Note: Descriptions are shown in the official language in which they were submitted.
WO 94/25764 21~ 0 ~ 9 3 PCT/SE94/00400
A ScREW
The present invention relates to a screw. The invention particularly has in view two types
of screw, namely self-drilling screws int~nded to secure sheet metal, especially stainless
s sheet metal, on roofs and house frontages, and self-tapping screws.
A multiplicity of clçm~nl1s, several of which are difficult to reconcile, are placed on a self-
drilling screw intçn-led to act as a securing çl~m~nt for st~inless sheet metal on roofs and
house frontages:
0 - 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 esper;~lly the surface of the screw
tip, shall possess a very high degree of hardncss,
15 - the screw shall possess very good inhere,lt rç~icl~nce to corrosion and no stress corro-
sion shall arise as a result of galvanic renctions between the screw and the stainless
sheet metal,
- at least the screw head shall, for n~h~ic reasons, be of the same colour as the sheet
metal,
20 - the screw must not be too eA~,ensi-Je.
Many di~erenl alloys, especially dirreléllL steel alloys, have been proposed for self-drill-
ing screws, these proposals inr.ll~ing martensitic stainless chromium steels. However,
none ofthe materials proposed hitherto has fulfilled all the above mentioned d~m~n~s.
25 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 I e4uire~,cnls that are placed on self-drilling screws which are
int~n~ed to be used as securing el~mçntc for st~ steel facings for roofs or frontages.
As far as self-tappi"g screws are concc-~,e,d, the most i"",o, l~lL thing is that the screw
30 shall possess a very high degree of h~rdness, 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 el -ç~.~ for st~inless sheet metal
or other stainless çl~m~.ntc the same dçm~n~s are placed on it in terms of corrosion re-
~ict~nce as are placed on self-drilling screws.
The above d~m~nds can be s~ticfied therein that the screw is characterized by what is
stated in the appending patent claims.
Wo 94/25764 216 0 7 9 3 PCT/SE94/00400
~, . . .
Further characteristic features and aspects of the invention will be evident from the fol-
lowing description. In the following description, I cre~ ence will be made to the accompa-
nying dlawings, of which
Fig. 1 shows a self-drilling screw of a design which is known per se, to which the in-
s vention can be applied;
Fig. 2 shows dia~ ;c~lly~ and on a very large scale, the structure of a surface layer
of the screw, and
Fig. 3 illustrates how the screws can be ~,~ged in a fixture during a lle~ l step.
lo The matrix of the screw 1, i.e. the whole screw with the exception of the surface layer,
con~;sls of austçnitiC stainless steel. This implies that the steel has a high content of
chrol ~UIll and nickel and a low content of carbon, i.e. max 0.1 % C, p-eÇerably max 0.05
% C; 18-25 % Cr; and 8-20 % Ni. In addition, the steel can contain other alloying ele-
ments, for ~. ~n.ple up to a m~Yimlln of 10 % Mo. Plercl~bly, the steel also colll~ills a
lS certain quantity of copper, suitably 2~ %, in order to improve the cold wolLbility of
the steel. A suitable co",po~ilion is 0.01 % C, 0.5 % Si, 0.6 % Mn, 18 % Cr, 9.5 % M,
and 3.5 % Cu, balance iron and i~yulilies
That which ~lictin~ hçs ~st~nitiC stainless steels is that they possess a very high degree
20 of corrosion rçsi~t~r~ce and a good level of to~lghnç~c 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 ~dequ~te surface ha~dncss in the drilling tip 2 of a self-drilling
25 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 merlillm
in the process. In this lle.,~ .l the screws are placed in a suitable fLxture which, in tum,
is placed in a furnace which is filled with a mixture of nil~oge. and }"~drogell gases. A
30 voltage of 1000 V is applied belweell 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 sul~aces ofthe screws, thereby heating the c~,llponenls to the desired nitriding tem-
p~lalule, so that no separate, external heating ofthe furnace is required. At the same
time, this bolllb&ldll~ t with ions supplies nitrogen to the screw surfaces, thereby
3s providing the desired nitriding effect. By means of this 1,~ , the gas and the screw
surfaces are heated to app.c,~ y 480C. The 1l..~ lasts for 30 h, r~c~lhi~ in a
surface-haldelled layer 3, Fig. 2, being obt~;~.ed which has a i' ^Lnçss of 0.01-0.2 mm,
wo 94/25764 21~ 0 7 9 3 PCT/SE94/00400
with an average depth of appro~. alely 0.05 mm and with a surface h~dness of at least
900 HV and, prere~bly, a hardllcss within the range of 1000-1300 HV. By contrast, the
hal dncss of the matrix is not ~re.,Led and retains a h~ dncss of ~ x;~ lly 250 HV, which
is sufficient for the screw to be of ~deqn~te strength so that it can be screwed, with the
5 screw at the same time having a very good level of tollghness.
A proble~n with ion nitriding is that the passivation layer ofthe st~inless ~lct~.nitic steel is
partially de~,oyed by the ionic bGllll,ard...~,n~, res.lki~ in a lowering of the corrosion
.c.~;c~ r,e. In order, inter alia, to restore the corrosion lçs~ -ce, the ion nitrided screws
o are - in accordal ce with one embodiment - covered with a thin layer of zinc by an elec-
tro-plating l- e~ .l This zinc layer 4 has a thir1rness of at least S llm, prcr~.ably of at
least 8 ~lm, but does not exceed 25 ~m in thirl~nesC An ~dition~l effect of the zinc layer
is that it imparts lubricating prope.~ies to the screw, which p-upe.Lies are adv~nt~eollc
when the screw is to be used as a self-drilling screw. The zinc layer also provides the
lS screw with an aestheti~ y appealing surface and colour. The galv~ni7.~tion is expe-
diently pc-ru---.ed by means of a dipping process in an acidic zinc bath after picl~lin~ 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
20 rçcict~nce 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 th~ ness of 1-5 llm, or approxi-
mately 2 ~lm. The chro--,atmg can be carried out by depo~;l;.~p 3-valent ch.(,.. iu..l - so-
called blue cl..c....ale - by a dipping process lasting app.u~ 1 minute.
25 Finally, the screw heads can be covered with a layer of lacquer, eYpeAiPntly 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 re.,.ains exposed while, at the same time, by
approp.iate choice of lacquer colour, a screw can be obla~ed which co.l,ple~ely harmo-
30 nizes with the sheet metal in which it is to be used.
Acco.dn1g to an alternative embo~limPntJ 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
35 resl.lting from the ion n il iding. It is particularly the peaks on the surface which are re-
moved so that an even surface finish is ob~1ed. While the surface-hardened layer re-
sulting from the ion nitriding has a thi~ess of 0.01-0.2 mm, with an average depth of
WO 94/25764 216 0 7 ~ 3 `~ PCT/SE94/~
about 0.05 mm, the surface layer removed by the electrolyte polishing amounts to a
maximum of only 20 ~lm, normally from 5 to 10 llm. Use of this lIeA~ 1 means that the
abovementioned galvanization can be di~,el1sed with for certain applications. If so de-
sired, the electrolyte polished screw can of course also be provided with a suitable lac-
5 quer paint.
When developing the present invention, the inventor first started from the p, ell~ise that it
should be possible to ion nitride the whole screw and that the corrosion rçcict~n~.e would
be restored in an entirely acceplable manner by the subsequent galvanization. However,
lo tests carried out as a so-called Ke~l~lllish test de...Gn~llaled that black pittings were
obla-ned on the exposed screw heads when the screws were mollnted 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 ofthe type which is shown in Fig. 3. The
fixture CQn~ 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 th:^~ness of which determines the height of the screw heads above the bottom 12.
The plate 14 has through-holes 16 ;,.. ed;~lP1Y opposite the holes 13 in the bottom 12.
20 The screws which are to be ion nitrided are mo~mted in the holes 16, 13, after which the
box 1 1 is covered by a lid 17. Both the box 11 and the lid 17 are made of metal and form
an anti-ioni7~tion 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 ofthe holes 13 and 16. For
one conceived case, this area has been deci~ted 9 in Fig. 1. The only part of each
25 screw 1 which is exposed to the plasma-nitri~in~ is thus the drilling screw tip 2 and the
rç.~ der 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 cl~o..~led in the same way as has been desc~ibed above. In
this case, ll.erefole, 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 _inc layer 4 and a chrolllale layer 5.
In order to test the ll~.?o-L~1ce ofthe zinc coati~ a number of screws, which had been
treated in di~t;l~ L ways, were eY~mined in a test series which comprised 3 di~elenL
tests.
Wo 94125764 216 0 7 ~ 3
In test No. 1, 5 screws were tested which were made of st~inless 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 h,.pu,ilies. 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
5 nitriding. The screws were then allowed to cool in air, a~er which they were pickled.
Clark's solution (37 % HCI, 20 g/l Sb203 and 50 gtl SnC12) was used as the pickling
solution. A~er the piç1~ling the p!.es 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
lo which was at least S llm in thi~nPss
In test No. 2, 5 screws were tested which were made of : less steel of the ASTM 305
type having the nominal composition of ". ~;... lly 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
5 erwise surface-hardened. However, the screws were pickled and coated with zinc gal-
vanically in the same manner as was e"",lbyed for the sample plepalalion for test No. 1.
In test No. 3, 10 screws were tested which were m--ade of the same steel as in test No. 1.
The screws were ion nitrided in a scræl~ing fixture in the same manner as for the sample
20 prep~a~ion for test No. 1, and were pickled but not coated with zinc.
Testing for corrosion was carried out in accordance with ISO 6988 (col~ ,ol~ds to DIN
50018), the so-called Kesternish test. Prior to the corrosion test, all the screws were
mo~mted in a plastic-coated metal facing sheet on a wooden board. The metal facing
25 sheet with the screws was in..lined at appro~ Ply 45 during the exposure. The screws
were exposed for two cycles. Each test cycle consisted of: 8 h, 40 + 3C, con~Pn~in~
moisture, 2.0 1 SO2 for a 300 1 test cl all~l)el, 16 h, drying in ~llb ~nt alll,os~vhere. A~er
each test cycle, the screws were e ~ ed visually and the propo~ lion of the area af-
fected with red rust was estim~ted. The following results were obt~;ncd.
Test 1 cycle 2 cycles
unaffected un~æled
2 approx. 1 % red rust approx. 5 % red rust
3 unaffected 1 sample,
approx. 1 % red rust
