Characteristics, Classification, and Applications of Gray Iron Castings

Characteristics, Classification, and Applications of Gray Iron Castings

The mechanical properties of gray iron castings. Because ordinary gray cast iron contains graphite—whose tensile strength is nearly zero—cast iron can be regarded as steel riddled with cracks or voids. Not only does graphite disrupt the continuity of the matrix, but it also reduces the effective cross-sectional area of the metal matrix under load, significantly increasing the actual stress. Meanwhile, stress concentrations tend to occur at the sharp corners of the graphite flakes, causing stresses at these points to far exceed the average stress level. The former phenomenon is known as "graphite reduction," while the latter is referred to as "graphite cutting." Consequently, gray cast iron exhibits much lower tensile strength and elastic modulus compared to steel, typically resulting in: 120 ~ 250 MPa σ B Right, the compressive strength is close to that of steel. 600 ~ 800 MPa , its plasticity and toughness are close to zero, classifying it as a brittle material. In gray cast iron, the greater the quantity, size, and distribution of graphite, the more significant its impact on mechanical properties. However, graphite has little effect on the compressive strength of gray cast iron, since compressive strength primarily depends on the matrix structure of the material—thus, gray cast iron exhibits compressive strength comparable to that of steel. When specimens contain sharp notches resembling graphite flakes found in gray cast iron, the stress concentration near the notch can reach levels up to several times the average stress value. 5 More than double. The stress concentration phenomenon present in gray iron castings occurs even under relatively small loads. ( Far short of the matrix's yield strength ) Additionally, the actual stress at the edge of the graphite matrix will also exceed its yield strength, leading to residual deformation of the metal—and in some cases, even cracking. ( When the actual stress exceeds the matrix's ultimate strength, ) The emergence of these graphite-edge cracks further reduces the effective cross-sectional area of gray iron castings under load, intensifying the stress-concentration phenomenon. At the same time, the stress concentration at the crack tip rapidly advances along with the crack, causing it to propagate swiftly and ultimately leading to brittle failure of the entire casting. Consequently, the reducing and cutting effects induced by the presence of graphite prevent the full development of strength in the cast iron's metallic matrix. According to statistics, the strength utilization rate of conventional gray iron matrices typically does not exceed 30% ~ 50% , indicating that gray cast iron has very low tensile strength. Additionally, the presence of graphite leads to severe stress concentration, triggering early crack formation and resulting in poor resistance to crack propagation. This, in turn, makes gray cast iron prone to brittle fracture, as its ductility and toughness are virtually absent. Clearly, the overall impact of flaky graphite on matrix degradation is not simply the algebraic sum of the two effects—rather, the cutting action typically causes far more damage to the matrix than the reduction process does. It is worth noting that when subjected to stress, ordinary gray cast iron often exhibits slight residual deformation due to stress concentrations at the graphite edges. Thus, the stress in gray cast iron - The stress-strain curve is not a straight line even at lower stresses—it exhibits constant curvature instead. Therefore, the elastic modulus of gray cast iron has only relative significance.

The hardness characteristics of gray iron castings. The ratio of steel's Brinell hardness to its tensile strength is constant, approximately 3; In cast iron, this proportion is widely dispersed. At the same hardness level, tensile strength can vary within a range. Similarly, at the same strength level, hardness also exhibits a range—this is because strength properties are significantly influenced by the graphite content, while hardness primarily reflects the matrix conditions. Many manufacturers use the hardness of cast iron as an estimate for its tensile strength, and in various contexts, they have proposed σ B With HBS The relationship. It should be noted that this estimation is reliable only when the process conditions are stable and the graphite flake parameters remain essentially consistent. The hardness of gray cast iron is determined by its matrix, which in turn is influenced by the steel ball pressed onto the test specimen. Since the size of the steel ball is relatively large compared to the graphite flakes, the external force is primarily borne by the matrix. Consequently, as the amount of pearlite in the matrix increases—leading to greater dispersion—the hardness rises accordingly. When harder constituent phases are present within the metallic matrix, ( Such as free cementite, phosphorus eutectic, and more. ) At that time, the hardness increases accordingly.