INGOT MOULDS USAGE AS AFFECTING MOULD LIFE
AS PER ISI NORMS

Proper procedure in the use of ingot moulds exert a profound influence upon the life of ingot moulds. Whatever care is taken in making a good ingot mould casting including control of chemical composition and metallographic structure, can be easily nullified if adequate care and a proper procedure is not followed in using the moulds.

Hence, preparation of the mould, bottom plate setting with refractory brick flow channels, mould setting over the bottom plate, mould assembly, teeming and ingot mould residence time are important to ensure adequate ingot mould life and quality of ingots. These are discussed below:

1. AGEING OF NEW MOULDS

This refers to manufacture of mould to start of first pouring of steel into the mould. This time period has a considerable influence on ingot mould life and may be attributed to internal residual stresses. Wherever ageing (natural stress relieving) is not adequate before first pouring of steel, a stress relieving heat treatment is required by heating for approximately three hours at degree C.

2. PREHEATING OF MOULDS

This is done at 100-120 C to reduce internal stresses.

3. COOLING OF TEEMED MOULDS

Immediately after stripping, the mould temperature ranges between 700 C and 950 C or even higher depending upon the steel, and ingot mould residence time. The moulds are to be air cooled even if rate of cooling is slow. Water cooling should be prohibited. If required, after stripping, the moulds are placed over special grates to allow better air circulation.. Too rapid a cooling increases stress build up, intensifying development of crazing and even cracking. Plants practising wholly air cooling have high mould life.

4. PREPARATION OF MOULD

(a) CLEANING OF MOULD

The moulds, especially the internal faces, are required to be thoroughly cleaned. Appropriate wire brushes may be used.

(b) REMOVAL OF ANY OVERFLOW METAL / FINS etc.

This is required as otherwise it will cause greater strains on the mould during next stripping and may cause transverse cracks.

(c) DRESSING OF MOULD

For better mould life and ingot quality, the cleaning and coating of the moulds is essential. The inside surface condition affects the ingot quality but the outside is most important from heat extraction point. If the mould is being used for the first time, the outer surface must be fully cleaned of the adhered sand which, if not removed, will cause higher temperature of the mould after teeming.

The object of mould coating is to prevent the metal from adhering to mould wall and to separate the mould wall and ingot to prevent crack and the break in ingot skin.

Coating is not to be done for atleast 30 minutes after the moulds have been cooled and cleaned with water to allow the water to evaporate out. The coating is normally done with a mixture of tar and varnish. Tar coating helps in improving the surface of the steel ingot and prolongs the life of the mould by retarding the oxidation and decarborisation of the internal surface of the mould and reduce sticking of the ingot in the mould.

Proprietary refractory coatings are also available which greatly improve the mould life and surface quality of the steel ingot.

The temperature of the mould at the time of coating needs control. A temperature lower than 90 C will give excessive thickness undecomposed volatile matter, while a temperature above 150 C will cause complete decomposition and cause safety hazard.

5. MOULD ASSEMBLY OVER BOTTOM PLATE

The bottom plate is placed flat on the ground and the hollow refractory bricks of the channels and pouring basin (trumpet) are carefully placed in position. The moulds are placed correctly centered, otherwise the mould wall at the bottom, nearer the steel entry gate will get eroded fast. While stripping the ingot from such a mould will eventually cause a piece of the mould bottom to tear off, making the mould unusable for further use. Cracks at the mould bottom will take place in off- centered placement of mould.

During mould setting, the bottom plate and the moulds should be perfectly matched. Inclined placing, uneven thickness of bottom plates or wrongly machined bottom face of moulds can cause trouble. If moulds are not straight, the metal stream will impinge ion the mould wall and damage the mould.

6. AVOID USE OF HOT MOULDS

This will hasten crazing and cracking, drastically reducing mould life. The mould re-use temperature should be 50 C - 80 C, which is most desirable.

Extra stock of moulds should be kept to avoid reuse of hot moulds. Hot and cold moulds both adversely affect the life of moulds and quality of the ingot as the maximum temperature developed in a mould is a function of the initial mould temperature. Higher the initial temperature, greater is the structural change and increased is the tendency to crazing, oxidation and growth.

7. TEEMING PRACTICE

(a) TYPE OF STEEL

The chemical composition of steel determines the teeming temperature. With killed steel, the average life of the mould is reduced, because of longer holding time before stripping. Rimming steels, being very low in carbon (0.05-0.08%) are tapped at higher temperatures.

(b) TEEMING TEMPERATURE

The steel pouring temperature during teeming affects mould life. Temperatures above 1570 C, affects moulds especially those used for the first time or during the first few heats. High incidence of mould stickers are a result of high pouring temperatures. Teeming temperatures should be as low as possible, in the process of mould filling.

(c) AVOID RUNNING STOPPER

For obvious reasons

(d) AVOID COLD HEATS

(e) AVOID METAL OVERFLOWS

The ingot moulds should be filled upto with atleast 3 inches less than the mould heights.

8. INGOT RESIDENCE TIME

Ingot residence time should be controlled to a minimum time, as this is the most important factor in mould life. Just after teeming, the inside mould wall acquires almost the temperature of the molten metal while the outside is hardly about 100 C. The outer surface however gradually gets heated to higher temperatures with increasing ingot residence time. Cast iron loses its strength very fast with rise of temperature. Any lateral pressure due to relative movement of ingot and ingot mould at the time of stripping may lead to bottom breaking of mould and / or scooping out of the mould metal from the inner surface. Scooping out of the mould metal also takes place if the ingot is sticking in any area of the mould, causing hard release during stripping.

Structural transformation is considerable leading to greater growth and a greater tendency to failure by crazing. Scooping out of the mould metal also takes place if the ingot is sticking in any area of the mould, causing hard release during stripping.

9. STRIPPING PRACTICE

Special care has to be exercised while stripping the ingot from the hot mould. The strength of the iron mould in a heated condition is very low and hence cracks any develop in the mould if it is handled roughly by hammering or hitting. Swinging of moulds against solid objects for removal of steel ingot should be prohibited or else moulds will crack. Mechanical strippers are useful