Knowledge

Taking 2Cr13 martensitic stainless steel as the research object, the effects of different nickel contents on the microstructure and comprehensive properties of 2Cr13 martensitic stainless steel were studied by means of metallographic microscopy, hardness testing and tensile testing at room temperature. The research results show that 2Cr13 martensitic stainless steel can obtain a fully martensitic structure after hot rolling in the single-phase austenite zone and air cooling. When w[Ni] is 0.1%, the martensite laths are relatively coarse. As w[Ni] increases to 0.3%, the hardness after heat treatment increases by 10HRC, and the yield strength, tensile strength and elongation after fracture increase from 614 MPa, 748 MPa and 30% to 670 MPa, 797 MPa and 33%, respectively. The mechanical properties of 2Cr13 martensitic stainless steel can be adjusted by increasing the tempering temperature and holding time to meet different order requirements.

 

1 Preface

2Cr13 martensitic stainless steel has good hardenability. Through quenching and tempering heat treatment technology, it can achieve a balance of strength, toughness and corrosion resistance, and thus is widely used in the manufacture of parts that work under corrosive and impact load conditions, such as ship engine blades, oil pipelines and natural gas transmission pipelines. With the rapid progress of the energy industry, especially in the development of deep and ultra-deep wells in oil fields under corrosive environments, the requirements for stainless steel pipes have become more stringent. The mechanical properties and corrosion resistance of metal pipes are mainly determined by the chemical composition and microstructure of the material itself. Therefore, by regulating the chemical composition and heat treatment process of the material, the service life of the steel can be significantly improved.

 

Nickel is a major austenite stabilizing element, which can to some extent prevent the precipitation of δ ferrite during high-temperature deformation and increase the Ms point, indirectly expanding the formation range of the austenite phase and significantly improving the hardenability of the material. In addition, the selection of heat treatment process parameters is also crucial. The optimization of the heat treatment system can provide technical references for improving the mechanical properties of 2Cr13 martensitic stainless steel. Yang Shunzhen et al. analyzed the influence of tempering systems on the microstructure and comprehensive performance of 1Cr13 martensitic steel, and found that the tensile strength and hardness decreased with the increase of tempering temperature. Chakraborty et al. studied the formation mechanism of carbides in AISI410 martensitic stainless steel during medium and high-temperature tempering. The results showed that the precipitation of M23C6 was more sensitive at high temperatures, and the embrittlement was the most severe at 550°C tempering. Zhang Xiaoke, Wei Zhengyan et al. systematically developed the optimal heat treatment process window for nickel-containing martensitic stainless steel. The test results showed that quenching and high-temperature tempering processes could significantly refine the grains and improve the comprehensive mechanical properties of the material, and inhibit temper embrittlement. In recent years, researchers have conducted extensive studies on the microstructure and performance control technology of martensitic stainless steel, mainly using microalloying to adjust the microstructure uniformity and mechanical properties of the material. Therefore, the research on the influence of the types and quantities of alloy elements in the material on the microstructure and performance after heat treatment has become particularly important. The microstructure and performance of two batches of 2Cr13 martensitic stainless steel tubes with w[Ni] of 0.1% and 0.3% respectively will be analyzed after quenching and tempering to determine the influence of alloy element content and heat treatment process parameters on microstructure and mechanical properties, providing basic data and theoretical analysis guidance for front-line production.

 

2. Experimental Materials and Methods

The experimental materials were 310 mm × 310 mm cross-section billets. The billets were cast in an electric furnace with a capacity of 90 tons, and the weight of the obtained cast billets was 1.16 tons. They were heated to 1240°C in a ring furnace, then pierced, continuously rolled (final rolling temperature was 850°C), sized, slit, and straightened. The final products were martensitic steel pipes with an outer diameter of 244.48 mm and a wall thickness of 11.99 mm.

 

For the above-mentioned hot-rolled steel tubes, quenching heating is first carried out at 1200℃, and then air cooling is adopted after removal from the furnace to obtain martensitic structure. Subsequently, the research on tempering temperature is conducted. The tempering process uses a step heating furnace, which is divided into three sections: preheating section, heating section, and holding section. The quenched steel tubes are subjected to high-temperature tempering at 785~800℃, with a holding time of 75 minutes for each tempering process. After high-temperature tempering, the steel tubes undergo hot straightening, geometric dimension inspection, and ultrasonic flaw detection to meet the required product standards. The mechanical property requirements for the steel tubes are: yield strength 557~650 MPa, tensile strength ≥ 655 MPa, elongation ≥ 23%, and Rockwell hardness 14~23HRC.

 

 

Due to the significant fluctuations in the mechanical properties of the bar after heat treatment caused by the variation in nickel content, which did not meet the order requirements, a series of studies were conducted on the influence of nickel content changes on the microstructure and mechanical properties of 2Cr13 martensitic stainless steel. Firstly, the equilibrium phase diagram of 2Cr13 martensitic stainless steel was calculated using the JMatPro thermodynamic analysis software. Then, the heat treatment of the rolled seamless steel tubes was carried out, followed by microstructure observation, hardness testing, and uniaxial tensile performance testing. The performance tests were conducted on three samples, and the average value was taken. The metallographic preparation was ground and polished, and then corroded with Kroll's reagent for 40 seconds. The metallographic structure observation and photography were completed on a Zeiss optical microscope. At the same time, the tempering process of the two types of steel tubes with different nickel contents was studied. In Scheme 1, the seamless steel tube was tempered at 785°C for 70 minutes and then air-cooled. In Scheme 2, the tempering temperature of the seamless steel tube was increased to 800°C, and the holding time remained at 70 minutes. Finally, samples under different heat treatment processes were sampled for observation, hardness testing, and tensile performance testing. The mechanical properties were tested on three samples, and the average value was taken. The hardness measurement was conducted using a Rockwell hardness tester, and three different areas on each sample were measured for hardness. The room temperature tensile test was completed on an electronic universal testing machine at a tensile speed of 3 mm/min.

 

3. Experimental Results and Analysis

3.1 Effects of Nickel Content on Microstructure and Properties after Quenching

Figure 1 shows the equilibrium phase diagram of 2Cr13 martensitic stainless steel calculated by JMatPro software. It can be seen that the temperature range of the single-phase austenite phase is 955~1212℃. Above this temperature range, ferrite will form, while below this temperature range, M23C6 will be produced.

Therefore, when the material is rolled at 1200℃, it is in the single-phase austenite region and has good plastic deformation ability. After deformation, quenching can obtain martensite structure, ensuring the mechanical properties of the steel pipe. Thus, the quenching heating temperature selected in this paper is 1200℃. Figure 2 shows the microstructure of two nickel contents of 2Cr13 martensitic stainless steel after rolling at 1200℃ and air cooling. It can be seen from the figure that the material can obtain all martensite structure after air cooling from the single-phase austenite region, but there are still a large number of rolling flow lines in the structure. The difference in nickel content cannot be seen from the low-magnification microstructure shown in Figure 2(a) and 2(b). However, the morphology of lath martensite can be seen from the high-magnification microstructure shown in Figure 2(c) and 2(d). The lath martensite in 2Cr13 martensitic stainless steel with 0.1% nickel content is coarse, and the grain size in 2Cr13 martensitic stainless steel with 0.3% nickel content is smaller. When the nickel content increases from 0.1% to 0.3%, the grain size after tempering decreases from 17.3 μm to 9.8 μm. This is probably due to the grain refinement caused by the addition of nickel. The average hardness value of 2Cr13 with 0.1% nickel content is 47HRC, while the average hardness value of 2Cr13 with 0.3% nickel content is 50HRC. The slight increase in hardness value is due to the grain refinement and improved hardenability caused by nickel.

 

Figure 3 shows the microstructure of 2Cr13 martensitic stainless steel with two nickel contents after tempering. Figures 3(a) and 3(b) show the microstructure after the same tempering process (785℃, 70 min, air cooling). Both figures retain the morphology of some lath martensite and precipitate a certain amount of carbides, which is closer to tempered troostite structure. According to the previous equilibrium phase diagram, it can also be verified that M23C6 type carbides will be produced during tempering at this temperature range. Most grains still retain the orientation characteristics of martensite lath bundles, and the grains in Figure 3(b) are significantly finer, about 10.4 μm, which is consistent with the grain size of the two materials after quenching in the previous text. The Rockwell hardness results show that the hardness value of 2Cr13 martensitic stainless steel with 0.3% nickel content is higher, at 31HRC, while the hardness value of 2Cr13 martensitic stainless steel with 0.1% nickel content is only 21HRC. Thus, it can be seen that the strengthening effect of trace Ni is more obvious after tempering. To meet the order requirements, the tempering process of 2Cr13 with 0.3% nickel content was adjusted to 800℃, 70 min, air cooling. The obtained microstructure shows that the martensite morphology basically disappears, and the granular carbides increase, presenting a distinct high-temperature tempered sorbite structure, as shown in Figure 3(c). At the same time, after this tempering process, the hardness value decreases to 23HRC. This is because during the adjusted tempering process, the higher tempering temperature is conducive to the diffusion of the alloy, further reducing the supersaturation degree of martensite, and the alloy exists in the form of carbide formation, thus reducing the hardness value of the material.

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The mechanical properties of two 2Cr13 steels with different nickel contents after tempering were obtained through uniaxial tensile tests at room temperature. With the increase of nickel content from 0.1% to 0.3%, the yield strength, tensile strength and elongation after fracture of 2Cr13 martensitic stainless steel tempered at 785℃ for 70 min increased from 614 MPa, 748 MPa and 30% to 670 MPa, 797 MPa and 33%, respectively. The strength values were significantly improved. When the tempering process of 2Cr13 martensitic stainless steel with 0.3% nickel content was adjusted to 800℃ for 70 min, the yield strength, tensile strength and elongation were 619 MPa, 755 MPa and 31%, respectively, which could obtain mechanical property indexes comparable to those of 2Cr13 martensitic stainless steel with 0.1% nickel content. The higher tempering temperature reduced the strength of 2Cr13 martensitic stainless steel to a certain extent.

 

In addition, the tensile test results show that when the nickel content or heat treatment temperature is appropriately changed, the elongation of the 2Cr13 specimens does not change significantly, remaining within the range of 30% to 33%. This indicates that the material is not sensitive to elongation under conditions of varying nickel content or tempering temperature, but the strength indicators will be significantly altered. In summary, nickel content has a significant impact on the microstructure and mechanical properties of 2Cr13 martensitic stainless steel. It not only enhances the material's hardenability and reduces grain size, but also alters its strength and toughness, thereby improving its overall performance [15]. The addition of certain alloys can increase the material's strength, but often at the expense of its plasticity and toughness. Some studies have also shown that adding appropriate amounts of nickel, molybdenum, and other alloys can significantly improve the mechanical properties and CO2 corrosion resistance of 2Cr13 martensitic stainless steel. For 2Cr13 type martensitic stainless steel, the strengthening and toughening effect of nickel addition can be achieved through adjustments in the heat treatment process.

 

4 Conclusions

1) After high-temperature rolling, 2Cr13 martensitic stainless steel can obtain a fully martensitic structure through air cooling. Nickel is an austenite-forming element. An increase in its content can enhance the stability of austenite in 2Cr13 martensitic stainless steel at high temperatures, improve the material's hardenability, and reduce the grain size. When the nickel content increases from 0.1% to 0.3%, the grain size after quenching decreases from 17.3 μm to 9.8 μm.

2) As the nickel content increases from 0.1% to 0.3%, the hardness value of 2Cr13 martensitic stainless steel after tempering does not change significantly. However, after tempering at 785°C for 70 minutes, the hardness value increases from 21HRC to 31HRC, and the yield strength, tensile strength, and elongation after fracture increase from 614 MPa, 748 MPa, and 30% to 670 MPa, 797 MPa, and 33%, respectively.

3) The mechanical properties of 2Cr13 martensitic stainless steel can be adjusted through tempering processes. To meet order requirements, the tempering temperature and holding time for martensitic stainless steel with 0.3% nickel content should be adjusted to 800°C and 70 minutes, respectively. The mechanical properties of this steel are comparable to those of 2Cr13 martensitic stainless steel with 0.1% nickel content.

 

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