The strength of stainless steel bolts depends greatly upon their alloy composition and crystalline structure. Bolts are rated by their yield strength, or the amount of force it takes to stretch the bolt to 0.2 percent longer than its original length, and their tensile strength, or their ultimate breaking point. The three classes of stainless steel crystalline structure are austenitic, martensitic and ferretic. Kilograms per square inch (KSI) is the industry standard for measurement.
Austenitic, Solution Annealed
Austenitic stainless fasteners are the most corrosion and temperature resistant of all the bolt types. They are also non-magnetic. This category includes stainless steel types 303-A, 304-A 316-A, 384-A and XM7-A. Yield and tensile strengths are 30 ksi for bolts and screws and 75 ksi for studs and nuts.
Austenitic, Cold Worked
This category includes stainless types 304, 305, 316, 384 and XM7. Yield and tensile strengths are 50 ksi for bolts and screws and 90 ksi for studs and nuts.
Austenitic, Strain Hardened
This is the most commonly used stainless bolt type. Yield and tensile strengths are 100 and 125 ksi for bolts up to 5/8 of an inch in diameter, and 125 ksi for nuts. For bolts from 5/8 to 1 inch, the ratings are 70 ksi and 105 ksi, and 105 ksi for nuts. Measurements are 50 and 90 ksi for bolts from 1 to 1 1/2 inches in diameter, and 90 ksi for nuts.
Martensitic, Hardened and Tempered
Martensitic steels are very tough, but aren't as corrosion resistant as the other two, and will hold a magnetic field. This category includes 410-H and 416-H. Yield and tensile strengths are 95 ksi for full-sized bolts and screws and 135 ksi for studs. Nut stress rating is 125 ksi.
Mertensitic, Hardened and Tempered (HT)
Although similar sounding to the "-H" category, these "HT" types are further hardened and tempered at 475 degrees to 575 degrees Fahrenheit, as opposed to the 1,000+ degree hardening of H-types. Yield and tensile strengths are 135 for bolts and screws and 180 for studs and nuts.
Ferretic Stainless Steel
Ferretic steels are cheaper than other types, and are easier to form and machine since they generally contain less nickel by weight. However, ferretic bolts are inherently weaker and more prone to splitting and cracking than other types. Yield strength and tensile strength are 40 ksi for bolts and screws and 70 ksi for studs and nuts.
- Photo Credit nuts and bolts 1 image by Vincenzo Novello from Fotolia.com
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