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Why is molybdenum used in stainless steel?

Views: 3     Author: Site Editor     Publish Time: 2022-11-23      Origin: Site

Molybdenum (Mo) is a valuable alloying element. The role of Mo in steel can be summarized as follows:

1) Improve hardenability and thermal strength, prevent tempering brittleness, improve remanence and coercivity, improve the corrosion resistance and prevent pitting tendency in some media (such as hydrogen sulfide, ammonia, carbon monoxide, water, etc.).

2) Molybdenum can strengthen ferrite in solution and improve the stability of carbide, thus improving the strength of steel.

3) Molybdenum plays a favorable role in improving the ductility, toughness and wear resistance of steel.

4) Molybdenum is the most effective alloying element to improve the thermal strength of steel because it increases the softening and recovery temperature and recrystallization temperature after deformation strengthening, and strongly increases the creep resistance of ferrite, effectively inhibits the accumulation of cementite at 450~600℃, and promotes the precipitation of special carbides.

1. Application of molybdenum in stainless steel

The application of molybdenum in the austenitic stainless steel Stainless steel in our country in the national standard‚ about half of the austenitic stainless steel grade is containing Mo‚ most of the stainless steel containing 2% ~ 3% Mo‚ peak Mo 5%‚ in austenitic stainless steel is the biggest containing Mo steel production and consumption of 316‚ followed by 317 and 316 l and 317 l.

1.1 Improve corrosion resistance

Mo improve austenitic stainless steel in the reducing medium (such as sulfuric acid, phosphoric acid, acetic acid, urea and some organic acids) in corrosion resistance, pitting and crevice corrosion resistance ‚ and the increase of Mo content including steel corrosion resistant performance enhancements. The results show that ‚ : in cr-ni austenitic stainless steel adding 2% ~ 3% Mo can effectively improve the austenitic stainless steel in sulfuric acid, phosphoric acid, acetic acid, urea, and in reducing the corrosion resistance of medium such as chloride solution. Therefore, the Mo content in the composition of 316, 316L, 317 and 317L steel is 2% ~ 3% Mo. In strong oxidizing nitrate solution ‚ Mo to add on the corrosion resistance of stainless steel are no good. Therefore, austenitic stainless steel containing Mo is not used for nitric acid purposes.

1.2 Improve mechanical properties

The beneficial effect of Mo is to improve the high temperature strength, creep strength, lasting strength and high temperature fatigue properties of austenitic stainless steel. The increase of Mo on the strength of Cr-Ni austenitic stainless steel at room temperature is not significant. On the other hand the increase in ‚ high strength steel deformation resistance and hot working difficulty  increase the heating way more time in the process of hot working. Due to form element contains  Mo Mo is ferrite steel group in the alpha phase and metal mesophase increase has also led to hot working hard ‚ scrap rate increase ‚ yield decrease.

1.3 Main uses of Mo austenitic stainless steel

316 (L), 317 (L) stainless steel with Mo content of 2% ~ 3% is widely used in the production of chemical, petrochemical, chemical fertilizer, atomic energy and other industrial departments of production equipment, process pipeline. For more demanding ‚ medium such as seawater medium corrosion conditions, gas desulfurization ‚, such waste gas processing, need to use the austenitic stainless steel with higher content of Mo. 00 cr25ni22mo2n steel with high intensity, stable organization ‚ is mainly used in co2 gas stripping tower structure material formulation urea plants; Mo4.5 Cu is resistant to sulfuric acid, phosphoric acid and acetic acid, and other general medium corrosion and resistance to chloride stress corrosion and crevice corrosion pitting corrosion ‚ ‚ is mainly used in the sulfuric acid and phosphoric acid industry ‚ can produce sulfuric acid production equipment: tower, tank, pipe, heat exchanger, etc. 00Cr18Ni18Mo5 steel is resistant to both seawater corrosion and ammonium chloride corrosion. It is mainly used as carbonization tower material in alkali plant.

2. Application of molybdenum in ferritic stainless steel

Ferritic stainless steel (also known as chrome steel or high chromium iron ferrite stainless steel) in addition to the uniform corrosion resistant ‚ has excellent ability to resist chloride local corrosion ‚ such as pitting, crevice corrosion, stress corrosion, etc. Reducing the content of C and N can improve the corrosion resistance, brittleness, welding performance and machining performance of ferritic stainless steel. Mo in ferritic stainless steel, the main function is to promote passivation ‚ Fe - Cr alloy to improve the corrosion resistance of steel in the reducing medium ‚ especially steel in chloride solution of pitting, crevice corrosion resistance and other local corrosion resistance. The experimental results show that Mo can effectively improve the chloride corrosion resistance of ferritic stainless steel. The higher the Mo content of ferritic stainless steel, the better the corrosion resistance.

Low C, N and high Cr contain Mo A new type of ferritic stainless steel with high corrosion resistance ‚ such as 00 cr18mo1 ‚ 00 cr18mo2 ‚ 00 cr26mo1 ‚ 00 cr25ni4mo4 ‚ 00 cr30mo2 ‚ 00 cr29mo4ni2 ‚ has been successfully used in petroleum chemical industry, automobile, alkali, chemical fertilizer, paper and other industrial sectors. 00Cr18Mo1 is used for automobile panels to prevent corrosion from road salt. 00 00 cr18mo2 corrosion resistance is superior to cr18mo1 ‚ not only can be used in the automotive industry ‚ and can be used as heat exchanger under the condition of weak corrosion, acetic acid, production equipment, water equipment, solar collection panels, hot water heater, etc. 00 cr26mo1 in pit corrosion, crevice corrosion resistance to chloride is better than 316 l ‚ resistance to chloride stress corrosion ‚ boil at 42% MgCl2 solution under the condition of yield stress test 1000 h Don't burst ‚ steel intergranular corrosion resistance, organic acid, formic acid, acetic acid and mixed acid corrosion ‚ used in lye, organic acids, bleaching liquid fluid equipment, urea, containing chloride. 

The diaphragm alkali process of legal system of fluid mud abrasion situation ‚ 00 cr26mo1 far better than that of the pure nickel. Main Applications: NaOH production enrichment equipment, oil and power industry for heat exchanger, pressure vessels and other equipment. 00 cr25ni4mo4 in seawater and contain chloride medium has very good corrosion resistance ‚ is mainly used in seawater or equipment ‚ containing chloride solution, such as washing machine, heat exchanger, condenser, etc. 00 cr30mo2 in the resistance to pitting corrosion, stress corrosion in chloride solution ‚ uniform corrosion resistance is superior to the 316, 317 and Cr25Ni5Mo2N steel. Corrosion resistance and Ni in NaOH quite ‚ in high temperature and NaOH containing NaClO3 oxidant is better than that of 26-1 and pure nickel. See Fig.14 and Fig.15 for test results. Main use: NaOH concentration equipment ‚ flue gas desulfurization equipment, power plant condenser. 

The seamless pipe made of steel has been used in the seawater condenser of Tokyo Bay thermal power plant in Japan for more than 12.5 years. 00 cr29mo4ni2 is high pure ferritic stainless steel of the higher Cr, Mo ‚ in many harsh corrosive medium has excellent corrosion resistance, such as chlorine, alkali, corrosion resistance in nitric acid, organic acid, urea ‚ than corrosion resistant alloy. Have excellent corrosion resistance in the dilute sulphuric acid ‚ anti-oxidation, vulcanization performance ‚ in many harsh medium can be instead of Ti and Ni base alloy. Usage: as the water of the sea, all kinds of chloride solution and organic acid solution in the reactor, the condenser ‚ NaOH concentration production equipment.

3. Application of molybdenum in duplex stainless steel

Many ‚ duplex stainless steel grade can be divided into four categories: namely, low alloy, the alloy, high alloy and super duplex stainless steel. Besides low alloy two phase stainless steel does not contain Mo ‚ in high-performance alloy and high alloy containing Mo ‚ its content in the range was 2% ~ 4%. Mo is important in duplex stainless steel alloy elements ‚ it is not only a strong alpha ferrite formation elements ‚ and corrosion resistance of duplex stainless steel has an important role. Mo and Cr composite effect significantly improved duplex stainless steel in reducing medium such as local corrosion resistance in the medium containing chloride ‚ such as improve in chloride solution of dual phase steel spot corrosion, crevice corrosion resistant properties and stress corrosion resistance, etc. Mo promotes the formation of metal mesophase in steel and thus increases the brittleness tendency of the material. 

In containing a high percentage of Mo ‚ increasing N content in steel by adjusting the phase equilibrium can inhibit or delay the precipitation of brittle phase‚ so containing Mo in the new generation of high N content in the duplex stainless steel is higher also‚ for example‚ high alloy containing 3% Mo dual phase steel with about 0.18% N‚ containing 3.5% ~ 4.0% Mo The addition of high alloy biphase steel is about 0.25% ~ 0.30% N. Duplex stainless steel ‚ especially containing Mo duplex stainless steel in the medium containing chloride with high corrosion resistance uniform corrosion resistance, pitting, crevice corrosion, stress corrosion resistance and good resistance to corrosion fatigue and abrasion resistance‚ in corrosion resistant performance is better than 340 l, 316 l austenitic stainless steel, etc. As the Mo content increased ‚ steel corrosion resistance improved. On mechanical properties of high strength duplex stainless steel has a good processing property and welding performance. Biphasic stainless steel in neutral chloride medium, oil refining, petrochemical, natural gas, fertilizer, energy, food, light industry and alkali, naval vessels, hydrometallurgy, phosphoric acid industry and other industrial sectors are very important applications.

4. Application of molybdenum in martensitic stainless steel

Martensitic stainless steel is a kind of stainless steel with high strength, high hardness and high wear resistance. Martensite stainless steel are usually divided into ordinary martensitic stainless steel and precipitation hardening martensitic stainless steel ‚ is 13 Cr and 17 Cr high Cr martensitic stainless steel ‚ latter is CrNi martensitic stainless steel. Into GB1220-92 standard of martensitic stainless steel with a total of 21 steel grade ‚ ordinary martensitic stainless steel in 17 precipitation hardened martensitic stainless steel 4. Contain the Mo steel grade five, four ‚ ordinary martensitic stainless steel, 1 ‚ precipitation hardened martensitic stainless steel, namely

1Cr13Mo      0.3%~0.6%Mo3Cr13Mo      0.5%~1.0%Mo9Cr18Mo      0.4%~0.7%Mo9Cr18MoV           1.0%~1.3%Mo0Cr15Ni7Mo2Al       2%~3%Mo

The role of Mo in martensitic stainless steel mainly is to improve the strength and tempering stability of martensitic stainless steel. Ordinary martensitic stainless steel is mainly used as turbine blades, shafts, valves, cutting tools, etc. Precipitation hardened martensitic stainless steel, high strength, high toughness, fatigue resistance, corrosion resistance, resistance to abrasion ‚ as 0 cr15ni7mo2al steel strength can reach 1820 ~ 1890 mpa is used mainly for requirements good corrosion resistance, high strength of container, pipe, springs, grinding, etc.

5. Application of molybdenum in high performance stainless steel

High performance stainless steel is also called super stainless steel. These stainless steels are high purity, high Cr, high Mo nitrogen containing stainless steels (except ferritic stainless steels without N). Increasing the content of Cr and Mo is an important way to improve the corrosion resistance and mechanical properties of stainless steel. High performance stainless steel has a high Mo content: high performance ferritic stainless steel and duplex stainless steel mostly contain 3% ~ 4% Mo; High-performance austenitic stainless steel containing 4% ~ 6% Mo 654 smo steel molybdenum content as high as 7%.

The characteristics of high performance stainless steel is pitting equivalent index (PREN) general in more than 35 ‚ these steel in oxidizing medium and reducing medium has excellent corrosion resistance ‚ seawater corrosion resistant to various chlorides uniform corrosion and local corrosion medium ‚ in sulfuric acid and phosphoric acid have good corrosion resistance ‚ as 654 smo The resistance of the steel to chloride corrosion and various local corrosion in seawater can reach the level of AlloyC-276 nickel-based alloy. The corrosion resistance of the highest level ‚ some performance of the steel grade, usage in front have been made. These stainless steel in seawater desalination and seawater corrosion resistant equipment, environmental pollution waste treatment equipment has a very important role. The main uses of high performance stainless steel: papermaking bleaching equipment, condenser pipeline, seawater treatment equipment, high temperature seawater heat exchanger, reverse desalination production equipment, waste incineration equipment, environmental protection and thermal power station waste gas wet desulfurization equipment (FGD), phosphoric acid filter and mixer parts, waste sulfuric acid concentration equipment, pollution-free pharmaceutical equipment, etc.