1. Surface defect-induced quenching cracks
When a certain unit rolls 26CrMo4s alloy structural pipes, small quenching cracks often appear on the inner wall. Photos of the polished micromorphology show that there are many surface defects such as pits and warping with a depth of no more than 0.2mm on the inner wall of the pipe. These defective parts produce stress concentration under the action of quenching stress and become the cause of quenching cracks. Surface defect-induced quenching cracks mostly occur in small-diameter, thin-walled steel pipes. On the one hand, the rolling elongation of small-diameter steel pipes is large, and original defects such as pits and scratches are prone to appear on the inner and outer surfaces of the pipe. At the same time, during the quenching and cooling process of thin-walled steel pipes, the surface tensile stress is generated at the defect location. The stress concentration effect is more significant, so defect-induced quenching cracks are prone to occur.
2. Stress cracking type quenching cracks
Stress cracking is a common type of quenching crack. It is a crack defect caused by the surface tensile stress exceeding the material strength during the quenching cooling process. The surface of the pipe body with stress-cracking quenching cracks is smooth and flat, without original defects, and the microstructure is uniform and fine. The cracks are caused by excessive surface tensile stress; the stress cracking quenching cracks are completely perpendicular to the surface of the pipe body, The extension in the direction of wall thickness also shows that this type of crack is entirely caused by excessive surface tensile stress.
3. Surface carburization-type quenching cracks
When using medium carbon Cr-Mo micro-alloy steel with a C content of about 0.30% to produce seamless steel pipes, quenching cracks often occur locally on the outer surface of the pipe. Microscopic analysis results show that the structure around the quenching crack has carburization, and the depth of the carburization layer is 0.5-2.0mm. The reason for the formation of this quenching crack is that there is local carburization on the outer surface of the tube, which leads to excessive stress in the carburized part during the quenching process, thus forming quenching cracks. According to the seamless steel pipe production process, it is speculated that the process that may cause the increase in C content on the surface of the steel pipe is: that the steelmaking high-carbon protective slag adheres to the surface of the silt pipe blank and penetrates the matrix during the high-temperature heating process of the annular furnace, resulting in localization of the surface of the capillary tube after the rolling is completed. Carburization; before the steel pipe enters the heat treatment furnace, high-carbon foreign matter such as oil stains and sawdust adheres to the surface. After high-temperature heat treatment, the C content on the surface is higher than that of the matrix.
4. Quenching cracks in crack-sensitive steel types
Some high-grade seamless steel pipes have high alloying element content and high pipe body strength, so the stress field intensity factor is high. They are crack-sensitive steel types. Microscopic defects on the surface or inside of the pipe are very easy to expand under the action of stress, thus Crack defects form. Surface quenching crack morphology and structure of S135 steel grade alloy structural pipe. The probability of quenching crack defects of this type of seamless steel pipe is significantly higher than that of other steel types. Because this steel type contains more Cr and Mo alloy elements, the pipe strength is higher, the pipe microstructure has a poor ability to coordinate plastic deformation, and the deformation storage capacity is poor. The release can only occur through the formation of new surface cracks, so it is a pipe with a high risk of quenching cracks.
Post time: Oct-09-2024