Four general strength categories
July 21, 2025
By Bob Lund | Engineering Manager
By Bob Lund | Engineering Manager
|
In the world of inch-based fasteners, three primary organizations play key roles in defining standards: ASME, ASTM, and SAE. While each has its own area of focus, they work collaboratively to ensure consistency and avoid redundancy. ASME is primarily responsible for dimensional and tolerance standards. ASTM and SAE, on the other hand, focus on material chemistry, mechanical properties, and certain manufacturing processes such as heat treatment and plating.
Rather than duplicating efforts, these organizations often reference each other’s standards. For example, ASTM or SAE may define the required strength of a fastener but refer to ASME for dimensional specifications. While there is some overlap—particularly between ASTM and SAE—their collaboration helps maintain a cohesive framework. Fastenal plays an active role in this ecosystem. As voting members on dozens of fastener committees—and chairs of many—we help represent the voice of distributors, a group often underrepresented in standards development. This involvement allows us to better support our customers by offering insights directly from the standards-writing process. The metric system, governed largely by ISO 898, offers a more streamlined approach to fastener strength classification. Strength grades such as 4.6, 8.8, 10.9 and 12.9 clearly indicate tensile and yield strength. The number on the left of the decimal point references the basic tensile strength in Mega Pascals (multiply by 100), and the number to the right tells you the yield strength of that fastener as a percentage of the tensile strength. These values are typically marked on the bolt heads. Nut grades are similarly intuitive. Just drop the yield for easy matching—like pairing a class 8 nut with an 8.8 bolt. In contrast, the inch system is more fragmented. The SAE J429 standard defines common fasteners like Grade 5 and Grade 8 hex cap screws, identified by three or six radial lines on the head. However, these markings don’t directly indicate strength, requiring reference to tables. Generally, both inch and metric systems can be grouped into four primary strength categories: 
While this table provides a helpful reference, it’s important to avoid assuming that parts within the same strength category are interchangeable. For instance, substituting a Grade 5 bolt for an ASTM A325 structural bolt may seem logical based on strength alone, but the two differ significantly in application and performance requirements.
While general-use stainless steel fasteners like 18-8 and 316 are often perceived as high-strength options, their usable strength—defined by yield rather than tensile strength—places them closer to the 60,000 psi category. Despite higher tensile ratings in standards, the low yield strength means these fasteners perform similarly to low-carbon steel options in many applications. Fastener strength tables, whether found online or in technical references, can be helpful—but they also present risks. A common mistake is assuming that fasteners within the same strength category are interchangeable. For example, substituting a Grade 5 bolt for an ASTM A325 structural bolt may seem logical due to similar tensile strength, but the two differ significantly in design, application, and compliance requirements. Grade 5 bolts are less expensive, making them tempting alternatives, but they are not suitable replacements in structural applications. Another frequent issue involves specifying ASTM A449 in small diameters. While A449 mirrors the Grade 5 specification, it is harder to source in smaller sizes and often requires special orders—resulting in higher costs. In many cases, simply specifying Grade 5 would be more practical and economical. Unlike the metric system, where bolt and nut grades are designed to pair intuitively, the inch system lacks a straightforward matching scheme. While SAE J995 defines Grade 5 and Grade 8 nuts to pair with their respective bolts, other combinations—such as ASTM A194 2H nuts with ASTM A193 B7 bolts—are less obvious. In these cases, you need to rely on more than just reference tables… look for expert guidance to ensure proper pairing. Fastenal’s engineering team is available to assist with these decisions, helping customers navigate the nuances of inch-based fastener standards and avoid costly errors. Another challenge arises when engineers assume that all standard-defined fasteners are readily available. For example, the ASTM F593 standard is the most common standard used for inch based stainless steel fasteners. The ASTM F593 lists numerous alloy options with varying tensile strengths. However, fewer than half of these options are available as off-the-shelf products at reasonable prices. While some customers knowingly request custom fasteners for specific needs, many are unaware that their specifications may trigger special orders and increased costs. This issue is especially common when prints specify exact materials. For instance, Alloy Group 1 in ASTM F593 is often referred to as 18-8 stainless, and 304 is a common alloy used in its production. However, if a print specifically calls for 304 stainless, suppliers must source that exact material—even though the standard allows for multiple alloys. This can lead to longer lead times and higher prices. The same applies to alloy steels like 4140, which are commonly used but not exclusively required by most standards, in the manufacturing of socket heads and other alloy steel parts. 4340 and many other materials can also be used to make the same part. Calling out a specific material can end up costing you a lot of money. To avoid these pitfalls, it’s important to understand how material specifications affect sourcing and cost. When exact material is not critical, using broader classifications like “Alloy Group 1” or “18-8 stainless” can offer greater flexibility and cost savings. For companies that frequently work with inch-based fasteners, owning a copy of the Industrial Fastener Institute (IFI) guide is highly recommended. Though priced around $600–$700, this comprehensive reference consolidates standards from ASTM, ASME, SAE, and IFI, offering valuable insights into fastener specifications. While it is not published every year, it may not include the latest updates for every standard, but it provides a solid foundation. Fastenal’s engineering team can help fill in any gaps with the most current information. Partnering for successFastenal is committed to helping customers make informed decisions about fastener selection, specification, and sourcing. Whether you're dealing with a redesign, a fastener failure, or simply need guidance on matching bolts and nuts, our engineering team is here to support you. We offer training, consultation, and technical resources as part of our “Growth through customer service” philosophy—one of the many reasons Fastenal is the largest fastener supplier in North America.
Fastenal Company disclaims all warranties, both express and implied, relating to the information, reports, opinions and analysis disclosed to the Customer by Fastenal Company. Fastenal Company shall not be liable for any errors or omissions or any losses, injuries or damages arising from the use of such information, reports, opinions and analysis by the Customer.
You may also like:Vertical Divider
|
Like what you're seeing here? Subscribe to the Blue Print for FREE and get the magazine sent right to your address.
|
