Bending Stainless Steel Sheet: Strategies to Conquer Springback

I. Stainless steel plate bending key points

Due to its high yield strength, hardness, and prominent cold-working effect, stainless steel plate bending has the following characteristics:

• Due to its lower thermal conductivity compared to regular low-carbon steel, it has a lower elongation rate and requires a larger deformation force.
• Stainless steel sheets have a stronger tendency to rebound when bending compared to carbon steel.
• The elongation percentage of stainless steel plate is lower than that of carbon steel, leading to a larger bending angle of the workpiece (R) or a possibility of cracks.
• Given the high hardness of stainless steel plate, which exhibits significant cold-working hardening, a punch die made of tool steel should be chosen, with a hardness of over 60 HRC after heat treatment. The surface roughness of the bending tools will be higher than that of carbon steel bending tools.

According to the above characteristics, generally speaking:

Under the same unit size, the thicker the plate, the more bending force is required. The bending force increases as the plate thickness increases.

Under the same unit size, the higher the tensile strength, the lower the elongation rate, the more bending force required, and the larger the bend angle should be.

When designing the plate thickness in relation to the bending radius, according to experience, the unfolded size of the workpiece with one bend should be calculated by adding the two right-angle sides and then subtracting two thicknesses. This will fully meet the design accuracy requirement. Using an empirical formula to calculate the quantity can simplify the calculation process and greatly improve production efficiency.

The higher the yield strength of the material, the greater the spring back. Therefore, the angle of the punch die for the 90-degree bend part should be smaller.

Compared to carbon steel, for the same thickness of stainless steel, the bending angle is larger. It is important to pay special attention to this point, as bending cracks can appear, affecting the strength of the workpiece.

II. The spring back of the stainless steel

The spring back phenomenon in stainless steel presents significant challenges in metal forming processes, attributable to several key factors:

1. Material Hardness: There is a direct correlation between material hardness and spring back magnitude. For instance, when working with 301-EH (Extra Hard) stainless steel, a spring back of approximately 14 degrees was observed. This high degree of spring back is due to the material’s increased yield strength and work hardening characteristics.
2. Bending Radius to Material Thickness Ratio: As this ratio increases, so does the spring back effect. This relationship is governed by the bending moment and stress distribution through the material thickness during forming.
3. Alloy Composition: Different stainless steel grades exhibit varying spring back behaviors. For example, SUS301 typically demonstrates greater spring back compared to SUS304. Under identical forming conditions, SUS304 may show approximately 2 degrees less spring back than SUS301. This difference is attributed to the variations in their chemical composition and microstructure, particularly the higher carbon and nitrogen content in 301, which increases its work hardening rate.
4. Geographical Source: Interestingly, Japanese-produced 301 stainless steel tends to exhibit greater spring back compared to its Taiwanese counterpart. This variation likely stems from subtle differences in alloying elements, processing techniques, or quality control standards between manufacturers.
5. Forming Methodology: Single-step forming processes generally result in greater spring back compared to multi-step forming approaches. This is because multi-step processes allow for stress redistribution and gradual plastic deformation, reducing the overall elastic recovery.

To address these challenges, an iterative approach to mold design and adjustment is crucial. After each forming test, the mold should be revised to account for both angular and radial spring back. This process requires close collaboration between skilled technicians and design engineers to implement necessary modifications efficiently.

In practice, processing batches of approximately 5 pieces at a time allows for quick iterations and adjustments. The efficiency of this process is heavily dependent on the technician’s expertise and experience in working with stainless steel forming.

To further optimize the forming process and mitigate spring back:

1. Consider using computer-aided engineering (CAE) simulations to predict spring back behavior before physical prototyping.
2. Explore advanced forming techniques such as warm forming or high-speed forming, which can reduce spring back in certain applications.
3. Implement precise process control measures, including consistent material properties, uniform temperature distribution, and accurate force application during forming.