When gray cast iron has a high carbon equivalent (4.10% CE or more), a trace amount of titanium can improve the mechanical properties of the cast iron and improve the uniformity of different sections of the casting. This paper studies the effect of different amounts of titanium on the quality of gray cast iron, such as structure, mechanical properties and density, under medium and low carbon equivalent (3.65% to 3.85% CE). The test results show that titanium promotes the formation of D-type graphite in cast iron; when D-type graphite reaches a certain proportion, especially when Si/C is high, its mechanical properties increase significantly; with the increase of titanium and aluminum, the density of gray cast iron decreases and the probability of shrinkage increases.
In the production process of cast iron, with the large-scale use of metal furnace materials such as pig iron and scrap steel, titanium more or less enters the molten iron. A large amount of research data points out that after adding titanium to gray cast iron, most of the titanium compounds exist in the metal matrix, but there is still a part of titanium nitride or carbonitride in the interface layer between ferrite and graphite. Titanium compounds with a hardness of 3200 HV or higher greatly reduce the machinability of cast iron. In addition, excessive titanium content (0.096%) produces thermal cracks when cutting castings. However, D-type graphite cast iron parts with titanium addition have good oxidation resistance, growth resistance and thermal fatigue resistance. Under 500-700℃ working conditions, the service life can be increased by 3 times compared with the commonly used CrMoCu cast iron. Titanium is strictly prohibited as an interfering element in ductile iron. However, in some vermicular cast iron, titanium is added to the vermicularizing agent as a beneficial element to expand the range of vermicular treatment. Under high carbon equivalent conditions, micro-titanium alloying can improve the strength and hardness of gray cast iron parts and improve the cross-sectional uniformity of castings. Trace amounts of titanium can also neutralize excessive nitrogen in cast iron to reduce the frequency of crack-like nitrogen pores.
1. The effect of titanium on the structure and mechanical properties of gray cast iron parts
Titanium can increase the austenite core in gray cast iron and refine the primary austenite grains. The American Foundry Association pointed out that titanium can act as a graphitizing agent and reducing agent for cast iron, and can also refine grains and improve the tensile strength and bending strength of cast iron. When the residual titanium content is 0.08% to 0.25%, it can improve the tensile strength of cast iron with high carbon equivalent and reduce the strength of cast iron with low carbon equivalent.
In dry casting with low titanium content, the supercooling tendency of molten iron is small, and A-type and E-type graphite are mostly produced. However, as the titanium content increases, the supercooling tendency increases, which promotes the increase of D-type graphite. When D-type graphite reaches more than 90%, the mechanical properties of the casting are significantly improved. Due to the large supercooling tendency of wet sand and the increase in titanium content, a large amount of D-type graphite also appears in the thick and large hot joints of the casting.
Titanium increases the supercooling tendency of gray cast iron, causing a large number of graphite branches. The numerous short and curved D-type graphites shorten the diffusion distance of carbon, causing the austenite near the graphite to transform into ferrite during the cooling process of the casting, resulting in reduced mechanical properties. However, due to the large number of austenite dendrites, the D-type graphite is short and the matrix splitting effect is small. In addition, the D-type graphite eutectic has a good spherical shape, and other reasons, compared with the cast iron of the same matrix, the D-type graphite cast iron has higher strength.
2. The influence of titanium and aluminum on density
Gray cast iron parts with lower titanium content do not have internal shrinkage, and the surface shrinkage is lighter. As the titanium content increases, the probability of internal shrinkage in the casting increases. The probability of shrinkage at 0.056% Ti and 0.016% Al is low, only 50%. When the titanium and aluminum contents in gray cast iron parts reach 0.17% and 0.023% respectively, the probability of internal shrinkage in the casting is 75%. At this time, shrinkage occurs not only in the hot section of the casting, but also in the non-hot section and small hot section.
There is no clear mechanism analysis on the cause of titanium shrinkage. Regarding the influence of aluminum, the data indicate that the increase of aluminum content in molten iron reacts with water molecules in the inoculant and wet sand under the action of hot molten iron to form hydrogen. The molten metal has a high hydrogen content, and the dissolved hydrogen in the residual liquid phase is continuously enriched during the solidification process, making the hydrogen content of the liquid phase in the isolated small molten pool higher. While the small molten pool is transformed from liquid phase to solid phase and the volume loss occurs, the dissolved hydrogen will also precipitate and occupy the space of the small molten pool, forming precipitation pores. At this time, the two defects of intergranular shrinkage and hydrogen precipitation pores are symbiotic and difficult to distinguish. Therefore, in order to eliminate intergranular shrinkage, countermeasures must be taken to prevent precipitation pores and minimize the hydrogen content of the molten metal. Therefore, this kind of micro-shrinkage and micro-pore composite defect is easy to form under the combined action of titanium and aluminum.
3 Conclusions
The test results show that under the condition of 3.65% to 3.85% CE, the following are true for gray iron castings:
3.1 A certain amount of titanium increases the tendency of supercooling of molten iron and promotes the formation of D-type graphite in gray iron castings. The D-type graphite in wet sand molding is significantly more than that in dry sand molding.
3.2 As the amount of titanium in molten iron increases, the amount of D-type graphite in gray iron castings increases continuously. When the proportion of D-type graphite reaches a high level, a large amount of D-type graphite also appears in the thick and large hot spots in the castings. The mechanical properties of castings with high carbon equivalent and silicon-carbon ratio increase significantly. When the carbon equivalent drops to 3.66%, the strength decreases significantly as the amount of titanium increases.
3.3 As the amount of titanium and aluminum increases, the density of gray iron castings decreases and the probability of shrinkage increases significantly. When Ti is 0.17% and Al is 0.023%, the shrinkage probability of the gray casting is 70%.
