Hot top cavity - ESR ingot defects

Hot top cavity - ESR ingot defects

Figure 1: Example of hot top cavites.

Figure 2: Example of hot top cavites.

Defect name: Hot top cavity
Record No.: 2423
Type of defect (Internal/Surface): Internal
Defect classification: ESR ingot defects
Steel name: Steel
Steel composition in weight %: No data.
Note: ESR is known as an alternative step on the continuous improvement in single ingot production for heavy forgings. In the recent past a new state of the art ESR remelting system was installed with the possibility to produce ingots in diameter from 1000 mm for 45 tons till 2000 mm for 145 tons. Consumable electrodes from traditional fabrication processes which consist of melting with electrical arc furnace, refining, degassing of the steel and bottom pouring will be used to produce one single heavy ingot with best cleanness with the help of electrode exchange technique. This paper tells about the furnace characteristics and the efforts to be made to ensure high quality level in material and furnace preparation as well as save automatic production performances during remelting and electrode exchange. Finally the results of ingots characterized by an elevated level of chemical homogeneity and internal quality such as to ensure the absence of defect greater than 0.5 mm AVG during the final inspection of the forged part from the ESR production will be presented. The ESR process serves for the reproducible quality in heavy ingot making and tightens the limits in specifications together with high material yield.

Consequences of our inexperience were evident in a marked undulation of the skin, in a too limited cap of slag at the end of remelting, in an evident ring in correspondence with the electrode change. All this points do not represent big problems themselves but they have to be read as external evidence of bigger possible internal problems: for examples, a small cap should immediately suggest a possible shrink hole wider than desired and a too manifest electrode change position can be easily linked to a non optimal internal structure of the ingot. At the end, the result of all the efforts done during remelting can be “measured” by the top and bottom scrap percentage. While the lower part of the ingot should be discarded because of some metallurgical reason related to the chemical composition variation and solidification direction strongly influenced by the copper base plate, the upper part is affected by porosity and shrink hole cavity strictly connected to how good is the hot top part of the melting profile (figs. 1 and 2) : better you are able to manage the closing of the process, more material you can save on your ingot. Unfortunately, this is absolutely not easy to obtain, in fact the hot top part of the recipe is the most difficult part to optimize during remelting and often it required more than one cast to obtain a satisfactory result.
Reference: Not shown in this demonstration version.