FORD_WD952

FORD WORLDWIDE FASTENER STANDARD Date: 1996-10-01 Engineering Standards & 1 of 4 Printed copies are uncontrolled. Systems Engineering (ESSE) WD 952 Mechanical and Performance Requirements of Through Hardened and Carbon Restored Thread Rolling Screws with Induction Hardened Points 1. SCOPE. This standard specifies the requirements for high performance through hardened and carbon restored thread rolling screws with induction hardened points which have performance capabilities beyond those normally expected of other types of tapping screws. The standard relates to screws with basic 60 screw thread form, metric series, in the range from 6mm up to and including 12mm nominal thread diameter and for use in applications where fastener toughness is a prime consideration. As partial or complete decarburization of the thread of thread forming screws seriously impairs the thread rolling properties of the screw, this condition is not permitted. Decarburization as permitted by ISO 898/1 is replaced in this standard by a requirement for carbon restoration as detailed in Clause 3.2. Fasteners produced to this standard possess mechanical properties in line with ISO 898/1 property class 10.9 (010) except surface hardness. Clause 3.4 defines thread forming capability in steel up to a hardness of 150 HV and thickness equal to the screw diameter. The parameters of material hardness and thread engagement may be varied by analysis of joint requirements and it is therefore recommended that the specific application be investigated to determine joint criteria. NOTE: Lubrication approved by Ford is permitted on parts to which this specification is applied unless otherwise specified. 1.1 REFERENCES. ISO 262 - ISO general purpose metric screw threads - Selected sizes for screws, bolts and nuts. ISO 898/1 - Mechanical properties of fasteners - Part 1: Bolts, screws and studs. ISO 898/2 - Mechanical properties of fasteners - Part 2: Nuts with specified proof load values. WSS-M99A3-A - Embrittlement Avoidance 2. MATERIALS. Screws shall be made from cold heading quality fully killed, fine grain, spherodized annealed steel wire. The following chemical composition is given for guidance only. Table 1. Chemical Composition - Ladle Property Class and Marking Symbol ISO 898/1 Equivalent Property Class Chemical Element Composition Limits (% by weight) Carbon 0.35 - 0.40 Manganese 0.70 - 0.90 Sulfur 0.040 Max 010 10.9 Phosphorous 0.035 Max Silicon 0.15 - 0.35 Molybdenum 0.20 - 0.30 FORD WORLDWIDE FASTENER STANDARD Date: 1996-10-01 Engineering Standards & 2 of 4 Printed copies are uncontrolled. Systems Engineering (ESSE) WD 952 Mechanical and Performance Requirements of Through Hardened and Carbon Restored Thread Rolling Screws with Induction Hardened Points 3. MECHANICAL PROPERTIES. Mechanical properties defined in ISO 898/1 apply except as detailed in Table 2. Table 2. Mechanical properties Criteria Clause Test Reference Wedge Tensile Strength 3.1 ISO 898/1 Decarburization 3.2 4.1 Surface Hardness 3.3 and Table 3 4.2 Thread Forming Capability 3.4 4.3 3.1 WEDGE TENSILE. Finished screws shall be hardened and tempered to achieve the minimum tensile strength for the applicable equivalent property class detailed in ISO 898/1 (See Table 1). 3.2 DECARBURIZATION. During the hardening process the carbon potential of the atmosphere shall be maintained at a level slightly in excess of the carbon content of the screws being processed. This process of carbon restoration is designed to eliminate partial and total decarburization of the screw thread form. Partial or complete decarburization of the screw thread form would seriously impair the thread rolling properties of the screw. Carbon enrichment up to 0.1mm maximum from the surface of the screw is permitted as a result of the carbon restoration process when tested in accordance with Clause 4.1. 3.3 SURFACE HARDNESS. The minimum tempering temperatures specified in ISO 898/1 shall apply (See Table 1). When tested is accordance with Clause 4.2, the hardness levels as given in Table 3 shall be achieved. Table 3. Surface hardness Property Class Surface Hardness (HV) 010 340 - 430 3.4 THREAD FORMING CAPABILITY. The screw shall, without permanent deformation of its own thread when viewed at 10x magnification, form a mating thread in a test plate in accordance with Clause 4.3. The thread formed by the screw in the test plate shall be capable of accepting a fastener threaded in accordance with ISO 262 tolerance class 6g and shall resist a proof load test in accordance with ISO 898/2 property class 8. 3.5 POINT HARDNESS. The lead threads shall be induction hardened to achieve a minimum hardness of 450 HV on one to three full threads to a depth of 0.2mm below the root as shown in Figure 1. Following induction hardening the part shall be stress relieved at 200oC - 230oC for one hour at temperature. 3.6 EMBRITTLEMENT. All through hardened and carbon restored thread rolling screws shall meet the requirements of WSS-M99A3-A. FORD WORLDWIDE FASTENER STANDARD Date: 1996-10-01 Engineering Standards & 3 of 4 Printed copies are uncontrolled. Systems Engineering (ESSE) WD 952 Mechanical and Performance Requirements of Through Hardened and Carbon Restored Thread Rolling Screws with Induction Hardened Points 4. TEST METHODS. 4.1 DECARBURIZATION. Longitudinal sections shall be taken through the thread axis approximately one nominal diameter from the end of the screw. The specimen shall be suitably mounted and prepared for metallographic examination at not less than 100 x magnification. Prior to examination the sample shall be etched in 2% Nital solution. 4.2 SURFACE HARDNESS TEST. The surface hardness shall be measured using the Vickers Hardness Testing Procedure in accordance with ISO 6507. The surface hardness shall be performed on the head after removal of any finish and suitable preparation (1200 grit grinding or better). Care should be taken to remove as little material as possible. For referee purposes, a micro-hardness instrument with a Vickers indenter and a 300g load shall be used. In such cases, measurements shall be made on the thread profile of a suitably prepared longitudinal metallographic specimen. 4.3 THREAD FORMING CAPABILITY. A mating thread must be formed by driving the sample screw into the test plate until a minimum of one pitch of the screw, excluding the tapered lead threads, extends beyond the plate. The axial load to ensure commencement of the thread forming process shall not exceed 20N. 4.3.1 TEST PLATE. The test plate shall be of low carbon rolled steel having a hardness of 115 to 150 HV30. Plate thickness shall be equivalent to the nominal screw diameter. Hole sizes shall be as given in Table 4. 4.4 INDUCTION HARDENING TEST. The hardness of the induction hardened zone may for routine control purposes be carried out on the screw end using a Vickers 5 kg load. As an alternative or where this method is not applicable, a Vickers micro-hardness test with a 300g load may be used on a suitably mounted and prepared specimen. This method shall be a referee in case of dispute. The extent of the induction hardened zone shall be determined by visual examination of a longitudinal section taken through the induction hardened portion and etched in accordance with standard metallurgical practice (See Figure 1). Figure 1. Induction hardened zone (pictorial representation) FORD WORLDWIDE FASTENER STANDARD Date: 1996-10-01 Engineering Standards & 4 of 4 Printed copies are uncontrolled. Systems Engineering (ESSE) WD 952 Mechanical and Performance Requirements of Through Hardened and Carbon Restored Thread Rolling Screws with Induction Hardened Points 4.5 TORQUE WRENCHES. Torque sensing power devices used in all tests shall be accurate within 3% of the specified torques. Alternatively, a torque wrench of equivalent accuracy may be used. For referee purposes a torque sensing power device shall be used. 5. MARKING. 5.1 SYMBOLS. Marking symbol is 010 (See Table 1). 5.2 IDENTIFICATION. Through hardened and carbon restored thread rolling screws with induction hardened points shall be marked by indenting or embossing with the marking symbol 010 as defined in Table 1. Marking is obligatory for hexagon screws with nominal diameters of 5mm or greater and for other products where the shape permits, preferable on the head. 5.3 TRADE (IDENTIFICATION) MARKING. The trade (identification) marking of the manufacturer is mandatory on all products which are required to be marked with the marking symbol. Table 4. Test plate thickness and hole sizes Nominal Thread Size and Plate Thickness (mm) 6 8 10 12 Hole Dia. (mm) Min. 5.4 7.3 9.2 11.1 Max. 5.43 7.336 9.236 11.143 NOTE: Plate thickness tolerance in accordance with ISO 5954 (for rolled plate)