The key points of spray welding process for iron castings

       

The key points of spray welding process for
iron castings are as follows

① There are quite a lot of parts made of
cast iron in vehicles and mechanical equipment. It is inevitable that there are
various problems in the process of manufacture or use. Adopting oxygen
acetylene flame spray welding process is not only an effective method to
strengthen cast iron, but also an ideal means to repair various defects and
damages (such as sand holes, air holes or wear and other damages in use) of
castings;

② Spray welding is mostly used to repair
local defects of iron castings, and the defects are different in size and
depth, so it is suitable for one-step spray welding. According to the workpiece
and repair parts, small power spray gun should be used as far as possible, so
as to reduce the heat input to the matrix. Generally, QH-1 / h, QH-2 / h, qh-4
/ h, etc. are selected;

③ In spray welding, because of the poor
weldability of cast iron parts, nickel based alloy powder (ni-b-si Series)
should be preferred. The melting point of nickel based alloy powder is
generally 950 ~ 1050 ℃, and the matrix will not be melted during remelting. At
the same time, the spray welding layer of nickel based powder with low carbon
content has low hardness and good plasticity, which can relax the spray welding
stress and prevent cracks. At this time, it is very important to operate
unskilled;

④ When selecting the spray welding
specification, the material of cast iron, the size of defects and the
requirements of working conditions should be considered. On the premise of
ensuring the necessary flame energy, the heat input to the matrix should be
reduced as far as possible. The lower limit of oxygen and acetylene gas
pressure should be taken. During powder spraying and remelting, the spray and
melting distance should be adjusted appropriately to control the heat input;

⑤ Small local defects, such as air holes
and sand holes, may not be preheated before spray welding, so as to minimize
the heating area around the spray welding layer and minimize the high
temperature area.

⑥ Spray welding is suitable for continuous
spray welding for local small and deep defects. This method has high powder
deposition rate, rapid thickness growth and high efficiency, but requires
skilled operation, so as to coordinate powder feeding rate with spray and
fusion speed, and achieve uniform spray and penetration;

⑦ For the defects with large area and deep
depth, in order to prevent the matrix from being heated too much and causing
the thermal stress to increase, the intermittent spray welding method can be
used. If necessary, the composite process of electric welding and spray welding
can also be used. The welding rod can be used to fill the bottom and spray
welding the upper part. If the wall thickness of the casting is large, the wire
can be planted before spraying, which can not only increase the bonding
strength, prevent the peeling of the spray welding layer from the base metal,
but also eliminate part of the thermal stress of spray welding;

⑧ The thermal stress of spray welding large
complex castings is large, so the measures should be taken to reduce the stress
accumulation, such as heating stress reduction method and sectional symmetry
method; Preheating before welding and slow cooling after welding can achieve
good results;

⑨ If the remelting temperature is too high,
not only the alloy elements will be burned, the matrix will be overheated, or
even the matrix will be melted, resulting in the carbon in cast iron entering
the spray welding layer, which will increase the carbon content in the spray
welding layer, improve the hardness, and reduce the plasticity. Because the
carbon content in the matrix is reduced, it is easy to appear white mouth.
Moreover, if the temperature is too high, it will cause greater thermal stress,
leading to cracks. The remelting temperature should not be too low, Otherwise,
it is easy to form ash inclusion or incomplete fusion, which will affect the
bonding strength. The control of the remelting temperature mainly depends on
the operator's attention to observe the change of the coating surface state
during remelting. With the help of the "mirror reflection" during
remelting, the "mirror reflection" indicates that the powder has
melted and the slag has floated up. At this time, the remelting gun should be
removed immediately, and the remelted part should avoid repeated heating.

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