The reabsorption tower manufactured by a company is a low temperature equipment, the working temperature is -18~-63°C, the materials are CO ₂ , H ₂ S, CH ₃ OH, etc., the shell material is ASMESA-203Gr.D, and the forging used is ASMESA. -350Gr.LF3, this low temperature steel introduced less information, and a series of tests were carried out to find a heat treatment process suitable for low temperature steel SA-350Gr.LF3.

1. Chemical composition and mechanical properties of SA-350Gr.LF3 forgings

The chemical composition of SA-350Gr.LF3 is shown in Table 1.

SA-350Gr.LF3 is delivered in normalized + tempered condition. The mechanical properties are shown in Table 2 and Table 3. The impact absorption energy of -101.1 °C in Table 2 allows a sample to have a minimum of 16 J. The shock absorption energy of -101.1 °C in Table 3 allows a minimum of 24 J for one sample. The impact energy required by the project is 50% higher than the ASME standard.

2. Alloy characteristics of low temperature steel

Steel generally increases in strength and decreases in ductility and toughness as the temperature of use decreases. For low temperature steel, in addition to meeting the normal temperature strength, stable structure at low temperature, good welding performance, processing performance and necessary physical properties, low temperature toughness is the most important performance index of low temperature steel. The main method to improve the low temperature toughness of steel is to refine the grains and improve the purity of the steel. Low temperature steels are usually refined by adding alloying elements.

Mn is one of the main elements for refining crystal grains and improving the toughness of steel. Mn can make the cementite appearing at the grain boundary after slow cooling, especially for hot rolling and normalizing steel, but it is more effective to add a small amount of Mn. When the amount is increased to a certain extent, the effect is saturated.

Ni is a pure solid solution element in steel and has a significant effect of lowering the cold and brittle transition temperature of steel. Ni forms an infinite solid solution with iron, and Ni refines the ferrite grains by its adsorption in the grains, thereby improving the impact toughness of the steel. Low temperature steel should greatly improve toughness, and it is often impossible to alloy with Ni.

C is an indispensable element in steel. However, although C has a high strengthening effect, it significantly reduces the toughness, so the C-carbon content of low-temperature steel is low.

Nb is a strong carbide-forming element and forms an extremely stable Nb(C,N) compound with carbon and nitrogen. Nb(C,N) particles distributed along austenite grains hinder austenite grain growth and significantly increase the original austenite roughening temperature, thereby refining ferrite grains and improving steel strength. And low temperature toughness. However, in the absence of carbon, the presence of Nb in solid solution in steel will delay the precipitation of pro-eutectoid ferrite and strongly delay the time at which austenite begins to decompose into pearlite. Therefore, the amount of Nb added is controlled so that Nb is dispersed as much as possible in the form of a compound in the steel.

3. Heat treatment test of SA-350Gr.LF3 forgings

According to the characteristics of the alloying elements of SA-350Gr.LF3 forging materials, several heat treatment process schemes were formulated, and some test pieces were forged. A series of tests were carried out to find a reasonable heat treatment process through the test results. Table 4.

Usually the normalizing temperature is Ac3+30~50°C, and sometimes the austenitizing is more uniform, which will increase the normalizing temperature accordingly. From the test results in Table 4, when the austenitizing temperature is increased to a certain temperature, the strength is remarkably improved, and the impact energy is decreased. This is because the too high normalizing temperature causes the austenite grains to grow. Generally, increasing the tempering temperature will lower the strength and increase the energy absorbed by the impact. From the data in Table 4, when the normalizing temperature is the same, the tempering temperature has a greater influence on the energy absorbed by the low-temperature shock, and the tempering temperature can increase the strength. The impact absorption energy decreases; the analysis may be that Mn and Ni lower the A1 line of SA-350Gr.LF3, and the high tempering temperature may precipitate carbides, thereby increasing the strength and reducing the impact absorption energy. From the above results, after normalizing at 840~870 °C, the mechanical properties of low temperature steel SA-350Gr.LF3 forgings after tempering at 660~670 °C are better.

4. Production verification

After the heat treatment process of SA-350Gr.LF3 forging products was tempered at 840~870°C and normalized at +660~670°C, the mechanical properties of several sets of forgings were randomly tested. See Table 5. It can be seen from Table 5 that the mechanical properties of the low temperature steel SA-350Gr.LF3 forgings are normalized at 840~870 °C and tempered at 660~670 °C. The specifications of this heat treatment process are more reasonable and Subsequent production of forgings was verified.

5 Conclusion

(1) The tempering temperature has a great influence on the impact energy of the low temperature steel SA-350Gr.LF3 forgings. The suitable tempering temperature is the key to the improvement of the low temperature impact energy.

(2) 840~870°C normalizing, 660~670°C tempering heat treatment process specification is suitable for low temperature SA-350Gr.LF3 forgings, and SA-350Gr.LF3 treated by this heat treatment process in actual production Forgings have a higher pass rate.

About the author: Lin Nan, Zhang Caixia, Wang Hongxue, Zhonghang Liming Jinxi Chemical Machinery (Group) Co., Ltd.