High-strength ductile iron with spheroidal graphite is recognized worldwide as a unique multifunctional material for products of the most critical applications across many industries. It has a favorable combination of strength characteristics and also exhibits high casting properties that often surpass those of steel (high fluidity, lower casting shrinkage, reduced tendency to crack formation, etc.), which make it possible to produce castings of any geometric complexity, configuration, shape, and mass.
Recently, there has been an intensive growth in the production of castings made of high-strength cast iron with spheroidal graphite (Fig. 1, b) in almost all industrially developed countries of the world.
Figure 1 – Microstructure of gray cast iron with lamellar graphite (a) and cast iron with spheroidal graphite (b) [1]
For example, in the USA, Germany, Korea, China, and Japan, the output of high-strength cast iron castings is 70–80% of the output of gray cast iron with lamellar graphite (Fig. 1). The output of high-strength cast iron castings exceeds the output of gray cast iron by 1.2–2.8 times in Turkey, Finland, France, Great Britain, Spain, Portugal, Denmark, Norway, and Austria (countries are listed in ascending order of proportion).
Currently, there are many different ways to produce high-strength spheroidal graphite cast iron. Each of them has its own advantages and disadvantages, as well as a rational scope of application.
In industry, the most common element-modifier, due to which spheroidal graphite is formed in the structure, the dominant place belongs to metallic magnesium. The boiling point of metallic magnesium is 1107°C, and the temperature of liquid iron during modification is approximately 1380…1480°C. Therefore, the addition of metallic magnesium to liquid metal is accompanied by splashes of cast iron from the ladle, pyroeffect and the release of white smoke of magnesium oxide. At the same time, the useful absorption of magnesium to the residual value required for spheroidization of graphite (0.03…0.05% Mgsalt) in cast iron is insignificant and unstable. To prevent splashes, pyroeffect, smoke release and stabilization of the coefficient of useful absorption, metallic magnesium is added to liquid iron by special, sometimes quite complex methods, some of which are shown in Fig. 2.
Magnesium metal is added to the bottom of the ladle with liquid cast iron in a refractory cup with holes on the rod through a hole in a heavy protective cover (Fig. 2, a), in a sealed autoclave under excess pressure, which increases the boiling point of magnesium to 1300…1380 °C (Fig. 2, c) or injected (blown) through a refractory lance in a stream of neutral kerosene (Fig. 2, b). The method of modifying cast iron with magnesium in a sealed converter-type revolving ladle (Fig. 2, d) is still widespread.
Instead of metallic magnesium, cast iron is modified with less active nickel alloys with 15…20% magnesium, or ferrosilicon (43…46% silicon) with 6…8% magnesium. The heavy nickel-magnesium modifier NMg-15…NMg-20 is mostly added to the bottom of the open ladle before filling with liquid cast iron (Fig. 3, a).
To avoid premature surfacing of the light ferrosilicon-magnesium modifier FSMg-6…FSMg-8, it is sometimes covered with calcium carbide with subsequent piercing of the sintered crust with a steel rod (Fig. 3, b); loaded with cast iron chips at the bottom of the ladle (Fig. 3, c), including in a special pocket of the ladle bottom lining – the so-called sandwich process (Fig. 3, d); loaded with a cast iron disk (Fig. 3, e) or mechanically held with a rod with holes lined near the bottom (Fig. 3, f). Reducing the pyroeffect is achieved by using a lid with a sprue (Fig. 3, g).
The methods used are adding a light modifier to a ladle with liquid cast iron in a refractory cup with holes on the rod or on the surface with simultaneous blowing of the cast iron with neutral gas through a porous refractory insert in the lining of the ladle bottom (Fig. 3, i).
There are several options for obtaining high-temperature carbon steel by processing molten iron with a flux-cored wire.
The wire can be inserted directly into the ladle using a special device, into the sprue and even into the mold through a special hole (Fig. 4)
Flux-cored wire modification technology is based on continuous or discrete processing of liquid metal. The wire sheath material and filler composition can be varied to obtain desired metal properties in castings.
One of the most modern and promising methods for obtaining high-strength cast iron with spheroidal graphite is the process of modifying cast iron in a casting mold, the so-called Inmold process (Fig. 3, k). Modification of cast iron in a casting mold consists in placing the crushed modifying mixture (ligature, modifier, additive) in a reaction chamber specially located under the feeder. At a certain combination of temperature and pouring speed, the modifier dissolves evenly and completely in the cast iron stream during pouring of the casting mold.
Figure 2 – Methods of treating molten magnesium metal:
a – ladle with a sealed lid and a refractory cup on a rod, b – ladle with a purge through a perforated refractory tube; c – sealed autoclave, d – sealed rotary ladle of the converter type [2]
Рисунок 3 – Найпоширеніші способи модифікування (оброблення) розплаву чавуну комплексними лігатурами (добавками): а – пур-овер; б – тригер; в – з прикриттям; г – сендвіч; д – з приван-таженням; е – з вогнетривкою сіткою;
ж – тандиш; и – газал; к – Інмолд-процес [2]
Figure 4 – Scheme of modification (processing) of the melt with flux-cored wire:
a – in the stream when pouring from a ladle; b – in a ladle [3]
The search for an effective, stable, safe method of spheroidizing cast iron continues to this day. Cast iron, which is treated with metallic magnesium or a nickel-magnesium modifier for the purpose of spheroidizing graphite, always crystallizes in a metastable system, i.e. with bleaching. Therefore, its simultaneous or secondary modification with ferrosilicon to eliminate bleaching in the cast state (without additional heat treatment of bleached castings) is a common practice in the production of high-strength cast iron. When spheroidizing cast iron with a ferrosilicon-magnesium modifier of the FSMg type, additional graphitizing modification of cast iron with ferrosilicon is not required.
List of links
1. Cheng X.A. Сomparative study on gray and nodular cast irons surface melted by plasma beam / X. Cheng, S. Hu, W. Song // Vacuum. – 2014. – Vol. 101. – P. 177-183
2. Methodological instructions for performing laboratory work on the discipline “Production of cast iron castings”, NTUU KPI named after I. Sykorsky, 2017, 37 p.
3. Ващенко К.И., Шумихин В.С. Плавка и внепечная обработка чугуна для отливок: Учеб. пособие. – К.: Вища шк., 1992. – 246 с.
How to contact us?
Write to us: info@ukrfavorit.com.ua
Call us: 0800 33 92 85 (toll-free).
Scan the QR code and join our social networks!
Together we move the industry forward!
Phone:
+38 (067) 333-92-85
+38 (066) 333-92-85
+38 (044) 333-92-85
Ukrfavorit Press Service
