All you want to know about bolts...Grade 5 or 8 or aircraft or whatever....

[Dan Case]

Quote:

”I've researched a little bit since the post started earlier. I was kinda confused about some comments that Dan Case made. He made the comment that Grade 8 bolts were more brittle than Grade 5....and that Grade 5 bolts would just plastically deform rather than suddenly fail.”

blykins

Response:

The typical steel grades and heat treatments used for Grade 5 type bolts will allow significant deformation before breakage in almost all cases. An assembly might get so loose as to rattle long before the bolt(s) breaks. My experience, mostly at work in very vertical (we buy hundreds of millions of pounds of raw materials and ship finished goods) manufacturing operation, is that the first sign of a Grade 8 bolt problem is its head laying in the isle and we have to figure out where it came from. I have found original British bolts (I don’t know what how they would classify but most are low hardness.) in Cobras I have worked on stretched over 1/16” but they were still keeping the car together. In what I have read under OEM websites you are better off having a lets say Grade 5 bolt torqued to 80% of its yield strength than a Grade 8 bolt tighten to anything less than 80% of its strength. I doubt you could tighten many Grade 8 bolts to 80% of their strength if you were holding two annealed cold finished steel parts together without damaging the parts you are trying to keep together. I have not tested every type fastener and material in an original Cobra but I have tested quite a few and there are few hardened parts in an original Cobra chassis I have found so far. I have also had to repair quite a few Cobra chassis parts that somebody damaged by under and over tightening fasteners. Since there is no place you can go buy exact dimension, look, and performance parts for original Cobras I reverse engineer and have made what I need. Part of doing so includes actually measuring the Rockwell hardness of original parts and research what the transportation/automotive industry would use in that type application and how they would process it into a finished part.

Unfortunately you can not cover everything in a simple fashion. Properties of Rockwell hardness, tensile, yield, and elongation is not the only ones that determine how a part will behave. The process used to produce the steel and the processes to make the part are also major players.
Example 1:

We have one device used to manufacture product that is a type of threaded valve stem. In 1973 when the device was first designed the engineers searched the world over for a commercial material and tool shop to fit the need. Early materials tried failed, i.e. parts snapped off at the first thread, within a day or two of use. Bear in mind they were doing this with full co-operation of the steel companies they were working with. It took two years of development to find a “steel” and a “process” to make parts that would last their desired design life. The application is one of the most severe I have ever encountered. A steel company was found that had a specialty tool steel that technically was an AISI “standard” material based only on the ingredients it contained. Their secret was in producing a grain structure no one else knew how to do. Fast forward from 1975 to 1989. In 1989 we started have sudden early breakage again. Failure analyses were done by the steel company and the world’s largest independent failure analysis company. What was wrong? The grain structure of the “new” material was nowhere close to the “old” material. We use the parts at a Rockwell C scale hardness of nearly 60. That is hard. The material also has an extremely high tensile strength. The old material would withstand bending and impact, the new would not. NEW AND OLD TESTED AS HAVING THE SAME ROCKWELL HARDNESS, SAME TENSILE STRENGTH, AND SAME CHEMISTRY. Their respective performances are worlds apart! What changed? The original company bought out another steel company and stopped making the steel themselves. The original company made 30 ton heats of material, processed it, and held it in inventory until somebody ordered it all. The new mill made 3 ton heats only after enough orders came in. Steel makers will tell you there is a world of difference in making tool steels in 30 ton versus 3 ton heats; you will not get the same properties. The steel company was totally unwilling to do anything to improve the performance. We selected a patented process material from another steel maker that is as far as AISI is concerned almost exactly the same steel and have had zero failures in years.

Example 2:
We use many millions of pounds of various cold finished low carbon steels a year. All are purchased to industry specifications and tested per ASTM methods. No two steel mills make large run commercial materials to a given specification that performs the exact same way except by random chance. This includes two mills from one company. They produce materials that fall into a range of chemistries and performance. This causes us endless trial runs to figure out how to use what purchasing bought. Steel purchased from a supplier from two of its mills can be night a day different. This is partly because industry standards are so broad but mostly due to equipment designs. Steel made the same day by the same company two states apart may vary in yield by 20% mostly because of the type equipment used at the separate mills. The steel companies could make the steels the same at all mills but nobody would want to pay their price for doing so. Even one mill will only typically guarantee a yield lot to lot within a range of 10% unless you want to pay a lot extra. Our plant has to deal with this constantly. That is part of my job.

Going back to my first 2 cents. Bolt grades are performance criteria and not necessarily anything to do with which AISI/SAE steel gets used, although more complex alloys are required to get higher tensile performance. Higher carbon content/complex alloys to get hardness and tensile up generally trade off with lower resistance to fatigue stresses in bending or impact. Pick up any steel supplier’s catalog and it will have some charts that show the trade offs of strength in tensile, wear, impact, fatigue, etc. among products. The higher the final hardness the less likely the material can stand anything but pure tensile or pure compression. A pot hole in the road is not likely to increase tensile load on components a lot but depending on what supports what connects to what some parts could be side loaded a lot. The Cobra control arm bent two plus inches I took off once sure wasn’t in tensile or compression. (The control arm bolts were bent too but they did not break. I bet you didn’t know there was such a thing possible as a 289 Cobra with a 90” wheel base on one side and 87” on the other after it had been restored!) Anybody could buy the “same grade” steel in name only as the big companies use and sell bolts saying they are made with the right material but that does not mean they meet the performance requirements. Counterfeiting of name brand top of the line fastener products with inferior material and processes is a very big business out there in the dog eat dog world. Just because the fastener has the grade markings don’t mean it is what is says it is.

Whatever you use, be sure it comes from a reputable supplier.