The material classification of stainless steel bolts includes austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, and precipitation-hardening stainless steel.
The selection of stainless steel bolts also follows certain principles. From which aspects should you start to choose the stainless steel bolts you need?
After a comprehensive and integrated consideration of the following five aspects, finally determine the grade, type, specification, and material standard of the stainless steel screws to be selected.
I. Ferritic Stainless Steel
430 type ordinary chromium steel has better corrosion resistance and heat resistance than 410 type, and it is magnetic. However, it cannot be strengthened by heat treatment. It is suitable for stainless steel screws with slightly higher corrosion resistance and heat resistance and general strength requirements.
II. Martensitic Stainless Steel
410 type and 416 type can be strengthened by heat treatment, with hardness ranging from 35 to 45 HRC. They have good machinability and are used for general-purpose heat-resistant and corrosion-resistant stainless steel screws. 416 type has a slightly higher sulfur content and is an easy-to-machine stainless steel. 420 type has a sulfur content of 0.15% and improved mechanical properties. It can be strengthened by heat treatment, with a maximum hardness of 53 to 58 HRC. It is used for stainless steel screws with higher strength requirements.
III. Precipitation Hardening Stainless Steel
17-4PH and PH15-7Mo can achieve higher strength than the common 18-8 type stainless steel, and are thus used for high-strength, corrosion-resistant stainless steel screws. A-286 is a non-standard stainless steel with higher corrosion resistance than the common 18-8 type stainless steel and good mechanical properties at elevated temperatures. It is used for high-strength, heat-resistant, and corrosion-resistant stainless steel screws and can be used up to 650 to 700°C.
IV. Austenitic Stainless Steel
The commonly used grades are 302, 303, 304, and 305, known as the “18-8” type austenitic stainless steel. They have similar corrosion resistance and mechanical properties. The selection is based on the production process of the stainless steel screws, which depends on the size and shape of the screws as well as the production quantity.
302 type is used for screws and self-tapping screws processed by mechanical means.
303 type has a small amount of sulfur added to improve machinability and is used for nuts processed from bar stock.
304 type is suitable for screws processed by hot forging, such as longer bolts and large-diameter bolts, which may exceed the range of cold forging.
305 type is suitable for screws processed by cold forging, such as cold-formed nuts and hex bolts.
309 type and 310 type have higher chromium and nickel contents than the 18-8 type stainless steel and are suitable for stainless steel screws used at high temperatures.
316 type and 317 type contain the alloy element molybdenum, thus having higher high-temperature strength and corrosion resistance than the 18-8 type stainless steel.
321 type and 347 type have improved resistance to intergranular corrosion due to the addition of the stable alloy elements titanium in 321 type and niobium in 347 type. They are suitable for stainless steel fasteners that are not annealed after welding or are used at temperatures between 420 and 1013°C.
The grades of stainless steel bolts are classified as 45, 50, 60, 70, and 80. The materials are mainly divided into austenitic A1, A2, A4, martensitic and ferritic C1, C2, C4. The representation method is, for example, A2-70, where the numbers before and after the hyphen represent the material of the bolt and the strength grade, respectively.
The grades of carbon steel bolts include 3.6, 4.6, 4.8, 5.6, 6.8, 8.8, 9.8, 10.9, and 12.9, etc.
The numbers before and after the decimal point represent the nominal tensile strength and the ratio of yield strength to tensile strength of the bolt material, respectively.
For example, a bolt marked as grade 8.8 indicates that the material has a tensile strength of 800 MPa and a yield strength ratio of 0.8, meaning the yield strength is 800 x 0.8 = 640 MPa.
1. Common bolt materials include Q215, Q235, 25, and 45 steel. For important or special-purpose threaded connections, alloy steels such as 15Cr, 20Cr, 40Cr, 15MnVB, and 30CrMnSi with higher mechanical properties can be selected. 2. SUS304 (0Cr18Ni9 – also known as A2) is mainly used in the manufacturing of common stainless steel bolts, screws, washers and nuts, with a general strength grade of 70 for bolts.
3. SUS 316 (0Cr18Ni12Mo2) and SUS 316L (00Cr17Ni14Mo2) – also known as A4 – are mainly used in the manufacturing of high corrosion-resistant stainless steel bolts, screws, washers and nuts, with a general strength grade of 70 for bolts, and can also reach a high strength grade of 80.
4. A represents austenitic, 2 represents the second material (in fact, 2 refers to 304), and -70 represents a strength grade of 700 MPa. Common materials are A2 and A4, which are 304 and 316 materials respectively.
5. There is no difference in material between stainless steel bolts A4-70 and A4-80. The 70 and 80 after A4-70 and A4-80 represent one-tenth of the minimum tensile strength of the fastener. A4-70 and A4-80 have no difference in material itself.
Since they are austenitic stainless steel, their tensile strength cannot be increased through heat treatment. The increase in tensile strength is achieved through dislocation blocking after deformation (commonly known as cold work hardening). A4-70 bolts are more common in the market.
6. A1, A2, A4 represent 302, 304, 316; 45, 50, 60, 70, 80 represent one-tenth of the minimum tensile strength of the fastener.
Group 7, 4 is an austenitic stainless steel developed for boiling sulfuric acid, thus named acid-resistant steel. It belongs to the sub-stable acid-resistant steel, with SUS316 as the typical representative.
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