FRACTURE FAILURE ANALYSIS IN TIE RODS OF STACKER RECLAIMER

This work aimed to analyze the causes of premature failure by fracture of a tie rod of a large rotary forklift or Stacker Reclaimer. A methodology that included visual inspection techniques, macrographic analysis, optical microscopy (OM), Vickers microhardness (HV) measurements, scanning electron microscopy (SEM), semi-quantitative chemical scattering spectroscopy (EDS) and quantitative chemical analysis by optical emission spectrometry and combustion was employed. It could be concluded that the chemical composition of the material and / or the manufacturing process were outside the specification of the material due to the unexpected presence of magnesium and carbon graphite nodules, the latter considered to cause fragility that may have led the material to fracture by fatigue.


INTRODUCTION
Stacker Reclaimer is an equipment to recover and stack granulated solid materials. This equipment is used to control the inflow and outflow of materials, such as coal, cement, gravel and steel, determining, for example, the stock and volume of ore in mine yards or vessels (EXCELNEX, 2018;MAGNOR, 2018; VAN, et al., 2019). Such equipment may have an arm that exceeds 50 meters in length and, due to its large measures, it is necessary to be careful with the dimensional design of the components that support the operating loads (EXCELNEX, 2018). The Stacker Reclaimer analyzed in this work is of the rotary type, in which the rotating buckets collect material from the yard. This equipment is used when it is necessary to mix material classes, or for storage yards with limited space. In this case, they have an arm of up to 67 meters and staking rates of 6,000 tons per hour for coal, and up to 10,000 tons per hour for iron ore (GLOBALASSETS, 2018). Design mistakes, manufacturing problems, operation errors, or external impacts are often responsible for failures in equipment that work with high loads (BOSNJAK et al., 2011).
Tie rods are mechanical elements used for tensile loads only and, due to high slenderness ratio, these elements can buckle under compression loads (CARJUNKY, 2018). The loads applied on this mechanical element in the Stacker Reclaimer are very high due to the large amount of ore transported simultaneously. The loads applied to the machine structure and transferred to the tie rods are variable over time due to the intervals between the buckets during the rotation movement. Figure 1 shows a Stacker Reclaimer, the arrows show the tie rods anchor points, the places where fractures occurred in the case analyzed in this work. This work aimed to analyze the failure due to the fracture of tie rods of a Stacker Reclaimer used to recover and stack ore in the raw material yard of a large steel plant. The tie rods fractured with a duty time significantly shorter than eight years.

MATERIAL AND METHODS
Four samples of two tie rods that fractured in operation were delivered for analysis. All samples contained the preserved fracture surfaces, one of which was chosen to perform metallurgical analysis and mechanical tests. The  According to the datasheet of the equipment, the component should be made of AISI 1050 steel (carbon steel containing 0.5 C wt. %). Figure 2 illustrates Stacker Reclaimer tie rods design.

RESULTS AND DISCUSSION
Visual inspection of the fractured surface revealed oxidized regions, and the presence of grooves and unevenness, aspects typically associated with the Chevron patterns, recurring in failures due to the fatigue mechanism (SHACKELFORD, 2008). Figure 2  Therefore, the microstructure of the material was not perfectly within specifications. Those nodules are typically present in nodular cast irons, and are associated with a combination of graphite alloying elements and cooling rates in the manufacturing process. The elements that favor the formation of graphite are silicon; cerium; and magnesium; however, steels of the type AISI 1050 do not have considerable percentages of those elements. The assumption of the presence of these elements was raised due to the graphite potential they have (OKAMOTO, et. al., 1984;SOINSKI, 2012).   Although carbon is difficult to identify by EDS, its high content allowed the confirmation of its presence at high peaks. In addition, the presence of a small magnesium concentration was observed. This presence was not expected for AISI 1050 steel and it could be responsible for the separation of carbon from the cementite and its coalescence, forming the graphite nodules (KIRBY, 1995;SOINSKI, et al., 2014).
To ensure that the result of the chemical analysis of the material was of good quality, the EDS signal capture lasted about 100 seconds per analyzed region. Thus, it was possible to detect about 0.16 wt. % of magnesium (Table   1).    For steel type AISI 1050, the presence of measurable levels of magnesium, silicon and chromium is unexpected. The Table 3 shows the typical chemical composition of steel type AISI 1050. The results of the quantitative chemical analysis corroborate the results by EDS, where the presence of the magnesium, an element with great graphitizing potential, was identified. The percentage of carbon present (Table 2) is within the specified for the tie rods, as shown in Table 3.
The hardness found for the material was 196.54 ± 16 HV, which is in accordance with the hardness specified for AISI 1050 steels (198 HV) (MATWEB, 2018).

CONCLUSIONS
Through the employed characterization techniques, it was concluded that the fracture failure occurred, most probably, due to the significant presence of graphite nodules distributed throughout the material. The graphite nodules occurred due to errors in the chemical composition and/or in the manufacturing process of the steel. The graphite regions are fragile, particularly when associated with regions machined with threads, regions with different residual stress and stress concentration points. The results of the macroscopic analysis give evidence of a fragile fracture mechanism due to fatigue, although this has not been confirmed by fractographic observations on a microscopic scale.