Introduction
4Cr14Ni14W2Mo or S32590, is a special austenitic stainless steel with a duplex structure. it can be classified as a stainless steel with 13%-15% of Ni and 13%-15% of Cr components. The history of this material is only 40 years as manufacture methods of duplex steel improved to allow more complex and special steel appear. [1] It has become widely used material under the server condition of high temperature and corrosion environment due to its great heat resistance and corrosion resistance. [2]
Chemical components
Expect for Fe, 4Cr14Ni14W2Mo consists 13%-15% Ni, 13%-15% Cr, 2%-2.7% W, 0.2%-0.4% Mo, 0.8% Si 0.4-0.5% C, 0.7% Mn, and impurity of P and S <1% (all in wt%) [2]
|
Elements |
Min (wt%) |
Max (wt%) |
|
C |
0.40 |
0.50 |
|
Si |
- |
0.80 |
|
Mn |
- |
0.70 |
|
P |
- |
0.35 |
|
S |
- |
0.03 |
|
Ni |
13.00 |
15.00 |
|
Cr |
13.00 |
15.00 |
|
W |
0.25 |
0.40 |
|
Mo |
2.00 |
2.75 |
Table 1. Chemical components of 4Cr14Ni14W2Mo [2]
Chromium and nickel are mainly added to obtain the austenitic organization which forms solid solution strengthening, thus greatly improve the corrosion resistance and oxidation resistance besides the heat and corrosion resistance the chromium and nickel would present themselves; Instead of more chromium, tungsten and molybdenum were added to carbide with carbon, which prevents the formation of chromium carbide, so that the content of chromium increases, which improves the corrosion resistance and heat resistance given by Cr. Besides, the insoluble carbide formed by W and Mo would cause grain boundary pinning effect, which prevents austenitic grain size grows, refining grain, thus makes solid solution better, contribute to the tenacity of the steel [3][4]
Property profile (short) [8]
Mechanical properties
|
Young’s modulus |
243 kN/mm2 |
|
Tensile strength |
705 MPa |
|
Tensile strength(after annealing） |
817 MPa |
|
Proof strength Rp0.2 |
246 MPa |
|
Proof strength Rp0.2(after annealing） |
363 MPa |
Thermal properties[1]
|
Melting point |
5989 - 6128 °C |
|
Thermal conductivity |
23 - 98 W/m*K |
|
Specific heat capacity |
450 - 460 J/kg*K |
|
Thermal expansion |
70 - 57 e-6/K |
Resistance properties[5]
|
Corrosion resistance |
Great |
|
Oxidation resistance |
Great |
4Cr14Ni14W2Mo shows good mechanical properties: with tensile strength 246 σb/MPa, yield strength 705 σb/MPa, and they could improve to 817MPa and 363MPa after annealing. The most significant properties are its great heat resistance, corrosion resistance and the oxidation resistance, but the poor heat conductivity and big linear expansion coefficient also reduce thermal fatigue properties. Besides, the material also demonstrates good weldability, machinability and toughness. [8]
Microstructure
Microstructure
The microstructure of 4Cr14Ni14W2Mo is duplex, which means the steel contains two primary phases, f.c.c austenite and b.c.c. ferrite. [5] As shown in the picture below, the ferritic shown as black phase and austenitic shown as white phase.[6]
Figure 1. Distribution of ferritic (black phase) and austenitic (white phase) [6]
The formation process of this duplex structure can be explained through microstructure: Firstly, define austenitic as γ, ferritic as α, initially solidify as σ. As Cr in favor of solidification of ferritic, Ni in favor of austenitic, and the concentration of Ni and Cr are similar, the distribution of two phase do not shows obvious difference. Then by recrystallization and deformation to refine crystal, 4Cr14Ni14W2Mo obtain typical pancake-like structure, which greatly increased strength and toughness based on the Hall-Patch principle. After that, the two-phase microstructure inhibits grain growth during deformation and heat treatment. The ferritic phase tends to coalesce while due to the rapid diffusion processes in f.c.c. lattice, while the austenite phase coarsens slowly. As the solubilities for interstitial elements in ferrite are approximately 100 times lower while the diffusion rates are 100 times lower than those in austenite. Which causes the formations of carbides, nitrides, and intermetallic phases is easy and occur in the ferritic phase only. [7] The formation of chromium carbides has a significant negative influence on the oxidation resistance and corrosion resistance.
Figure 2. Phase diagram of Fe-Ni-Cr [5]
The formation of the duplex structure offers higher strength and better thermal conductivity than austenitic steels; higher impact value than ferritic steels, besides the solid solution during the duplex formation would increase corrosion resistance. However, due to the complex precipitation and transformation behaviour, the formation of carbides, nitrides, and intermetallic phases would also cause reduction in corrosion resistance.
Processing and manufacture
Processing and manufacture can be concluded into five steps:
- Melting：raw materials melted and mixed in electric furnace
- Argon-oxygen decarburization (AOD): [9] In order prevent the reduction of the corrosion resistance due to formation of carbides, nitrides, and intermetallic phases, advanced refining processes are required, so the Argon-oxygen decarburization method is used. It can be divided into the following steps:
- The mixture of argon and oxygen is injected to the melted steel at 1700°C. [10]
- Decarburization： Argon is added to decrease the partial pressure of CO, drive the reaction forward
- Reduction: remove additional oxidized elements
- Desulfurization
Figure 3. Schematic of an Argon-oxygen decarburization vessel [9]
- Casting and forming: mostly hot-forming, [10] can be formed into tubes, plates, pipes, etc.
- Solution annealing: The annealing temperature effects the distribution of austenite and ferrous, the required annealing condition by the Chinese government is: Annealing temperature: 820-850℃ fast cooling [11]
- Descaling, cutting and finishing [12]: The first step is electro cleaning, which uses steel as anode and outer cathode. Then use phosphoric acid to electrify steel, remove the outer layer. After that, mechanical cutting or plasma jet cutting can cut steel into the final size and shape. Besides, cold working could refine grain size, which must be applied in certain applications.
Environmental impact
Like other stainless steels, S32590 is 100% recyclable, the current recycling rate is approximately 60%. Without considering the recycle, the carbon emission of stainless steel is 2.9kg/kg, the major contribution is the energy costs for heating and electricity. [13]
Figure 4. Contribution of waste emission in different processes in steelmaking [14]
Applications:
With outstanding corrosion resistance and oxidation resistance, 4Cr14Ni14W2Mo is widely used in valves and machine manufacturing. Good weldability, machinability of 4Cr14Ni14W2Mo makes it easy to process into long and short shafts and is then assembled on both sides of large equipment such as butterfly valves. Besides, Its wear-resistance and corrosion-resistance properties are also helpful for use and processing. Expect for normal bars, plants, coils, and pipes, 4Cr14Ni14W2Mo can also be used in the production of various molds such as creep forming die of titanium alloy, strong corrosive glass-forming die and core mold for die casting. In general, 4Cr14Ni14W2Mo plays an important role in today's manufacturing industry due to its excellent mechanical and chemical properties.[12]
Current researches and further developments
Since different treatment methods and chemical composition of 4Cr14Ni14W2Mo will lead to different properties, there remains a huge potential for improving the property of 4Cr14Ni14W2Mo according to the required application. For example, The experimental results show that the 4Cr14Ni14W2Mo steel can obtain preferable mechanical properties after solution at 1170 ℃ for 60 min and age at 760 ℃ for 5 h, which can meet the standard requirements of superalloy bars for internal combustion engines and valve steel.[15][16]
There are Also researches focusing on the surface of 4Cr14Ni14W2Mo, for example, Chan et. al. focusing on the effect of laser surface melting and subsequent re-ageing on microstructure and corrosion behaviour of aged S32950 duplex stainless steel, which found laser surface melting can reduce the hardness and brittleness of surface of the aged S32950, thus restored the pitted corrosion resistance.[17]
Figure. 5. Polarization curves of solutionised and aged S32950 (for 200 h); and laser-melted S32950 after re-aging for 200 h in 3.5 wt% NaCl solution at 25 °C. [17]
At present, the research on treatment, structure, and property of 4Cr14Ni14W2Mo is still going on, which provides a promising future for the application of 4Cr14Ni14W2Mo in special conditions and machine manufacturing.
References
[1] Krivsky, W.A. The linde argon-oxygen process for stainless steel; A case study of major innovation in a basic industry. MT 4, 1439–1447 (1973). https://doi.org/10.1007/BF02667991
[2] 4Cr14Ni14W2Mo forging [Internet]. Tool & Die Steels. 2020 [cited 4 March 2020]. Available from: https://www.tool-die-steels.com/products/forging-rolling/46/139/45Cr14Ni14W2Mo-forging.html?tdsourcetag=s_pcqq_aiomsg
[3] Christian Doppler Laboratory for Early Stages of Precipitation The Effects of Alloying Elements on Steels (I) [Internet]. Technische Universität Graz; 2020 [cited 19 March 2020]. Available from: https://online.tugraz.at/tug_online/voe_main2.getvolltext?pCurrPk=32837
[4] Effect of alloying elements on steel properties [SubsTech] [Internet]. Substech.com. 2020 [cited 20 March 2020]. Available from: https://www.substech.com/dokuwiki/doku.php?id=effect_of_alloying_elements_on_steel_properties
[5] Weber J. Encyclopedia of Materials - Science and Technology, Stainless Steels: Duplex, Volumes 1-11. 2nd ed. Knovel; 2001.
[6] Knyazeva M, Pohl M. Duplex Steels: Part I: Genesis, Formation, Structure. Metallography, Microstructure, and Analysis. 2013;2(2):113-121.
[7] Knyazeva M, Pohl M. Duplex Steels. Part II: Carbides and Nitrides. Metallography, Microstructure, and Analysis. 2013;2(5):343-351.
[8] Steels T. 4Cr14Ni14W2Mo - Stainless Steel,Heat-Resistant Steel and Special Alloy Steel - steel, 4Cr14Ni14W2Mo Datasheets, Supplier, Chemical composition, properties [Internet]. Tool-die-steels.com. 2020 [cited 28 May 2020]. Available from: https://www.tool-die-steels.com/grades/Stainless-Steels-Special-Steel/31/140/4Cr14Ni14W2Mo.html.
[9] Krivsky, W.A. The linde argon-oxygen process for stainless steel; A case study of major innovation in a basic industry. MT 4, 1439–1447 (1973). https://doi.org/10.1007/BF02667991
[10] Weber J. Encyclopedia of Materials - Science and Technology, Austenitic Stainless Steels, Volumes 1-11. 2nd ed. Knovel; 2001.
[11] Practical Guidelines for the Fabrication of Duplex Stainless Steel. 4th ed. London: The International Molybdenum Association; 2014.
[12] General Administration of Quality Supervision, Inspection and Quarantine of the P.R.C；Standardization administration of the P.R,C. GB/T 1221-2007 Heat-resistance steel bar. 2007.
[13] Sun W, Zhou Y, Lv J, Wu J. Assessment of multi-air emissions: Case of particulate matter (dust), SO2, NO and CO2 from iron and steel industry of China. Journal of Cleaner Production. 2019;232:350-358.
[14] Sandberg, H., Lagneborg, R., Lindblad, B., Axelsson, H. and Bentell, L., 2001. CO2 emissions of the Swedish steel industry. Scandinavian Journal of Metallurgy, 30(6), pp.420-425.
[15] Bao Wenquan, Liu Guijiang, Yang Jianhua, Guo Qiang. Effect of heat treatment on Microstructure and properties of 45cr14ni14w2mo steel [J]. Metal heat treatment, 2016,41 (05): 153-155
[16] Xiong Guangyao1,He Bolin1,Zou Rui.The Research of Microstructure and Properties of The 4Cr14Ni14W2Mo Steel With QPQ Salt-bath Nitriding[C]. Dalian University of Technology,2007:181.
[17] Chan W, Kwok C, Lo K. Effect of laser surface melting and subsequent re-aging on microstructure and corrosion behavior of aged S32950 duplex stainless steel. Materials Chemistry and Physics. 2018; 207:451-464.