Patent Application
20150376764
The invention relates to a process for manufacturing a fixing device and a fixing device made by such a process. In particular, albeit not exclusively, the present invention relates a fixing device for application in fixing to concrete and like materials/substrates.
Conventional threaded fixing devices such as screws are difficult to secure in masonry substrates since it is difficult for a conventional thread to find secure location within a bore in such a substrate. Conventional screw thread fixings are accordingly conventionally secured within bores in masonry substrates by first lining the bore with a lining of relatively soft material into which the threaded fixing can cut its own thread, at the same time compressing the lining against the walls of the bore within the masonry substrate. A typical example of such a lining is that sold under the trademark Rawlplug. Such linings are available in fibrous and plastics material form and in a wide variety of configurations reflecting a very considerable activity in the art over the years to improve upon the security and ease of use of screw threaded fixing devices used with such liners.
Adopting a somewhat similar principle, alternative forms of fixing device are of metallic material and structured so as to be expansible after introduction into a bore in a masonry material whereby compressive forces against or impingement into the internal surfaces of the bore resist withdrawal of the fixing device from the bore. Reflecting similarly substantial activity in the art, a wide variety of such devices is available. For example, various devices of this kind are available under the trademarks Fischer, Hilti and Rawlplug.
Applicant Company has been responsible for providing a number of fastening screws that have a shank provided with sometimes several helical threads which provide self-tapping capacity for the coarse deep thread. For example, U.S. Pat. No. 5,531,553 discloses a masonry fixing device which comprises a steel shank which in the form of the blank is right circular cylindrical form. A ridge-groove-ridge configuration extends helically along the lower portion of shank and comprises a pair of parallel opposed ridges upstanding from an adjacent land. Each ridge defines with the adjacent ridge a groove. At least the forward end of the lower portion of shank is configured so as to provide a self-tapping facility. In use, the fixing device is introduced into a pre-drilled bore in a masonry substrate such as brickwork by turning so as to form a thread on the interior walls of the bore.
In order to maintain the integrity of a structure it is important that a fixing does not corrode. Fixings employed in construction are frequently exposed to inclement conditions, by way of being exposed in all sorts of weather, attached to damp or wet structures, subject to variations to temperatures and humidity. Corrosion is a normal, natural process. Corrosion can seldom be totally prevented, but it can be minimized or controlled by proper choice of material, design, coatings, and occasionally by changing the environment. Various types of metallic and non-metallic coatings are regularly used to protect metal parts from corrosion. The use of linings can affect corrosion—and not necessarily for the better.
Modern and traditional construction fastening bolts are made from a variety of steels which are manufactured to increase strength. Presently extreme endurance fastening bolts are manufactured with boron steel, which are cold forged, thread rolled and then subjected to a heat treatment and yellow passivation. Boron steels possess hardenability equivalent to that of much higher carbon steels and more expensive low alloy heat treatable steels. Tempering, following oil or water quenching after forming, toughens boron steels. The addition of only 0.001-0.003% soluble boron to a suitably protected base steel can produce an increased hardenability compared to that obtained by additions of about 0.5% manganese, chromium or molybdenum, but with little effect on the as-rolled, normalized or annealed strength. In addition, during the hardening process scaling is formed when hot boron steel reacts with the oxygen in the air.
There are a number of problem areas associated with the manufacture and subsequent use of hardened boron steel, namely, the hardening process and the corrosion protection. Boron steel lacks the zinc coating that HSS steels have for the corrosion protection. The application of coatings to enhance the surface resistance coating of boron steels has tended to result in a reduced hardness, with annealing taking place when heated with protective coatings, which effect is notable especially on cold worked steels. This market is becoming increasingly competitive and there is an increasing need for fixings to become cheaper while providing good corrosion protection.
Certain types of stainless steels have been found not to be suitable for high tensile fixings since some grades have been found to be susceptible to stress corrosion cracking, which is an insidious type of failure. Stress corrosion cracking can occur without an externally applied load or at loads significantly below yield stress. Thus, failure can occur without significant deformation or obvious deterioration of the component. Stress corrosion cracking is a failure mechanism that is caused by environment, susceptible material, and tensile stress. Whilst all metals are susceptible to stress corrosion cracking in the right environment, stainless steel is well known for stress corrosion cracking problems. Ferritic stainless steels generally have better engineering properties than austenitic grades, but have reduced corrosion resistance, because of the lower chromium and nickel content. They are also usually less expensive. Martensitic stainless steels are not as corrosion-resistant as the other two classes but are extremely strong and tough, as well as being highly machinable, and can be hardened by heat treatment.
Presently, applicant manufactures fixings from a grade of steel containing boron. Once cold forged and thread rolled it is subjected to heat treatment and surface finish. The standard finish is bright zinc plate plus yellow passivation. This finish is good for 100 hours of salt spray testing. Salt spray test is a common test for various types of all metal coating, plating, and surface finishing in military, marine and aerospace engineering. A typical salt spray test for this would be set in the region of 100 hours. A 24 hours salt spray test is considered by some to be the equivalent to 10-15 years for 304 stainless steel, but the correlation of such tests is not particularly good; it is best to consider salt spray testing as a screening test to confirm that a product is good for a particular duration. The environment around the part also matters: if the part will see physical damage, heat, water, and/or salt, then the finish will need to be improved.
Notwithstanding the above there is an increasing tendency for requirements in large construction projects for fixings to exceed certain standards, such as the corrosivity category C5-M standard—a very severe (marine) atmospheric-corrosivity category, corresponding to exterior coastal and offshore areas with high salinity or interior buildings and other areas with almost permanent condensation and with high pollution. This corrosion resistance category C5, is the toughest standardized environmental condition with respect to corrosion.
Material surface plays a key role to control the service life of materials that is deteriorated by the environmental attacks, especially corrosion and wear. Annually, a large number of economy losses in various industries come from the corrosion and wear damages on machines and components. It is known to apply zinc based coatings to hardened fasteners, with thick coatings (80-100 μm) being applied for long term outdoor use and thinner coatings, (40-60 μm) being applied for less arduous conditions. Such coatings, however, are liable to scratching and ultimate degradation of the fasteners. Various systems across the world employ diffusion coatings with a particular emphasis on hardening of the material, but this does not necessarily provide a high degree of corrosion resistance and vice versa.
The present invention therefore seeks to provide a fixing device for fixing to concrete, other like substrates that overcomes, or at least reduces some of the above-mentioned problems of the prior art. The present invention also seeks to provide a fixing device that has enhanced corrosion resistance.
Accordingly, in a general aspect, the invention provides a process for manufacturing a fixing device for fixing to concrete and like substrates, the process comprising:
The present invention thus provides a specialized hardening and controlled corrosion protection treatment. The hardening applied by the nitriding process can be applied by a number of known nitriding processes. Applicants have determined that subsequent to the nitriding process, diffusion of zinc powders, the powder size being in the range of 5-80 μm, creates a superb corrosion resistance, and during which, by that addition of further powders selected from a range including zinc, tin, iron, aluminum, magnesium, can provide further advantages.
This development on the traditional nitriding treatment is designed to give a hard resultant surface condition that readily accepts the levels of zinc diffusion required by the sherardizing process, performed in a rotating barrel, conveniently in a non-oxidizing atmosphere, conveniently a nitrogen atmosphere.
The conversion of the steel surface layer into a zinc rich surface layer happens at the atomic level.
Conveniently, the steel is 34CrMo4 steel, preferably having a low silicon content. Unlike plating or coating, the finish is not subject to flaking, peeling, wear-off from rubbing (as when employed as a fastener), or rust when scratched.
Preferably the steel is rolled in a cold-forming die comprising first and second die members; each die member being arranged with respect to the other die member such that grooves defined in a first die member are positioned such that, in use, the dies are reciprocated in parallel planes with respect to each other the steel is rolled whilst the die members are brought together in proximity, whereby the corresponding upstanding members in the steel mate with corresponding grooves in the second die member.
The sherardizing diffusion process comprises the heating of the fixings are heated together with a zinc powder, with the powder size being in the range of 5-80 μm, whereby zinc-alloy is formed in a diffusion process to markedly provide a significant increase in corrosion resistance.
The sherardizing diffusion process can be performed over a sufficient period of time to ensure that from the surface, inward to the core, distinct regions—commonly referred to as diffusion layers of zinc-iron alloy.
The sherardizing diffusion process can be performed at temperatures of between 340 and 500° C. for a period of between 30 minutes and 180 minutes.
The sherardizing diffusion process can be performed at temperatures below 300° C., for a period of between 30 minutes and 360 minutes, where the zinc powder sublimates.
The corrosion proofing process provides a zinc-iron surface alloy. The corrosion proofing process can comprise heating zinc powder at a temperature between 340 and 500° C. Alternatively, the corrosion proofing process comprise sublimating zinc powder at a temperature between 200 and 300° C. Applicants have extensive test results indicating that the particular process routes provide particular long life indications for superior hardened materials, as compared to traditional stainless steels, including marine grade stainless steels.
In accordance with a further aspect of the invention, there is provided a fixing device manufactured in accordance with the preceding paragraphs.
For a better understanding of the present invention, reference will now be made, by way of example only, to the Figures as shown in the accompanying drawing sheets.
a and 4b detail features of the die parts in detail.
a and 5b show sample bolts prior to salt spray testing.
a and 6b show sample bolts after 3250 hours of salt spray testing.
a and 7b show sample bolts at the end of the salt spray test.
There will now be described, by way of example only, the best mode contemplated by the inventor for carrying out the present invention. In the following description, numerous specific details are set out in order to provide a complete understanding to the present invention. It will be apparent to those skilled in the art, that the present invention may be put into practice with variations of the specific.
In order to explain the invention in detail, reference shall now be made to a screw bolt manufactured in steps according to the invention. In a brief overview of one embodiment of the present invention, there is shown in
The fixing devices shown in
The fixing device shown in
Groove 5 is defined between two parallel marginal ridges 6 and 7, formed of shank material displaced from the groove 5 by the plastic deformation that occurs during thread rolling.
Top section 3 of shank 2 is formed with a conventional male screw thread 8 to enable threaded engagement of articles to the fixing element 1 when the fixing element 1 is secured in a masonry structure.
The axial extremities of the shank 2 are formed having regard to the practicalities of the fixing device 1 in use. Thus, for example, the extremity or the bottom portion 4 of shank 2 has a frustoconical configuration to assist bore entry of that extremity. The extremity of the top portion 3 of shank 2 has a domed configuration and is provided with a screwdriver engagement slot, cross-recess, hexagon or square drive (not shown) or screw head enlargement such as one configured with a countersink.
It will be noted that a land 9 is provided between the turns of the ridge-groove-ridge configuration. In the embodiment shown in
The fixing device embodiment shown in
The thread-rolling station shown in
a and 4b relate to surface detail of the die surfaces. Die grooves corresponding to ridges 6 and 7 on the fixing are shown at 6a and 7a in
Once a fixing has been formed in shape, further surface treatments can be applied. The fixing is subjected to a two stage hardening process, comprising an initial hardening and tempering process followed by a secondary, nitriding hardening process.
The hardening and tempering process can be performed by a neutral hardening, which process shall now be described—although other hardening and tempering processes can also be employed.
Provides parts with an optimal combination of high strength, toughness and temperature resistance.
In neutral hardening processes, the chemical composition of the steel surface of the parts is not intended to be changed during the process.
Direct quench hardening in oil is then performed which is the most common practice for hardening of steel. The first step is to heat up in stages to the hardening temperature which is 830-870° C. At a temperature above 730° C. a transformation of the microstructure into austenite takes place.
The second step is to hold at this hardening, austenitizing temperature to simultaneously fully equalize the temperature of the parts, and transform the microstructure into austenite, which provides a reduction in the specific volume.
The third step is quenching the part direct from the austenitizing temperature in a cold medium. This kind of quench medium is oil. The quenching speed must be high enough to prevent the material from transforming back into the original soft structure and is quite rapid for small piece parts such as fixings, although consideration must be give n to applying the process too rapidly.
Nitriding imparts a high surface hardness that promotes high resistance to wear, scuffing, galling, and seizure. Fatigue strength is increased mainly by the development of surface compressive stresses and nitriding is employed for a range of applications including motor vehicle engine parts such as gears, crankshafts, camshafts, cam followers, valve parts.
Gas nitriding is a low temperature (typically 520° C./970° F.), low distortion “thermochemical” heat treatment process carried out to enhance the surface properties of finished or near finished ferrous components. The layer usually consists of two zones—the compound layer (white layer), which can be a cubic or hexagonal nitride and the diffusion layer below with dissolved nitrogen and hard nitride precipitations. The compound layer on the surface of the parts is responsible for the major benefit of high resistance to wear, scuffing, galling and seizure. The diffusion layer contributes improved fatigue strength and works as a support for the hard compound layer. By controlling and adjusting the process atmosphere, the constitution of the layer can be influenced from thin compound layers for fatigue strength improvement to thick nitrogen and carbon rich compound layers in case of gaseous nitrocarburizing and post oxidation if good wear and corrosion resistance is desired.
In plasma nitriding, the reactivity of the nitriding media is not due to the temperature but to the gas ionized state. In this technique intense electric fields are used to generate ionized molecules of the gas around the surface to be nitrided. Such highly active gas with ionized molecules is called plasma, naming the technique. The gas used for plasma nitriding is usually pure nitrogen, since no spontaneous decomposition is needed (as is the case of gas nitriding with ammonia). There are hot plasmas typified by plasma jets used for metal cutting, welding, cladding or spraying. There are also cold plasmas, usually generated inside vacuum chambers, at low pressure regimes. Plasma nitriding is an expensive process and accordingly tends not to be commonly deployed.
In a final stage, the processing of the present invention is completed by the performance of a variant of a sherardizing process, wherein the fixings are heated together with a zinc powder, whereby zinc-alloy is formed in a diffusion process to markedly provide a significant increase in corrosion resistance. The powder can be provided in a rotating barrel.
At temperatures of 380° C. and above, in presence of zinc powder, zinc reacts with iron oxides. Accordingly, the iron surface is deoxidized and “cleaned”, followed by the zinc diffusing into the surface. Conveniently, to prevent substrate oxidation, the reaction barrel is has a non-oxidizing atmosphere—nitrogen can conveniently be employed.
Preferably, the process is performed over a sufficient period of time to ensure that from the surface, inward to the core, distinct regions—commonly referred to as diffusion layers—of zinc-iron alloy are produced, each diffusion layer being harder and more corrosion resistant, noting that the material of the fastener is treated rather than coated as such. The process can conveniently be performed in a rotating barrel arrangement at elevated temperatures of between 340 and 500° C. The diffusion process can be performed at reduced temperatures, i.e. at temperatures below 300° C. where the zinc powder sublimates, penetrating the steel structure to form zinc-iron alloy i.e. the steel surface layer is converted into a zinc rich surface layer, at the atomic level. Typical process times being of the duration of 30 minutes to two hours, with lower temperature procedures taking longer for the process to take place to achieve a desired diffusion depth. Coverage of the product is effectively all-over, without any “bald” patches due to hanging supports as in other types of coating systems, since the products that are treated are provided in a rotating barrel, within the predominantly zinc based composition.
The coating powder can comprise a number of additives: further powders selected from a range including zinc, tin, iron, aluminum, magnesium, can provide further beneficial properties in addition to the general corrosion resistance, for example by way of accelerating the rate of diffusion and thus depth of diffusion layer. These powders can be provided in a range of percentages from 0.1-5% of the overall powder weight, with the powder size being in the range of 5-80 μm. Further additives may be employed, for example, clay materials, such as kaolin (Al2O3•2SiO2) which is typically available with sizes of 10 μm or less and which is believed to help improve an evenness of coating. The kaolin powders can be provided in a range of percentages from 0.1-2% of the overall powder weight.
34CrMo4 Alloyed steel is a heat-treatable steel with a typical tensile strength of 800-1100 N/mm2. A typical composition of 34CrMo4 in percentage terms is as follows: C 0.34 Si 0.25 Mn 0.70 Cr 1.10 Mo 0.25 S <0.035. The steel is easily worked in a number of processes such as hot forging/hot rolling plus annealing/normalizing plus tempering/quenching plus tempering. Additionally, 34CrMo4 steel provides sufficient strength for fixings, as can be determined from the table below:
As will be appreciated, other heat treatable steels can be employed. Fasteners made in accordance with the present invention have been manufactured form this steel, since it provides a readily available rolled bar (and other configurations) in a variety of sizes. Reference may be had to number of specification sheets.
Applicants have had independent test results in respect of salt spray testing to the UK Water Industry Specification for anti-corrosion coatings on threaded fasteners, WIS 4-52-03:1994 Issue 1. With reference to
The above conditions were stabilized for two hours prior to introducing the samples. The samples were carefully cleaned using demineralized water and a mild solvent. The samples were introduced in a horizontal orientation close to the upper part of the cabinet. The temperature, salt fall out rate, salt pH and salt specific gravity were monitored as the test progressed as follows:
The following table summarizes the sample of the data
Note that after trials and re-testing the surface hardness specification for the fixings made in accordance with the present invention provided a figure of 580-600 hv.
It will be appreciated from the foregoing that the invention provides a realizable and controllable systems and method of providing fixings with high tensile strengths that are impervious to corrosion as is anticipated to occur in a typical fixing over several tens of years. The method applies certain known techniques in new processes to provide significant advantages in fixings over known materials, without resorting to the use of boron alloying processes as is known and to provide significant improvements in corrosion resistance.
It will also be appreciated that although only one particular embodiment of the invention has been described in detail, various modifications and improvements can be made by a person skilled in the art without departing from the scope of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1217576.6 | Nov 2012 | GB | national |
| TBA | Nov 2013 | GB | national |
This application is the national stage entry of and claims priority to PCT patent application serial number PCT/GB2013/000469, titled, “A PROCESS FOR MANUFACTURING A FIXING DEVICE”, which was filed on Nov. 1, 2013, the entire specification of which is incorporated herewith by reference in its entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/GB2013/000469 | 11/1/2013 | WO | 00 |
