MX2008003450A - Corrosion resistant conduit systems with enhanced surface hardness - Google Patents

Abstract

A corrosion resistant conduit system with enhanced surface hardness is made by forming a pre-assembly comprising a stainless steel conduit and a stainless steel fitting, or a portion of such a fitting, and then subjecting the pre-assembly to low temperature carburization.

Description

SYSTEMS OF RESISTANT DUCTS TO CORROSION WITH IMPROVED SURFACE HARDNESS BACKGROUND OF THE INVENTION Low temperature carburization ("LTC") of stainless steel has been described in a number of publications including U.S. Pat. No. 5,92,282, EPO 0787817, Japanese Patent Document 9-14019 (Kokai 9-268364), US Pat. No. 6,165,597 and Patent of E.U.A. No. 6,547,888. The description of these documents is incorporated herein by reference. In this technology, a workpiece is brought into contact with a gas containing carbon at an elevated temperature of less than 538 ° C. As a result, high concentrations of diffuse elemental carbon on the workpiece surfaces without formation of carbide precipitates. The result is that the hardness of the surface and the corrosion resistance of the work piece are significantly increased. This technology has been used to harden the surface of the components of the splint-based adaptations used to form a stainless steel conduit system. The surface hardness of the duct itself, however, is not normally done.

According to this description, the stainless steel conduit used in said system is the cover itself hardened by carburization at low temperatures, this hardening of the cover being made by previously connecting the ferrule and optionally other parts of said accessory to the stainless steel conduit and then submitting the preassembly thus formed to LTC. The technology of this disclosure can also be used more widely to apply any diffusion-based surface treatment process for shaped metal articles made of metal work pieces by pre-assembling these work pieces and then subjecting the pre-assembled assemble thus formed to the process of surface treatment based on diffusion. Therefore in a particular embodiment, this description provides a process for forming a corrosion resistant conduit system with improved surface hardness in which a preassembly formed of a stainless steel conduit and an accessory based on a stainless steel ferrule, or a Stainless steel component, it is carburized at low temperature so that the surfaces of the stainless steel tube and accessory or accessory component are a hardened cover without formation of carbide precipitates.

More broadly, this disclosure provides a process for applying a diffusion-based surface treatment to an article made from a first cooperative workpiece by defining a first equalization surface and a second cooperating workpiece defining a second equalization surface, the first and second surfaces being below the substantial compression stress when the article is in the final form, the process comprising forming a pre-assembly of these metal workpieces and then subjecting the pre-assembly to the surface treatment based in diffusion. In addition, this description also provides a pre-assembly for use in the formation of a corrosion resistant conduit system with improved surface hardness, the pre-assembly comprising a stainless steel conduit having an attachment based on a pre-connected ferrule or a component of said accessory, the preassembly having been a hardened cover by low temperature carburization so that the surfaces of the stainless steel tube and accessory or accessory component are essentially free of carbide precipitates. In addition, this description also proceeds a corrosion resistant conduit system with improved surface hardness, the conduit system comprising a conduit stainless steel hardened cover attached to an accessory based on stainless steel splint, or a stainless steel component of said accessory. Conveniently, this conduit system was formed by pulling the accessory body and the casing nut of a splint-based fitting into a pre-assembly comprised of a stainless steel conduit having a stainless steel ferrule pre-connected in at least one end, the preassembly having been hardened by low temperature carburization so that the surface of the stainless steel tube and splint are substantially free of carbide precipitates.

DETAILED DESCRIPTION Terminology In this description, the reference to the carburization stainless steel "without formation of carbide precipitates" means that the amount of carbide precipitates formed, if any, is too small to adversely affect the corrosion resistance of the stainless steel. . In addition, the "tubes", "" pipes "," lines "and "conduit" will be understood as referring to the same, without any attempt being made to make a difference in its meaning. In relation to this, the difference between "pipe" and "pipe" is basically a nomenclature and arising convention for historical reasons. In particular "pipe" has the term traditionally used to refer to the pipe having particular internal diameters, while "pipe" has the term traditionally used to refer to the pipe having particular external diameters. Therefore, "50.8 mm pipe" refers to a pipe having an internal diameter of 50.8 mm, while "50.8 mm pipe" is understood to refer to a pipe having an outer diameter of 50.8 mm. The wall thicknesses may also have been different. Then, the convention regarding "pipe" changed so that it is standardized in fixed external diameters as well. At present, pipes and tubes are made by the same processes and have the same structure. Thus; "conduit" is used herein to refer to both pipes and pipes, unless otherwise indicated. In addition, "joint" is used herein to refer to the combination of a conduit and an accessory, not just the accessory. Also, "cooperative work pieces" of an article means two or more pieces or parts of the article that is in direct physical contact with each other when the article is assembled in the fine form.

Similarly, "final form" as it relates to workpieces in an article refers to the relationship of these treaty pieces between them when the article is in its final fully assembled configuration in contrast to the relationship of these work pieces before finish the article's manufacture. For example, a duct and splint of a splint-based fitting are in the "final shape" when the fitting is fully tightened so that the connection of the fitting to the duct is complete.

Alloys The technology of this disclosure is most commonly used to form a hardened surface or "cover" by low temperature diffusion of carbon atoms on iron surfaces or nickel-containing alloys. Such materials are well known and are described for example in the U.S. Patent. previously noted No. 5,792,282, U.S. Patent. No. 6,093,303, Patent of E.U.A. No. 6,547,888, EPO 0787817 and Japanese Patent Document 9-14019 (Kokai 9-268364), the descriptions of which are incorporated herein by reference. The technology of this description finds particular applicability in the case of hardened steels, especially steels containing from 5 to 50, preferably from 10 to 40,% by weight of Ni. Preferred alloys contain from 10 to 40% by weight of Ni and from 10 to 35% by weight of Cr. Stainless steels are preferred, especially steels of series AISI 300 and 400. Of special interest are stainless steels AISI 316, 316L, 317, 317L and 304, alloy 600, alloy 625, alloy 825, alloy C-22, alloy C-276 and alloy 20 Cb, to name a few examples.

Other Diffusion-Based Surface Treatments This description concentrates on the cover made of hardened stainless by carburization at low temperature. However, the technology of this description is also used to apply other analogous surface treatments to configure metal articles made of multiple metal workpieces. In carburization at low temperature, the atomic carbon diffuses interstitially into the workpiece surfaces, that is, the carbon atoms travel through the spaces between the metal atoms. Because the processing temperature is low, the carbon atoms form a solid solution with the metal atoms of the workpiece surfaces. Honest metal atoms do not react to form other compounds. Carburization at low temperature is therefore different from carburization carried out at higher temperatures in wherein the carbon atoms react to form carbide precipitates, i.e., specific metal compounds such as M23Ce (e.g., Cr23Ce or chromium carbide), M5C2 and the like, disposed in the form of separate discrete phases and apart from the metal matrix in which they are contained. Other analogous processes are known for coating the surface characteristics of a metal workpiece. That is, other processes are known in which the hardness, corrosion resistance and / or other surface characteristics of a metal workpiece can be altered by interstitial diffusion of atoms on the workpiece surfaces to form solid solutions with the atoms of metal in them without forming new compounds in separate phases. Examples include the nitration of alloys based on iron, chromium and / or nickel, carbon nitration of iron, chromium and / or nickel alloys, and nitration of titanium-based alloys to name a few. For convenience, all these processes will be collectively referred to as "surface treatments based on diffusion". All of these diffusion-based surface treatments can be applied using the technology of this description. That is, each of these diffusion-based surface treatments can be applied to articles configured metallic made of multiple metal workpieces using the technology of this description forming a pre-assembly of the metal workpieces first and then subjecting this pre-assembly to the surface treatment based on diffusion.

Conduit Systems The technology of this description is particularly suitable for producing stainless steel duct systems having hardened cover by carburization at low temperatures. By "duct system" is meant a fluid handling system composed of at least one duct and at least one additional fluid handling component such as a coupling, valve, gauge, etc. Commonly referred to as an "accessory". Although these stainless steel ducts and stainless steel fittings with carburized ferrules at temperature have been used to make stainless steel duct systems, the duct systems as a whole and particularly the ducts forming such duct systems have not been Carburized at low temperatures. According to the technology of this description, however, the entire conduit system, or at least the portions of the conduit system entering into contact with extreme corrosion conditions and / or mechanical stress, are formed from the hardened stainless ring cover. Therefore, for example, both conduits and fittings of a hydrogen supply system in a hydrogen-powered car may be the hardened cover according to the technology of this disclosure to prevent corrosion and physical damage to the salt of the road and other road debris. Because at least some of the components forming said duct system (including at least one duct) are preassembled. And, this is spatially so when some components of the conduit system in the pre-assembly are "pre-assembled" (ie, they join in their final form without joining, as defined below) under significant stress before the carburization at low temperature. Any type of duct systems can be processed with the technology of this description. Normally, the duct system in which the fastening and sealing of the fittings to the ducts is achieved mechanically (ie without bonding by welding or adhesion) will be processed. Examples include duct systems using tapered fittings, duct system using accessories based on splints that are described more fully below, duct systems using VCO systems (using O-rings for sealing) such as those shown in the Patent of E.U.A. No. 3,288,949 and duct systems using VCR (using flat or annular gaskets for sealing) as shown in the US Patent. DO NOT. 3,521,910. The descriptions of these patents are incorporated herein by reference. In some of the duct systems (v.gr, conical accessories and some accessories based on splints) the ends of ducts are deformed plastically before the final connection of the accessory to the duct. In other tabs (eg, VCO and VDR fittings) or other structures may be connected to the ends of the conduit such as by welding or other techniques. However, the attachment of the accessory to the end of the preformed duct systems is performed mechanically.

Conduit Systems with Fittings Based on Ferrules Are of special interest the conduit systems that are formed of accessories based on splints. In this context, an "accessory based on splints" is an accessory in which the primary mechanism by which the attachment holds and seals the conduit is performed mechanically by a splint. Splinter-based accessories are well-known items in the trade. Normally, they are composed of an accessory body adapted to fit over the end of the duct, an accessory nut and a complementary splint. The accessories that are intended to be used with metal conduit are almost always made of metal, although other materials are possible. Some accessories based on splints using splints while three or more splints are theoretically possible. The accessory body, accessory nut and splints are designed in such a way that the last squeeze of the nut in the body of the attachment (known as "traction") causes the splint, the portion of the duct that connects the splint or both, they deform plastically to a greater or lesser degree. Four general types of splints-based accessories are commonly used. The most common can be considered as a splint of the compression type. Splinter-based accessories that are common and purchased daily at the nearest store are a good example of this. The accessories formed from said splints show only deformation of localized, smaller conduits, with the clamping force created by the splint which is mainly due to friction. The second type of adjustment based on the splint can be considered as a stamping type adjustment. In these accessories, the clamping force created by the splint is mainly due to stamping, that is, deformation Radial important but not cut off from the duct surfaces. The third type of splint-based accessory can be considered as a punch or cut type accessory. In these accessories, the clamping force created by the splint is due to a combination of forces. As in cutting type accessories, the important clamping force is created as a result of the main edge of the splint being cut on the surface of the duct. In addition, the additional clamping action is generated outside this deformation of the shearing step of the splint during traction. Splinter-based accessories of this type are shown, for example, in the U.S. Patent. No. 6,629,708 B2, the description of which is also incorporated herein by reference, especially in Figs. 2-28. As indicated above, any type of conduit system can be processed with the technology of this description. For example, a conduit system may include a conduit and an accessory formed of multiple components. In one embodiment a conduit system may include a stainless steel conduit and an accessory formed of multiple components, at least one of which is made of stainless steel. In an illustrative embodiment, a pre-assembly to be used in order to provide a conduit system with a treatment of Enhanced surface includes a stainless steel duct and at least one stainless rim components for an accessory that is physically combined with the other, with the pre-assembly being a cover hardened by carburization at low temperature. Said illustrative conduit system includes an accessory of two highly successful and commercially available splints used to form the pipe, which is illustrated in Figs. 1 and 1A. Figs. 1 and 1A are taken from the U.S. Patent. No. 6,629,708, the description of which has been incorporated by reference as indicated below. Fig. 1 shows the accessory components in a preparatory airtight tactile position at a final adjustment, while Fig. 1A shows the accessory after the final adjustment. As shown, the attachment comprises a body 10 having a cylindrical opening 12 counter-formed to receive the end of the tube 13. A frusto-conical, tapered cam mouth 14 is located at the axial outer end of the counter-hole. A front ferrule 16 having an even cylindrical internal wall 18 is received closely in the tube. The front splint has a frustoconical outer surface 20 to be received in the cam. Associated with the front splint 16 and located axially outwardly thereof is a posterior splint 22 configured as shown with a nose portion tapered 24 and a rear flange 26 having an inclined end surface 28. The inclined end surface of the rear splint 22 provides a radial component as well as an axial component of the tensile forces acting on the end surface as will be apparent to the skilled artisan. in the matter. The tapered nose 24 enters a tapered cam surface on the rear surface of the front ferrule. The ferrules 16, 22 are enclosed by a drive nut member 30 threaded to the body 10. During adjustment and conformation of the fit, the inner end face, flange or shoulder 32 of the nut acts against the rear wall end surface 28 of the posterior splint for driving the splints backward in the fully engaged position shown in Fig. 1A. The technology of this disclosure was advantageously used to form the hardened cover conduit system with the ferrule-based fittings in which the splint or ferrules, and optionally the accessory body, the accessory nut or both are made of stainless steel. In addition, this technology is especially useful for the hardening cover of duct systems based on the accessories of the type of subjection by deformation of stamping, perforation and / or collection where the steps of work and assembly are necessary. important mechanics to form a complete gas supply system. This is due, rather than subjecting each individual and accessory conduit (or accessory part) to a separate carburizing step, most if not all of the low temperature carburization can be done in one step (or just a few steps). carburization steps) after these steps of mechanical work and assembly have been completed or substantially completed. Also, when splints-based fittings are used, a lubricant may be applied to the adjacent duct and out of the contact zone between the splint and the duct. See, for example, provisional patent application of E.U.A. No. 60 / 652,631 (case 22188/06884), the description of which is incorporated herein by reference.

Metal Set and Metal Workpieces Although the technology of this disclosure is particularly useful for forming hardened roof duct systems, it can also be used to form hardened roof metal articles of any configuration and structure that is composed of at least two mechanically bonded cooperating metal workpieces defining respective equalization surfaces under substantial compression stress when the workpieces are mechanically joined (hereinafter "wider technology"). In this context, "mechanically joined" means that the metal workpieces are joined to each other in their final form and unbonded relationship such as by welding or by adhesive. In addition, the "equalizing surfaces" in this context means that the surface of each work piece that is in physical contact with the other workpiece. Finally, "substantial compression stress" in this context means more than the incidental compression stress. Therefore, for example, the equalizing surfaces of a tight nut and bolt are under "substantial compression stress" due to the stress created as a result of their adjusted condition will normally prevent the nut and bolt from separating. Similarly, the equalizing surface of a ferrule and conduit in a fully-fitted fitting, or the equalizing surface of a completely tight conical fitting formed of pre-formed tapered conduit and an associated conical fitting are under "substantial compression stress", due to They will not move relative to each other due to the compression tensions created by the settings. In contrast, the watch extensible parts rotatably connected described in Example Bl of the U.S. Patent. No. 6,905,758 Bl do not define surfaces of respective equalization under substantial compression stress when these watch extensible parts are mechanically joined, due to the "connecting parts" to which they are attached allowing the free rotary movement of these connecting parts and band parts with respect to the others . According to the broader technology of this description the low temperature carburization of the cooperating metal workpieces have equalizing surfaces which are, or will be, under the compression stress is achieved by the pre-assembly of two or more of these metal workpieces and then subjected to the previous assembly thus formed to decrease the carburization temperature. Then, the previously assembled work pieces are mechanically joined to form the completed metal article. Alternatively, the workpieces may be mechanically joined before carburization at low temperature. In Altuna another temperature, the workpieces can be mechanically joined partially before a carburization at low temperature, with the rest of the mechanical union occurring after the carburization at low temperature. The "pre-assembly" and "pre-assembly" in this context means that metal workpieces are physically combined so that contact with another or they can contact each other in such a way that at least one of the parties can be supported or carried by the others. For example, a ferrule in sliding contact with a conduit, a nut bolted loosely on a bolt, two links of a chain and a metal plate with holes having bolts could be "physically combined" between them because each combination can move from one place to another by manipulating only one member of the combination without touching each other, although the unmanipulated member may be free to leave the manipulated member. Note, in particular, that the "pre-assembled" workpieces in this context can be assembled in the final form, that is, joined mechanically, as when a nut and bolt have been finally adjusted. In addition, the "pre-assembled" work pieces can also be assembled in a preliminary form, such as when a nut is screwed loose on its bolt so that the final adjustment can be made afterwards. Due to a previous assembly of some or all of the components that make up the product configured with the metal article are carburized at low temperature, the carburization at low temperature of the article as a whole can be done more easily than if each part will be carburized at low temperature individually before assemble This makes the overall manufacturing process easier, faster and more cost effective. Note that the final shaped metal article can not only be produced by this technology (that is, the final product produced) has any configuration, but in addition to the metal work pieces that make up this final product can also have any configuration. In addition, the configuration can be formed by any means including bending, stretching, working, machining, etc. In addition, the metal work pieces forming this product can be integrally formed, that is, they are composed of a single piece of material, or they can be composed of multiple metal parts welded or otherwise secured. In addition, metal workpieces may be formed from multiple cooperating parts, such as a valve or the like. In addition, they can include portions or parts that are themselves non-metals, such as the plastic seals of a valve.

Workpieces Subjected to the Flow of Plastic Before Pre-Assembling In one aspect of this wider technology, at least one of the surface is equalized under compression stress in the finally produced article is configured at least in part by plastic flow. A common example It is a conical fitting joint in which the end of a metal tube is formed conical before joining a complementary accessory combo. A similar example is when a splint is stamped into a conduit as shown, for example, in the U.S. Patent. No. 6,834,524 B2, the description of which is incorporated herein by reference. In these examples, the plastic deformation of this equalizing surface occurs before the pre-assembly of the metal work pieces in comparison (and therefore before the carburization at the temperature of this pre-assembly). The additional plastic deformation of this leveling surface may occur after pre-assembly, after low temperature carbuption or both, if desired. When the stainless steels are carburized at low temperature, the hardness of the steel surface is significantly increased as previously indicated. In addition, its resistance to corrosion is substantially improved over that of native stainless steel, which is thought to be due to the high concentration of carbon on the surfaces of ACRO. Because the plastic deformation carried out after the carburization at low temperature can be physically difficult due to its hardness, it may be convenient to complete the plastic deformation as much as possible before the carburization at low temperature of the carburization. pre-assembly to avoid the need for additional effort to plastically deform the hardest parts.

Workpieces Subjected to Plastic Flow After Pre-Assembly Another example wherein an equalization surface under compression stress is configured at least in part by plastic flow in certain types of ferrule-based fittings in which the conduit, splint or both are subjected to plastic deformation as part of traction (adjustment) of the accessories. In this example, the plastic deformation of this equalizing surface occurs after the pre-assembly as a result of the joining of the work pieces of metals in comparison together. In other words, the configuration of a "first" equalizing surface is at least partially formed by plastic deformation through contact with a "second" equalizing surface when the two cooperating work pieces that form these equalizing surfaces they join. Normally, all the plastic deformation required to achieve the mechanical joining of the work pieces in the final form will be parched at this time, before the carburization at low temperature. However, the deformation of additional plastic on the leveling surface can also occur after carburization at low temperature if desired. Also, carburization at pre-assembled temperature of these workpieces can be done without relieving the compression stress previously applied to this pre-assembly to cause plastic deformation. Alternatively, this compression tension can be relieved, partially or completely, the pre-assembly then carburized at low temperature, and another compression tension is then applied to the pre-assembly after carburization at low temperature to cause the first and second pieces of I work cooperatively to assume its final form.

Plastic Flow Through Differential Hardness In a particularly interesting approach, the plastic flow (or deformation) of one or a "first" equalization surface by a cooperative or "second" equalization surface is facilitated by selecting the second surface of equalization to be harder than the first equalization surface. As a result, when the two equalizing surfaces are subjected to mutual compression tension, the second equalizing surface plastically deforms the first equalizing surface due to its harder nature. The selection of an equalizing surface to be harder than the other can be done in a variety of different forms. For example, the cooperating metal workpieces defining these equalizing surfaces can be formed from different alloys having different hardnesses. Alternatively, cooperating metal workpieces may be made of the same alloy but may be pre-assembled to achieve different surface hardness. For example, the second equalization surface may be made harder than the first equalization surface by subjecting the second equalization surface to a separate, additional, low temperature carburization treatment, pre-assembly bodies of the metal workpieces in cooperation. Other surface hardness techniques including nitration, carbonization, hot or cold treatment and the like can also be used. The combinations of this approach can also be used.

Pre-assembled Workpieces As indicated above, in one aspect of the technology of this description, herein referred to as "pre-joining" work pieces are mechanically joined (i.e., the work pieces are joined in a final) before carbuption at low temperature. For example, a nut and bolt can finally be tightened before carburization at low temperature. Similarly, a splint-based fixture can finally be adjusted before carburization at low temperature. Surprisingly, although the equalizing surfaces of these workpieces may have the case effectively hardened by carburization at room temperature, at least when activation (depassivation) is performed with a compound having the halogen or gas such as HCl, HF , Cl2, F ?, NF3, etc. Stainless steel is stainless steel because it forms a coherent protective layer of chromium oxide ((Cr03) essentially instantaneously upon exposure to the atmosphere.This layer of chromium oxide is impermeable to the diffusion of carbon atoms. therefore, it is necessary before the carburization at low temperature forms workpiece surfaces transparent to the diffusion of carbon atoms, this treatment is usually referred to as "activation" or "depassivation." This can be done by a variety of different techniques including contacting the workpiece with a halide gas and hydrogen such as NC1 or HF at elevated temperature (e.g., 260 to 316.5 ° C), contacting the workpiece with a strong base, electroplating the piece of work with iron, putting the work piece in contact with solid liquid and putting the work pieces in contact with a batch of molten salt, including cyanide. hatred, when the work pieces are mechanically joined before the carbupzation at ba at temperature in accordance with this mode, the equalizing surface of these workpieces could be expected to not carburize because physical access to these surfaces is blocked. However, in practice, it has been found that in most cases the activation and then the carburization at ba temperature will occur adequately even on its equalizing surface, as long as the activation is carried out with a halogen-containing compound. or gas such as HCl, HF, Cl2, F2, NF3, etc. Although a few modalities of this technology have been described before, it should be appreciated that many modifications can be made. All these modifications are intended to be included within the scope of this description, which is limited only by the following claims.

Claims (25)

1. - A process for applying a diffusion surface treatment based on a duct system composed of at least one hardened cover steel duct and at least one accessory, the accessory or at least one component of the accessory has been made hardened deck steel, the process comprises the mechanical joining of the hardened cover steel conduit and the fitting or the hardened cover making component of the fitting to form a pre-assembly and then the pre-assembly is subject to the treatment of surface based on diffusion.

2. The process of claim 1, wherein the surface treatment based on diffusion is a low temperature carburization.

3. The process of claim 2, wherein the conduit system is a ferrule-based conduit system wherein the ferrule is made of hardened cover steel and further where the hardened cover steel conduit and the ferrule are joined together. mechanically to form a pre-assembly.

4. The process of claim 3, wherein the conduit is subjected to plastic flow before carburization at low temperature.

5. - The process of claim 4, wherein the hardened cover steel conduit and the fitting are pre-joined prior to carburization at low temperature.

6. The process of claim 3, wherein the ferrule-based duct system includes a hardened cover steel duct and an accessory, the fixture has an accessory body, a hardened steel deck ferrule and the nut of the ferrule. accessory and further where the process comprises carburization at low temperature a pre-assembly comprises the hardened cover steel conduit, the ferrule and at least one of the accessory body and the accessory nut.

7. The process of claim 6, wherein the body of the fixture, the nut, the fixture or both are made of hardened cover steel.

8. The process of claim 4, wherein one end of at least one conduit is pre-stamped prior to carburization at a low temperature.

9. The process of claim 8, wherein the splint is a stamping type fixture, a bite-type fixture or a grip-type fixture of collective deformation.

10. The process of claim 8, wherein a lubricant is applied to the hardened steel jacket duct within the outboard end of the ferrule. adjacent and outboard of the contact area between the splint and the tube.

11. A process for applying a diffusion-based surface treatment to an article made of the primary cooperation workpiece defines a primary bonding surface and a secondary cooperation workpiece defines a secondary bonding surface, the surface of Primary and secondary unions were under substantial compressive stress when the article is in final form, the process comprises forming a pre-assembly of the cooperated metal workpieces and then subjecting the pre-assembly to surface treatment based on diffusion.

12. The process of claim 22, wherein the diffusion-based surface treatment is low temperature carburization of hardened cover steel.

13. The process of claim 12, wherein the shape of the primary bonding surface is at least partially formed by plastic deformation through contact with the secondary bonding surface where the article is a final shape, the process further comprises causing the secondary bonding surface to plastic deformation of the primary bonding surface after the pre-assembly is formed but before the pre-assembly is subjected to the diffusion-based surface treatment.

14. - The process of claim 13, wherein the secondary bonding surface is more difficult than the primary bonding surface.

15. The process of claim 13, where: the primary and secondary cooperation work pieces are joined to form a pre-assembly, a tension is then applied to the pre-assembly to cause the secondary bonding surface to plastically deform the primary bonding surface, the tension is released, the pre-assembly is then subjected to surface diffusion based treatment, and another tension is then applied to the pre-assembly to cause the primary and secondary co-operation pieces to assume their final shape.

16. The process of claim 13, wherein: the primary and secondary cooperative work pieces are joined to form a pre-assembly, and a tension is then applied to the pre-assembly to cause the secondary bonding surface for plastically deforming the primary bonding surface, and the pre-assembly is then subjected to diffusion-based surface treatment without substantially releasing the tension.

17. - The process of claim 15, wherein the work pieces are joined in their final form prior to surface treatment based on diffusion.

18. The process of claim 11, wherein the work pieces are joined is their final form prior to surface treatment based on diffusion.

19. A pre-assembly for use in providing a duct system with an increased surface treatment, the duct system has been formed of hardened cover steel duct and an accessory formed of multiple components of at least one of the which is formed of hardened cover steel, the assembly comprises the hardened cover steel conduit and at least one hardened cover steel component of the physical apparatus combined together, the pre-assembly has been forced by low temperature carburization .

20. The pre-assembly of claim 19, wherein the pre-assembly comprises a hardened cover steel conduit having a hardened steel ferrule pre-connected in at least one end.

21. The pre-assembly of claim 20, wherein the splint is a component of a splint-based apparatus includes an accessory body and accessory nut. 22.- A corrosion resistant duct system with increased surface hardness, the duct system It comprises a steel conduit with a hardened cover attached to an accessory. 23. The conduit system of claim 22 wherein the conduit system comprises a steel conduit with hardened cover attached to a splint-based fitting. 24. The conduit system of claim 23, wherein the conduit system is formed by attracting the accessory body and the nut of the ferrule-based conduit fitting in a pre-assembly composes a hardened cover steel conduit that It has a pre-attached ferrule at one end, the pre-assembly has been reinforced by carbuption at low temperature so that the surfaces of the hardened cover steel tube and the ferrule are essentially freed from carbon precipitates. 25.- A pre-assembly for use in providing an article with an increased surface treatment, the article has been made of at least one primary and secondary cooperation metal workpiece, the primary and secondary cooperation pieces respectively define to the primary and secondary joining surfaces where the junction with one and the other when the article is in final form, the secondary bonding surface has been more difficult than the primary bonding surface and the shape of the primary bonding surface has been formed Partially deformation plastic applied by the secondary joining surface, the pre-assembly comprises at least one primary and secondary co-operating metal work piece physically combined with each other, the primary joint surface has been partially formed from plastic deformation applied by the surface of secondary union, the pre-assembly has been reinforced by carburization at low temperature.