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.
Failure analysis of recurrent cases of fatigue fracture in flash butt welded rails.
(2015) Godefroid, Leonardo Barbosa; Faria, Geraldo Lúcio de; Cândido, Luiz Cláudio; Viana, Thiago Gomes
Medium carbon steel rails are commonly jointed by flash butt welding procedures. To ensure an adequate service life for the structural component, commonly subjected to fatigue loading, a rigorous quality control should be performed, both in welding parameters and in the surface quality of the welded joint. This work investigated the causes of recurrent failures in railroads, with cracking always initiated in the rail in the region of the welded joint. Two case studies are presented. Aiming to execute the failure analysis, all standard procedures were applied, with a careful assessment of the material characteristics and the fractured surface. It was concluded that, for both cases, fatigue cracks initiated near the weld bead and spread in a brittle mode, leading to premature fracture of the material. An in-depth analysis of the rails showed that the surface finishing of the weld bead was not appropriate, generating undesirable stress concentration. Some undesirable microstructural characteristics were also found in the weld region. A recommendation to control welding parameters and to change the procedure of surface finishing of welded joints was held, aiming to decrease the number of accidents on the railroads.
Fatigue failure of a welded automotive component.
(2014) Godefroid, Leonardo Barbosa; Faria, Geraldo Lúcio de; Cândido, Luiz Cláudio; Araújo, Sidney Cardoso
Automotive components are commonly tested in fatigue before the final assembly of the vehicle, to ensure the accomplishment of technical specifications of the customer, and to avoid premature failure and accidents with autos. This work investigated the cause of failure of a control arm (component of the automobile movement system), made of high strength and low alloy steel, stamped and welded by robot procedure. The fatigue cracking of the component occurred during the application of standardized blocks of fatigue loading. All standard procedures of failure analysis were applied, with a careful assessment of the material characteristics and the cracked surface. It was concluded that fatigue cracks initiated near the weld bead, at the beginning of the welding procedure. An in-depth analysis of this region showed that there was presence of stress concentration, and the microstructure of the steel has been completely changed, due to an excessive amount of heat at the beginning of the welding. A recommendation to change the welding procedure was performed, ensuring the lifetime specified for the component in the fatigue test.

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