The present invention relates to a screw. The invention particularly has in view two types of screw, namely self-drilling screws intended to secure sheet metal, especially stainless sheet metal, on roofs and house frontages, and self-tapping screws.

A multiplicity of demands, several of which are difficult to reconcile, are placed on a self- drilling screw intended to act as a securing element for stainless sheet metal on roofs and house frontages: - the material shall have a cold workability such that it can be shaped in the cold condi¬ tion into a screw having a tip which can drill the screw through a sheet of stainless steel,

- the screw, or at least a surface layer thereof, and especially the surface of the screw tip, shall possess a very high degree of hardness, - the screw shall possess very good inherent resistance to corrosion and no stress corro¬ sion shall arise as a result of galvanic reactions between the screw and the stainless sheet metal,

- at least the screw head shall, for aesthetic reasons, be of the same colour as the sheet metal, - the screw must not be too expensive.

Many different alloys, especially different steel alloys, have been proposed for self-drill¬ ing screws, these proposals including martensitic stainless chromium steels. However, none of the materials proposed hitherto has fulfilled all the above mentioned demands. One object of the invention is to provide a screw of such a material, and treated in such a manner, that it fulfils all the requirements that are placed on self-drilling screws which are intended to be used as securing elements for stainless steel facings for roofs or frontages.

As far as self-tapping screws are concerned, the most important thing is that the screw shall possess a very high degree of hardness, at least in a surface layer thereof, in order to be able to function as "its own thread tap" in order to produce threads in a drill hole. In those cases where the screw is to be used as a securing element for stainless sheet metal or other stainless elements, the same demands are placed on it in terms of corrosion re¬ sistance as are placed on self-drilling screws.

The above demands can be satisfied therein that the screw is characterized by what is stated in the appending patent claims.

Further characteristic features and aspects of the invention will be evident from the fol¬ lowing description. In the following description, reference will be made to the accompa¬ nying drawings, of which

Fig. 1 shows a self-drilling screw of a design which is known per se, to which the in- vention can be applied;

Fig. 2 shows diagrammatically, and on a very large scale, the structure of a surface layer of the screw, and

Fig. 3 illustrates how the screws can be arranged in a fixture during a treatment step.

The matrix of the screw 1, i.e. the whole screw with the exception of the surface layer, consists of austenitic stainless steel. This implies that the steel has a high content of chromium and nickel and a low content of carbon, i.e. max 0.1 % C, preferably max 0.05 % C; 18-25 % Cr; and 8-20 % Ni. In addition, the steel can contain other alloying ele¬ ments, for example up to a maximum of 10 % Mo. Preferably, the steel also contains a certain quantity of copper, suitably 2-4 %, in order to improve the cold workability of the steel. A suitable composition is 0.01 % C, 0.5 % Si, 0.6 % Mn, 18 % Cr, 9.5 % Ni, and 3.5 % Cu, balance iron and impurities.

That which distinguishes austenitic stainless steels is that they possess a very high degree of corrosion resistance and a good level of toughness and cold workability, but are of low hardness. Thus, the matrix of the screw has a hardness which does not exceed 250 HV.

In order to obtain an adequate surface hardness in the drilling tip 2 of a self-drilling screw, the screw was surface hardened by means of so-called ion nitriding, also termed plasma-nitriding. This is a method which has received its name from the fact that a plasma, in other words an ionized gas, is employed as the heating and nitriding medium in the process. In this treatment, the screws are placed in a suitable fixture which, in turn, is placed in a furnace which is filled with a mixture of nitrogen and hydrogen gases. A voltage of 1000 V is applied between the screws and the furnace wall. This has the effect of ionizing the gas, with the ions, which possess a high level of kinetic energy, striking the surfaces of the screws, thereby heating the components to the desired nitriding tem¬ perature, so that no separate, external heating of the furnace is required. At the same time, this bombardment with ions supplies nitrogen to the screw surfaces, thereby providing the desired nitriding effect. By means of this treatment, the gas and the screw surfaces are heated to approximately 480°C. The treatment lasts for 30 h, resulting in a surface-hardened layer 3, Fig. 2, being obtained which has a thickness of 0.01-0.2 mm,

with an average depth of approximately 0.05 mm and with a surface hardness of at least 900 HN and, preferably, a hardness within the range of 1000-1300 HN. By contrast, the hardness of the matrix is not affected and retains a hardness of maximally 250 HN, which is sufficient for the screw to be of adequate strength so that it can be screwed, with the screw at the same time having a very good level of toughness.

A problem with ion nitriding is that the passivation layer of the stainless austenitic steel is partially destroyed by the ionic bombardment, resulting in a lowering of the corrosion resistance. In order, inter alia, to restore the corrosion resistance, the ion nitrided screws are - in accordance with one embodiment - covered with a thin layer of zinc by an elec¬ tro-plating treatment. This zinc layer 4 has a thickness of at least 5 μm, preferably of at least 8 μm, but does not exceed 25 μm in thickness. An additional effect of the zinc layer is that it imparts lubricating properties to the screw, which properties are advantageous when the screw is to be used as a self-drilling screw. The zinc layer also provides the screw with an aesthetically appealing surface and colour. The galvanization is expe¬ diently performed by means of a dipping process in an acidic zinc bath after pickling in an acid bath in order to remove oxides on the surface of the screw.

However, the zinc layer, too, can have certain defects. In order to improve the corrosion resistance still further, the zinc layer can, in an additional operation, be coated with a very thin layer of chromium 5. This layer typically has a thickness of 1-5 μm, or approxi¬ mately 2 μm. The chromating can be carried out by depositing 3-valent chromium - so- called blue chromate - by a dipping process lasting approximately 1 minute.

Finally, the screw heads can be covered with a layer of lacquer, expediently a polyester lacquer which is sprayed onto the heads in powder form, after which the lacquer is hard¬ ened in a manner which is known per se. This layer of lacquer improves the corrosion protection still further on that part which remains exposed while, at the same time, by appropriate choice of lacquer colour, a screw can be obtained which completely harmo- nizes with the sheet metal in which it is to be used.

According to an alternative embodiment, the screw can, after the ion nitriding and pick¬ ling, be electrolyte polished, i.e. treated in an electrolytic bath in accordance with princi¬ ples which are known per se, so that a very thin layer is removed from the rough surface resulting from the ion nitriding. It is particularly the peaks on the surface which are re¬ moved so that an even surface finish is obtained. While the surface-hardened layer re¬ sulting from the ion nitriding has a thickness of 0.01-0.2 mm, with an average depth of

about 0.05 mm, the surface layer removed by the electrolyte polishing amounts to a maximum of only 20 μm, normally from 5 to 10 μm. Use of this treatment means that the abovementioned galvanization can be dispensed with for certain applications. If so de¬ sired, the electrolyte polished screw can of course also be provided with a suitable lac- quer paint.

When developing the present invention, the inventor first started from the premise that it should be possible to ion nitride the whole screw and that the corrosion resistance would be restored in an entirely acceptable manner by the subsequent galvanization. However, tests carried out as a so-called Kesternish test demonstrated that black pittings were obtained on the exposed screw heads when the screws were mounted in a wooden board and a piece of plastic-coated metal facing sheet was fitted between the screw heads and the wooden board.

New screws were now mounted in a fixture 10 of the type which is shown in Fig. 3. The fixture consists of a flat box 11 with a plane bottom 12 which is provided with small orifices 13 for the screws 1 which are to be ion nitrided. In the box, there is also a plate 14 the thickness of which determines the height of the screw heads above the bottom 12. The plate 14 has through-holes 16 immediately opposite the holes 13 in the bottom 12. The screws which are to be ion nitrided are mounted in the holes 16, 13, after which the box 11 is covered by a lid 17. Both the box 11 and the lid 17 are made of metal and form an anti-ionization screen for the screw heads 6 and for those parts of the shank 7, Fig. 1, which are located inside the fixture 10, i.e. within the area of the holes 13 and 16. For one conceived case, this area has been designated 9 in Fig. 1. The only part of each screw 1 which is exposed to the plasma-nitriding is thus the drilling screw tip 2 and the remainder of the shank 7, i.e, the section 8. After the ion nitriding and subsequent pick¬ ling in an acid bath, the screws were provided with a thin layer of zinc by galvanic treat¬ ment in order, finally, to be chromated in the same way as has been described above. In this case, therefore, the structure of the surface layer which is shown in Fig. 2 only ap- plies within the parts 2 and 8, while the head 6 and also the upper part 9 of the shank 7 of the screw have a surface layer structure which is void of the ion nitrided layer 3, while these parts of the screw nevertheless have a zinc layer 4 and a chromate layer 5.

In order to test the importance of the zinc coating, a number of screws, which had been treated in different ways, were examined in a test series which comprised 3 different tests.

In test No. 1, 5 screws were tested which were made of stainless steel having the follow¬ ing composition: 0.01 % C, 0.5 % Si, 0.6 % Mn, 18 % Cr, 9.5 % Ni and 3.5 % Cu, bal¬ ance iron and impurities. The screws were mounted in a fixture of the above-described type, after which the tips 2 and the lower part 8 of the screw were subjected to ion nitriding. The screws were then allowed to cool in air, after which they were pickled. Clark's solution (37 % HC1, 20 g/1 Sb2θ3 and 50 g 1 SnCl2) was used as the pickling solution. After the pickling, the samples were thoroughly rinsed with pure water. The whole of each screw was zinc-coated galvanically in the manner which has been de¬ scribed above such that the screw head, the shank and the tip all received a zinc layer which was at least 5 μm in thickness.

In test No. 2, 5 screws were tested which were made of stainless steel of the ASTM 305 type having the nominal composition of maximally 0.06 % C, 18.5 % Cr and 11.5 % Ni, balance iron and impurities. These screws were not ion nitrided and nor were they oth- erwise surface-hardened. However, the screws were pickled and coated with zinc gal¬ vanically in the same manner as was employed for the sample preparation for test No. 1.

In test No. 3, 10 screws were tested which were made of the same steel as in test No. 1. The screws were ion nitrided in a screening fixture in the same manner as for the sample preparation for test No. 1, and were pickled but not coated with zinc

Testing for corrosion was carried out in accordance with ISO 6988 (corresponds to DIN 50018), the so-called Kesternish test. Prior to the corrosion test, all the screws were mounted in a plastic-coated metal facing sheet on a wooden board. The metal facing sheet with the screws was inclined at approximately 45° during the exposure. The screws were exposed for two cycles. Each test cycle consisted of: 8 h, 40 ± 3°C, condensing moisture, 2.0 1 SO2 for a 3001 test chamber; 16 h, drying in ambient atmosphere. After each test cycle, the screws were examined visually and the proportion of the area af¬ fected with red rust was estimated. The following results were obtained.

Test 1 cycle 2 cycles

1 unaffected unaffected

2 approx. 1 % red rust approx. 5 % red rust

3 unaffected 1 sample, approx. 1 % red rust