1.Why do double phase diagrams exist? What is the practical significance of the existence of double phase diagram for the production of cast iron? What is the practical significance of the effect of silicon on the dual phase diagram?
A: 1) From the thermodynamic point of view, under certain conditions, according to Fe-Fe3C phase diagram transformation is also possible, so there is a duality; 2) Through double equivalence, it can be clearly seen that the temperature of stable equilibrium during eutectic transition and eutectoid transition is higher than that during mesostable equilibrium, while the C content required for eutectic and eutectoid transition and the carbon content in r after transformation are lower than that in mesostable equilibrium. According to this rule, the microstructure of cast iron after solidification can be controlled by controlling the temperature composition. 3) The role of silicon: a: the carbon content of eutectic point and eutectoid point decreases with the increase of silicon content; b: The addition of silicon makes a triple coexistence zone of eutectic and eutectoid transition appear on the phase diagram; c: The eutectic and eutectoid temperature ranges change, the higher the silicon content, the more the eutectic temperature of the stable system is higher than that of the mesostable system: d: The increase of the silicon content shrinks the austenite region on the phase diagram.
2. Analysis and discussion of flake graphite, spherical graphite, wormlike graphite growth process and formation conditions.
1) Flake graphite: According to the crystal growth theory, the normal growth mode of graphite is optimized along the base surface to form flake tissue. There are many defects in actual graphite crystals, and the spiral dislocation defects can promote the formation of flake graphite. The spiral dislocation provides two vertical growth directions a and c for the growth of graphite. When the growth rate of a direction is greater than that of c direction, flake graphite will travel. 2) Spherical graphite: The rotating grain boundary defect in the graphite crystal can promote the formation of spherical graphite, in addition, in the spiral dislocation, when the growth rate of the c direction is greater than the growth rate of the a direction will form spherical graphite. The formation of spherical graphite generally begins with a crystal nucleus composed of sulfide and oxide of calcium and magnesium, and after spheroidization, it is also conducive to the growth of spherical graphite. There are two necessary conditions for the growth of spheroidal graphite: large supercooling degree and large interfacial tension between liquid iron and graphite; 3) Wormlike graphite: there are two formation processes: 1> Small spherical ink → distorted spherical ink → vermicular graphite; 2> Small flake graphite → local enrichment of vermiculate elements → vermicular graphite.
3, Try to discuss the classification of phosphorus eutectic, precipitation process and how to control the shape (thickness) and quantity of phosphorus eutectic.
Answer: According to the different composition of phosphorus eutectic can be divided into binary phosphorus eutectic and ternary phosphorus eutectic. The formation of phosphorus eutectic is caused by the segregation of phosphorus. The part of phosphorus that is separated first contains less P, and P is continuously enriched. When the content is high to a certain degree, phosphorus eutectic is formed. Practice has proved that if the graphitization ability of cast iron is strong or the cooling rate is low, stable ternary phosphorus eutectic will be formed, the form is similar to binary phosphorus eutectic, and vice versa, metastable ternary phosphorus eutectic will be formed. In gray cast iron, mainly stable primary phosphorus eutectic.
Carbon equivalent: According to the influence of each element on the actual amount of carbon at the eutectic point, the amount of these elements is converted into an increase or decrease in the amount of carbon. (CE=C+1/3 (Si+P)) eutectic: the ratio of the actual carbon content of the cast iron to the actual carbon content of the eutectic point.
4.Segregation: The phenomenon of uneven distribution of the component elements in the alloy during crystallization is called segregation.
Austenite diameter segregation characteristics: there is enrichment of silicon in the initial austenite, low manganese, and high carbon, high manganese and low silicon in the residual liquid between dendrites.
Partition coefficient: Kp= concentration of elements in austenite xA/ average concentration of elements in liquid iron xI (phase heterogeneity)
Segregation coefficient: Kl= concentration of elements in the core of austenite dendrites/concentration of elements at the edge of austenite (inhomogeneous)
5.Eutectic clusters:
Such a graphite-austenite biphase symbiotic growth eutectic grain is formed at the center of each graphite core
6, spherical graphite structure characteristics and formation conditions:
Spherical graphite has a polycrystalline structure, radiating outward from the core, and each radial Angle is formed by the accumulation of parallel graphite surfaces perpendicular to the ball, and the graphite ball is composed of about 20 to 30 such cone-shaped graphite single crystals. Conditions: The solidification of liquid iron must have a large degree of supercooling and a large interfacial tension between liquid iron and graphite.
Act 2
1.What are the characteristics of the metallographic structure and properties of gray cast iron?
A: The metallographic structure of gray cast iron is composed of metal matrix and flake graphite. The metal matrix consists of pearlite, ferrite and pearlite plus ferrite. The shape, size, quantity and distribution of graphite are the main factors that determine the performance of gray cast iron: 1) the strength performance is poor, because of the reduction and notch effect of graphite; 2) Hardness characteristics, the hardness of gray cast iron is mainly determined by the matrix, ferrite is soft, low strength, pearlite strength hardness is high, but toughness is lower than ferrite, because the strength is mainly affected by graphite, hardness is mainly affected by the matrix, so the same strength, hardness has a range, the same hardness; 3) Low notch sensitivity; 4) Good shock absorption; 5) Good anti-friction; 6) Good castability; 7) Good machinability.
2.How does the cooling rate affect the cast iron structure?
A: The cooling rate increases, the supercooling degree of liquid iron increases, the distance between the eutectic reaction platform and the Letenite eutectic line is getting closer and closer, and it is easy to form a white hole. In actual production, the influence of the cooling rate is generally reflected by factors such as the wall thickness of the casting, casting conditions and pouring temperature.
3, Quality coefficient: quality coefficient Qi is the ratio of maturity RG and hardening HG.
The maturity RG is the ratio of the tensile strength measured by a test rod with a diameter of 30mm to the tensile strength calculated by the eutectic degree. The RG value is better in the range of 1.15~130, and proper overheating and inoculation can improve the RG value. If… 1 indicates poor breeding, low production level, and failure to play the potential of the material.
The hardness is the ratio of the measured hardness to the hardness calculated from the eutectic degree. The lower the HG, the higher the strength of gray iron, the lower the hardness and the better the machinability.
Why can it measure the metallurgical quality of cast iron? A: The higher the Qi value, the better the metallurgical effect, fluctuating between 0.7 and 1.5, > 1 is good.
4, What is the main way to improve the performance of gray cast iron?
1) Reasonable selection of chemical composition. Under the condition of keeping the carbon equivalent unchanged, an appropriate increase in the Si/C ratio, such as 0.5 to 0.75, will have the following effects: 1> The increase of primary austenite has the effect of strengthening the matrix. < 2> G reduction, reduction, cutting effect is reduced; < 3> The amount of silicon dissolved in iron increases and the ferrite is strengthened. < 4> With the increase of eutectic transition temperature, the pearlite is slightly coarsened, which is unfavorable to the strength property. < 5> Silicon increase, promote graphitization, reduce white mouth tendency
2) Inoculation treatment. The purpose is to promote graphitization, reduce the tendency of white bite, reduce the sensitivity of cross section, refine the grain, increase the number of eutectic pellets and promote the formation of fine flake pearlite.
3) Low alloying, adding a small amount of alloying elements, often have the following effects: refining graphite, ferrite reduction or even disappearance, pearlite refinement, ferrite solid solution strengthening, and thus have higher strength properties. While trying to improve the strength of cast iron, we must pay attention to: to maintain a high carbon equivalent, in order to maintain the casting performance of cast iron, so as to give full play to the advantages of gray cast iron, measures are: a certain degree of overheating, enhanced inoculation treatment, low alloying.
5, The influence of common gases on the graphitization of cast iron
Hydrogen: can make the shape of graphite become thicker, and at the same time have a strong stability of cementite and hinder the ability of graphite precipitation. In addition, there is a tendency to form anti-whiteness. With the increase of hydrogen, the mechanical properties and casting properties of cast iron will deteriorate.
Nitrogen: hinders graphitization, stabilizes cementite, promotes the formation of D-type graphite, and also promotes the formation of wormlike graphite. Nitrogen has the effect of stabilizing pearlite, so it can improve the strength of cast iron.
Oxygen: hinder graphitization, increase the tendency of white mouth, increase oxygen content, increase the cross-section sensitivity of cast iron, when oxygen increases, it is easy to produce pores in the casting, increase the consumption of inoculant and metamorphic agent.
6. How to evaluate the purpose and effect of inoculation
The purpose is to promote graphitization, reduce the white tendency, reduce the section sensitivity, control the graphite form, eliminate the supercooled graphite, appropriately increase the eutectic cluster number and promote the formation of fine flake pearlite.
How to evaluate the effect: (1) the number of white population decreased; (2) The number of eutectic clusters increased; (3) Reduce the degree of supercooling.
Inoculation treatment: Before the iron pouring, under certain conditions, a certain amount of material is added to the iron liquid to change the solidification process of the iron liquid, improve the as-cast structure, so as to achieve the purpose of improving the performance of the treatment method.
Chapter III Toughening cast iron
- Analyze the reasons why ductile iron is more sensitive to the incision than gray iron, and the shock absorption and thermal conductivity are poor.
The sensitivity, shock absorption and thermal conductivity of cast iron depend on the microstructure of the metal matrix and graphite. Gray cast iron has a large number of flake graphite, which is equal to a large number of gaps in the interior, thus reducing the sensitivity of external gaps to mechanical properties, the same large number of flake graphite cuts the matrix, organized the transmission of vibration, and can be converted into heat and diverges, so it has good shock absorption. The structure of nodular cast iron is a metal matrix and small round graphite, graphite and the metal matrix have no damage effect, so the notch sensitivity than gray cast iron is poor shock absorption. In the same way, due to the good thermal conductivity of graphite, a large number of graphite flakes of gray cast iron is conducive to heat transfer, and nodular cast iron is round and spherical, no flake transfer is good, so nodular ink has poor thermal conductivity than gray cast iron.
2, ductile iron production chemical composition selection principle is? How is it different from gray cast iron?
The selection should not only be conducive to the spheroidization of graphite to obtain a satisfactory matrix, but also to make the cast iron have better casting performance, and appropriately increase the Si/C ratio (such as from 0.5-0.75) for gray cast iron under the condition that the carbon equivalent remains unchanged.
3.What are the main directions of nodulating element magnesium in the nodulating process? How to improve magnesium absorption rate?
Where magnesium goes – desulfurization, deoxygenation – spheroidization of liquid iron – burning and floating gasification. Methods The pressure plus magnesium method, rotating pack method and magnesium alloy method were established by ourselves.
The heat treatment of austenitic bainitic ductile iron was analyzed:
- The influence of changing heating temperature and isothermal quenching temperature on the formation structure and performance.
(1) In order to obtain bainite structure, it is necessary to carry out isothermal quenching treatment of ductile iron. Lower bainite can be obtained by isothermal quenching at low temperature, and austenite and upper bainite can be obtained by isothermal quenching at high temperature.
(2) Austenite-bainite structure is also affected by isothermal temperature. When the isothermal temperature is higher than 330~350 (generally 350~370), the matrix is mainly bainite and austenite, the strength and hardness are lost, and the wear resistance is good, in addition, the difference in isothermal temperature will also make the amount of residual austenite in the matrix different.
- What is the difference between the purpose of inoculation treatment of malleable cast iron and gray cast iron and ductile cast iron?
The purpose of inoculation treatment of gray cast iron; Promote graphitization, reduce white tendency, reduce section sensitivity, control graphite form, eliminate supercooled graphite, appropriately increase eutectic cluster number and promote the formation of fine pearlite. The purpose of inoculation of nodular cast iron is to eliminate the tendency of supercooling, promote the nodulation of graphite and reduce the segregation between grains. The purpose of inoculation treatment of malleable cast iron is to promote the formation of cementite structure during primary crystallization, and have no effect on or promote the formation of graphite during the subsequent graphitization annealing process. The biggest difference is that malleable cast iron expects cementite rather than graphite when first crystallized.
3, strong cast iron?
Ductile cast iron is the general name of ductile cast iron, vermicular cast iron and malleable cast iron.
4, several kinds of ductile iron organization? The production process?
The normal structure of ductile iron is a small round graphite ball plus a metal matrix, in the cast state, the metal matrix is usually a mixture of ferrite and pearlite. Production process: molten iron qualified, (composition and temperature) nodulization treatment, inoculation treatment, pre-furnace inspection, casting iron, cleaning and heat treatment, casting quality inspection.
5, for the smelting requirements of ductile iron, commonly used nodulating agent, nodulating treatment method?
Requirements for smelting high-quality liquid iron should be high temperature, low sulfur, low phosphorus content and low impurity content (such as oxygen and denitrification element content). Rare-earth magnesium alloys are commonly used in China, while magnesium alloys and pure magnesium alloys are mostly used abroad.
Nodularization treatment methods: (1) Magnesium as nodulating agent, self-built pressure adding magnesium, rotating pack, magnesium alloy method; (2) rare-earth magnesium alloy injection method, internal nodularization method.
6, why should the production of ductile iron inoculation treatment?
Objective To eliminate the tendency of crystallization supercooling, promote graphitization and reduce intergranular segregation
7, nodular cast iron solidification characteristics
1, with a wide range of eutectic solidification temperature
2, paste solidification characteristics
- Large eutectic expansion
8, the common defects of ductile iron?
Common defects such as shrinkage porosity, slag inclusion, subcutaneous pores, graphite floating and spheroidization decay.
- Main causes of spheroidization decline and measures to be taken
Reason: Magnesium, rare earth elements continue to escape from liquid iron, escape usually through oxidation loss, sulfur and combustion loss, and in addition to the continuous decline of inoculation.
Measures:
- Maintain sufficient nodulating element content in liquid iron
2, reduce the sulfur content of the original iron liquid and prevent the oxygen of iron liquid
- Shorten the residence time of liquid iron after spheroidization
4, after the liquid iron is nodulated and slagging, in order to prevent magnesium and rare earth elements from escaping, the surface of the liquid iron can be covered with a covering agent to isolate air to reduce escape.
- Performance characteristics of vermicular graphite cast iron and where is it commonly used?
1, strength properties: the tensile strength of vermicular graphite cast iron is much less sensitive to carbon equivalent changes than ordinary gray cast iron.
2, toughness and elongation: the impact toughness and elongation of vermicular cast iron are lower than that of ductile cast iron but higher than that of gray cast iron, low creep rate or high content of ferrite in the matrix, toughness and elongation are high.
3, the thermal conductivity of vermicular graphite cast iron mainly depends on the shape of graphite, when the creep rate is high, the thermal conductivity is basically equivalent to gray cast iron, and when it is low, it is close to ductile iron.
4, casting performance has good fluidity.
Application: 1. Due to the high strength and small sensitivity to the section, the casting performance is good and can be used to manufacture complex large parts such as transmission boxes
2, because the vermicular graphite cast iron has high mechanical properties and good thermal conductivity, it is commonly used to manufacture parts such as automotive brake discs that work under heat exchange and large temperature gradients.
11, the classification of malleable cast iron, the composition of malleable cast iron selection principle.
Categories Ferritic malleable iron, pearlite malleable iron, white core malleable iron. Chemical composition is the main factor that determines the mechanical properties and heat treatment time of malleable cast iron.
Selection principle: (1) To ensure that the entire section of the casting can be obtained in the casting state, there is no pitting, otherwise it will significantly reduce the mechanical properties.
(2) The graphitization process should be fast to ensure that the graphitization annealing is completed in the shortest possible time and the production cycle is shortened.
(3) It is conducive to improving mechanical properties to ensure high-quality products. 4) Under the premise of ensuring mechanical properties, it has better casting performance to facilitate the acquisition of sound castings.
- Graphitization process and influencing factors of malleable cast iron
(1) Graphite nuclei are formed on the austenite grain boundary;
(2) cementite continuously dissolves into Austenite;
(3) The diffusion of carbon atoms from the high concentration region to the low concentration region;
(4) Carbon atoms are deposited into the graphite core, that is, graphite grows.
