How to produce high-strength gray cast iron parts without adding precious metals such as alloys?

Detailed process flow of producing HT300 high-strength gray cast iron without alloying

Phase 1: Ingredients and Smelting - Laying the Foundation

1. Selected furnace materials: pig iron: high-purity pig iron or high-quality cast pig iron is used, which is characterized by low content of trace elements (such as Ti, V, As, Sb, etc.). These trace elements can interfere with the morphology of graphite, which is not conducive to strength enhancement. The iron block size should be uniform. Scrap steel: The proportion of addition needs to be significantly increased, usually accounting for 30% -40% of the furnace charge. The use of low-carbon, low sulfur clean scrap steel, such as stamping parts, carbon steel waste, etc., aims to dilute carbon and impurities in the molten iron. Recycled materials: Use sprues and waste castings of the same brand to ensure stable composition. Strictly control its proportion and cleanliness to avoid introducing too many impurities. 2. Accurate ingredient calculation: Core idea: Low carbon equivalent. The goal is to strictly control the carbon equivalent (CE) within a narrow range of 3.8% to 4.0%. Carbon (C): The target value is set at 2.9% -3.2%. Suppressing carbon content through a high proportion of scrap steel. Silicon (Si): The initial silicon in the furnace is controlled at 1.2% -1.5%, leaving sufficient incremental space for subsequent incubation treatment. The balance between manganese (Mn) and sulfur (S) is crucial. The goal is to control the sulfur content between 0.07% and 0.12%, and then calculate the amount of manganese added according to the formula Mn% ≈ 1.7 × S%+0.3%. Based on this, the manganese content is usually between 0.8% and 1.0%. This ensures the formation of beneficial MnS compounds and promotes the formation of pearlite. Phosphorus (P): It must be strictly limited to below 0.08%, as phosphorus can reduce the toughness and strength of cast iron. 3. High temperature melting: Medium frequency induction furnace is used for melting to ensure uniform composition and precise temperature control. The tapping temperature must be above 1520 ℃. The purpose of high-temperature melting is to fully reduce the gas (hydrogen, nitrogen) content of molten iron. Fully float non-metallic inclusions to obtain pure molten iron. Provide sufficient heat reserves for subsequent processing and pouring.

Phase 2: Pre furnace treatment and pouring - precise control

1. Rapid analysis and adjustment of furnace components: Take iron liquid samples for spectral analysis or thermal analysis to quickly obtain the actual content of C, Si, Mn, P, and S. Fine tune according to the results to ensure that all elements are within the target window, especially the carbon equivalent. 2. Efficient incubation treatment: This is the soul of the entire process. Under low-carbon equivalent conditions, the tendency of molten iron to white mold is extremely high, and it is necessary to eliminate white mold and refine graphite through strong inoculation. Selection of inoculants: Choose inoculants with strong resistance to decay and nucleation ability, such as strontium (Sr) - containing ferrosilicon or barium (Ba) - containing ferrosilicon. Incubation process: Adopting the flow inoculation method. At the moment when the molten iron flows from the ladle to the pouring cup, a dedicated inoculation feeder is used to uniformly add inoculants with a particle size of 0.2-0.7mm to the molten iron flow. Addition amount: controlled at 0.3% -0.5% (by weight of molten iron). Effect: Instantaneous incubation can provide a large amount of graphite crystal cores before the solidification of molten iron, thereby obtaining A-type graphite (fine flakes, uniform distribution) and effectively preventing the appearance of cementite at the edges. The refinement of graphite directly leads to the refinement of the matrix pearlite. 3. Pouring and cooling control: Pouring temperature: On the premise of ensuring sufficient filling, a lower pouring temperature is used, usually between 1320 ℃ and 1350 ℃. Low temperature casting helps to increase undercooling and refine eutectic clusters. Casting process: The preferred method is sand covered iron casting, which is the most effective technique for achieving high strength. Use a metal mold (iron type) as the outer shape, and cover its working surface with a 4-8 millimeter thick sand lining. This process can greatly improve the cooling speed and force the rapid solidification of molten iron. The benefits of rapid cooling: greatly fine ink flakes. Greatly refining the interlayer spacing of pearlite is the key to improving strength. Make the overall structure of the casting denser and more uniform. The use of cold iron: For ordinary sand casting, it is necessary to place external cold iron reasonably in the thick and hot parts of the casting to force these parts to solidify synchronously with the thin-walled parts, prevent shrinkage and loosening, and refine the local structure.

Phase Three: Post Processing and Inspection

1. Sand cleaning and heat treatment: After the casting solidifies, it is left in the mold for a sufficient period of time until it is below the phase transition temperature, and then the box is filled with sand to avoid excessive internal stress. Perform stress relief annealing, usually at a temperature of 520 ℃ -550 ℃, hold for 2-4 hours, and then cool with the furnace. Special attention: The annealing temperature must not exceed 720 ℃, otherwise the fine flake like pearl will undergo spheroidization, resulting in a decrease in strength and hardness. 2. Strict quality inspection: Mechanical properties: Single cast or attached test bars are poured along the line, and the tensile strength is measured on a universal testing machine to ensure stability of over 300MPa. Metallographic examination: Check the metallographic structure of the specimen or test bar. The target organization is: ≥ 95% of fine lamellar pearlite+small, uniformly distributed A-type graphite (graphite length of 3-4 grades is preferred)+no free cementite. Hardness test: Measure the Brinell hardness on the casting body. The hardness of HT300 without alloy is usually between 190-220HBW, which is a normal phenomenon.

Summary and core tips:

The successful production of alloy free HT300 relies on a precise chain of interlocking components: high-purity furnace material+low-carbon equivalent formula+high-temperature pure melting+precise Mn/S balance+efficient instantaneous incubation+forced rapid cooling. Any loss of control in any of these links may lead to insufficient strength or the appearance of hard and brittle phases. This is a process that requires extremely high management and technical execution, but once mastered, it can significantly reduce production costs and enhance product competitiveness.