Fragilização por hidrogênio de parafusos cementados / Hydrogen embrittlement of carburized bolts

AUTOR(ES)
FONTE

IBICT - Instituto Brasileiro de Informação em Ciência e Tecnologia

DATA DE PUBLICAÇÃO

28/07/2011

RESUMO

The main aim of this work is to investigate the effects on hydrogen embrittlement of bolts due to acid pickling, baking, and strip processes performed during zinc plating. Carburized bolts type "hexagon flange head tapping screws", with 6 mm of diameter and 16 mm of thread length were organized into 39 different groups. Acid pickling was evaluated using volumes of hydrochloric acid of 700 ml, 900 ml, and 1000 ml, presence, or absence, of acid inhibitor, and pickling periods of 15, 30, 45, and 60 minutes for each condition of acid volume and inhibitor used. Baking was evaluated using 1000C, and periods of 1, 2, and 3.5 hours for bolts that were subjected to acid pickling with a solution of 1000 ml of acid, absence of inhibitor, and pickling periods of 15, 30, 45, and 60 minutes. Strip was evaluated using bolts with zinc layer thickness of 8 ?m, stripped for 4 minutes in a solution of 700 ml of acid, absence of inhibitor, and 300 ml of water, performing the hypotheses of single and double strip without baking, and single strip followed by baking (1000C - 2 hours). Preloading tests for the detection of hydrogen embrittlement were conducted in bolts that were subjected to the preparations mentioned, while torsional tests were also conducted at these bolts, as well as in those obtained after heat treatment. Chemical and metallographic analysis, Vickers microhardness and tensile tests, and fractographic analysis using scanning electron microscopy (SEM) were also conducted. Metallographic analysis revealed a ferritic matrix composed of equiaxed grains with spheroidized carbides for the wire, while the bolts showed a ferritic-pearlitic microstructure at the center and tempered martensite at the hardened layer. Microhardness and tensile tests revealed that wire and bolts have strength levels as expected according to the conditions used, while no significant differences between the breaking torque values were obtained in both situations analyzed. Acid pickling revealed that in the groups, which did not use inhibitor, the number of failures increases at a first stage with increasing pickling periods, however it starts to decrease after a certain pickling period related to the volume of acid used. The number of failures is greatly reduced with the use of the inhibitor, but even at low concentrations of iron, the use of larger amounts of acid associated with longer pickling periods increases the risk of hydrogen embrittlement. Failures were observed only at 1000C - 1 hour as baking parameters, showing the importance of proper temperature controls, homogenization procedures, and temperature and time parameters. There were no failures related to strip, revealing the importance of adequate procedures for strip (period and solution used) and baking procedures as observed. The fracture surface analysis revealed only intergranular and dimples micromechanisms of fracture, where the latter being more often associated with regions closer to the core of the bolts, also showing the presence of secondary cracks

ASSUNTO(S)

metais - fragilização por hidrogenio aço - decapagem desidrogenação metals steel pickling baking

ACESSO AO ARTIGO

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