Common problems in welding
(1) White mouth structure after welding
It is generally believed that under the conditions of rapid cooling of 30-100°C/s, the carbon melted in the iron is too late to precipitate in the form of graphite, but appears as cementite, which is the so-called white mouth . The white mouth structure is hard and brittle, making the weld difficult to machine after welding, and may even cause cracking.
Preventive measures:
Preheat before welding and heat preservation after welding; appropriately adjust the chemical composition and cooling rate of the filler metal; improve the chemical composition of the weld metal.
(2) Cracks appear in welded joints
Due to the extremely poor plasticity of gray cast iron, almost no plastic deformation can occur, and the strength is low, so under the combined action of welding stress and the stress of the casting itself (structural stress), when the local stress is greater than the strength limit, cracks will occur.
Preventive measures:
A. Use arc cold welding to reduce welding stress
Choose welding materials with good plasticity, such as nickel, copper, nickel-copper, high-vanadium steel, etc. as filler metals, so that the weld metal can relax the stress through plastic deformation and prevent cracks; hammering the weld can eliminate stress and prevent cracks; Allow the weld to shrink freely without hindrance when cooling, so as to avoid excessive force and cause cracks.
B. Use heat welding and control the temperature
When the temperature is higher than 600℃, due to certain plastic deformation, some internal stress is eliminated. Generally, thermal stress cracks will not occur when welding above 600℃.
