Product failure analysis is an important discipline in the production industry and it is used in development of new products and improvement of existing products on the market.
Complex problems on the production line are not easily solved. In such cases, often the companies do not have enough resources, knowledge and analytical methods to ensure a comprehensive approach to the problem. LOTRIČ Metrology laboratories are your partner in problemsolving, using chemical and mechanical methods in the field of chemistry, mechanics, polymer technology and metallurgical sciences.
The customer asked LOTRIČ Metrology to determine why screws are cracking at relatively very low torque moment.
First, we performed a penetrant analysis and checked that no cracks in the screws have occurred before the screw was installed. We have found that the material has the required hardness and that the screws were surface-protected according to the instructions. By SEM analysis, we looked at the fracture surfaces and determined that this was an intercrystalline fragile fracture, suggesting that the most probable cause is either temper brittleness due to inadequate heat treatment or hydrogen embrittlement due to excessive irrigation in surface protection agents.
In order to determine whether this is a case of hydrogen embrittlement, we have carried out a tensile test of screws tempered at different temperatures. Some screws were exposed for 5 hours at 200 ° C, resulting in diffusion of hydrogen from the metal and were then compared with the control group. It has been shown that the mechanical properties are somewhat improved, but remain substantially worse than expected.
We found that the temperature of austenitization and hardening and the method of tempering correspond to those in the literature. According to the client's data, however, the screws were tempered at a temperature of 350 ° C, which was almost 200 ° C higher than the actual temperature. In order to confirm the hypothesis that temper brittelness is responsible for the failure of the screws, the untreated screws were irrigated at the recommended temperatures and subsequently tempered at two different temperatures - we carried out low temperature deterioration at temperatures below the area of the appearance of temper brittleness and high temperature deterioration in this region. Then the same tensile test we performed on the screws which were cracking, was carried out on these treated screws. The test showed a strong increase in mechanical properties, especially strength and elongation after break. The SEM analysis was used to examine the breaks produced by the tensile tests. It turned out that the fracture morphology at other tempering temperatures is completely different.
Analyses have shown that the cause of the cracking of the screws is the incorrect heat treatment of the screws resulting in temper brittleness , while a small part of the failure can also be ascribed to hydrogen embrittlement. Thus, we proposed tempering at higher temperatures to prevent temper brittleness. Had material hardness been the problem, we could also use lower tempering temperatures, but this is a sensitive issue. In any case, we could choose a more appropriate material which would achieve the prescribed hardness in an area where it would not be sensitive to temper brittleness.