Fasteners can be either a key component that ensures a project's success—or the reason it fails. Each fastener serves a specific purpose and is designed for a particular application. After all, you wouldn’t use the same fastener to hang a picture frame as you would to secure heavy construction equipment. To make these differences clear, an entire grading system exists. This system classifies fasteners based on factors like material composition, tensile strength, and intended use. Understanding these grades helps ensure you're choosing the right fastener for the job, whether it's a simple household task or a demanding industrial application. There are three main, commonly used grading systems:
1. The first, and most commonly used in the US, system is SAE, it was created by the Society of Automotive Engineers and is used to classify various products, including bolts, steel, and lubricating oils.
How to Identify It: Grade 2 bolts usually have no head markings (on U.S. standard bolts). Their lack of identifying lines or symbols is a key visual indicator that they are not heat-treated and are intended for low-strength applications.
How to Identify It: Grade 5 bolts can be identified by three radial lines on the head. These markings indicate that the bolt has been heat-treated and meets the strength requirements for this grade.
How to Identify It: Grade 8 bolts have six radial lines on the head. These markings are a clear indication of their high strength and heat-treated condition, distinguishing them from lower-grade fasteners. 2. The next one is ASTM (by American Society for Testing and Materials) fasteners, which are commonly used in the construction, structural, and civil engineering industries. The grading focuses more on application and use rather than general strength classification.
How to Identify It: Marked as “307A” on the bolt head. It typically has a plain finish, with minimal head markings beyond the grade and manufacturer code.
How to Identify It: Head markings vary by manufacturer but usually include “A325” along with specific code symbols.
How to Identify It: Clearly marked with “A490” on the head. These bolts may also have restrictions such as no galvanization due to hydrogen embrittlement risks. 3. The last one on our list is ISO Standard. Metric fasteners follow standards set by the International Organization for Standardization, the most used standard in the world, especially in Europe and Asia. Metric fasteners use a two-number system (like 8.8 or 10.9). The first number indicates tensile strength in MPa divided by 100 (e.g., 8 = 800 MPa), and the second number is the ratio of yield strength to tensile strength (e.g., 0.8 = 80%).
How to Identify It: Look for the numbers “8.8” stamped on the bolt head. These are typically either raised or indented, depending on the manufacturer.
How to Identify It: Stamped with “10.9” on the head. It may also have manufacturer marks depending on the standard followed. While SAE, ISO, and ASTM fastener grades may look similar at a glance, they serve different purposes and are designed around distinct standards and applications:
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