52CrMoV4 Leaf Spring Evolution: Fatigue Life Improvement

Transactions of the Canadian Society for Mechanical Engineering, Vol. 00, No. 0, 0000 1
EVOLUTION OF 52CRMOV4 FROM 51CRV4 MATERIAL TO WITH STAND FIELD SEVERITY OF
PARABOLIC LEAF SPRING SUSPENSION IN HEAVY DUTY COMMERCIAL VEHICLE
P.Thangapazham1), L. A.Kumaraswamidhas1) *, D.Muruganandam2) T. Dharanivendhan3)
1) Dept of Mining Machinery Engg, Indian Institute of Technology (ISM), Dhanbad, 826004, India.
2) Dept of Mechanical Engg, Jeppiaar Institute of Technology, Chennai-631604, India.
3) Dept of Mechanical Engg, Indian Institute of Technology, New Delhi-110016, India.
E-mail : [email protected]
Received July 2018, Accepted December 2018
No. 00-CSME-00, E.I.C. Accession Number 0000
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ABSTRACT
The investigative study is mainly focused on improving the fatigue life of leaf spring through
the following protocols. Protocol-1: The parabolic leaf spring is manufactured with 51CrV4
material through the normal production process, which results in low residual compressive stress
and high decarburization. The proto sample does not support severe field application. This issue
is resolved by optimizing heat treatment and shot peening process. The proto part is prepared
and tested in the Rough Road conditions and the vehicle withstands the field severity up to 10%
higher than the design load. However, in highly severe field operation, the severity goes up to
30% higher than design load. Hence, the above process improvements could not sort out the
failures of 51CrV4 material. An alternate material is identified 52CrMoV4 and investigated in
the protocol 2: In this investigation, the spring proto part is manufactured directly through
optimized process. The residual compressive stress, decarburization and mechanical properties
are obtained at the desired levels. The proto part is tested in Rough Road conditions; the
suspension system can now withstand 30% of field severity. The vehicle was tested in the test
track and covered 335 thousand Kilometers in off road distance, with met durability
requirements.
Keywords: Fatigue life, Residual compressive stress, Hardness, Microstructure and Tensile strength.
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RÉSUMÉ
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résumé principal sont en français. Par contre, il ne faut pas traduire le texte « Received…Accession
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1. INTRODUCTION
In mining applications, parabolic leaf spring suspension system is mainly used with high strength
materials. The vehicle under consideration has two front axles, which are fitted to the chassis with
parabolic leaf spring suspension system. The axle supports the steering components. The main function
of the suspension system absorbs the impact forces and vibrations and reduces its impact to chassis
frame. A double acting shock absorber is attached with a U-bolt clamping in order to dampen the
vibration. The other components include the anti-roll bar is mounted between the front axle and the
vehicle frame to protect it from the vehicle rollover behavior. The individual leaf of the multi leaf
suspension system is in the shape of a parabolic with variable thickness from center to both ends to
achieve optimum spring rate and stress distribution. The rear side of vehicle, it is fitted with an anti-roll
bar for better control, stability and reduce roll angle. The general overview of the parabolic leaf spring is
shown in Fig.1.
Fig.1. Parabolic leaf spring suspension overview
The microstructure of a leaf spring greatly determines the fatigue life of leaf spring, which can be
attained by appropriate surface treatment (Fragoudakis et al. 2017). Normally, the surface treatment by
shot peening as well as heat treatment improves the mechanical properties and fatigue life of the
components. The first evaluation of parabolic leaf spring made of 51CrV4 is chosen in this investigation
and it is one of the conventionally used spring steel for heavy-duty commercial vehicle suspension
system. The main merits of parabolic leaf spring used in suspension system are high fatigue strength and
lightweight (Soner et al. 2011). The processes like quenching, tempering and shot peening are important
to sustain the amount of residual compressive stresses in the component (Todinv 2000). The leaf spring
is manufactured using 51CrV4 material by normal shot peening and heat treatment process, the residual
compressive stress increases only up to 310 MPa and decarburization level is found to be 0.30mm. The
achieved values of mechanical properties are not sufficient to withstand the field severity and hence the
surface treatment by shot peening and heat treatment process were optimized. Specifically, the tensile
strength of material increased up to 1535 MPa in 51CrV4 and 1720 MPs in 52CrMoV4. Higher
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intermolecular attraction is achieved over the entire component through tempering process. In this
process unwanted gases come out of the component, which enables finer grain size. Therein, the
component changes from tempered maternsite to fine martensite with good intermolecular attraction. The
proto model is prepared with material of 51CrV4 and tested in Rough Road conditions; no premature
failure occurred and withstood field severity of 10%. However, in mining application, the filed severity
goes up to 30% extra of the design load. The failure in a leaf spring normally occurs based on the
following factors-low quality of material, poor design, and poor manufacturing techniques (Fuentes et al.
2008). The leaf spring ensured higher level of safety during dynamic and static conditions (Soner et al.
2013). The first material of 51CrV4 did not support the required field severity conditions. To sort out this
issue, an alternate material of 52CrMoV4 was identified. The new material was used to manufacture the
parabolic leaf spring in the optimized process. The results shows that the induced residual stress reaches
max of 744 MPa, decarburization level reduces to 0.0781 mm, desired levels of micro structure and
hardness are also obtained and the tensile strength of material increases to 1720MPa with percentage of
elongation of 6.5%. Using the material of 52CrMoV4, the proto model is prepared and tested on Rough
Road conditions. Metallographic Microscope measures the decarburization level, Brinell hardness testing
machine measures hardness, residual compressive stress is measured by X-Ray Diffractometer and
Universal Testing Machine measures the tensile strength. To ensure durability, the components were
tested for success until particular expected life cycles. The damage calculation results were used to
evaluate the durability of the component. The durability limit set in terms of kilometers run on Rough
Road conditions of the test track, has an established co-relation to actual field operation life expected. No
premature failure occurs in normal load and system withstands the field severity of 30% of more than the
design load and the vehicle does not fails after completing 335 thousands kilometers equivalent operation
in normal off road conditions.
2. FAILURE LOCATION
The failure of leaf spring is mainly due to field severity. The field severity is nothing but the vehicle’s
ability to withstand load more than the design load. The following are various types of field severity that
affect the failure of leaf spring-loading material on the scoop area, overloading in the load body, vehicle
moving in downward gradient transfer of rear load to front axle pot holes or hair pin bend in which the
left side load transfer to right side and vice versa. In rough road conditions, the vehicle is usually
operated in controlled speed and load. In actual mining sites, the operation of the vehicle is done under
poor terrain conditions and they are overloaded with heavy rocks.
Fig.2a. Field severity withstand 51CrV4 &52CrMoV4
Sample tested -100 Nos
Mining Truck- 93 Nos
Failures in normal shot
peening-%1CrV4 spring
51CrV4
30% withstand field
severity 52CrMoV4 Spring
Optimized
10% withstand field
severity 51CrV4 Spring
Optimized
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From the Fig.2a, it can be inferred that the same vehicle part, which achieved target durability life in
most of operational, sites but failed in few. In these sites, the vehicle is operated with varying
acceleration and deceleration. Also during loading in mining operation, the canopy above the truck’s
cabin that is meant to protect the cabin from rock falling is also abused by loading with material
inadvertently. This additional load on front of vehicles is transferred to first front axle Fig.2b.
Fig.2b. Overloading in Scoop and normal area
In addition to above, the parabolic suspension system also withstands the vertical, horizontal forces and
lateral forces acting on the body. The major load on the axle is the vertical load due to the weight. An
additional vertical load also acts on the components during braking, since the entire load tends to shift to
the front axle due to the inertia. According to the field data, the suspension system fails at two locations;
at the eye end and at the main leaf behind U-bolt area, indicated as 1 and 2 respectively. From the Fig.3,
Fig.3. Field severity -51CrV4 Spring Failure Location
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