Segregation Control Technology of Centrifugal Casting High Speed Steel Roll

Centrifugal casting roll has the advantages of a simple production process and equipment, high production efficiency, compact structure and less processing, and occupies a very important position in the field of high-speed steel roll manufacturing. Due to the effect of centrifugal force and the large density difference between various alloy elements and carbides formed in high-speed steel, ordinary centrifugally cast high-speed steel rolls are prone to segregation.

Segregation causes uneven performance of the working layer of the high-speed steel roll. Inside the working layer of the roll, as the distance from the roll surface increases, its mechanical properties and thermophysical properties change significantly, which seriously affects the use effect of the roll.

Segregation is one of the main defects of centrifugally cast high-speed steel rolls. In severe cases, the rolls will be scrapped, and segregation defects cannot be eliminated by heat treatment. Therefore, discovering the law of element segregation in centrifugal casting high-speed steel rolls and seeking measures to reduce and eliminate element segregation can improve the performance of rolls.

 

1. Effect of segregation on the microstructure and properties of centrifugally cast high-speed steel rolls

 

Centrifugally cast high-speed steel rolls

 

1.1 Effect of segregation on the microstructure of centrifugally cast high-speed steel roll

 

The radial distribution of alloy elements after solidification of horizontal centrifugal casting high-speed steel rolls under the moulding speed of 950r/min, pouring temperature of 1480℃, casting outer diameter ∅360mm, inner diameter ∅200mm, length 800mm and mould air cooling.

When the density of the elements (such as W and Mo) is higher than that of the metal solution, as the distance from the roll surface increases, the element content decreases continuously, while the density of the elements (such as V and C) is smaller than that of the metal solution, as the distance from the roll surface increases. Increase, its elemental content is increasing continuously. However, the segregation of elements (such as Cr) with a density close to that of the metal solution is not obvious. Roll segregation intensifies as the mould speed increases.

Element segregation leads to obvious segregation of roll structure, especially the obvious change of carbide distribution. The outer layer contains more M2C and M6C type carbides, while the amount of MC type carbides is less, and the inner layer is just the opposite. It is worth noting that a thin layer of metal close to the surface of the mould, due to the chilling effect of the mould, solidifies quickly and the element segregation is not obvious.

In addition, during the solidification process of the roll, it is not an ideal one-way heat transfer. The solidification speed of the inner surface of the roll is higher than that of the sub-inner surface of the roll. Therefore, the element segregation of the sub-inner surface of the roll is greater than that of the inner surface.

 

1.2 Effect of segregation on the performance of centrifugally cast high-speed steel rolls

 

The existence of segregation leads to obvious differences in the performance of high-speed steel rolls at different radial positions. The outer layer (<5mm) high-speed steel roll basically does not segregate due to chilling, and the performance of the outer layer high-speed steel roll can approximately reflect the performance of the roll without element segregation. Although the outer structure of the high-speed steel roll contains more W and Mo, the amount of V and C is low, resulting in a low amount of C in the matrix, poor hardenability, and poor wear resistance. In the inner structure of the roll, the content of W and Mo is low. When the high-speed steel is quenched, the MC-type carbide containing V is more stable, and the dissolution rate is slower than the M2C and M6C type carbide containing W and Mo. W and Mo in the quenched matrix structure have low content and poor wear resistance. In addition, segregation also promotes cracking during roll solidification and heat treatment.

 

2. Force and motion analysis of high-speed steel rolls in a centrifugal force field

 

During centrifugal casting, the rotating liquid metal occupies a certain space. If any particle M of the liquid metal is taken in this space, and its mass is m, the particle M will be subjected to centrifugal force F and gravity mg pointing outward along the radial direction. The dual role of, where F is

F a=mω²r (1)

In the formula:

ω—rotation angular velocity, r/s
r — the radius of rotation of the metal particle, m

The space occupied by the rotating metal that generates centrifugal force during centrifugal casting usually becomes a centrifugal force field, and in the centrifugal force field, each metal particle is subjected to the centrifugal force shown in formula (1). Since the particle is affected by the gravity mg in the gravitational field, corresponding to the gravitational acceleration g, the centrifugal acceleration in the centrifugal force field is ω²r, and the ratio G of the centrifugal acceleration to the gravitational acceleration is called the gravity coefficient, that is

G=ω²r/g (2)

Practical experience shows that, in order to obtain a dense structure when centrifugally casting rolls, a certain gravity coefficient must be guaranteed, and the value of G is required to reach tens or even more than one hundred. Since the centrifugal force is far greater than the gravity, in order to simplify the analysis, only the influence of the centrifugal force on the segregation of the high-speed steel roll is considered for the particle in the centrifugal force field.

According to the law of buoyancy, when heterogeneous particles (such as atomic clusters, first crystallized, etc.) coexist with the metal liquid, if their density is different from that of the metal liquid, these particles may float or sink in the gravitational field. The buoyant force F on the particle is

F f=πd³/6(ρ1-ρ2)g

In the formula:

d—heterogeneous particle diameter, m
ρ1—heterogeneous particle density, kg/m³
ρ2—density of molten metal, kg/m³

Similar to the situation in the gravitational field, under the centrifugal force field, the heterogeneous particles can obtain the radial buoyancy F and the radial moving speed V. Since all the material particles are aggravated according to the gravity coefficient, the buoyancy and The formula for calculating the moving speed is

F f=πd³/6(ρ1-ρ2) ω²r (4)

V a=d²(ρ1-ρ2) ω²r/18η (5)

Therefore, in centrifugal casting, if ρ1<ρ2, then V is less than 0, and the heterogeneous particles move to the inner surface of the roll. If ρ1>ρ2, then V is greater than 0, and the heterogeneous particle moves to the direction of the mould wall (that is, the outer surface of the roll).

Therefore, during the solidification process of high-speed steel rolls, as long as there is a density difference between the heterogeneous particles and the metal solution, element segregation of high-speed steel rolls is inevitable no matter whether horizontal centrifugal casting, vertical centrifuge or inclined centrifuge is used.

 

3. Segregation mechanism of centrifugally cast high-speed steel rolls

 

3.1 Research on MC-type carbide segregation

 

The literature points out that the segregation of the centrifugal casting high-speed steel roll is mainly the segregation of MC-type carbide, and the MC-type carbide is mainly the segregation of primary crystal VC, so the density difference between VC and molten steel is caused by a large difference. In order to prevent VC segregation, the measures of adding niobium element can generate MC-type composite carbides (W, V, Mo, Nb carbides) with higher density, whose density is close to that of molten steel, so that VC can be reduced and effectively controlled. MC-type carbide segregation. However, the segregation cannot be completely eliminated by only adding niobium to the high-speed steel roll. Niobium only has an effect on reducing the segregation of vanadium, but has no effect on the segregation of tungsten and molybdenum. It can be seen that the segregation of the centrifugally cast high-speed steel roll cannot be considered as the segregation of the primary crystallization VC, and its segregation mechanism is far more complicated than that of the primary crystallization VC segregation.

According to the Fe5%, Cr5%, W5%, Mo3%~4%V-C phase diagram provided by the literature, the cast high-speed steel roll is a hypoeutectic structure, and the primary phase austenite will be precipitated when it solidifies and cools from the liquid state. As the temperature decreases, more and more primary phases are precipitated, and the amount follows the lever law of the phase diagram. Before reaching the eutectic temperature, the primary phase and the mother liquor are in a state of two-phase coexistence. These first-precipitated primary phases become self-generated heterogeneous particles because there are fewer solid-dissolved carbon and alloy elements in the primary phase.

Energy spectrum analysis results of various elements in primary austenite w/%

Element C W Mo Cr V Si Fe
Content – 2.28 2.18 3.93 2.27 1.84 87.5

 

3.2 Theoretical analysis of atomic cluster formation in HSS solution

 

By analyzing the physical and chemical properties of various elements in high-speed steel rolls, it is found that the segregation of centrifugally cast high-speed steel rolls is mainly caused by the strength of alloying elements’ ability to form carbides and the degree of bonding between atoms. As we all know, when transition metal elements and carbon form carbides, the binding force between metal atoms and carbon atoms is mainly determined by the number of electrons in the outer shell of the transition metal element atoms because the valence electrons of carbon atoms will fill in the secondary electrons not filled by the metal. The d electron layer makes the atoms in the formed carbide have the property of metal bonding. Therefore, the fewer electrons in the sub-d electron layer, the more valence electrons are provided by carbon atoms when forming carbides, and correspondingly, the greater the binding force between metal element atoms and carbon atoms. From a thermodynamic point of view, the greater the absolute value of carbide formation heat, the higher its stability.

If the size difference between the metal atom and the carbon atom is greater, that is, the smaller Rc/Rme, it means that the electron excitation energy of the metal atom is smaller, and the easier it is to obtain electrons from the carbon atom to form carbide, and the carbide is more stable. The ratio of carbon atoms to the atomic radius of the main metal elements in high-speed steel rolls is shown in the table

Element Fe Cr V Mo W Nb
Rc/Rme 0.61 0.61 0.57 0.56 0.55 0.53

In addition, for any chemical bond, the greater the degree of bonding between atoms, the stronger the interaction energy between atoms, and the greater the bond energy. The degree of bonding between atoms can be reflected by the relevant physical parameters of the elemental substance. Generally speaking, the larger the value of these parameters, the stronger the ability of the element atom to participate in the bonding, and the greater the bond energy. This relationship not only exists between atoms of the same type but also has a similar relationship between atoms of different types. The greater the high-temperature specific heat, the smaller the self-diffusion coefficient, and the greater the binding force between the atoms of the element.

Before the eutectic reaction of high-speed steel, because Nb, V, W, and Mo have strong carbide capabilities, clusters rich in Nb-C, W-C, V-C and Mo-C will appear in the molten metal. In addition, due to the strong ability of element atoms such as W, Mo and Nb to participate in bonding, the bond energy of the formed chemical bond is also large, and there will also be rich W, Mo and Nb and W-Fe, Mo-Fe and Nb in the molten metal.

4. Conclusion

4.1 Centrifugally cast high-speed steel rolls are prone to segregation, which reduces the hardness, red hardness, impact toughness and wear resistance of the roll.

4.2 The main reason for the segregation of centrifugally cast high-speed steel rolls is that there are atomic clusters in the high-speed steel with different densities from the metal solution. Layer inner surface moves. The main reason for the formation of atomic clusters is that the physical and chemical properties of various elements in high-speed steel rolls are different.

4.3 The main factors affecting the segregation of centrifugal casting high-speed steel rolls are:

The number of revolutions of the centrifuge, the solidification cooling rate of molten metal and the properties of atomic clusters. The higher the rotation speed, the smaller the solidification cooling rate and the more serious the segregation. Atom clusters with the same volume and density are in the form of agglomerates or strips, which have a larger spatial extension than when they are spherical, which is beneficial to reduce segregation.

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