Nb-free low-alloy 890 MPa grade seamless steel pipe
Keywords
Low alloy 890 MPa seamless steel tube, Heat treatment process, Thermodynamic simulation, Microstructure observation
Introduction
Low alloy 890 MPa seamless steel tube without Nb is one of the most important materials for engineering applications. Its mechanical properties and meso-structure morphology are the main factors affecting its performance. The proper heat treatment process can effectively improve the strength, toughness and other properties of low alloy 890 MPa seamless steel tube without Nb. In this paper, a series of laboratory experiments, thermodynamic simulation and microstructure observation were conducted to research the effect of different heat treatment process on the performance of low alloy 890 MPa seamless steel tube without Nb.
Laboratory Experiment
A low alloy 890 MPa seamless steel tube without Nb was selected as the sample in this study. The chemical composition of the sample was as follows in weight percent: 0.2% C, 0.15% Si, 0.8% Mn, 0.07% P, 0.03% S, 0.008% N and 0.0004% Nb. The samples were heated at temperatures ranging from 900℃ to 1150℃ for different holding times. The quenching medium used for cold treatment was water. After quenching, the samples were tempering at temperatures ranging from 160℃ to 600℃ for one hour, after which mechanical properties and microstructures were tested and analyzed.
Thermodynamic Simulation
Thermodynamic simulation and calculation were used to predict and analyze the evolution of the alloy composition and properties during the heat treatment process. The simulation model was established by combining thermodynamics calculation with Scheil-Gulliver and Prout-Tompkins models. The simulation results were compared with the experimental results to evaluate the accuracy of the model.
Microstructure Observation
The microstructure of low alloy 890 MPa seamless steel tube without Nb was observed by and photographed using a metallographic microscope after the heat treatment process. The morphology, size and distribution of grains, as well as the location of inclusions and secondary phases, were analyzed.
Results and Discussion
The mechanical properties of the sample after different heat treatment processes were measured and compared. It can be seen from the test results that the hardness and tensile strength of the sample increase with increasing heat treatment temperature or with further tempering. At the same time, the elongation and impact toughness decrease with increasing heat treatment temperature or with further tempering. The thermodynamic simulation results agreed well with the experimental data and revealed the influence of FeSi and FeNb alloys on softening and hardening. The microstructure observation showed that the grain refinement and distribution were homogeneous, and the inclusions were evenly distributed in the matrix.
In this paper, the heat treatment process of low alloy 890 MPa seamless steel tube without Nb was studied. The laboratory experiments, thermodynamic simulations, and microstructure observations showed that the samples had higher mechanical properties and better microstructure morphology after heat treatment. The thermodynamic simulation results agreed well with the experimental results, which could provide theoretical guidance for the heat treatment process.
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