Causes of surface quenching crack, torque overrun and hydrogen
embrittlement of fasteners and improvement measures
Fasteners are a kind of mechanical parts used for fastening and
connection and are widely used.
Fasteners are widely used in all walks of life. All kinds of
fasteners can be seen in all kinds of
machinery, equipment, vehicles, railways and so on. They are one of
the most widely used
mechanical basic parts. It is characterized by a wide variety of
specifications, different performance
and uses, and the degree of standardization, serialization and
generalization is also very high.
Once the fastener fails, it will cause serious impact. Therefore,
it is necessary to strengthen the
analysis of the causes of fastener failure and find the
corresponding improvement measures.
Based on the knowledge of fasteners, China standard parts network
shares the following:
1. Surface quenching crack
Surface quenching crack refers to the crack produced in the process
of quenching or placing at
room temperature after quenching, which is also called aging crack.
In the process of quenching,
when the stress produced by quenching is greater than the strength
of the material itself and
exceeds the plastic deformation limit, it will lead to cracks.
Quenching cracks often occur shortly
after the beginning of martensitic transformation. The distribution
of cracks does not have a certain
law, but they are generally easy to form at the sharp corners and
abrupt changes of section of the
workpiece. Quenching cracks caused by too fast cooling in the
martensitic transformation zone are
often transgranular distribution, and the cracks are straight, with
no branching small cracks around.
Quenching cracks caused by too high quenching heating temperature
are distributed along the grain,
with sharp and fine crack ends and overheating characteristics.
Coarse acicular martensite can be
observed in structural steel and eutectic or angular carbides can
be observed in tool steel. The high
carbon steel workpiece with decarburized surface is more likely to
form network cracks after quenching,
because the volume expansion of the surface decarburized layer
during quenching and cooling is
smaller than that of the non decarburized core, and the surface
material is pulled and cracked into
a network due to the expansion of the core. Quenching cracks on the
surface can cause sudden
fracture of bolts, and the fracture source of such fracture is
located on the surface.
2. Torque overrun
Torque alarm often occurs in the bolt assembly process of
controlling torque by angle method.
Failure modes and causes of fastener torque overrun include:
(1) After assembly, the final torque of the parts is higher than
the upper control limit or lower than the
lower control limit. The reason is that the assembly torque control
range of parts is unreasonable,
which is manifested in that the set control range is too small and
the control range deviates upward
or downward.
(2) If it is not pre tightened to the preset angle, the torque
reaches the upper limit alarm. The reason
is that the friction coefficient of the parts exceeds the upper
limit, the matching friction coefficient of
the parts exceeds the upper limit, and the interference between the
parts causes a sharp increase
in the assembly torque.
(3) Normal installation, lower torque limit alarm. The reason is
that the friction coefficient of the part
itself exceeds the lower limit or the matching friction coefficient
of the part exceeds the lower limit.
When the part is screwed in, the fitting torque is greater than the
initial torque (that is, the screw in t
orque consumption is too large). It is common to tighten the lock
nut.
3. Hydrogen embrittlement
Fasteners are prone to hydrogen embrittlement, which is the main
cause of fastener fracture.
Hydrogen embrittlement is a phenomenon when hydrogen atoms enter
and diffuse into the whole
material matrix. When hydrogen atoms enter the material matrix, the
material matrix produces lattice
distortion, destroys the original equilibrium state, and is easy to
crack under external force. When the
external load is applied to the screw, the hydrogen atoms migrate
to the high stress concentration area,
resulting in great stress between the crystal boundary edges,
resulting in the fracture between the
crystal particles of the fastener. When the fastener contains
critical hydrogen before installation, it will
break within 24h. When hydrogen enters the fastener, it is
impossible to predict when fracture will occur.
4. Improvement measures
4.1 measures to prevent surface quenching cracks:
(1) Reasonably adjust the gap between the induction quencher and
the workpiece, select appropriate
medium frequency power supply parameters and quenching process
parameters in strict accordance
with the process requirements, ensure uniform temperature rise of
the product circumference, and
prevent local temperature from exceeding the normal quenching
temperature.
(2) Improve the quenching inductor structure, change the circular
section structure of the upper end
and tail end of the inductor into a rectangular section structure,
reduce the heating speed of the end
and tail inductor, and prevent the rapid temperature rise of the
end and tail from exceeding the process
control temperature and over burning, resulting in cracks.
(3) Reduce the number of conductive magnets of the quenching
inductor in the transition area after
quenching, and appropriately reduce the heat there.
(4) The preheating heating cooling quenching method is adopted to
make the heating temperature of
the product uniform.
(5) Appropriately extend the delayed cooling time after medium
frequency heating.
(6) Implement self tempering. In strict accordance with the process
technical parameters, reasonably
control the quenching coolant pressure, flow, temperature and
cooling time. After stopping the liquid
spraying, use the waste heat of the workpiece to recover the
temperature of the hardened layer, so
as to carry out self tempering, so as to maintain high surface
hardness and good wear resistance,
timely stabilize the quenched structure and reduce the peak tensile
stress.
4.2 torque system
Torque control method is to screw the bolt to a small torque,
generally 40% ~ 60% of the tightening
torque (formulated after process verification), and then screw a
specified angle from this point. This
method is based on a certain angle, which means that the bolt
produces a certain axial elongation
and the connector is compressed. The purpose of this is to screw
the bolt to the close contact surface,
and overcome some uneven factors of surface concavity and
unevenness, while the required axial
clamping force is generated by the rotation angle. After
calculating the rotation angle, the influence
of friction resistance on axial clamping force no longer exists, so
its accuracy is higher than that of
simple torque control method. The key point of torque control
method is to measure the starting point
of rotation angle. Once the rotation angle is determined, quite
high tightening accuracy can be obtained.
4.3 prevention measures of hydrogen embrittlement
(1) Normal electroplating and strict hydrogen removal. Using the
reversibility of hydrogen in metal to
remove hydrogen from electroplated bolts is an important method to
slow down or eliminate hydrogen
embrittlement. During treatment, the plated steel bolts are placed
in the oven for heating. The baking
temperature is about 200 ° C, and the baking time varies according
to the strength of the steel. The
higher the strength, the longer the baking time. The hydrogen in
the bolt material forms hydrogen
overflow at high temperature to achieve the purpose of hydrogen
removal.
(2) Low hydrogen embrittlement electroplating. Low hydrogen
embrittlement electroplating is a process
developed in the 1960s and 1970s for the study of hydrogen
embrittlement of aircraft parts, including
low hydrogen embrittlement cadmium plating, low hydrogen
embrittlement cadmium titanium plating,
low hydrogen embrittlement zinc plating, etc. Low hydrogen
embrittlement electroplating requires stress
relief tempering before plating. Instead of pickling with strong
acid, sand blasting is used to remove
oxide scale and surface dirt, or vacuum heat treatment is used to
avoid oxide scale. In the electroplating
process, on the one hand, adjust the bath formula, on the other
hand, reduce the voltage, strictly control
the current density and reduce the adsorption of hydrogen
particles. The subsequent process also
requires strict baking hydrogen removal, and the hydrogen removal
time shall be at least more than 18h.
Stainless Steel Adjustable Feet Parameter
Stainless Steel Adjustable Feet Size
Stainless Steel Adjustable Feet Material
| Name | KTC Foot |
| Model | KTF-PR |
| Type | Light Duty Purpose Series |
| Application | Conveyor, Automatic facilities, Light duty equipment |
Stainless Steel Adjustable Feet Describe
Application.
Conveyor, Automatic facilities, Light duty equipment.
Stainless Steel Adjustable Feet Scope of application
About us.
KTC (Bejing Korea Technology Caster Co., Ltd.) was founded in 2007.
KTC is a modern
company specialzingin the production of multi function &high
quality CASTER and
KT FOOT based on best design and design technology.
KTC.
The company is headquartered in Niulanshan Industrial Zone,Shunyi
District, Beijing. KTC
adopts international advanced production testing equipments,
professional intelligent
production management system and professional design flow. Now the
company produces
multi function & high quality production, such as KT CASTER and KT
FOOT, among which
high quality of height adjusting CASTER,shockproof type CASTER,
high quality of height
adjusting CASTER, shockproof type CASTER,high load CASTER and level
adjusting FOOT
are widely used in the world's advanced automation equipment and
semiconductor e
quipment industry.
In order to manufacture the world's highest level of products,KTC
use the best processing
technology and equipment. And KTC continuously launches world top
level of new products.
KTC's s also has set up a branch in Korea, so it is favored by
theusers of various fields around
the world.
