News

Discussion on the influence of titanium and zirconium on cast iron and other factors

Melting HT300 machine tool cast iron, the strength and hardness of cupola castings in production are always better than that of electric furnaces. Although the five major elements of molten iron are basically the same and also increase sulfur content in electric furnace molten iron, the tensile strength generally differs by 30~20MPa, and the hardness HB of the bed guide rail surface after rough machining differs by 10~30. In the end, the only solution was to reduce the carbon content of the electric furnace molten iron, barely meeting the strength and hardness requirements of the castings.
In the past, it was generally believed that the overheating temperature, chemical composition, and purity of molten iron in electric furnaces were better controlled than those in cupola furnaces; The solidification undercooling of molten iron is relatively high, and the grain size is refined; The strength and hardness of electric furnace castings should be better than those of cupola furnaces when the raw materials and iron treatment are completely consistent. After more than a month of comparing the chemical composition of cupola and electric furnace molten iron, it was found that the main difference lies in the titanium and nitrogen content in the molten iron. The titanium content of electric furnaces is generally between 0.035% and 0.055%, while that of cupola furnaces is between 0.019% and 0.025%. The nitrogen content in the molten iron of the cupola is around 90-120ppm, while the nitrogen content in the molten iron of the electric furnace is below 100ppm.
When the nitrogen content in the molten iron is high, the ferrite will be supersaturated by nitrogen when the casting cools. At room temperature, with the prolongation of time, nitrogen gradually precipitates in the form of Fe4N. The strength and hardness of the casting increase, but plasticity and toughness decrease. The higher the dissolved nitrogen content in cast iron, the lower the degree of graphitization, the less carbon precipitated, and the more Fe3C generated. Therefore, nitrogen promotes the formation of pearlite in cast iron and suppresses ferrite in the matrix. To melt high-grade gray cast iron in cupola and medium frequency electric furnace, it is necessary to develop a molten iron treatment process based on principles to solve the mechanical properties of high-end gray cast iron. Nitrogen in the air will dissolve into the molten iron, and the equilibrium concentration of N in the molten iron is about 100 × 10-6. The nitrogen in pig iron, scrap steel, and carburizing agents is also an important factor in increasing the nitrogen content of molten iron.
A nitrogen content greater than 110 × 10-6 in molten iron can lead to cracked nitrogen pores in castings. The graphite of pig iron with extremely low nitrogen content is coarse and can be passed on to subsequent production of cast iron. The low nitrogen content in molten iron makes it difficult to generate Fe3C, making it difficult to form pearlite. Pearlite is an organic complex that combines the interface between ferrite and carbides through eutectoid decomposition in a certain proportion. Increasing the nitrogen content below the limit is beneficial for increasing the pearlite content of cast iron, thereby enhancing its strength.
However, for large wind power castings, using QT350-22 and QT400-18 with an all ferrite matrix is necessary to achieve stable castings with good impact toughness at both room temperature and low temperature, which will inevitably reduce the nitrogen content of the molten iron. Ti and Zr belong to the periodic table IVA group and have many similar chemical properties. For example, they all have strong nitrogen fixation ability to generate TiN and ZrN with N.
1. The effect of titanium and zirconium on nitrogen in molten iron
Metallurgical thermodynamics mainly studies the changes in the state, chemistry, and structure of metal liquids, as well as the direction and limitations of metallurgical reactions. There are alloy elements and impurities dissolved in the molten iron, which reflect the mutual influence of the activity of each component in the multi-component molten iron. The addition of Ti and Zr in molten iron reduces the activity coefficient of N, which is 1-2 orders of magnitude lower than that of Al, C, Cr, Cu, Mn, Mo, O, P, S, and Si, and Zr is 2.3 times that of Ti. It can be seen that zirconium has a significant limiting effect on nitrogen in molten iron.
Domestic pig iron generally has a high content of Ti, and the metallurgical reaction in the cupola can reduce the titanium content in the molten iron. Electric furnace remelting of the molten iron will not reduce the titanium content in the molten iron. So the high Ti content limits the effect of N, reduces pearlite, and lowers the strength of cast iron.
On the basis of optimizing graphite morphology, modern cast iron refines the matrix structure to regulate austenite, improves the cleanliness and cross-sectional uniformity of cast iron, thereby improving the mechanical properties, toughness, fatigue strength, corrosion resistance, and reducing the cold brittle transition temperature of cast iron, expanding the performance of cast iron, and adapting to the special requirements of high-end equipment manufacturing industry for mechanical parts.
2. Fertilizers containing zirconium
Data shows that inoculation treatment is the addition of inoculants to the molten iron to change its metallurgical state, thereby improving the crystallization characteristics, metallographic structure, and properties of cast iron. And these improvements in performance cannot be explained by the changes in chemical composition after adding inoculants to the molten iron. In short, inoculation treatment is the process of adding an inoculant to produce a large number of crystal nuclei, reduce the degree of undercooling, change the graphite morphology to promote the acquisition of A-type graphite in gray cast iron, promote the roundness of ductile iron graphite, increase the number of eutectic clusters, and promote the formation of fine pearlite. For a long time, cast iron workers have taken the improvement of graphite morphology as an opportunity to enhance the mechanical properties of cast iron and expand its application scope.
The combination of zirconium and graphite inoculant can obtain high-strength cast iron with high impact value and low white tendency. Zirconium is a deoxidizer that reacts with sulfur and oxygen in molten iron and has a strong nitrogen fixation effect. Adding trace amounts of zirconium to molten iron can become an effective graphitization element, but exceeding a certain amount can become a strong carbide forming element.
Silicon strontium zirconium inoculant is currently one of the best inoculants for gray cast iron, widely used abroad and mainly used for flow inoculation. Strontium inoculation element has the best ability to eliminate white spots in castings, without significantly increasing the number of eutectic clusters. After inoculation, the tendency of shrinkage and porosity in castings is minimized; The aluminum content is extremely low, reducing the probability of pinholes in castings. Therefore, it is suitable for the production of automotive castings, especially engine cylinder blocks and cylinder heads. Titanium, one of the nitrogen fixing elements, is a de spheroidizing element. The high titanium content in gray cast iron affects machining performance and intensifies tool wear. Therefore, silicon zirconium becomes an ideal inoculant for ensuring an all ferrite matrix, obtaining high impact value in the cold state, generating refined dendrites, and casting high-strength ferrite ductile iron castings.
Zirconium forms ZrC as the crystalline core of graphite in molten iron, improving the graphite structure and reducing the tendency of white spots, resulting in uniform and fine A-type graphite. Manganese containing silicon zirconium inoculant, due to its low melting point, melts quickly in molten iron and is suitable for the pouring temperature of large castings. The addition of silicon zirconium inoculant can eliminate the negative impact of nitrogen. Ti, Zr, and Hf are transition elements of the IVB group metal elements in the periodic table. In 1956, Didoff published a melting point of 1855 ± 15 ℃ for zirconium, and Zr is a typical dispersed phase element with weaker stability than titanium and stronger stability than hafnium. In alloys, the size ranges from 0.01 to 0.1 μ The dispersed phase of m suppresses metal recrystallization and grain growth, thereby refining grain strengthening alloy properties. Zr either solidly dissolves in the matrix, or after adding Zr, small particles such as Al3Zr, ZrN, ZrC, etc. are dispersed and distributed in a strip like manner on ferrite dendrites, which are co lattice with the matrix to pin dislocations, hinder dendrite growth and grain boundary migration, and generate refined dendrites and fine grains.
For gray cast iron, the elements that play a nurturing role are carbon, aluminum, calcium, strontium, zirconium, titanium, barium, magnesium, and rare earth. Silicon only serves as the carrier of these nurturing elements and rapidly dissolves and disperses them in the molten iron. This reminds us of the reason why industrial pure silicon cannot be nurtured.
3. Zirconium and cast iron microalloying
The technologies of wire feeding, alloying, microalloying, clean steel, and pretreatment in steel smelting have all been introduced into the iron liquid treatment of cast iron to improve its quality. In the 21st century, advanced smelting technologies such as BOF blowing and RH vacuum cycle degassing are adopted in steelmaking to reduce the carbon content in steel. Titanium niobium is added to fix carbon and nitrogen elements, thereby obtaining pure ferrite steel without interstitial atoms (referred to as IF steel). The high-strength and complex cold deep drawing formed ultra-low carbon sheet steel used in the new generation of automobiles has replaced boiling steel (first generation stamping steel 08F) and aluminum killed steel (second generation stamping steel 08Al).
The yield strength of metal materials increases with the decrease of grain size, and grain refinement is the only way to improve the strength of metal materials without damaging their toughness. The microalloying of cast iron mainly promotes the formation and refinement of pearlite, and strengthens ferrite. The addition of microalloying elements can increase the carbon silicon equivalent of gray cast iron and improve its casting performance. Austenite in an unstable state below the critical temperature A1 is called undercooled austenite. The decomposition product of undercooled austenite is pearlite. In a multicomponent system, solutes interact with each other, and the activity coefficient is often 1+1 greater than 2. If strong carbide forming elements, medium strong carbide forming elements, weak carbide forming elements, non carbide forming elements, and internally adsorbed elements are organically combined, the stability of austenite can be improved by hundreds and thousands of times. Austenite eutectoid decomposition increases alloying factors and forms alloy carbides or special carbides. The carbides forming elements also need to diffuse and redistribute, resulting in a slow diffusion rate of alloying elements in austenite, which is an important factor in delaying eutectoid transformation. The dissolved nitrogen in cast iron is a factor that hinders graphitization. Iron with higher sulfur content has a lower dissolution rate of nitrogen in liquid iron, and high silicon in cast iron also significantly reduces the dissolved nitrogen. However, in order to reduce the toughness and brittleness temperature of wind power large section ductile iron castings, the silicon content must be relatively low, and the sulfur content is usually controlled very low. Even if the lead content in cast iron is 0.0007%, it will still produce Weinstein graphite. Coarse austenite grains will promote the formation of ferrite in the Vickers structure. The lead in cast iron comes from enamel scrap, paint, free cutting scrap, copper (non electrolytic copper), engine deposits, lead plated steel plates, etc. Regardless of the lead content in cast iron, it is harmful. After adding zirconium, strengthening graphitization and refining dendrites and grains can avoid the formation of acicular ferrite.
4. Clean molten iron
Clean molten iron is a high-quality metallurgical molten iron. Assessment of purity of molten iron: ① The content of non-metallic and harmful elements such as sulfur and phosphorus in the molten iron; ② Gas element nitrogen, hydrogen, and oxygen content; ③ Content of non-metallic inclusions and trace interfering elements. High oxygen content in cast iron increases non-metallic inclusions, reduces plasticity, toughness, and fatigue life. The properties of zirconium are similar to those of rare earth metals, and foreign steel and metallurgical enterprises attach great importance to the application of zirconium. Research has shown that the deoxygenation ability of zirconium is stronger than that of aluminum at 1650 ℃, and steel with extremely low oxygen content can be obtained by adding a small amount of zirconium for final deoxygenation. Therefore, in industrialized countries, the temperature for casting and smelting iron is maintained at 1520-1550 ℃. With the increase of overheating temperature, the nitrogen and hydrogen content in the molten iron slightly increase, but the oxygen content decreases significantly after 1450 ℃, and the purity of the molten iron is improved. The hydrogen in cast iron mainly comes from the chemical reaction between the molten iron and the moisture in the mold, which hinders graphitization. The amount of dissolved hydrogen increases the undercooling of cast iron crystallization, thereby increasing the tendency for white spots. Magnesium, zirconium, and rare earth can reduce the hydrogen content of molten iron. Zirconium generates ZrC, Al3Zr, and ZrN in molten iron, which reduces the dissolved nitrogen in the iron, increases precipitation, and refines austenite dendrites.
More than 80% of the damage to metal structural components in modern manufacturing is caused by fatigue failure. The slag pores, pores, and low melting point trace elements formed by the solidification of cast iron are the fatigue sources or fracture points in cast iron structural components. When the N content is less than 90ppm, gray iron castings will not produce nitrogen precipitation pores. The incubation of Ca, Ba, and FeSi results in a decrease in the number of dendrites and a shorter length. The silicon zirconium manganese composite inoculant significantly increases the number of austenite cores, resulting in an increase in the number of primary austenite. The toughness of inclusions, Fe3P, phosphorus eutectic, FeS type segregation phase, graphite, cementite, carbides, intermetallic compounds, etc. in cast iron is worse than that of the matrix, which is called brittle phase. The sulfide manganese inclusions in cast iron can be spheroidized and modified by adding rare earth, zirconium and other elements to improve the fracture toughness of cast iron.
Foreign scholars have classified non-metallic inclusions in molten iron into three categories: Type I is iron manganese oxide and spherical sulfide; Class II refers to sulfides distributed in thin films or chains; Class III includes Al2O3, angular sulfides, and irregularly shaped oxygen sulfides. Thin film or chain distributed sulfides reduce the tensile strength of cast iron; Angular sulfides and irregularly shaped sulfur oxides are sources of fatigue and stress concentration in materials.
An effective way to improve the non-metallic inclusions inside cast iron is to improve the metallurgical quality of cast iron and enhance the cleanliness of the molten iron. The second is to improve the morphology and distribution of non-metallic inclusions in cast iron. The spheroidization modification of silicon based calcium, strontium, barium, zirconium manganese, and rare earth composite inoculants can improve the morphology and size of Class II and III non-metallic inclusions, and obtain Class I spherical oxygen sulfide composite inclusions. Moderate rare earth elements can reduce slag inclusion and shrinkage.
5. Conclusion
In the silicon zirconium manganese composite inoculant, zirconium generates ZrC, Al3Zr, and ZrN in the molten iron, reducing the dissolved nitrogen in the molten iron, generating a large number of crystalline cores, increasing precipitation and refining austenite dendrites, increasing graphite crystalline cores to promote the graphitization of the molten iron, promoting the stable acquisition of ferrite matrix, and improving the strength of cast iron.
The carbides and oxides of zirconium, strontium, and barium have lattice constants similar to graphite, and their affinity for oxygen is weak, resulting in delayed oxidation reactions. According to the data, "inoculation treatment is the process of deoxidation of molten iron, and inoculation decline is the process of re oxidation of molten iron." Because the silicon based inoculants of zirconium, strontium, and barium have the above advantages, they have strong resistance to inoculation decline. Silicon zirconium composite inoculant effectively eliminates casting white spots and enables gray cast iron to obtain A-type graphite. The effect of zirconium on iron-based metals still needs further research and exploration.

[ 上一页 ] Welding of industr...[ 下一页 ] How to carry out c...