Cause analysis of surface micro-cracks on 12Mn pipeline steel

¹ School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, China
² National Engineering Research Center of Continuous Casting Technology, Central Iron and Steel Research Institute, Beijing, China

^* Corresponding author: yushi_tian@tom.com

Abstract

In order to find out the causes of defects in 12Mn pipeline steel produced by a Chinese steel pipe plant, the characteristics of the defects are analyzed and discussed by means of metallographic microscope, scanning electron microscope, ASPEX scanning electron microscope and transmission electron microscope and so on. The physical and chemical tests results show that there are several typical micro-cracks after slight grinding the pipe outer skin in which a few large particles of brittle alumina reside, which possess the macrocharacteristics of continuously curving and extending along the radial direction as well as many clear and smooth inner edges and few secondary branches, so that the cracks are not experienced a long and hightemperature oxidation process. The industrial tests indicate that most of the defects have been proved occurred in the quenching and tempering process of the steel pipe production. The main cause of steel pipe cracking is the stress concentration arose from some large brittle inclusions in the quenching process. When the critical strength of the steel pipe is exceeded by quenching stress, a micro-crack will generate and propagate along the grain boundary. These defects of steel pipes could be eliminated by means of taking steelmaking process optimization measures.