Weldability studies on steel grades used in agriculture equipment

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Ashok Kumar.Sanjana Vadapalli 3Ravi Thakur 4Bijoy Rajak SSuryanaraya 6Kanwer Arora
‘Principal Researcher, 2Manager, PAG, 384 Ræarcher, SHead PAG 6Head MWJ group
Research and Development, Tata steel, Jharkhand,
Email: ashok.pa@tatasteel.com ; Ph: +91-7763807009

Abstract:
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 characterization results show that the microhardness of the cold-
rolled 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 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 intemal 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.

I. 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 for the
foreseeable future due to the varying growth of agricultural mechanization and the various

operations and scales of agricultural output [II. A compact set of essential lawn mower
supplementary items, the tractor-driven 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

Material selection and its weldability finds an place in the design of this type of
agricultural equipment, and how steel is welded (2n have a significant impact on its application
and service life. Hence, the 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 analvsis:
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 cnllected 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 cause for the failure [31. The failure cause could be any of the
following factors:
(i ii) Material compatibility
Weld parameters
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:

Further, both material properties after welding and compatibility with the adjacent weld and
heat-affected zone were characterised using microhardness tests.
(ii) Weldinq 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 be a problem because the welding is performed
on a rotating fixture in a horizontal position
(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 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 cnncentration at the weld toe.

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 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.

By considering above factors, welding experiments on actual components were planned with
modified weld angle as shown in the Figure 6 a & b.

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.

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 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.

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
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.

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.

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
and 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.

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