The present invention refers to a process and a plant for the anti-oxidising surface treatment of steel parts.

Various types of anti-oxidising surface treatments are known in the art, to which steel parts are subjected, which, at the end of such treatment, are put in such a condition as to efficiently resist to wear and corrosion, to which they are subjected according to the working environment in which they operate.

In particular, for components used in the motor vehicle field, such as filters, gas springs, etc., a galvanic treatment can be performed, such as chromium-plating, nickel-plating or partially zinc-plating, etc. Another type of treatment, this time of the thermal type, is nitriding, followed by an oxidising step performed in salt baths at the presence of cianide, cianate and sodium carbonate, which therefore is rather obnoxious from the ecologic and working safety points of view.

In the field of parts and components for motor vehicle filters, in order to build the external envelopes for protecting and containing ^" such filters, the corrosion resistance feature is important, and is obtained, using the current technology, by painting such envelopes with a suitable corrosion-preventing paint: also this treatment has severe ecologic problems, due to the toxic materials being used.

Object of the present invention is solving the above prior art problems, by providing a new process for the anti-oxidising treatment of steel parts that is ecologic, does not show the above pollution and working safety problems and provides the same levels of wear and corrosion resistance of current echnologie ^' s ; such process further allows extending the length of such resistance, in components ^' such as filters, because it will not be necessary any more to paint them, since such property will be embedded into the structure itself of the material being treated.

Another object of the present invention is providing a process as mentioned above that has a better resistance to saline mist, and allows a better oxidising, with respect to known processes. A further benefit of such process is that it is much safer than the previous ones, using a step of pre-heating with nitrogen instead of air, as happened in prior processes.

A further object of the present invention is providing a plant for performing the above process, that is much more simplified-,, from the construction point of view, with respect to known prior plants, providing, in the most common cases, for the absence of the steam generator that was necessary before. Such plant remains anyway capable of reaching the high production volumes required for manufacturing components aimed for mass productions, such as the motor vehicle production in general, the production of oil platforms or components for mechanics in general.

The above and other objects and advantages of the invention, as will appear from the following description, are obtained with a process and a plant as claimed in the respective independent claims. Preferred embodiments and non-trivial variations of the present invention are the subject matter of the dependent claims.

It is intended that all enclosed claims are an integral part of the present description.

The present invention will be better described by some preferred embodiments thereof, provided as a non-limiting example, with reference to the enclosed drawings, in which:

- Figure 1 is a schematic view of an embodiment of the plant according to the present invention; and

- Figure 2 is a side sectional view of a pit furnace used in the plant of Figure 1.

With reference to the Figure, a preferred embodiment of the process and plant of the present invention is shown and described. It will- be immediately obvious that numerous variations and modifications (for example related to shape, sizes, arrangements and parts with equivalent functionality) can be made to what is described, without departing from the scope of the invention as appears from the enclosed claims.

The process of the invention is used for the anti-oxidising surface treatment of steel parts, and comprises the steps of:

washing or de-greasing a charge of steel components to be treated;

- nitriding under gaseous phase the charge, after its washing, wherein the step of nitriding is composed of. the sub-steps of:

• pre-heating with nitrogen till a temperature of 350 °C;

· heating from 350 "C ^' .to 450 °C in an atmosphere composed of C0 ₂ ;

• heating from 450 . °C to 520 - 600 °C for half an hour in an atmosphere containing N ₂ ;

· maintaining, at the same temperature of

520 - 600 °C, for . a period from 2 to 10 hours, in an atmosphere composed of a mixture containing NH ₃ , C0 ₂ and N ₂ ; washing with nitrogen the environment where nitriding occurred; and

steam oxidising, consisting in the immission of overheated steam at a temperature in the range between 490 °C and 540 °C for the necessary time to obtain a complete oxigen- iron exchange.

Preferably, but not in a limiting way, the steel parts can be components for motor vehicle use, or for oil platforms, or still more to be used in mechanics. Obviously, the inventive process is also applied to several other types of components that need nitriding and oxidising.

In particular, the mixture with which the sub -step of maintaining at the same temperature of 520 - 600 °C is performed, can contain, in volume, 70% of NH3, from 10% to 15% of 2 and, respectively, from 20% to 15% of C0 ₂ .

Still in particular, the mixture with which the sub-step of maintaining at the same temperature of 520 - 600 °C is performed, can contain, in volume, 70% of NH ₃ , 10% of N ₂ and 20% of C0 ₂ .

The invention also deals with a plant for performing the above-described anti-oxidising surface treatment of steel parts. Such plant substantially comprises:

- a washing machine 5;

a pit furnace 7 ;

oxidising means 9; and

handling means 2 (of the winch type) of a charge 3 with the parts to be treated;

wherein, through the handling means 2, the charge 3 is first inserted into the washing machine 5, to be washed and de-greased (removing impurities from previous working, such as for example oil), and is then inserted into the pit furnace 7 to be subjected to the steps of nitriding and washing the furnace 7 with nitrogen.

The oxidising means 9 are connected to the pit furnace 7 in order to perform in a single plant the step of steam oxidising after the step of washing with nitrogen the furnace 7.

At the end of the nitriding process, the charge 3 is extracted from the furnace 7, through the handling means 2, and is sent to following workings.

Figure 2 describes a typical pit furnace 7 used for the plant of Figure 1. With reference to Figure 2, the furnace 7 comprises an external structure 11 for support and heat insulation, composed of ceramic fiber and containing, inside, on its side walls and its lower wall, a plurality of electric resistances 13 for heating the internal environment. The charge 3 is inserted centrally and coaxially ^' into the furnace 7 using its upper opening; the charge 3 is slinged in a supporting structure 15 ending with a cover 17 that facilitates its various handling operations. After having inserted the charge 3, the furnace 7 is closed by a plug 19, modelled in order to support a motored fan 21, whose blades 23 work in order to move the gas mixture contained inside the furnace, in order to keep its temperature uniform. The furnace 7 is further equipped, in its upper part, with an exhausting device 25, which is a barrel from which reaction gases from various processes go out: such gases are burnt by the flame generated by such. exhausting device 25. Function of the exhausting device 25 is also signalling the moment in which the oxidising step (that has different operating times according to the number and sizes of treated parts) ends, because at such time the small flame contained therein is extinguished.

As some possible non-limiting variations of the invention, the oxidising means 9 can be composed of an intermittence-type pump suitable to send distilled water onto the charge 3 for its oxidation, or be composed of a steam generator adapted to steam-oxidise the charge 3, or still more be oxidising means adapted to perform the oxidising of the charge 3 by using the same C0 ₂ already present in the nitriding plant.