Learn to identify stainless steel screw grades by reading head markings like A2 and A4, plus other methods to confirm grade.
Choosing the right stainless steel screw for a project often comes down to knowing the material grade. Different grades of stainless steel offer different levels of corrosion resistance, strength, and magnetic properties. While the easiest way to verify a screw's grade is through the manufacturer's packaging or certification, the head markings on the screw itself can provide a quick clue. This article explains the most common markings found on stainless steel fasteners and how to interpret them, along with other identification methods you can use on the job.
Stainless steel screws are available in several grades, each designed for specific applications. The three grades you will encounter most often in general construction and hardware are 18-8 (also called 304), 316, and 410. Each grade has a distinct chemical composition that affects its performance, especially in corrosive environments.
Grade 18-8 refers to the 300 series of stainless steel, which contains approximately 18% chromium and 8% nickel. This is the most common and popular stainless steel for fasteners. On the head of a screw, this grade is often marked with the designation A2. The head marking 'A2' identifies 304 grade stainless steel. If you see A2 on a stainless steel screw head, you can be confident it is 304 stainless. This grade offers good corrosion resistance for most indoor and outdoor applications, including general construction and marine environments where salt exposure is moderate.
Grade 316 stainless steel adds molybdenum to its alloy, which gives it superior resistance to chlorides and harsh chemicals. It is commonly used in marine environments, chemical plants, and coastal construction. The head marking for 316 stainless steel screws is A4. So when you spot an A4 marking on a fastener, you are looking at 316 grade. This grade is often preferred for applications where long-term exposure to saltwater or aggressive cleaning agents is expected.
Grade 410 is a martensitic stainless steel that can be heat-treated to achieve higher hardness than the austenitic grades (304 and 316). It is frequently used for self-drilling screws where the screw needs to penetrate metal. Grade 410 does not have a universal head marking as clear as A2 or A4. The research pack does not specify a standard head marking for 410. Instead, it may be identified by its magnetic properties (it is magnetic) or by its manufacturer's markings. Some 410 screws will have no grade marking at all, so you must rely on the product packaging or a magnet test for initial identification.
Most stainless steel bolts and screws that fall under the common property classes will have either an A2 or A4 stamp on the head. These markings are typically raised or indented and appear on the top of the screw head. The letters are usually followed by a number that indicates the property class, such as A2-70 or A4-80. The property class relates to the yield stress of the fastener. There are now four property classes assigned across various austenitic stainless steel fastener grades. However, the most important thing to remember is that A2 equals 304 and A4 equals 316. If you need a higher strength stainless screw than those grades, you may need to look at grades like A286, which are not covered by the A2/A4 marking system.
It is worth noting that not all stainless steel screws come with a grade marking. Small-diameter screws, such as those used in electronics or light-duty furniture, often lack head markings simply because the head is too small to stamp clearly. In those cases, you must rely on other identification methods or purchase from a reputable supplier who specifies the grade.
Head markings are not always present or legible. When they are not, you can use a few simple tests to narrow down the grade. However, keep in mind that these tests are not 100% foolproof. The only sure way to know the exact grade of a stainless steel fastener is to have it tested chemically.
Many people assume that stainless steel is non-magnetic, but this is not entirely true. Every stainless steel grade, including 316, can be slightly magnetic if the raw stainless steel was formed using a hot process. In general, 304 and 316 (austenitic grades) are mostly non-magnetic in the annealed condition, but cold working or forming can make them slightly magnetic. Grade 410, being martensitic, is strongly magnetic. So a simple magnet test can help you rule out 410 if the screw is non-magnetic, but it cannot reliably distinguish between 304 and 316 because both can show weak magnetic attraction depending on how they were manufactured.
Stainless steel screws all have a silver-gray appearance, but there can be subtle differences. Grade 410 sometimes has a slightly duller or more matte finish compared to the brighter luster of 304 and 316. However, this is not a reliable method because surface treatments, passivation, and polishing can change the appearance. Do not depend on color alone to identify a grade.
While A2 and A4 markings are a good indicator of grade, they are not always present on every screw. Smaller fasteners, especially those under a certain diameter, may not have enough head space for any stamping. Additionally, counterfeit or mislabeled products exist in the marketplace. If the application is critical, such as in food processing, marine structures, or structural connections, you should verify the grade through the supplier's documentation or by chemical analysis. The research pack confirms that testing the stainless steel chemically is the only sure way of knowing the grade. For everyday use, however, a combination of head markings, packaging labels, and a magnet test will give you enough confidence to select the right fastener.
Another limitation is that property class markings (like 70, 80, etc.) are sometimes confused with grade markings. A stainless steel screw marked A2-70 is still 304 stainless steel; the 70 indicates the tensile strength. Do not mistake the number after the letter for a different grade. Always focus on the letter designation first.
A2 indicates the screw is made of 304 grade stainless steel, which is the most common 18-8 stainless steel. A4 indicates 316 grade stainless steel, which offers better corrosion resistance, especially against chlorides. These markings are usually stamped on the head of bolts and larger screws.
Yes, 18-8 is a general term for the 300 series of stainless steel that typically contains 18% chromium and 8% nickel. Grade 304 is the most common variant of 18-8. So when you see a screw described as 18-8, it is almost always equivalent to 304 stainless steel.
You can use a magnet test to separate strongly magnetic grades like 410 from the mostly non-magnetic 304 and 316. However, this test cannot distinguish between 304 and 316. Chemical testing is the only definitive method when markings are absent or unclear.
Grade 316 (A4) is generally best for outdoor environments, especially near saltwater or in coastal areas. Grade 304 (A2) is also suitable for most outdoor applications but may rust faster in harsh marine conditions. Grade 410 is less corrosion-resistant and is better suited for dry indoor uses or where hardness is needed.
No. While austenitic grades like 304 and 316 are usually non-magnetic in their annealed state, they can become slightly magnetic after cold working or hot forming. Grade 410 is strongly magnetic. So a screw that is magnetic could be 410, or it could be a cold-worked 304 or 316.
Identifying stainless steel screw grades by markings is a practical skill that can save time and prevent using the wrong fastener. Look for A2 for 304 and A4 for 316 on the screw head, and use the magnet test as a secondary check. Always purchase from a reputable supplier who provides clear grade information. For critical applications, request material test certificates or perform chemical analysis to be certain.
