Development and production of Bainite Ductile Iron grinding ball (2)
2.2 Casting and smelting process
After analyzing the elements of grinding ball material for Bainite Ductile Iron Balls, goes to the next step– casting and melting process. Wet sand molding, alloy melting in a one-ton cupola.
(1) metal charge: High Quality No. 18 Benxi pig iron, return iron, ferromolybdenum, ferrosilicon, Ferromanganese, scrap steel, return charge, and spheroidized alloy. These metal furnaces are expected to be free of contaminants and to be cleaned of surface rust by a drum cleaner. The chemical composition of the metal charge should be clear.
(2) molten iron: the pig iron, scrap steel, recharging charge according to the requirements of the batch furnace melting, after adding ferroalloy furnace, the temperature of iron out is controlled above 1450 °C.
(3) spheroidizing treatment: The spheroidizing agent is rare earth magnesium silicon alloy, the dosage is hot metal 1. 5% ~ 1. 7% , the particle size is 10 ~ 20mm.
(4) inoculation treatment: adding 0.5% to the spheroidized alloy before tapping. 80% of 75 ferrosilicons, particle size is 10 ~ 12mm, at the same time adding quantitative ice, crystal powder, slag and add cryolite, with plant ash cover, to be poured.
(5) pouring: pouring temperature is 1380ー1420 °C, pouring time is controlled within 10 min, and the ladle is transferred instantaneously by adding 0.12% 75 ferrosilicons with the particle size of 3ー5 mm./n(6) uncrating: when the casting is cooled to 900 ~ 950 °C, uncrating is carried out, and heat treatment is carried out.
2.3 Heat treatment process
At present, the cooling methods of bainite ductile iron include isothermal quenching, continuous cooling quenching, normalizing and as-cast cooling. The following is a brief description of isothermal quenching, continuous cooling quenching, and as-cast residual heat graded isothermal quenching.
2.3.1 Isothermal quenching
The austempering process of ductile iron is easy to control, which can avoid the stress, distortion, and embrittlement that may occur when rapidly cooling from austenitizing temperature to room temperature. The bainite transformation of ductile iron can be realized by controlling the austenitizing temperature, adjusting the austenite and carbon content. During austempering bainite transformation of ductile iron, the nucleation and growth of ferrite slab are not accompanied by cementite, and bainite ferrite is continuous. Carbon-rich austenite exists stably at room temperature under certain conditions. At last, the ductile iron with austenite and bainite mixed structure matrix was obtained. Isothermal quenching needs much equipment, the dependence on the equipment is strong, the craft is complex, the production cycle is long, our country’s use reduces gradually.
2.3.2 continuous quenching
After austenitizing at high temperature, the castings are immediately quenched into a special quenching medium, and the austenite bainite structure is obtained by continuous cooling. However, after continuous quenching, the workpiece has complex internal stress, and the organization is not stable, so tempering is a necessary follow-up process.
2.3.3 Stepwise isothermal quenching of casting waste heat
As shown in Fig. 1, the process of isothermal quenching of cast waste heat is carried out by opening the box at 900 ~ 950 °C for rapid cooling (oil quenching) to slightly below the MS point avoiding the upper bainite formation zone. The lower bainite isothermal transformation was carried out in an isothermal furnace at Ms. Point. When the lower bainite transformation is finished, air cooling is finally produced. Compared with the as-cast process, the addition amount of the alloy is greatly reduced, and the production cost is lower. Compared with the continuous quenching process, the austenitizing process can be omitted, and the addition amount of the alloy is relatively more economical.
2.4Influence of different processes on performance
The effects of different heat treatment methods on hardness and impact toughness of bainite ductile iron, such as graded isothermal quenching, quenching isothermal quenching, continuous quenching, normalizing and as-cast, are shown in Table 2.
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