WO1996014514A1 - A screw and method for the manufacturing and treatment of a screw - Google Patents

A screw and method for the manufacturing and treatment of a screw Download PDF

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Publication number
WO1996014514A1
WO1996014514A1 PCT/SE1995/001264 SE9501264W WO9614514A1 WO 1996014514 A1 WO1996014514 A1 WO 1996014514A1 SE 9501264 W SE9501264 W SE 9501264W WO 9614514 A1 WO9614514 A1 WO 9614514A1
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WO
WIPO (PCT)
Prior art keywords
screw
maximum
layer
steel
ion
Prior art date
Application number
PCT/SE1995/001264
Other languages
French (fr)
Inventor
Tonny Westlund
Original Assignee
Fästsystem I Karlskoga Aktiebolag
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Fästsystem I Karlskoga Aktiebolag filed Critical Fästsystem I Karlskoga Aktiebolag
Priority to AU38615/95A priority Critical patent/AU3861595A/en
Publication of WO1996014514A1 publication Critical patent/WO1996014514A1/en

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Classifications

    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F16—ENGINEERING ELEMENTS AND UNITS; GENERAL MEASURES FOR PRODUCING AND MAINTAINING EFFECTIVE FUNCTIONING OF MACHINES OR INSTALLATIONS; THERMAL INSULATION IN GENERAL
    • F16B—DEVICES FOR FASTENING OR SECURING CONSTRUCTIONAL ELEMENTS OR MACHINE PARTS TOGETHER, e.g. NAILS, BOLTS, CIRCLIPS, CLAMPS, CLIPS OR WEDGES; JOINTS OR JOINTING
    • F16B35/00—Screw-bolts; Stay-bolts; Screw-threaded studs; Screws; Set screws
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/0093—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for screws; for bolts
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/36—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases using ionised gases, e.g. ionitriding
    • C23C8/38—Treatment of ferrous surfaces
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/04—Hardening by cooling below 0 degrees Celsius

Definitions

  • the present invention relates to a screw consisting in the main of stainless steel with a shank with a point and at least one threaded portion.
  • the invention aims especially at two types of screw, namely self- drilling screws intended for fastening sheet metal, in particular stainless sheet metal, on roofs and house facades, and self-tapping screws.
  • the invention also relates to a method for manufacturing and treating a self-tapping or self- drilling screw and also use of a specific steel material for this manufacturing.
  • a self-drilling screw intended as a fastening element for stainless sheet metal on roofs and house fa ades has to satisfy a number of requirements, some of which are difBcult to combine: the material must have cold-workability such that it can be shaped in the cold state into a screw with a point which can drill the screw through a stainless metal sheet, the screw, or at least an outer layer, and in particular the surface in the screw point, must have very great hardness, the screw must have very good inherent corrosion-resistance and no stress corrosion must arise as a result of galvanic reactions between the screw and the stainless metal sheet, at least the screw head must have the same colour as the stainless metal sheet for aesthetic reasons, the screw must not be too expensive.
  • EP- 0 523 298 proposes using a conventional austenitic stainless steel of the 316 type which is nitrided in order to produce a hard outer layer, after which the nitrided outer layer is covered by a metal layer or by a plastic layer in order to prevent rust formation on the nitrided layer.
  • One aim of the invention is to offer a screw made of such a material and treated in such a manner that it satisfies all the requirements made of self-drilling screws intended to be used as fastening elements for stainless roof or facade covering.
  • self-tapping screws it is of primary importance that the screw has very high hardness, at least in an outer layer, in order to be able to function as "its own screw tap" in order to bring about threads in a bore hole.
  • the screw being used as a fastening element for stainless sheet metal or other stainless elements, the same requirements with regard to corrosion resistance exist in the case of self-drilling screws also.
  • the screw is characterized, according to a first aspect of the invention, in that at least the point and a length of said at least one threaded portion has a surface-hardening layer produced by ion nitriding which has a thickness of 0.01-0.2 mm and a hardness of at least 900 HV, preferably 1000-1300 HV.
  • ion nitriding which has a thickness of 0.01-0.2 mm and a hardness of at least 900 HV, preferably 1000-1300 HV.
  • the screw point and at least a front portion of the threaded screw shank have said surface-hardening layer produced by ion nitriding, while at least the screw head lacks such surface-hardening layer.
  • the ion nitriding according to the method according to the invention amounts to the screw being exposed to electric high voltage in a furnace which is filled with a mixture of nitrogen gas and hydrogen gas so that the gas is ionized and at least the point of the screw and at least a portion of the threaded part of the screw shank are supplied with nitrogen by ion bombardment for a time of 5-50 h and at a temperature of 450-600°C, as a result of which said screw surfaces are ion nitrided to create said surface-hardening layer which has a thickness of 0.01-0.2 mm and a hardness of at least 900 HV.
  • the screw can be placed in a hole in a fixture so that the point and said portion of the threaded shank project from the fixture and are exposed to the ion bombardment in the furnace, while the rest of the screw is shielded by a protection which covers the screw head.
  • the stainless steel contains a maximum of 0.1% carbon, preferably a maximum of 0.08% carbon, and, as a stabilizing element, at least one of the elements niobium, tantalum and titanium in a total content amounting to at least five times the carbon content, but a maximum of 1%.
  • stabilized austenitic stainless steel e.g. stabilized stainless steel of 18/9 type
  • the risk can arise that the hard outer layer is peeled off from the underlying steel during the drilling operation.
  • Fig. 1 shows a self-drilling screw of a design known per se, in the case of which the invention can be applied
  • Fig. 2 shows diagrammatically and on very large scale the construction of an outer layer on the screw within a ringed area A
  • Fig. 3 shows diagrammatically and on very large scale the construction of an outer layer within the ringed area of the screw according to another preferred embodiment
  • Fig. 4 illustrates how the screws can be arranged in a fixture during ion nitriding.
  • the starting material for the screw like the basic mass of the finished screw, i.e. the entire screw with the exception of the outer layer, consists of carbon-stabilized stainless steel.
  • types of stabilized stainless steel are stabilized austenitic stainless steel and stabilized precipitation-hardening stainless steel.
  • Examples of specific steel alloys which can be used for the screw according to the invention are given in the table below, in which steels no. 1-3 are constituted by stabilized stainless steel and steels no. 4 and 5 represent stabilized precipitation-hardening stainless steel. Table 1
  • a screw has been generally designated 1.
  • ion nitriding also called plasma nitriding.
  • This is a method which has obtained its name from the fact that a plasma or in other words an ionized gas is utilized as heating and nitriding medium in the process.
  • the screws 1 are placed in a fixture 10, Fig. 4. This consists of a flat box 11 with a plane bottom 12 which is provided with small openings 13 for the screws 1 which are to be ion nitrided.
  • the box there is also a plate 14, the thickness of which determines the height of the screw heads 6 above the bottom 12.
  • the plate 14 has through-holes 16 in front of the holes 13 in the bottom 12.
  • the screws which are to be ion nitrided are mounted in the holes 16, 13, whereupon the box 11 is covered by a cover 17.
  • Both the box 11 and the cover 17 consist of metal and form a shield against the ionization for the screw heads 6 and for the parts 9 of the shank 7, Fig. 1, which are situated inside the fixture 10, i.e. within the area of the holes 13 and 16.
  • the only part of the screws 1 which is exposed to the plasma nitriding is thus the drilling screw point 1 and the front portion 8 of the shank 7.
  • the fixture 10 with the screws 1 is placed in a furnace which is filled with a mixture of nitrogen gas and hydrogen gas.
  • a voltage of 1000 V is connected between the screws and the furnace wall.
  • the gas is ionized, the ions striking with great kinetic energy the screw surfaces which are exposed outside the fixture 10, i.e. the point 2 and the front portion 8 of the shank 7.
  • the components are heated to the desired nitriding temperature so that no separate outer furnace heating is required.
  • nitrogen is supplied to the screw surfaces, which produces the desired nitriding effect.
  • the temperature can be allowed to rise to 550-600°C.
  • the temperature lies between 450 and 600°C, preferably between 500 and 600°C.
  • the treatment is carried out for a time of 5- 50 h, preferably for a maximum time of 30 h, which is possible if the temperature lies within the higher range.
  • the treatment should preferably be carried out for 10-30 h.
  • a surface-hardened layer 3 is obtained.
  • ion nitriding is that the passivation layer of the stainless steel is partially destroyed by the ion bombardment, which means that the corrosion resistance is reduced.
  • the ion nitrided screws are covered - according to one embodiment, Fig. 2 - with a thin zinc layer by gah anotechnical treatment.
  • This zinc layer 4 has a thickness of at least 5 ⁇ m, preferably at least 8 ⁇ m, but does not exceed 25 ⁇ m thickness.
  • a further effect of the zinc layer is that it gives the screw lubricating properties which are advantageous when the screw is to be used as a self-drilling screw.
  • the zinc layer also gives the screw an aesthetically attractive surface and colour. Galvanization is carried out expediently by immersion in an acid zinc bath after pickling in an acid bath to remove oxides on the surface of the screw.
  • the zinc layer can also have defects.
  • the zinc layer can in a further operation be coated with a very thin chromium layer 5.
  • a typical thickness of this is 1-5 ⁇ m or approximately 2 ⁇ m.
  • the chromating can be carried out by depositing trivalent chromium - so-called blue chromate - by means of immersion for approximately 1 minute.
  • the screw heads can be covered with a lacquer layer, expediently a polyester lacquer, which is sprayed onto the heads in powder form, whereupon the lacquer is hardened in a manner known per se.
  • a lacquer layer gives a further improvement of the corrosion protection on the part which remains exposed, at the same time as, by selecting a suitable colour of the lacquer, it is possible to obtain a screw can which completely matches the metal sheet in which the screw is to be used.
  • the screw can, after ion nitriding and pickling, be electrolytically polished, i.e. treated in an electrolytic bath according to principles known per se, so that a very thin layer of the raw surface which is produced by ion nitriding is worked off In particular peaks on the surface are worked off so that a smooth surface finish is obtained.
  • the layer surface-hardened by ion nitriding has a thickness of 0.01-0.2 mm, with an average depth of approximately 0.05 mm
  • the surface layer worked off by electrolytic polishing is only 1-5 ⁇ m, normally 2 to 3 ⁇ m, in the ion nitrided areas 2, 8 of the screw and in the remaining areas 10-40 ⁇ m, normally of the order of 20 ⁇ m
  • the electrolytically polished screw also can, if desired, of course be provided with a suitable lacquer colour on the screw head 6.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Organic Chemistry (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • General Engineering & Computer Science (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)
  • Heat Treatment Of Articles (AREA)

Abstract

A screw consisting mainly of stainless steel with a head (6) and a shank (7) with a point (2) and at least one threaded portion, characterized in that the stainless steel contains a maximum of 0.1 % C, preferably a maximum of 0.08 % C, in that it is stabilized with at least one of the elements niobium, tantalum and titanium in a total content amounting to at least 5 times the carbon content, but a maximum of 1 %, and in that at least the screw point (2) and a length (8) of said at least one threaded part have a surface-hardening layer (3) produced by ion nitriding, which has a thickness of 0.01-0.2 mm and a hardness of at least 900 HV, preferably 1000-1300 HV. The invention also relates to production of a stainless screw by means of a method which comprises ion nitriding.

Description

A SCREW AND METHOD FOR THE MANUFACTURING AND TREATMENT OF A SCREW
The present invention relates to a screw consisting in the main of stainless steel with a shank with a point and at least one threaded portion. The invention aims especially at two types of screw, namely self- drilling screws intended for fastening sheet metal, in particular stainless sheet metal, on roofs and house facades, and self-tapping screws. The invention also relates to a method for manufacturing and treating a self-tapping or self- drilling screw and also use of a specific steel material for this manufacturing.
A self-drilling screw intended as a fastening element for stainless sheet metal on roofs and house fa ades has to satisfy a number of requirements, some of which are difBcult to combine: the material must have cold-workability such that it can be shaped in the cold state into a screw with a point which can drill the screw through a stainless metal sheet, the screw, or at least an outer layer, and in particular the surface in the screw point, must have very great hardness, the screw must have very good inherent corrosion-resistance and no stress corrosion must arise as a result of galvanic reactions between the screw and the stainless metal sheet, at least the screw head must have the same colour as the stainless metal sheet for aesthetic reasons, the screw must not be too expensive.
Many different alloys, in particular different steel alloys, have been proposed for self- drilling screws, inter aha martensitic, stainless chrome steel, but stainless austenitic steel also has been proposed for this purpose. EP- 0 523 298 proposes using a conventional austenitic stainless steel of the 316 type which is nitrided in order to produce a hard outer layer, after which the nitrided outer layer is covered by a metal layer or by a plastic layer in order to prevent rust formation on the nitrided layer. As far as the applicants know, however, none of the previously proposed materials has satisfied all the requirements indicated above. One aim of the invention, therefore, is to offer a screw made of such a material and treated in such a manner that it satisfies all the requirements made of self-drilling screws intended to be used as fastening elements for stainless roof or facade covering. As far as self-tapping screws are concerned, it is of primary importance that the screw has very high hardness, at least in an outer layer, in order to be able to function as "its own screw tap" in order to bring about threads in a bore hole. In the event of the screw being used as a fastening element for stainless sheet metal or other stainless elements, the same requirements with regard to corrosion resistance exist in the case of self-drilling screws also.
In order to achieve the abovementioned and other aims, the screw is characterized, according to a first aspect of the invention, in that at least the point and a length of said at least one threaded portion has a surface-hardening layer produced by ion nitriding which has a thickness of 0.01-0.2 mm and a hardness of at least 900 HV, preferably 1000-1300 HV. Preferably only the screw point and at least a front portion of the threaded screw shank have said surface-hardening layer produced by ion nitriding, while at least the screw head lacks such surface-hardening layer. The ion nitriding according to the method according to the invention amounts to the screw being exposed to electric high voltage in a furnace which is filled with a mixture of nitrogen gas and hydrogen gas so that the gas is ionized and at least the point of the screw and at least a portion of the threaded part of the screw shank are supplied with nitrogen by ion bombardment for a time of 5-50 h and at a temperature of 450-600°C, as a result of which said screw surfaces are ion nitrided to create said surface-hardening layer which has a thickness of 0.01-0.2 mm and a hardness of at least 900 HV. In order that only the screw point and at least a front portion of the threaded screw shank are surface-hardened by ion nitriding, the screw can be placed in a hole in a fixture so that the point and said portion of the threaded shank project from the fixture and are exposed to the ion bombardment in the furnace, while the rest of the screw is shielded by a protection which covers the screw head.
According to another aspect of the invention, the stainless steel contains a maximum of 0.1% carbon, preferably a maximum of 0.08% carbon, and, as a stabilizing element, at least one of the elements niobium, tantalum and titanium in a total content amounting to at least five times the carbon content, but a maximum of 1%. By these means, it is possible to counteract local depletion of chromium as a result of the formation of chromium carbides at the grain boundaries, which could otherwise take place during ion nitriding treatment, which in turn could entail a risk of grain boundary corrosion.
For self-drilling screws with small or modest diameters, e.g. diameters up to approx. 4.8 mm stabilized austenitic stainless steel, e.g. stabilized stainless steel of 18/9 type, can have not only adequate corrosion resistance but also adequately high hardness, normally a maximum of 250 HV. For self-drilling screws of larger dimensions, however, the risk can arise that the hard outer layer is peeled off from the underlying steel during the drilling operation. At least for such applications, it may be more expedient to select as starting material a stabilized stainless steel which is precipitation hardening by ageing treatment. After ageing treatment, it is possible to obtain with such a steel a hardness in the steel mass of 300-500 HV.
Further aspects and characteristics of the invention emerge from the following description of exemplary embodiments and from the subsequent patent claims.
In the following description, reference will be made to attached drawing figures, of which
Fig. 1 shows a self-drilling screw of a design known per se, in the case of which the invention can be applied, Fig. 2 shows diagrammatically and on very large scale the construction of an outer layer on the screw within a ringed area A, Fig. 1, according to a first embodiment, Fig. 3 shows diagrammatically and on very large scale the construction of an outer layer within the ringed area of the screw according to another preferred embodiment, and Fig. 4 illustrates how the screws can be arranged in a fixture during ion nitriding.
The starting material for the screw, like the basic mass of the finished screw, i.e. the entire screw with the exception of the outer layer, consists of carbon-stabilized stainless steel. Examples of types of stabilized stainless steel are stabilized austenitic stainless steel and stabilized precipitation-hardening stainless steel. Examples of specific steel alloys which can be used for the screw according to the invention are given in the table below, in which steels no. 1-3 are constituted by stabilized stainless steel and steels no. 4 and 5 represent stabilized precipitation-hardening stainless steel. Table 1
Steel C Si Mn Cr Ni Other Rest no.
1 < 08 ≤l.O <2.0 17-20 9-13 lOxC≤Nb+Ta <1.0 Fe an unavoidable impurities
2 ≤.08 ≤l.O <2.0 17-20 9-13 5xC≤Ti<0.5
3 .01 0.5 0.6 18 9.5 3.5Cu, 5xC≤ Ti<0.5
4 < 07 ≤l.O ≤l.O 15-17.5 3-5 ≤0.5Mo, 3-5 Cu, 5xC≤Nb+ Ta<0.5
5 < 07 ≤l.O ≤l.O 15-17.5 6-8 1A1, 5xC≤ Ti< 0.5
In Fig. 1, a screw has been generally designated 1. In order that, for a self- drilling screw, it has adequate surface hardness in the drilling point 2, the screw has been surface- hardened by so-called ion nitriding, also called plasma nitriding. This is a method which has obtained its name from the fact that a plasma or in other words an ionized gas is utilized as heating and nitriding medium in the process. During this treatment, the screws 1 are placed in a fixture 10, Fig. 4. This consists of a flat box 11 with a plane bottom 12 which is provided with small openings 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 6 above the bottom 12. The plate 14 has through-holes 16 in front of the holes 13 in the bottom 12. The screws which are to be ion nitrided are mounted in the holes 16, 13, whereupon the box 11 is covered by a cover 17. Both the box 11 and the cover 17 consist of metal and form a shield against the ionization for the screw heads 6 and for the parts 9 of the shank 7, Fig. 1, which are situated inside the fixture 10, i.e. within the area of the holes 13 and 16. The only part of the screws 1 which is exposed to the plasma nitriding is thus the drilling screw point 1 and the front portion 8 of the shank 7.
The fixture 10 with the screws 1 is placed in a furnace which is filled with a mixture of nitrogen gas and hydrogen gas. A voltage of 1000 V is connected between the screws and the furnace wall. By these means, the gas is ionized, the ions striking with great kinetic energy the screw surfaces which are exposed outside the fixture 10, i.e. the point 2 and the front portion 8 of the shank 7. At the same time, the components are heated to the desired nitriding temperature so that no separate outer furnace heating is required. By ion bombardment, nitrogen is supplied to the screw surfaces, which produces the desired nitriding effect. By virtue of the fact that the steels are stabilized, the temperature can be allowed to rise to 550-600°C. Normally, the temperature lies between 450 and 600°C, preferably between 500 and 600°C. The treatment is carried out for a time of 5- 50 h, preferably for a maximum time of 30 h, which is possible if the temperature lies within the higher range. The treatment should preferably be carried out for 10-30 h.
As a result, a surface-hardened layer 3 is obtained. One problem with ion nitriding, however, is that the passivation layer of the stainless steel is partially destroyed by the ion bombardment, which means that the corrosion resistance is reduced. In order inter alia to restore the corrosion resistance, the ion nitrided screws are covered - according to one embodiment, Fig. 2 - with a thin zinc layer by gah anotechnical treatment. This zinc layer 4 has a thickness of at least 5 μm, preferably at least 8 μm, but does not exceed 25 μm thickness. A further effect of the zinc layer is that it gives the screw lubricating properties which are advantageous when the screw is to be used as a self-drilling screw. The zinc layer also gives the screw an aesthetically attractive surface and colour. Galvanization is carried out expediently by immersion in an acid zinc bath after pickling in an acid bath to remove oxides on the surface of the screw.
However, the zinc layer can also have defects. In order further to improve the corrosion resistance, the zinc layer can in a further operation be coated with a very thin chromium layer 5. A typical thickness of this is 1-5 μm or approximately 2 μm. The chromating can be carried out by depositing trivalent chromium - so-called blue chromate - by means of immersion for approximately 1 minute.
Finally, the screw heads can be covered with a lacquer layer, expediently a polyester lacquer, which is sprayed onto the heads in powder form, whereupon the lacquer is hardened in a manner known per se. This lacquer layer gives a further improvement of the corrosion protection on the part which remains exposed, at the same time as, by selecting a suitable colour of the lacquer, it is possible to obtain a screw can which completely matches the metal sheet in which the screw is to be used.
According to an alternative embodiment, Fig. 3, the screw can, after ion nitriding and pickling, be electrolytically polished, i.e. treated in an electrolytic bath according to principles known per se, so that a very thin layer of the raw surface which is produced by ion nitriding is worked off In particular peaks on the surface are worked off so that a smooth surface finish is obtained. While the layer surface-hardened by ion nitriding has a thickness of 0.01-0.2 mm, with an average depth of approximately 0.05 mm, the surface layer worked off by electrolytic polishing is only 1-5 μm, normally 2 to 3 μm, in the ion nitrided areas 2, 8 of the screw and in the remaining areas 10-40 μm, normally of the order of 20 μm By means of this treatment, the abovementioned galvanization can be eliminated for certain applications. The electrolytically polished screw also can, if desired, of course be provided with a suitable lacquer colour on the screw head 6.

Claims

PATENT CLAIMS
1. Screw consisting mainly of stainless steel with a head (6) and a shank (7) with a point (2) and at least one threaded portion, characterized in that the stainless steel contains a maximum of 0.1% C, preferably a maximum of 0.08% C, in that it is stabilized with at least one of the elements niobium, tantalum and titanium in a total content amounting to at least 5 times the carbon content, but a maximum of 1%, and in that at least the screw point (2) and a length (8) of said at least one threaded part have a surface-hardening layer (3) produced by ion nitriding, which has a thickness of 0.01-0.2 mm and a hardness of at least 900 HV, preferably 1000-1300 HV.
2. Screw according to Claim 1, characterized in that only the screw point (2) and at least a front portion (8) of the threaded screw shank (7) have said surface-hardening layer (3) produced by ion nitriding, while at least the screw head (6) lacks such surface-hardening layer.
3. Screw according to Claim 1 or 2, characterized in that, within the surface-hardened areas, it has a surface structure which can be obtained by ion nitriding for producing said surface-hardening layer and an electrolytic poUshing following thereafter for working off the outermost layer of said surface-hardening layer.
4. Screw according to Claim 1 or 2, characterized in that the surface-hardening layer is covered by at least a gah anically applied metal coating (4).
5. Screw according to Claim 4, characterized in that said galvanic metal coating includes a zinc coating (4) which is at least 5 μm, preferably at least 8 μm, thick.
6. Screw according to Claim 5, characterized in that the zinc layer coated on the surface- hardening layer is at most 25 μm thick.
7. Screw according to Claim 5 or 6, characterized in that the zinc coating is in turn covered by a 1-5 μm thick chromium layer.
8. Screw according to any one of Claims 1-7, characterized in that at least the screw head is covered by a lacquer layer, preferably a polyester lacquer layer.
9. Screw according to any one of Claims 1-8, characterized in that the stainless steel is constituted by a steel which is precipitation hardenable by ageing, and in that the steel in the screws has been hardened by ageing treatment to a hardness of 300-500 HB.
10. Screw according to any one of Claims 1-9, characterized in that the screws are produced from a steel with the composition, in percentage by weight maximum 0.07 C maximum 2.0 Mn maximum 1.0 Si
15-20 Cr
3-10 Ni
5 x %C < (Nb + Ta +Ti) < 1.0
2-6 Cu maximum 10 Mo the rest essentially iron and impurities in normal contents.
11. Screw according to Claim 10, characterized in that the screws are produced from a steel with the composition, in percentage by weight, maximum 0.07 C maximum 2.0 Mn maximum 1.0 Si
15-17.5 Cr
3-5 Ni
5 x %C < (Nb + Ta +Ti) < 1.0
3-5 Cu maximum 1.0 Mo the rest essentially iron and impurities in normal contents.
12. Screw according to any one of Claims 1-8, characterized in that the steel, in addition to iron and unavoidable impurities, contains maximum 0.1% C 17-25% Cr 8-20% Ni maximum 10% Mo maximum 4% Cu.
13. Screw according to Claim 12, characterized in that the steel contains a maximum of 0.05% C.
14. Screw according to either of Claims 12 and 13, characterized in that the steel contains 2-4% Cu.
15. Screw according to Claim 14, characterized in that the nominal composition of the steel is maximum 0.05% C maximum 1.0% Si maximum 2.0% Mn
18% Cr
10% Ni
3.5% Cu the rest iron and unavoidable impurities.
16. Method for manufacturing and treating a self-tapping or self-drilling screw, characterized in that, from a starting material in the form of a stainless steel which contains a maximum of 0.1% C, preferably a maximum of 0.08% C, and as a stabilizing element at least one of the elements niobium, tantalum and titanium in a total content amounting to at least 5 times the carbon content, but a maximum of 1% C, the screw is formed so that it obtains its essential final shape with head and a shank which is provided with threads and point (2), and in that the screw is exposed to electric high voltage in a furnace which is filled with a mixture of nitrogen gas and hydrogen gas so that the gas is ionized and at least the point (2) of the screw and at least a portion (8) of the threaded part of the screw shank are supplied with nitrogen by ion bombardment for a time of 5- 50 h and at a temperature of 450-600°C, as a result of which said screw surfaces are ion nitrided to create a surface-hardening layer (3) which has a thickness of 0.01-0.2 mm and a hardness of at least 900 HV, preferably 1000-1300 HV.
17. Method according to Claim 16, characterized in that the screw is placed in a hole (13) in a fixture (10) so that the point (2) and at least a portion (8) of the threaded shank project from the fixture and are exposed to ion bombardment in the furnace, while the rest of the screw is shielded by a protection (17) which covers the screw head.
18. Method according to Claim 16 or 17, characterized in that the screw, after ion nitriding, is electrolytically polished and in that the screw head is covered by a lacquer layer.
19. Method according to Claim 18, characterized in that, by means of the electrolytic polishing, a 1-5 μm thick outer layer is worked off from the ion nitrided outer layer and a 10-40 μm thick outer layer within the parts of the screw which have not been ion nitrided.
20. Method according to Claim 16 or 17, characterized in that the surface-hardening layer (3) produced by ion nitriding is covered by at least a 5-25 μm thick metal coating, preferably 8-25 μm thick metal coating, which is applied galvanically.
21. Method according to Claim 20, characterized in that said galvanically applied metal coating is covered by a 1-5 μm thick chromium layer.
22. Method according to any one of Claims 16-21, characterized in that the screw head is covered by a lacquer layer.
23. Method according to any one of Claims 16-22, characterized in that, as starting material, a wire made of stabilized stainless steel which precipitation hardens by ageing treatment is selected, and in that the screw is ageing treated at a temperature between 450 and 600°C so that the steel mass obtains a hardness of 300-500 HB by precipitation hardening.
24. Application of a stainless steel which precipitation hardens by ageing treatment and which contains at least one of the elements niobium, tantalum and titanium as carbon- stabilizing element in a content of at least 5 times the carbon content, but in total at most 1.0%, for producing screws intended to be ion nitrided in a furnace at a temperature of 450-600°C for a time of 5-50 h.
PCT/SE1995/001264 1994-11-02 1995-10-25 A screw and method for the manufacturing and treatment of a screw WO1996014514A1 (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU38615/95A AU3861595A (en) 1994-11-02 1995-10-25 A screw and method for the manufacturing and treatment of a screw

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
SE9403773A SE9403773L (en) 1994-11-02 1994-11-02 Screw as well as ways to manufacture and process a screw
SE9403773-6 1994-11-02

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Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2315079A (en) * 1996-07-08 1998-01-21 Rhp Bearings Ltd Ion nitriding surface treatment of rolling element bearing steels
EP1253334A1 (en) * 2001-04-25 2002-10-30 Aoyama Seisakusho Co., Ltd. Threaded fastener and method of clamping using the same
EP1900945A1 (en) * 2005-06-13 2008-03-19 Saga Tekkohsho Co., Ltd Screw and screw fastening structure
JP2011256412A (en) * 2010-06-07 2011-12-22 Tanaka:Kk Stainless steel screw
JP2014506287A (en) * 2010-12-20 2014-03-13 エヨット ゲーエムベーハー ウント カンパニー カーゲー Low alloy carbon steel screw and method of manufacturing the screw
TWI714846B (en) * 2018-05-09 2021-01-01 國聯螺絲工業股份有限公司 Nitriding heat treatment process for 300 series stainless steel screw fasteners

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE3235447A1 (en) * 1981-10-27 1983-05-11 Yugen Kaisha Shinjo Seisakusho, Osaka Drilling screw
EP0523298A1 (en) * 1991-07-18 1993-01-20 Daidousanso Co., Ltd. A hard austenitic stainless steel screw

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE3235447A1 (en) * 1981-10-27 1983-05-11 Yugen Kaisha Shinjo Seisakusho, Osaka Drilling screw
EP0523298A1 (en) * 1991-07-18 1993-01-20 Daidousanso Co., Ltd. A hard austenitic stainless steel screw

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2315079A (en) * 1996-07-08 1998-01-21 Rhp Bearings Ltd Ion nitriding surface treatment of rolling element bearing steels
GB2315079B (en) * 1996-07-08 1999-03-24 Rhp Bearings Ltd Surface treatment of rolling element bearing steel
EP1253334A1 (en) * 2001-04-25 2002-10-30 Aoyama Seisakusho Co., Ltd. Threaded fastener and method of clamping using the same
EP1900945A1 (en) * 2005-06-13 2008-03-19 Saga Tekkohsho Co., Ltd Screw and screw fastening structure
EP1900945A4 (en) * 2005-06-13 2010-06-30 Saga Tekkohsho Co Ltd Screw and screw fastening structure
US7976258B2 (en) 2005-06-13 2011-07-12 Saga Tekkohsho Co., Ltd. Screw and screw fastening structure
JP2011256412A (en) * 2010-06-07 2011-12-22 Tanaka:Kk Stainless steel screw
JP2014506287A (en) * 2010-12-20 2014-03-13 エヨット ゲーエムベーハー ウント カンパニー カーゲー Low alloy carbon steel screw and method of manufacturing the screw
TWI714846B (en) * 2018-05-09 2021-01-01 國聯螺絲工業股份有限公司 Nitriding heat treatment process for 300 series stainless steel screw fasteners

Also Published As

Publication number Publication date
SE9403773D0 (en) 1994-11-02
SE9403773L (en) 1996-05-03
AU3861595A (en) 1996-05-31

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