Ingot casting
Ingot casting is a method of casting of semi-finished products from metals and their alloys . Wrought alloys are cast instead of the usual cast alloys in the foundry .
In ingot casting, the liquid metal is poured into a standing mold and solidifies in it. The metal can be supplied from above (falling cast) or from below (rising cast). After solidification , the mold is stripped, i. H. pulled up with a crane. For this reason, cast blocks are always slightly conical in shape.
The cast ingot is further processed in the rolling mill or in a forge .
Ingot casting has now largely been replaced by continuous casting . The output is significantly worse with ingot casting and the productivity is lower. For this reason, ingot casting is used for products that cannot (yet) be produced using continuous casting, either because the required dimensions are too large, the alloy does not solidify sufficiently homogeneously ( segregation ), the batch sizes are too small or the alloy is prone to cracking.
Economical meaning
Blockuss has meanwhile been largely displaced by continuous casting . Only about 5% of the total amount of steel is produced using ingot casting. But it is still used for large forgings, e.g. B. crankshafts for large diesels with weights up to 100 t, indispensable. Furthermore, types of steel that are sensitive to cracks or segregation are cast as blocks, or special types of steel for which only small quantities are required.
Procedure
The liquid and ready-to-cast steel is distributed from the transport ladle through a refractory channel into the molds. The steel solidifies in these molds and is then removed from the molds (stripped).
construction
For increasing cast, a team is set up with which several molds are filled at the same time. The combination consists of a clamping plate , the channel system made of refractory bricks, the funnel and several molds.
On the Königstein, with which the melt is distributed, stands the funnel, in which the melt is poured from the ladle. The funnel consists of a reusable permanent mold which is refractory for each cast. For mechanical stabilization, a filling sand is placed between the refractory material and the permanent mold, which can be cleaned and reused.
The liquid steel is fed into various molds via the channel system.
In the case of multi-part molds, this can consist of a foot, the actual mold and a hood (also known as a skull). Thermal insulation for single use is attached to the side of the hood.
Before the start of casting, some casting powder is placed in the mold, either hung with a certain distance or placed on the floor.
In the falling casting, each mold is filled individually from the ladle.
procedure
The liquid steel, ready for casting, is moved to the casting area in a transport ladle. There is an opening in the bottom that is opened by means of a hydraulic slide. For thermal protection of the slide, the inlet in the refractory material is filled with a special slide sand.
- Sprue: For casting, the ladle is moved over a sprue mold and the slide is opened. The slider sand falls out and ideally the liquid steel flows out. With this sprue, the slide is heated so that the liquid steel above the slide does not freeze later during short transport trips. If the liquid steel does not come directly after opening the slide, a solid steel plug has formed above the slide, which has to be burned open with an oxygen lance. The number of slide openings at which liquid steel emerges directly without burning is called the opening rate .
- Pouring jet protection: After casting, the molds are filled one after the other with the pan. When the ingot is falling , the ladle is moved over the mold and felt with the liquid steel. With the rising ingot cast (see scheme), the pan is moved over a funnel. The system of ladle, pouring stream and funnel acts like a jet pump : At the pouring jet surface, a collision of the air molecules with the metal atoms close to the surface in the pouring jet causes an impulse to be transmitted downwards and a layer of air is carried along. The diffuser is missing, so that initially only air convection is excited. The content of oxygen and nitrogen in steel is usually significantly lower than their solubility , so that both oxygen and nitrogen are dissolved in the steel. This creates a pumping effect similar to a sorption pump . To reduce nitrogen and oxygen uptake, the transition area is flooded with the inert gas argon . In principle, argon also dissolves in steel or is entrained in gaseous form, but it is not an alloying agent and does not influence the properties of the steel (behaves inertly).
- Block base: The liquid steel has a casting temperature of more than 1500 ° C, while the refractory material and the molds in the combination have significantly lower temperatures (<100 ° C). So that the liquid steel does not freeze immediately and clog the channels, a large amount of liquid steel is poured off at the beginning of the casting. During the actual filling of the molds, the casting speed is reduced via the opening of the slide.
- Casting speed: The funnel and the molds, together with the channels in the base, form a communicating vessel with flow resistances F W (see adjacent graphic). If liquid steel is poured into the funnel, it flows with resistance into the molds. This resistance leads to the liquid column in the funnel being d H higher than in the molds. A flow of liquid occurs in the sub-floor to compensate for this difference in height. If the pouring speed of the ladle is constant, a dynamic equilibrium is established: the inflow rate from the ladle into the funnel is equal to the outflow rate from the funnel into the molds. The height difference between the funnel and the mold is adjusted according to the flow resistances. If the casting speed is too high or the flow resistance increases (e.g. due to the freezing of a channel), the funnel can overflow. The height of the funnel must be such that it does not overflow due to the required height difference.
- If the individual channels in the base have too different flow resistances , this leads to different fill levels in the molds. If the casting speed is reduced too much at the transition to the hood, this can lead to the steel flowing back out of individual molds.
- If there are two molds one behind the other on a channel (as in the graphic opposite), the volume velocity d V in the piece between the funnel and the first mold is the sum of the volume velocities d V 1/2 flowing into both molds . The erosion of the channel material depends on the relative speed, so that in this area increased erosion and associated non-metallic foreign bodies can be expected.
- Body casting: During the casting of the molds, the steel surface is covered with a casting powder. The casting powder should u. a. reduce contact with oxygen. The casting powder is selected in such a way that an area melts on the underside and forms a thin liquid layer. This layer should completely wet the steel. On the side of the mold, a thin layer is drawn in between the liquid steel and the mold and forms a separating layer so that heat welding between the two steels (block and mold) is avoided. The composition of the casting powder must be selected so that it melts sufficiently quickly and completely wets the liquid steel. If the wettability is not sufficient, the casting powder layer breaks open and in the worst case can lead to welding to the mold.
- The flow of steel into the mold is initially turbulent and leads to a cast baldness on the upper side . The degree of casting baldness decreases with increasing filling of the mold until it is no longer visible. It depends on the inflow velocity and the inflow geometry. The larger the inflow opening, the less the cast patch.
- Casting slag can be emulsified in the steel in the edge area of the casting bald spot. The emulsified casting slag has a significantly lower density and would float. However, due to the convection flow, the casting slag can be distributed throughout the block. If the droplets are too small, the ascent speed alone is not sufficient for the droplets to float within the solidification time and lead to non-metallic inclusions in the steel.
- If the amount of casting powder is insufficient, additional casting powder is sprinkled.
- Cover casting: The casting speed is reduced again at the transition to the cover. Finally, the mold is covered at the top for better thermal insulation. This usually happens with purely insulating material or with exothermic reacting material.
- Service life: After the steel has completely solidified (the service life is often determined experimentally), the ingot is removed from the mold. Before it is used in the next step, the block is visually examined and clearly marked.
solidification
An edge shell solidifies in the mold during casting, so that by the end of casting, around 10% of the steel has already solidified. The edge shell in the foot area is thicker than on the shoulder. In addition to the taper of the mold, this leads to an inclined course of the solidification front. During casting, the casting stream feeds convection in the steel melt, which continues after the end of casting and later changes into thermal convection. The density of liquid steel shows the usual temperature dependency, so that colder melt has a higher density and thus sinks on the outside. The even warmer melt in the middle is transported upwards.
Steel has a solidification area that is characterized by the liquidus and solidus temperatures. During solidification, growing solid dendrites are formed in this temperature range , between which there is still liquid residual melt. The space between the two temperatures is the solidification area, which looks like a sponge from the topology. Towards the solidus temperature, the sponge becomes more and more dense and the remaining liquid spaces become thinner. During solidification, the density increases significantly or the volume decreases. Melt must be replenished in order to fill the gaps with liquid steel. If the ferrostatic pressure of the residual liquid is not sufficient to push the liquid steel into the pores between the dendrites, microscopic cavities (shrinkage holes) are created. The spatial distance between the liquidus line and the solidus line is greatest in the middle of the block, so that microporosities tend to arise here.
During solidification, there is a mixed structure in the spatial area between the liquidus and solidus temperature, in which individual elements can separate (segregation). In the residual melt between the dendrites, the concentration of individual elements is significantly higher than in the solid phase. And the concentration of these elements in the solid phase is significantly lower than in the original melt. However, this higher-alloyed residual melt has a different solidus temperature, so that the residual melt only solidifies at significantly lower temperatures in some cases. This results in segregation between the dendrites on a microscopic level (microsegregation). If the solidification time is sufficiently long or if diffusion annealing is carried out afterwards, the diffusion speed of the alloy components is sufficient to largely compensate for this difference in concentration.
At the same time, it can happen in the residual melt that the solubility product of individual oxides or gases is exceeded. Oxides then precipitate as non-metallic inclusions and gases generate gas bubbles (right half in the graphic below), which lead to a porous material. For oxygen, a distinction is made between calmed and unkilled steel: In unkilled steel, the amount of dissolved oxygen in the melt is so high that a kind of boiling over can already be observed during casting. In the case of killed steel, the oxygen is bound by means of aluminum or calcium and the concentration is reduced to below 50ppm.
The hood serves as a feeder to compensate for the decrease in volume . There is additional material here that can flow in during solidification.
Due to the thermal shrinkage, the solidifying block contracts and detaches from the mold. A gas gap is formed in which the thermal energy is mainly transmitted by radiation. If there is direct contact between the ingot and the mold, the heat transport is significantly better, so that solidification proceeds more slowly once the gas gap has formed.
exam
Due to the coarse cast structure , a non-destructive test of the complete ingot is only possible visually. Internal defects such as segregation, non-metallic inclusions or porosities / cavities cannot be detected before further processing.
During the visual control, the following block errors can be identified, which indicate deviations in the manufacturing process and lead to measures within the framework of an FMEA .
- Casting powder pockets : Deep impressions in the steel surface, some of which still contain powdery casting powder. This flaw is mainly to be found at the block base and indicates that the sprue speed is too high (process deviation). The casting powder was flooded by the molten steel, sintered and forms a solid pocket.
- Depending on the location and characteristics, this error must be removed by grinding.
- Off-center sprue: The block base clearly shows that the sprue was not centered. The solidification no longer takes place symmetrically. The error is due to the incorrect assembly of the trailer.
- Flat impression on the side surface: Isolated impressions can be caused by the rope that is used to hang the casting powder. If the casting powder is hung over the edges of the mold with two ropes, remnants of the rope will hang on the side of the mold after the casting powder has burned on. Due to the oxygen-poor atmosphere, the rope cannot burn and leaves an imprint on the surface. This flaw must be sanded depending on its form.
- Casting powder noses: Individual elongated impressions are present parallel to the casting direction, which may still contain casting powder residues. This can occur if the casting powder does not melt homogeneously, but rather forms lumps that are drawn in in the gap to the mold and rolled long through the steel.
- With this error, the composition of the casting powder does not match the casting temperature or steel alloy.
- Steel noses: While casting powder noses are impressions in which the surface is arched at these points, steel noses are arched outwards. These arise in surface defects in the molds. Depending on how the block is stripped, small imperfections in the block can result in the stripping of the surface along the stripping direction. This error increases with each use until pronounced steel noses are visible.
- Casting heel: A notch running all around the block indicates an interruption in casting, which can have various causes. Depending on the shape of the casting step, the block must be scrapped.
- Overflow: The shoulder can overflow. There is still liquid material in the hood. The shrinkage of the solid block can create a small gap in the refractory lining, in which liquid steel can flow into the cavity between the block and the mold.
- If these overflows are not removed, the underlying boundary layer is worked into the block by rolling or forging and leads to internal defects.
- Wavy surface: Wavy surfaces are usually transverse to the direction of casting. It can often be seen that this defect does not yet exist in the foot area and only increases with the height of the block. The distances between the waves roughly match the visible vibration of the surface, so that this error correlates with the rhythmic bath movement during pouring.
- Canopy rear run: At the transition from the fuselage to the canopy, there are narrow webs where liquid steel has run behind the canopy insulation. This can lead to an unintended solidification.
- Head Lunker: In the middle of the block head a clear funnel is visible, which shows a head Lunker. In the best case, the cavity surface is wetted with casting slag, in the worst case with an oxide layer. In the following steps, this leads to clear internal defects, so that blocks with head blows are usually scrapped.
- The cause is insufficient insulation of the block hood by rice husk ash.
- Overflow: If the casting is not finished in time, some liquid steel runs over the mold and leads to webs on the block head. In the best case, it is sufficient to separate these bars.
- Transverse crack: Pronounced crack, preferably in the hood, is often a stress crack. Internal block stresses arising during solidification cannot be absorbed and lead to these cracks. Depending on the type of steel, this crack occurs if the ingot is not removed from the mold in good time and is initially stress-relieved in a separate furnace.
- No head collapse: A missing head collapse indicates insufficient isolation of the block head. The surface solidifies too quickly and forms a bridge. As the solidification continues, a head cavity forms below this bridge.
- Flower head formation: During solidification, the alloy concentrations in the solid and liquid change continuously, which leads to segregation or segregation. As a rule, the alloying elements accumulate in the residual liquid melt, which also changes the solubility of gases in the residual melt. The dissolved gases also accumulate in the residual melt.
- If the gas concentration exceeds the solubility at any point in time, bubbles form. These bubbles, with a significantly lower density than steel, cause the material to rise, comparable to the rise of a cake in an oven.
- The cause is either inadequate degassing or incorrect design of the alloy by defining an excessively high nitrogen concentration in the steel.
Block formats
The steel mills only keep a limited number of different formats in stock so that only graduated block sizes can be produced. Depending on the application, various geometries have proven themselves:
- Heavy plate uses slab formats that have a rectangular cross-section with a significantly greater width than length.
- Rolling mills (sheet metal and billets ) use square cross-sections, some of which can also be designed as non-uniform octagons. The aim is to have slender blocks with a high ratio of height to diameter.
- Forge inserts use square cross-sections for small weights. With larger weights, polygonal cross-sections are used. The H / D ratio is smaller than in the rolling mill.
- ESU insert uses circular cross-sections with little or no conicity . As a result, the purity and porosity of the blocks are poorer, but these blocks are melted again.
Advantages and disadvantages
In general, in both processes, ingot casting and continuous casting, the start and end areas must be scrapped before further processing. Continuous casting significantly reduces the scrap content in continuous casting. The more pans that are processed in a sequence, the more economical the process.
Disadvantages compared to continuous casting
In the case of ingot casting, the head and foot of the ingot are scrapped because the metallurgical purity in these parts is insufficient. The proportion of usable fillet weight ( output ) is significantly reduced compared to continuous casting. The output is in the range of 80%, approx. 20% of the block is melted down again.
In continuous casting, the material has no conicity, which simplifies subsequent processes. Due to their conicity, blocks must first be pre-machined in order to be able to be rolled to their final dimensions in a roll stand .
Advantages over continuous casting
The benefits focus on the following four topics:
For special applications, dimensions are required that cannot (yet) be realized in continuous casting, such as B. Tools for deep drawing engine hoods. In these dimensional ranges, ingot casting competes with mold casting . While die casting produces near net shape, block casting is followed by a forging operation. In this following process route, the material structure can be adjusted more specifically than in mold casting.
In continuous casting, materials that are sensitive to segregation tend to segregate ( segregate ) individual elements in the cross-section , so that no homogeneous cross-section is created. These materials also segregate in ingot casting. However, in ingot casting, the solidification preferably takes place from the bottom up. The segregation therefore takes place in the vertical direction. The cross-section is significantly more homogeneous than in continuous casting. Due to the segregation, the proportion of head scrap is higher.
In principle, there are limits to the degree of purity of both methods. For the production of high-purity and homogeneous steels, the material is remelted at least once. For the production of blooms for remelting plants, ingot casting is more economical.
The advantage of continuous casting in terms of economy is also a disadvantage: a minimum amount is required for a strand for economical production . Usual batch sizes are in the 50-120to range. With an order quantity of 100 tons, continuous casting is more uneconomical than ingot casting.