Detecting the cause of screw cracking
14. 05. 2020
Problem
The customer turned to LOTRIČ Metrology with the question of why screws crack at relatively very low screw turning moments.
Initial investigations
We first performed an analysis with petentrants and checked that there were no cracks in the screws that would have formed before the installation of the screw. We made sure that the material has the required hardness and that the screws are surface protected according to the instructions. We examined the fracture surfaces with SEM analysis and determined that it was an intercrystalline brittle fracture, which indicated that the most probable causes were either temper brittleness due to inadequate heat treatment of tempering or hydrogen brittleness due to excessive soaking in surface activation baths (HCl, HF, ().
Confirming hypotheses
To determine whether this is hydrogen fragility, we performed a tensile test of screws of different temperature treatments. A few screws were exposed to 200 ° C for 5 h, causing hydrogen diffusion from the metal, and compared with the control group. Mechanical properties have been shown to improve slightly, but remain significantly worse than expected.
We found that the austenitization and quenching temperature and the quenching method were appropriate according to the literature, but according to the client, the screws then loosened at 350 ° C, which is almost 200 ° C higher than the actual temperature according to the literature. .
To confirm the hypothesis that the brittleness of the screws is responsible for the brittleness of the screws, we tempered the untreated screws from the recommended temperature and then loosened them at two different temperatures – low temperature loosening at temperatures below the brittleness and high temperature weakening behind this area. Subsequently, the same tensile test was performed on the screws treated in this way as before on the screws that were cracked. The test showed a strong increase in mechanical properties, especially strength and elongation after rupture.
The fractures resulting from tensile tests were examined by SEM analysis. It turned out that the morphology of the fracture at other yield temperatures is completely different.
Conclusion
We have proven by analyzes that the cause of screw cracking is incorrect heat treatment of tempering, which is reflected in the embrittlement fragility, and a smaller share can also be contributed by hydrogen embrittlement. Thus, we proposed a change in upward tempering temperatures to avoid the occurrence of tempering fragility. If the problem was in hardness, lower tempering temperatures could also be used, but this is more delicate. In any case, you could also choose a more suitable material that would achieve the prescribed hardness in an area where it would not be sensitive to temper brittleness.