FR2649415A1 - PROCESS FOR MANUFACTURING PARTS WITH HIGH MECHANICAL CHARACTERISTICS FROM UNTREATED STEEL - Google Patents

Abstract

Process for the manufacture of components from untreated steel, comprising a succession of thermal treatment and forming operations, characterised by the following successive operations: - heating the steel to a temperature above the transformation (austenitisation) point, - isothermal quenching in a fluidised bed bath immediately following the austenitisation heating, endowing the steel with a bainitic structure, and - forming of the components at mild heat. t

Description

The present invention relates to a method of manufacturing parts to

  high mechanical properties from untreated steel, including a succession of treatment operations

thermal and forming.

  Conventional methods for manufacturing steel parts treated from untreated steel in bars, strips or rings, for example springs, stabilizer bars, elastic fasteners

  railway tracks, include a large number of operations.

  As an example, the following successive operations of a conventional spring manufacturing process will be given: - unwinding of the crowns, dressing, peeling and grinding, - austenitization heating in an atmosphere furnace, - forming hot, - oil quenching, - tempering in an oven, - cooling, - blocking, cooling to room temperature, - shot peening, - tempering after peening, - protection (painting or plasticization),

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  - baking or crosslinking, - cooling to room temperature,

- control.

  This conventional process in particular comprises a large number of temperature variation phases, each of which involves considerable energy expenditure and costly installations. Hot forming and oil quenching are also operations that take place under difficult working and safety conditions. Finally oil quenching is an operation generating pollution by the release of harmful fumes and vapors and by washing which

  parts must be submitted after oil quenching.

  The process according to French patent application no. 2 391 789 already partially remedies the drawbacks listed above. This process, intended in particular for the manufacture of rail fasteners, comprises the following stages: - cold forming or preforming of the parts,

  - austenitization heating of the rooms, preferably in -

  a fluidized bed or induction bath, - quenching of the parts in a fluidized bed bath, - tempering of the parts preferably in a fluidized bed bath, - shot blasting, - possibly final cold forming of the parts,

- protection.

  However, this known method still has a certain number of drawbacks. In particular, it involves two temperature rise operations (austenitization heating before quenching, tempering after quenching). Furthermore, since the parts undergo cold forming or at least preforming, before any heating, it is impossible to carry out this heating on continuous material,

for example in strip.

  The subject of the present invention is a process for manufacturing parts made of treated steel which makes it possible to produce parts of equivalent quality or even better than that obtained by lower cost.

the usual procedures.

  The process according to the invention for manufacturing parts from untreated steel comprises the following successive operations: heating the steel to a temperature above the transformation point (austenitization), - isothermal quenching in a fluidized bed bath , immediately following the austenitization heating,

- warm parts forming.

  Contrary to a prejudice according to which only a formation after austenitization heating and martensitic quenching made it possible to obtain parts with high characteristics, it turned out surprisingly that the parts obtained by lukewarm forming after isothermal quenching (bainitic structure) had mechanical and geometrical characteristics (in particular fatigue strength, stress corrosion, precision, tighter tolerances)

markedly improved.

  In the context of the invention, the austenitization heating can advantageously be carried out in a fluidized bed bath or by induction, on continuous products (strips, crowns) or on blanks or pieces. It is also possible, within the framework of the invention, to carry out a cold preforming, before austenitization heating, to give the parts a shape different from their final shape, in which case the warm forming carried out after heating isothermal austenitization and quenching is intended above all to give the parts a more precise geometry and higher mechanical characteristics (thermomechanical treatment

  lukewarm such as blocking in the case of springs).

  Finally, after lukewarm forming, it is possible, within the framework of the invention, without further temperature rise of the parts, to a shot peening with lukewarm lukewarm and to a lukewarm protection (painting with cooking, plasticization), in

  benefiting from the residual heat of the rooms.

  The numerous advantages of the process according to the invention can be classified as follows:

ENERGY SAVING

  The conventional hot forming process before quenching requires heating of the parts as a whole to a temperature much higher than the transformation point to take account of the cooling which the parts undergo during the operation of

  hot forming and during transfers.

  In the process according to the invention, since the quenching operation (which gives the steel a bainitic structure) immediately follows the austenitization heating operation, the heat losses are very low and the heating temperature

  lower, since it is only used for tempering.

  According to the process according to the invention, it is moreover only necessary to heat, therefore to treat the working part of the

  metal, which constitutes an additional energy gain.

  It should be noted that in this case, the untreated part of the metal plays a complementary role for quenching and following

  operations, by heat diffusion.

  The choice of induction or fluidized bed heating makes it possible to carry out heat exchanges in reduced spaces, which significantly reduces the losses of the installation in operation and the start-up and shutdown times of the installation, therefore the losses who as a result. The replacement of oil quenching by fluidized bed quenching eliminates the need to suck oil fumes and those to wash and dry the parts. In addition to the energy consumed by these operations, according to the invention, the additional heating necessary to compensate for the savings is saved.

  losses due to oil quenching and washing operations.

  The replacement of the two quenching and tempering operations of the usual processes by isothermal quenching allows other significant energy savings. Thus, the energy to bring the part to the tempering temperature is removed. The holding time at the isothermal quenching temperature represents less than half of the holding time at the tempering temperature according to the usual methods. Most of the energy induced during austenitization is returned to the fluidized quench bath

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  isothermal, which allows this energy to be reused for example for

heating of buildings.

  At the end of the isothermal quenching, the parts with bainitic structure are at a temperature higher than the martensite point, that is to say at a temperature of the order of 280-350 C depending on the steel grades, and the residual energy they contain can be used as the following operations are carried out

  lukewarm (forming, blocking, prestressing, gearing, protection).

  The process according to the invention thus makes it possible to recover the

  most of the energy provided during austenitization.

  IMPROVED PRODUCT QUALITY

  Warm forming gives parts better precision and increases the mechanical properties of the finished product. by notably favoring the blocking of dislocations induced during forming, in particular because of the increase in mobility of carbon atoms linked to temperature. Rapid heating followed by

  isothermal quenching eliminates any risk of decarburization.

  The successive operations of the process according to the invention are carried out in a precise order making it possible to make the most of the energy stored in the material. The choice of temperature for each operation aims to optimize the beneficial effects, in particular on a metallurgical level for shot peening and pre-forming (or blocking), and on the electrochemical level for anti-corrosion protection. Unlike the usual methods, the method according to the invention does not require the formation of a so-called bonding layer generally obtained by chemical combination (phosphating) with the metal to be protected, the formation of this bonding layer causing a

  decrease in the surface mechanical characteristics of the parts.

ECONOMY OF MATERIALS

  The elimination of the oil quenching operation and its replacement by quenching in a fluidized bed bath saves the consumption of petroleum products such as quench oil and

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  washing products and water. Insofar as the process according to the invention provides better quality parts, it makes it possible to reduce the quantity of material necessary for a same function. On the other hand, it is possible, according to the invention, to use less alloyed steels to obtain parts having

comparable characteristics.

  The possibility of using raw rolling and crown metal results in a significant reduction in material losses and

of the price of the raw material.

  SURFACE AND INVESTMENT SAVINGS

  The process according to the invention reduces the number of operations, in particular temperature rise and the temperature maintenance times. The facilities required for the installation

  of the process are therefore of reduced dimensions.

  By way of example, the investments and the surface area required for an installation using the method according to the invention, compared to a conventional installation for producing suspension springs for motor vehicles, are divided by a

factor of about five.

  IMPROVED FLEXIBILITY OF THE INSTALLATION

  The process according to the invention makes it possible to considerably reduce the work in progress. Thus, for a production of 600 suspension springs per hour, the outstanding amount is only 40 springs for the process according to the invention, while it is 1,200 in the conventional process. A few minutes are enough to empty the installation of its parts for the installation

  work of the process according to the invention.

  IMPROVED WORKING AND SAFETY CONDITIONS

  Thanks to the elimination of oil quenching and thanks to lukewarm forming, the process according to the invention eliminates all harmful fumes and vapors and makes work much less

annoying.

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  The cleanliness of the facilities for implementing the process according to the invention and their reduced dimensions provide a quality of working atmosphere and productivity far superior.

to those of the conventional process.

POLLUTION REDUCTION

  The process according to the invention eliminates all the releases (fumes, vapors, detergents, etc.) from the usual process of

  hot forming and oil quenching.

  The sequence of operations will be described below, with indication of the temperatures and times required, for a conventional method of manufacturing springs and, by way of comparison, an example of the method according to the invention for the manufacture of springs of comparable characteristics intended for suspensions of

motor vehicles.

CLASSIC PROCESS

STEEL 60 S CS

  Continuation of operations Temperature Time 1 - coils unwinding 2 - cutting dressing 3 - peeling rectification 4 - heating in atmosphere oven 950 15 '5 - superior hot forming 800 6 - oil quenching (cooling) 50 7 - tempering in oven 450 60 '8 - cooling 250 9 - blocking or pre-stressing or pre-shaping 250 10 cooling to room temperature approx. 20 11 - shot peening 12 - tempering after peening approx. 220 13 - protective paint 14 - baking paint approx. 200 3 15 - cooling to room temperature approx. 20 16 - tracking control

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  PROCESS CONFORM TO THE INVENTION STEEL 55 C3 or 50 CV4 Continuation of operations Temperature Time 1 - unwinding crowns 2 - heating (induction or fluidized bed approx. 850 15 '3 - isothermal quenching (cooling) approx. 320 15' 4 - warm forming 300 - pre-stress blocking with warm and control 280 6 - pre-stress shot blasting 260 7 - protection (painting, plasticization) 220

Claims (6)

  1. A method of manufacturing parts from untreated steel comprising a succession of heat treatment and forming operations, characterized in that it comprises the following successive operations: - heating the steel to a higher temperature at the transformation point (austenitization), - isothermal quenching in a fluidized bed bath, immediately following the austenitization heating, (giving the steel a ba-initial structure) and - warm parts forming.   2. Method according to claim 1, characterized in that a cold preforming is carried out, before austenitization heating, to give the parts a shape different from the final form.   3. Method according to claim 1 or 2, characterized in that the austenitization heating is carried out in a fluidized bed bath. 4. Method according to claim 1 or 2, characterized by the   the fact that the induction austenitization heating is carried out.   5. Method according to any one of the claims   previous, characterized by the fact that a blocking of lukewarm pretension.   6. Method according to any one of the claims   above, characterized by the fact that a warm-up shot peening is then carried out while benefiting from the heat residual parts.   A method according to any of the claims   above, characterized by the fact that protection (painting, plasticization) of the warm parts is then carried out   benefiting from the residual heat of the rooms.