1) The end face of the welded steel bar is unqualified. For example, the gap between the two steel bars is too large, and there are debris on the end face. 

 (2) The flame is not adjusted well during the welding process, the carbonization flame is not used in the initial heating, or the flame violates the crimping surface, forming the appearance of oxidation of the end face. 

 (3) During the welding process, the flame heating time is too long, and many carbon molecules are burned or the heat-affected zone is too large, and the grain thickens, which reduces the strength of the steel bar. 

 (4) If the heating temperature is too low or the heating time is too short, the base of the steel bar connecting to the steel bar will start to be pressurized before the welding temperature is reached. It is because some molecules at the base of the welding end face cannot be fully immersed and dispersed to form a weak strength. 

 (5) In the late stage of heating, the heater moves back and forth, forming uneven heating of steel bars. 

 The characteristics of the rebar connecting sleeve and the precautions during construction：

 1. The steel connecting sleeve can greatly reduce the use of materials, and it is easy to manipulate, and it is not limited by the composition of steel bars. 

 2. The steel connecting sleeve can be prefabricated in advance, and different specifications can be formulated according to needs. It does not occupy the construction period and has high processing efficiency. 

 3. It can connect horizontal, vertical and oblique HRB335, HRB400 steel bars of the same diameter or reduced diameter. It is suitable for steel bar connectors in steel concrete structures of various structures that withstand the two-way forces of tension and pressure. 

 4. The connection is convenient, fast and simple. The construction does not require electricity, rain or shine, and can be constructed all day long.