1. In the gray cast iron, there is graphite, and the tensile strength of graphite is almost zero, and the cast iron can be regarded as a steel full of cracks or voids. Graphite not only destroys the continuity of the matrix, but also reduces the effective cross-sectional area of the metal matrix under load, so that the actual stress is greatly increased. At the same time, the stress concentration is easily caused at the sharp corner of the graphite, so that the stress at the sharp corner is much larger than the average stress. The former is called the reduction of graphite, and the latter is called the cutting action of graphite. Therefore, the tensile strength and elastic modulus of gray cast iron are much lower than that of steel. Usually, σb is about 120-250 MPa, the compressive strength is close to that of steel, generally up to 600-800 MPa, and the ductility and toughness are close to zero. Brittle material. The more the number of graphite sheets in gray cast iron, the larger the size and the more uneven the distribution, the greater the influence on the mechanical properties. However, the presence of graphite has little effect on the compressive strength of gray cast iron, because the compressive strength mainly depends on the matrix structure of gray cast iron, so the compressive strength of gray cast iron is similar to that of steel. When there is a sharp gap like a graphite piece in gray cast iron in the sample, the stress value near the notch can reach more than 5 times the average value. This stress concentration phenomenon causes the gray cast iron to withstand a relatively small load (far from reaching the yield strength of the substrate), and the actual stress of the matrix at the edge of the graphite exceeds its yield strength, so here There will be residual deformation of the metal and even cracks (when the actual stress exceeds the strength limit of the substrate). The appearance of such graphite edge cracks further reduces the effective cross-sectional area of the gray cast iron bearing load, and further exacerbates the phenomenon of stress concentration. The tip of the stress concentration also moves rapidly with the crack, so that the crack spreads quickly. The brittle failure of the entire casting occurs. Therefore, due to the reduction and cutting effect caused by the presence of graphite, the strength of the cast iron metal matrix cannot be fully exerted. According to statistics, the utilization rate of the strength of the common gray cast iron matrix generally does not exceed 30% to 50%, which is manifested as the tensile strength of the gray cast iron. The intensity is very low. In addition, due to the severe stress concentration caused by the presence of graphite, the early occurrence of cracks and the ability to resist crack development are poor, resulting in brittle fracture, so the plasticity and toughness of gray cast iron are almost impossible to show. Obviously, the overall effect due to the presence of flake graphite is not the algebraic sum of the two, and the damage of the matrix to the matrix is often much stronger than the reduction. Incidentally, when the ordinary gray cast iron is subjected to stress, a small amount of residual deformation is often caused at the edge of the graphite due to stress concentration. Therefore, the stress-strain curve of gray cast iron does not have a straight line even under the action of lower stress, but has a certain curvature. Therefore, the elastic modulus of gray cast iron has only a relative meaning.
2. Hardness characteristics of gray cast iron
In steel, the comparison of Brinell hardness and tensile strength is constant, about equal to 3, and in cast iron, this ratio is very dispersed. At the same hardness, there is a range of tensile strength. Similarly, there is a range of hardness at the same strength because the strength properties are greatly affected by graphite, and the hardness basically reflects only the matrix. Many factories estimate the tensile strength of cast iron, and many papers also propose a relationship between σb and HBS. It must be pointed out that this estimate is only reliable if the process conditions are stable and the parameters of the graphite sheet are substantially close. The hardness of gray cast iron is determined by the matrix. This is because the hardness is measured by pressing the steel ball on the test block. The size of the steel ball is quite large compared to the graphite crack, so the external force is mainly on the substrate, so As the amount of pearlite increases in the matrix, the dispersion becomes larger and the hardness is correspondingly increased (Fig.). When a hard phase is formed in the metal matrix (such as free cementite, phosphorus eutectic, etc.), the hardness is correspondingly increase.
3. Other performance characteristics of gray cast iron
Although graphite reduces the mechanical properties of gray cast iron, it brings a series of other excellent properties to gray cast iron.
1 Good casting properties When casting gray cast iron parts, not only its fluidity is good, but also because of the precipitation of graphite with larger specific volume during solidification, the solidification shrinkage is reduced, and excellent castings are easily obtained, showing good casting properties. .
2 Good vibration damping Graphite acts as a buffer for the vibration of the cast iron parts, weakens the transmission of vibration energy between the grains, and converts the vibration energy into heat energy, so the gray cast iron has good vibration damping.
3 Good wear resistance Graphite itself is also a good lubricant. Graphite falling off on the friction surface can be lubricated, so gray cast iron has good wear resistance.
4 Good cutting performance When cutting, graphite acts as a friction reducing and chip breaking. Since the graphite is detached to form a micro-cavity, and the oil storage function is maintained, the continuity of the oil film can be maintained, so the gray cast iron has good cutting performance and small tool wear.
5 low notch sensitivity flake graphite is equivalent to many tiny notches, which reduces the sensitivity of the casting to the notch, so the surface processing quality is not high or the structural defects have less adverse effects on the fatigue strength of the cast iron than the steel.