Standard For Grade 8.8 Stainless Steel Screws

We often encounter customers asking about the property class of stainless steel screws. In fact, the so‑called "Grade 8.8 stainless steel screw" is not an accurate term. Grade 8.8 refers to the mechanical property class of fasteners, which is inherently related to material standards. Therefore, in daily use and description, we identify stainless steel screws by their material such as SUS304 or SUS316, rather than by grades like 8.8 or 4.8-these designations apply primarily to carbon steel bolts.

Stainless steel screws have their own standardized strength specifications. For example, 304 stainless steel screws are designated as A2‑50 or A2‑70, where the numbers 50 and 70 represent their strength class. Users familiar only with carbon steel grades such as 8.8 or 4.8 often find these markings unfamiliar.

Strictly speaking, stainless steel screws should be described by their material type: martensitic or austenitic stainless steel. Although technically precise, this terminology is too specialized for non‑professionals to understand, which is why customers frequently ask for "Grade 8.8 stainless steel screws."

For rough comparison only: the strength of SUS304 stainless steel screws is roughly similar to Grade 4.8 carbon steel bolts, and SUS316 is roughly similar to Grade 8.8 carbon steel bolts. However, this is only an analogy for easier understanding and does not represent the actual rated strength class. It cannot be used for performance testing, as the primary function of stainless steel screws is corrosion resistance, not ultra-high strength.

Among common stainless steel products, austenitic stainless steel is the most widely used. It offers stable performance, excellent corrosion resistance, and sufficient strength. Nearly all hexagon bolts, nuts, and washers are made from this type of material, with common national grades including 1Cr18Ni9 and 0Cr19Ni9.

Another category is duplex stainless steel, which is a separate type-not austenitic stainless steel. It combines the characteristics of austenitic and ferritic stainless steels, with superior heat and corrosion resistance, mainly used in special fields such as pressure vessels, and rarely in general industry. Martensitic stainless steel screws are also uncommon due to poor formability, making hexagon head forming difficult; this material is mostly used in welding applications.

Whether the chemical composition of raw materials meets standards directly determines the performance of finished bolts. Bolt quality therefore depends first on material, then on surface treatment and machining accuracy. There are over ten chemical elements, but only key indicators need to be remembered.

Stainless steel screws are mainly divided into austenitic and martensitic types. Most screws use austenitic stainless steel, typically containing approximately 18% chromium (Cr) and 8% nickel (Ni), providing good toughness and corrosion resistance. Martensitic stainless steel is based on chromium, with very low nickel content-this is a major difference from austenitic stainless steel. To judge whether a stainless steel screw meets standards, a basic check is whether chromium and nickel contents are within specification, especially that chromium is at least 12%.

Many people assume stainless steel contains no carbon, which is incorrect. Carbon is intentionally included to improve strength. In modern engineering materials, carbon is one of the most important elements for enhancing mechanical properties and structural strength. Stainless steel fasteners resist corrosion mainly due to chromium, not nickel; their strength comes mainly from carbon. Carbon and chromium are highly compatible and form complex microstructures that greatly increase strength.

The two most common stainless steel fasteners-304 and 316-differ in corrosion resistance and strength mainly due to differences in nickel and carbon content. Different carbon levels lead to different reactions with chromium, resulting in different mechanical properties. To produce standard 304 and 316 fasteners, chromium content must be at least 12%, and carbon content is adjusted to achieve the desired performance.

In current industrial stainless steel fasteners, carbon content is generally low-usually around 0.3%. Only a very small number have carbon content above 0.4%. The primary requirement for stainless steel fasteners remains corrosion resistance, while high carbon levels are typical of high‑strength carbon steel fasteners.

Furthermore, excessive carbon impairs weldability. For this reason, weldable fasteners have similar carbon levels to standard stainless steel fasteners. This explains why carbon is present in stainless steel fastener materials.