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Mechanical Properties and Identification MarkingsforThreaded FastenersByK. P. ShahEmail: kpshah123[at]gmail.com (Please replace [at] with @)Committed to improve the Quality of LifeFor more articles on mechanical maintenance, visit www.practicalmaintenance.netThe information contained in this booklet represents a significant collection of technicalinformation about mechanical properties and identification markings for threaded fasteners.This information will help to achieve increased reliability at a decreased cost. Assemblage ofthis information will provide a single point of reference that might otherwise be timeconsuming to obtain. Most of information given in this booklet is mainly derived fromliterature on the subject from the sources as per the references given at the end of thisbooklet. For more information, please refer them. All information contained in this booklethas been assembled with great care. However, the information is given for guidancepurposes only. The ultimate responsibility for its use and any subsequent liability rests withthe end user. Please view the disclaimer uploaded on http://www.practicalmaintenance.net.(Edition: September 2020)1Mechanical Properties and Identification Markings for Threaded Fastenerswww.practicalmaintenance.net

ionMechanical PropertiesMechanical Properties of Fasteners Made of Carbon Steel andAlloy Steel as per ISO Standard (ISO 898)Mechanical Properties of Steel Nuts as per DIN StandardsMechanical Properties of Corrosion Resistant-Stainless SteelFasteners as per ISO Standard (ISO 3506)ASTM Standards for FastenersASTM Standards for Carbon and Alloy Steel Externally ThreadedFastenersASTM Standards for Carbon and Alloy Steel NutsASTM Standards for Alloy Steel, Stainless Steel and Nickel AlloyThreaded FastenersThreaded Fasteners as per SAE InternationalASTM Standards for Nonferrous Threaded FastenersWashersIS 1367: Technical Supply Conditions for Threaded Steel FastenersSelection of Fastener MaterialMechanical Properties and Grade Marking as per ASTM and SAESpecificationsReferencesPage No.341025283841566176808285879496For more information on threaded fasteners, please visit www.practicalmaintenance.net2Mechanical Properties and Identification Markings for Threaded Fastenerswww.practicalmaintenance.net

IntroductionA standard or a norm is a document containing agreements, specifications or criteria about amaterial, product, process or service. They are used to ensure that materials, products,processes and services are fit for their purpose. Standards are established within acompany, an organization, a consortium of organizations or recognized standardizationbodies.When a standard exists, a product / material requires little or no further description. Most ofthe features of a product / material are described in the standard. Most industrial threadedfasteners are covered by two basic standards: one for materials and properties; the other, fordimensions and tolerances.In ordinary usage of steel bolts, the chemical composition of the material and themanufacturing process are of little interest to the user, provided that the servicerequirements are met.Fastener material specification systems have been developed according to this reasoning.Most specifications are basically performance specifications. They emphasize performancecriteria such as strength, hardness, ductility, and impact resistance. The materialcomposition is usually flexible. This allows the manufacturer to choose the best material forthe fastener.There always seems to be some confusion regarding mechanical versus metallurgicalproperties. Mechanical properties are those associated with elastic or inelastic reaction whenforce is applied, or that involve the relationship between stress and strain. Metallurgicaltesting includes chemical composition, microstructure, grain size, carburization anddecarburization, and heat treat response. The chemical composition is established when thematerial is melted. Nothing subsequent to that process will influence the basic composition.The microstructure and grain size can be influenced by heat treatment. Carburization is theaddition of carbon to the surface which increases hardness. It can occur if heat treat furnaceatmospheres are not adequately controlled. Decarburization is the loss of carbon from thesurface, making it softer. Partial decarburization is preferable to carburization, and mostindustrial standards allow it within limits.There are two main systems for measuring weights and distances: the metric system andthe imperial system. For material standards, the primary standardization organizations forthe metric system fasteners are the International Organization for Standardization (ISO) andthe German Institute for Standardization (DIN) whereas for the imperial fasteners they arethe American Society for Testing and Materials (ASTM) and the Society of AutomotiveEngineers (SAE). In view of this, information about mechanical properties and identificationmarkings as per them is given in this booklet.Note:In ISO standards, comma (,) is used as a decimal marker. However, to maintain uniformitywith general practice, I have used full point (.) as the decimal marker for the informationgiven about ISO standard in this booklet.3Mechanical Properties and Identification Markings for Threaded Fastenerswww.practicalmaintenance.net

Mechanical PropertiesMost fastener applications are designed to support or transmit some form of externallyapplied load. For joint designing, fasteners are covered by standards/specifications thatdefine required mechanical properties such as tensile strength, yield strength, proof load andhardness. Other mechanical properties are shear strength, fatigue strength and torsionalstrength. In view of this, information about bolted joints and mechanical properties is given inthis chapter.Bolted JointsLoads can be applied to bolted joints in a number of different ways, each of which producesunique effects on the joint. These effects result from the way the joint is loaded, as well ashow the joint responds to the load. Some of the various load types include tensile, shear andbending. The type of bolted joint derives its name from the external load applied to the joint.The stress in the bolt when the bolt has been tightened to the design extent is known as thepre-stress. The tensile load corresponds to the force that clamps the joint members together.Torsion in the bolt results from friction between the threads in the bolt and the nut.Some bolts are also exposed to shear loads which occur when the external force slides themembers of the joint in relation to each other perpendicular to the clamping force. In aproperly designed joint the external shear force should be resisted by the friction betweenthe components. A joint of this kind is called a friction joint. If the clamping force is notsufficient to create the friction needed, the bolt will also be exposed to the shear load. Jointsare frequently designed for a combination of tensile and shear loads.Tensile PropertiesThe tensile properties; tensile strength and yield strength are determined by a tensile test.To find out tensile properties of a fastener material, a machined test piece is fitted in the jawsof the tensile testing machine (Universal Testing Machine) and subjected to a tensile force.The applied force and the resulting elongation of the test piece are measured. The processis repeated with increased force until the test piece breaks. The gauge length of the testpiece is the length of the parallel portion of the test piece between gauge length marks onwhich elongation is measured at any moment during the test. The percentage elongation ofthe test piece is the elongation expressed as a percentage of the original gauge length. Theelongation of the test piece is measured as extension by an arrangement consisting of dialgauge and clamps called the extensometer.4Mechanical Properties and Identification Markings for Threaded Fastenerswww.practicalmaintenance.net

The stress (intensity of force) at any moment during the test is the force divided by theoriginal cross-sectional area of the test piece. The strain is the elongation of the test piece,increase in the extensometer gauge length at any moment during the test. It is expressed asa percentage extension, extension (increase in extensometer gauge length) expressed as apercentage of the extensometer gauge length. Using the readings of the test, a stress strain (percentage extension) diagram is plotted to find out tensile properties.Tensile StrengthAs shown in above stress - strain (percentage extension) diagram, tensile strength (Rm) isthe stress corresponding to the maximum force, Fm.Fracture is phenomenon which is deemed to occur when total separation of the test pieceoccurs.Yield StrengthYield strength is the strength when the metallic material exhibits a yield phenomenon, stresscorresponding to the point (called yield point) reached during the test at which plasticdeformation occurs without any increase in the force.5Mechanical Properties and Identification Markings for Threaded Fastenerswww.practicalmaintenance.net

As shown in above figure, upper yield strength (ReH) is the maximum value of the stress priorto the first decrease in force and the lower yield strength (ReL) is the lowest value of thestress during plastic yielding, ignoring any initial transient effects.It may be noted that after reaching yield point, extension/elongation begins to increasedisproportionately with increasing tensile force. A plastic elongation remains after relief.Proof Strength or Non-Proportional Extension/ElongationAs it is difficult to determine yield strength of harder materials, proof strength (Rp) is definedfor them. Proof strength (Rp) is determined from the stress - strain curve by drawing a lineparallel to the linear portion of the curve and at a distance from it equivalent to theprescribed plastic percentage extension, e.g. 0.2 %. The point at which this line intersectsthe curve gives the stress corresponding to the desired proof strength plastic extension.As shown in above figure, proof strength or non-proportional extension/elongation (Rp) alsocalled plastic extension is the stress at which the plastic extension is equal to a specifiedpercentage of the extensometer gauge length. A suffix is added to the subscript to indicatethe prescribed percentage, e.g. Rp0.2.The most widely associated mechanical property associated with standard threadedfasteners is tensile strength. Tensile strength is the maximum tension, applied load thefastener can support prior to or coinciding with its fracture. Ultimate tensile load a fastenercan withstand is determined by the formula:Fm Rm AsWhere,Fm Ultimate tensile load (N, lb.)Rm Tensile strength (MPa, psi)As Tensile stress area in thread (mm2, in.2)When a standard threaded fastener fails in pure tension, it typically fractures through thethreaded portion (this is characteristically it’s smallest area). The tensile stress area in threadis calculated through an empirical formula. The empirical formula and tables stating this areaare provided in fastener standards (e.g. ISO 898-1).6Mechanical Properties and Identification Markings for Threaded Fastenerswww.practicalmaintenance.net

Note:For more information on tensile testing, please see ISO 6892-1, “Metallic materials - Tensiletesting - Method of test at room temperature”.Proof LoadYield strengths of machined test specimens are easily determined because of their uniformcross-sectional area throughout the stressed length. It has been noted that the yield strengthcharacteristics of test specimens do not always parallel those of the full size fastener fromwhich they are taken. This is because the beneficial effects of cold working may becompletely lost when the test piece is machined from the parent product. It is difficult to testfull size fasteners for yield strength because of the different strain rates in areas such as: thefully threaded portion; the thread runout; and the unthreaded shank which comprises thestressed length. Because of this, the proof load test was introduced as an approvedtechnique for testing a fastener’s deformation characteristics.By definition, the proof load is an applied tensile load/force that the fastener (bolt/screw)must support without permanent (plastic) deformation. In other words, the material mustremain in its elastic region when loaded up to its proof load and the bolt/screw returns to itsoriginal shape (or size) once the load is removed.In most (but not all) bolting applications, it is important not to tension a bolt past its yieldstrength. If a threaded fastener has been tightened past its yield strength, it is no longerreusable and will have to be thrown away if loosened because if it is tightened past its yieldstrength, it will not return to its original shape on loosening. Proof load is the limit of theelastic range of the threaded fastener. Designing of threaded fasteners according to proofload can help prevent plastic deformation. As long as a fastener is never tensioned beyondits specified proof load, you can be assured that it has maintained its original shape, andmay be reused safely. It may be noted that structural bolts are tensioned well beyond yieldstrength.It may also be noted that proof load is a force measurement. The units are newton orpounds. Yield strength is a stress measurement. The units are MPa or psi. The stress underproof load (Sp) is typically 90% (between 85-95%) of the yield strength. Stress under proofload, for various materials is quoted in fastener standards (e.g. ISO 898-1).In a proof load test, a headed bolt is placed in a testing machine with a nut on the threadedend, and a wedge under the head. The wedge varies from 4-10 degrees depending on thesize and configuration of fastener, and serves to evaluate the ductility of the bolt. In amachine specimen test, you test ductility by measuring elongation and reduction of area, butthose are impossible during a full size test, so head deflection is used in its place. Proof loadtesting is typically performed at 90% of the expected minimum yield strength and is a simplepass/fail test. The bolt length is measured, and after being subjected to the published proofload value for 10 seconds, if it has not elongated more than 0.0005″, it is deemed to havepassed.HardnessHardness is a measure of a material’s ability to resist abrasion and indentation. Generallyspeaking, hardness is the resistance which the material offers to the penetration of a testbody under a defined load. It is measured by the Brinell (ISO 6506), Rockwell (ISO 6508) orVickers (ISO 6507) methods.7Mechanical Properties and Identification Markings for Threaded Fastenerswww.practicalmaintenance.net

For carbon steels, Brinell and Rockwell hardness testing can be used to estimate tensilestrength properties of the fastener.Shear StrengthShear strength is defined as the maximum load that can be supported prior to fracture, whenapplied at a right angle to the fastener’s axis.As shown in above figure, the joint design is said to be in single shear if the fastener wouldonly need to shear in one plane for the joint to fail; whereas the joint design is said to be indouble shear if the fastener would need to shear in two planes for the joint to fail. Obviouslythe double shear joint is stronger and more stable than the single shear joint, and should beused at every available opportunity when joint strength and reliability are of concern.Since most bolts are used as clamps, not as shear pins, most bolting specifications andstandards list only one or more forms of tensile strength (proof, yield, or ultimate) and notshear strengths.When no shear strength is given for common carbon steels with hardness up to 40 HRC orso, 60 % of their ultimate tensile strength is often used as their shear strength. However, thisshould only be used as an estimation. The stainless steels are an exception to this rule ofthumb; they have shear strengths which are about 55% of their ultimate tensile strengths.To determine the shear strength of the material, the total cross-sectional area of the shearplane is important. For shear planes through the threads, we could use the equivalent tensilestress area (As). However, to take full advantage of strength properties, the preferred designwould be to position the full shank body in the shear plane as shown in above figure.8Mechanical Properties and Identification Markings for Threaded Fastenerswww.practicalmaintenance.net

Fatigue StrengthA fastener subjected to repeated cyclic loads can suddenly and unexpectedly break, even ifthe loads are well below the strength of the material. The fastener fails in fatigue. The fatiguestrength is the maximum stress a fastener can withstand for a specified number of repeatedcycles prior to its failure.Torsional StrengthTorsional strength is a load usually expressed in terms of torque, at which the fastener failsby being twisted off about its axis. Tapping screws and socket set screws require a torsionaltest.DuctilityDuctility is a measure of the degree of plastic deformation that has been sustained atfracture. In other words, it is the ability of a material to deform before it fractures. A materialthat experiences very little or no plastic deformation upon fracture is considered brittle. Areasonable indication of a fastener’s ductility is the ratio of its specified minimum yieldstrength to the minimum tensile strength. The lower this ratio, the more ductile the fastenerwill be.NoteA maintenance engineer is required to know tensile strength (hardness can be used toestimate tensile strength), yield strength and proof load properties of fasteners becausepreloading / tightening of fasteners depend on these properties. In view of this, informationabout tensile strength, yield strength, proof load and hardness is given in various chapters.For information on other properties like elongation, reduction of area, etc. and chemicalcomposition, please see the relevant standard/specification.9Mechanical Properties and Identification Markings for Threaded Fastenerswww.practicalmaintenance.net

Mechanical Properties of Fasteners Made of Carbon Steeland Alloy Steel as per ISO Standard (ISO 898)ISO 898 consists of the following parts, under the general title Mechanical properties offasteners made of carbon steel and alloy steel:Part 1: Bolts, screws and studs with specified property classes - Coarse thread and fine pitchthreadPart 2: Nuts with specified property classes - Coarse thread and fine pitch threadPart 5: Set screws and similar threaded fasteners with specified hardness classes - Coarsethread and fine pitch threadPart 7: Torsional test and minimum torques for bolts and screws with nominal diameters 1mm to 10 mmSome useful information about the mechanical properties as per the standard is given in thischapter. It may be noted that the information is given for education purpose only. Forcommercial use, please see the latest version of the standard.ISO 898-1: Bolts, Screws and Studs with Specified Property ClassesISO 898-1 specifies mechanical and physical properties of fasteners (the term used whenbolts, screws and studs are considered all together) made of carbon steel and alloy steelwhen tested at an ambient temperature range of 10 C to 35 C. Fasteners conforming to therequirements of ISO 898-1 are used in applications ranging from 50 C to 150 C. Theymight not retain the specified mechanical and physical properties at elevated temperaturesand/or lower temperatures.ISO 898-1 is applicable to bolts, screws and studs made of carbon steel or alloy steel,having triangular ISO metric screw thread in accordance with ISO 68-1 with coarse pitchthread M1.6 to M39, and fine pitch thread M8 1 to M39 3.Designation System for Property ClassesProperty class defines the strength of a bolt or nut. For metric fasteners, property classesare designated by numbers where increasing numbers represent increasing tensilestrengths. The symbol for property classes consists of two numbers, separated by a dot.The number to the left of the dot consists of one or two digits and indicates 1/100 of thenominal tensile strength, Rm,nom, in megapascals (N/mm2).The number to the right of the dot indicates 10 times the ratio between the nominal yieldstrength and the nominal tensile strength, Rm,nom, (called yield strength ratio). The nominalyield strength is: lower yield strength ReL,nom, or nominal stress at 0.2 % non-proportional elongation Rp0,2 nom, or nominal stress at 0.0048d non-proportional elongation Rpf,nom.Hence, yield strength ratio ReL,nom / Rm,nom or Rp0,2 nom / Rm,nom or Rpf,nom / Rm,nom.An additional zero to the left of the property class designation indicates that fasteners havereduced loadability.10 Mechanical Properties and Identification Markings for Threaded Fastenerswww.practicalmaintenance.net

For example, in property classes 10.9 designation; the first number indicates 1/100 of thenominal tensile strength in N/mm2. So, nominal tensile strength for property classes 10.9designation is 10 100 1000 N/mm2. The second number indicates 10 times the ratio9lower yield strengthbetween lower yield strength and nominal tensile strength. So, 10 nominal tensile strength. Asnominal tensile strength is 1000 N/mm2, the lower yield strength is 900 N/mm2.It may be noted that the multiplication of the nominal tensile strength and the yield strengthratio gives the nominal yield strength.A fastener with material properties of property class 8.8 but with reduced loadability isdesignated by 08.8.Following table specifies the limits for the chemical composition of steels and minimumtempering temperatures for the different property classes of bolts, screws and studs.PropertyClass4.64.85.65.86.8Material and Heat TreatmentCarbon steel or carbon steelwith additives.Chemical Composition Limit, %CPSBmax.min. max. max. .0500.0600.0600.060TemperingTemperature C min.NotSpecifiedCarbon steel with additives(e.g. Boron or Mn or Cr)0.15 0.40 0.025 0.025quenched and temperedor8.8Carbon steel quenched and0.25 0.55 0.025 0.0250.003temperedorAlloy steel quenched and0.20 0.55 0.025 0.025temperedCarbon steel with additives(e.g. Boron or Mn or Cr)0.15 0.40 0.025 0.025quenched and temperedor,9.8Carbon steel quenched and0.25 0.55 0.025 0.0250.003temperedor,Alloy steel quenched and0.20 0.55 0.025 0.025temperedCarbon steel with additives(e.g. Boron or Mn or Cr)0.20 0.55 0.025 0.025quenched and temperedor,10.9Carbon steel quenched and0.25 0.55 0.025 0.0250.003temperedor,Alloy steel quenched and0.20 0.55 0.025 0.025temperedAlloy steel quenched and12.90.30 0.50 0.025 0.0250.003temperedCarbon steel with additives(e.g. Boron or Mn or Cr or12.90.28 0.50 0.025 0.0250.003Molybdenum) quenched andtemperedNote: For remarks about the text of above table please see the specification (ISO 898-1).11Mechanical Properties and Identification Markings for Threaded 380

The bolts, screws and studs of the specified property classes shall, at ambient temperaturemeet all the applicable mechanical and physical properties in accordance with the followingtable. They shall also meet the minimum ultimate tensile loads and proof loads as per thetables given in the specification.No.Mechanical or Physical Property1Tensile strength, Rm, N/mm22Lower yield strength, ReL, N/mm234567Stress at 0.2 % non-proportionalelongation, Rp0.2, N/mm2Stress under proof load, Sp, N/mm2Percentage elongation after fracturefor machined test pieces, A, %Vickers hardness, HVF 98 NBrinell hardness, HBWF 30 D2Rockwell hardness, HRB8Rockwell hardness, HRC9 Impact strength, KV, J10 Head soundnessd is the nominal thread diameter, mma - Values do not apply to structural bolting.b - For structural bolting d M12.k - Value for K is under .max.min.max.min.max.min.max.min.Property Class6.88.89.8 10.9 12.9/12.9d 16 d 16 d 16mma mmb mm400500600800900 1000 1200400 420 500 520 600 800 830 900 1040 1220240 - 300 240 - 300 640 640 720 900 1080640 660 720 940 1100225 310 280 380 440 580 600 650 830 9704.64.85.65.822-20--12121098120 130 155 160 190 250 255220250 320 335114 124 147 152 181 245 250209238 316 33167 71 79 82 8995.099.522233234272727No 3239273944kIt may be noted that above table is incomplete (information on some mechanical or physicalproperties and some remarks for the table text are not given).The minimum ultimate tensile load and proof load can be calculated using above table, andthe following formulas.Minimum ultimate tensile load, Fm,min As,nom Rm,min, NProof load, Fp As,nom Sp,nom, NNominal stress area in thread, As,nom 𝜋 𝑑2 𝑑3 2( 2 )4mm2where,d2 is the basic pitch diameter of external thread in accordance with ISO 724d3 is the minor diameter of external thread𝐻d3 d1 6d1 is the basic minor diameter of external thread in accordance with ISO 724H is the height of the fundamental triangle of the thread in accordance with ISO 68-1For ready reference, values of the nominal stress areas, As,nom, for ISO metric coarse pitchthread and ISO metric fine pitch thread are given in the following tables.12Mechanical Properties and Identification Markings for Threaded Fastenerswww.practicalmaintenance.net

Nominal Stress Areas, As,nom, for ISO Metric Coarse Pitch ThreadNominal Stress Area, As,nom, 9M30561M33694M36817M39976Note: Where no thread pitch is indicated in a thread designation, coarse pitch is specified.Thread, dThread, minal Stress Areas, As,nom, for ISO Metric Fine Pitch ThreadNominal Stress Area, As,nom, 8651030For information on various test methods (for example, tensile test under wedge loading offinished bolts and screws; tensile test for finished bolts, screws and studs for determinationof tensile strength, Rm; proof load test for finished bolts, screws and studs; tensile test formachined test pieces; head soundness test; hardness test; decarburization test; etc.) pleasesee the specification. Following is the brief information on some of the test methods.Tensile Test Under Wedge Loading of Finished Bolts and Screws (Excluding Studs)The purpose of this tensile test is to determine simultaneously: the tensile strength onfinished bolts and screws, Rm; and the integrity of the transition section between the headand the unthreaded shank or the thread.13Mechanical Properties and Identification Markings for Threaded Fastenerswww.practicalmaintenance.net

For this test, tensile test is carried out after a wedge (as per test specification) is placedunder the head of the bolt or screw as shown in above figure.For integrity of transition section between head and unthreaded shank/thread: the fractureshall not occur in the head; for bolts and screws with unthreaded shank, the fracture shallnot occur in the transition section between the head and the shank.Proof Load Test for Finished Bolts, Screws and StudsThe proof load test consists of two main operations: application of a specified tensile proofload and measurement of permanent elongation, if any, caused by the proof load.For applicability, apparatus, testing device and test procedure, please see the specification.The test result requires that the total length of the fastener after unloading, l1, shall be thesame as before loading, l0, within a tolerance of 12.5 μm allowed for uncertainty ofmeasurement.Some variables, such as straightness, thread alignment and uncertainty of measurement,can result in apparent elongation of the fastener when the proof load is initially applied. Insuch cases, the fastener shall be retested using a 3 % greater load than the proof loadspecified in the proof load tables.The total length after the second unloading, l2, shall be the same as before this loading, l1,within a tolerance of 12.5 μm allowed for uncertainty of measurement.Head Soundness TestThe purpose of the head soundness test is to check the integrity of the transition sectionbetween the head and the unthreaded shank or the thread by striking the head of the14Mechanical Properties and Identification Markings for Threaded Fastenerswww.practicalmaintenance.net

fastener on a solid block to a given angle (β) as shown in above figure. A hammer shall beused to strike the head of the bolt or screw with several blows so that the head bends to anangle of 90 β. Values of angle β are specified in the standard. The test requires no sign ofcracking at the transition section between the head and the unthreaded shank shall bevisible. For screws threaded to the head, this requirement is fulfilled even if a crack appearsin the first thread, provided the head does not fracture off.This test is generally used when the tensile test under wedge loading cannot be carried outdue to the too short length of the fastener.Hardness TestThe purpose of the hardness test is to determine the hardness of the fastener for allfasteners which cannot be tensile tested and to determine the hardness of the fastenerwhich can be tensile tested in order to check that the maximum hardness is not exceeded.MarkingThe trade (identification) marking of the manufacturer is mandatory on all products which aremarked with property classes.Marking symbols for property classes for fasteners with full loadability shall be as per thefollowing table (It may be noted that due to their geometry, some fasteners are tested withreduced loadability. For more information on loadability, please see ISO 898-1).Marking Symbols for Fasteners

9 ASTM Standards for Alloy Steel, Stainless Steel and Nickel Alloy Threaded Fasteners 61 10 Threaded Fasteners as per SAE International 76 11 ASTM Standards for Nonferrous Threaded Fasteners 80 12 Washers 82 13 IS 1367: Technical Supply Conditions for Threaded Steel Fasteners 85 14 Selection of Fastener Material 87 .