8 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

The low yield strength of the material indicates that the material is easy to yield, has small rebound after forming, and has good die fitting and shape setting properties.

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

The tensile strength of the material is large, and the material is not easy to break during deformation, which is conducive to plastic deformation.

3. Yield ratio

(σ0.2/σb)

The yield strength ratio has a great influence on the stamping formability of materials.

The yield strength ratio is small, the plastic deformation stage of sheet metal from yield to fracture is long, and the risk of fracture in the forming process is small, which is conducive to stamping forming.

Generally speaking, a small yield ratio is beneficial to the crack resistance of sheet metal in various forming processes.

Table: Yield ratio of common stainless steel materials

Type of SteelYield strength (N/mm2)Tensile strength (N/mm2)Yield ratio
SUS3043006700.45
SUS304(Cu)2956400.46
SU53163126250.50
SUS316L2455250.47
SUS4303505100.69
SUS409L2414100.59

4. Elongation

(mechanical symbol, English abbreviation EL)

Elongation is the ratio of the total elongation length of the material from plastic deformation to fracture to the original length, namely:

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

The elongation of the material is large, the allowable plastic deformation of the sheet metal is large, and the crack resistance is good, which is beneficial to drawing, flanging, and bulging.

In general, the flanging coefficient and bulging property (Ericsson value) of materials are in direct proportion to elongation.

5. Strain Hardening Index (n)

The strain hardening index is commonly referred to as the n value, which represents the cold work hardening phenomenon of materials and can reflect the stamping formability of materials.

The large strain hardening index shows that the local strain capacity of the material is strong, and the ability to prevent local thinning of the material is strong, that is, increasing the instability limit strain makes the deformation distribution tend to be uniform, 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

It refers to the stability of austenite.

The smaller the A value is, the more unstable the austenite is.

The structure of the steel is easy to be affected by cold and hot working, which will change the structure and affect the mechanical properties of the steel.

The common austenite-forming elements in stainless steel are: Ni, Mn, C, N, which help to form and stabilize austenite and are indispensable in austenitic stainless steel, especially Ni element.

It can also be seen from the austenitic equilibrium coefficient that the increase of the content of these four elements can increase the austenitic equilibrium coefficient, thus making the austenitic structure more stable.

Common ferrite forming elements are: Cr, Mo, Si, Ti, Nb. These elements help to form and stabilize the ferrite structure.

It can also be seen from the above formula that increasing the content of Cr can reduce the austenite equilibrium coefficient.

SUS304 stainless steel is a pure austenitic structure, and the austenitic structure has its stability.

After cold working, SUS304 becomes hard, mainly because part of the austenitic structure changes to martensite structure, which is called cold working induced martensite.

For austenitic stainless steel, the balance coefficient is small, and it is easy to produce martensite transformation or more martensite during cold working, so the cold work hardening degree is intense.

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

It refers to the temperature at which 50%α martensite is generated after cold deformation of 30% of the true strain, indicating that the higher the content of alloy elements in austenitic stainless steel, the lower the martensite transformation point Md (30/50).

It is not easy to produce induced martensite during cold working deformation, and the degree of cold work hardening is small.

The cold work hardening of stainless steel is mainly caused by two factors:

One is work hardening caused by dislocation increase;

One is work hardening caused by structural transformation (transformation from austenite to martensite).

For SUS430 steel, there will be no structural transformation during the process of deformation, and its cold work hardening phenomenon is all caused by the increase of dislocations.

Therefore, it is of no practical significance to talk about cold work induced martensite for SUS430 steel.

In the cold deformation process of SUS304 steel, there is hardening caused by dislocation increase and hardening caused by martensitic structure transformation, and the hardening caused by structure transformation is the main reason, which is why the cold work hardening phenomenon of austenitic stainless steel is more obvious than that of ferritic stainless steel, and the work hardening coefficient (value) is large.

In austenitic stainless steel, the effect of Ni content on the transformation point of induced martensite is obvious.

With the increase of Ni content, the transformation point of martensite decreases, and the hardening degree of the material during cold deformation is small.

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.
Relationship between crystal size and mechanical properties of SUS304

The higher the grain size N level is, the more grains per unit cross-sectional area are, the finer the grain size, the greater the strength and the better the elongation of the material will be.

Generally speaking, steel with N>5 (256 grains/mm) days is called fine-grain steel.

When the grain size is large, it is beneficial to increase the plastic strain ratio (R) of the material, and reduce the yield strength ratio and yield elongation.

However, when the grains are large, they have different orientations on the surface of the sheet metal, and the difference in deformation is obvious, so the “orange peel” phenomenon is easy to appear on the surface of the material.

Refined grains can reduce the occurrence of orange peel, but if the grains are too fine, the value will decrease, and the yield strength ratio and yield elongation will increase, which is not conducive to forming.

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