Effect of Mo, Al, Cu, W on steel properties

MO element

1. It can obviously improve the hardenability and heat strength of steel, prevent temper brittleness, and improve remanence and coercivity;
2. Molybdenum can refine the grain of steel, improve hardenability and heat strength, and maintain sufficient strength and creep resistance at high temperature (long-term stress at high temperature, deformation, called creep). The addition of molybdenum to the structural steel improves mechanical properties. It is also possible to suppress brittleness of alloy steel due to fire. Redness can be improved in tool steel.
3, molybdenum can increase the maximum strength and hardness of steel, so it is also very important in alloy steel.
a. It can improve the tensile and creep strength of steel at high temperatures.
b. In the case of working red heat, the hardness of the steel can be kept constant.
c. High-speed tool steel with molybdenum provides better cutting performance.
d. The addition of molybdenum to the alloy steel removes temper brittleness.
4. Improve the hardenability of steel. When the content is 0.5%, it can reduce the temper brittleness and have secondary hardening effect. Improve heat strength and creep strength, content 2% to 3%
Improve the resistance to organic acids and reducing media;
5. The good effect of molybdenum is:
1) The effect of refining the crystal grains is stronger than W, so the tendency of overheating of the steel can be lowered, and the strength, hardness, and thermal stability can be improved.
2) Mo in the steel will make forgings σb, σs, HB ↑, and make δ, ψ, αk ↓. Improve the tempering stability of the M body, and combine with Cr and Ni to greatly improve the hardenability.
The grain can be refined, the toughness is improved, and the forging process is easy.
3) Reducing temper brittleness, for some structural steels can eliminate temper brittleness (such as 24CrMoV5), so it can improve strength and plasticity does not decrease, molybdenum can improve the impact toughness of steel. 1 Furthermore, it is said that alloying elements (including Mo) can only suppress the temper brittleness and cannot eliminate the temper brittleness.
The effect of Mo is that a content of 0.2% has a good effect. Therefore, the ordinary alloy structural steel contains Mo 0.25 ~ 0.4% for the long-term work of the tempering brittle temperature range of 550 ~ 600 °C, the Mo content is 0.5 ~ 0.6%, when the Mo content exceeds a certain value (for low carbon steel) This limit is 1.0%), which will make the high temperature tempered water-cooled steel brittle. Mo steel is easy to temper for a long time and becomes brittle. 2 When P and Mo are high, temper brittleness cannot be avoided even if Mo or W is present. 3 Talk about the method of reducing the temper brittleness (see the previous paragraph).
4) Improve the coercivity of steel and improve magnetic properties.
5) The carbides are also very stable, and it prevents other carbides from being precipitated. High temperatures are also difficult to transfer to solid solutions.
6) Molybdenum can be substituted for tungsten (because the atomic weight is in a half relationship, 1% Mo can be used instead of 2% W).
7) Similarly, Mo can also improve the austenite stability for valve steel.
8) It can improve the resistance of intergranular corrosion of Cr and Ni stainless steel.
9) It is easy to passivate acid-resistant stainless steel in some reducing media to improve corrosion resistance. (such as sulfurous acid, boiling phosphoric acid and acetic acid, oxalic acid, formic acid, etc.).
10) The heat resistance of the pearlitic heat-resistant steel can be improved, and the heat-resistant steel can be added in a single amount, and the amount thereof is about 0.5 to 1% (when the alloying element is used alone, the steel has a tendency to graphitize).
6, the adverse effects of molybdenum:
1) Volatile, when heated, brown fumes (molybdenum oxide) will evaporate.
2) Promote decarburization , so the quenching temperature should be reduced by 10 to 20 °C to prevent decarburization, which hinders graphitization.
3) Mo is a ferrite forming element , so in order to obtain austenite, austenite forming elements such as Ni and Mn should be added in combination. Otherwise, when the Mo content is large, the ferrite δ phase or other brittle phase tends to occur and the toughness is lowered.
4) Mo reduces the thermal conductivity of steel by the same effect as W, but Mo prevents overheating .
5) Mo steel has higher deformation resistance than carbon steel.
7. Molybdenum content in general alloy steel:
Alloy structural steel and tool steel: Mo=0.15～0.30% (Example 5CrNiMo, 35CrMo, 40CrMnMo,
38CrMoAlA, etc.)
Stainless acid-resistant steel: Mo = 1 to 2.6% (such as Cr17Mo2Ti, Cr25MoTi, Cr18Ni18Mo2Cu2Ti, etc.)
Heat-resistant steel: Mo = 0.4 to 1% (16Mo, 20CrMo, Cr5Mo, 25Cr2Mo1VA, 15Cr11MoV, etc.)
Aerospace superalloy: Mo = 0.35 ~ 7% (such as 901 alloy = Cr12Ni43Mo6Ti3BMo2Ti)

Al element

1. It can refine the grain structure of steel and suppress the aging of low carbon steel. Improve the toughness of steel at low temperatures, and also improve the oxidation resistance of steel, improve the wear resistance and fatigue strength of steel.
2. Aluminum is a commonly used deoxidizer in steel. A small amount of aluminum is added to the steel to refine the grains and improve the impact toughness, such as 08Al steel for deep-drawing sheets. Aluminum also has oxidation resistance and corrosion resistance. Aluminum is combined with chromium and silicon to significantly improve the high temperature and corrosion resistance of steel. The disadvantage of aluminum is the influence on the hot workability, weldability and machinability of the steel.
(1) Used as a deoxidizing and nitrogen-fixing agent in steel making, refining crystal grains, suppressing the aging of low-carbon steel, improving the toughness of steel at low temperatures, and particularly reducing the brittle transition temperature of steel;
(2) Improve the oxidation resistance of steel. The oxidation resistance of iron-aluminum alloy has been studied extensively; 4% AI can change the structure of the scale, and adding 6% A1 can make the steel have oxidation resistance below 980C. When aluminum and chromium are used in combination, their oxidation resistance is greatly improved. For example, an alloy containing 50% to 55% of iron, 30% to 35% of chromium, and 10% to 15% of aluminum still has a relatively good oxidation resistance at a high temperature of 1400C. Due to this effect of aluminum, in recent years, aluminum has often been added as an alloying element to heat-resistant steel.
(3) In addition, aluminum can also improve the corrosion resistance to hydrogen sulfide and V2O5.
Disadvantages: 1 If the amount of aluminum is too much during deoxidation, it will promote the tendency of graphitization of steel. 2 When the aluminum content is high. Its high temperature strength and toughness are low.
3. a. It is easy to combine with oxygen to form alumina, which is a strong deoxidizer.
b. Can inhibit grain growth.
c. It is an important element of steel for nitriding.
4, the main role is to refine the grain and deoxidation, in the nitriding steel can promote the nitriding layer, when the content is high, can improve the high temperature oxidation resistance,
It is resistant to the corrosion of H2s gas, has a large solid solution strengthening effect, improves the heat resistance of the heat resistant alloy, and has a tendency to promote graphitization;
When Al is less than 3 to 5% in steel, it is one of high resistance to oxidation and electrical resistance.
1 Al, added as a strong deoxidizer, produces highly fine, ultramicroscopically beneficial elements. The oxide of its action is dispersed in the volume of the steel. Therefore, the grain growth when the steel is heated can be prevented (including Al<10%, refining effect when heating <1200°C, otherwise the effect is very small) and improving the hardenability of the steel. Therefore, these oxides become the center of crystallization, and promote the decomposition of the A body when the steel cools.
As an alloying element, it contributes to the nitriding of steel, thereby improving the thermal stability of steel. Therefore, AlN itself has high stability when heated.
Both 1 and 2 are beneficial to weaken the tendency of the steel to overheat.
3 can improve the oxidation resistance of steel, consider 2 and 3,
4 can improve the electrical resistance of steel, together with Cr for the manufacture of high-resistance chrome-aluminum alloy: such as Cr13Al4, 1Cr17Al5, 1Cr25Al5. Al increases the resistance to a much higher degree than CCr. Adding Al to CCr steel will make the coarse crystals brittle, so the amount is generally not more than 5%, and the individual is only 8 to 9%.
5 For silicon steel, Al can reduce the loss of α core and reduce the magnetic induction intensity. The combination with oxygen can weaken the magnetic aging phenomenon, but the oxide of Al can deteriorate the magnetic properties. Al (>0.5%) also makes the silicon steel brittle.
6. The adverse effect of Al 1 promotes the graphitization of steel and reduces the carbon concentration in the alloy phase, so the hardness and strength are lowered.
2 Accelerated decarburization When the Al content is increased to 3 to 5%, 8 to 9% will greatly promote the columnar crystallization process of the steel ingot. Therefore, the difficulty of mechanical thermal processing of steel is greatly increased, and the steel is extremely easy to decarburize. (The reason why the thermal processing is difficult is because the alloy steel ingot has a coarse crystal structure, and the cleavage of the crystal is extremely weak, so the thermal conductivity is low, a large temperature difference is easily generated during heating, and the steel is ingot, even the peeling process of the steel ingot is The grain boundary is oxidized and destroyed. In addition, it is easily brittle when it is stopped at a high temperature of 800 ° C or higher for a long time.
The amount of Al in general alloy steel:
Alloy structural steel: Al = 0.4 ~ 1.1% (38CrAlA, 38CrMoAlA, 38CrWVAlA, etc.)
Heat resistant stainless steel: Al = 1.1 ~ 4.5% (Cr13SiAl, Cr24Al2Si, Cr17Al4Si, etc.)
Electrothermal alloy: Al=3.5～6.5% (Cr13Al4, 1Cr17Al5, Cr8Al5, 0Cr17Al5, etc.)
Even Al = 8% Cr7Al7: Considering that the electrothermal alloy has little load, it can be used although it is brittle.

Cu element

1. Its outstanding function is to improve the atmospheric corrosion resistance of ordinary low alloy steel, especially when combined with phosphorus;
2. Copper can improve strength and toughness, especially atmospheric corrosion. The disadvantage is that it is prone to hot brittleness during hot working, and the copper content is significantly reduced by more than 0.5%. When the copper content is less than 0.50%, there is no influence on the weldability;
3, the content of copper in alloy steel can not exceed 1.5%, otherwise it will make the steel brittle, in addition
a. Copper has resistance to atmospheric corrosion in steel. Low carbon steel contains 1% copper, which is about four times higher than atmospheric corrosion resistance. Adding 3–4% of copper to stainless steel also helps to prevent corrosion of stainless steel.
b. Can increase the strength of steel, but should not exceed 0.2%.
4. When the content is low, the effect is similar to that of nickel. When the content is high, it is unfavorable for thermal deformation processing, such as exceeding O. At 30%, it causes high-temperature copper brittleness during hot deformation processing, and the content is higher than O. At 75%, aging strengthening can be produced after solution treatment and aging. In low carbon alloy steels, especially with phosphorus,
It can improve the corrosion resistance of steel, and 2% to 3% of copper can improve the corrosion resistance and stability to stress corrosion of sulfuric acid, phosphoric acid and hydrochloric acid in stainless steel.
5. The beneficial effects of copper:
1) It can improve the stability of austenite in steel, so it can improve the hardenability and hardenability. Adding 2 to 4% Cu to the A-steel steel can improve the acid resistance.
2) It has the function of strengthening ferrite. Adding Cu to ferrite can improve its corrosion resistance in certain reducing media and improve the toughness of steel.
3) Adding about 0.20% of Cu to low-alloy steel, especially when used in combination with P, can improve the corrosion resistance of steel to the atmosphere. When the amount of Cu exceeds 0.75%,
The strength of the steel can be increased by precipitation hardening.
4) Cu is a strong graphitizing element used in the refining of graphite steel. (The age hardening of Cu steel is because the solubility of Cu in α iron varies greatly,
It is 3.5% at 830 °C and 0.35% at 20 °C, so it can promote age hardening with Cu>0.35%. )
6. The adverse effects of copper are:
1) When the amount of Cu is high, the steel will be hot brittle, which makes the hot forging process difficult.
2) Excessive Cu content increases coercivity and hysteresis loss, which is unfavorable for magnetic steel.
3) "Copper brittleness" - in the article on steel defects, it is pointed out that when Cu>0.2%, the heating process is selectively oxidized on the surface, causing Fe to oxidize before Cu, and the surface Cu content is relatively increased to form a film. Then, a Cu-containing network is formed to diffuse, and it is easily forged at 1030 °C. Appropriate addition of Ni can produce a Cu-Ni solid solution with a higher melting point, which can reduce "copper embrittlement".
In general, Cu<0.7% is dissolved in α-Fe, which promotes carbon oxidation and has no significant effect on magnetic properties. When Cu is 0.5%, the rust prevention ability can be increased by 15 times;
When Cu>0.7%, a heterogeneous mixture will appear, which will increase the coercive force and hysteresis loss and make the copper brittle.
(3) Copper content in general alloy steel: 1) Silicon steel: containing Cu = 0.2 to 0.3%
2) Bearing steel: Cu≤0.25%, and Cu+Ni≤0.50%, because Cu will cause age hardening and affect the bearing precision, Ni will reduce the hardness of the quenching layer.
3) Low-alloy steel: Cu ≤ 0.2% (for low-alloy high-strength steel, the alloying element content is generally limited to ≯0.2%).
4) Stainless steel: containing Cu = 2 to 4% (Cr18Ni9Cu3Ti, etc.). 5) Graphite steel: containing Cu=0.6%

W element

1. It can improve the red hardness and heat strength of steel and improve the wear resistance of steel;
2. Tungsten has a high melting point and a large specific gravity. It is an alloying element of precious earth. Tungsten and carbon form tungsten carbide with high hardness and wear resistance. Adding tungsten to tool steel can significantly improve red hardness and heat strength for cutting tools and forging dies;
3, tungsten can withstand high temperatures, and dissolved in steel will form carbides with carbon called tungsten carbide, which can improve the strength of steel. In addition,
a. Tungsten can increase the quenching temperature of steel.
b. Strengthen the microstructure of the steel section to resist tempering and softening.
c. It can reduce the tendency of grain growth of steel during quenching.
d. Tungsten steel knives have red hot hardness.
e. It can increase the magnetic retention of steel, so it can be incorporated into steel to make permanent magnetic steel.
4, there is a secondary hardening effect, so that the steel has red hardness, improve wear resistance, the impact on the hardenability, tempering stability, mechanical properties and thermal strength of steel are similar to molybdenum, slightly reduce the oxidation resistance of steel Sex
5. The good effect of W is:
1) Refining the crystal grains, (the effect is stronger than Cr, so the tendency of the steel to be overheated can be lowered, and the strength, toughness and thermal stability can be improved.
2) Improve the stability of the M body and greatly improve the hardenability. The 18CrNiWN can be completely hardened at any cooling rate, and the M body is obtained with W18%.
With Cr4% matching, the stability of M body can reach 600 °C, and it is red hard.
3) Prevent the development of temper brittleness, so it can increase the strength without reducing the plasticity and improving the toughness.
4) Improve the coercive force of the quenched steel and prevent the aging of the steel.
5) The carbide is extremely stable, and it is difficult to dissolve into the solid solution at high temperature, so it can be used as a high speed tool steel.
6) W steel generally has good hardness and wear resistance, small heat treatment deformation, is not easy to be quenched, and has good tempering stability.
7) Because it can improve the stability of the A body, it is used in the valve steel to reduce the γ area and the same Cr.
8) It can improve the heat strength of pearlitic heat-resistant steel and increase the recrystallization temperature.
9) Adding 2 to 4% W to high-alloy Cr and Ni steel can increase the yield point, fatigue strength and thermal stability of steel, and reduce the tendency of crystal orientation corrosion due to the formation of carbides (Fe3W2 and Fe2W are both poles). Stable compound, highly diffused, so it can improve strength and thermal stability.
W element
6, the adverse effects of W:
1) Increase decarburization (carbide stabilization) to prevent graphitization.
2) W is a strong carbide element, which should prevent the unevenness of carbide from affecting the performance of waste products (can increase the number of plucking and normalizing treatment).
3) When W>9%, the hardness is remarkably improved, while δ and ψ are significantly lowered.
4) W reduces the thermal conductivity of steel, including W>10%, and its thermal conductivity is only 0.7 times that of pure iron.
5) The inclusion of W increases and the range of forgeable temperature decreases.
7. The content of W in general alloy steel:
Alloy structural steel: W=0.3~1.0% (Example 20Cr3MoWVA, 18Cr3MoWVA, etc.)
Heat-resistant stainless steel: W = 0.3 ~ 3.2% (the last two kinds of anti-hydrogen steel is also this type, and also like Cr15Ni36W3Ti, etc.)
Alloy tool steel: W=1～18% (CrWMn, W, W2, 3Cr2W8V, P9, P18, etc.)
It is not appropriate to add more than 20 to 22% of W to the steel. Therefore, exceeding this value W does not have a better effect on the performance of the steel, so it is uneconomical to add more.
The methods generally mentioned to reduce the temper brittleness are as follows:
1) Add 0.3 to 1.0% of Mo or 1 to 1.5% of W to the steel.
2) Use tempering for rapid cooling, water or oil cooling.
3) Increasing the tempering temperature, this method is less useful because the performance (strength) is lowered and it is not fully utilized.
4) Extend the tempering time or increase the number of repeated high temperature tempering.
5) Reduce the quenching temperature, or use the secondary quenching 1AC3 or more and normal quenching 2AC1 ~ AC3 between incomplete quenching to eliminate temper brittleness.
6) Add V to the steel (about molybdenum content + 0.1% equivalent) to improve the tempering stability of the steel and suppress temper brittleness, but this effect is minimal.
7) Using deformation heat treatment, that is, heating to AC3 or higher for deformation (15 to 20% deformation), quenching and tempering immediately, and improving toughness
3 times.
8) High speed (1000 ° C / sec) heating and cooling, the last two methods are not used because of the difficulty in production.
Note: The temper brittleness is often referred to as the second type of temper brittleness (ie, 450 to 570 ° C can occur at 650 to 700 ° C), while the first type of temper brittleness (250 to 400 ° C), all steel is inevitable Exist, but repeated tempering can be eliminated, so it is no longer discussed.

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