Abstract: Forming and heat treatment process of high strength 4145 steel was developed, but two types of defects were resulted during the producing and research processes. The macro and micro characteristics of the defects were compared, the causes were analyzed, and countermeasures were proposed. The results show that the first type defect is the folding caused by the poor surface quality of rolling tools in the rolling forming process, which forms a certain angle with the axis direction of the steel pipe, the depth is about 0.9 mm, and with the decarburized layer thickness of 0.090-0.157 mm. The second type defect is perpendicular to the axis of the steel pipe, cracking through or the depth exceeding 2 mm, and the thickness of the decarburized layer is not more than 0.1 mm, which is determined as the quenching crack caused by rapid quenching cooling rate. Then the rolling defects are avoided by controlling the quality of rolling forming tools, and quenching crack defects are solved by optimizing the chemical composition and quenching process. After testing and verification, the quality of the steel pipe produced by the improved measures is qualified.

Key words: high strength 4145 steel, hot rolling forming, heat treatment, microscopic characteristics, decarburized layer, quenching crack