The difference between stainless steel bolts and iron bolts

What is the difference between stainless steel bolts and iron bolts?

Stainless steel screws and iron screws, can not say which is better, mainly depends on the use of occasions.

In terms of corrosion resistance, stainless steel screws are obviously better than iron screws. Because of the anti-corrosion of stainless steel, it relies on the inside of the chromium to react with the oxygen in the air to form a chromium oxide protective layer to protect the stainless steel screws from corrosion. If the chromium oxide protective layer is destroyed due to impact or the like, the chromium inside will automatically react with the oxygen in the air to continue to generate chromium oxide protection screws.

The iron screws, although there are black, electroplating and other surface treatment methods, anti-corrosion performance is also good. However, once the surface coating is destroyed, it will rust quickly because there is no ability to automatically regenerate the protective layer.

In terms of mechanical properties, the strength grade of stainless steel is generally equivalent to 8.8 grade of carbon steel, generally slightly lower than 8.8, A2-80 or A4-80 is higher than 8.8, but it is relatively rare, because processing is more difficult. Although carbon steel is relatively soft, it can only be made directly to 4.8, 5.8 and 6.8, but carbon steel can be heat treated, so the strength grade can reach 8.8, 10.9, 12.9 or even 14.99. It can meet more demanding requirements such as tensile, twisting and toughness, and has wider application.

In particular, the continuous development of new alloy steel materials, used in the field of fasteners, has made great progress in terms of product requirements such as better corrosion protection, lighter weight, smaller size and longer life. The relative development of stainless steel is relatively slow. In addition to strain hardening (increased hardness and strength, but limited effect), solid solution treatment (increased toughness and corrosion resistance, and reduced magnetic properties), the other did not hear any breakthrough progress. . On the contrary, the price is too expensive (the raw materials are too expensive, and the processing is also more difficult than carbon steel). Where stainless steel is used, it is being replaced by a large number of carbon steel screws (the surface treatment technology of carbon steel screws is developing very rapidly).

Of course, these two types of screws will still be used in a large number of different occasions. After all, there are many places that cannot be replaced. Under normal circumstances, from the safety point of view, carbon steel screws of class 8.8 and below can basically be replaced by stainless steel screws, but only if you have enough strength, expensive products can be accepted by the market.

Stainless steel screw performance grade

The performance rating of stainless steel bolts 8.8 means that the material has a tensile strength limit of 800 MPa and a yield limit of 640 MPa.

The performance grades of stainless steel bolts, studs and studs are divided into 10 grades: from 3.6 to 12.9. The number before the decimal point represents 1/100 of the tensile strength limit of the material, and the ratio of the yield limit of the representative material after the decimal point to the tensile strength limit is 10 times.

Stainless steel bolt performance level profile

The performance grade of the nut is divided into 7 grades, from 4 to 12, and the figures roughly indicate that the stainless steel nut is guaranteed to withstand 1/100 of the minimum stress. For uniform inch thread, the external thread has three thread grades: 1A, 2A and 3A, and the internal thread has three grades: 1B, 2B and 3B, all of which are clearance fits. The higher the level number, the tighter the fit.

1, 1A and 1B grade, very loose tolerance class, which is suitable for tolerance fit of internal and external threads.

2, 2A and 2B grades are the most common thread tolerance grades specified by the Inch series of mechanical stainless steel fasteners.

3, 3A and 3B grade, screwed to form the tightest fit, suitable for tight tolerance stainless steel standard parts, for critical design of safety.

Metric thread, external thread has three thread grades: 4h, 6h and 6g, internal thread has three thread grades: 5H, 6H, 7H. The thread fit is preferably combined into H/g, H/h or G/h. For the refined fastener threads such as bolts and stainless steel nuts, the standard is recommended to use 6H/6g.

Carbon steel: The strength grade marking code consists of two parts separated by ".". The meaning of the front part of the "." in the mark code indicates the nominal tensile strength. For example, the "4" of the 4.8 grade indicates 1/100 of the nominal tensile strength of 400 N/MM ² .

The meaning of the "." and the numerical part after the dot in the mark code indicates the yield ratio, that is, the ratio of the nominal yield point or the nominal yield strength to the nominal tensile strength. For example, the yield point of the 4.8 grade product is 320 N/mm ² .

The stainless steel product strength rating mark consists of two parts separated by "-". The symbol "-" in the logo code indicates the material. Such as: A2, A4 and other signs "-" after the intensity, such as: A2-70.

Carbon steel: Metric stainless steel bolt mechanical performance grade can be divided into: 3.6, 4.6, 4.8, 5.6, 5.8, 6.8, 8.8, 9.8, 10.9, 12.9 a total of 10 performance levels.