Crankshaft is an important part of the car
engine, both in the car crankshaft R & D or actual production stage, need
to test the fatigue performance of the car crankshaft. In the actual operation
of the car engine, the crankshaft to bear the form of the load for the bending
and torsional load, so the actual case shows that the main form of failure of
the car crankshaft fatigue damage. In addition, if the engine crankshaft output
power is large, the greater the torque generated, will produce a certain degree
of torsional fatigue damage. Early automotive engine crankshaft used by the
material for the 40Cr and 42CrMo and other carbon quenched and tempered steel.
However, since the late 1990s, a large number of automotive crankshaft began to
widely use non-quenched and tempered steel, especially for heavy-duty engine
crankshaft production, the materials used mainly include 48MnV and C38N2, these
two materials and medium carbon Compared with the quenched and tempered steel,
in the case of ensuring the same strength, a lot of conditioning process is
omitted. In recent years, the use of ductile iron production of engine
crankshaft has better mechanical processing performance and bending
fatigue strength, especially in the use of heavy-duty engine to show a good
application prospects.
Automotive crankshaft in the design, the
choice of raw materials, forging, machining and heat treatment and surface
strengthening of all aspects, if any one part of the mistakes will cause the engine
crankshaft failure. A company combined with the actual bearing
production case, introduced and analyzed the car engine crankshaft common
failure form and internal reasons.
The factors that affect the fatigue
strength of the crankshaft of the vehicle mainly include the stress
concentration, the size, the surface condition and the environment medium at
the journal. In these factors, stress concentration and size and surface state
are most important. The factors that affect the stress distribution of the
crankshaft mainly include the design shape of the crankshaft and the basic
properties of the material. The bending fatigue strength of the crankshaft can
be improved by changing the kind and size of the material, but this will
greatly increase the manufacturing cost of the material. In the actual
application process, the surface strengthening process is usually used to
improve the fatigue strength of engine crankshaft.
Under the effect of variable load, the maximum stress should appear in a range
near the rounded corners of the crankshaft due to the abrupt change in the
shape of the crankshaft journal Inside. Research indicates:
(1) Fatigue test of automobile crankshaft
The fatigue limit of crank moment is 96.7MPa, the crankshaft stress
concentration coefficient is 6.72, and the fatigue test process is in the
middle of crankshaft journal. The safety factor at 99% was 1.31.
(2) SEM microstructure analysis of the
microstructure and crack fracture of the 40Cr crankshaft journal shows that the
microstructure after 40Cr quenching and tempering is tempered sorbite + ferrite
+ few bainite, conforms to the relevant standard The Fracture analysis shows
that the 40Cr crankshaft fracture is a typical dimple and step mixed fracture,
and no obvious inclusions.
(3) From the chemical composition and
tensile properties of the material and the hardened layer of the journal
surface, it can be seen that the reason of the fatigue failure of the
crankshaft can exclude the influence of the above factors.
(4) crankshaft drilling hole in the journal
internal wall caused by stress concentration, more prone to defects; crankshaft
journal under the action of alternating stress is prone to fatigue cracks and
expansion, resulting in early fracture of the crankshaft.
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