modeling and development of a forging ingot of … · foundry production. in the process of forging...

Journal of Chemical Technology and Metallurgy, 53, 5, 2018

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MODELING AND DEVELOPMENT OF A FORGING INGOT OF RATIONAL DESIGN AND MASS

Aleksandr N. Rogotovsky, Alexey A. Shipelnikov, Natalya A. Bobyleva

ABSTRACT

The present communication presents results of an investigation aimed at developing a ratio between the top mass and the body mass of the forging octagonal dead-melted steel ingot. Technological solutions for the application of various exothermic and thermal insulating materials in the exterior part of the ingot are advanced on the ground of an analysis of the shape and length variation of the pipe zone and shrinkage porosity in conformity with the numerical computer modeling of crystallization.

Keywords: steel ingot, modeling, solidification, shrinkage cavity, shrinkage porosity, mold, steel casting.

Received 15 March 2018Accepted 15 June 2018

Journal of Chemical Technology and Metallurgy, 53, 5, 2018, 936-942

Lipetsk State Technical University, Department of Metallurgical Technologies, 30 Moskovskaya Str., 398055 Lipetsk, Russia,E-mail: [email protected]

INTRODUCTION

The modern trends in the metal products market set high requirements both on the quality of cast billets for the manufacture of critical forgings and on the cost of their production. The greatest influence on the quality of the ingots obtained is exerted by the development of macrostructure formation processes at a directed move-ment of the crystallization front from the ingot walls to the center of the billet. The creation of conditions for maximum slowdown of metal cooling and crystallization in the hot top affects favorably the decrease of the pipe value and the top height, so that the defect zone does not penetrate the ingot body. Meanwhile, there is also a tendency to limit the development of the center shrink-age porosity of the ingot so that the existing microdefects disappear while the casting is processed by pressure.

In order to achieve it, the hot top is insulated in prac-tice using exothermic and heat insulating inserts of the metal heel of the top. Synthetic exothermic substances are also introduced. Additional heat is transferred during the combustion. It is directed to the bulk of the top metal. In some cases it contributes to the reduction of the height

of the ingot classical cone part thereby increasing the metal yield per each ton cast into the mold. Especially relevant is the influence of the hot top configuration on the pipe size and shape in view of decrease of the hot top metal consumption. This is possible because of the development of a modern market of relatively cheap heat-insulating and exothermic inserts and mixtures [1 - 6].

At present, the use of engineering analysis systems for the development and optimization of various foundry technologies, including the production of quality ingots, provides the improvement of the product quality, the increase of the casting yield and the significant time costs decrease. and to provide important information on the processes that occur when the mold is filled with melt and the ingot solidifies [7 - 9].

The aim of the present investigation is to perform 3D modeling of the process of casting steel into a closed-bottom mold which expands upwards under various initial and boundary conditions affecting the value of the pipe and shrinkage porosity. The value and shape of the concentrated pipe and the shrinkage porosity zone are estimated.


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EXPERIMENTAL A series of computer experiments were conducted in

the laboratory on “Computer modeling of metallurgical processes” of the metallurgical institute at Lipetsk State Technical University.

The ingot working drawings were put up together with a 3D model of the casting set (mold, ingot, top) in CAD licensed system “Compass 3D_V14”. The

ingot casting and crystallization process was mod-elled using “LVM Flow_CV” software package (the complex is based on all thermophysical calculations performed according to the finite difference method, or more precisely, to its modification – the finite vol-umes method). This software complex belonged to the class of CAE-systems for computer-aided engineering analysis of technological processes and was success-fully used by practicing metallurgical technologists in developing and/or adjusting the technological process of producing cast billets, steels and cast iron castings. The “LVM Flow_CV” software complex used in modeling provided to trace the dynamics of the process of filling the mold with metal and that of crystallization of the casting in the mold, as well as to obtain information in the field of the velocity, the pressure, the temperature, the liquid phase, the data on the slag inclusion distribution and the shrinkage defects [10].

The initial and boundary conditions for modeling forg-ing ingot casting were as follows: a mold material – carbon steel 35L, a siphon – fireclay; a base – grey cast iron; a hot top – fireclay, exothermic fill from “LVM Flow_CV” data-base with a combustion onset temperature of 1000°C and a calorific value of 2000 KJ/kg, heat insulating inserts with a thermal conductivity not exceeding 0.2 KW/M from the database on the basis of corundum. The grid pitch was 25 mm. The “melt-wall” heat transfer model was used without taking into account the convective heat transfer.

Fig. 1. Technological assembly of an ingot and a mold 3D model in “KOMPAS-3D Viewer”: a - with a conical top, b - with a cylindrical top.

а b

Fig. 2. Shrinkage (a) and shrinkage porosity (b) of a 12-ton ingot with a conical top and exothermic fill (the mold is heated to 120°C).


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RESULTS AND DISCUSSIONIn total, four variants of an ingot design with a hot

top are analyzed varying the ingot mass, the hot top type and height, the ingot mold temperature, and the options for top heat insulation. The main results of the calcula-tions refer to the data on the size of the pipe and the Niyama criterion characterizing the shrinkage porosity.

It is seen from the analysis of the results of the first variant of the ingot design (Table 1) that the shrinkage and the shrinkage porosity increase with increase of the ingot mass, and that the ratio of the height of the ingot body to the average reduced diameter (H/Dav) increases with increase of the depth. Thus, the shrinkage defects pen-

etrating the ingot body, i.e. the share of the axial shrinkage pipe or the axial V-shaped heterogeneity increases.

The development of shrinkage defects in the second variant (Table 1) is compared in case of changing the top design type: a classical conical design (Fig. 2) and a cylindrical design (Fig. 3).

Thus, it is possible to use cylindrical and conical hot tops on classic forging ingots, but strong axial poros-ity develops in the first case, while a concentrated pipe which can penetrate the ingot body without additional heat insulation is found in the second one. The third variant of ingot design modeling (see Table 1) considers a 12-ton forging ingot with the use of a classical coni-

Mod

ellin

g va

riant

s

Ingo

t mas

s, t

H/D

av

ratio

Mol

d te

mpe

ratu

re, º

С

Shrin

kage

,%

Shrin

kage

po

rosi

ty

Top

and

othe

r m

ater

ials

1

10 3.19

120

3.19 advanced center porosity

conical fireclay, exothermic mixture 30 mm

12 2.76 3.31 strong porosity in the

ingot deepening into the ingot

15 2.75 3.32 shrinkage porosity in the ingot top

19 3.31 3.22 strong shrinkage

porosity deepening into the ingot

2 12

2.76 120 3.31 porosity in the ingot top conical fireclay, exothermic mixture 30 mm

2.76 120 3.24 strong shrinkage

porosity deepening into the ingot

cylindrical top, exothermic mixture

3 12 2.76 20 3.37

strong shrinkage porosity, biting part of the ingot upper section

conical fireclay, exothermic mixture

2.76 20 3.35 shrinkage porosity is in the top

cylindrical top, heat insulating inserts

4 12

2.35 120 3.30 advanced center porosity

conical fireclay cut by 30%, heat insulating inserts,

exothermic mixture 70 mm

2.35 20 3.29 advanced center porosity

conical fireclay cut by 30%, heat insulating inserts,

exothermic mixture 70 mm

Table 1. Modeling results.


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cal top (Fig. 4) with fill and a cylindrical top with heat insulating inserts (Fig. 5) at a mold initial temperature of 20°C. There is a V-shaped pipe and shrinkage porosity deepening into the ingot body in case of using a conical fireclay top and a cover or fill. The best option would be to use a cylindrical top and heat insulating inserts which retain heat in the ingot top for a longer time minimizing the shrinkage defects.

The fourth variant (Table 1) considers the applica-tion of a conical top shortened by 35 % heightwise with the use of heat insulating inserts together with 70 mm exothermic fill at an initial mold temperature of 120°C (Fig. 6) and 20°C (Fig. 7).

According to the calculations results referring to a shortened conical top with heat insulating inserts in the ingot, there is an insignificant pipe in the top and

Fig. 3. Shrinkage (a) and shrinkage porosity (b) of a 12-ton ingot with a cylindrical top and exothermic fill (the mold is heated to 120°C).

Fig. 4. Shrinkage (a) and shrinkage porosity (b) of a 12-ton ingot with a conical top and exothermic fill (a mold tempera-ture of 20°C).


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well advanced porosity center. Since the heat insulating inserts and the exothermic fill help to keep the top hot even after 24 h when the ingot is cooled below 2000°C, the billet is forged if it does not contain significant ac-cumulation of slag. There is practically no furnace slag in steel-casting ladles when “remelting” technologies are used for smelting carbon steel from the returns of foundry production. In the process of forging ingots,

the developed center porosity disappears. This means that the inserts and the fills assist reducing the metal consumption of the top, thereby increasing the product conditional yield by a ton of cast steel.

For example, it is common practice in molding shops to cast 7 ton - 9 ton quality ingots with a small concentrated pipe and with a top height not exceeding 150 mm. It is not frequently cut after being knocked out

Fig. 5. Shrinkage (a) and shrinkage porosity (b) of a 12-ton ingot with a cylindrical top and heat insulating inserts (a mold temperature of 20°C).

Fig. 6. Shrinkage (a) and shrinkage porosity (b) of a 12-ton ingot with a shortened conical top, heat insulating inserts and exothermic fill (an ingot temperature of 120°С).


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of the mold and ingot surface examination, but is forged together with the ingot body. This decreases significantly the metal consumption per ton of forgings. Thus, in casting carbon steel with a siphon on a 12-ton forging ingot, a conical top could be recommended: 30 % - 35 % less height can be used with heat insulating inserts and exothermic fill, which reduces the metal content of the top, thereby increasing the conditional yield by a ton of cast steel. This is not the case with the cylindrical top with exothermic inserts.

The modeling results show that the volume of the pipe and the porosity zone in the head part of the ingot depend on the ingot mass, the top size and, to a small extent, on the mold temperature. The geometric shape and length of the pipe and the porosity zone in respect to its height are determined by the design of the hot top and the conditions of the heat removal through the top lining depending on the type of the top metal heat insulation. The most effective method of reducing shrinkage defects in the upper part of the forging ingot body of a classical shape is the application of a cylindrical top along with heat insulating inserts.

It should also be noted that LVM Flow CV mod-eling makes provides the improvement of the quality of the casting obtained by the most economical method, decreases the production costs, provides important in-

formation on the processes which occur when the mold is filled with melt and the ingot solidifies in the mold. The program saves information on the casting (initial and boundary conditions, data on the finite difference grid, modeling results) to be used for different calculation options, which allow a comparative analysis.

CONCLUSIONS

The shrinkage defects (pipes and porosity) in the up-per third of the ingot height are the main problem in the production of forging ingots. It is promising to improve the hot top design with the application of modern heat insulating materials and exothermic inserts and mix-tures. Their application is based on the heat dissipation inhibition effect in the upper part of the cast billets as it ensures a maximum prolonged liquid metal feed in the top of the underlying layers of the ingot body with the aim of reducing the pipe length. A threat of shrinkage porosity at the center of ingots remains as it may not fill up in the process of billet forging.

The most promising is the use of exothermic mix-tures on the heel of the metal in the top and of heat insulating inserts which are laid on the lining or directly on hot top casing. Thus, the heat flow through its side surface is significantly decreased.

Fig. 7. Shrinkage (a) and shrinkage porosity (b) of a 12-ton ingot with a shortened conical top, heat insulating inserts and exothermic fill (a mold temperature of 20°С).


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AcknowledgementsThe reported study was funded by RFBR and Lipetsk

Regional Administration according to research project №17-48-480203р_а.

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