K-factor stainless steel 304
The K-factor in bending of annealed AISI 304 stainless steel sheet metal is typically 0.40, with a common working range between 0.35 and 0.45 for standard thicknesses. This dimensionless value represents the relative position of the neutral axis with respect to the material thickness during plastic bending deformation. 304 stainless steel, with its moderate thermal conductivity of 14.4 W/m·K at 20 °C, requires precise K-factor compensation to achieve accurate flat pattern developments without oversizing or cracks in the tension zone. The combination of its yield strength of 205 MPa in the annealed condition and its elongation of 40% allows minimum inside bend radii ranging from 0.5 to 1 times the sheet thickness.
Relevant properties for K-factor calculation
Section titled “Relevant properties for K-factor calculation”The mechanical and physical properties of 304 stainless steel directly influence the position of the neutral axis during bending. The following table lists the fundamental values in the annealed delivery condition at 25 °C, extracted from technical reference sources for rolled sheet metal.
| Property | Value in annealed condition (25 °C / 77 °F) |
|---|---|
| Tensile strength | 515 MPa / 74,700 psi |
| Yield strength (0.2%) | 205 MPa / 29,700 psi |
| Elongation in 50 mm / 2 in | 40% |
| Reduction of area | 50% |
| Thermal conductivity at 20 °C / 68 °F | 14.4 W/m·K / 8.3 BTU·in/(hr·ft²·°F) |
| Typical hardness (Rockwell B) | 70 – 92 HRB |
The high ductility of the material, evidenced by the 40% elongation, allows the neutral axis to shift toward the inside of the bend as the radius increases. The thermal conductivity of 14.4 W/m·K at 20 °C, although low compared to carbon steels, does not compromise dimensional stability during cold forming in controlled environments between 15 °C and 30 °C.
Bending parameters and K-factor by thickness
Section titled “Bending parameters and K-factor by thickness”The K-factor correlates directly with the ratio of inside bend radius to sheet thickness (R/T). As this ratio increases, the neutral axis moves away from the inner face and the K-factor increases. The values in the table represent cold bending conditions with a V-die and standard punch for 304 stainless steel sheet in the annealed state.
| Sheet thickness (mm / in) | Inside bend radius (mm / in) | R/T ratio | Estimated K-factor |
|---|---|---|---|
| 0.5 mm / 0.020 in | 0.5 mm / 0.020 in | 1.0 | 0.35 |
| 1.0 mm / 0.039 in | 1.0 mm / 0.039 in | 1.0 | 0.36 |
| 1.5 mm / 0.059 in | 1.5 mm / 0.059 in | 1.0 | 0.38 |
| 2.0 mm / 0.079 in | 2.0 mm / 0.079 in | 1.0 | 0.40 |
| 2.0 mm / 0.079 in | 3.0 mm / 0.118 in | 1.5 | 0.42 |
| 3.0 mm / 0.118 in | 3.0 mm / 0.118 in | 1.0 | 0.40 |
| 3.0 mm / 0.118 in | 4.5 mm / 0.177 in | 1.5 | 0.43 |
| 4.0 mm / 0.157 in | 4.0 mm / 0.157 in | 1.0 | 0.41 |
| 4.0 mm / 0.157 in | 8.0 mm / 0.315 in | 2.0 | 0.45 |
For thicknesses above 4 mm or R/T ratios above 3.0, the K-factor tends to stabilize between 0.45 and 0.48, approaching the geometric center of the section. At the other extreme, with aggressive bend radii where R/T is less than 0.5, the risk of cracking in the outer fiber forces the K-factor to be limited to conservative values of 0.33 at most.
Advantages and limitations of 304 stainless steel in sheet metal bending
Section titled “Advantages and limitations of 304 stainless steel in sheet metal bending”Advantages
- High dimensional accuracy in flat pattern development when a calibrated K-factor between 0.38 and 0.42 is applied for thicknesses from 1.5 mm to 3 mm, with typical deviations less than ±0.2 mm in developed length.
- Excellent ductility with 40% elongation in the annealed condition, allowing reduced bend radii down to 0.5T without cracking.
- Low sensitivity to strain rate during conventional press brake bending, maintaining predictable neutral axis behavior.
- Good K-factor stability in repetitive mass production processes due to the microstructural homogeneity of cold-rolled sheet material.
Limitations
- Tendency to work hardening, progressively raising the yield strength from 205 MPa to values above 500 MPa, altering the K-factor in multiple bending operations on the same part.
- Thermal conductivity of 14.4 W/m·K at 20 °C, lower than that of carbon steels such as SAE 1020 which reaches 50–60 W/m·K, limits heat dissipation in temperature-assisted bending.
- Appreciable springback risk, with return angles that can reach 2° to 3° in 90° bends with large radii, requiring compensatory overbending.
- Susceptibility to cracking in chloride environments if severe deformation induces stress-induced martensite in the tension zone.
K-factor selection guide by application
Section titled “K-factor selection guide by application”The selection of the appropriate K-factor depends on the combination of thickness, bend radius, and material condition. For precision work in annealed 304 stainless steel sheet with thicknesses between 1 mm and 3 mm and an R/T ratio between 1 and 2, it is recommended to start with a value of 0.40 and adjust using test specimens. In components with demanding dimensional tolerances of ±0.15 mm on the developed length, the K-factor must be verified through test bends and measurement of the resulting part, correcting the value in increments of 0.01 according to the observed deviation. For thin thicknesses below 0.8 mm with R/T = 1, the K-factor drops to values of 0.33–0.35 due to the greater relative effect of compression on the inner face. In cold-rolled material condition with partial hardening (tensile strength near 620 MPa), the K-factor shifts slightly toward the inner face, recommending an initial value of 0.36 for R/T = 1. When the process involves bending in rotary dies or edge bending (hemming) in thicknesses from 0.5 mm to 1.5 mm, the K-factor can drop to 0.30 due to extreme material compression.
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”-
What is the K-factor value for 304 stainless steel of 1.5 mm thickness? The recommended value is 0.38 for an inside radius equal to the thickness, with an R/T ratio = 1. This value produces an accurate flat pattern development with a typical tolerance of ±0.2 mm on the total length of the bent part. If the bend radius is increased to 2.25 mm (R/T = 1.5), the K-factor rises to 0.43.
-
How does sheet thickness affect the K-factor in 304 stainless steel? With greater thickness and constant R/T, the K-factor tends to increase slightly. For 0.5 mm sheet with R/T = 1, the K-factor is 0.35, while for 4.0 mm sheet with the same R/T it reaches 0.41. This variation of 0.06 in the K-factor translates into a difference of up to 1.2 mm in the development of a part with four 90° bends.
-
What K-factor should be used when 304 stainless steel is cold-rolled hardened? For material with a tensile strength of approximately 620 MPa, typical of cold-rolled condition without subsequent annealing, a K-factor of 0.36 is recommended for R/T = 1. This represents a reduction of 0.04 relative to annealed material, due to the shift of the neutral axis toward the compression face as a result of the higher yield strength of 310 MPa.
-
What is the minimum inside bend radius for 2 mm 304 stainless steel sheet? The practical minimum inside radius is 1.0 mm (0.5T), although the recommended value for stable production is 2.0 mm (1T). With a radius of 1.0 mm and R/T = 0.5, the K-factor drops to 0.33 and the risk of cracking in the outer fiber increases significantly. In critical parts with high structural responsibility, it is advised not to go below 1T to preserve surface integrity.
-
Does temperature influence the K-factor of 304 stainless steel during bending? Yes. At temperatures above 100 °C, the thermal conductivity of 14.4 W/m·K at 20 °C limits homogeneous heat dissipation, which can generate localized zones with different plasticity. In cold bending between 15 °C and 30 °C, the K-factor remains stable within the published range. Below 0 °C, ductility decreases and it is recommended to increase the minimum radius to 2T to avoid brittle fracture.
-
How is the K-factor adjusted when the bend is made at 60° instead of 90° in 304 stainless steel? The bend angle has a second-order effect on the K-factor. For a 60° angle, the value remains within ±0.02 of the K-factor corresponding to 90° for the same R/T. The difference in the resulting development is less than 0.15 mm for thicknesses up to 3 mm, so in applications with general tolerances of ±0.5 mm, no additional adjustment is required.