A baler is an implement used as a tractor attachment to convert crop residue like hay, flax straws, etc. In general, baler is manufactured by using several operations, such as the welding of many cups and shafts which are interconnected through gear mechanisms. In the present study, the cup material was cold-rolled steel, which was welded to the shaft using the MIG welding process. After a few months of service, a few connections reportedly failed at the weld interface. The metallurgical characterization results show that the microhardness of the coldrolled steel HAZ dropped significantly compared to the weld zone. To eliminate weld interface failure, hot-rolled grades were considered feasible for the cup application. The use of metallography, fractography, and instrumented indentation techniques proved useful for understanding the microscopic to macroscopic transitions near the failure and the mechanical properties of the structural elements. The proposed hot-rolled-grade material HAZ, on the other hand, did not show a significant hardness drop. The results show that, near the weld, the heat produced by welding caused a transformation to austenite, and the rapid removal of heat produced a low-ductility region with improved hardness (the HAZ) compared to the cold-rolled grades. This type of metal can be cooled at room temperature without damage, meaning it won’t have a lot of internal stress that results from other work-hardening processes. The paper investigates the material selection for the cup material as well as the effect of welding parameters on mechanical properties and microstructures in several mechanical tests.
Keywords: Baler cup-shaft, MIG welding, Cold rolled and hot rolled steels.
1. Introduction
The baler has steadily evolved into a necessary and efficient tool for the harvest of feed grass and straw as a result of the quick development of agriculture and animal husbandry. Small square balers will continue to be the most popular square baler machine models for the foreseeable future due to the varying growth of agricultural mechanization and the various operations and scales of agricultural output [1]. A compact set of essential lawn mower supplementary items, the tractordriven round straw baler (Fig. 1) is mostly employed in south Indian agricultural regions. The round straw baler frequently fails during the real baling process due to variables including the machine's quality, the working environment, use, and maintenance, which delays the timely harvest of fodder grass and straw and raises the cost of operation and maintenance. Improving the adaptability and reliability of such parts is an important direction in the design and development of agricultural equipment.
Fig 1: Tractor driven round straw baler and its operations.
Material selection and its weldability finds an important place in the design of this type of agricultural equipment, and how steel is welded can have a significant impact on its application and service life. Hence, the performance and longevity of these machines depend largely on the type and quality of materials used in its parts [1, 2]. Even if the grades and specifications between multiple steel properties are the same, the manufacturing process can change the chemical composition of the steel and the mechanical properties change completely after welding. Mostly, this type of welded cup-shaft parts undergoes torsional loads during service life, sometimes it leads to failure at the weld interface. The service life of such welded structures can be improved by using the proper welding technique and controlling various parameters, such as material selection and welding electrodes, among other things.
2. Background:
Tata Steel supplies various types of cold-rolled and hot-rolled grades to several agricultural equipment manufacturers. One manufacturer makes deep drawn cups of 30mm depth out of 3mm-thick IS513 CR2 grade, which are welded to the baler shaft of diameter 25.4mm made of EN8D grade material. As recommended by the design team, an interface plate of E250 grade is being used between the shaft and cup to strengthen the welded assembly. The interface plate dimensions are 5 mm in thickness and 70 mm in diameter. Such multiple cup-shaft assemblies are interconnected using gear mechanisms, which rotate the shafts at an angular speed of 240 RPM. The detailed assembly is shown in Figure 2.
Fig 2: Cup shaft assembly – (a) Cup, (b) Interface plate, (c) Weld location between interface plate and cup, (d) Weld location between shaft and interface plate, (e) Shaft.
2.1 Problem statement and analysis:
As reported by the equipment manufacturer, a few of the balers were reported to have premature failure at the upper roller cup-shaft weld interface after a few months of operation. As shown in Figure 3, the failure location is in the hat affected zone of the cup side. To understand the reasons for the crack, failure samples were
Fig 3: Cup shaft assembly failure – (a) weld line along the interface plate edge (b) Crack along the HAZ towards cup side.
collected from the manufacturers, and fractography studies were carried out on the crack propagation surface. Further, based on the information received from the manufacturer, a few observations were made to identify the root cause for the failure [3].
The failure cause could be any of the following factors:
(i) Material compatibility
(ii) Weld parameters
(iii) Weld toe stress concentration
Accordingly, a detailed study was carried out to understand the effect of each factor.
(i) Material compatibility:
The existing cup and interface plate material chemistry details are shown in Table 1:
Table 1: Cup and Interface plate properties:
Further, both material properties after welding and compatibility with the adjacent weld and heat-affected zone were characterised using microhardness tests.
(ii) Welding parameters:
At the manufacturer's end, the assembly of the cup shaft consists of a lap joint weld between the interface plate and cup carried out using the GMAW process. The welding parameters are as follows: welding current of 115–130 amps, voltage of 21–22 volts, and angular speed of 2 RPM. The welding is carried out in a single layer to maintain a fillet size of 4 mm. Welding defects such as a lack of penetration could be a problem because the welding is performed on a rotating fixture in a horizontal position [2].
(iii) Weld toe stress concentration:
Because the welding is done in a limited axis position, controlling the weld pool and maintaining the concave weld bead profile becomes difficult. To overcome this problem, it was proposed to carry out a welding experiment with edge chamfering of the interface plate. As shown in Figure 4, the weld profile shape is changed from convex to concave by chamfering the interface plate (5 mm) to 30 and 45 degrees from the vertical plane. After welding experiments at 30 and 45 degrees, samples were extracted to verify if the desired concave profile was achieved without any stress concentration at the weld toe.
Fig 4: Chamfering to interface plate (a) 30 degree (b) 45 degree.
3. Experimental procedure:
A 3mm-thick CR02 sheet with dimensions of 300 x 300mm was used for bead-on-plate welding trials, as shown in Figure 5. Welding parameters are identified by varying the travel speed from 50 mm/min to 500 mm/min to achieve the desired bead width and complete penetration. At maximum travel speed, narrow bead width was observed with limited penetration; at minimum travel speed, for the given thickness, burn through occurred, whereas at a travel speed of 80 mm/ min, wider bead width and optimum root penetration were achieved.
Fig 5: Bead on plate welding experiment using CR02 flat sheets.
By considering above factors, welding experiments on actual components were planned with modified weld angle as shown in the Figure 6 a & b.
Fig 6: Welding experiments (a) Rotating fixture set up used (b) Torch angle details.
The experimental setup consists of a rotating fixture that holds the shaft and cup assembly in the centre of the fixture in such a way that the shaft's centre axis makes a 90-degree angle with the fixture's rotating plane. Also, the welding torch is positioned with an offset of 5 mm from the centre of the rotating plan and makes an angle of 15 degrees to achieve complete weld penetration in the horizontal position. A welding travel speed of 80 mm/min, or 1 RPM, is necessary to control the heat input of less than 1.5 kJ/mm. The modified welding parameters are shown in Table 2. After welding, samples were extracted to understand the effect of welding parameters on the bead profile and metallurgical behaviour, as shown in figure 7.
Table 2: Welding parameters used:
Fig 7: Sample location (a) Welded cup with visible HAZ (b) Cut location covering interface plate cup (c) extracted specimen (d) Cut specimen location from the cup inside.
4. Results
4.1 Failure analysis:
The fractography samples received from the manufacturer did not show any signs of distortion due to potential shaft misalignment during rotation. Clear evidence of the occurrence of a fatigue failure mechanism with dimples on the fracture surface was shown by macro fractographic analysis (Fig. 8). No obvious shaft deformation and a largely flat fracture surface showed that the complicated rotational, in-plane, and reversed bending loading mechanisms were the cause of the operating
Fig 8: Fracture sample received from manufacture – (a) Shaft -cup assembly before fracture (b) fracture location after separating from the cup (c) Dimples on the fracture surface.
fatigue. Torsional overload and fatigue failures occurred in agricultural machinery such as baler cup shaft assemblies that were subjected to a variety of complex stress states, such as the transmission of rotational motion or acting as cantilever beams enduring high bending moments.
4.2 Welding analysis:
The modified welding parameters' macrographs show complete weld penetration towards the cup side as well as the interface plate side, with no weld defects. An approximate 2 mm-wide heat-affected zone was observed on the cup and plate sides. As shown in Figure 7, the chamfered edge samples with 30° and 45° showed better bead quality with a concave profile.
Fig 7: Weld bead profile (a) without chamfering (b) 30o chamfering and (c) 45o chamfering.
To understand the weld zone properties, microhardness plots were generated from the centre of the weld location to the cup side and to the interface plate side. Figure 8, depicts the specifics. In the case of the interface plate material, E250, the hardness values were distributed from the fusion zone to the base material from the maximum value of 250HV to the minimum value of 150HV. Whereas in the case of cup materials, i.e., CR02, the hardness values were very low, up to 120 HV, compared to the fusion zone. Also, an abrupt transition of the microhardness plot from the HAZ to the fusion zone was observed. However, the weld heat input has not influenced the HAZ in both the cup and plate, hence the hardness values were not changed between the HAZ and base material.
Fig 8: Microhardness plots across the plate (E250) and cup (CR02) weld zones.
Further metallurgical characterization studies were carried out on both cup- and plate-welded samples. Widmanstatten ferrite was observed in the fusion zone in both cup and plate samples. At the same time, polygonal ferrite was observed in the HAZ. This may be the reason for the sharp transition of hardness values from FZ to HAZ. The details of the microstructure are shown in Figure 9.
Fig 9: Optical micrographs of the weld interface zones at 100x (a) cup side (b) plate side.
5. Discussion:
According to the fractography results, the crack begins adjacent to the weld zone and spreads from the heat-affected zone to the cup base material side. Fatigue crack in the present case is clearly initiated from cup plate weld toe and propagated to the interior forming a characteristic macroscopic smooth surface pattern [4]. Evidence of finely defined beach marks can be seen in the surface topography as concentric circles (Fig. 4). Beach marks or crack arrest marks are indications of the propagation of a crack front caused by intermittent loading or the emergence of a compressive stress condition prior to the crack tip towards the weaker zone [5].
Even though the modified welding parameters provide improved weld quality with complete fusion and without any stress concentration at the weld toe, there is a significant difference between the microhardness values observed from the cup material to the weld zone. On the other hand, a smooth transition in the hardness values was observed between the interface plate material and the weld zone (refer to Fig. 8). Hence, it clearly shows that material compatibility could be the main reason for the failure. According to the functionality of the cup shaft assembly, the shaft is designed to withstand torque loading of up to 640 N-m. But as the welded cup material is unable to take up such torque loading, this leads to crack initiation during the component service stage.
Accordingly, an alternative grade of IS 1079 HR3 was proposed for the cup material, with deep drawing quality also to match the hardness values of the interface plate and the weld zone. The properties of the IS 1079 HR3 grade are shown in Table 3.
Table 3: IS 1079 HR3 and E250 grade properties. The bead on plate welding experiments were repeated to verify the hardness values in the heat affected zone. The results are shown the figure 10.
Table 3: IS 1079 HR3 and E250 grade properties.
Fig 10: Microhardness plots across HR03 weld zone.
Fig 11: HR03 welded sample (a) Bead on plate macrostructure (b) at 100x Weld zone with acicular ferrite (c) at 100x HAZ (d) at 100x Base material polygonal ferrite.
As shown in Figure 9, the hardness values were distributed from the fusion zone to the base material from the maximum value of 250 HV to the minimum value of 120 HV. At the base metal location, the hardness values were very low, up to 120 HV, compared to the HAZ and fusion zone. The microhardness plot, however, did not abruptly transition from the HAZ to the fusion zone. A smooth transition of the hardness profile between the HAZ and the fusion zone indicates good material compatibility with the interface plate properties [6]. The details of the heat-affected microstructure are shown in figure 11(c), which represents a combination of polygonal ferrite and acicular ferrite with average hardness values of 175HV. As compared to the CR 02 grade, the proposed HR 03 grade also has PF and pearlite in its base material. But CR02 grade has lower hardness (100 HV max) than HR3 (125 HV), as HR03 grains are finer than CR02 grains. This may be because of the grain growth stage during annealing in CR02.
6. Conclusion:
1. A detailed fractography analysis was performed on the actual baler cup shaft samples, and no shaft deformation was observed; in this case, it is identified as a fatigue crack with visible dimples on the surface, which clearly initiated from the cup plate weld toe & propagated to the cup heat affected zone.
2. A modified welding procedure proposed to enhance the weld accessibility on the rotation fixture with the welding torch is positioned with an offset of 5 mm from the centre of the rotating plan and makes an angle of 15 degrees to achieve complete weld penetration in the horizontal position. A welding travel speed of 80 mm/min, or 1 RPM, is necessary to control the heat input of less than 1.5 kJ/mm. To avoid the stress concentration at the weld toe, a chamfering angle of 45o can be used.
3. It was observed that, material compatibility in the assembly is the main reason for the failure, even though as the existing CR02 is weldable material it has lower hardness values compared with the adjacent interface plate, hence an alternative grade HR03 was proposed, which has improved drawability, weldability and better hardness values.
4. This study also concludes that material selection plays an important role in agricultural equipment, where the equipment manufacturers and steel suppliers need to understand the final application of the components and design the assemblies accordingly to eliminate the cost and delay of the failure analysis. Further, it provides an opportunity for a more economical design, a better material, or a more efficient fabrication procedure.
References
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2. Morel, M.K., Robots Weld Round Baler Rolls at New Holland Plant, in AUTOMATION NEWS & RESOURCES. 2008, Association for Advanced Automation.
3. Pense, A.W., Failure Avoidance in Welded Fabrication. National Board BULLETIN, 1988.
4. Serje, D.A., E.E. Niebles, and S.K. Lascano, Failure Analysis of Fan Motor Shafts of a Tunnel Dryer. Journal of Failure Analysis and Prevention, 2018. 18(5): p. 1053-1061.
5. Das, S., G. Mukhopadhyay, and S. Bhattacharyya, Failure analysis of axle shaft of a fork lift. Case Studies in Engineering Failure Analysis, 2015. 3: p. 46-51.
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