PROCESS OF PRODUCTION

[0001] The present invention relates to rings of cast nitridable steels for pistons of internal combustion engines. The present invention proposes the obtainment of piston rings of nitridable steels utilising processes of casting with alloys having relatively high contents of nitrogen, comprising the composition of the alloy and the process of manufacture .

STATE OF THE ART [0002] In recent years, having the objective of minimising the emission of environmentally harmful gases, and of particulate materials and/or other GHGs (greenhouse gases) , a series of technologies has been incorporated into engines. The reduction in emissions of gases is related, inter alia other factors, to the increase in the thermal performance of the engine and, consequently, the reduction in the specific consumption of fuel. [0003] As a consequence, engines are developing greater power per volume of displacement of the piston in the cylinder. Combustion engines are working under greater mechanical stresses, at higher rotation and higher combustion temperature. In this manner, the components thereof must be likewise dimensioned to support these harsher operating conditions with the objective of ensuring both the reliability of the assembly and the maintenance of the expected working life. This greater operational stress is translated, likewise, into greater stress experienced by the components, inter alia, the piston and the rings associated with the piston. With greater rates of compression, combustion pressure, temperature and rotation, the rings also exert a greater pressure upon the piston and upon the cylinder walls, leading likewise to greater wear or fatigue of the rings and, consequently, increasing the play between the ring and the cylinder and causing problems related to the wear of the cylinder liner and/or of the piston itself, oil leakage, increase in fuel and/or oil consumption and, even, breakage of the ring. [0004] The materials commonly utilised for manufacture of piston rings are cast irons and stainless steels. The process of manufacture of the rings from cast iron may be summarised in the following stages: (1) melting of the alloy and correction of composition; (2) pouring into sand moulds or centrifuging; (3) heat treatment of the rings obtained; (4) machining to define the final dimensions; (5) surface treatments such as nitridation or coatings to obtain a surface having high hardness. There is a technical limitation related with the mechanical strength on the utilisation of cast iron rings in engines under high loading or in engines requiring small cross-sectional dimensions of the rings. [0005] For these applications wherein the loads are high, the cast iron rings are substituted by rings of martensitic stainless steels. These are obtained through mechanical shaping of wires of drawn stainless steel, for example as revealed in the document US 20070187002 ( ^Piston ring excellent in resistance to scuffing, cracking and fatigue and method for producing the same, and combination of piston ring and cylinder block' ) , and they pass through the aforedescribed stages (4) and (5) . However, there are limitations on the dimensions of the rings which may be produced by this process of manufacture, in addition to technical disadvantages in the definition of geometric shape, requiring a more complex process of manufacture and finishing. In order to satisfy the diverse dimensions it is necessary to maintain a large stock of material, increasing the costs involved. [0006] A relevant technical aspect linked to the process of obtainment of rings by mechanical shaping of drawn stainless steel wires is the presence of microcracks flowing from decohesion between carbides and matrix caused by the cumulative cross-sectional reductions in the stages of rolling and drawing, notably in the regions of the microstructure wherein there is agglomeration of carbides. These problems are resolved by stainless steel rings produced through casting processes similar to those utilised for the production of cast iron rings.

[0007] The document US 20120090462 (^Nitratable steel piston rings and steel cylindrical sleeves , and casting method for the production thereof ) describes an alloy and possible casting processes for the production of piston rings of nitridable steels. In this invention the principal elements in the composition of the alloy proposed are: silicon (Si) with contents from 2 to 10 % by weight, chromium (Cr) with contents between 4 and 20 % by weight, and nickel (Ni) between 2 and 12 % by weight. In this material the high content of Cr, as found in the stainless steel wires, accounts for the good nitridability . Subsequent to the obtainment of the cast rings, the process described presents the following stages: heat treatment with austenisation (above the Ac3 line of the alloy) and quenching (preferably in oil), tempering (between 400 °C and 700 °C) , and nitridation. [0008] The document US 20120090462 describes as principal concept thereof the effect of silicon on the reduction in the temperature of commencement of solidification of the alloy (liquidus temperature) , which would facilitate the process of manufacture. However, even with high contents, such as the content of 3 % utilised in the example of embodiment of the invention, thermodynamic calculations demonstrate that the silicon provides a small contribution in the decrease in the liquidus temperature of the alloy (approximately 30 °C) .

[0009] In addition thereto, the microstructure of the material obtained according to the description in the document US 20120090462 presents coarse eutectic carbides formed during solidification (Figure 1). Coarse carbides facilitate the nucleation and propagation of cracks in the steels. The presence of these carbides in the final microstructure of the rings greatly restricts the employment thereof in conditions of high structural stress by virtue of the fact that it diminishes the fatigue strength occasioned by the initiation and propagation of cracks.

[0010] In the material obtained according to document US 20120090462, these coarse carbides formed during solidification are stable, remaining in the microstructure in all the stages of production of the material, including the final product. Figure 1 shows the microstructure of a material obtained according to the description of the example of embodiment of the document US 20120090462 in the as-cast state and subsequent to tempering (final stage of the development of the microstructure) . It may be observed that the coarse eutectic carbides precipitated in a continuous manner in the interdendritic regions are not altered through the application of the cycles of heat treatment, including the tempering. These eutectic carbides are formed as a consequence of the separation of elements of alloy, principally chromium and carbon, in the interdendritic regions during the solidification of the primary austenite. [0011] Figure 2, obtained by means of the use of the thermodynamic modelling software ThermoCalc, presents the sequence of solidification of the material having a composition described in the document US 20120090462 and shows that the eutectic carbide is of the M ₇ C ₃ type. The thermodynamic analysis indicates that the M ₇ C ₃ carbides are stable in the band of temperatures of heat treatments described in this document. This stability of the M ₇ C ₃ carbides is not observed in similar alloys having low contents of silicon (contents of up to 1 %) .

[0012] Figure 3 shows the microstructure of a material obtained according to the description of the example of embodiment of the document US 20120090462 in the tempered state, subsequent to Villela and Murakami differential chemical attacks. These chemical attacks permit differentiating the carbides of the M ₇ C ₃ and M2 ₃ C ₆ types, wherein the carbides of the M ₇ C ₃ type are light in colour and those of the M2 ₃ C ₆ type present colouration. It may be observed in Figure 3 that the eutectic carbides are of the M ₇ C ₃ type (carbides light in colour) and, consequently, have not undergone transformation. It is also noted that these carbides do not undergo significant alteration of morphology in relation to the as-cast state.

[0013] Continuous networks of eutectic carbides, such as the carbide M ₇ C ₃ , are preferential sites for the nucleation and propagation of fatigue cracks and, in this manner, the presence thereof implies detriment to the performance of piston rings. DESCRIPTION OF THE FIGURES

[0014] Figure 1 presents a micrograph showing the microstructure (a) in the as-cast state and (b) subsequent to tempering at 600 °C of the material produced according to the description of the example of embodiment of the document US 20120090462. [0015] Figure 2 shows the graph of the development of the fractions of phases as a function of the temperature during the solidification of the material having a composition described in the document US 20120090462.

[0016] Figure 3 presents a micrograph showing the microstructure subsequent to tempering at 600 °C of a material produced according to the description of the example of embodiment of the document US 20120090462, subsequent to Villela and Murakami chemical attack.

[0017] Figure 4 presents a micrograph showing the microstructure of the material obtained in the example of embodiment of the present invention in the as-cast state.

[0018] Figure 5 presents a micrograph showing the microstructure of the material obtained in the example of embodiment of the present invention subsequent to the heat treatment at 1040 °C and quenching in still air .

[0019] Figure 6 presents a micrograph showing the microstructure subsequent to tempering at 600 °C of a material produced according to the description of the example of embodiment of the present invention, subsequent to Villela and Murakami chemical attack.

[0020] Figure 7 shows a graph of the development of hardness of the material obtained in the example of embodiment of the present invention in the different stages of the process of manufacture. DETAILED DESCRIPTION OF THE INVENTION

[0021] The stainless steels contemporaneously utilised for piston rings present in the microstructure thereof eutectic carbides formed during the stage of solidification of the alloys. These eutectic carbides are relatively large and continuous and act as stress concentrators, as revealed by Tanaka et al (Tanaka, S., Yamamura, K. and Oohori, M., Excellent Stainless Bearing Steel (ESI). Motion & Control. May 2000. Vol. No. 8, pp. 23 - 26), and sites for nucleation and propagation of cracks. In the process of manufacture of the rings through mechanical shaping of steel wire these carbides are broken down mechanically and distributed in the microstructure. As a result they present cracks and decohesion between carbides and matrix .

[0022] In the process of manufacture by casting described in the document US 20120090462 there occurs the formation of these eutectic carbides, they being stable in all the stages of manufacture of the piston ring. The formation of these carbides and their permanence in the microstructure of the final product has been proven through the reproduction of the state of the art in accordance with the example of embodiment presented in the document US 20120090462, as shown in Figures 1, 2 and 3. [0023] The elimination or the reduction of these eutectic carbides in the microstructure of these materials is fundamental for improvement in the fatigue strength of piston rings. That is to say, a microstructure having carbides being small and distributed in a homogeneous and discontinuous manner would lead to a significant increase in the fatigue strength of the material. [0024] The reduction in the fraction of carbides in the microstructure of stainless steels may be realised through the reduction in the contents of C and Cr, as revealed by Tanaka et al . , op. cit. However, in order to maintain the hardness and the corrosion resistance, the contents of these elements cannot be disproportionately reduced. As revealed by Tanaka et al . , op. cit., even reducing the content of C to 0.7 % by weight and of Cr to 13 % by weight the eutectic carbides are observed in the microstructure. This finding is corroborated by thermodynamic simulations employing the Scheil model. Steels alloyed with C and N present greater fatigue strength, increase in the strength of the martensite (Tanaka, et al . , op. cit.), and greater abrasion resistance, as revealed by Berns (Berns, H., Increasing the wear resistance of stainless steels. Mat.-wiss. u. Werkstofftech. 6, 2007, Vol. 38).

[0025] The present invention describes an alloy and the process for the obtainment of cast parts suitable for use in components of internal combustion engines, preferentially piston rings of nitridable steels having a relatively high content of N and having low contents of Si. In this invention, the quantity of M ₇ C ₃ eutectic carbides is reduced to fractions of less than 2.0 % by weight through the substitution of part of the C by N and, furthermore, these eutectic carbides are transformed into M2 ₃ C ₆ carbides during the stage of heat treatment. This transformation is accompanied by an alteration in the morphology of the carbides, reducing the maximum size thereof and their continuity in the microstructure of the material. This microstructural alteration of the material of the present invention is due to the chemical composition (low contents of Si and relatively high contents of N) , and appropriate heat treatments . [0026] Table 1 shows the limits of composition for the material of the present invention.

Table 1: Limits of composition of the alloy

Concentration in % by weight

Element

min . max .

C 0.60 0.90

Cr 9.00 20.00

Fe Balance Balance

Mn 0.20 1.20

Mo 0.50 1.50

N 0.06 0.20

Si 0.20 1.90

V 0.10 0.35

P - 0.05

S - 0.05

[0027] In the present invention different types of casting processes may be utilised for the production of parts appropriate for the obtainment of piston rings. There may be utilised moulds of green sand, typical of casting for cast iron, or sand-resin. Furthermore, there may be obtained rings having dimensions proximate to the final dimensions or tubes from which the rings are obtained by transverse cutting. Optionally, the centrifuging process may be utilised for production of tubes from which piston rings are obtained. In the centrifuging process the molten alloy is poured into a centrifuge having a metal body, preferentially having ceramic coating in the internal part.

[0028] The raw materials utilised in the process of the present invention may be scrap of low alloy steels, recycled production, and ferroalloys, not being limited to these materials. [0029] The melting of the material for preparation of the alloy is realised preferentially in induction furnaces. The furnace utilised for the preparation and melting of the alloy may have atmosphere and pressure control, it not being, however, strictly necessary. The pouring of the alloy into the moulds must be realised at temperatures of between 1500 °C and 1650 °C.

[0030] Subsequent to cooling, the parts are removed from the moulds and the system of casting channels, should it exist, is removed, separating the rings or tubes. The rings or tubes are annealed at temperatures of between 600 °C and 800 °C. During this stage of heat treatment there occurs the transformation of the M ₇ C ₃ eutectic carbides formed in the solidification of the material into M2 ₃ C ₆ . This transformation is accompanied by alteration in the morphology of the carbides, there being a reduction in sizes and in continuity of the eutectic structure. This alteration in the morphology leads to an improvement in the fatigue strength of the material, being an important property for application in piston rings.

[0031] Subsequent to annealing, the parts pass through an initial machining. Subsequently, heat treatments are carried out on the parts, having the objective of obtaining microstructures and properties appropriate for the rings. The heat treatments consist in heating the parts at temperatures of between 1000 °C and 1100 °C (austenitisation) and quenching in air (forced or still) or in oil. These heat treatments may be realised in furnaces without a controlled atmosphere, but preferentially having a controlled atmosphere to prevent alterations in composition of the surfaces of the parts. Subsequent to the heat treatment the parts must pass through a stage of tempering at temperatures of between 500 °C and 700 °C. [0032] As is customary for piston rings of nitridable steels, surface treatments are realised having the objective of obtaining high hardness (of approximately 1000 HV) , low friction against the lining, and wear resistance. The nitridation is a process of surface treatment commonly utilised on piston rings, being realised preferentially by gas, by plasma or in a salt bath. [0033] Optionally, anti-wear coating may be applied upon the face of contact with the cylinder. For application of this coating the processes customarily applied on piston rings may be utilised. [0034] The present invention presents cast rings of nitridable steels having low contents of silicon and high contents of nitrogen, having the following principal characteristics: they are appropriate for common casting processes, they do not present microstructural characteristics detrimental to the mechanical properties of the piston rings, such as coarse eutectic carbides typical of the cast stainless steels having high silicon content, and they do not present microcracks or decohesion between carbide and matrix, typical of the mechanically shaped stainless steels. These characteristics, in addition to representing an economic benefit, signify a greater capacity of resistance to breakage during manufacture and greater fatigue strength of the rings subjected to high loading.

EXAMPLE OF EMBODIMENT OF THE INVENTION

[0035] A charge containing low alloy steel, recycled production, and ferroalloys containing elements of alloys of the composition was melted in an induction furnace without a chamber for pressure control. To prevent the excessive oxidation of the molten metal there was utilised an injection of argon onto the surface of the bath. Subsequent to the complete melting of the charge and verification of temperature, the molten metal was poured at 1530 °C into a mould of sand-resin. During the pouring samples were taken for analysis of chemical composition, the result whereof is shown in Table 2.

Table 2: Chemical composition measured on sample

obtained in the example of embodiment

[0036] A tube was cast having internal and external diameters of dimensions proximate to those of a piston ring. Following cooling, the tube was cleaned and passed through an annealing stage at 750 °C. Subsequently, initial machining and cutting of the tube into rings was carried out. Then the rings were subjected to a heat treatment with heating to 1040 °C and cooling in still air, and to a tempering treatment at 600 °C. Finally the rings were nitrided employing gas nitridation.

[0037] Figure 4 shows the structure of the material obtained in the as-cast state and Figure 5 shows the structure of the material following the heat treatment of quenching and tempering. In the as-cast structure (Figure 4) coarse and continuous eutectic carbides formed in the interdendritic region may be observed, however through the chemical composition and the thermal treatments utilised in the present invention these eutectic carbides formed during solidification (M ₇ C ₃ type) are transformed into carbides of the M2 ₃ C ₆ type during the stage of heat treatment. This transformation is accompanied by an alteration in the morphology of the carbides, reducing the maximum size of the carbides in the microstructure of the material. This microstructural alteration of the material is due to the chemical composition (low contents of Si and relatively high contents of N) and appropriate heat treatments. Figure 6 shows the microstructure of a material obtained in the present invention following tempering and being passed through Villela and Murakami chemical attack. It may be observed in Figure 6 that the carbides in the interdendritic region are of the M2 ₃ C ₆ type (region with colouration) and do not present continuity, substantiating the aforedescribed transformation .

[0038] The M2 ₃ C ₆ carbides have a hardness lower than the M ₇ C ₃ carbides, as revealed by Li, Yefei, et al (Li, Yefei, et al . The electronic, mechanical properties and theoretical hardness of chromium carbides by first- principles calculations . Journal of Alloys and Compounds. 2011, Vol. 509, pp. 5242-5249) , which renders the M2 ₃ C ₆ carbides less fragile in relation to the M ₇ C ₃ carbides. This characteristic of the M2 ₃ C ₆ carbides renders the material obtained tougher than the materials of the state of the art possessing M ₇ C ₃ carbides . [0039] Figure 7 shows the development of the hardness of the material obtained during the process of manufacture. Whilst the annealing does not appreciably reduce the hardness, this stage is fundamental for the determination of the properties of the material, taking into account the microstructural alterations with transformations of the carbides as aforedescribed . As anticipated, the quenching increases the hardness of the material with the formation of martensite. The resulting hardness of the material obtained subsequent to tempering is 600 HV, being substantially greater than the result of 420 HV found in the state of the art

(US 20120090462) . This result of greater hardness of the material is due to the chemical composition

(relatively high content of N) and to the heat treatments realised.

[0040] The obtainment of rings of cast nitridable steels having low contents of Si and high contents of nitrogen has overcome the limitations imposed by the processes and/or materials of the present state of the art, these being: limit of loading whereunto the rings of cast iron may be subjected, dimensional limits of the process of shaping wires of stainless steels, and formation of coarse carbides in the alloys proposed in the document US 20120090462.