Stainless steel screws: tensile strength, torque, yield strength and material composition

The performance standards of stainless steel screws as discussed in this article do not refer to their specific size regulations.

Instead, these performance standards refer to several mechanical properties, including material composition (chemical composition), tensile strength (the amount of kilogram of force it can withstand), failure torque (the amount of torque required to break it), guaranteed voltage and yield. strength, among others.

This article mainly explains the performance of stainless steel screws produced using 304 and 316 austenitic stainless steel as raw materials.

This article is only intended for peers who are new to using stainless steel fasteners, as well as purchasing personnel and technical personnel of companies that use stainless steel fasteners. We simplify tedious explanations of national standards, add parts not mentioned in national standards, and briefly explain them together with industry rules.

This is done to allow viewers to quickly navigate and understand relevant knowledge. In other words, it's not that strict. If you are a senior employee, this document may be what you are looking for: GB/T3098.6-2000 Stainless Steel Bolt Test Standard.

Screw material SUS304 and 316 stainless steel

Speaking of materials, the stainless steel screw materials we commonly use are divided into two types, SUS304 and SUS316. Of course, there is also the 400 series, namely SUS410 or SUS416. This falls into the stainless iron range, which we won't discuss.

There are also some special requirements, such as 316L, 304L, etc. As they are rarely used, we will not discuss them here. As for other series such as series 201, series 668, these are misleading tricks and will not be mentioned.

The chemical composition of SUS304 is as follows:

The chemical composition of SUS316 is as follows:

The table above shows that the main difference between 304 and 316 is in the nickel and chromium content, with 316 being slightly higher.

Performance Grades of Stainless Steel Bolts

Typically, we see head markings on screws like:

• A2-50
• A2-70
• A4-70
• A4-80

These represent the performance grades of stainless steel screws. Simply put:

• A2 means 304 stainless steel, “70” represents the tensile strength of this bolt (or nut) being “700N per square millimeter, the unit is N/ ^mm2 , (the number “2” represents square, this unit is also known as MPa). Similarly,
• A4 means 316 stainless steel, “80” represents a tensile strength of 800N per square millimeter. Now you understand:
• A2-70 represents: grade 304 material, tensile strength 700N/mm ²
• A4-80 represents: grade 316 material, 800N/ ^mm2

So what does A4-70 mean?

A4-70 also represents grade 316 material, but the tensile strength is not 800, but 700N/mm ² . You read right, the standard SUS316 hex bolt is A4-70 type, only SUS316 nuts (of course, excluding thin nuts) can reach A4-80. In other words, not all 316 fasteners are grade 80.

Likewise, not all 304 screws can reach grade 70, for example, M4 and smaller screws cannot reach grade 70, which is why A2-50 exists.

So what type of screws use the A2-70 standard and what type of screws use the A2-80? The national standard does not specify this. If you are not a picky scholar, I can publish the industry standard performance degree application patterns as follows:

Here is the standard list describing the tensile strength, yield strength, elongation and guaranteed tension of stainless steel screws of various grades:

(Reference: GB/T3098.6-2000 test standard, applicable everywhere)

The table above specifies the range of thread diameters. This means that for diameters greater than this range there are no regulations established by national standards and negotiation between the supplier and the applicant would be necessary.

Oh right, the table above appears to be missing the destructive torque standard, which is commonly referred to as “how much torque”. The pattern is as follows:

Essentially, this concludes the description of the performance of 304 and 316 series stainless steel screws.

The above torque patterns are easy to understand. For example, what is the torque value for a SUS304 M6*25 hex screw?

1. First, refer to the above performance rating standards as it belongs to level A2-70.

2. Refer to the torque standard, M6 corresponds to 13N.M.

So, what is the tensile parameter of a SUS304 M6*25 hex bolt?

The astute among you will notice something amiss. The “standard of tensile strength, yield strength, elongation, guaranteed tension” above does not directly provide parameters for specific specifications. This requires calculation on your own.

Speaking of calculations, I can imagine you are complaining – even though you are a woman – because it involves the effective stress cross-sectional area of ​​each bolt specification.

Well, I can very well calculate everything for your reference.

Effective tension cross-sectional area of ​​bolts

There is a formula to calculate the cross-sectional area of ​​a screw, which is as follows:

As=0.7854*(d-0.9382d) ²

In the formula above:

• How: represents the stress cross-sectional area
• d: represents the nominal diameter of the thread, such as the nominal diameter of an M6 screw is 6

The table showing the cross-sectional areas of common threads (here referring to the cross-sectional areas that carry stress) is as follows:

The tensile strength, yield strength and proof stress standards for stainless steel bolts of grades A2-50, A2-70, A4-70, A4-80, etc. can be calculated with the given cross-sectional area.

Here are the parameters for A2-70 and A2-50:

The following are the performance parameter standards for A4-70 and A4-80: