Analysis and improvement measures for nitrogen porosity defects in sand-covered gray iron castings with iron molds

The automotive brake drum castings produced by the iron-type sand casting process overcome the limitations of traditional sand casting processes, such as the high casting shrinkage, low strength, and low hardness. The disadvantages of the traditional casting process are overcome, and the rapid cooling process of iron-type sand covering is fully utilized Features: dense structure, high mechanical properties, cooling and solidification of castings The product is good and has no internal defects, especially for automotive brake drums. Products with wear resistance and thermal conductivity requirements during use can achieve high Carbon, high strength, high hardness, can improve the service life of brake drum products. A batch of brake drums produced by a sand-covered iron mould technology have been rejected. The circular casting defects mainly concentrate on the upper corner of the brake drum product’s large outer circle. The corner will show up after 1~2mm machining, and in severe cases, it will appear on the blank There will be obvious holes, which are irregularly dense and continuous around the outer circle of the product. The distribution of defects is shown in the figure, and the defects occur in a circumferential direction.

foundry process

Casting production uses medium frequency induction furnaces, using recycled materials, waste The smelting method of synthetic cast iron with carbon additive in steel, the proportion of scrap steel added is 60% to 70%, the rest is recycled material, including pouring risers and machining Iron chips and waste products. The use of graphitized carbon additive, fixed C (mass) the ash content is less than 2%, and the volatile matter content is less than 1%. The hair fraction is ≤1%, and the particle size is 1~5 mm. The order of adding furnace materials is from the bottom of the furnace. 50% of the re-melted material (iron scrap) is added with 70% of the carbonization agent and then the scrap steel, During the scrap steel process, the remaining 30% of the carbonizer is added to the furnace in two The absorption rate of high carbonizing agent is increased in 3 times during the feeding process, which This will ensure that the furnace bottom quickly forms a high-carbon molten pool after the furnace is powered on, making it possible to refine scrap steel The melting point is reduced due to surface carburization, which plays a role in melting. Temperature: 1500~1530℃, high temperature standing at 1530~1550℃ Remove the slag for 5 minutes and repeat. The temperature of the molten iron is 1490~1530℃ when it comes out of the furnace. The iron is inoculated with 75# ferrosilicon during the casting process, and the pouring temperature of the final piece is not lower than 1 360 ℃, the pouring time of the whole package should not exceed 15 minutes.

The used sand is made of recycled coated sand, and the new sand is made of Inner Mongolia Tongliao sand. The particle size is 70-140 mesh, the amount of new sand added is 5%-20%, and the rest is recycled sand. The precoated sand is recycled by thermal method, and the main process flow includes: Crushing→screening→magnetic separation→thermal regeneration → removal of fine powder→cooling→ Storage → inspection and other processes. The tensile strength of coated sand at room temperature is ≥2.5 MPa, The bending strength at room temperature is ≥6 MPa, and the tensile strength at room temperature is ≥1.3 MPa. The flexural strength is ≥3.8 MPa, the loss on ignition is ≤2.6%, and the gas evolution is ≤20 mL, melting point 95~110℃.

pouring system

The middle top pouring method is used, and the pouring cup is funnel-shaped, with the top of the cup forming the maximum outer circle with the upper plane of the casting. The pressure angle is not less than 40°, and a 15 PPI ceramic filter is placed in the gate cup Filter the molten iron, and set a gate with a minimum R50 below the pouring cup The cavity plays a buffering role for the molten iron entering the cavity, avoiding the generation of iron liquid Highly turbulent. Using a semi-closed pouring system, ΣS : ΣS horizontal : ΣS straight =1:4.17:2.04, set 4 or 5 internal sprues according to the product structure, The cross-sectional area of a single ingate is not less than 240 mm2, and the casting time for a single casting type is No more than 25 seconds, two stress slots are set on both sides of the internal gate, and It plays a role of breaking when pouring system, protecting the casting body from damage. The width of the runner is the same as that of the internal runner, and the thickness is determined according to the flange wall thickness of the casting. Generally, it is set at 1.25 times the wall thickness of the flange to ensure sufficient liquid metal, flow smoothly into the internal sprue, while initially injecting low-temperature gas and slag The molten metal is stored to prevent slagging.

Defect analysis

Perform energy spectrum analysis on the defects and their surroundings using an EDS spectrometer, The defect surface scanning image shows that there are emulsion dendrites in the defect area. The sink shape is irregular, perpendicular to the casting surface, and penetrates the casting by 2-5 mm. As shown in Figure 3, the defects in the casting mainly include C, N, Fe, O, and Si The elements of N, C, and Fe are highly segregated, but the content of other elements such as Si, P, and Al is not significantly different between the inside and outside of the defects. The detection results show that the element C is mainly enriched inside the defects, The N element is mainly distributed in the surrounding area of the defects, and the distribution is relatively uniform. As can be clearly seen from Figure 3, there are continuous or discontinuous defects inside the defects. The graphite film separates out, and connected to it is the surrounding decarbonized matrix group This is a typical nitrogen porosity defect caused by high N content.

Formation mechanism of nitrogen pores

Nitrogen in the liquid iron is solid dissolved in iron to form a interstitial solid solution, which solidifies During solidification, as the temperature decreases, the solubility of nitrogen also decreases. During the solidification process of molten iron, the solubility of nitrogen in the first solidified molten iron decreases. The nitrogen content in the post-solidified molten iron gradually increases, and when the nitrogen content in the molten iron reaches a certain level, When the content exceeds the solubility of liquid iron, nitrogen will be released, and due to the high reactivity of iron, The cooling rate of the sand-covered casting process is fast, which can lead to the solidification of the surface layer of molten iron. The nitrogen cannot be smoothly discharged from the shell, resulting in nitrogen hole defects.

Analysis on the causes of nitrogen pores

The reason for the formation of nitrogen pores is due to the high content of ω(N) in the molten iron, which can be For iron-type sand-covered gray iron brake drum castings, when the iron liquid contains ω(N) When it exceeds 120 ppm, nitrogen pores are prone to occur, and the source of nitrogen There are mainly three aspects: (1) Scrap steel: The proportion of scrap steel used in the smelting of synthetic cast iron is relatively high. And the demand for gray cast iron casting, especially ordinary carbon steel, high manganese steel, etc. in scrap steel The amount of waste steel with high ω(N) is relatively high, such as low carbon steel ω(N) About 40-60 ppm, screw steel ω (N) about 90 ppm, guide rail steel ω (N) About 110-120 ppm. (2) Carbon additive: The coal quality carbon additive ω (N) is about 2000-7000 ppm, That is, 0.2% to 0.7%; ordinary calcined petroleum coke carburizer ω (N) is generally About 1000 ppm; high temperature calcination of petroleum coke carburizer ω (N) 300-500 ppm; graphitized carburizer ω(N) does not exceed 300 ppm. (3) Coated sand: Coated sand mainly consists of raw sand, recycled sand, thermoplastic resin, and other additives. phenolic resin, curing agent and additives, which are widely used in the industry The agent is urotropine, which will be decomposed when heated to 230℃ or above. It will be decomposed in large quantities, so during the heating and curing of precoated sand and pouring process, Vaporize a large amount of NH3 。

Improvement of coated sand quality

(1) Change the supplier of resin-coated sand raw materials, especially to ensure the thermoplasticity The quality of phenolic resin and urotropine. (2) Increasing the amount of new sand added to the resin-coated sand mix, from the current 5% to 20% adjusted to 20% to 40%, reducing the residual nitrogen content in recycled sand, Reduce the risk of nitrogen porosity. (3) Differentiated management of recycled sand for shell and iron mould lines Due to the different performance requirements of the two production lines for coated sand, the sand particle size Different, the amount of new sand added is different, and the mixing of recycled sand can lead to process problems.

Improvement of production process

According to the characteristics of the iron mold sand casting production process, the following improvements have been made in process control due to the high N content in the precoated sand: (1) Add exhaust plugs to the mold, especially at the bottom and corners. Adding an exhaust channel is beneficial for the gas discharge during the curing of precoated sand, and also Improve the quality of sand injection by using precoated sand mold filling. (2) Urotropine in precoated sand will decompose significantly at 230 ℃, The curing temperature of iron-type sand-coating process is generally 220~240℃, which is Therefore, the mold temperature is set to 230-250℃, and the iron temperature is set to 240~280 ℃, accelerate the gas evolution of coated sand. (3) To exhaust the gas volatilized from the resin-coated sand in the cavity, The number of iron-type exhaust channels has increased from 8 to 20. At the same time, after curing and closing the box, The starting time for pouring has been extended from 10 minutes to no less than 20 minutes, which is beneficial. discharge of gases. (4) Appropriately increasing the pouring temperature and reducing the viscosity of the molten iron are beneficial to The gas invading into the molten iron floats up, thereby reducing the formation of porosity defects in the casting. Ok.


Through the adjustment of precoated sand raw material suppliers and the recycling of used sand, Distinguish, adjust the proportion of new sand added to the sand blending ratio to 20% to 40%, At the same time, the exhaust channels of the mold and iron mould are added, and the production process is improved. Improve the iron mold temperature, curing time, pouring temperature and other measures for continuous production There are no nitrogen holes in more than 100,000 products.

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