Propriedades de fadiga de parafusos de alta resistencia mecanica / Fatigue properties of hith strengh bolts

AUTOR(ES)
DATA DE PUBLICAÇÃO

2008

RESUMO

High strength bolts, class ISO 10.9, were manufactured with two different steels in order to investigate the influence of tempering temperature, the sequence of manufacture process (threads rolled before and after heat treatment), and the inclusions amount contained in steels, in the bolt fatigue behavior and also what criterion of fatigue failure existent in literature is the best for application on high strength bolts with the threads rolling before heat treatment. M8 bolts with coarse threads were separated in seven different groups, three manufactured with Brazilian steel (AISI 4135) and heat treated, after the process of thread rolling, with tempering temperatures of 490, 520, and 550 ºC, and four groups were manufactured with Japanese steel (SCM 435H), however, three groups were identical of the bolts manufactured with national steel and a last one group had the threads rolled after heat treatment with tempering temperature of 550 ºC. In all groups were carried out metallographic and inclusion analyses, tension, microindentation hardness, and fatigue tests, and a fractographic analysis of bolts failed in the fatigue tests, using a scanning electron microscope. The fatigue tests were carried out with constant preload of 700 MPa, stress ratio about 0.8, and it was used the staircase statistical method for determine the fatigue limit. Besides, additional fatigue tests were carried out, with low stress ratio ( 1 . 0 ??R ), in order to analyze the existents criteria of fatigue failure. The results of metallographic analysis showed that the characteristic microstructure in all groups was tempered martensite and the only difference between the groups was the alignment of the grains around the root of the thread that occurred in the group which was heat treated before the threads rolling. The inclusion analysis revealed that the Brazilian steel has an amount of inclusions very superior than Japanese steel. The tension and microindentation hardness tests showed that the bolts with threads rolling after heat treatment have strength level lower than solely the groups tempered at 490 ºC. All the groups with threads rolling before heat treatment presented practically the same fatigue limit. It shows that variations of up to 60 ºC in the tempering temperature don t interfere in the fatigue limit. On the other hand, the group with threads rolling after heat treatment presented fatigue limit approximately 9% superior than the group treated in the same conditions, however with threads rolling before heat treatment. This improve is related with the alignment of the grains around the root of the thread and the introduction of residual compressive stress near the surface of the bolts. With the results of the additional fatigue tests, it was possible to analysis some criteria of fatigue failure and show that the best criterion of failure, existent in the literature, for high strength bolts with thread rolling after heat treatment is the VDI 2230, which presents a constant alternating stress independently of the stress ratio

ASSUNTO(S)

aço - propriedades mecanicas aço - fadiga steel parafusos e porcas steel bolts and nuts steel aço - tratamento termico

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