Section 1: Application of ductile iron in high-speed rail

Ductile iron casting is widely used in the production of high-speed rail parts with its excellent tensile resistance and high load and friction. The axle box of high-speed rail bogie, the shell and end cover of high-speed rail traction motor, the gear box and brake parts of high-speed train are all ultra-low temperature impact resistant ductile iron castings. Generally, QT 400-18AL (-50℃, -60℃) low-temperature impact resistance material.

Section 2: Material standard of high-speed rail ball iron parts

QT 400-18AL (-50℃, -60℃) resistant material, material requirements:

Material standard: 400-18AL,

Tensile strength of 400 Mpa,

Yield strength of 240 Mpa,

18%,

Hardness HBW 130-150,

Low-temperature shock value of-50℃, -60℃,

Impact work: 12 J / cm2,

Glographic matrix organization requirements:

Ball ping rate: over 90%,

Number of graphite balls: 100 cells / mm2,

Collective organization: 100% iron bodies,

Graphite size 6 a 7, no phosphorus co-crystal and carbide

100% ferrite matrix is the first condition for ductile iron to improve the low temperature impact value, even 1%~2% of beads will lead to the reduction of low temperature shock value. The same ferrite ductile iron, low phosphorus, low silicon annealing state of all ferritin ductile iron, its low temperature impact value is the best.

The sphericity rate to reach 90%~95% is necessary to ensure the low temperature shock value. The crack of ductile iron expands along the boundary of ductile. The more round the sphere, the less difficult it is to crack. Therefore, so the higher the spherical rate, the more conducive to the improvement of dynamic load performance such as impact performance and fatigue strength.

The number of graphite balls affects the low temperature impact: the number of graphite balls is conducive to the improvement of upper limit impact performance; the number of ducink is conducive to the improvement of low temperature impact performance. For the bogie axle box with the main wall thickness of 30 mm, take 100~200 pieces / mm2 appropriately. The phosphorous cocrystal and carbide on the grain boundary will greatly reduce the low-temperature shock absorption work of spherical iron.

Section 3, high iron ductile iron shaft box production process

1, raw molten metal requirements

Chemical composition of raw iron solution before pelleting the reaction:

C 3.9% to 3.98%

Si 0.65% -0.75%

Ni 0.75% to 0.83%

Mn ≤0.10%

P ≤0.025%

S ≤0.015%

Ti ≤0.015%

Cr ≤0.015%

Other trace alloys of 0.01%.

The level of Si content directly affects the tensile strength and impact value; when Si reduction cannot meet the intensity requirements, the problem of insufficient strength caused by Si reduction is solved by adding Ni.(This scheme plus Ni)

2. Raw materials

1>, pig iron: using longfengshan high-quality pig iron. The sum of pig iron trace elements is 0.10%.

2>, scrap steel: using carbon scrap steel.

(1) All scrap steel shall not be mixed with dirt. Non-ferrous metals or any type of foreign materials shall not have excessive rust and corrosion.

(2) The P and S content are not greater than 0.030%.

The mass score of Cr was not greater than 0.1%.

The mass score of Cu was not greater than 0.1%.

3>, Carburizing agent: high temperature full graphitization carbon increasing agent, C 98.5%, S 0.1%, N 0.03

Use more raw iron, less scrap steel, furnace material, calculation as close as possible to the ideal chemical composition, leaving a small amount of space for fine tuning. The purpose of adding raw iron is to ensure the number of original graphite core, and reduce the amount of carbon agent and reduce the sulfur of carbon agent. Commonly used formula pig iron accounts for 88% to 92%, scrap steel for 8% to 11%, ferrosilicon for 1.7% to 2.0%, carbon increasing agent for 0.2% to 0.24%, etc.

4>, spherical agent:

Brand: Lanthanide agent La 11

Mg 6-6.5%

La 1.0-1.2%

Si 43-44%

Ca 2.2-2.5%

Ba 1-1.5%

MgO <0.5

Particle size: 5 to 20mm

Adquantity: 1.2-1.3%

5>, gestation agent: using a long-acting compound gestation agent,

One gestation: silicon barium J a 4

Si 68 to 70%

Ba 4.5 to 5.5%

AL <1.0%

Size size 3-8mm,

Add quantity: 0.8 to 1.0%

Secondary gestation: silicon barium J a 3

Si 68 to 70%

Ba2.5 to 3.5%

AL<1.0%

Particle size of 1-3mm,

The addition volume was 0.2-0.4%

Flow with gestation: silicon bismuth J-1

From Si 70 to 72%

Ba 1 to 1.5%

And Ca 2 to 3%

Bi 1.5-2.5%

AL <0.8%

Particle size of 0.2 to 0.7mm,

Add to a volume of 0.1%

Incubation agent requires a strong effect on promoting graphitization, and can maintain a long time, high absorption rate and stable, so gestation is divided into the furnace before the reverse package gestation with the flow, is indispensable.

The use of high Ba anti-recession and the gestation agent containing Bi before the furnace can improve the sphericity condition of the central part of the section, make the spherical diameter small and the number of balls many, and improve the ferrite content and improve the casting performance.

3. Melting process:

The melting process of ductile iron is basically the same, the melting temperature is generally controlled at 1450 to 1520, the high temperature is static, the temperature is controlled at 1520 to 1550, the standing time is about 5 minutes, the oven temperature is 1480 to 1500,

Pour the water temperature: 1420

After high temperature standing, when the temperature in the furnace drops to 1480℃ -1500℃, wash into the molten iron for spherulization. After the reaction will be 2-3 times after the slag.

4. Chemical composition after spherization:

C 3.50 to 3.80%,

Si 2.05-2.15%,

Mg 0.035-0.045%，

La 0.005-0.009%

Ni 0.75-0.83%,

Mn ≤0.10%，

P ≤0.025%，

S ≤0.01%，

Ti ≤0.018%，

Cr ≤0.023%，

Other alloy elements: 0.01%.

Tensile strength of 405 to 415 PMa,

Yield strength of 260 to 275 PMa,

Elongation rate of 18% to 23%,

Bulping level 1, level 1 and level 2,

The shock absorption energy is set at 13 to 15J,

All meet the technical requirements of the grade material.

Section 4: Key points of rushing into the ball

We must ensure that the selected sphericity agent, gestation agent and other pure, there is no inclusion, impurities, oxides, etc., these substances can not be treated through the sphericity, the slag removal and finally into the casting to cause defects.

1. The temperature fluctuation of spherization is controlled at ± 15 degrees Celsius.

2. The ratio of the depth of H: 1.5 to 2.0 (the depth and inner diameter of iron liquid in actual treatment)

3. After adding spherizing agent, gestation agent, covering agent and other materials, the dam should be more than 20mm higher.

4. Turn the ball bag. Dry the ball bag thoroughly before use, and burn the bag in advance if necessary.

5. The order of packing is bulking agent, gestation agent with covering agent or iron cutting, must be rammed without gap

6. The optimal time for the general spherulation reaction is 1 to 1.5 minutes, and the compaction of the alloy and the thickness of the covering material are adjusted according to the reaction time.

7. 40 to 60% of the total (all the incubation agents added), 30% of the reverse incubation agent, and 10-15%.

8. The pouring time is controlled within 13 minutes from the end of the ball.

Section 5: Why lanthanide globation was chosen

At present, the rare earth used in China is mainly divided into three categories.(Except for the rare earth slag used by the defective manufacturers)

1,30 Rare earth silicon La accounted for 5% of the total RE

2. Lanthanum and cerium rare earth metal La accounted for 35% of the total RE

3. Pure metal lanthanum La accounted for 99.9% of the total RE

Although the main useful globular elements are mainly lanthanum, cerium,

In actual production, the comparison of the casting after spherization is as follows:

1. Number of graphite balls: the number of graphite balls produced by pure metal lanthanum is 1.5-2 times that of the spherical agent produced by lanthanum and cerium metal, and 2~3 times that of that produced by ordinary 30 rare earth silicon.

2. spherical rate: for the spherizing agent produced by pure lanthanum, the spherical rate is 10-15% higher than the spherical agent produced by lanthanum and cerium metal, and 15-25% higher than that of the spherical agent produced by ordinary 30 rare earth silicon.

3. Pearite content: with the spherical agent produced with pure metal lanthanide, the pearless content is 10-20% less than that of the spherical agent produced by lanthanide and cerium metal, and 30-40% less than that of ordinary 30 rare earth silicon.

4. Graphite sphere distribution. With the spheruizing agent produced by pure metal lanthanum, the graphite ball is small in size and round and many, and the distribution is obviously inclined distribution. It is better than lanthanum and cerium metal production, which is better than ordinary 30 rare earth silicon production globtizing agent.

5. White mouth tendency. The spherical agent produced with pure metal lanthanum has more white mouth resistance, and the 5mm sample section has no carbide. The tendency of sphericity produced with lanthanum and cerium is significantly reduced. Are better than the ordinary 30 rare earth silicon production of cryptizing agent.

6. Pinening tendency. With a spherulizing agent produced with pure metal lanthanum, the heat section of the cross shape specimen can be eliminated. The heat joints of the cross samples produced by lanthanum and cerium metal are greatly reduced, and the cross cross samples of 30 rare earth silicon production do not change significantly.

Section 6: High-temperature heat treatment

The matrix iron content of QT 400-18AL (-50℃, -60℃) must be 100%. Since a certain amount of Ni is added to improve the intensity, a trace amount of pearles must be produced. The purpose of high temperature heat treatment is to completely convert the pearlite into austenite, and then slow cooling according to the stable phase change, to achieve the pearlite all transformed into ferrite, in order to achieve 100% ferrite.

The 870℃ high temperature heat treatment affects the tensile strength, yield strength, and shock absorption energy, but has no effect on the elongation. The 870℃ high temperature heat treatment reduces the tensile strength by 5MPa, but the shock absorption energy increases by 3J, which can meet the requirements. Therefore, the QT 400-18℃ high temperature heat treatment process is used to produce the 18AL (-50℃, -60℃) low temperature shock material castings.

Section 7: Note 1,100% ferrite matrix is required to ensure the low temperature shock value, but 100% ferrite is difficult to ensure the tensile strength of 400 MPa, the matrix must be consolidated and strengthened to improve the tensile strength. This requires the use of higher Si in chemical composition or Ni and other alloy elements to strengthen the ferrite, so that they reach the requirement of 400 MPa.2. The strengthening of ferroiron bodies often leads to the increase of plastic-brittle transition temperature and the decrease of low temperature shock value. Therefore, it is necessary to adjust the relationship between tensile strength and low temperature shock value, and explore its solutions.3. When the metallographic organization is relatively perfect and basically meets the requirements, the low-temperature impact value should be further improved, which depends on the purification of the crystal boundary inclusion. How to purify the grain boundary will be the key factor to ensure the low temperature shock value.4, the impact performance of ductile iron decreases with the decrease of temperature is the inherent characteristic of ductile iron. In order to ensure the safe operation of parts at low temperature, it is not only necessary to ensure the impact value at low temperature, but also to study the plastic-brittle transition temperature curve, that is, the change curve of impact value with temperature. The more gentle the curve drops, the safer. It is not advisable to fluctuate the impact value greatly due to a slight change in temperature.