Bending: ever stronger, ever thicker

The heavy machinery market seeks to improve the performance and efficiency of their products. To reduce the maintenance cost and to prolong the life time of this equipment, heavy gauge advanced high strength steel grades are increasingly used.

Heavy gauge advanced high strength steel grades are mainly, if not solely deformed by bending. Therefore, the bendability of those grades must be evaluated as part of the qualification process.

Evaluation of bendability

In general, the bendability of a material is expressed by means of the minimum guaranteed bending ratio, i.e. the smallest possible bending ratio for which no cracks can be observed by visual inspection.

The bending ratio is defined as the ratio of the punch radius Rp to the sheet thickness t, Rp/t.

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A material’s bendability is typically assessed through lab scale bending tests during which samples are bent to either 90° or 180°. During those tests, the punch radius is gradually decreased.

New 1500 kN bending press

In view of the metallurgical development of ever stronger and thicker steel grades, OCAS invested in a new servo-hydraulic press with a capacity of 1500 kN. The press is equipped with a fixture for three point air bending making it possible to bend samples up to 25 mm thick. The press is instrumented to capture force, punch displacement and bending angle, providing useful information such as required bending force and springback angle.

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OCAS uses an acoustic emission (AE) monitoring system to detect crack initiation during bending tests, making it possible to interrupt the test once the first crack has been detected. This helps to identify which microstructural feature is causing crack initiation, improving our understanding of the relation between microstructural/metallurgical features and bendability.

“The instrumented bending press not only allows determining the minimum guaranteed bending ratio, but also provides other useful information for our customers, such as the required bending force and the springback angle.”

Steven Cooreman, Senior Research Engineer, Applications & Solutions Department, OCAS