Tensile strength classes of stainless steel screws and nuts

Basic knowledge Joining

The tensile strength of steel and stainless steel is a critical parameter when it comes to the load-bearing capacity and safety of threaded joints. It is made up of two key parameters for steel screws based on which the tensile strength and the yield strength can be easily determined. But starting at a certain size, stainless steel screws and stainless steel nuts are marked with a letter-number combination (e.g. A4-80). The letter-number combination arranged in front of the hyphen assigns the screw so marked to a material group and steel group and thus provides information on the basic material properties of the screw. The number 80 to the right of the hyphen indicates the strength class. This article shows how to use the strength class of stainless steel bolts to determine their limits of use and provide an overview of the load-bearing capacity of typical strength classes.

What is tensile strength?

Tensile strength describes the maximum mechanical stress value a material can tolerate before it fails. This is usually expressed in megapascal (MPa) or in Newtons per square millimeter (N/mm²). For fasteners, tensile strength is particularly important because it determines the maximum load the element can absorb without failing.

Formula for calculating tensile strength

There is no universal tensile strength formula to calculate tensile strength as it depends on different conditions (material, alloy composition and processing). As a rule, tensile strength is determined empirically by material testing and then summarized in tensile strength tables for screws and stainless steel in material data sheets or standards. However, it is possible to calculate the stress a material experiences when it is subjected to a tensile load. The general formula for stress (σ) in a material under tensile load is:

\sigma=\frac{F}{A}

where:

σ = the stress in Pascal (Pa)
F = the tensile force acting on the material in Newton (N)
A = the cross-sectional area of the material subjected to the tensile force in square meters (m²).

To determine the tensile strength of e.g. steel and stainless steel, the maximum tensile force (F) and the smallest cross-sectional area (A) at which the material was tested can be used. Tensile strength is typically expressed in Megapascal (MPa).

Overview of stainless steel grades

Stainless steel is an generic term for approximately 120 different grades of corrosion-resistant steel. The individual alloys serve different purposes and use cases. However, all of them contain chromium, and possibly also nickel, which are responsible for corrosion-resistance. The elements react with oxygen from the air to form a thin, protective oxide layer on the surface of the steel. This oxide layer is called a passive layer and prevents further oxygen from coming into contact with the underlying iron, thus preventing corrosion and rusting. Compared to low-alloy steels or structural steels, the tensile strength of stainless steel is relatively high. For example, stainless steel 1.4301 has a tensile strength of 500 - 700 N/mm² (comparison: for conventional structural steel, the tensile strength is 300 - 590 N/mm²). DIN 17440 and DIN EN ISO 3506 apply for stainless steels. The main categories of stainless steels are as follows.

Austenitic stainless steels

These stainless steels are characterized by their high chromium and nickel content, usually above 16% chromium and above 8% nickel. They are non-magnetic and have excellent corrosion resistance and weldability. The letter A in the name of a stainless steel, such as V2A, indicates that it is an austenitic stainless steel.

Ferritic stainless steels

Ferritic stainless steels contain a high amount of chromium, but little or no nickel. They are magnetic and offer good corrosion resistance in non-aggressive environments. The letter F is used in the name of stainless steels to indicate that they are ferritic stainless steels.

Martensitic stainless steels

Martensitic stainless steels are characterized by their high carbon content and moderate to high chromium content. They are magnetic and have excellent hardness and wear resistance. Martensitic stainless steels are often used in knives and tools. The letter C in the martensitic stainless steel name indicates the carbon content.

Duplex Stainless Steels

Duplex stainless steels have a mixture of austenitic and ferritic structure in their alloy. They provide good corrosion resistance and strength and are particularly useful in environments with high chloride concentrations, such as in the chemical industry and in seawater applications.

Stainless steel screws and nuts

Stainless steel screws and nuts are known for their robust corrosion resistance. It is nevertheless possible that stainless steel can also start to rust, e.g. due to foreign rust from contact with rusting metals. The strength class of screws gives information about how much load they can absorb.

Tensile strength of stainless steel screws

The tensile strength of stainless steel screws is specified as a strength class. The strength class is a combination of the tensile strength and the yield strength of the tensile strength:

Below the yield strength, the material returns to its original state after deformation, above the yield strength the deformation remains. However, the 0.2% yield strength is often specified instead of the yield strength since the yield strength is not always clearly identifiable by the tensile test. This describes the stress at which the permanent elongation after relief equals 0.2%. There are tables for the screw strength classes as a guide. In the past, it was common to specify the tensile strength of screws in kg in tables, but nowadays, it is primarily expressed in Pascal (Megapascal) or Newton (per square millimeter).

The strength class directly influences the maximum torque that can be exerted on the screw or nut. When selecting the correct stainless steel screw or nut, one should therefore first determine what torque is required for safe installation and whether the strength class allows this torque.

In addition to the strength class - 80 in the example here - the marking A4 specifies the material group and steel type of the stainless steel screw or nut. It refers to the chemical composition of the stainless steel and the associated properties of corrosion resistance and resistance to acids. A2 for example denotes an austenitic stainless steel with certain chromium and nickel components. Stainless steel with the label A4 is also resistant to acid or seawater.

MISUMI offers a wide range of stainless steel screws with a wide range of strength classes.

Tensile strength of stainless steel nuts

The classification of nuts is slightly less specific than those of screws, as they are often used in combination with screws and should therefore be primarily matched to the screw. In general, stainless steel nuts are often designed to match the same or higher tensile strength class as the associated screw.

The mechanical properties of stainless steel nuts, as well as steel nuts, are expressed by strength classes and by the guaranteed load stresses that corresponds to the strength classification of the paired screw.

Because stainless steel nuts are used with matching stainless steel screws, the material thickness of a stainless steel nut is usually determined by the material thickness of a screw made of the same material. Because of this agreement, it is often assumed that there will be no shear failure on the threads when a combination of stainless steel screws and nuts is used. However, it is important to note that low-profile nuts with only a few interlocking threads should be used with caution to avoid shear failure on the threads.

Stainless steel tensile strength table

See the table below for an overview of the tensile strength of various stainless steel grades. The indicated values represent general guidelines and may vary slightly depending on the manufacturing process, heat treatment and specific alloy. It is advisable to consult material data sheets and technical specifications for detailed information on your requirements.

Stainless steel tensile strength
Steel grade	Strength class	Tensile strength R_{m, min.} N/mm 2	0.2% elongation limit R_{p 0.2. min.} N/mm2	Elongation at break A_min.	Hardness HV
Austenitic
A1, A2, A3, A4, A5	50	500	210	0.6 d	-
A1, A2, A3, A4, A5	70	700	450	0.4 d	-
A1, A2, A3, A4, A5	80	800	600	0.3 d	-
Martensitic
C1, C4	50	500	250	0.2 d	155 to 220
C1, C4	70	700	410	0.2 d	220 to 330
C1	10	1100	820	0.2 d	350 to 440
C3	80	800	640	0.2 d	240 to 340
Ferritic
F1	45	450	250	0.2 d	135 to 220
F1	60	600	410	0.2 d	180 to 285

The number in the strength class corresponds to 1/10 of the nominal value of the minimum tensile strength.
Example: The strength class A2-50 for austenitic materials refers to a material with a tensile strength of 500 N/mm², a yield strength of 210 N/mm² and an elongation at break of 0.6 d.
Stainless steels are classified into types (austenitic, martensitic, and ferritic), and strength classes for martensitic steels are determined based on differences in heat treatment.

Link to more blog articles about screws

If you wish to dive deeper into the topic of screws, you can find more information in the following blog articles:

Calculating shear strengths and tensile strengths for screws

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