The 8 Must-Know Mechanical Properties of Stainless Steel

1. Yield strength

(mechanical symbol σ0.2, abbreviation YS)

σ0.2=P0.2/F0

• P0.2 – load borne by a tensile specimen with plastic deformation of 0.2%
• F0 – original sectional area of tensile specimen

A low yield strength of a material means that it is prone to yielding, has minimal bounce-back after being formed, and has favorable properties for die fitting and maintaining shape during forming.

2. Tensile strength

(mechanical symbol σb, abbreviation TS)

σb=Pb/F0

• Pb – maximum load borne by tensile specimen before fracture
• F0 – original sectional area of tensile specimen

A high tensile strength of a material means that it is resistant to breaking during deformation, making it suitable for undergoing plastic deformation.

3. Yield ratio

(σ0.2/σb)

The yield strength ratio has a significant impact on the formability of materials during stamping.

When the yield strength ratio is low, the stage of plastic deformation from yielding to fracture in sheet metal is prolonged, reducing the risk of fracture during forming, making it easier to stamp.

In general, a low yield strength ratio enhances the resistance to cracking in sheet metal during various forming processes.

Table: Yield ratio of common stainless steel materials

4. Elongation

(mechanical symbol, English abbreviation EL)

Elongation refers to the ratio of the total increase in length of the material from plastic deformation to fracture, compared to its original length. It is expressed as:

• δ – elongation of material (%);
• L – length of the sample when it is pulled off (mm);
• L0 – length of specimen before tension (mm).

A high elongation of a material means that it can undergo greater plastic deformation and has good crack resistance, making it favorable for drawing, flanging, and bulging.

Typically, the flanging coefficient and bulging property (Ericsson value) of a material are directly proportional to its elongation.

5. Strain Hardening Index (n)

The strain hardening index, also known as the “n value,” reflects the cold work hardening of materials and its impact on the formability during stamping.

A high strain hardening index indicates that the material has strong local strain capacity and can effectively prevent local thinning. This means that increasing the instability limit strain results in a more uniform deformation distribution, and the overall forming limit of the material is high during forming.

6. Austenite equilibrium coefficient (A)

A(BAL) = 30(C+N)+0.5Mn+Ni-1.3Cr+11.8

The stability of austenite is indicated by the “A value.” A smaller A value means that the austenite is less stable.

The structure of steel is susceptible to changes from cold and hot working, which can affect its mechanical properties.

Ni, Mn, C, and N are common elements that help form and stabilize austenite in stainless steel, especially Ni. An increase in the content of these elements can increase the austenitic equilibrium coefficient and make the austenitic structure more stable.

Cr, Mo, Si, Ti, and Nb are elements that help form and stabilize the ferrite structure. An increase in the content of Cr can reduce the austenitic equilibrium coefficient.

SUS304 stainless steel is a pure austenitic structure with its own stability. After cold working, it becomes hard due to a portion of the austenitic structure changing to martensite, known as cold working induced martensite.

Austenitic stainless steel has a small balance coefficient, making it prone to martensite transformation or further martensite formation during cold working, resulting in a high degree of cold work hardening.

7. Cold working induced martensite transformation point Md (30/50)

Md(30/50)= 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.5Mo

The martensite transformation point (Md(30/50)) is the temperature at which 50% of the material undergoes martensite transformation after undergoing 30% true strain from cold deformation. The higher the content of alloy elements in austenitic stainless steel, the lower the martensite transformation point.

Austenitic stainless steel with a lower martensite transformation point is less prone to induced martensite during cold working and has a low degree of cold work hardening.

Cold work hardening in stainless steel is caused by two factors: work hardening due to an increase in dislocations and work hardening due to structural transformation (from austenite to martensite).

SUS430 steel does not undergo structural transformation during deformation and its cold work hardening is solely caused by an increase in dislocations.

In contrast, the cold work hardening of SUS304 steel is mainly due to the transformation from austenite to martensite, with a smaller contribution from an increase in dislocations. This is why the cold work hardening of austenitic stainless steel is more pronounced than that of ferritic stainless steel.

The Ni content has a significant effect on the martensite transformation point in austenitic stainless steel. An increase in Ni content leads to a lower martensite transformation point and a smaller degree of cold work hardening.

8. Grain size (N)

The physical meaning of grain size can be understood according to the following formula:

ξ=2N+3

• ξ-Number of grains per square millimeter of the sectional area;
• N-grain size.

A higher grain size N level means that there are more grains per unit cross-sectional area, making the grain size finer. This results in higher strength and better elongation of the material.

Steel with N>5 (256 grains/mm) is considered fine-grain steel.

Large grain size can increase the plastic strain ratio (R) of the material, but also decreases the yield strength ratio and elongation.

However, with large grains, there can be different orientations on the surface of the sheet metal, leading to unequal deformation and causing the “orange peel” effect on the material surface.

Refining the grain size can reduce the occurrence of orange peel, but if the grain size is too fine, the plastic strain ratio will decrease, and the yield strength ratio and elongation will increase, making it less favorable for forming.