Analysis and preventive measures on the causes of quenching cracking of 40CrMnMo steel axial steel pipes

Underground oil mining tools work in wells thousands of meters deep, in harsh environments and complex stress conditions. Normally, mining tools have to withstand not only tensile stress, and torsional bending stress, but also strong friction and impact. At the same time, the tools are also To withstand high temperatures, high pressure, and environmental corrosion.

This requires the material properties of underground mining tools to have excellent comprehensive mechanical properties, which must not only ensure high strength, but also ensure excellent impact toughness, and at the same time be resistant to corrosion by seawater and mud. Given the performance requirements of downhole working conditions, the material selection of downhole tools is usually alloy structural steel containing corrosion-resistant elements such as Cr and Mo, and then through appropriate heat treatment and tempering processes to ensure that it meets the strength and impact toughness requirements. This article focuses on the process of downhole pipe string processing. When one of the axial pipe workpieces made of 40CrMnMo steel was quenched and tempered, severe cracking occurred many times during the quenching process, resulting in the workpiece being scrapped and causing certain economic losses. To this end, the causes of quenching cracks were analyzed from the aspects of the chemical composition, structure, heat treatment process, and crack morphology of the axial tube material, and improvements and preventive measures were proposed.

1. Description of the failed workpiece: The raw material is a solid forged and cast material of 40CMnMo steel with a diameter of φ200 mmx1 m. Process flow: rough turning→drilling and boring (to the wall thickness of about 20mm)→quenching→tempering→finishing. The outline of the axial tube workpiece is a pipe with a length of about 1m, a diameter of φ200 mm, and a wall thickness of 20 mm. Heat treatment process: first slowly heat it to 500°C in a box furnace, then put it into a salt bath furnace to heat it to the quenching temperature of 860~880°C. The heating time in the salt bath furnace is about 30 minutes and then quenched at about 40-60°C. Quench in oil for about 10 minutes. After taking it out, temper it in a box furnace and keep it at 600°C for 10 hours to cool down in the furnace. Crack situation: The crack develops along the axis of the central tube, is visible from the edge, and has cracked in the radial wall thickness direction.

2. Detection and analysis

2.1 Chemical composition detection: The quenched cracked axial tube workpiece will be sampled by local wire cutting for composition analysis. Its chemical composition complies with GB/T3077--1999 "Chemical Composition and Mechanical Properties of Alloy Structural Steel".

2.2 Metallographic inspection and analysis Experts took two samples of the quenched and tempered axial tube longitudinally, treated them with fire (insulated at 850°C for 15 hours and cooled in the furnace), then polished them with sandpaper and polished on a polishing machine, using 4% Corrode with nitric acid alcohol and observe its metallographic structure. Sample 2 was directly ground with sandpaper and then polished and corroded, and its metallographic structure was observed.  Comparing the detected metallographic structure with GBT 13299-1991 "Method for Evaluation of Microstructure of Steel", it was found that the banded structure in sample 1 was grade 3 to 4, of which white was eutectoid ferrite and gray-black was pearlescent. body, the pearlite structure accounts for about 60%, which is higher. The metallographic structure of sample 2 is tempered troostite and a small amount of tempered troostite.

3. Analysis of cracking causes and solutions

3.1 Crack shape and heat treatment process. Observe the shape of the crack in the axial tube. It is a longitudinal crack. It occurs along the axial direction and the crack is deep. It is even obvious that the crack has cracked along the radial direction on the edge of the axial tube. Therefore, It is concluded that the stress causing the cracking of the axial tube is the surface tangential tensile stress, which is caused by the later structural stress. At the same time, because the material of the axial tube is medium carbon alloy structural steel, the structural stress also dominates during the quenching process. Martensitic transformation occurs, the plasticity decreases sharply, and the structural stress increases sharply at this time, causing the tensile stress formed by the quenching internal stress on the surface of the workpiece to exceed the strength of the steel during cooling, causing cracking, which often occurs in the fully quenched part. The occurrence of such cracks is mainly due to the large structural stress caused by improper quenching process. Since the quenching heating temperature of the axis tube is 860~880℃, which is relatively high, it is quickly put into the quenching oil of 40~60℃. When the temperature is above the Ms transition temperature, the quenching heating temperature is high. The thermal stress is large, and when cooling below the MS transformation temperature, the quenching oil temperature is relatively low, and the quenching time of 10 minutes is relatively long. During the rapid cooling process, more martensite is produced. The different specific volumes of different structures, in turn, produce greater tissue stress, which is one of the causes of quenching cracking of the axis tube.

3.2 Uniformity of the raw material structure. Through metallographic analysis of the intercepted sample 1 after annealing (insulation at 850°C for 15 hours and cooling in the furnace), it was found that the axial tube with cracks still had obvious banding after annealing. The existence of structural segregation indicates that the copper material itself has severe band-like structural segregation and uneven structure. The existence of a band-like structure will increase the tendency of quenching cracking of the workpiece. Relevant literature points out that the band-like structure in low- and medium-carbon alloy steel refers to the band-like structure formed along the rolling direction or forging direction of the steel. The bands mainly composed of proeutectoid ferrite and the bands mainly composed of pearlite are stacked on each other. The cast structure is a defective structure that often appears in steel. Because the molten steel selectively crystallizes during the ingot crystallization process to form a dendrite structure with unevenly distributed chemical components, the coarse dendrites in the ingot are elongated along the deformation direction during rolling or forging and gradually become consistent with the deformation direction. , thus forming depleted bands (strips) of carbon and alloying elements and depleted bands stacked alternately with each other. Under slow cooling conditions, the depleted bands of carbon and alloying elements are first formed (overcooled austenite has lower stability). ) precipitates eutectoid ferrite, and discharges the excess carbon into the enriched zones on both sides, eventually forming a zone dominated by ferrite: a carbon and alloying element enriched zone, whose supercooled austenite is more stable After that, a band mainly composed of pearlite is formed, thus forming a band-like structure in which bands mainly ferrite and bands composed of pearlite alternate with each other. The different microstructures of adjacent bands in the banded structure of the axial tube, as well as the differences in morphology and grade of the banded structure, cause the expansion coefficient and the difference in specific volume before and after phase change to increase during the heat treatment and quenching process of the axial tube, resulting in The large organizational stress will eventually increase the quenching distortion of the axial tube. If the quenching process is improper, the tendency of the band structure to cause quenching distortion and cracking will increase, making it easier to cause quenching cracking.

3.3 Solutions and Effects Through the above analysis of the causes of cracking of the axial tube during the quenching process, we first improved the heat treatment and quenching process, reducing the quenching temperature by about 10°C, and increasing the quenching oil temperature to about 90°C. , and also shorten the time the axis tube is in the quenching oil. The results showed that the axial tube did not crack during quenching. It can be seen that the main cause of quenching cracking of the axial tube is an improper quenching process, and the band-like structure in the raw material will increase the tendency of quenching cracking of the axial tube, but it is not the main cause of quenching cracking. A sealing test was conducted on the axial tube, and it was able to maintain a stable pressure for 10 minutes at a pressure of 3500 psi (equivalent to 24 MPa), which fully meets the sealing requirements of downhole tools.

4. Conclusion The main cause of quenching cracking of the axial tube is an improper quenching process. The band-like structure in the raw material increases the quenching cracking direction of the axial tube, but it is not the main cause of quenching cracking. After improving the heat treatment process, the axial tube did not undergo quenching cracking again, and when the sealing test was conducted on the axial tube, it was able to stabilize the pressure for 10 minutes at a pressure of 3500 psi (equivalent to 24MPa), which fully meets the sealing requirements of downhole tools. To prevent the axial tube from being damaged during the quenching process For medium cracking, please note:

1) Keep good control of raw materials. It is required that the band structure in the raw materials is ≤3, various defects in the raw materials such as looseness, segregation, non-metallic inclusions, etc. must meet the standard requirements, and the chemical composition and microstructure must be uniform.

2) Reduce machining stress. Ensure a reasonable amount of feed to reduce machining residual stress, or perform tempering or normalizing before quenching to eliminate machining stress.

3) Choose a reasonable quenching process to reduce structural stress and thermal stress. Appropriately lower the quenching heating temperature and increase the quenching oil temperature to about 90°C. At the same time, the residence time of the axis tube in the quenching oil is also shortened.