KNOWLEDGE CENTRE
 
WIRE GUAGE OF WOOD SCREWS
Screw Guage No Measurement Unit
2 82.00 - 83.00 Micrometer
3 92.00 - 93.00 Micrometer
4 106.00 - 107.00 Micrometer
5 116.00 - 117.00 Micrometer
6 127.00 - 128.00 Micrometer
7 138.00 - 139.00 Micrometer
8 155.00 - 156.00 Micrometer
9 169.00 - 170.00 Micrometer
10 185.00 - 186.00 Micrometer
12 205.00 - 207.00 Micrometer
14 228.00 - 230.00 Micrometer
16 267.00 - 268.00 Micrometer
 
 
STRENGTH CHARACTERISTICS
 
Mechanical Properties
Hardness is a measure of a material’s ability to resist abrasion and indentation. For carbon steels Brinell & Rockwell hardness testing can be used to estimate tensile strength.
   
Toughness is a material’s ability to absord impact or shock loading, and is rarely a specification requirement.
   
Ductility is the measurement of a material to deform before it fails and fractures.
   
Elongation is the extension of a material in a tension test at any point and is probably the characteristic that renders the most complications in fasteners. A material that exhibits little or no plastic deformity at its fracture point is considered brittle.
 
Tensile Strength
Itis the ultimate strength of a given alloy or product that determines how much load it can withstand before breaking, or being pulled apart. It is calculated by performing a tension test and determining at what tensile strength maximum load is reached. i.e. how much force is needed in psi, pounds per square inch, to break it. Tensile strength figures in PSI are used as a common standard in comparing different metals
 
Proof Load
It is applied tensile load that fastener must support without permanent deformation and represents the usable strength of a certain standards.
 
Thread Strength
It should be taken into consideration as the threaded section is the weakest part of the fastener so it remains a critical aspect of strength and clamping force. The proper assembly should always have fasteners of equal grade working to meet their ultimate tensile strength.
 
Creep
At ordinary temperatures metals under load normally change their dimensions only when the load they are under is changed. At elevated temperatures, however, dimensional changes take place even under constant load. Metals, and other materials creep. For example, a bolt under a constant tensile load at high temperatures will elongate continuously. The higher the temperature the fastener will elongate, i.e., the higher the creep rate.
 
Fatigue Strength
It is subjected to repeated cyclic loads can suddenly and unexpectedly break, even if the loads are well beneath the strength of the material. Fatigue strength is the maximum stress it can withstand for a specified number of repeated load cycles prior to its failure.
 
Shear Strength
Shear Strength is the maximum load that can be supported prior to fracture when applied at a right angle to the fastener’s axis. It is opposite from tensile strength in that tensile strength is a measured longitudinal pull, while shear strength is caused by a push or pull 90’ from the longitudinal axis. A single shear joint occurs when there is a load occurring in one transverse plane and if the shear strength was exceeded the fastener would be broken into two pieces. Double shear is a load applied in two planes where the fastener could be broken into three pieces.
 
Torque
It is a twisting force that causes the rotation of a shaft or will set up a twist in a stationary shaft and is generally expressed in foot-pounds (ft-lbs) or inch-pounds. Properly threaded products achieve their clamping load from the tension or torque that is derived from the mating of the external and internal threads.
 
Torsional Strength
This is a load usually expressed in terms of torque, at which the fastener fails by being twisted off its axis. Tapping screws and sockets set screws require a torsional test.
 
Strength-to-Weight-Ratio
In application where load supporting parts are to be lifted or moved against the pull of gravity the strength-to-weight ratio (SWR) becomes an important figure. SWR is defined as the ratio of the tensile strength to the density of the material, the density being the weight per unit volume.
 
Temperature Effects: Elevated and Cryogenic
Each come with a set of static and dynamic strengths that are determined by their manufacturing process, composition, and size. In application, a range of factors including temperature variation affects the availability of these strengths. Every bolting material has a temperature above which it would be unsafe to use; this is often times referred to as the high temperature service limit. Although a fastener loses strength as the temperature increases, the service limit is usually determined by an occurrence known as stress relaxation. A fastener is bolted into a joint, which places it under significant stress. When the temperature rises the bolts begins to relieve itself of a significant amount of stress. Science stress and preload are related, the clamping force that the bolt is exerting to hold the joint together will be greatly reduced.
At very cold temperatures these same molecular bonds lose their ability to expand and flex, which is needed to relieve fastening stress under application load. As a result, it can become brittle at low temperature and also provides less than room temperature fastening performance.
 
 
STANDARDS
 
- ANSI (American National Standards Institute)
 
- ISO(The International Organization of Standardization)
 
- DIN (Deutsches Institute for Normung, or The German Engineering Society)
 
- BSI(British Standard Institute)
 
- JIS(Japanese Industrial Institute) Fasteners Standards
 
- ASTM(The American Society for Testing and Materials)
 
- ASME(American Society of Mechanical Engineers)
 
- SAE(The Society of Automotive Engineers)
 
- AFNOR( Association Francaise de Normalisation)
 
- AISI( American Iron and Steel Institute)
 
- UNI(Ente Nazionale Italiane de Unificazione)
 
- VDI(Verein Deutscher Ingenieure, Dusseldorf)
 
- EN(European Organization for Standardization)
 
 
MATERIAL
 
Fastener Materials
Almost 90% of all fasteners are made of carbon steel. A plain carbon steel is one in which carbon is the only alloying element added to the iron base. The amount of carbon in the steel controls its hardness, strength, and ductility.
 
Low Carbon Steel
Low carbon steel generally contains less than 0.25% carbon and cannot be strengthened by heat-treating; strengthening may only be accomplished through cold working and case hardening. The low carbon material is relatively soft and week, but has outstanding ductility and toughness; in addition, it is machinable, weldable and is relatively inexpensive to produce. The most commonly used chemical analyses include AISI 1006, 1008, 1016, 1018, 1021, and 1022.
 
Medium Carbon Steel
Medium carbon steel have carbon concentrations between about 0.25% and 0.60%. these steels may be heat treated by austenizing, quenching and then tempering to improve their mechanical properties. The plain medium carbon steels have low hardenabilities and can be successfully heat treated only in thin sections and with rapid quenching rates. Notices on the SAE J429, ASTM A325, and ASTM A449 specifications that their strength properties “step down” as the diameters increase. The popular chemical analyses include AISI 1030, 1035, 1038, and 1541 Alloy Steel.
 
High and Very High Carbon Steel
Steels with a carbon range of 0.45% to0.75% are classified as high-carbon and those with 0.75% to 1.7% carbon as very-high-carbon steels. Both of these steels respond well to heat treatment. As a rule, steels up to 0.65% carbon can be welded with special electrodes, although preheating and stress relieving techniques must often be used after the welding is completed. This form of steel has the best hardness, strength and ductility. The areas best suited for this steel are in tools, drills, saws, knife blades, and bearing. High carbon content reduces the wear and deformation of the steel.
 
Alloy Steel
Carbon steel is classed as an alloy steel when it contains more than 1.65% manganses, 0.60% silicon, or 0.60% copper, or when chromium content is less than 4%. Dozens of different carbon alloy steels are used for fasteners: AISI 1335(Mg), 4037(Mo), 4140(Cr, Mo) M 4340(Ni,Cr, Mo), 8637(Ni, Cr, Mo), and 8740(Ni, Cr, Mo), for examples.
 
Manganese (Mn) contributes strength, moderately improves hardenability, and is beneficial to surface quality.
   
Nickel (Ni) provides strength, improves toughness at low temperature, benefits corrosion resistance, and adds to the heat treat process, which assure more consistent results.
   
Chromium(Cr) increases hardenability, reduces susceptibility to temper brittleness, and has a powerful positive effect on temperature, tensile and creep strengths.
   
Sulphur(S) increases machinability, but causes lower strength at elevated temperatures.
 
Stainless Steel
Stainless steel is a family of iron-based alloys that must contain at least 10.5% chromium. The presence of chromium creates an invisible surface film that resists oxidation and makes the materials “passive” or corrosion resistance. Other elements, such as nickel or molybdenum are added to increase corrosion resistance, strength or heat resistance. Stainless steels can be simply and logically divided into three classes on the basis of their microstructure; martensitic, ferritic or austenitic. Each of these classes has specific properties and basic grade or “type”.
 
Austenitic : 18-8 ( the commercial grade) or 300 series (303 & 304 are common). This chromium-nickel type contains about 18% chromium and 8% nickel, is not hardenable by heat treatment, non magnetic ( in it’s annealed condition-it will become more so due to cold-working), and offers the greatest degree of corrosion resistance. 316 has a higher nickel content and offers higher corrosion resistance in certain chemical and seawater environments. About 80% of all stainless steel fasteners are produced from this type of steel.
   
Martensitic : Contains from 12-20% chromium, is magnetic, and because they are heat-treated they are high-strength. Type 410 and 416 are of this group and common fastening alloys. Suitable for industrial and medical applications, 400 series martensitic steel is much more corrosion-resistant than carbon steel and can be sharpened to equally-keen edge sharpness.
   
Ferritic : These are also chromium stainless alloys that can be case hardened. Type 430 falls into this class. This type of stainless steel is magnetic, non-hardenable by heat treatment and has very poor weld characteristics. They should not be used in situations of high corrosion resistance requirements, such as marine use or on building exteriors .The most common places to use Stainless Steel are in areas of corrosion and tempering, or were strength is required. Because of its corrosion resistant qualities and ability to attain a mirror-like finish, it is one of the most versatile of all metals.
   
Sulphur(S) increases machinability, but causes lower strength at elevated temperatures
 
 
CORROSION AND PREVENTION
 
Corrosion
Is the wearing away or alteration of metal by Galvanic (electro-Chemical) reaction, or by a direct chemical attack, such as the rusting of Iron & Steel. Corrosion can be thought of as an electro chemical reaction in which one metal is changed into a chemical, or simply eaten away.
 
Galvanic Corrosion
Is the combination of two dissimilar metals with an electrolyte is all that is needed to form a corrosive reaction. The use of dissimilar metals in structural design is common, especially where the fastener is a different material from the structures being joined. The necessary ingredient to induce corrosion, the electrolyte, may be present in the form of rain, snow dew, high humidity, ocean salt spray or even air pollution. Cell corrosion and pitting are similar types of corrosion because each requires only one metal and electrolyte to set up a corrosive attack system.
 
Stress Corrosion
A term representing particular condition where cracks are induced and propagated in a fastener under combined effects of stress and corrosive environments .The initial corrosion may occur at a point of high stress that contributes to crack initiation, which can be either intergranular or transgranular. Continued exposure to the corrosion environment will propagate the crack and can result in serious, and possibly catastrophic, failure.
 
Hydrogen Embrittlement
It is associated with fasteners made of carbon and alloys steels. It is a type of deterioration that can be linked to corrosion –control process. There are three main ways to fight to fight hydrogen embrittlement:
 
Use the proper plating procedures and bake fasteners correctly.
   
Use fastener coatings that do not involve electroplating.
   
Use a softer bolt material. As with stress corrosion, harder, stronger materials are more susceptible to this type of failure than weaker, softer ones.
 
Methods for Fighting Hydrogen Embrittlement
There are three methods used to fight hydrogen embrittlement, Hardness is a major contributor to hydrogen embrittlement. Harder, stronger materials are more susceptible to failure than weaker, softer ones. If the hardness is less than 35 HRC, there will probably be difficulty with hydrogen embrittlement and if the hardness is above 40 HRC, problems are more likely to occurs.
While coating process can also induce hydrogen embrittlement. Use a coating process that does not introduce hydrogen into material. If electroplating is still desired, ensure that plater uses the proper procedures and bakes the fasteners correctly based on its hardness. ASTM F 1941 has a hydrogen embrittlement relief requirement for coated fasteners made from steel heat treated to a hardness of HRC 40 or above, case- hardened Fasteners made from hardened steel .The extract time and temperature of the bake is not specified ,but times is between 2 and 24 hours at temperature between 350 to 450 F are listed as suitable depending on type, size, geometry etc.
The proper selection of the material for the services environment can reduce the risk of embrittlement. The potential for hydrogen embrittlement cracking is accelerated if the fasteners is acting as the cathode in a galvanic couple . Caustic or sour environments may require much lower hardness levels to lower the susceptibility to hydrogen embrittlement.
 
 
PLATINGS AND COATINGS
 
Plating and Coatings
Platings are the deposition of an adherent metal onto the surface of a base metal .Hot Dip Galvanizing, metal spray, vacuum, sherardizing , or mechanical plating accomplishes practically all deposition.
 
Hot Dip Galvanizing
Hot Dip Galvanizing produce a coating that thoroughly covers the work, including edges ,seams, and welds. Because of heavy coating buildup ,galvanizing requires special processes for small or fine threads and hot-dipped galvanized bolts must be used with galvanized nuts whose threads are oversized to compensate for the thick coating. It is widely used and inexpensive means of protecting fasteners .This process actually produces a thicker coating than electroplating and provides more projection against corrosion. The typical coating thickness is approx 0.0015 in., or about 10 times the thickness of the zinc electroplating. Hot dipping a fastener will dramatically alter the thread fit because of the thickness of the plating.
 
Electroplating
Is the deposition of a metallic coating onto an object by putting a negative charge onto the object and immersing it into a solution which contains a salt of the metal to be deposited.The metallic ions of the salt carry a positive charge and are attracted to the part. when they reach it, the negatively charged part provides the electrons to reduce the positively charged ions to metallic form. Plating by electrolysis, or electroplating , is commonly because it permits the control of the thickness of the plating.
 
Metal Spray (Metallizing)
It refers to the various process of applying zinc,as well as other plating materials, to the fastener surface by mean of the spray or blast of molten or semi molten metallic particles.
 
Vacuum Plating
A metallizing processes that is mainly used for decorative purposes, and can include several steps in addition to the actual evaporation of the thin metallic coating. Since the metallic vapor travels from heated source through a vacuum to the substrate in molecular form, a thin coating is deposited one molecular at a time. The coating therefore reproduces the exact contour or roughness of whatever it is coating.
 
Sherardizing
A cementation process where zinc dust is heated to a temperature near its molten point and its brought into intimate contact with the steel surface to form an iron and zinc coating on the steel by diffusion.
 
Mechanical Plating
A process where coating is applied by impacting particles of the plating material against the parts and cold-welding them to the surface.
 
Chromating
Chromate films are the chemical conversion coatings .The substrate metal participates in the coating reaction, becoming a component of the coating and has a profound influence on the properties of the coating. Chromate coating improve corrosion resistance, appearance of metal, and adhesion of organic topcoats.