EM - Escola de Minas

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Failure analysis of a steel slide ring of a tubular ball mill used in an iron ore mining plant.
(2020) Godefroid, Leonardo Barbosa; Cândido, Luiz Cláudio; Guimarães, Claudinei Roberto; Araújo, Sidney Cardoso
This work identified the root cause of an early failure occurring near welded joints that connect the slide ring with the head of a ball mill used in an iron mining plant. The ASTM-A516-G60 steel specified for use at this location had a lower carbon content than required. The microstructure presented by this steel consisted of a totally heterogeneous distribution of ferrite grains and pearlite colonies, with variations in grain size and volume fraction. Moreover, the union between the slide ring and the mill head created a significant geometric stress concentration and the welding process used, although meeting the specification, contributed to the microstructural heterogeneity, due to the thermal input required for this operation. These microstructural and geometrical characteristics led to the development and growth of fatigue cracks near the welded joints, that crossed the thickness of the slide ring. As a consequence, the iron ore pulp was contaminated with the lubricating oil, decreasing the adequate oil level and generating excessive wear on bearings and loss of pressure from the lubrication system. All these facts made impracticable the operation of the mill. Three different steels that meet the specification for application in pressure vessels were analyzed and compared with the steel of the failed mill by means of tension, Rockwell hardness, Charpy impact, fracture toughness (J-R curves) and fatigue tests (σa × Nf curves and da/dN × ΔK curves). It was concluded that the material selection for the mill component needs to consider a suitable chemical composition and microstructure of the steel for that application, to ensure adequate values of mechanical properties compatible with its use.
Strain ageing in welded joints of API5L X65Q seamless pipes.
(2017) Batalha, Rodolfo Lisboa; Godefroid, Leonardo Barbosa; Faria, Geraldo Lúcio de; Porcaro, Rodrigo Rangel; Cândido, Luiz Cláudio; Trindade, Vicente Braz
In this work, we investigated the effects of the phenomenon of strain ageing in joints obtained by gas-shielded arc welding (gas metal arc welding, GMAW), of seamless pipes of API5L X65Q steel. Test specimens obtained from the welded joints were submitted to cold plastic strain of 3% and then aged for 1 h at 250 °C, so as to induce static ageing. For evaluating dynamic ageing, the test specimens were strained to 3% and loading was maintained for 1 h at 250 °C. The aged specimens were submitted to tensile testing and representative samples were examined in the light microscope and the scanning electron microscope. It was observed that the phenomenon altered the volume fraction of secondary constituents in the weld metal and in the HAZ, with increase in the proportion of ferrite–carbide aggregates. Regarding the mechanical properties, it was found that the phenomenon reduced the elastic ratio of the welded joints due to an increase in the ultimate strength, besides increasing the total elongation, but without impairing the tensile properties of the welded joints. On comparing static ageing with dynamic ageing, it was observed that the increase in the capacity for plastic strain of the welded joints was greater after static strain ageing.
Envelhecimento por deformação em juntas soldadas de tubos de aço API5L X65Q sem costura.
(2015) Batalha, Rodolfo Lisboa; Godefroid, Leonardo Barbosa; Faria, Geraldo Lúcio de; Porcaro, Rodrigo Rangel; Cândido, Luiz Cláudio; Trindade, Vicente Braz
Neste trabalho, investigaram-se efeitos do fenômeno de envelhecimento por deformação em juntas obtidas por soldagem a arco elétrico com proteção gasosa (GMAW), de tubos sem costura de aço API5L X65Q. Corpos de prova (CPs) obtidos a partir das juntas soldadas foram submetidos à uma deformação plástica a frio de 3% e posteriormente envelhecidas por 1h a 250°C, de forma a se induzir envelhecimento estático. Buscando avaliar o envelhecimento dinâmico, os CPs foram deformados até 3% e o carregamento foi mantido durante 1h a 250°C. Os CPs envelhecidos foram submetidos a ensaios de tração e amostras representativas foram analisadas ao microscópio óptico e microscópio eletrônico de varredura. Observou-se que o fenômeno modificou a fração volumétrica de constituintes secundários no metal de solda e na ZTA, com aumento na proporção de agregados ferrita carbonetos. Com relação às propriedades mecânicas, constatou-se que o fenômeno reduziu a razão elástica das juntas soldadas devido a um aumento no limite de resistência, além de aumentar o alongamento total, não prejudicando, portanto, as propriedades mecânicas em tração das juntas soldadas. Ao se comparar o envelhecimento estático com o envelhecimento dinâmico, observou-se que o aumento na capacidade de deformação plástica das juntas soldadas foi maior após envelhecimento estático por deformação.

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