When the carbon seamless steel pipe is cold drawn, there are
defects such as cracks in the hot rolled tube blank. For example,
after the high-precision cold-drawn tube is made into an oil
cylinder, the fracture that occurs during use has almost no plastic
deformation, and is generally brittle fracture. Brittle fracture is
caused by a variety of reasons. For example, when there are
precipitates on the grain boundary, no matter their strength is
stronger or weaker than that of the matrix, it is the cause of
cracks; the segregation of inclusions on the grain boundary is also
the cause of fracture; in addition, even at the intersection far
less than the yield limit Under the action of variable load,
fatigue fracture will also occur.
Generally speaking, the mechanical performance indicators used in
the design of hydraulic (pneumatic) parts assume that the material
is uniform, continuous and isotropic. According to this method, the
design is considered to be safe, and sometimes accidents may occur.
breakage accident. It has been found that during low-stress brittle
fracture in high-strength metallic materials, the material
structure is far from homogeneous and isotropic. There will be
cracks, inclusions, pores and other defects in the structure, and
these defects can be regarded as micro-cracks in the material.
cold drawn seamless steel pipe
In addition, brittle fracture is also related to the service
temperature of the component. Through research, it has been found
that when the temperature is lower than a certain temperature
value, the material will transform into a brittle state, and its
impact absorption work will decrease significantly. This phenomenon
is called cold brittleness. Therefore, the design should also be
based on the working temperature of the component. Choose a
material with a suitable cold-brittle transition temperature.
At present, the material of cold-drawn seamless steel pipe (a53
seamless) in my country is ordinary 45# or 20# steel 27SiMn steel,
which improves the metal strength through cold-drawing deformation;
however, it is at the expense of sacrificing the plasticity and
toughness of the metal. High-precision cold-drawn seamless steel
pipe is based on its high dimensional accuracy and high-strength
performance. It must ensure that the amount of deformation is
within a certain range in order to maximize the performance of the
material and reduce the adverse effect on the material.
The deformation of the cold-drawn seamless steel pipe is too small
to meet the requirements of surface finish and dimensional
accuracy, and the strength index of the component cannot be
achieved; if the deformation is too large, the plasticity and
toughness of the seamless steel pipe will be reduced too much, and
the grains will be pulled too much. Elongated, forming a fibrous
structure, the metal will have obvious anisotropy. The axial
direction of the cold-drawn seamless steel pipe is parallel to the
elongation direction of the grain, and the strength increases; the
radial direction of the cold-drawn seamless steel pipe is
perpendicular to the elongation direction of the grain, and the
strength decreases instead, and the maximum stress of the hydraulic
cylinder Existing in the radial direction of the seamless steel
pipe, so too large deformation is unfavorable to give full play to
the performance of the cold drawn pipe.
For the fiber structure and crystal defects such as dislocations
and vacancies in the steel, heat treatment methods such as
annealing or normalizing are generally used to eliminate them. The
purpose of annealing is to refine grains, eliminate structural
defects, reduce hardness, improve plasticity, and facilitate cold
working. In the production of cold-drawn seamless steel pipe,
annealing equipment is indispensable. Therefore, formulating a
suitable annealing process is a necessary condition to ensure that
the cold-drawn seamless steel pipe is qualified and the product
does not have fracture defects. If the manufacturer reduces the
annealing process in order to unilaterally reduce the production
cost, it will definitely have an adverse impact on the product
quality.
