Grades Of Stainless Steel Bolts And Classification Of Stainless Steel Screws

I. Bolt Property Classes

Bolts and studs are divided into 10 property classes, ranging from 3.6 to 12.9. The property class designation consists of two parts separated by a decimal point: the figure before the decimal point represents 1/100 of the nominal tensile strength, and the figure after the decimal point represents 10 times the ratio of the nominal yield strength to the nominal tensile strength (yield-to-tensile ratio).

For example, a Class 8.8 bolt indicates a nominal tensile strength of 800 MPa and a yield-to-tensile ratio of 0.8, i.e., a nominal yield strength of 640 MPa. A Class 4.8 product has a nominal tensile strength of 400 N/mm² and a yield point of 320 N/mm².

Metric bolt mechanical property classes are: 3.6, 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 9.8, 10.9, and 12.9, totaling 10 property classes.

II. Nut Property Classes

Nuts are divided into 7 property classes, from 4 to 12. The figure roughly represents 1/100 of the minimum stress that the nut is guaranteed to withstand.

III. Strength Grade Designation for Stainless Steel Bolts

The strength grade designation for stainless steel products consists of two parts separated by a hyphen. The symbol before the hyphen indicates the material group, such as A2, A4, etc.; the figure after the hyphen indicates the strength grade. For example, A2-70 denotes austenitic stainless steel (Type 304) with a nominal tensile strength of 700 MPa.

IV. Thread Tolerance Grades

Unified Imperial Threads

For unified imperial threads, external threads have three tolerance grades: 1A, 2A, and 3A; internal threads have three tolerance grades: 1B, 2B, and 3B. All are clearance fits. The higher the grade number, the tighter the fit.

Grades 1A and 1B: Very loose tolerance grades, suitable for allowance fits of internal and external threads.

Grades 2A and 2B: The most commonly specified thread tolerance grades for mechanical fasteners in the imperial series.

Grades 3A and 3B: Produce the tightest fit when engaged, suitable for standard fasteners with tight tolerances and used in safety-critical designs.

Metric Threads

For metric threads, external threads have three tolerance grades: 4h, 6h, and 6g; internal threads have three tolerance grades: 5H, 6H, and 7H. Recommended thread fit combinations are H/g, H/h, or G/h. For precision fastener threads such as bolts and nuts, the standard recommends a 6H/6g fit.

V. Material Classification of Stainless Steel Screws

Stainless steel screws are classified into four categories by material: austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, and precipitation-hardening stainless steel. When selecting stainless steel screws, comprehensive consideration should be given to service conditions, corrosion resistance requirements, strength requirements, processing technology, and cost, to finally determine the grade, type, specification, and material standard of the stainless steel screw.

1. Austenitic Stainless Steel

The most basic alloying elements of austenitic stainless steel are chromium and nickel. The representative grade is chromium-nickel austenitic stainless steel containing approximately 18% chromium and 8% nickel, commonly known as 18-8 stainless steel. The ratio of chromium to nickel essentially ensures a stable austenitic structure.

Commonly used grades are 302, 303, 304, and 305, the so-called "18-8" austenitic stainless steels. Their corrosion resistance and mechanical properties are similar. The starting point for selection is the manufacturing process of the stainless steel screw, which in turn depends on the size and shape of the screw, as well as the production quantity.

Type 302: Used for screws and self-tapping bolts manufactured by machining.

Type 303: Contains a small amount of sulfur to improve machinability, used for nuts machined from bar stock.

Type 304: Suitable for stainless steel screws manufactured by hot heading, such as longer-specification bolts and large-diameter bolts, which may exceed the scope of cold heading.

Type 305: Suitable for stainless steel screws manufactured by cold heading, such as cold-formed nuts and hex bolts.

Types 309 and 310: Contain higher Cr and Ni than 18-8 stainless steel, suitable for stainless steel screws operating at high temperatures.

Types 316 and 317: Contain the alloying element Mo, therefore their high-temperature strength and corrosion resistance are higher than those of 18-8 stainless steel.

Types 321 and 347: Type 321 contains the stabilizing alloying element Ti, and Type 347 contains Nb, which improves the intergranular corrosion resistance of the material. Suitable for stainless steel standard parts that are not annealed after welding or operate at 420–1013 °C.

2. Ferritic Stainless Steel

Type 430 plain chromium steel has better corrosion resistance and heat resistance than Type 410, is magnetic, but cannot be strengthened by heat treatment. It is suitable for stainless steel screws requiring moderately high corrosion and heat resistance with general strength requirements.

3. Martensitic Stainless Steel

Types 410 and 416 can be strengthened by heat treatment, with a hardness of 35–45 HRC and good machinability. They are used for general-purpose heat-resistant and corrosion-resistant stainless steel screws.

Type 416 has a slightly higher sulfur content and is a free-machining stainless steel. Type 420 has a sulfur content of ≥0.15%, improved mechanical properties, and can be strengthened by heat treatment, with a maximum hardness of 53–58 HRC. It is used for stainless steel screws requiring higher strength.

4. Precipitation-Hardening Stainless Steel

17-4PH and PH15-7Mo can achieve higher strength than conventional 18-8 stainless steel, and are therefore used for high-strength, corrosion-resistant stainless steel screws.

A-286 is a non-standard stainless steel with higher corrosion resistance than commonly used 18-8 stainless steel, and it retains good mechanical properties at elevated temperatures. It is used for high-strength, heat-resistant, corrosion-resistant stainless steel screws, and can be used up to 650–700 °C.