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Study on Stainless Steel Stretch Forming Process in Rail Vehicle Manufacturing

stretch forming machine

In the production of stainless steel bodies for rail vehicles, many beam and column components require stretch forming, such as the upright columns of the side walls and the curved beams of the roof. Due to their complex shapes, high precision requirements, and large dimensions, it is challenging to achieve the desired shape and accuracy using conventional forming methods, which can lead to quality issues such as wrinkling and springback. Stretch forming technology, by redistributing internal stress within the section, reducing springback, eliminating wrinkling defects at compressed edges, and significantly improving forming precision, has become a core technology in stainless steel body manufacturing for rail vehicles. It plays a crucial role in enhancing the quality and production capacity of rail vehicles while reducing production costs.

Principles of Stretch Forming Process

stretch forming machine

Stretch forming refers to the bending process of a material with simultaneous stretching force applied, resulting in a profiled shape. This technique utilizes the redistribution of internal stress to achieve the desired shape and dimensional accuracy.

Application Range of Stretch Forming Process

The stretch forming process is mainly used for bending parts with large lengths and radii of curvature. When using conventional methods, such parts experience mostly elastic deformation, resulting in significant springback and difficulty in forming. By applying the stretch forming process, the internal stress distribution within the section can be altered, reducing springback and improving forming precision. It also helps eliminate wrinkling defects at compressed edges and enhances forming quality.

Factors Influencing Stretch Forming Process

The stretch forming process is a complex bending technique influenced by various factors such as friction and contact, geometric structure, shape and material properties of the forming tool, shape parameters of the profile, and process parameters. These factors mainly affect how to overcome the impact of springback.


Springback is an inevitable part of metal deformation as long as there is elastic deformation. After the surface of the formed part enters the plastic zone, the center remains in an elastic state. Therefore, springback occurs when the external force is removed. It involves the adjustment of residual stresses within the workpiece after forming to achieve a balanced state.

Factors Affecting Springback:

  • Material mechanical properties
  • Bend radius and wall thickness
  • Bending angle
  • Shape of the formed part

Methods to Reduce Springback:

Improving the distribution of internal stresses within the section to achieve uniform stress distribution and reduce springback, thus enhancing forming precision.

Distribution of Strain States in Stretch Forming Process

The distribution of strain states in the stretch forming process is critical for achieving desired forming results. By carefully controlling the strain distribution, particularly ensuring tension stress above and below the neutral axis, springback can be effectively reduced, leading to improved dimensional accuracy.

Key Features of CNC Stretch Forming for Stainless Steel Profiles

  • Difficult-to-form materials, resulting in significant springback and challenging control of profile accuracy.
  • Thin wall thickness and poor rigidity lead to significant cross-section distortion, making it difficult to control the dimensional accuracy of the section.

Analysis of Defects and Solutions in Stainless Steel Stretch Forming Process

Figure 2 shows four typical bent beam components for urban rail stainless steel cars, all made of SUS301L stainless steel. Here, we discuss the main defects encountered during the stretch forming process of stainless steel profiles for urban rail vehicles and provide feasible solutions using the example of the first bent beam section shape.

Common Defects

  • Wrinkling and fractures

Solutions to Common Defects

  • Wrinkling and fracture control: Overcome wrinkling defects by adjusting the tension parameters to ensure the minimum strain in the section is positive. Pay attention to control the maximum strain value of the curve, keeping it below the material’s forming limit to avoid fractures. The longitudinal strain values during stretching are shown in Figure 4.
Stretch Forming Process of Aluminum Alloy Automobile Roof Rack【Stretch Forming Machine】

CNC Stretch Forming Equipment

Stretch forming equipment mainly consists of CNC stretch forming machines. They are categorized as CNC and non-CNC based on the operating system. The bending arm’s position can be vertical or horizontal. According to the working table’s motion, there are rotary table types and swing arm types.

The selection of stretch forming machines depends primarily on the specific production requirements.


This study proposes a stretch forming process for lightweight stainless steel components in urban rail vehicles. It combines the situation in the bending industry at home and abroad with the actual production conditions of domestic urban rail vehicles. The proposed technique has been extensively applied in actual production and possesses significant research potential and practical economic value. The article discusses the main defects and quality issues that may arise during the stretch forming process of stainless steel components and provides viable solutions and methods. It has broad guiding significance for improving the industry’s technological level and the manufacturing of lightweight stainless steel urban rail vehicles.