FR2503191A1 - Highly stressed engine component made of steel - is pptn. hardened to obtain tough core and is boronised on surface - Google Patents

Abstract

Machine parts, esp. i.c. engine components, which can be highly loaded and have a ductile high strength core and a harder, wear resistant and corrosion resistant case, are produced from a steel of compsn. up to 0.8% C, up to 3% Mn, less than 0.6% Si, up to 0.3% V, balance Fe by soln. heat treating, at 860-1000 deg.C and simultaneously boriding to form a Fe2B case of up to several tenths of a millimetre thickness, and then cooling at a rate of 2-50 deg.C/min. The process is useful in the prodn. of i.c. engine piston crowns, piston ring carriers, cylinder heads, cylinder bushes, crankshafts, camshafts, cam discs or gear wheels. Components having surfaces resistant to high temp. frictional wear and corrosive attack and having cores which are fatigue resistant are produced by a method which is less expensive than conventional methods.

Description

 The present invention relates to a method for manufacturing mechanical parts with a high load factor, in particular mechanical parts of an internal combustion engine, having a resistant and very solid core as well as a hard surface layer which is resistant to oil and to corrosion even at high operating temperatures.

 When during the operation of a machine, parts of it must be used at high working or operating temperatures and under continuously changing conditions, for example in the event of a sudden large increase followed by significant reduction in pressure, and possibly in a corrosive atmosphere, in particular when these parts cooperate in a final form and for example produce reversing movements, as is the case for many parts of an internal combustion engine, they must, for be able to perform their function for a sufficiently long time, be designed to be able to withstand the extraordinarily complex constraints which act on them.

 Mechanical parts of this type are generally made of highly alloyed steel which, after the corresponding shaping is heat treated by quenching and tempering, or hardened and / or subjected to a thermochemical diffusion treatment in products generating carbon, nitrogen or boron. The type of process to be used depends first of all on the purpose fixed for each mechanical part and on the constraints envisaged during its use, the costs of the processes of this type are relatively high, in particular for the mechanical parts which require not only a thermochemical treatment , but also a posterior or anterior heat treatment.

 An object of the present invention is to provide a process for manufacturing mechanical parts with a high load factor of the type mentioned above, making it possible to obtain surfaces of mechanical parts which are resistant to overheating of about 4000C, also to a impact or friction stress and also a deficiency in lubrication or a possible attack by a corrosive product and making it possible to obtain parts which can undergo stresses under high and variable loads without fatigue, said parts being able to be also obtained at costs much lower than those of the processes known hitherto.

This object is achieved by the present invention which in fact relates to a process for manufacturing mechanical parts with a high load factor, in particular mechanical parts of an internal combustion engine, having a resistant and very solid core as well as a surface layer. hard and resistant to wear and corrosion even at high operating temperatures, said process being characterized by the following stages
a) Firstly said parts are made from a base material, namely from a microalloy steel, poor in carbon, which has a content of up to about 0.2% by weight of vanadium as a carbide or carbonitride generator ;;
b) They are then subjected to a deboruration process during which, at a temperature corresponding to the solution temperature of vanadium above the transformation temperature of the base material in phase X of approximately 860-10000C, vanadium on the one hand dissolves in phase X found in the base material and, on the other hand, the boron, having diffused inside the surface of the mechanical parts, forms a surface layer of Fe2B of several tenths millimeters thick and,
c) Finally, they are subjected to a cooling step at a cooling rate of approximately 10 to 300 ° C. per minute, in order to avoid the appearance of cracks in the borated surface layer and this results in a fine-grained perlite structure in the base material as well as a hardening by structural hardening providing a core of mechanical part resistant to high solidity.

 With regard to the process of the invention, it should be noted that the use of microalloyed steels with a fine-grained structure or the like, as well as boriding or the like are known for the manufacture of mechanical parts. On the other hand, what is not known is to use, for a mechanical part, a low-carbon micro-alloyed steel having up to 0.2% by weight of vanadium, as starting material for the manufacture of a metal part, then boronizing this part at a temperature corresponding to the temperature of dissolving the vanadium and cooling it in such a way that one obtains in the base material a structure of fine-grained perlite and a hardening by structural hardening.

 A mechanical part obtained according to the method of the invention does not require any additional heat treatment and nevertheless exhibits at least the same good properties as a mechanical part obtained from a more highly alloyed, borided and hardened steel.

An important advantage of the method according to the invention lies in that the use of cheaper micro-alloy steel than a more highly alloyed steel already makes it possible to save on the cost of the material. Another reduction in the cost lies in the fact that, by the process of the invention, on the one hand, the very storytelling step of heat treatment by quenching and tempering necessary for obtaining the required properties is eliminated and, on the other On the other hand, the boronization process is the most economical thermal process with a view to obtaining a surface layer with very high hardness and a high resistance to wear and corrosion.

 Advantageously, the relatively simple heat treatment according to the invention makes possible an extreme hardening of the surface and a great solidity of the friend even for parts of complicated shape which cannot be subjected without disadvantages to several rapid heating and cooling in as part of a heat treatment after extreme thermochemical hardening of the surface by means of a relatively thin brittle layer.

 In addition, the heat treatment according to the invention gives a high solidity to the core of the parts and at the same time also allows the formation of a hard surface layer which is as thick as possible depending on the alloy constituting the starting material. chosen according to the invention.

 The mechanical parts are first of all obtained from a micro-alloy steel which generates fine grains. Micro-alloyed steels are normally characterized by small additions, approximately up to 0.2% by weight of aluminum, niobium, titanium, ziconium and vanadium, as generators of carbide or carbonitride.

 For the process of the invention, a steel is used above all with up to 0.2% by weight of vanadium as a carbide or carbonitride generator, since only vanadium has the property of passing completely into solution in austenite at the temperature below 10000C at which boriding can be carried out.

 Said micro-alloyed steel has, in addition to vanadium as the basic constituent of iron, preferably less than 0.4% by weight of carbon and, moreover, the usual alloy additives. Among these additives, mention should be made first of all of the manganese used in order to increase the solidity in an amount of up to 3% by weight and also a lower quantity of silicon of less than 0.6% by weight.

As other additives to the micro-alloyed steel used in the present invention, it is also advantageous to use molybdenum, nickel, copper and also optionally chromium with a content, for each of these additives, of between 0 and 0.5 % in weight.

 The addition of molybdenum is used above all to reduce the cooling rate necessary for the formation of perlite, which makes it possible to reduce the risk of formation of cracks in the borated surface layer.

Furthermore, it may be desirable for the steel used according to the invention to contain, in addition to vanadium, niobium as another generator of fine grains, however according to a content lower than that of vanadium.

 After obtaining the final shape of the mechanical part, it is subjected during the second step to a boriding process. To do this, the mechanical part is surrounded in a container of a boron-generating product.

 The part is then heated to a temperature of approximately 860-10000C, higher than the transformation temperature of the base material, said temperature corresponding to the temperature of dissolving vanadium in the base material, and it is maintained at this temperature. temperature for a period of several hours, this period being dependent on the thickness desired for the layer. It follows, on the one hand, a dissolution of the vanadium in the phaser of the base material and, on the other hand, a diffusion of the boron released by the boron generating product on the surface of the mechanical part and therefore the formation of 'a surface layer of Fe2B a few tenths of a millimeter thick.

 The thickness of the surface layer thus formed depends on the carbon and certain alloying constituents present in the steel.

 The low carbon content of the micro-alloyed steel used according to the invention promotes the formation of the borated layer. Other components of the steel alloy are also important for the production of gears and the adhesion of the boronated surface layer to the base material. Manganese and nickel have little influence on the thickness of the layer and the manufacture of gears. Nickel like carbon and silicon are pushed inward by the boron having diffused and accumulate under the surface layer of Fe2B. Vanadium would be detrimental to the thickness of the layer and to the quality of the gear only if its content exceeded about C.28 by weight. This cannot be the case for the micro-alloyed steel used in the present application.

 After the boriding process, the metal parts are subjected to a cooling process in a third step. The latter is mainly carried out at a cooling rate of 10 to 300 ° C. per minute. As indicated above with regard to the addition of molybdenum, fine-grained perlite structure formation and structural hardening quenching in the base material can also be achieved with an even lower cooling rate by means of small appropriate amounts of alloy additives. Any residual ferritic zones are either negligible or gathered inside the part so that they cannot cause any adverse influence on the qualities required for the part.

 A relatively slow speed permissible for the change of structure, at the same time avoids the appearance of cracks in the borated surface layer.

 By the process of the invention, machine parts are obtained which are resistant to oscillations which, thanks to their borated surface layer, have a hardness of about 2000 HV at room temperature and are also very resistant to wear and corrosion. higher temperatures, for example 4000C.

 The process according to the invention is particularly suitable for the production of high load factor internal combustion engine parts, such as piston heads with fully or only boronised surfaces in the groove areas of the piston rings and piston bottoms. , or segment supports with surfaces fully or only in the groove areas of the segments. In addition, the method according to the invention is suitable for the manufacture of cylinder heads and liners with borated surfaces on the side of the combustion chamber.

 It is also possible to use the method according to the invention for manufacturing crankshafts, camshafts or discs of which all or part of the surface is to be borided.

 It is also possible to manufacture pinions for drive mechanisms which may have, at least in the engagement zone, a borated surface layer. The process according to the invention can be used for the production of liners with borated cylindrical surfaces. The use of the method according to the invention is not limited to the manufacture of the mechanical parts mentioned above but extends to other mechanical parts which must satisfy the constraints of the same type.

Claims (9)

 1) - Method for manufacturing mechanical parts with a high load factor, in particular mechanical parts of an internal combustion engine, having a resistant and very solid core as well as a hard surface layer which is resistant to wear and even corrosion at high operating temperatures, said process being characterized by the following steps:  a) - firstly said parts are made from a base material, namely from a microalloy steel, poor in carbon which has a content of up to about 0.2% by weight of vanadium as a carbide or carbonitride generator,  b) - they are then subjected to a boriding process during which, at a temperature corresponding to the solution temperature of vanadium above the transformation temperature of the base material in phase, of approximately 860-10000C, vanadium on the one hand dissolves in phase X found in the base material and, on the other hand boron, having diffused inside the surface of the mechanical parts, forms a surface layer of Fe2B of several tenths of a millimeter thick, and,  c) - they are finally subjected to a cooling step at a cooling rate of approximately 10 to 300 ° C. per minute, to avoid the appearance of cracks in the borated surface layer and this results in a fine-grained perlite structure in the base material as well as a hardening by structural hardening obtaining a core of mechanical part resistant to high solidity.  2) - Method according to claim 1, characterized in that the mechanical parts are obtained from a steel generator of fine grains which contains, in addition to vanadium as carbide generator or carbonitride as base of iron, less than 0, 4% by weight of carbon as well as usual alloy additives, in particular up to 3% by weight of manganese to increase the solidity, less than 0.6% by weight of silicon, as well as other additives of alloy such as molybdenum, nickel, copper and possibly chromium, for each of these latter additives, with a content of between 0 and 0.5% by weight.  3) - Method according to any one of the preceding claims, characterized in that the steel comprises in addition to vanadium niobium as another generator of fine grains according to a content by weight lower than that of valadium.  4) - A method according to claim 1, characterized in that said method is used for the manufacture of piston heads of an internal combustion engine whose surface is entirely boronized or is only boronized in the zone of the grooves of segments and the bottom of piston  5 - A method according to claim 1, characterized in that said method is used for the manufacture of segment supports for piston of an internal combustion engine with a fully borated surface - or only in the region of the segment grooves.  6) - Method according to claim 1, characterized in that said method is used for. the manufacture of cylinder heads of internal combustion engines with boron surfaces on the side of the combustion chamber.  7) - Method according to claim 1, characterized in that said method is used for the manufacture of liners with cylindrical cylindrical surfaces.  8) - Method according to claim 1, characterized in that said method is used for the manufacture of crankshafts, camshafts or discs with a surface in whole or in part borurate.  9) - Method according to claim 1, characterized in that said method is used for the manufacture of pinions of drive mechanisms with at least one surface layer boronized in the engagement zone.