Abstract:
A process for treating a steel surface, comprising the steps of immersing the surface in a bath of molten tin at a temperature and for a period of time suitable to cause a layer of tin containing diffused iron to adhere to the surface and, after removal of the surface from the bath, removing a surface layer in which iron and tin accord substantially with the formula of Fe Sn 2  so as to expose a layer in which iron and tin accord substantially to the formula Fe Sn.

Description:
FIELD OF THE INVENTION 
     The invention relates to surface treatment processes for steels, of the sort which involve the immersion of steel slugs (pieces) in a bath of molten tin. 
     THE PRIOR ART 
     It is known that these processes result in the surface formation of complex layers containing iron and tin and also, in the case where the treated alloy additionally contains nickel and chromium, (the case of nickel-chrome steels) certain amounts of nickel and chromium which diffuse from the said alloy. 
     However, up to the present time it has not been known how to make use of the properties of these layers, in particular as regards the surface hardness. It was known that layers of FeSn and FeSn 2  were obtained but there had been no means of separating these layers. 
     SUMMARY OF THE INVENTION 
     The principal object of the invention is to treat the piece in such a way that a layer of FeSn preferably containing considerable amounts of one of the metals Ni, Cr or possibly magnesium incorporated therein is finally revealed on the surface of the treated piece. 
     It has in fact been found that the layer of FeSn especially where it contains amounts of at least one of the afore-mentioned addition metals incorporated therein has hardness and corrosion resistance characteristics better than those of the FeSn 2  layer. 
     According to the invention there is provided a process for treating a steel surface, comprising the steps of immersing the surface in a bath of molten tin at a temperature and for a period of time suitable to cause a layer of tin containing diffused iron to adhere to the surface and, after removal of the surface from the bath, removing a surface layer in which iron and tin accord substantially with the formula FeSn 2  so as to expose a layer in which iron and tin accord substantially with the formula FeSn. 
     Preferably the steel contains a metal selected from the group consisting of nickel and chromium and the FeSn layer contains a proportion of such metal. 
     As has been shown in practice, such a process enables a layer of FeSn having a very high degree of hardness of the order of 500 to 700 V (Vickers) and also good corrosion resistance properties to be obtained, even in the case of a combined friction and corrosion process, in other words in the case of moving parts in a corrosive medium. This is a new feature compared with known surface treatments, in which the increase in hardness and improvement in friction properties resulting therefrom often have as a counterpart a decrease in the corrosion resistance. 
     The said process can be applied either to nickel-chrome steels such as in particular the 18/10  austenitic type steels, in which case the nickel and chromium contributing to the remarkable qualities of the FeSn layer diffuse theretowards from the underlying steel, or to other steels such as carbon steels, in which case the nickel and/or chromium or any other addition metal are added to the tin bath and diffuse into the said layer from this bath. 
     Means which may be used to remove the poorer quality surface layer of FeSn 2  are for example machining or grinding, which leaves only the FeSn layer remaining, or a chemical or electrochemical descaling. 
     Chemical descaling employs, in particular, soaking in a nitric acid solution to which hexamethylenetetramine has been added. 
     Electrochemical scaling involves, for example, applying a constant or variable voltage to the piece in a 1N sulphuric acid bath or other bath. 
     Another embodiment of the invention consists in slowly cooling (for example 100°/hour) the pieces after they have been treated, which tends to prevent fissuration due to the difference in the coefficients of expansion of steel and the FeSn-type compound. 
     Apart from these features and embodiments, the invention also includes certain other provisions which are preferably used at the same time and which will be discussed in more detail hereinafter. 
     The invention will be better understood with the help of the following description and the accompanying drawings relating thereto, said description and drawings being given of course only by way of example. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 shows diagrammatically (in a magnified section through the surface layers) the various layers obtained at the surface of a steel slug or piece treated by the process according to the invention. 
     FIG. 2 is a diagram (corresponding to FIG. 1) showing the variation in the surface hardness in such a piece. 
     FIG. 3 is a diagram (corresponding to FIG. 1) showing the distribution of the constituents in the layers initially obtained. 
     FIG. 4 shows an arrangement for the electrolytic descaling of the FeSn 2  layer, according to one of the embodiments of the invention. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In accordance with the invention, and more especially in accordance with those methods of application and embodiments of its various parts which are preferred, a process is provided in general for treating a steel in a tin bath in order to obtain a very hard and corrosion-resistant surface, said process being carried out as follows or in a similar manner. 
     It is assumed first of all that the piece being treated is nickel-chrome stainless steel, for example an 18/10 type austenitic steel. 
     The piece is first of all descaled before the treatment in the molten tin bath. 
     This descaling is very important since it prevents any chromium oxide film remaining on the surface of the piece, which would oppose the diffusion of nickel and chromium into the tin, and this diffusion is essential for the correct operation of the process, as will be seen later. The said descaling is carried out for example in a suitable steel bath, in particular one containing hydrochloric acid. However, alkaline solutions, particularly those containing soda, may also be used. 
     When the piece has been descaled it is immersed in a bath containing molten tin and is kept therein for an appropriate period of time at temperatures of the order of 400° to 700° C. 
     The treatment time is in particular approximately 48 hours, preferably up to 100 hours, but these figures are in no way limiting. As regards the temperature, it is desirable that it should be sufficiently high but it should not exceed the value at which steels of this type begin to be sensitised with, as in known, the formation of chromium carbide. A temperature of the order of 500  to 600° C is particularly appropriate. 
     It should also be noted that at low temperatures the treatment has tendency to promote the formation of the compound FeSn 2 , whereas, as we shall see, it is particularly the compound FeSn which is important. 
     It should finally be noted that the use of times and temperatures which are greater than those suggested above, in particular 96 hours at 650° C or above, favours the formation at the surface of the austenitic stainless steel of a ferrite phase which is able to improve the adherence and resistance of the FeSn type compound at the surface of the steel during friction. Such treatment conditions would therefore be of great value in the case of stabilised (particularly with molybdenum or titanium) austenitic stainless steels which do no exhibit the sensitisation phenomenon already mentioned. 
     Tests have been carried out, particularly on a nickel-chrome steel having the following composition: 
     
         __________________________________________________________________________C   Si Mn S   P   Ni Cr  Mo Ti V   Nb__________________________________________________________________________0.028    0.26  1.78     0.019         0.022             9.70                18.31                    0.33                       none                          0.026                              0.030__________________________________________________________________________ 
    
     and the following results, given simply by way of example, were obtained. 
     After a treatment at 650° C for 48 hours, the following surface phases were obtained, as are also shown in FIGS. 1 to 3: 
     A. In the interior sub-layer the austenitic steel subjected to treatment has a hardness of the order of 150 V, as may be seen in FIG. 2, this layer apparently not having been sensitised by the treatment at 650° C, and the intergranular diffusion of tin into the austenitic structure appears to be very slight. 
     B. Above the previous phase is a layer of uniform appearance having a thickness of the order of 50 to 100 microns, particularly 50 to 70 microns, and a high degree of hardness, of the order of 450 to 500 V and even as high as 700-750 V, and analysis of this layer shows the presence of a relative amount of iron and tin corresponding to the specific compound FeSn, with in addition large amounts of chromium and nickel which have diffused from the base steel. 
     C. Next is a more extended phase having a thickness of the order of 100 microns, which is less compact than the preceding phase and has a lower degree of hardness, around 140 V, and whose composition corresponds in particular to a specific compound FeSn 2  with very small amounts of chromium and nickel (below 1%). 
     D. Finally there is a layer of tin with some polyhedric crystals of the FeSn 2  type. 
     The curves in FIG. 3 shown clearly the changes in the concentration of the alloy elements in the various phases. 
     In general a large decrease in nickel and chromium concentrations is found in the direction of the outermost layer of the treatment. It is also found however that the compounds of the FeSn and FeSn 2  type contain chromium and nickel. In actual fact it is the compound FeSn, situated at the boundary of the underlying steel, which contains most chromium and nickel, and this, it would appear, is present in the form of a specific compound of the type Fe(CrNiSn) in a layer of the order of 10 to 20 microns (the plateau of layer B, FIG. 2). The proportions of Ni and Cr in this layer, at least in the initial plateau region, may be of the order of 3 to 7% for chromium and 0.5 to 3.5% for nickel. 
     The presence of the said compound appears to increase the already high value of the hardness of the said layer B. 
     In addition, the fact that this layer B is still rich in chromium and nickel imparts good corrosion resistance properties. 
     The invention thus essentially consists of using a tin bath to obtain a surface layer B of the type Fe(Cr Ni Sn), and in addition removing a surface layer C of the type FeSn 2  of poorer quality. 
     This removal of the layer C may in particular be effected by machining, that is to say grinding the pieces to a sufficient depth so that the desired layer B appears. Other means of removal will be mentioned hereinafter. 
     As regards the hardness of the layer B which has thus been exposed, the considerable increase in hardness already mentioned above and which is seen in FIG. 2, is obtained. As regards the friction effect, which has been investigated under load using ceramic friction member, it has been found that the coefficient of friction is independent of the load beyond a limiting value of the latter, the said coefficient changing from 0.4 to about 0.25, that is to say being divided by a factor of one-and-a-half, for a hardness of the order of 500 V. In the case of metal friction members, the coefficient of friction was three times less than the value for the non-treated metal. 
     As regards the corrosion resistance, tests have been carried out in a 1N solution of sulphuric acid, and as a rule the tests included a study of the passivity curves. 
     It was found that although the value of the passivity current was slightly higher than in the case of untreated steels, it was nevertheless very low. It can be said that it remains 20 times lower than that obtained in the case of known treatment processes, such as sulpho-nitriding treatments. 
     The life of a steel treated according to the invention in a corrosive medium is thus much higher than that of a nitridated steel. The experimental values obtained indicate that the life of a tin diffusion treatment layer having a thickness of 100 microns is about 2 to 3 months in a 1N solution of sulphuric acid, whereas a sulpho-nitriding treatment layer of the same thickness will be destroyed after 3 or 4 days. 
     It should finally be said that these good corrosion resistance conditions remain, though to a less extent, even when friction and corrosion occur together. 
     Various other embodiments and preferred features will now be described. 
     According to one embodiment, at least one metal such as nickel and/or chromium, and even at least one other metal, in particular magnesium, is added to the molten tin bath. 
     Thus, a certain amount of nickel may be added to the tin bath, which may even be saturated with nickel, that is to say the amount of nickel may be greater than 6% by weight. 
     It is then found that not only does the amount of nickel diffusing into the compound FeSn increase, but also compounds of the type Ni 3  Sn 4  are formed, which help to increase markedly the corrosion resistance. 
     This resistance appears to be three to four times higher than in the case where the nickel is merely allowed to diffuse from the piece assumed to be of nickel-chrome stainless steel. A reduction in the fragility is also noted, without the wear and tear resistance being affected. 
     The following procedure is adopted for the preparation of the tin bath saturated with nickel, taking into account the high melting point of the latter. 
     It is sufficient to add nickel (powder for example) to the liquid tin bath at 650° C. After keeping the bath at this temperature for a few hours, optionally combined with stirring of the bath, nickel is added to the bath to give a concentration of about 6%, any excess nickel remaining in the form in which it was added. This excess enables the nickel concentration in the tin bath to be maintained constant during treatment. This excess does not obstruct the operation in any way, on condition that it does not restrict contact between the surface to be treated and the bath. 
     The above method, consisting of incorporating an addition metal in the tin bath, may be applied to other addition metals. 
     Thus, magnesium may be added to the tin bath, in particular in an amount of 5 to 10%. On cooling it forms a compound of the type Mg 2  Sn which has the effect of improving the mechanical resistance of the tin-plated layers. 
     This method, consisting of incorporating additional metals in the tin bath, may be used in the treatment of steels other than nickel-chrome steels. In particular, it could be applied in the treatment of carbon steels in a tin bath. The additional metals of the bath, which would for example be nickel and chromium, will then diffuse into the FeSn layer or into the FeSn and FeSn 2  layers, thereby contributing to the improvement in the qualities of these layers, as in the case of the treatment of nickel-chrome steels. 
     This application is particularly important. In fact, if a carbon steel is merely treated in a tin bath a surface layer of FeSn is indeed obtained, but the corrosion resistance of this layer is 20 times less than that of the type Fe(CrNiSn) obtained by treating a nickel-chrome stainless steel in such a tin bath. If however in the treatment of a carbon steel, for example of the XC type, nickel and chromium are added to the tin bath, a layer of the type Fe(CrNiSn) is obtained, as in the case of the treatment of a stainless steel. However it should be noted that the concentrations of chromium and nickel are not necessarily identical to those mentioned in the case of the treatment of a stainless steel. 
     In each case it is in general important, as mentioned above, to remove the residual tin and the surface layer of FeSn 2  having less good qualities. 
     Instead of using a machining procedure for this purpose, in accordance with another provision of the invention there may be used a descaling operation in a bath, under conditions which enable the unreacted tin on the surface of the pieces to be recovered, and also enable the above-mentioned layer of FeSn 2  to be removed without however attacking the useful layer of FeSn. 
     Several solutions are possible to this problem of removing the first layer without causing any passivity of the second layer. 
     Very good results have been obtained by immersion in a concentrated solution of nitric acid (35% concentration, i.e. 10N), to which hexamethylenetetramine has been added in an amount of 1 g/liter. 
     At ambient temperature such a solution rapidly dissolves the tin and the FeSn 2  layer at a rate of 0.76/g Sn/h, i.e, in one hour a layer 1 mm thick per 1cm 2  of surface area exposed to the solution is removed. The FeSn type compound is not attacked by this bath since it becomes passive spontaneously on contact with a concentrated solution of nitric acid. In addition, hexamethylene tetramine acts as a corrosion inhibitor for iron ions as soon as the compound appears on the surface. The rate of dissolution of the compound is nearly 150 times less than that of tin, i.e. is 0.0053 g/g compound/hour. It is even approximately six times less than that of the base stainless steel after the thermal treatment at 650° C for 48 hours. The use of such a bath thus promotes a rapid dissolution of the tin; the attack stops spontaneously as soon as the compound Fe(CrNiSn) appears on the surface. 
     In practice, after the pieces have been treated they are cleaned by simply immersing them in the bath in question. The removal of descaling products from the surface is accelerated by stirring the bath. 
     In order to prevent any possible escape of gases (always slight under these conditions), the descaling bath may be heated to a temperature slightly above the ambient temperature (30° to 40° C). 
     It is advantageous to recover the tin thus dissolved by the descaling bath. 
     In this connection it should be noted that the attack on the tin by such a bath basically leads to the formation of hydrated β stannic oxide. This stannic oxide SnO 2  is insoluble in the solution and is thus easily recovered. In actual fact it constitutes the principal tin ore and can be reduced by one of the numerous known industrial or laboratory methods to obtain the metal. 
     Instead of using a descaling bath to remove the FeSn 2  layer, an electrochemical method may be used. 
     This method basically consists of applying to the surface or piece being descaled and immersed in an appropriate solution, a constant or variable voltage appropriate for the dissolution of tin, and for this several methods of operation may be considered. 
     According to a first method of operation a constant voltage is applied, appropriate for the dissolution of the tin and promoting the passivity of the compound Fe(CrNiSn) as soon as it appears at the surface. 
     Using a 1N solution of sulphuric acid, a potential of about -550 mV (with respect to a sulphate electrode) brings about a rapid dissolution of the tin without attacking the compound. 
     According to another method of operation, a potential which varies linearly according to an appropriate cycle, that is to say in an alternating manner, is applied to the surface being descaled. 
     For this purpose an automatically controlled potentiostat which provides the desired cycle is used. Thus, for a sulphuric acid solution such as mentioned above, the voltage may be varied in an alternating manner within a range of the order of -900 to +300 mV at a rate of several hundreds of mV per minute. Moreover, the potentiostat will be regulated to enable the dissolution of the tin and the removal of the descaling products to be controlled accurately. 
     FIG. 4 shows an arrangement which can be provided for such a descaling. 
     The voltage generator or potentiostat 1 is controlled by a pilot member 2. The generator applies a constant or variable voltage to the piece 4 to be descaled which is immersed in the solution 5, via a reference electrode 3 (a sulphate electrode for example). The resultant current which passes between the standard electrode 3 and a counter electrode 6 is measured by a milliam-meter 7. The magnitude of the current defines the duration of the descaling operation. An electronic millivolt meter 8 regulates the voltage applied by the potentiostat 1 to the piece. 
     Whatever the method adopted, the electrochemical dissolution process will cease as soon as a very low passivity current density corresponding to the compound Fe(CrNiSn), i.e. about 0.1 mA/cm 2  , appears. 
     The tin may advantageously be recovered from the descaling baths thus employed, and this may be effected by various methods, for example: 
     by directing reduction of the salts in an acid medium using reducing agents such as Zn, Al or Mg, 
     by cathodic reduction after previous conversion, by hydrochloric acid, of stannic sulphate to stannic chloride. 
     In the latter case electrolysis of the stannic chloride is then carried out using a cathode, particularly a platinum cathode, on which the tin is deposited. 
     The final result of the above-specified methods of chemical or electrochemical descaling is to produce a metal surface containing practically only the compound of the type FeSn or Fe(CrNiSn). If necessary, that is to say in the case where the surface obtained is not absolutely in accordance with requirements from the mechanical point of view, the treated piece may be polished or lightly machined. 
     The invention also includes another preferred feature which enables cracks to be avoided, which can form in the surface of the piece after treatment in the tin bath as a result of differences which exist between the coefficients of expansion of steel and the compound FeSn. This arrangment consists in ensuring that the pieces are cooled slowly, for example in a furnace, at a rate of about 100° C per hour after they have been treated in the said bath. 
     Clearly, whichever embodiment is adopted steel pieces or slugs may be produced whose work surfaces may be substantially improved both as regards hardness or friction, and as regards corrosion resistance. 
     The treatment process is simple and is thus inexpensive. 
     In order to provide pieces according to a desired quota, the procedure may be carried out as for various known surface treatments, in this case taking into account the mechanical, chemical or electrochemical operations the piece must be subjected to in order to reveal the effective layer such as that designated hereinbefore by B. 
     It will be noted that, optionally, any stirring means can be provided to accelerate the reactions, and that in particular the pieces, if suitably supported, can be rotated or subjected to any other movements. 
     It goes without saying and furthermore also follows from what has already been said that the invention is not limited solely to those preferred embodiments which have been considered more particularly; on the contrary, it covers any other variations within the scope of the claims.