APPLICATION NOTE Bend failure mechanism of high-strength steel Efficient and effective root cause analysis using the Axia ChemiSEM Introduction Steel is frequently employed in applications where it needs to be shaped and curved; hence, a certain degree of ductility is required to enable the bending without allowing cracks. For these type of applications, a specific type of alloy steel is used: highstrength, low-alloy (HSLA) steel. HSLA steels are characterized by lower content of carbon, chromium, nickel and molybdenum than other alloy steels like AISI 4140, where the decrease in carbon enables higher formability and weldability. For this reason, prior to being manufactured in its final shape, HSLA steel is evaluated with a bending test. The main reason is to ensure that no cracks or fractures will occur during or after the bending. Figure 2. Image of a failed bend test. However, in the case of any failure, it is crucial to understand the root cause. Common types of cracking include hydrogen cracks (often in weld heat affected zones), hot tears (contamination by copper or tin during solidification), and subsurface non-metallic inclusions. The latter may originate from native or endogenous steel-making inclusions or from foreign or exogenous inclusions, such as slag or refractory. Steel production requires deoxidation to decrease the dissolved oxygen content to a very low level. Commonly, aluminum is used for this process, where the resultant oxides are micron-sized, high melting-point aluminum oxide particles. During refining and casting, the pure aluminum oxide inclusions may become contaminated with magnesia, calcia, or slag droplets. Figure 3 shows the Ca-Al-Mg ternary diagram of oxide inclusions analyzed in the steel near the bend test defect, which has been run by a Thermo Scientific™ Phenom™ ParticleX Steel Desktop SEM thresholding the features greater than 1 µm. Figure 1. Bend test (top) and tested piece (botto

Results and discussion The imaging of the area of interest and the quantitative elemental information of the investigated features were acquired at 15 kV. A moderately low beam current (0.22 nA) was used in order to reduce the beam spreading and maximize the spatial resolution. Figure 5 shows a lower magnification overview of the crack, which is also visible in the optical image in Figure 4. Outer radius Inclusion count: 382 DMAX 0.0-2.0 [133] Figure 3. Base inclusion analysis was collected on this sample by the Phenom ParticleX Steel Desktop SEM. Molten metal transfer from ladle to tundish,...

Figure 6 highlights some of the inclusions present in the stringer. The top image shows the secondary electron signal, which gives topographical information and shows the presence of voids. Due to the presence of inclusions, voids are often formed during the hot rolling process as the deformed steel surrounding the foreign particles undergoes high tensile stress along the rolling direction. The bottom image provides the backscattered electron signal, which gives compositional information based on the atomic number of the imaged material. The information gained from this image is not enough to...

Conclusions Bend testing is conducted on steel and other materials to determine ductility and resistance to fracture. When the material of interest is subjected to fractures or cracks, we can surmise that it will fail if employed in similar applications. For this reason, understanding the cause of failure is key to prevention. In this application note, we have presented a fast and efficient method to achieve the root cause of a failed bend test on a highstrength steel. With the new ChemiSEM Technology approach, the morphological characterization of the area of interest is no longer separated...