FR3084375A1 - PROCESS FOR REMOVAL OF HYDROGEN FROM A MECHANICAL PART OF TEMPERED AND REVENTED STEEL - Google Patents

Abstract

The invention relates to a process for removing hydrogen from a mechanical part of quenched and tempered steel, coated with an electrolytic or chemical coating, in which a thermal degassing treatment of hydrogen is carried out, characterized in that the degassing heat treatment is carried out at a pressure below atmospheric pressure.

Description

The invention relates to the technical field of manufacturing mechanical steel parts which may be weakened by the presence of hydrogen in their structure.

We call "mechanical parts" parts intended to be energized during their use, such as screws, springs and clips for example.

Processes for manufacturing mechanical steel parts are known, comprising the following steps:

a step of developing and shaping the mechanical part (cold, hot, by machining, ...);

a stage of austenitization and quenching of the part thus obtained, that is to say a heating of the steel part making it possible to transform all of its crystallographic structure into austenite; quenching transforms, through accelerated cooling, austenite into martensite in order to give mechanical parts their mechanical service characteristics;

a surface treatment step, such as: degreasing / acid pickling operations, conversion treatments (phosphating for example), electrolytic surface treatments (zinc plating, cadmium plating, and chrome plating for example) or chemical (nickel chemical) .

It is known that these last two steps are hydrogenating operations (chemical or electrolytic). Hydrogen can also be introduced during boilermaking, such as during flow forming, machining and drilling operations, due to the decomposition of inappropriate lubricants, as well as during welding and brazing operations.

[005] However, the presence of hydrogen weakens the mechanical parts. Indeed, hydrogen, by entering the crystalline network of hardened steel, during the manufacture of the part, can greatly reduce the elongation and necking properties of the part, when the part is tensioned . This has the consequence of causing the abrupt and premature rupture of the part once mounted in its system.

This phenomenon is explained by a setting in motion of the atoms of hydrogen in the zones constrained in traction (example: stress fields at the level of zones with high concentration of stress) which then impede the movement of the dislocations.

FIG. 1 illustrates a slow tensile test on pretreated and hydrogenated screws (37Cr4) for 1 to 7 hours (loading from 1 to 7 hours) chemically in a solution of ammonium thiocianate at 50 ° C. The abscissa axis indicates the deformation in percentage, and the ordinate axis indicates the stress in MPa. It is found that the greater the hydrogenation, the more the deformation is limited, even non-existent.

Among the major families of products impacted by embrittlement by hydrogen (also called stress cracking by hydrogen or delayed brittle rupture due to the introduction of hydrogen): screws, springs, clips, ...

The reduction, or even the elimination of the plastic domain of a steel normally having an elastoplastic character (in the absence of hydrogen) is the source of numerous mechanical malfunctions very quickly leading to premature rupture (lower at 96h for static energization) of the hydrogenated component by one or more operations in the manufacturing range.

In order to overcome this embrittlement problem linked to the presence of hydrogen in the finished part, it is known to add at the end of the manufacturing process, a degassing step carried out at atmospheric pressure, after the treatment has been carried out. of hydrogenating surface.

Degassing is a process of heating parts for a defined time at a given temperature, so as to minimize the risk of embrittlement by hydrogen.

This operation is governed by a European standard NF ISO 9588 entitled Treatments after coatings on iron or steel to reduce the risk of embrittlement by hydrogen. This standard determines a degassing time as a function of the mechanical tensile strength (Rm in MPa) of the part for an air treatment carried out between 190 ° C and 220 ° C. The minimum holding time (t in hours) recommended by the standard follows the following law: t = 0.02 * Rm -12. The table below shows some values.

Table 1

Tensile strength (Rm in MPa) 1000 1400 1800 Degassing time (t in hours) 8 14 24

The standard also specifies a maximum interoperation time. The interoperation time corresponds to the time between the first hydrogenating operation of the part manufacturing process and the start of the degassing step.

This duration must be as short as possible, that is to say that the degassing step must begin as soon as possible. A maximum interoperation time of 3 hours is indicated by this standard.

However, the duration of degassing is too long for the manufacture in large quantities of certain mechanical parts (the holding time is a function of the mechanical tensile strength of the part), and the maximum interoperation time of 3 hours cannot not be observed for all parts manufactured in a production line.

As a result, the degassing process is not effective enough to loosen the mechanical parts under acceptable industrial conditions, and the complete elimination of embrittlement by hydrogen cannot be guaranteed, as the standard indicates. NF EN ISO 4042.

The invention aims to remedy these drawbacks by providing a process for removing hydrogen based on degassing carried out at a pressure below atmospheric pressure, in particular degassing under vacuum.

In general, the invention relates to a process for removing hydrogen from a mechanical part of hardened and tempered steel, and coated with an electrolytic or chemical coating, in which a degassing heat treatment is carried out hydrogen, in which the degassing heat treatment is carried out at a pressure below atmospheric pressure.

The advantage of degassing at a pressure below atmospheric pressure lies in the fact that the thermal effect of the return of plasticity is supplemented by a suction effect associated with the working pressure within the laboratory.

Thus, the degassing time is greatly reduced and it is independent of the mechanical resistance in traction.

The method according to the invention thus saves cycle time during the manufacture of mechanical parts and improves the efficiency of the degassing process in order to limit embrittlement by hydrogen.

In addition, the method makes it possible to dispense with an interoperation duration incompatible with production requirements.

The method can also include one or more of the following characteristics, taken alone or in combination:

- the degassing heat treatment is carried out at a pressure below 500 Pa;

- the degassing heat treatment is carried out under primary or secondary vacuum;

the degassing heat treatment is carried out by heating the steel to a heating temperature between 25 ° C and 300 ° C, preferably between 190 ° C and 220 ° C;

the degassing heat treatment is carried out by heating the steel for a period of less than 4 hours;

- the mechanical part has a mechanical tensile strength greater than

800 MPa.

The invention also relates to a process for manufacturing a mechanical part of hardened and tempered steel, and coated with an electrolytic or chemical coating, said process comprising:

a stage of development and shaping of the part;

a step of austenitization and quenching of the part thus obtained;

- at least one step of surface treatment;

a step for heat treatment of degassing of hydrogen according to the invention.

The process for manufacturing a mechanical part can also include one or more of the following characteristics, taken alone or in combination:

- the mechanical part includes a mechanical tensile strength greater than 800 MPa;

the mechanical part is likely to be put under static or dynamic tension during its use;

the surface treatment step is chosen from at least one of the following treatments: degreasing operation, acid pickling operation, conversion treatment such as phosphating, electrolytic surface treatment, such as zinc coating, cadmium plating, or chromium plating, chemical surface treatment;

- the interoperation time between a first hydrogenating operation and the start of the degassing heat treatment is more than one week.

The invention will be better understood on reading the appended figures, which are provided by way of examples and have no limiting character, in which:

- Figure 1 illustrates a slow tensile test on screws (37Cr4) pretreated and hydrogenated for 1 to 7 hours chemically in a solution of ammonium thiocianate at 50 ° C;

FIG. 2 illustrates the process according to the invention.

According to the invention, the process for removing hydrogen from a mechanical part of quenched and tempered steel, and coated with an electrolytic or chemical coating, comprises a step for heat treatment of degassing of hydrogen. , this degassing heat treatment being carried out at a pressure below atmospheric pressure, advantageously under vacuum.

A degassing heat treatment is a thermal process carried out over a given temperature range and duration. The range and duration are chosen so as not to cause any alteration of the metallurgical structures of the steel and of the coating produced, but in order to allow hydrogen degassing of the part.

According to the invention, the degassing heat treatment is carried out at a pressure below atmospheric pressure, preferably at a pressure of 500 Pa.

According to a preferred embodiment, the degassing heat treatment is carried out under primary vacuum, at a pressure between 0.1 and 100 Pa.

According to another embodiment, the degassing heat treatment is carried out under secondary vacuum, at a pressure between 10 ^-5 and 0.1 Pa.

Thus, the method comprises a step of introducing the mechanical part of hardened and tempered steel, and coated with an electrolytic or chemical coating, in a vacuum oven capable of being able to heat the part to a pressure below the atmospheric pressure, for example under vacuum.

The oven is characterized in particular by the dimensions of its chamber, suitable for receiving mechanical parts, its vacuum level (for example down to 10 ⁵ Pa), and its temperature range. According to an exemplary embodiment, a class 5 oven is used.

Advantageously, the degassing heat treatment is carried out by heating the steel to a heating temperature between 25 ° C and 300 ° C. Preferably at a temperature between 190 ° C and 220 ° C.

Advantageously, the degassing heat treatment is carried out by heating the steel for a period of less than 4 hours.

Thus, according to a particular embodiment, the process for defragraging a mechanical part of hardened and tempered steel, and coated with an electrolytic or chemical coating, comprises the introduction into a vacuum oven of mechanical parts , then a lowering of the pressure inside the oven below atmospheric pressure, in particular below 500 Pa, preferably below 100 Pa, then heating of the parts to a heating temperature of between 25 ° C and 300 ° C, preferably between 190 ° C and 220 ° C for less than 4 hours.

The pressure is then raised to atmospheric pressure, then the oven is opened and the mechanical parts are removed from the oven.

The invention also relates to a process for manufacturing a mechanical part of hardened and tempered steel, and coated with an electrolytic or chemical coating.

With reference to FIG. 2, this manufacturing process comprises the following steps:

1. a stage (EMF) of development and shaping of the mechanical part (cold or hot or by machining);

2. an austenitization (AUS) and quenching (TRE) step for the mechanical part thus obtained (hydrogenating operation);

3. at least one surface treatment step (TRS) - hydrogenating operation;

4. a step of thermal degassing treatment (DSV) of hydrogen according to the invention. The step of developing and shaping the part makes it possible to form a mechanical part comprising a mechanical tensile strength greater than 800 MPa after heat treatment, and capable of being put into static or dynamic tension during its use.

The surface treatment step is chosen from at least one of the following treatments: degreasing operations, acid pickling operation, conversion treatment such as phosphating, electrolytic surface treatment, such as zinc plating, cadmium plating, or chrome plating, chemical surface treatment.

According to an exemplary embodiment, the interoperation time between the first hydrogenating operation (step 2) and the start of the degassing heat treatment (step 4) is greater than one week.

Examples of embodiments The method according to the invention, for removing hydrogen from a mechanical part of hardened and tempered steel, and coated with an electrolytic or chemical coating, was implemented and compared to degassing under air on mechanical parts, axes, steel type C75E quenched zinc-plated.

We carry out the step of shaping the part by drawing, in order to manufacture axes.

Then, the heat treatment step (austenitization and air quenching) is carried out, which makes it possible to obtain axes whose hardness at core is greater than 650HV0.3 while the surface hardness is of the order 170HV0.05 (decarburization phenomenon associated with air treatment).

Then, the surface treatment step is carried out (hydrogenating operation), by carrying out electrolytic zinc plating on these axes making it possible to produce a coating 15 μm thick. The axes are weakened by the hydrogen which is released at the level of the part placed as a cathode during the surface treatment.

In order to demonstrate this loss of ductility, a qualitative bending test at 90 ° was carried out. The interpretation of this test involves observing the axes after tests. If the axes deform at 90 °, the plasticity of the part is guaranteed. On the other hand, if the axes break during the 90 ° bending test, the parts have lost all plasticity.

The results of the tests carried out on uncoated, coated and coated axes followed by degassing in air or in primary vacuum are grouped in Table No. 2. Interoperation time was also a variable during these trials.

Table 2: Results of the bending tests carried out on axes without degassing or with degassing in air or in primary vacuum.

Lots of 20 pieces Degassing temperature (° C) Mataining time (H) Time interoperation Number of broken pieces Number of folded pieces Lot 1: uncoated axes without degassing 0 20 Lot 2: zinc plated axes without degassing - - - 20 0 Lot 3: galvanized axes with air degassing 220 ° C 24 3h 0 20 Lot 4: zinc plated axes with air degassing 220 ° C 0.5 3h 20 0 Lot 5: galvanized axes with degassing under primary vacuum 220 ° C 0.5 3h 0 20 Lot 6: zinc plated axes with air degassing 220 ° C 2 6 months 20 0 Lot 7: galvanized axes with degassing under primary vacuum 220 ° C 0.5 6 months 0 20

We see that all the axes of hardened C75E galvanized steel then degassed under 10 primary vacuum bent at 90 °. Consequently, vacuum degassing makes it possible to rediscover the level of plasticity making it possible to ensure folding at 90 ° in the image of what is obtained with an uncoated axis.

Claims (11)

1. A process for removing hydrogen from a mechanical part of quenched and tempered steel, coated with an electrolytic or chemical coating, in which a hydrogen degassing heat treatment is carried out, characterized in that the degassing heat treatment is carried out at a pressure below atmospheric pressure. 2. Method according to the preceding claim, wherein the degassing heat treatment is carried out at a pressure below 500 Pa. 3. Method according to one of the preceding claims, in which the degassing heat treatment is carried out under primary or secondary vacuum. 4. Method according to one of the preceding claims, in which the degassing heat treatment is carried out by heating the steel to a heating temperature between 25 ° C and 300 ° C, preferably between 190 ° C and 220 ° C . 5. Method according to one of the preceding claims, in which the degassing heat treatment is carried out by heating the steel for a period of less than 4 hours. 6. Method according to one of the preceding claims, wherein the mechanical part comprises a mechanical tensile strength greater than 800MPa. 7. Method for manufacturing a mechanical part of hardened and tempered steel, and coated with an electrolytic or chemical coating, said method comprising: a stage of development and shaping of the part; a step of austenitization and quenching of the part thus obtained; - at least one step of surface treatment; a hydrogen degassing heat treatment step according to one of the preceding claims. 8. The method of claim 7, wherein the mechanical part comprises a mechanical tensile strength greater than 800MPa. 9. Method according to one of claims 7 and 8, wherein the mechanical part is capable of being put in static or dynamic tension during its use. 10. Method according to one of claims 7 to 9, in which the surface treatment step is chosen from at least one of the following treatments: degreasing operation, acid pickling operation, conversion treatment such as as phosphating, electrolytic surface treatment, such as zinc plating, cadmium plating, or chromium plating, chemical surface treatment. 11. Method according to one of claims 7 to 10, in which the interoperation time between a first hydrogenating operation and the start of the degassing heat treatment is greater than one week.