High Carbon Equivalent, High Strength Gray Cast Iron: Four Key Furnace-Side Control Points in Electric Furnace Melting

Thanks to its good castability, vibration damping and wear resistance, and relatively low cost, gray cast iron remains the world's most-produced casting alloy. Against the rapid development of ductile iron, compacted graphite iron and light-alloy castings, gray iron still dominates critical castings such as diesel engine blocks/heads and machine tool beds, thanks to process advances that combine high carbon equivalent (CE) with high strength. Combining domestic production practice with conclusions from the American Gray Iron Research Committee (AFS), this article outlines the key control points for stably producing high-strength gray iron under electric-furnace melting.

Carbon Equivalent and the Five Elements: Balancing "High CE" and "High Strength"

CE≈C+1/3Si is the primary factor governing gray iron strength: the higher the grade and strength, the lower the selected CE should be; however, at the same strength level, a higher CE gives better castability and machinability, so thin-walled parts should use a moderately higher CE while heavy sections should use a lower one. Mn acts in layers: it first forms high-melting MnS (about 1620°C) with S to neutralize S's anti-graphitizing effect, and only excess Mn actually stabilizes pearlite; an empirical ratio is Mn≈1.7×S+0.3%. Although S is usually regarded as harmful, when melting in induction furnaces the iron is often below 0.05% S and must be resulfurized to above 0.06%, otherwise insufficient MnS nucleation leads to chill and uneven graphite flake distribution.

Trace Elements: The "Hidden Variables" That Cannot Be Ignored in Electric-Furnace Melting

Cupolas have a strongly oxidizing atmosphere that carries most trace elements into the slag; after switching to induction furnaces this "self-cleaning" disappears, and the effect of trace elements becomes prominent. Pb comes mainly from scrap steel: above 20 ppm and acting with hydrogen, it readily forms Widmanstätten graphite on thick, slowly cooled sections (spiky graphite growing on coarse flake sides, colloquially "hairy graphite"), reducing strength and hardness by about 50%, so scrap-steel Pb should be kept below 15 ppm. Moderate N promotes graphite nucleation, refines pearlite and produces interstitial solid-solution strengthening; the suitable range is 70–120 ppm, above 180 ppm giving nitrogen porosity and microcracks. Ti comes mainly from pig iron and preferentially binds with N to form TiN, consuming the free nitrogen that strengthens the iron, so pig-iron Ti should be kept below 0.8% and matched at N:Ti≈1:3.42. Accordingly, the charge mix should be 50–70% scrap steel and 5–20% pig iron.

Alloying and Pearlite Stabilization: Keeping the Matrix as It Should Be

Alloying elements fall into four groups: graphitizing, carbide-forming, pearlite-stabilizing and pearlite-refining. Cr (0.1–0.6%) and V (0.1–0.4%) are common pearlite formers, while Mo (0.1–0.8%) and Ni (0.1–1.5%) mainly stabilize pearlite. Sb is cheap and efficient at 0.02–0.06%: it suppresses type B/D/E undercooled graphite, promotes type A graphite and prevents pearlite decomposition at high temperature, but above 0.1% it hardens and lowers strength; its utilization in electric furnaces is about 80%. Cu (0.4–0.8%) combines weak graphitizing with pearlite-stabilizing action, reducing section-to-section structural differences and improving machinability. Sn (0.02–0.1%) stabilizes pearlite by blocking carbon diffusion to graphite flakes during eutectoid transformation; excess increases brittleness. Production commonly combines Cu+Cr+Sn so that the positive effects offset the carbide tendency.

Inoculation and the Furnace-Side Temperature Closed Loop

Inoculation shares three traits: stream treatment at the furnace, trace addition (a few per mille to per ten-thousand), and nucleation; the shorter the time from inoculation to pouring, the better. In practice, superheat the iron to 1500–1520°C to fully dissolve residual undissolved graphite particles and eliminate pig-iron heredity, then inoculate with 75SiFe at 1420–1460°C, with grain size 2–5 mm, addition 0.3–0.6%, and an effective inoculation time of about 10 minutes. For large castings or two-ladle pouring, supplementing with ladle float-silicon inoculation (about 0.1%) or stream inoculation effectively prevents inoculation fade. Combined with the chill wedge (CW) value and spectrometer quick check, this forms a three-in-one furnace-side criterion of "temperature–composition–chill."

FAW Foundry stably produced HT300 heavy-duty diesel engine blocks/heads under high CE using all-scrap induction melting, Cu-Cr-Mo-Sn combined alloying and stream inoculation; Yantai Binglun produced horizontal machining-center beds exported to Japan with type A graphite on the guide surface, pearlite volume fraction above 98%, tensile strength 310–340 MPa and hardness 180–200 HB. These cases show that high-strength gray iron does not rely on a single element or alloy but on system-level control of composition–charge–inoculation–temperature. Workshops are advised to use a molten-iron thermal analyzer and fast temperature tools in daily production for furnace-side rapid detection, linking CW value, eutectic characteristic temperature and tapping/inoculation temperatures into a closed loop to further reduce batch-to-batch structural and property variation.