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Causes and Preventive Measures of Quenching Cracks in Seamless Steel Pipes (Part One)

Posted: 04/12/2021 11:22:56 Hits: 83

Abstract: According to the characteristics of manufacturing processes of seamless steel pipes, combined with macroscopic morphology and microstructure analyses, it is believed that the main causes of quenching cracks in seamless steel pipes are surface defects, stress cracks and surface recarburization. The formation mechanism of different quenching cracks is analyzed, and corresponding preventive measures are proposed. Improving the surface quality of the pipe, optimizing the quenching process parameters and improving the uniformity of the billet's composition can effectively reduce the occurrence of quenching cracks of seamless steel pipes through the analysis.

The main processes of seamless steel pipes are: refining → continuous casting of solid steel bars→ heating of solid steel bars→ piercing → hot rolling → reducing diameters → heat treatment. In the heat treatment process, quenching cracks sometimes appear. A quenching crack is a kind of crack which is perpendicular to the surface of the steel pipe. The crack tip is sharper and of less decarburization, which is caused by structural stress generated by rapid cooling and thermal stress exceeding the strength of the pipe. According to the different morphology of quenching cracks, the causes are also various. The causes of several typical quenching cracks are analyzed and corresponding improvement measures are proposed in this article to improve the quality of seamless steel pipes.

1. Types and causes of quenching cracks
1.1 Quenching cracks induced by surface defects
When a 26Cr-Mo4s alloy structural pipe with a size of 114.30mm x 8.56mm is rolled, small quenching cracks often appear on the inner wall, as shown in Figures 1 and 2. The polished micro-topography photo shows that there are many surface defects such as pits and peeling skin with a depth of less than 0.2 mm on the inner wall of the pipe. These defects produce stress concentration under the quenching stress and become the cause of quenching cracks.

Quenching cracks caused by surface defects mostly appear in pipes with small diameters and thin walls. The rolling elongation of steel pipes with small diameters is great, and the original defects such as pits and scratches are prone to appear on the inner and outer surfaces of the pipe; at the same time, the stress concentration effect generated by the surface tensile stress is more significant in the process of quenching and cooling processes of steel pipes with thin walls, so quenching cracks caused by surface defects easily happen. Besides, when rolling pipes in steel 20 with a size of 88.90mm x 6.45mm, we found that quenching cracks caused by surface defects were on the inner and outer surfaces.

1.2 Stress cracking type of quenching cracks
Stress cracking is a common type of quenching crack. It is a kind of crack caused by the surface's tensile stress exceeding the material strength in the quenching and cooling process.

Figure 1 and Figure 2

The morphology and surrounding structure of quenching cracks on surfaces of casing in steel N80Q with a size of 177.80mm ×11.51mm are shown in Figures 3 and 4. It can be seen from Figures 3 and 4 that pipes with stress cracking have smooth and flat surfaces, without original defects, and the microstructure is uniform and small. Cracks are caused by excessive tensile stress on surfaces. Stress cracking type of quenching cracking is completely perpendicular to the surface of the pipe body and extends along the direction of wall thickness, which also shows that this type of crack is entirely caused by excessive tensile stress on surfaces.

Figure 3 and Figure 4

In addition, when casing in P110 steel grade 29MnCr6 with a size of 139.70mm×10.54mm is rolled, stress cracking type of quenching cracking also appears.

Excessive stress on the surface of the pipe body is mostly caused by uneven cooling of the inner and outer walls of the steel pipe. Therefore, through experiments, the effect of different cooling methods on tensile stress on surfaces of seamless steel pipes in the quenching process is analyzed.

The quenching cooling method of a heat treatment production line is external water spraying plus internal spraying water quenching, and the internal spraying is delayed by 3 seconds compared with the external spraying. The delayed quenching process with internal spraying will cause tensile stress in the circumferential direction of the pipe body; the water flow will be too great, and the cooling rate will be faster, resulting in generating great residual stress. The quenching test was carried out on the rolled Cr-Mo alloy steel pipe with a size of 127.00mm × 9.19mm. The sample was heated to 890℃ and kept for 30 minutes before leaving the furnace. The No. 1 sample was internally sprayed and then cooled for 10 seconds, and then the whole pipe was water hammered. The No. 2 sample was directly subjected to the whole water quenching and cooling. The results of residual stress measurement (Figures 5 and 6) show that the residual stress of the pipe body of the No. 1 sample is -105.8MPa (compressive stress), and that of the pipe body of the No. 2 sample is 219.8MPa (tensile stress). It can be seen that improper cooling methods will lead to great residual tensile stress on the surface of the pipe body and increase the risk of stress cracking type of quenching cracking.

Figure 5 No. 1 sample (compressive stress) Figure 6 No. 2 sample (tensile stress)

1.3 Surface carburized quenching cracks
When the medium carbon Cr-Mo microalloy steel with a C content of about 0.30% is used to produce seamless steel pipes, quenching cracks often appear on the surface of the pipe body, as shown in Figure 7. The results of microscopic analysis showed that there was recarburization in the structure around the quenching crack (Figure 8), and the depth of the recarburization layer was 0.5 to 2.0mm. The reason for the formation of the quenching crack is that there is recarburization on the partial outer surface of pipes, which leads to excessive stress in the recarburization part in the quenching process, thereby forming a quenching crack.

Figure 7 and Figure 8

According to the production process of seamless steel pipes, it is speculated that the processes which may cause the increase of C content on the surface of the steel pipe are:
① Casting powder containing much carbon is bonded with surfaces of steel solid bars, which penetrates the matrix in the high-temperature heating process of the ring furnace, resulting in recarburization on partial surfaces after rolling.
② The surface of the steel pipe bonds with high-carbon foreign matter such as oil stains and wood chips before entering the furnace for heat treatment. After high-temperature heat treatment, the C content of surfaces is higher than that of the substrate.

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