as indicated above, the injection of a neutral gas such as nitrogen (N 2 ) or argon (Ar) into the carburizing enclosure 2. Thus, as soon as the enrichment step 10 is completed, neutral gas is injected into the carburizing enclosure 2.

[0035] The gas sweep generated during the entry of neutral gas makes the elimination of the carburizing gas molecules on the surface of part 1 more efficient. In fact, this elimination of gas molecules de facto stops the enrichment in progress. In a vacuum atmosphere, the elimination of these molecules is much slower.

[0036] More particularly, diffusion step 12 consists of two phases: a cementing gas purge phase with a duration t 2 of between 5 and 60s (preferably 10±5s), and a diffusion phase proper, with a duration t 3 of between 10 and 2000s (preferably 30 to 2000s). The duration of diffusion step 12 is therefore, as visible on the figure 2 , of (t 2 +t 3 ).

[0037] In the chosen example, the values ​​t 1 , t 2 t 3 have been taken as identical for simplification. In fact, depending on the desired processing fineness, they are often called upon to be modified (adjusted) from one cycle C1, C2, C3, C4 to the next.

[0038] The difference between the two phases of diffusion step 12 lies in the injection rate of the neutral gas. During the duration t 2 of the purge phase, the neutral gas is injected into the carburizing chamber 2 at a rate of between 1000 and 10000 NI / h, i.e. between 2.778 x 10 -4< and 2.778 x 10 -3< Nm 3< / s, preferably equal to 6000±500 Nl / h, i.e. 1.667 x 10 -3< ± 1.389 x 10 -4< Nm 3< / s, at a neutral gas pressure in the chamber 2 of between 0.1 and 7 bar, preferably equal to 230±50 mbar (23,000 ± 5000 Pa). During the duration of t 3 of each diffusion phase, after the purge phase, the neutral gas is injected at a flow rate of between 500 NI / h and 3000 NI / h, i.e. between 1.389 × 10 4< and 8.333 × 10 -4< Nm 3< / s, preferably equal to 1800±500 NI / h, i.e. between 5 × 10 -4< ± 1.389 × 10 -4< Nm 3< / s, the pressure in the cementation chamber 2 being between 2 and 25 mbar (between 200 and 2500 Pa), preferably between 7 and 13 mbar (between 700 and 1300 Pa).

[0039] As visible on the figure 3 , at the end of the diffusion step 12, it is noted that the carbon rate (C) 20 as a function of the depth of the part 1 presents a substantial plateau, for a depth ranging from 0 to 0.1 mm, before decreasing to reach a rate equal to a value close to 0 (for the sample of pure iron whose values ​​are illustrated in figure 3 ) for a depth of 0.2 mm.

[0040] On the figure 2 , we see that the lower surface carbon rate (C) 22 predetermined at the end of the diffusion step 12 (comprising a purge phase and a diffusion phase), is lower than the higher surface carbon rate (C) 14 which had been obtained at the end of the enrichment step 10. This predetermined lower surface rate 22 may however be higher than the initial surface rate 16.

[0041] It is however important to keep in mind that the lower surface rate does not correspond to a physical value, nor to a characteristic of the material, nor to a fixed or controlled value. The value of this lower surface rate 22 is obtained downstream of the methods of implementing the cementation targeted by the present invention, namely, very generally: the material treated and the targeted cemented depth, the characteristics of the cementation installation used. The precise value of this lower surface rate 22 is ultimately neither measured nor really known.

[0042] The process being cyclical, after the diffusion step 12 of a given cycle C1, C2, C3, we move on to a new cycle C2, C3, C4 and therefore to a new enrichment step 10. The enrichment steps 10 and diffusion steps 12 follow one another in a cyclic manner and the number of cycles C1, C2, C3, C4 is between 1 and 30, preferably between 1 and 15. As can be seen in the figure 1 , the upper 14 and lower 22 carbon (C) rates on the surface obtained at the end of the different steps 10, 12 remain unchanged throughout the process. As an example, a number of four cycles C1, C2, C3, C4 has been illustrated in figure 2 .

[0043] A change in the number of cycles impacts the desired maximum carbon rate (C) 14 and the predetermined lower surface rate 22, as well as the total diffused depth and the final carbon profile.

[0044] When the desired number of cycles C1, C2, C3, C4 has been reached, after the last diffusion phase of the last diffusion step 12 of the last cycle C4, a final diffusion step 24 then a cooling step are implemented, and a drop in the surface carbon content (C) is observed. Cooling can be carried out outside the carburizing chamber 2, in a dedicated cooling cell (not shown). Cooling makes it possible to gradually reduce the temperature of the part 1 from its carburizing temperature to a temperature suitable for handling the part 1. The cooling rate of the carburizing chamber 2 or of the dedicated chamber is between 7°C / min and 200°C / min, preferably 120±50°C / min.

[0045] Following this final diffusion step 24, the final surface carbon (C) rate 26 is lower than the lower carbon (C) rate 22 obtained at the end of diffusion step 12, but it may be higher than the initial rate 16, as visible on the figure 2 .

[0046] The final surface carbon (C) rate 26 aims to be as close as possible to the theoretical optimal surface rate (0.5% by mass for Ferrium steels).

[0047] Depending on the chemical composition of the steel and the nature of the areas to be carburized, the lower 22 and higher 14 surface rates targeted and obtained result from the physical parameters of the part 1 to be carburized and those of the carburizing enclosure 2 used.

[0048] Different combinations of the different parameter value ranges given above allow for different thermochemical treatments of the treated part 1. These differences include the carburized depth and the type of carburized structure obtained. The shape of the carburized part 1 (e.g. parts with teeth) must also be taken into account when adjusting the carburizing process parameters.

Claims

1. A method for low-pressure carburizing of a workpiece (1) comprising steel, said steel comprising, in weight percent: - from 0.10% to 0.20% in carbon, - from 0.1% to 20% in cobalt, - from 2% to 15% in nickel, - from 1% to 10% in chromium, characterized in that said method comprises: a) a step of placing said workpiece (1) in a carburizing enclosure (2), the pressure in the carburizing enclosure (2) is between 200 Pa and 2500 Pa, and b) implementing between 1 to 30 consecutive carburizing cycles (C1, C2, C3, C4), each cycle (C1, C2, C3, C4) comprising: i) a step (10) of injecting carburizing gas selected from propane C3H8 and acetylene C2H2 into the carburizing enclosure (2) at a pressure between 10 000 Pa and 300 000 Pa, so as to enrich the surface of the workpiece (1) with carbon and to increase the surface rate of carbon of the workpiece (1) up to a predetermined higher surface rate (14), the carburizing gas being injected into the enclosure (2) at a flow rate of between 2,778×10-4 Nm3 / s and 8,333×10-4 Nm3 / s, the temperature of the enclosure (2) being between 950°C and 1050°C, and for a period of time t1 of between 30 and 250s, and ii) a step (12) of injecting a neutral gas selected from dinitrogen N2 and argon Ar into the carburizing enclosure (2) at a pressure between 10 000 Pa and 700 000 Pa, so as to diffuse the carbon from the surface to the interior of the workpiece (1), and to decrease the surface rate of carbon of the workpiece (1) to a predetermined lower surface rate (22), the neutral gas injection step (12) comprising a first phase of neutral gas injection, at a flow rate of between 2,778×10-4 Nm3 / s and 2,778×10-3 Nm3 / s, and for a period of time t2 of between 5 and 60s, followed by a second phase of neutral gas injection, at a flow rate of between 1,389×10-4 Nm3 / s and 8,333×10-4 Nm3 / s, and for a period of time t3 of between 10 and 2000s.

2. The method according to the preceding claim, characterized in that the predetermined higher surface rate (14) of carbon is the same for all cycles (C1, C2, C3, C4).

3. The method according to any of the preceding claims, characterized in that the carburizing gas has a dilution ratio of between 0 and 75%.

4. The method according to any one of the preceding claims, characterized in that it also comprises, after implementing the carburizing cycles (C1, C2, C3, C4), a final diffusion step (24) followed by a cooling step during which the cooling rate is between 7°C / min and 200°C / min.

5. The method according to the preceding claim, characterized in that the cooling step following the final diffusion step (24) is implemented outside the carburizing enclosure (2), in a dedicated cooling cell.

6. The method according to any of the preceding claims, characterized in that the workpiece (1) consists of said steel.

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