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STAINLESS – A SYSTEMATIC APPROACH TO THEIR SELECTION

A DESIGNERS’ HANDBOOK SERIES NO 9003

Produced by Distributed by AMERICAN IRON AND STEEL INSTITUTE INSTITUTE FASTENERS – A SYSTEMATIC APPROACH TO THEIR SELECTION

A DESIGNERS’ HANDBOOK SERIES NO 9003

Originally, this handbook was published in 1976 by the Committee of Stainless Steel Producers, American Iron and Steel Institute. The Nickel Institute republished the handbook in 2020. Despite the age of this publication the information herein is considered to be generally valid. Material presented in the handbook has been prepared for the general information of the reader and should not be used or relied on for specific applications without first securing competent advice. The Nickel Institute, the American Iron and Steel Institute, their members, staff and consultants do not represent or warrant its suitability for any general or specific use and assume no liability or responsibility of any kind in connection with the information herein.

Nickel Institute @nickelinstitute.org www.nickelinstitute.org CONTENTS

Introduction...... 3

Fastener Materials...... 3

Stainless Identification...... 5

Choosing the Right Type...... 8

Stainless Steel Properties...... 13 Tensile & Strength Shear Strength

High- and Low-Temperature Service...... 16

Magnetic and Electrical Properties...... 19

Electrical conductivity...... 19

Manufacture of Stainless Steel Fasteners ...... 19

Plated Coatings...... 20

Effective Utilization of Stainless Steel Fasteners...... 22

Designing for Optimum Corrosion Resistance...... 23

Types of corrosion...... 24

Availability of Stainless Steel Fasteners...... 26

References...... 26

ROUND FLAT PLAIN

The designer should consider a Preface A systematic fastener as a system, and regard the assembled joint as a critical There was a time when the approach to and integral portion of the design periodic repair of mechanical and -not as an afterthought-because electrical components was taken stainless steel the joint is normally an area under for granted. Today, with labor costs highest stress and often the place at record levels and going up, fastener selection where failure is most likely to occur. greater consideration at the design Selecting the optimum fastener A designer should start with the level is being given to the reduction can be an awesome task for any optimum fastener and design the -or even elimination-of main­ designer. There is a staggering joint around that, rather than start­ tenance. In cases where a joint diversity of fastener types available ing with the joint and then looking must be taken apart and reassem­ (over 500,000 standard items), and for the fastener that seems most bled, the corrosion resistance of for any one type there can be a adequate. the fastener is particularly important large number of sizes from which so that corrosion will in no way to choose. For example, one of the hamper or prevent its removal. The smallest standard fasteners has a Fastener materials cost of removing rusty bolts, and head of 0.01 inch (.254mm); the Fasteners have been made of a replacing them with new ones, is largest has a head 4 feet (1.219m) more expensive than using cor­ vast number of materials for one in diameter. To further complicate reason or another. The designer rosion resistant fasteners to the problem of fastener selection, begin with. there are special designs, which Other costs resulting from are not considered to be "off-the­ fastener failure, such as downtime shelf" items, and a wide variety of and lost production, make an even different materials. stronger case for stainless steel Stainless steel is one of these fasteners. fastener materials, and it is used extensively throughout industry for both original equipment manufac­ ture as well as for replacement. The purpose of this publication is to help designers trace an orderly path through the fastener complex­ ity to arrive at a stainless steel fastener system that best fills the need.

• . 0.

3 CID 0 ◊ D

FLAT HEAD CAP iSCREW OR HEX HEAD SQUARE HEAD ' iBOLT TI BOLT

can specify anything from hard oak CARBON STEELS (AISI Type): They are not hardenable by to lnvar, a low coefficient-of­ 1008, 1010, 1015 are low-carbon heat treatment but are hard­ expansion material. There are steels used for the great enable by , such indeed many cases wherein an majority of machine , as by heading and thread exotic material must be used wood screws, etc. Sometimes rolling. They are nonmagnetic because of design requirements. called mild steel, they have a in the annealed condition and However, the common materials tensile strength of about 55 ksi only slightly magnetic in the used to produce both standard and (379MPa), which cannot be cold worked condition. special fasteners should be con­ increased by heat treatment. 384 is a stainless steel that has sidered first. The common materials 1018, 1020, 1022 are medium lower work-hardening char­ are a first choice because of the low-carbon steels that can be acteristics than Type 305. It is ease in manufacturing fasteners heat treated for strength to a in the 18-8 family of stainless from these materials, because they minor degree (except 1018), steels and is included in most provide adequate physical charac­ and they can be case hardened AN and MS specifications. teristics at more economical cost, to form an outer layer of hard, 316 stainless steel has the and because they are readily tough, but brittle carburized highest degree of corrosion available from suppliers throughout steel. They are used for sheet resistance of the commonly­ the and Canada. The screws, thread-cutting used 18-8 stainless steels. brief outline which follows identifies screws, and for grade 2 410, 416, 430 are magnetic some different common fastener (bright) hexhead cap screws. stainless steels having less materials. About 59 ksi (407MPa) is the corrosion resistance than the usual tensile strength for 300 Series types. Types 41 0 these materials. and 416 are hardenable by 1035, 1038 are high-carbon, heat treatment to 125-180 ksi medium steels that can tensile (862-1241MPa). be heat treated to 120 ksi Designers also have choices to (827MPa) tensile strength. Cap make from among a large group of screws are a typical use. nonferrous fasteners, and fasteners ALLOY STEELS (AISI Type): that have been plated either for 4037, 4137 are alloy purposes of corrosion protection or steels that can be heat treated for appearance. to 150 ksi tensile (1034MPa) This publication is concerned and above. Typical uses only with stainless steels; however, include hex socket and cap some comments will be made screws and for spline concerning the other materials fasteners. FREE-CUTTING STEELS (AISI Type): 1110, 1112, 1113 are low-carbon, free- steels used for low-strength nuts that are used in all but critical applications. STAINLESS STEELS (AISI Type): - 303, 304, S30430 (UNS), 305 are in the "18-8" family of stain­ less steels that have tensile strengths in the 80 ksi (552MPa) and up range.

4 fied by the new Unified Numbering System (UNS), which encompasses all commercial and rare earths. For example, Type 304 is Type S30400 in UNS. Special analysis and proprietary stainless steels are identified by trade names, some of which may resemble AISI numbers. The terms austenitic, martensitic, terrific and precipitation hardening serve to identify categories of stainless steels on the basis of their metallurgical structure. De­ signers should recognize these terms and understand what they mean, because stainless steels so classified tend to have similar characteristics with respect to corrosion resistance, hardenability, and fabricability. Austenitic stainless steels are -nickel-manganese and chromium-nickel composi­ tions identified by 200 and 300 Series numbers, respectively. They are hardenable only by cold working and are non­ magnetic in the annealed con­ because an evaluation requires a the American Iron and Steel Institute dition. Typical of the austenitic comparison of available materials. as standard compositions. A com­ group is Type 304, which con­ plete listing of all AISI stainless tains nominally 18% chromium steels and a description of each and 8% nickel, hence the Stainless steels are contained in the AISI publica­ 18-8 name. Stainless steel is a family of tion, "Steel Products Manual Ferritic stainless steels are iron-base alloys containing about -Stainless and Heat-Resisting straight-chromium steels in the 10.5% chromium or more, plus Steels, December 197 4.1'' 400 Series that are not harden­ other alloying elements such as In addition to the standard AISI able by heat treatment and only nickel, manganese, molybdenum, types, many special analysis and slightly hardenable by cold sulfur, selenium, , etc. The proprietary stainless steels are working. All are magnetic. Type chromium is chiefly responsible for produced in the United States 430 is typical of this group. corrosion and heat resistance; the and Canada. Martensitic stainless steels are other alloying elements are present straight-chromium, 400 Series in stainless steel to enhance that can be hardened by heat corrosion resistance and to impart Identification treatment only. All are magnetic. certain characteristics with respect Most AISI stainless steels are Type 410 is typical of this group. to strength and fabricability. identified by a system of numbers Precipitation hardening stainless A total of 57 commercial stain­ that are in either 200, 300, or 400 steels are hardenable by a less steel types are designated by Series. In addition, all are identi- combination of a low-temperature 5 FASTENER BASICS

Driving Head Shoulder Recess Styles Form Types

q;p � § HEX SLOTTED SQUARE SQUAREw RIBBED•

� � ROUND () PAN PHILLIPS FULL SIZE � ft BINDING HEX© SOCKET FILLISTER FIN NECK ROUNDH V TRUSS wFLAT @ COUNTERSUNK POZIDRIV

� 9 M UNDERSIZE OVAL ROUND WASHER OVAL CLUTCH8 9 HEX WASHER � FREARSON 6 An analysis of standard fasteners, Thread Point such as bolts and screws, Form Styles reveals that all have certain characteristics in common. Further, their differences can be classified as shown here. Each bolt and is, in effect, made up of a series of component D 0 parts; thus, the fasteners may OVAL SPHERICAL have some or all of these: (a) a head; (b) a driving recess; WHITWORTH BUTTRESS METRIC (c) a shoulder ; (d) an unthreaded shank ; (e) a threaded shank; and D (f) a point. PILOT CHAMFER Certain combinations of these components, because of usage are considered standard. Others D are non-standard, but nearly any CONE CUP combination can be readily produced. This analysis of fastener parts is presented in the hope that it will AMERICAN WOOD TAPPING D assist the user in understanding UNIFIED SCREW FLAT NEEDLE and specifying bolts and screws. STANDARD Source: ITT Harper

NAIL

ACME BRITISH HEADER SPECIAL 7 I I I

CAP SQUARE MACHINE CASTLE NUT () WING NUT SCREW NUT

aging treatment and cold working. tions, most organic chemicals and siderably higher sulfur content that The AISI types are identified by dyestuffs, and a wide variety of enhances machining character­ UNS numbers only (such as Type inorganic chemicals. It resists nitric istics. If a fastener is made by S17400), although many are acid well, the sulfur acids mod­ machining, such as large bolts or referred to in literature by propri­ erately, and the halogen acids specials that cannot be cold headed, etary trade names such as poorly. It is used for cryogenic ap­ more than likely it will be Type 303. 17-4PH. The precipitation hard­ plications, and is easily fabricated. On the other hand, Types UNS­ ening stainless steels are especi­ Types 303, UNS-S30430, 305, S30430, 305, and 384 were ally useful because fabrication and 384 are variations of Type 304 developed specifically for cold­ can be completed in an annealed that were developed to meet spe­ forming operations. Type UNS­ condition and uniform harden­ cific manufacturing requirements. S30430 contains and is ing achieved without a high­ For instance, Type 303 has a con- used for cold headed fasteners. temperature treatment that can Regardless of the diversity in result in distortion and scaling. numbering, the above-mentioned stain less steels are essentially 18-8 materials in that their composition is very close to the nominal com­ Choosing the right position of Type 304, which is 18% stainless steel chromium and 8% nickel. Because they have similar corrosion resis­ The stainless steels used most tance properties, these 18-8 ma­ frequently as fasteners are Types terials are usually interchanged in 303, 304, UNS-S30430, 305, 384, fastener applications. If an applica­ 316, 410, 416, and 430. Selection tion calls for Type 304, the designer of the right type for a fastener can generally specify an 18-8 application is greatly simplified fastener material. when they are classified according However, it should be recognized to metallurgical categories de­ that while the 18-8 stainless steels scribed on page 5. are considered to have similar Austenitic -Types 303, 304, corrosion-resistance properties, UNS-S30430, 305, they are not identical. The sulfur in 384, and 316 Type 303, for example, causes it to Ferritic -Type 430 have somewhat lower corrosion Martensitic-Types 410 and 416 resist?nce than Type 304, especially The selection process is further when in direct and continuous simplified when the designer contact with water or some chem­ understands the characteristics of ical solution. Accordingly, a each category and the relationship designer should specify Type 304 of alloys to one another in that when it is known that Type 303 is group. not suitable for the application. The austenitic stainless steels are When in doubt, the designer should characterized as having excellent consult with a corrosion engineer. corrosion resistance, with the If an application calls for a higher alloyed materials accordingly material with corrosion resistant having superior resistance. For properties better than that of Type example, Type 304 is the most 304, Type 316 is tlie next logical widely used material that withstands candidate. Type 316 stainless steel ordinary rusting. It is virtually im­ is a higher alloyed material contain­ mune to foodstuffs, sterilizing solu- ing 2-3% molybdenum, which gives

8 it corrosion-resistant properties alloyed types, in which case the cameras, vending , superior to Type 304, especially in designer can consider one of the counters, appliances, showcases, environments containing chlorides. straight-chromium types, some of and a host of other products that It has wide application in pulp and which may be lower in cost. For need dressing up or "eye appeal" paper mills, for example, because it example, Type 430 stainless steel to increase their salability. is more resistant to sulfurous acid contains about 18% chromium, but On the other hand, there is Type compounds. It is also widely used no nickel. Although it has lower 410, which contains 11.5-13.5% in phosphoric and acetic acids that corrosion-resistance properties chromium, and accordingly has tend to cause pitting corrosion in than the 18-8 types, it has wide ap­ lower resistance to corrosion than the 18-8 types. plication for decorative trim because Type 430 or the 18-8 materials. It Many fastener applications do not when it is buffed it closely will resist corrosion in mild atmo­ require the high degree of corro­ resembles a material that has been spheres, fresh water, mine water, sion resistance offered by the 18-8 chromium plated. Typical applica­ steam, carbonic acid, crude oil, stainless steels or the higher tions include trim on automobiles, gasoline, blood, alcohol, ammonia,

TABLE 1-CHEMICAL COMPOSITION LIMITS OF RAW MATERIAL (IFl-104) GENERAL SPECIFICATIONS1 DESCRIPTION GRADE COVERING CHEMICAL COMPOSITION, % MAX OF RAW MATERIALS (UNLESS SHOWN AS MIN OR MAX/ MIN LIMITS GIVEN) MATERIAL ..l.. REQUIREMENTS STAINLESS STEEL C Mn p s Si Cu Mo Ni Cr Other Austenitic ASTM A276 Type 303 8.00 17.00 See 303 Stainless QQ-S-764 Class 303 0.15 200 0.20 0.15 1.00 0.603 to to Note Steel AISI 303 min 10.00 19.00 (3) Austenitic ASTM A276 Type 304 8.00 18.00 304 Stainless QQ-S-763 Class 304 0.08 200 0,045 0.030 1.00 to to Steel AISI 304 12.00 20.00 Austenitic ASTM A276 Type 305 10.00 17.00 305 Stainless QQ-S-763 Class 305 0.12 2 00 0.045 0.030 1.00 - to to Steel AISI 305 13.00 19.00 Austenitic ASTM A276 Type 316 ±2.00 1000 16.00 316 Stainless QQ-S-763 Class 316 0.08 2.00 0.045 0 030 1.00 to to to Steel AISI 316 3.00 14.00 18.00 Austenitic 3.0 8.00 17.00 XM7* Stainless ASTM A493 Type XM7 0.10 2.00 0.045 0.030 1.00 to to to Steel 4.0 1000 19.00 Austenitic 17.00 15.00 384 Stainless AISI 384 0.08 2.00 0.045 0.030 1.00 to to Steel 19.00 17.00 Martensitic ASTM A276 Type 410 11.50 410 Stainless QQ-S-763 Class 41 O 0.15 1 00 0040 0.030 1.00 - - to Steel AISI 410 �� 13.50 Martensitic ASTM A276 Type 416 12.00 416 Stainless QQ-S-764 Class 416 0.15 1.25 0.06 0.15 1.00 0.602 to Steel AISI 416 min 14.00 Ferritic ASTM A276 Type 430 14.00 430 Stainless QQ-S-763 Class 430 0.12 1.00 0.040 0030 1.00 - to Steel AISI 430 I I 18.00 NOTES TO TABLE 1 1. Legend of specification designations - 2. May be added at manufacturer's option. * Type S30430 ASTM-American Society for Testing and Materials 3. ASTM A276 permits addition of molybdenum, and also AISI-American Iron and Steel Institute 0.12/0.30% lead at manufacturer's option. AISI requires QQ-X-XXX-Federal Government the addition of molybdenum but permits no lead.

9 a•,,a.ra ...... - WASHER HEX WASHER OVAL SLOTTED

TABLE 2-MECHANICAL REQUIREMENTS FOR STAINLESS STEEL BOLTS, SCREWS, STUDS AND NUTS (IFl-104) MECHANICAL REQUIREMENTS BOLTS, SCREWS AND STUDS NUTS 1 FULL SIZE SOLTS, MACHINED TEST SPECIMENS OF 1 GENERAL PROOF GRADE SCREWS, STUDS SOL TS, SCREWS, STUDS HARDNESS DESCRIPTION HARDNESS LOAD 2 2 ROCKWELL YIELD TENSILE YIELD TENSILE ELONG- ROCKWELL STRESS STRENGTH STRENGTH STRENGTH STRENGTH ATION3 min ksi min ksi min ksi min ksi % Min Min ksi Min Austenitic 303-A Stainless Steel- 30 75 30 75 20 B75 75 875 Sol. Annealed ustenitic 304-A Stainless Steel- 30 75 30 75 20 B75 75 875 Sol. Annealed Austenitic 304 Stainless Steel- 50 90 45 85 20 B85 90 885 Cold Worked Austenitic See See 304-SH Stainless Steel- See See See 15 C25 C20 Note 6 Note 6 Note 6 Strain Hardened Note 6 Note 6 Austenitic 305-A Stainless Steel- 30 75 30 75 20 870 75 870 Sol. Annealed Austenitic 305 Stainless Steel- 50 90 45 85 20 885 90 885 Cold Worked i Austenitic See See 305-SH Stainless Steel- See See See 1 15 C25 C20 Note 6 Note 6 Note 6 t Note 6 Note 6 Strain Hardened r Austenitic 316-A Stainless Steel- 30 75 30 75 20 870 75 870 Sol. Annealed Austenitic 316 Stainless Steel- 50 90 45 85 20 885 90 B85 Cold Worked i Austenitic See See See See See r 316-SH Stainless Steel- t t 15 C25 C20 Note 6 Note 6 Note 6 Note 6 Note 6 Strain Hardened Austenitic XM7-A* Stainless Steel- 30 75 30 75 20 870 75 870 Sol. Annealed Austenitic XM7* Stainless Steel- 50 90 45 85 20 B85 90 B85 Cold Worked Austenitic 384-A Stainless Steel- 30 75 30 75 20 B70 75 B70 Sol. Annealed Austenitic 384 Stainless Steel- 50 90 45 85 20 885 90 B85 Cold Worked Martensitic Stainless Steel- 410-H 95 125 95 125 20 C22 125 C22 Hardened and Tempered Martensitic Stainless Steel 135 180 135 180 12 C36 180 C36 410-HT Hardened and Tempered Martensitic Stainless Steel- 95 125 95 125 20 C22 125 C22 416-H Hardened and Tem(2ered Martensitic Stainless Steel- 135 180 135 180 12 C36 180 C36 416-HT Hardened and Tern ered Ferritic 40 70 40 70 20 875 70 875 430 Stainless Steel-

10 NOTES TO TABLE 2 1. Legend of grade desig nations: - 4. Loads at minimum yie ld strength and minimum ultimate tensile A-solution annealed strength for full size produc ts may be computed by multiplying the SH- strain hardened yield strength and tensile strength stresses as given in Table 2 by H-h ardened and tempered at 1100 F min. the stress area for the product size and thread series as give n in HT- hardened and tempered at 525 F ± 50 F Table 2a. below. 2. Yield strength is the stress at which an offset of 0.2% of 5. Proof loads of nuts in pound s may be computed by multi ply ing the length occurs for all stainless stee ls. proof load stress as given in Table 2 by the stress area for the nut 3. Elongation is determined using a gauge length ol 2 in. or 4 diameters size and thread series given in Table 2a. of test specimen in accordance with Federal Standard 151, Method 6. Austenitic stainless steel, strai n hardened bolts , screws, studs and 211. nuts shall have the follow ing strength properties.

BOLTS, SCREWS, STUDS NUTS - TESTED FULL SIZE MACHINED TEST PRODUCT SPECIMENS SIZE PROOF - - LOAD YIELD TENSILE YIELD TENSI LE STRESS STRENGTH STRENGTH STRENGTH STRENGTH in. min ksi min ksi min ksi min ksi ksi - --- - to% in. 100 125 90 115 125 over% to 1 in. 70 105 65 100 105 over 1 to 1½ in. 50 90 45 85 90

TABLE 2a-TENSILE STRESS AREAS AND THREADS PER INCH

COARSE THREAD FINE THREAD PRODUCT SIZE DIA. in. STRESS AREA, THREADS STRESS AREA, THREADS sq in. per in. sq in. per in.

6 0.00909 32 0.0 1015 40 8 0.0140 32 0.01474 36 10 0.0175 24 0.0200 32 12 0.0242 24 0.0258 28

¼ 0.03 18 20 0.0364 28 5/16 0.0524 18 0.0580 24 ⅜ 0.0775 16 0.0878 24 7/1616 0.1063 14 0.1187 20 ½ 0.1419 13 0.1599 20

9/16 0.1820 12 0.2030 18 ⅝ 0.2260 11 0.2560 18 ¾ 0.3340 10 0.3730 16 ⅞ 0.4620 9 0.5090 14 0.6060 8 0.6630 12

1⅛ 0.7630 7 0.8560 12 1¼ 0.9690 7 1.0730 12 1⅜ 1.1550 6 1.3150 12 1½ 1.4050 6 1.5810 12

Tensi le stress areas are comput ed usin g the following formula : Where A,= tensile stress area in square inches • Type 30430 9 43 2 D = nominal size (basic major diameter) in inches A, = 0.7854 [ D - O : ) n = numb er of threads per inch

11 [1 r CLOSED EYE BOLT I U BOLT , J BOLT DOUBLE END SQUARE HEAD TREADED RIGH T ANGLE BOLT STUD SET SCREWS ROD

determin ation , however , should be used for we lded fabricat ions that FIGURE 1. YIELD STRENGTH­ based on tests conducted under wi ll be exposed to ac id environ ­ DETERMINED BY OFFSET actua l working conditions. If this ments. Other stainless stee ls are is not practical for a des igner , he espec ially suited for high-temper­ should consult with a corrosion ature environments , suc h as Types engineer having experienc e w ith 309 , 310, 314, 32 1, 347 , 348 , 436 , 40 stai nless steel s. Assistance can be 442 , and 446. obta ined throu gh the companies Anothe r group of stain less steels I listed on th e back cover of this has improved mechanical prop­ 0.2% OFFSET pub I ication. erties, suc h as high strength and UJ The stainless steels desc ribed so a: hardness. Included in this group far represent only the common ones PARALLEL LINES are Types 304 N, 316N, 422 , 431 , cove red by fastener industr y speci ­ 440A , 440B, and 440 C, plus the fic ation s.' Other stainless steel types precipitation hardening grades­

.005 .010 are available to meet specif ic re­ Types UNS-S1 3800, UNS-S155 00, requ irements of fabr ication or end UNS-S 17400 , and UNS-S17700 . STRAIN In /in use, many of which are used in Nearly all of the stainless stee ls

To convert to metric: 1000 psi= approx. 6.9 MPa fastener form . For exam ple , there can be made into fasteners , 1 in. = 25.4 MM are var iati ons of the 18-8 grou p althoug h special arrangements with having lower-carbon contents. such a fastener manufacturer may be , soap, sugar so lutions and as Types 304L and 316L. These are nec essary in some cases. other reagents. It also has good sca ling and oxidat ion resistanc e up FIGURE 2. TENSILE STRENGTH OF NINE FASTENER ALLOYS to 1200F (649C), whic h is dis­ cussed in greater detail beginn ing on page 16. Type 416 is a free­ 180 machining variat ion of Type 410 and has simil ar characteristics. 110

To review briefly , the relative 100 resista nce to corros ion among the 90 co mmon stainless steel fastener 80 materials is as follows Corrosion 70 Category Type Resistance 60 Austeni tic 316 Superior 50 (18-8) Excellent 40 Ferritic 430 Good Marte nsitic 410 Fair The cor ros ion Data Survey pub­ lish ed in 1968 by the National Ass ociation of Corrosion Engineers (NACE) contains data on the use of these materia ls in various chemica l environme nts, with specific info r­ mation on temperatures and con­ centrations. These data can be very helpful in narrowing dow n the cho ice of materials for any given Source : ITT Harper co rrosive enviro nment. The fin al

12 Tensileand yieldstrengths Tensile or ultimate strength is that property of a material which determines how muc h load it can withstand unti l fai lure. Yie ld strength is a measure of the resistance of 0 a material to deformation; that is, to taking of a permanent set under load. For stainless steels, the stainless steels used for bolts , yield strength is calcula ted on a Stainlesssteel screws , studs, and nuts up to and stress-strain diagram, (Figure 1) including 1½ inch (38.1 mm ) in and it is a point at which a line fastenerproperties diameter. The specification is drawn parallel to and offset 0.2% Once the design engineer has especially useful because it is from the straight line portion of the determ ined cand idate fastener based entirely on the properties of curve intersects the curve. mate rials on the basis of their full -size finished fasteners , which It can be seen from the data in corrosion-resistant propert ies, his in many cases are different from Table 2 that there is a considerable next concern probably will be the the proper ties of the bar or spread between the tensile and mec hanical and physical propert ies from which they are made. Tables yield strength values , which is of these materia ls. Once aga in , the 1 and 2 present some of the data characteristic of stainless steels. fam ily of stainless steels covers a conta ined in the IFl-104 specifica ­ Yield strength is used in design wide cho ice. The choice , however , tion. should not be a diff icu lt one to make if the designer uses the TABLE 3-STRENGTH-TO-WEIGHT RATIO guide lines ava ilable to him , such as Typica l Strength-to- Dens ity the specificat ions published by the Material Tensile Strength Weight Ratio Lbs. / Cu. In. 2 Industrial Fasteners Institute. ksi Inches x 10 Martensitic Stainless Many an enginee r who has 180 .280 6.4 Steel (410 , 416) attempted to design a product using stainless steel fasteners has Aluminum (2024-T4) 60 .098 6.1 learned that meaningful data on Austenitic Stainless fastener properties are sometimes Steel (18-8) 125 .290 4.3 difficult to find. In many situations , Strain hardened the des igner has had to rely on Titanium 50 163 3.1 technical data based on the Commercially pure mechan ical properties of the 12 .041 2.9 mater ials from which the fasteners Austenitic Stainless are made. All too often these 80 .290 2.8 Steel (18-8) , annealed properties vary considerably from the actual properties of the Mone! 400 80 .319 2.5 manufactured fasteners . Silicon 75 .308 2.4 The Industrial Fasteners Institute (IFI), however , has a specifica­ 60 .308 2.0 tion-lFl - 104-that covers the Mild Steel 50 .282 1.8 mechanica l, metallurgical , and quality requirements of the common Source: ITT Harper

13 HELICAL SPRING FLAT WASHER EXTERNAL TOOTH INTERNAL TOOTH COUNTERSUNK LOCK WASHER LOCK WASHER LOCK WASHER FINISHING WASHER

calcu lations , however , because it TABLE 5- stands to reason that if the yield CREEP STRENGTHS OF TYPICAL STAINLESS STEEL FASTENER MATERIALS strength is exceeded , defor mation will occ ur in the fastener . LOAD FOR 1 % ELONGATION IN 10,000 Hr, ksi AISI In looking at the data in Table 2, TYPE it is noted that three different sets 1000 F 1100F 1200 F 1300 F 1500 F of properties are given for Types 303 16.5 11 .5 6.5 3.5 0.7 304, 305, and 316, and two sets of proper ties for Types S30430 and 304 20 12 7.5 4 1.5 384. The reason is that 300 Series sta inless steels increa se in strength 305 19 12.5 + 8 4.5 2 as a result of cold working. For examp le, full size bolts of 309 16.5 12.5 10 6 3 18-8 stainles s steel in the annealed 310 33 23 15 10 3 condition wi ll have a minimum yield + strength of 30 ksi (207 MPa). If the 316 25 17.4 11 .6 7.5 2.4 bolt is cold worked 15-20%, its yield strength level will increase to 321 18 17 9 5 1.5 50 ksi (345MPa) minimum. Cold wo rked from 35 to 40%-indicated 347 32 23 16 10 2 in Table 2 as "strain hardened"- 430 8.5 4.7 2.6 1.4 the minimum yield strength level is as high as 100 ksi (690MP a), de - 446 6.4 2.9 1.4 0.6 0.4 pending upon size of the fastener. In the 400 Series, only the 410 11.5 4.3 2 1.5 martensitic stainless steels are hardenable , and that is by therma l 416 11 4.6 2 1.2 treatment. Two sets of values are Source: Ind us try Data

TABLE 4-ALLOWABLE SHEAR STRESS FOR STAINLESS STEEL BOLTS4

Allowable Shear Minimum Tensile Requirements Stress (ksi) Diameter Condition d Type Finish and Specification No Threads Threads (In) 0 2% Yield Tensile Strengt h in Shear in Shear (ksi) I I Strength (ksi) Plane I -- Plane Condition A (Anneale d) 302** in ASTM A276- 71 304 Hot Finish ed all 30.0 75.0 15.0 10.5 Class 1 (sol ution treated) 316 in ASTM A193-71

302 Condition A (Annea led) 304 Cold Finished 45.0 90.0 18.0 12.6 in ASTM A276- 71 ~½ 316

Condition B (cold -worked) 302* * in ASTM A276- 71 304 Cold Finished Class 2 (solu tion treated ~ ¾ 100.0 125.0 25.0 17.5 316 and strain hardened) I in ASTM A193-7 1 * I I I • For Class 2: B8M in ASTM A 193, the allowable shear shear plane, or 15.0 ksi when threads are in the shear plane. stress is 22.0 ksi when threading is excluded from the * * ASTM A276- 71 only.

14 given in Table 2 for Types 410 and TABLE 6- 416, representing two levels of THERMAL EXPANSION OF CORROSION-RESISTANT FASTENER ALLOYS hardening and tempering. Type 430 Mean Coefficient of is not hardenable. Alloy Temperature Range °F Thermal Expansion Figure 2 is a relative comparison In./ In. / °F (10 6) of strength va lues between stainless J Type 304 Stainless Steel 32 to 212 9.6 steels and other corrosion resistant 32 to 572 9.9 fastener materials. 32 to 1112 10.4 In selecting a stainless steel on Type 316 Stainless Steel 32 to 212 8.9 the basis of mechanica l and 32 to 572 9.0 physica l properties, designers 32 to 1112 10.3 should keep in mind the following considerat ions: Type 41 0 Stainless Steel 32 to 212 6.1 32 to 1000 7.2 Thread Strength Thread forms on Brass 68 to 572 11.3

Naval Bronze 68 to 572 10.0

Silicon Bronze 68 to 572 11.8

Monel 68 to 212 7.8

Titanium 32 to 68 4.7 32 to 1600 5.6

Aluminum (2024) -76 to 68 21.4 68 to 212 22.8 68 to 392 23.9

Source: ITT Harper in (1) more accurate, more uniform given in Tab le 3. Of part icul ar thread dimensions , giving a better interest is the sim ilari ty between fit between threaded parts and Type 410 stain less stee l and alum­ fewer concentrated loads at points inum , and the fact that Type 410 of misfit; (2) smoother thread sur­ has a higher strength-to-weigh t faces and , thus , fewer scratches ratio than aluminum 2024-T4. and other markings to initiate Shear Strength cracks, or ga lling; and (3) higher Shear is transverse rupture. It is yie ld , tensile , and shear properties caused by a push ing or pulling to better withstand service loads. force at 90 ° from the axis of a part. Strength-To-Weight Ratio In appli ­ Thus , a used as a pulley axle cations where weight is an im­ will shear if the load on the pulley portant consideration, designers exceeds the shear value of the look to strength-to-weight ratios for rivet. Shear strength is defined as fasteners are manufactured by an indication of the most efficient the load in pounds to cause rupture , cutting, rolling, or grinding. The materia l to use. The strength-to­ divided by the cross-sectional area best qual ity highest-strength thread, weight ratio is defined as the ratio in square inches of the part along however, is achieved by thread of tensile strength to density. Some the rupture plane. rol ling. This is because the plastic typical properties of corrosion­ The allowab le shear stresses for deformation-or cold working­ resistant fastener materials, includ­ stainless steel bolts are given in involved in rolling threads results ing strength-to-weight ratios, are Table 4, which is based on the AIS I

15 -- ..

PAN BINDING TRUSS

publication, "Stainless Steel Cold­ TABLE 7-SCALING (OXIDATION) RESISTANCE OF Formed Structural Design Manual, TYPICAL STAINLESS STEEL FASTENER MATERIALS 1974 Edition.•" The allowable shear AISI MAX. CONTINUOUS MAX. INTERMITTENT stress for bolts with no threads in TYPE SERVICE, AIR, °F SERVICE, AIR, °F the shear plane was taken as 60% - 303 1650 1400 of the minimum tensile strength divided by a safety factor of 3.0. 304 1650 --- 1550 This allowable shear stress provides 305 1650 - - a minimum safety factor of about 309 1950 1850 1.2 against shear yielding of the 310 2050 1900 bolt material. When threads are in­ - 316 1650 1550 cluded in the shear plane, 70% of -- ·->-- the nominal allowable shear stress 321 -- ·---- 1650 1550 is used due to the fact that the 347 1650 1550 actual shear stress in bolts is to be 430 1550 1650 calculated on the basis of the gross cross-sectional area or nominal 446 - 1950 2050 area, and that the ratios of stress 410 - 1300 1450 � area to nominal area range from 416 1250 1400 0.65 to 0.76 for diameters of bolts Source: Stainless Steel Industry Data

varying from ¼to¾ inch (6.3- FIGURE 3. EFFECT OF CHROMIUM CONTENT ON SCALING RESISTANCE OF CHROMIUM-IRON ALLOYS (At 1 S00"F or 982°C) 19.1 mm). This practice is comparable to that for high-strength carbon steel structural bolts. However, it is slightly more liberal because of the 45 generally shorter joint lengths in cold-formed stainless steel con­ 40 struction. For bolts not listed in Table 4, the allowable shear stress 35 can be determind in the same .2 manner 30 'ii 25 High-and I .,, 20 low-�emperature .5 service 15 The selection of stainless steel 10 fasteners for high-temperature service is complex because of the 5 many factors involved. Mechanical and physical properties have to be considered together with corrosion 16 2 4 6 8 10 12 14 18 20 22 24 resistance. % Chromium Source: Industry Data In all bolted joints, the fasteners are tightened to some initial elastic

16 TABLE 8-CRVOGENIC PROPERTIES OF STAINLESS STEELS

Test Yield Strength Tens ile Elongation lzod-lmpact AISI Temperature 0.2% Offset Strength % % Reduction TYPE in 2" of Area °F °C ksi kg/ mm2 ksi kg/ mm 2 (5.08 cm) It-lb kg-m 304 -40 t -40 34.0 24.0 155.0 109.0 47.0 64.0 110 15.2 -80 -62 34.0 24.0 170.0 120.0 39.0 63.0 110 15.2 -320 -196 39.0 27.0 221.0 155.0 40.0 55.0 110 15.2 -423 -252 50.0 35.0 243.0 171.0 40.0 50.0 110 15.2

316 -40 I -40 41.0 29.0 104.0 73.0 59.0 75.0 110 15.2 -80 -62 44.0 31.0 118.0 83.0 57.0 73.0 110 15.2 -320 - 196 75.0 53.0 185.0 130.0 59.0 76.0 Not Available - 423 -252 84.0 59.0 210.0 148.0 52.0 60.0 Not Available

430 -40 -40 41.0 29.0 76.0 53.0 36.0 72.0 10 1.4 -80 -62 44.0 31.0 81.0 57.0 36.0 70.0 8 1.1 -320 -196 88.0 62.0 92.0 65.0 2.0 4.0 2 0.3

410 -40 -40 90.0 63.0 122.0 86.0 23.0 64.0 25 3.5 -80 -62 94.0 66.0 128.0 90.0 22.0 60.0 25 3.5 --320 -196 148.0 104.0 158.0 111.0 10.0 11.0 5 0.7

Source: Stainless Steel Industry Data

FIGURE 4. WHEN COLD WORKING IS EMPLOYED, FIGURE 5. MAGNETIC PERMEABILITY OF AUSTENITIC ALLOYS SOME NORMALLY NON-MAGNETIC AUSTENITIC SUBJECTED TO COLD WORKING CAN ALSO BE EXPRESSED STEELS BECOME SUBSTANTIALLY MAGNETIC AS A FUNCTION OF TENSILE STRENGTH

10. 10. 302 304 304 /.

2.0 2.0

0 0 0 0 305 0 0 1 .1 305 C\J 1.1 #/

E E 316 1.01 1.01 -- 316 ------

1.001 1.001 0 20 40 60 80 100 75 100 125 150 175 200 225

Cold reduction , per cent Tensile strength , psi x 1000 Increasing cold work

17 GD @ 0 •

FLAT 100I DEGREE FLAT ITRIM HEAD HEX.HEADI i HEAD MACHINE MACHINE SCREW MACHIN� _SCREW THUMB SCREW

stress and corresponding strain. At FIGURE 6. MACHINING elevated temperatures, creep occurs in which some of the elastic strain is transformed to plastic strain with a corresponding reduc­ tion in stress. This behavior is termed relaxation. When bolts relax they no longer maintain a tight joint. Resistance to creep, or relaxa­ tion, is an important consideration Illustrated above is the same part produced by machining from a large for fastener systems at elevated diameter bar or wire, shown to the right. Grain or metal flow lines are broken temperature. Table 5 shows creep through the head and washer sectjon, which creates planes of weakness. values for several widely used stainless steels, some of which are readily available as "off-the-shelf" FIGURE 7. COLD-HEADING fasteners. Other considerations for elevated-temperature service in­ clude thermal expansion character­ t-, istics and oxidation resistance. ► The thermal expansion of a • fastener should match the expan­ sion characteristics of the materials being fastened, with a logical Illustrated above is a cold headed part formed from the diameter of wire conclusion that stainless steel shown to the right. Unbroken metal flow lines (grain) greatly increase fatigue life and enhance load-carrying ability. fasteners are best for stainless steel base metal joints; otherwise, there constant temperature condition is, Of primary importance to the can be overstressing and possible for the most part, related to chrom­ development of the world's natural failure, or a rapid loss in clamping ium content, as illustrated in Figure gas supplies is the handling and stress. Table 6 shows the thermal 3. In Table 7, scale resistance is storage of liquid natural gas (LNG). expansion characteristics of several expressed as temperature at maxi­ Fasteners have a role in LNG corrosion resistant fastener materials. mum continuous service in air or processing for piping and heat The oxidation or scaling resis­ maximum intermittent service (in exchanger flanges, pumps, and tance of stainless steels under which temperature cycling occurs). various related equipment. As the high-temperature environ­ Austenitic stainless steels are the ment becomes contaminated by most widely used materials in compounds of sulfur, carbon, cryogenic applications, especially hydrogen, and the halogens, the Type 304, because it does not problem of materials selection be­ become brittle as it is chilled. Not comes even more complex. Never­ only does Type 304 remain tough theless, stainless steels are widely and ductile at LNG temperature­ used in these environments. A fairly minus 260F ( -162C)-but it retains comprehensive discussion of their excellent properties with liquid application is found in a publication hydrogen ( -423F or -253C) and by The International Nickel Com­ liquid helium at -452F ( -268C). pany, "Corrosion Resistance of the Table 8 shows low-temperature Austenitic Chromium-Nickel Stain­ mechanical properties of several less Steels in High-Temperature stainless steels used in cryogenic Environments. 5" service. 18 is shown in Figure 4. The magnetic Magneticand permeabi lity of the same group , but expressed as a function of tensile I electricalproperties strength, is show n in Figure 5. The straight -chrom ium , 400 Magnetism , for purposes of this Ser ies stainless stee ls are always discussion , is the ability of a par t strongl y magneti c. The degree of to be attracted by a magne t and magnetic permeability, howev er , is not the part 's abili ty to function as affected by chemical composition a magnet It is mor e accurate ly and heat treatment. For highest expressed as magnetic perme­ initial permeability , the carbon con­ ability , and it can be an important tent should be kept low; Types 416 design considera tion . One reason and 430 shou ld be fully annealed is the need to have a magnetic for the best magnetic behavior. mater ial for automatic assembly Durin g annealing , a dry hydroge n operations On the ot her hand , atmosphere should be used to keep some highly sophist icated elec­ surfaces bright and free of con­ tron ic equipment may req uire ma­ tamination, such as carbo n or terials with very low or nil magnetic nitrog en co ntamin ation wh ich can permeability Stainless steels can dec rease permeability. Chemical c leanin g (pass ivat ion), which removes iron partic les from the surface , may also improve permeabili ty. Electrical conductivity

The conduct ivity of a fastener material may be important , althou gh good design se ldom calls for a fastener to be a current-carry ing • member . Following are some con­ ductivity va lues expressed as a percent of the Interna ti onal Anneal ed Copper Standard at Manufactureof satis fy either requireme nt 68F (20C) stainlesssteel The austen itic group of stainless Low silicon bronze steels have essentia lly low magnet ic (col d-formed fasteners) 12% fasteners permeabi lity in the annealed con ­ High sil icon bronze dition ; i e , they will not be attracted (hot-formed and machined) 7% While designers seldom get in­ by a magnet Some of the austenitic Naval bronze 26% volved in the manufacture of materials , however , are weakly Brass ( cold formed) 27% fasteners, it ca n be useful to know attracted by a magnet after severe Brass (machined) 26% something abou t the processes co ld working. The effect of cold Electrolytic copper 101 % involved. Thi s can be especially true working on magne tic prope rties fo r Stainl ess Stee l (18-8) 5% if th e product requires a fastener a few common 18-8 sta inless stee ls Aluminum (2024- T4) 30% of spec ial design, such as the many

19 ©) @ @ o:::D (Ill COJ a::o IT:Tl

REGULAR HEX NUT FINISHED HI:)( NUT HEX SLOTTED FULL AND JAM FULL AND JAM NUT

"specials" illustrated in the photo­ TABLE 9-TORQUE GUIDE graphs throughout this booklet. There are two basic methods for BOLT SIZE TYPE 304 ST. ST. _... TYPE 316 ST. ST. - producing fasteners-machining 2-56 2.5 2.6 - 2-64 3.0 3.2 - and cold heading-both of which 3-48 3.9 4.0 are applicable to stainless steels. 3-56 4.4 4.6 4-40 5.2 5.5 Machining is the oldest method 4-48 6.6 6.9 for fastener production, and it is still 5-40 7.7 8.1 specified for very large diameters 5-44 9.4 9.8 6-32 9.6 10.1 and for small production runs. 6-40 12.1 12.7 Machining, however, has a signifi­ 8-32 19.8 20.7 cant disadvantage, as illustrated in 8-36 22.0 23.0 .__- 10-24 22.8 23.8 Figure 6. It disrupts metal grain 10-32 31.7 33.1 - flow and creates planes of weak­ - 75.2 78.8 )-�0 - ness in the critical head-to-shank - ¼"-28 94.0 99.0 - - ¾',"-18 132 138 - fillet area. The result is some loss in -- - U,"-24 -- 142 147 load-carrying ability and a drastic %"-16 236 - -- 247 reduction in fatigue resistance. 6 - �"-24 259 271 - ¾'.''-14 -- 376 393 Cold heading is a method of - ?<," -20 -- 400 -- 418 forming wire into various shapes by ½"-13 -- 517 542 causing it to plastically flow into - _l"2"-20 - -- 541 565 - �i/'-12 -- 682 713 and cavities without pre­ --- Yi/'-18 -- 752 787 heating the material (Figure 7). - %"-11 -- 1110 1160 Bolts, screws, nails and have ¾"-18 -- 1244 13C,1 - ¾"-10 1530 1582 long been made by cold heading, - -- ....___--- ¾"-16 1490 1558 - but recent developments in this !/8"-9 2328 2430 field have expanded the market for - ½"-14 2318 2420 - 1 "-8 3440 3595 special fasteners. - 1 "-14 3110 ._ 3250 ---·- --- 1½"-7 F 413 __ 432 Cold heading has many important -- - 1½"-12 390 4(J3 advantages in both quality and -- - 1¼"-7 -- 523 546 economy. Production rates, for - 1¼_"-12 -- 480 �04 example, can exceed 12,000 parts - 1½"-6 -i -- 888 930 1½"-12 703 732 per hour. Cold heading also cold works stainless steels, which results Source: ITT Harper in significant increases in strength Suggested Max Torquing Values-a guide based upon industry tests on dry products wiped for the 300 Series types. clean. Values thru 1" diameter are stated in inch pounds; over 1" diameter, in foot pounds. The ¾" diameter and under metal products were roll-threaded and, where size range Following heading, the blank is permitted, were made on Bolt Maker equipment. ready for threading, which is fre­ quently done by roll threading, match, it is far more desirable to another cold forming technique that Plated coatings use a fastener in which corrosion preserves grain flow patterns. Some designers may be inclined protection is inherent within the There are other operations in­ to think of plated fasteners as a material itself and not just added to volved in fastener production, such low-cost solution to corrosion. the surface. as head slotting, shank slotting (for While plated fasteners do serve a Stainless steels do not need any thread-cutting screws), head ­ useful purpose in some applica­ form of protective coating for resis­ ing, etc. But the making o,= and tions, such as when a plated tance to corrosion, in contrast with threading the blank are the two coating is added for purposes of plain steel and some nonferrous major processes. creating a special finish or color fastener materials. While plated or

20 FIGURE 8. FASTENER LUBRICATION

2000 DRY

1111111LUBRICATED 1600

I 1200 z 0 z w I- 800 0

400

0 20 40 60 80 100 120 140 TORQUE (lb .-in.) Effect ot lubrication on torque-tension to 1250 lb. tension range . relationships is shown by the chart which Torque values are affected in various is based on results obtained with 9/16-18 ways by different types of lubricants . Wax steel bolt driven into aluminum. For a non­ on either the bolt or nut, or both, also acts lubricated bolt, torques of 115 to 125 lb.-in. to reduce the torque requirements. were required to develop tensions of 800 to 1400 lb. For a lubricated bolt, torque values ranged from 65 to 75 lb.-in. for 1000 Source: Skidmore-Wilhelm Mfg. Co. galvanized steel fasteners are ade­ failure . Such discontinuities result quate where corrosive conditions from or driver damage , are not severe , many designers poor plating practices , or simp ly conside r the extra cost of stainless from the turning action of thread steel fasteners as inexpensive . agai nst thread. insurance against possib le fai lure Furthermo re, while the cost of or loss of eye appeal. When the stainless steel fasteners may be cost of failure is considered , in more than plated fasteners, the conjunction with the ease in which overa ll cost of the finished product damage can occur to a protective (from a small app liance to a large coating , it makes good sense to plant) wil l genera lly be affected specify a fastener made of a ma­ only by an insignificant amount. terial which is inherent ly corrosion Interestingly , plated coat ings are resist ing. Often, a minute discon­ applied to stainless steels for tinuity in a plated surface is all that purposes of chang ing appearance. is needed to lead to corrosive For instance , the designer may 21 (])

PAN HEAD BINDING HEAD FILLI STER HEAD MACHINE SCREW MACHINE SCREW MACH I NE SCREW

how the fasteners wil l be installed. can be adjusted to speci fic torque With any product, effective values. The most tro uble occu rs utilization requires knowledge of when replacement is being made the product's characteristics as well under condit ions wher e tor que as its proper use. Failure to fol low tools are not available. There are accepted practices can lead to some gu idelines for these difficulties, such as seizing and ci rcums tances: ga lling , which can be encoun tered 1) Tighten the nut fing er tight­ wit h fasteners made of any materia l about one foot-p ound of • includin g stainless steels. There are torque or less. several courses of action open to 2) Tighten the nut one additional designers that will minimize or turn , 360 degrees, for proper elimin ate such difficulties. torque. This is an arbitrary Fasteners made in accordance figu re that applies primarily to with nationally recognized stan­ 300 Series fasteners . For dards , such as pub lished by the Ame rican Nationa l Standa rds In­ stitute , Inc., (ANSI), wi ll assure that nuts and bolt s are uniformly thr eaded. One of the common causes for ga llin g is mismatched threads, or threads that are not uniform from shank or shoulder to po int. Standards provide for product uniformity supported by quality control practices. Reasonabl e care should be exercis ed in the handling of fas­ teners to keep threads clean and free of dirt , espec ially coarse grime or sand. If threads are tightened down on sand, the chance of gall­ ing or seizing-in any fastener want a black fastener , or a highly mater ial-increases sign ificant ly. reflective chrome plated fastener to Torque , another consideration match the surfaces being joined. in a proper ly fastened jo int, is the Such requirements can be accom­ twisting force appl ied to a fastener. modate d in stainless steel. Table 9 offers some suggested maximum torque values for stain­ less steel fastene rs. This table is a Effectiveutilization gu ide based on industry tests that provide maximum clamping values of stainlesssteel with minimum risk of seizing. The fasteners values are based on fasteners that are dry-free of any lubricant­ In order to have an effective and wiped clean of chips and fastener system , the designer foreign matter. should also be concerned with Most prod uction lines are proper utilization, espec ially as to equ ipp ed with assembly tools that

22 cants have different effects also . FIGURE 9. GALVANIC SERIES OF METALS AND ALLOYS Wax, for example , on either the bolt or nut, or both , acts to reduce the torque requirements . Magnesium Magnesium Alloys If a lubricant is going to be used , Zinc tests should be conducted to de­ Aluminum 1100 termine torque requirements and to Cadmium Anodic More Likely to Be evaluate the compatibility of the Aluminum 2024-T4 Attacked lubricant to the environment-such Steel Iron as high temperature. Among the Cast Iron popular lubricants are those which Lead-Tin Lead contain substantial amounts of Tin molybdenum disulfide , graphite, Copper mica, talc, copper or zinc fines , or Copper-N ickel All oys zinc oxide. However , the zinc­ More Noble Stainless Type 430 (Passive) bearing lubricants are not recom­ Cathodic Stainless Typ e 304 (Passive) mended for use with stainless steel Stainless Type 316 (Passive ) Silver at elevated temperatures . Graphite Gold

Source: ITT Harper

hardened and tempered 400 can prevent this from happening Series fasteners , this may be Some engineers are of the opin­ too high. In any event , a trial ion that the only way to avoid seiz­ test should be conducted with ing and galling is to lubricate the a for best threaded joint before it's assembled. results. Adding a lubricant can affect the In service at elevated temper­ torque-tension relationships , as ature , the buildup of oxides or shown in Figure 8. A lubricated scale on fastener surfaces may fastener requires less torque to Designingfor "fuse" threaded surfaces together. achieve the same degree of tension Regular loosening and retightening or clamping force. Different lubri- optimumcorrosion resistance

The best approach in effectively combatting the destructiveness of corrosion in a fastener system is prevent ion. Since corrosion resistance is an important aspect of product reli­ abilit y, inherent in any attempt to prevent corrosion is the careful selection of fastener materials. A common practice in industry is to use fasteners made of metals or allo ys that are more corrosion

23 COMMON . BOAT NAIL, AND COTTER TAPER PIN ESCUTCHEON PIN

resistant than the materials they join. Thi s practice is justified be­ cause the fasteners may have to withstand higher loads with greater unit stress than the parts they hold together. Also , corrosion-weakened fasteners may lead to a more im­ mediate failure with more serious consequences than the same amount of corrosive attack else­ where in the assembly.

Typesof corrosion

Corrosion protection for a fas­ tened jo int encompasses much more than a consideration of the corrosion resistance of the fastener itself. Actually required is an analysis of the entire assembled joint as a system. This system in­ cludes structural design , materials , When two different metals are in located nearest each other. Con­ stresses , product life expectancy contact , in polluted water for versely , the chance of serious and environmental conditions. example , in effect a batter y is corrosion effect is greater the Because of the variety of special created , current flo ws, and one of further apart the metal s are located environmenta l conditions possible, the metals corrodes. In considering in the series. For example, one of the subject of corrosion is very a bimetallic couple, it is important the worst galvanic joints would complex. To simplify the subject, to know which of the two metals is consist of magnesium and Type however, this discussion will at­ more anodic (least noble). A guide 316 stainless steel. tempt to put some of the more to this is the arrangement of metals A very important factor to con­ important factors in perspective . in the galvanic series chart shown sider in evaluating the potent ial for Corrosion is the wearing away or in Figure 9. Any metal in this series problems is the alteration of a metal either by direct will tend to have corrosion accel­ relative surface area of the two chemical attack or by electro­ erated when it is coupled , in the different metals that are in direct chemical reaction. There are several presence of an electrolyte , with a (electrical) contact in an assembly. basic types of corrosion that may metal in a lower position. The For example , steel is located above occur singly or in combination. The corrosion of this lower metal will stainless steel in the galvanic series most prevalent form of attack in tend to be reduced . and is accordingl y subject to fasteners, however , is electro­ Some of the metals in the gal ­ accelerated corrosion when a chemical in nature and can best be vanic series , it will be noted , are in galvanic couple is established. But understood by a discussion of groups . Under ordinary circum ­ the extent of this galvanic acti on galvanic corrosion. stances no serious galvanic action depends on the relative area of Galvanic Corrosion can occur will result from the coupling of each material. For instance , if small when dissimilar metals are in con­ metals within the same group . As is steel fasteners , such as rivets , are tact in the presence of an electro­ the case with the series as a whole , used to join stainless steel plate s. lyte , which may be nothing more the safest galvanic combinations and the assembly is immersed in than a wet industrial atmosphere. are those produced by metal s water , the steel rivets will cor rode

24 will provide the needed corrosion resistance. Stainless steels have resistance to a broad range of aggressive environments. Concentration Cell Corrosion takes place where metals are in close proximity and , unlike galvanic cor­ rosion, does not require dissimilar metals. When two or more areas on the surface of a metal are ex­ posed to different concentrations of the same solution , such as under dirt or in crevices , a difference in electrical potential results , and corrosion takes place To avoid such corrosion , keep surfaces smooth and minimize or eliminate lap joints, crevices, and seams. Surfaces should be clean of organic material and dirt. Bolts and nuts should have smooth surfaces , especially in the seating areas . Flush-head bolts should be used when possible. quickly. If, on the other hand, stain­ A general rule to remember is to Fretting corrosive attack may occur less steel rivets are used to join use the more-noble metal for the between contacting , high-loaded steel plates in water, both rivets and part with the smaller surface area. metal surfaces subjected to very plates will suffer negligible galvanic This makes a good case for using slight vibratory motion. It is most attack, even in the immediate stainless steel fasteners for joining likely to occur in high tensile , high­ vicinity of the rivets. Aircraft de­ metals that are less corrosion frequency dynamically loaded ap­ signers , for instance , who specify resistant. Table 10 provides guide­ plications , and it can be prevented stainless steel fasteners in alum­ lines for the selection of fasteners by specify ing fasteners with high inum structures depend upon this for various base metals . If the hardness and requiring maximum area-relationship principle. The potential is high for galvanic cor­ preloading of the fastened joint. A greater the relative area of the rosion in a fastened joint , it is higher clamping force results in a anodic metal (the metal that cor­ possible to insulate the fastener. more rigid joint with less move­ rodes} , the less severe the Other types of corrosion of con­ ment. corrosion. cern to the designer are direct Oxidation is the reaction with oxy­ The area relationship depends attack, concentration cell corrosion gen , or other gases , at high not only on the relative area of the or crevice attack, fretting , oxidation , temperatures . Some stainless steels materials in the structure , but also stress-corrosion cracking , and have good oxidation resistance at on the number of fasteners . Some­ corrosion fatigue . high temperature. (See discussion times an acceptable balance of Direct Attack is the most common of stainless steels for high-temper­ incompatible metals may be form of corrosion affecting all ature service on page 16.) achieved by adjusting the number metals and structures . It is a direct Stress-Corrosion Cracking is be­ of fasteners used , or by relocating and general chemical reaction of lieved to be caused by the com ­ the fasteners to distribute them the metal with a corrosive environ­ bined and mutually accelerating more uniformly to avoid a local ment-liquid , gas, or even a solid. effects of a tensile stress and a condition of low relative area. Usually a material can be found that corrosive environment-such as 25 -

HEX WASHER Fl- LUSTER HEXAGON

t

.? l ll

with copper-base alloys in a sulfur be purchased. With the name and tenitic Chromium-N ick el Stainless environment or with some stainless address of each company is a list Steels in High-Tem perature En­ steels in chloride environments at of the fastener types available from vironments ," The Internat ional elevated temperature Methods of that company. There are 62 fastener Nickel Compan y, 1963. combatting stress-corrosion crack­ types listed , including bolts, nuts , 6. Varrese, F. R., "A User 's Guide ing include relieving the stress , screws, staples, washers, rivets, to Fastener Reliabil ity," METAL removing the critical environment, lock nuts, nails, studs, threaded PROGRESS , Decembe r 1974. or selecting a more corrosion­ rod, , cap screws, hooks , hose resistant stainless steel or a stain­ clamps, and specials. less steel (straight-chromium 400 Acknowledgement series or proprietary grade) that is The Committee of Stainless Steel not subject to this phenomenon. References Producers acknowledges the data Corrosion Fatigue is accelerated 1. " Steel Products Manual-Stain­ and materials that were received fatigue failure occuring in a cor­ less and Heat-Resisting Steels," from the following: rosive medium. It differs from American Iron and Steel Institute , Accurate Threaded Fasteners, stress-corrosion cracking in that December 1974. Inc. dynamic alternating stress is the 2 "Compi lation of Trade Names, Adjustable Bushing Corp. contributing factor. Factors extend­ Specifications , and Producers of Ajax Hardware Corporation ing fatigue performance are ap­ Stainless Alloys and Super­ Avibank Mfg. Inc. plication and maintenance of a high alloys," ASTM Data Series OS Clendenin Bros. Inc. preload , and proper alignment to 45, American Society for Testing Dzus Fastener Co., Inc. avoid bending stresses. and Materials. Harvey Hubbell, Inc. 3. " Mechanical and Quality Re­ Hi-Shear Corporation Availability quirements for Stainless Steel Huck Manufacturing Company and Nonferrous Bolts , Screws , Lamson & Sessions of stainlesssteel Studs, and Nuts," IFl-104, a Maclean-Fogg Lock Nut Co. standard published by the Indus­ Mid-West Fabricating Co. fasteners trial Fasteners Institute of Cleve­ West Bent Bolt Division A booklet, "Stainless Steel land, Ohio. Rosan , Inc. Fasteners Suggested Source List ," 4. "Sta inless Steel Cold-Formed Standard Pressed Steel Co. published by the Committee of Structural Design Manual, 1974 Stillwater Assoc. Stainless Steel Producers in 1975, Edition," American Iron and Townsend Division of identifies over 300 companies Steel Institute. Textron Inc. where stainless steel fasteners can 5. "Corrosion Resistance of Aus- 26 TABLE 10 – GUIDELINES FOR SELECTION OF FASTENERS BASED ON GALVANIC ACTION

Fastener Metal

Brasses, Austenitic Zinc & Aluminum & Copper, Martensitic Stainless Galvanized Aluminum Steel & Cast Bronzes, Stainless Types 302/304 Base Metal Steel Alloys Iron Type 416 303, 305

Zinc & A B C C C C Galvanized Steel

Aluminum & A A B C Not B Aluminum Alloys Recommended

Steel & Cast Iron AD A A C C B

Terne (Lead-Tin) ADE AE AE C C B Plated Steel Sheets

Brasses, Copper, ADE AE AE A A B Bronzes, Monel

Ferritic Stainless ADE AE AE A A A (Type 430)

Austenitic Stainless ADE AE AE AE A A (Type 302/304)

Key:. A The corrosion of base metal is not increased by D. The plating on the fastener is rapidly consumed, the fastener. leaving the bare fastener metal. B. The corrosion of the base metal is marginally E. The corrosion on the fastener is increased by the increased by the fastener. base metal. C. The corrosion of the base metal may be markedly NOTE: Su rface treatment and environment can increased by the fastener material. change activity.