grades of inch-sized bolts

SAE grades

The best-known grades for inch-sized steel bolts are those defined by the SAE, a sequence of grades from 0 to 8, on the basis of the metal from which the bolt is made and the manner of manufacture. Available grades run from 2 to 8, with 8 the strongest. Higher grade numbers almost always mean increased strength (an exception is that some grade 6 bolts were stronger than grade 7). 

Grade What kind of bolts Size Minimum strength
in thousands of pound-force
per square inch
Marking
Proof Yield Tensile
1 Low- or medium-carbon steel. In practice this grade is obsolete, grade 2 being supplied in its place. ¼″ – 1½″ 33  36 60 drawing of grade 1 head
2 Low- or medium-carbon steel. ¼″–¾″ 55  57 74 drawing of grade 2 head
>¾″–1½″ 33 36 60
3 Obsolete.       drawing of grade 3 head
4 Medium-carbon cold drawn steel. Used for studs. ¼″ – 1½″ 65  100 115  
5 Medium-carbon steel, quenched and tempered. ¼″-1″ 85  92 120 drawing of grade 5 head
>1″- 1½″ 74 81 105
5.1 Low- or medium-carbon steel, quenched and tempered.         drawing of grade 5.1 head
5.2 Low-carbon martensitic steel, fully killed, fine grain, quenched and tempered. ¼″ – 1″ 85  92 120 drawing of grade 5.2 head
6 Obsolete       drawing of grade 6 head
7 Medium-carbon alloy steel, quenched and tempered. ¼″ – 1½″ 105  115 133 drawing of grade 7 head
8 Medium-carbon alloy steel, quenched and tempered. ¼″ – 1½″ 120  130 150 drawing of grade 8 head
8.1 Drawn steel for elevated-temperature service. Medium-carbon steel or 1541 steel.      130    drawing of grade 8.1 head
8.2    ¼″ – 1″ 120  130 150 drawing of grade 8.2 head

SAE J429

ASTM International grades

Specification Size Minimum strength
in thousands of pound-force
per square inch
Mark
Proof Yield Tensile
A307 grade A ¼″ – 1½″ 33 36 60 drawing of A307 head
grade B          
A449 1 1/8″ – 1½″ 74 81 105 drawing of A449 head
  1¾″ – 3″ 55 58 90
A325 type 1 ½″ – 1″ 85 92 120 drawing of head of ASTM 325 type 1 bolt
1 1/8″ – 1½″ 74 81 105
type 2 ½″ – 1″ 85 92 120  drawing of A325 type 2 head
1 1/8″ – 1½″ 74 81 105
type 3 ½″ – 1″ 85 92 120 drawing of A325 type 3 head
1 1/8″ – 1½″ 74 81 105
A354 grade BB         drawing of A354 grade BB head
grade BC ¼″ – 2½″ 105 109 125 drawing of A354 grade BC head
2¾″– 4″ 95 99 115
grade BD ¼″ – 1½″ 120 130 150  
A490 type 1 ½″ – 1½″ 120 130 150 A490
type 2         A490
type 3 ½″ – 1½″ 120 130 150 A490

Is a higher grade always better?

Replacing a bolt with a stronger one can be a bad idea, cost aside.

Some bolts were deliberately chosen so that they are weak enough to fail before the stress or strain damages some more expensive or critical part of the equipment. For the same reason, in making furniture cabinetmakers use glues that are weaker than wood. That way, if the furniture is overloaded, the joints break. It is much easier to reglue a broken joint than to replace a piece of broken wood.

Perhaps a more important reason is that the same processes that make a fastener harder and stronger make it more liable to fatigue and corrosion. To quote Alexander Blake, “The impression that we get a better product for the money because of the increased strength can eventually hurt us since higher strength means greater susceptibility to stress corrosion and fatigue failure.”

Alexander Blake.
What Every Engineer Should Know about Threaded Fasteners.
New York: Marcel Dekker, Inc., 1986.

Page 189.

Counterfeit bolts

In the 1980s, large numbers of counterfeit bolts appeared in the United States, almost all imports. For this reason, grade markings can no longer be trusted unless one knows exactly who made and graded the bolt. Aerospace-grade bolts are also being counterfeited (even NASA has been duped, to the tune of one million dollars to disassemble the Astro 1 space lab to remove counterfeit and defective fasteners).1

“In March 2000, Boeing discovered that it had 330,000 fasteners that were made from the wrong alloy.The company had to slow down production to remove and replace these fasteners on planes going through assembly and also had to recall 20 planes that had already been put into service. The manufacturer of the fasteners said it had received incorrect alloy material from its metal supplier. The material test report (MTR) indicated it was the correct alloy grade, but, in fact, it was not.”2

1. See http://articles.latimes.com/1989-01-27/news/mn-1479_1_single-counterfeit-part/2

2. From "Quality Assurance of Critical Fasteners", see https://www.niton.com/docs/literature/pmi_fasteners_appsummary_final_2013jan18.pdf?sfvrsn=2. (retrieved 2 April 2015). They cite an AP article of 8 March 2000 titled "20 Boeings Found Faulty", in the [Charleston] Post and Courier.

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