FASTENERS FOR WINDOW FABRICATION

Transcription

1 1.0 INTRODUCTION FASTENERS FOR WINDOW FABRICATION A SPECIFIER'S GUIDE TO FASTENERS A window's performance is only as good as its weakest link. Fasteners represent the smallest unit percentage in total window costs; but if the wrong product is used the costs of repair or replacement can be prohibitive. This Specifier's Guide will cover the following points Corrosion - Problem, Types and Solution 3.0 Security 4.0 Safety 5.0 Applications and Fastener Types 6.0 Summary

2 2.0 CORROSION. 2.1 The Problem - One of the major requirements in any window replacement programme is a reduction in the requirement for maintenance. Windows by their very nature are exposed to both air and water; the combination of both these elements and metal fasteners will generally lead to corrosion. At best there is the aesthetic problem of surface rust; at worst a total mechanical breakdown with all the risks that entails. The problem is significantly increased where dissimilar metals are in contact. A prime example of this is the attachment of stainless steel friction stays with carbon steel or martensitic stainless steel (400 series) fasteners. Where fasteners attach hardware, crevice corrosion can occur beneath the head of the fastener caused by a micro climate effect which is different from the surrounding climate. Crevice corrosion can cause stress corrosion in hardened martensitic stainless steels leading to a failure of the fastener without prior warning. 2.2 Types of Corrosion that can effect window fasteners. General corrosion Galvanic corrosion Chemical corrosion Biochemical corrosion Aeration cell corrosion Crevice corrosion Stress corrosion Localised corrosion ( pitting )

3 2.3 Specifier Requirements - B.S : 1992 Durability of buildings and building elements. What standard can the specifier use to base and judge his/her specification upon. B.S :1992 details the intended design life of the proposed building and individual components or assemblies as required and defined by the client or his agent. Housing refurbishment has an estimated design life of 30 years whilst new housing, health and educational buildings have an estimated design life of 60 years. Most external cladding, doors and windows will last the life of the building, years, with periodic treatment. Therefore to avoid damaging maintenance costs fasteners must have at least an equal service life to the particular system in which they are incorporated. Are fasteners installed in window systems accessible and maintainable? Do you want the responsibility of having to maintain and replace fasteners with the associated costs involved? Appendix 2 provides a table of design life requirements 2.4 Codes of Practice The BPF publication Guidelines for "THE SELECTION AND APPLICATION OF FASTENERS FOR THE MANUFACTURE OF PVC-U WINDOWS AND DOORS", specifies that stainless steel friction stay hinges are secured with austenitic stainless steel fasteners to prevent bi-metallic corrosion. Section 4. Details the materials and finishes. Fasteners used for the attachment of hardware shall have a corrosion resistance in excess of 240 hours in neutral salt spray test to BS Fasteners used to attached stainless steel hardware shall be protected against corrosion to provide in excess of 500 hours in neutral salt spray test to BS Carbon steel fasteners are not recommended for the attachment of stainless steel hardware because of the risk of galvanic corrosion between dissimilar metals. Carbon steel fasteners will not provide 500 hours corrosion resistance when tested in application attaching stainless steel hardware and therefore will not meet the requirements for this application. The only alternative is stainless steel fasteners, which meet the 500 hour corrosion resistance salt spray test. Note It should be noted that there is no direct correlation between 500 hours salt spray testing and real time natural environment exposure. 2.5 Independent Testing - Salt Spray Testing Salt Spray Testing / Plating In 1993 SFS Stadler commissioned a test programme by Birmingham City Laboratories, formerly known as IRL, to examine the corrosion resistance of a wide range of fasteners that were recommended by fastener suppliers. All fasteners, with the exception of austenitic stainless steel, showed evidence of significant red rust, some fasteners showing corrosion as early as 120 hrs. The "in

4 application" test is especially important, to highlight the potential problems of bimetallic corrosion, as some plated fasteners will resist salt spray for up to 1000 hours when they have not been used in application. Since the completion and publication of the above report, the fastener industry has made significant improvements in the plating used to offer corrosion resistance for fasteners. However no plating or material can claim to offer a guarantee of resistance to corrosion over a period of 10 years, which is the demanded by most specifiers - with the exception of austenitic stainless steel. Many fastener suppliers can offer test certificates showing one batch of their product has achieved 500 hours exposure to salt spray without corrosion, but no supplier ever shows the certificate on batches that fail. Even testing on the same batch at monthly intervals can show widely varying results. It should also be noted that salt spray tests do not reveal either crevice or stress corrosion which can have a severely damaging effect on the mechanical performance of the fastener. 2.6 Stainless Steel - Common grades used in the window industry When the results of the IRL tests were published many fabricators and specifiers were confused to read about stainless steel rusting. The explanation is hopefully quite straightforward. Stainless steel is a generic term covering over 200 steels which are stain - less. However, the 3 main types are as follows:- i) Austenitic: 18-20% Chromium 8-1 0% Nickel AISI Series 200 and 300 are considered to be the steels most resistant to corrosion in industrial and coastal environments * - The most popular grade is The alloys are non-hardenable and non-magnetic - They have a low thermal conductivity ii) Ferritic: 12-30% Chromium AISI Unsuitable for manufacturing fasteners, generally used in the manufacture of friction stays for the window industry. iii) Martensitic: 11-14%Chromium

5 AISI 410 have a poor corrosion resistance * especially when quenched and tempered. They are mainly used for cutlery and indoor applications. - These alloys are hardenable and magnetic. - When hardened are at risk of stress corrosion (sub 2.1) * According to "Handbook of Stainless Steels" McGraw-Hill, New York Real Life Testing - Swedish Corrosion Institute Report R62 124:1995 Information for the specifier to make judgements upon based on fact: The Swedish Government has long been at the forefront in the fight against corrosion. In 1985 government and industry combined to commission real life application testing of fasteners to gain knowledge on the state of corrosion resistant fastener materials. The testing was carried out over a 10 year period and was completed in 1995, the report shows the corrosion resistance of various fasteners tested in real life applications in all climatic conditions. Part of the testing programme was to carry out in house laboratory corrosion tests to compare the results of real life applications and laboratory conditions. The results of the testing showed that salt spray tests could not be relied upon to give an accurate representation of corrosion resistant products in real life applications. The real life testing proved that austenitic stainless steel, along with hot dipped galvanised plating were the only fastener materials/treatments to remain corrosion free in all climatic conditions. Martensitic stainless steel proved only to be suitable for rural applications and not recommended for urban, industrial or coastal areas. 2.8 Summary - All platings are designed to sacrifice themselves to protect the base metal. Even the most superior coatings will break down exposing the base metal which will lead to red rust corrosion on all but austenitic stainless steel. 500 hour salt spray testing is the minimum industry requirement but should not be used to base long term specifications upon. 2.9 Conclusions - To prevent aesthetic or mechanical corrosion specify austenitic stainless steel fasteners to secure stainless steel hardware. It would be advisable to specify austenitic stainless steel for all load bearing fasteners e.g. pivot gear on reversible windows. Specifying austenitic stainless steel for all applications will remove the risks and costs of red rust corrosion completely.

6 3.0 SECURITY 3.1 Industry Requirement - A secure window is best achieved by specifying a window which achieves the requirements of BS 7412/7950 ( formerly BSI PAS ). 3.2 Test Loads - To meet the requirements of BS 7412/7950 : a window must withstand a lateral load of 1 kn while a load of 3kN is applied to all locking points. 3.3 Problem Area - Many windows even if fully reinforced do not have reinforcement running right into the corner of the vents or frames. This resulted in many windows failing pre-test trials as standard fasteners securing the friction stay pulled out of unreinforced PVC u. One solution is to mitre reinforcement but this is labour intensive and slows production down, which could lead to a significant increase in the finished cost of windows. 3.4 Fastener Solution - SFS Stadler have, through the development and manufacture of a purpose designed security fasteners enabled many system companies and fabricators to comply with the requirement of BS 7412/7950 even in unreinforced profile. 3.5 Conclusion - Specify SPT-S-PD8/SRI 4.8 x length

7 4.0 SAFETY 4.1 Potential Problem - All opening vents are held in place by window hardware, which in turn is secured to vent and frame by fasteners. The implications of any fastener failure could have far reaching consequences not only in cost but personal injury litigation. 4.2 Material - It is recommended that all load bearing fasteners should be manufactured in austenitic stainless steel for total corrosion protection. There have been several instances where carbon steel and aluminium fasteners have corroded to the extent where windows have become unsafe. 4.3 Securing Hardware to Reinforced PVC-U - The use of pilot holes for self tapping fasteners should be avoided wherever possible. Self drilling / self tapping fasteners self drill a pilot hole in an exactly defined ratio to the diameter of the fastener, ensuring maximum pull - out values - if correctly installed. Stripped fasteners should be removed and replaced with a 4.8mm diameter repair fastener which should be installed by hand. 4.4 Securing Hardware to Unreinforced PVC-U - In some cases fasteners of the right design can give better performances into unreinforced PVC-U than reinforced profile. In tests carried out for the BPF Fastener Performance Working Party, fasteners with a single high thread flank 4.3mm diameter - full thread flank self piercing point, retained 65% of pull out values even when stripped. Fasteners of differing designs dropped to between 23-28% of original values - see appendix 4 The 4.3mm diameter thread design was subjected to 20,000 open/close cycles when stripped without failure. A different fastener design failed at 28 cycles see appendix Conclusion - Specify that fasteners should when installed meet the requirements of B.S : part 2 : 1987 see appendix 4 for further details To prevent window failure, ensure that the fasteners specified provide the optimum performance, even when stripped, and are austenitic stainless steel. See model specification for details.

8 5.0 APPLICATION AND FASTENER TYPE 5.1 Reinforcement Retainment - To ensure optimum mechanical performance only self drilling / self tapping fasteners should be used. Carbon steel is the preferred fastener material for this application, as no long term benefit is gained by the specifier in the use of austenitic stainless steel. Fastener head styles may be pan or countersunk to suit window system. At least 2 full threads of fastener must project through reinforcement. 5.2 Securing Standard Window Hardware To reinforced PVC-U As 5.1 but head style must be compatible with the furniture to be attached To unreinforced PVC-U To ensure optimum mechanical performance, even when stripped, fasteners should have a single high thread flank - minimum diameter 4.3mm - with fully threaded self piercing point. Head style must be compatible with the furniture to be attached. Fastener must penetrate at least 2 wall thickness or equivalently thickened wall section of PVC-U profile. 5.3 Securing Stainless Window Furniture Material Fasteners should be manufactured from austenitic (non magnetic) stainless steel as prescribed in BS.3111 part 2 : 1979 to prevent galvanic corrosion. Austenitic stainless steel which have been hardened using the nitriding process should not be used as this process significantly reduces corrosion protection To reinforced PVC-U Fasteners should incorporate a welded carbon steel point to ensure optimum mechanical performance but all threads into reinforcement must be austenitic stainless steel to provide corrosion free anchorage point. Where aluminium reinforcement is used drill point may be manufactured in austenitic stainless steel To unreinforced PVC-U As but in austenitic stainless steel. To comply with the requirement of BS 7412/ , top corner track and fourth arm fastener must be single high thread flank 4.8mm diameter with fully threaded self piercing point Head style Fastener head style must be compatible with the furniture to be attached. Fasteners for friction stays should have a special low profile pan head to prevent fastener head interfering with movement of the hinge. It is recommended that friction stay fasteners have defined underhead serations to reduce over tightening and prevent lateral movement of the hinge.

9 5.4 Mechanical Jointing - Fastener should be a self tapping fastener - 4.8mm diameter - capable of forming a thread into aluminium or PVC-U screwports. Fasteners should be of sufficient length to allow 35mm penetration into screwport and thread length should not exceed 35mm to prevent jacking apart of profiles. Where increased mechanical performance is required a system such as KB thread barrel section can be used. 5.5 Frame Fasteners - The range of frame fasteners is wide and varied to meet the demands of a wide variety of substrates. It is recommended that specifiers consult the Code of Practice for the survey and installation of windows. This may be obtained from either the British Plastics Federation or Glass and Glazing Federation.

10 6.0 SUMMARY 6.1 Specification - The proceeding pages and appendices are intended to give a full explanation of the reasons for specifying fastener material and types. The following page is a distillation of these points and if implemented will eradicate many of the fastener problems which currently exist. As one would expect there are slightly increased costs resulting from the specification of high performance materials. It must be remembered that a window's performance is only as good as its weakest link. Fasteners represent the smallest unit percentage in total window cost; but if the wrong product is used, the cost of repair or replacement can be prohibitive. Below is a comparison expressed in units which clearly indicates the small difference in cost between different specifications. Specification Cost Austenitic stainless steel fasteners for stainless furniture 100 and carbon steel fasteners for all other applications. Austenitic stainless steel fasteners for all applications 102 Carbon steel fasteners for all applications 98.5

11 ACKNOWLEDGEMENTS Swedish Corrosion Research Institute Bulten Kanthal BSI Customer Services - B S 7543 : 1992, B S 6375 : 1987, B S 7412 : 1991 BPF/GGF - The selection and application of fasteners for the manufacture of PVC-U windows and doors. Birmingham City Laboratories - B C L BPF/GGF - Code of Practice for the survey and installation of white high impact modified PVCu windows Handbook of stainless steels, McGraw-Hill 1977 Heinz Wieland: Graduate Civil Engineer Swiss Federal Institute of Technology FMPA Stuttgart University