FR2531105A1 - Process for thermochemical treatment of steel and alloy articles and articles treated in accordance with the said process. - Google Patents

Abstract

The invention relates to metallurgy. The process forming the subject of the invention is of the type in which the surface of the articles is saturated with silicon by diffusion, with formation of a silicide layer and is then saturated with antifriction substances, and is characterised in that the saturation of the surface of the articles with antifriction substances is performed by starting with the tempering medium during the isothermal tempering of the articles. The invention applies especially to the treatment of the rubbing surfaces of steel and cast iron articles working in rubbing pairs of the shaft-smooth bearing type or sliding under conditions of limited greasing, high loads, low sliding speed and increased temperatures.

Description

 The present invention relates to metallurgy, more precisely the treatment of the surface of parts made of steels and alloys, and in particular relates to a process for thermochemical treatment of parts made of steels and alloys, as well as the parts treated in accordance with said method.

 The invention can be applied with maximum effectiveness to the treatment of the friction surfaces of steel and cast iron parts, working in friction friction pairs of the smooth shaft-bearing type or sliding under conditions of limited lubrication, heavy loads. , low sliding speed and increased temperatures, namely, the bearings of bearings subjected to heavy loads, the axes or plates (links) of conveyors, the friction assemblies of transporters of agricultural equipment.

 In addition, the invention can be applied in the manufacture and repair of various types of sealing sub-assemblies, guiding elements for tools for press forming, electrical contacts, etc.

Thermochemical treatment processes for parts of steels and alloys are widely known, in which the surface of the parts is saturated with nitrogen, carbon, chromium and other elements, which leads to an increase in the surface hardness of rooms. These processes ensure an elevation of the mechanical characteristics of the surface of the parts, including their resistance to abrasion wear. However, in
o Couples with friction in materials close to each other by their properties (steel on steel, steel on cast iron), the parts are susceptible to tearing and jamming and require permanent lubrication. This implies the use of costly anti-friction alloys in the couple.

 Among the thermochemical treatment methods, there is known a method providing saturation by diffusion of the surface of the parts with silicon, which is mainly applied as a method of increasing the hot resistance to chemical agents and of protecting against corrosion. . However, it has been proven that on steel and cast iron, siliconized hoses, endowed with porosity, soak up liquid lubricant well and do not give it anything to bronze with regard to anti-friction properties. In recent years, a series of methods have been developed for saturating the siliconized porous layer with anti-friction materials by boiling, rubbing, etc.

 A thermochemical treatment process for parts of steels and alloys is known in particular, in which the parts are saturated with silicon by diffusion, with the formation of a siliconized layer which is saturated with anti-friction materials. During siliciding at the prescribed regimes, it forms in the silicided layer of the pores opening onto the surface of the part. The subsequent saturation of the silicided layer with an anti-friction material (carbon) is carried out by case hardening of the parts, preferably by gas hardening. It is known that silicon, by dissolving in iron, stabilizes ferrite and prevents the dissolution of carbon in steel. This is why, during cementation, carbon does not dissolve in steel; the carbon deposited by pyrolysis on the surface of the part fills the pores of the silicided layer. This results in an improvement in the adsorption of liquid lubricants on the surface of the parts, which increases the anti-friction properties of the parts in the friction torques in the presence of liquid lubricant.

However, the parts having undergone siliciding, followed by carburizing by carbon, are in the annealed state; their mechanical properties are insufficient
and the carrying capacity of their surface is low.

In addition, when the parts thus treated are used in sub-assemblies with limited (periodic) lubrication, they do not provide sufficient anti-friction properties at low sliding speeds under heavy loads, at high temperatures, when the lubricant is volatilizes or burns quickly, under conditions of frequent starts, stops and reversals of the slip
It has therefore been proposed to create a process for thermochemical treatment of parts made of steels and alloys, in which the operation of saturating the surface of the parts with anti-friction materials would give these parts high mechanical properties and a large capacity. load, in combination with high anti-friction properties of their surface under conditions of limited lubrication - low sliding speeds and increased temperatures
The solution consists in a process of thermochemical treatment of parts in steels and alloys, of the type in which the surface of the parts is saturated with silicon by diffusion, with the formation of a siliconized layer, then is saturated with antifriction materials, -characterized, d after the invention, in that the saturation of the surface of the parts with the anti-friction materials is effected from the quenching medium, during the isothermal quenching of the parts.

 Thanks to the fact that the surface of the parts is saturated with the anti-friction materials takes place during isothermal quenching, the anti-friction material being in the quenching medium and contacting the hot surface of the part well fills the pores of the layer. siliconized, which ensures the retention of the lubricant on the surface in sufficient quantity to maintain high anti-friction properties of the surface when the supply of grease from the outside is limited, that the sliding speeds are low and that the temperatures are increased.

The diffusion of silicon in the steel (the alloy) causes a backflow of carbon deep into the part, in which a layer with increased carbon concentration is formed compared to that of the core. It follows that, during the isothermal quenching, a lower bainite or tempering martensite structure is formed under the siliconized layer, which gives the part high mechanical characteristics and a high load capacity.

 If the carbon content in the steel is low, it is advantageous to use, as a medium for isothermal quenching, a Wood alloy at a temperature of 150 to 2500 C. For the treatment of steel and alloy parts at high quenching penetration, it is advantageous to use as a quenching medium a regulator at a temperature of 390 to 500 C.

 Depending on the composition of the steel (alloy) and the structure of the silicided layer and the transition layers, it is possible to use, as quenching medium, oils for cylinders or tin at a temperature of 280 to 3500C, products that are used for this purpose.

 Since the use of lead as a quenching medium is undesirable due to the toxicity of lead baths, but since lead is also an anti-friction material which is particularly suitable for filling pores, it is advantageous to use as a quenching medium cylinder oils added with very fine lead powder.

 To improve the properties of use of the siliconized layer on the surface of the parts of which the saturation in anti-friction materials is effected by isothermal quenching, it is possible to introduce therein alloying elements ensuring the obtaining of a predetermined porosity in the layer. and pushing the carbon deep from the cemented layer into the part to form a layer of increased hardenability.

 Thus, to increase the mechanical strength by formation of intermetallic and carbide phases in the cemented layer, chromium can be introduced in sufficient quantity so that its rate in the layer preferably reaches up to 4 só at most, because the formation of 'Too much carbides would cause decarburization of the transition layer.

 It is advantageous to introduce aluminum into the siliconized layer, in order to increase its resistance to heat to chemical agents thanks to the formation of intermetallic compounds and complex oxides, in sufficient quantity so that its level in the layer reaches preferably up to a maximum of 2%, since carburizing with aluminum increases the porosity of the layer.

 The invention will be better understood and other objects, details and advantages thereof will appear better in the light of the explanatory description which will follow of various embodiments given solely by way of nonlimiting example.

 The process for thermochemical treatment of parts of steels and alloys which is the subject of the invention consists of the following.

 The surface of the parts is saturated with silicon by diffusion, the parts being placed in a mixture of powders containing the constituents known by specialists, in particular ground ferrosilicon, chamotte and a halogen activator, or else in a paste containing constituents analogues, in which they are heated to a temperature of 800 to 12000C and kept in residence at a predetermined regime. After cooling, the parts undergo the necessary cleaning and final machining at the prescribed dimensions. Then the parts are saturated with anti-friction materials.

 According to the invention, the saturation of the surface of the parts with the anti-friction materials takes place during isothermal quenching, starting from the quenching medium. For this, the quenching medium containing the anti-friction materials is heated to the isothermal quenching temperature, which is kept constant during the course of the quenching. The pieces are heated to the prescribed temperature, kept in stay for the temperature up to the prescribed depth, then they are placed in the quenching medium where they are kept in stay for the time fixed for the saturation of their surfaces by anti-friction materials and the completion of structural transformations occurring during quenching.

 The quenching medium used is a Wood alloy at a temperature of 150 to 2500C, in the case where the parts are made of low carbon steels or if it is necessary to obtain a structure of the martensite type in the layer of transition and at the heart of the room.

 In order to obtain intermediate structures of the bainite type in the transition layer and at the core, tin quenching medium at a temperature of 280 to 3500 ° C. or cylinder oils is used as the quenching medium.

 In the case where it is advantageous that the anti-friction material is lead, cylinder oils are used as quenching medium, into which a very fine lead powder is introduced.

 For alloys with high penetration of quenching, the quenching medium used is a regulator at a temperature of 390 to 5000C.

 The introduction of alloying elements into the siliconized layer is carried out by adding to the constituents of the mixture of powders or of the paste in which the saturation by diffusion takes place, silicon, ferrochrome, ferroaluminium, and other compounds containing chromium and aluminum. The quantity of added compounds is fixed at a value ensuring in the cemented layer a chromium content of up to 4% and an aluminum content of up to 2%.

 Below are described, for a better understanding of the invention, concrete but nonlimiting exemplary embodiments.

 Example 1.

 We have treated a bearing bush (support) of a rolling roller, working under conditions of limited lubrication, frequent reversals and stops, at low sliding speeds, under average loads of 6 to 8 NPa. Usually, such sockets are made of bronze with a low tin content, added with lead and zinc. This bushing was made of mild steel with a carbon content of 0.22 /%. The silicon sleeve was saturated by diffusion into a mixture of powders containing ferrosilicon, chamotte and a halogen activator.

This resulted in the formation on the active surface of the sleeve of a siliconized layer with a thickness of 0.4 to 0.5 mm, with pores arranged radially and opening onto the surface, which was determined by analysis. metallographic
This analysis also revealed the presence of a transition layer 0.2 to 0.25 mm thick with a steel structure and a carbon content of approximately 0.496,
After cooling, the sleeve was taken out of the mixture, it was cleaned and it was corrected to the dimensions specified in the drawing, specific extra thicknesses being left for this purpose during manufacture
Then the bush was heated to a temperature of 9000C and immersed in cylinder oil of the "Vapor" type, warmed up to 2200C, where it was kept for 30 to 40 min. The rollers were used for 2 years, from an average revision to the next, a roller having been mounted on a bronze bushing and a steel bushing treated by the process described.

 The surface of the bronze bushing had deep furrows, tears there were traces of bronze on the journal of the roller The surface of the steel bushing had small furrows, without traces of seizure.

The change in linear dimensions of the active surface of the steel bush was 2 to 2.5 times smaller than that of the bronze bush, and the change in taper, 3 to 4 times smaller The bronze bush had to be replaced, while the steel sleeve could still be used.

 Example 2.

 We treated the axes of a conveyor chain of a painting section in an automobile factory.

During service, the chain passes through a drying oven heated to 30 ° C. The axes were made of 0.43% carbon steel. According to the mass production method, they were hardened by high frequency induction and worked in couple with the eyelet of a plate also subjected to a heat treatment.
The silicon axes were saturated by diffusion, with simultaneous introduction of chromium and aluminum into the siliconized layer. This treatment was carried out in a paste based on organic binder, composed of ferrosilicon, recharging powder, containing chromium, aluminum powder and aluminum oxide, and a halogen activator.

 Metallographic analysis at the end of the axis revealed the presence of a siliconized layer 0.15 to 0.2 mm thick with pores emerging at the surface, as well as the presence of a transition layer. 0.1 mm thick with a carbon content of around 0.8%.

The spectral analysis of the layer, carried out using a high frequency spark generator, revealed the presence in the layer of about 3.5% chromium and 1.5 o / o of aluminum. .

 Attempts to introduce more than 4% chromium into the layer resulted in the decarburization of the transition layer, and the introduction of more than 2% aluminum greatly increased the porosity of the layer, its brittleness.

 After cleaning, the parts were subjected to saturation with an anti-friction material. For this, the parts heated by high frequency induction were immersed in a Wood alloy heated to 1800C.

The duration of stay was 20 to 30 min.

 The conveyor chain whose axes had been treated by the process described was used for 4.5 months. No lubricant was introduced into the torques formed by the machined shafts and the chain plates.

An axle removed after the specified time had a lapped surface without tears and no seizures, with insignificant wear. The other axes were still used for an extended time The axes produced in series, in the absence of periodic lubrication performed after each passage through the drying oven, were put out of use in 2 or 3 weeks, the linear wear up to 2 mm and even more.

 Example 3.

 The plain bearings of a stainless steel conveyor were treated, working without lubrication. in a medium constituted by technical carbon having a temperature of approximately 2000C. The bearings made of gray cast iron were saturated with silicon by diffusion with simultaneous introduction into the siliconized layer of 2.5% chromium. In a mixture of powders consisting of ferrosilicon, ferrochrome, quartz sand and a halogen activator, the presence of a greater amount of chromium (almost 7%), obtained by increasing the amount of ferrochrome in the mixture of powders, resulted in a large increase in the microhardness of the layer and tears appeared on the trunnions of the conveyor screw.

The bearings saturated by diffusion were heated in the oven to a temperature of 8500C and immersed in cylinder oil, reheated to 2500C and added with lead powder with particle size of about 0.5 au. The stay in oil was approximately one hour.
The service life of a bearing thus treated increased from 1.5 to 1.7 times, which correspondingly increased the duration of uninterrupted operation of the production chain.

 Example 4.

 The links of the belt of the bar lift of a potato harvester made of steel with 0.35 SS carbon were treated. The links were saturated with silicon by diffusion, with simultaneous introduction, into the silicided layer obtained, of 2.5% chromium, from a paste based on mineral binder, containing ferrosilicon, a chromium reloading powder. , a halogen activator and quartz sand.

The links thus saturated were then heated to 8800C and placed in a tin bath at a temperature of 2900 C. They were kept there for 0.5 hour. The links treated by the process described were used in pairs with steel bars for springs, containing 0.659 g of carbon and alloyed with manganese, without lubrication, with the presence of abrasive between the rubbing parts, for 100 hours.

The increase in the pitch of the elevator conveyor bars in the case of treated links, in combination with bars treated thermally by the ordinary method, was twice as small as in the case of untreated links, and, in case the use of bars having undergone a friction hardening treatment, the increase in said pitch was four times smaller.

 Example 5.

 The cheeks of a compression mold were treated to form an insulating material from a powder, made of steel containing 0.5% carbon, 1.2% chromium, 0.8% nickel and 0.4% molybdenum. The cheeks were saturated with silicon by diffusion, in a mixture of powders, with the formation of a siliconized layer 0.2 mm thick. After cleaning and rectification, the cheeks were heated to 9000C and immersed in a regulating bath having a temperature of 4000C. They were kept there for 15 min. The cheeks thus treated had a service life three times greater than that of the cheeks having undergone an ordinary quenching in oil.

 Thanks to the thermochemical treatment which is the subject of the invention, it is possible to substitute structural steel and cast iron for bronze, regulates, brass and sintered alloys in friction couples.

The anti-friction and wear resistance properties of "steel on steel" coupled friction sub-assemblies are significantly improved.

The increase in mechanical strength, load carrying capacity and resistance to tearing of parts, compared to non-ferrous metals and sintered alloys, makes it possible to reduce the size of the friction sub-assembly, the reduction of the amount of metal to be used and the increase in workloads
The surface compositions obtained by the treatment method reduce the running-in and lubrication costs of the parts.

 The treatment according to the invention is carried out in chamber, conveyor, rotary ovens, or with high-frequency induction heating, at a temperature of 800 to 12000C. Its duration is from a few minutes to 3-4 hours, depending on the kind of process

Claims (10)

 1. A method of thermochemical treatment of steel and alloy parts of the type in which the surface of the parts is saturated with silicon by diffusion, with the formation of a siliconized layer, then it is saturated with anti-friction materials, characterized in that the saturation of the surface of the parts by the anti-friction materials is carried out from the quenching medium, during the isothermal quenching of the parts.  2. Method according to claim 1, characterized in that the quenching medium is a Wood alloy at a temperature of 150 to 2500 C.  3. Method according to claim 1, characterized in that the quenching medium is a regulates at a temperature of 390 to 5000C.  4. Method according to claim 1, characterized in that the quenching medium is tin at a temperature of 280 to 3500C.  5. Method according to claim 1, characterized in that the quenching medium is' constituted by oils for cylinders.  6. Method according to one of claims 1 and 5, characterized in that one adds to the cylinder oils a lead powder of great finesse.  7. Method according to one of the preceding claims, characterized in that, during the silicon saturation by diffusion, is introduced into the siliconized layer of alloying elements ensuring the obtaining of a predetermined porosity of the siliconized layer and the deep discharge of carbon from the silicided layer into the part.  8. Method according to claim 7, characterized in that as an alloying element, chromium is introduced in the silicon layer in sufficient quantity so that its rate in the silicided layer reaches 4% at most.  9. Method according to one of claims 7 and 8, characterized in that as an alloying element is introduced into the silicon layer of aluminum in sufficient quantity so that its tawç in the silicon layer reaches 2% to the maximum.  10. Parts of steel or alloys, characterized in that they are treated in accordance with the process which is the subject of one of claims 1 to 9.