Keywords: ductile iron castings; CNC machine tool column; casting process; shrinkage porosity; shrinkage cavity; chills; guide rail; quality control
Since the beginning of the 21st century, the rapid progress of science and technology, the continuous upgrading of industrial products, and the uninterrupted construction of national key projects and local infrastructure have significantly increased the demand for machine tools in almost every sector of the national economy. Machine tool products themselves are evolving rapidly. In both domestic and international markets, large high-precision CNC machine tools have been developed to solve the manufacturing problems of impellers, turbine blades, marine propellers, heavy generator rotors, steam turbine rotors, large diesel engine crankshafts, and many other critical components. These massive machine tools have a decisive influence on the aerospace industry, military industry, scientific research, precision instrumentation, and high-end medical equipment. For such machines, the large castings mainly include beds, columns, worktables, cross beams, spindle headstocks, tailstock bodies, and other structural parts. Among them, the column is one of the most important structural parts because it supports the spindle headstock and ensures the precise Z-axis movement of the spindle. Therefore, the column casting must possess excellent rigidity and thermal stability. In the work described in this paper, the column was designed as an A-shaped structure, which can thoroughly eliminate the offset and sway that often occur during machining, thereby ensuring the machining accuracy of the whole machine tool.
The quality demands for this kind of ductile iron castings are extremely strict. The guide rail surfaces on the column must have high guiding accuracy, excellent wear resistance, and sufficient rigidity. Furthermore, the casting dimensions must be accurate, and the guide rail surfaces are not allowed to contain shrinkage cavities, shrinkage porosity, cracks, gas holes, slag inclusions, or any other casting defects. These defects can seriously impair the wear resistance and rigidity of the guide rail, reduce the service life of the column, and, in severe cases, cause the machine tool machining center to lose control, which can lead to serious safety accidents. Thus, the production of high-quality ductile iron castings for machine tool columns requires not only careful process design but also strict control of every production step.
1. Casting Structure Analysis and Technical Requirements
The column casting discussed in this paper is produced for an international CNC machine tool manufacturer. The maximum contour size of the casting is 2296 mm × 1598.5 mm × 672 mm. The casting has three guide rails. The wall thickness is generally rather large; the average wall thickness is about 35 mm, while the thickest section reaches 78 mm. The casting material is spheroidal graphite cast iron QT600-3, which is one of the most widely used grades for high-strength ductile iron castings in machine tool applications.
The mechanical properties and chemical composition requirements of the ductile iron castings are summarized in the following table.
| Cast iron grade | Tensile strength / MPa | Yield strength / MPa | Elongation / % | C / % | Si / % | Mn / % | P / % | S / % |
|---|---|---|---|---|---|---|---|---|
| QT600-3 | ≥600 | ≥370 | ≥3 | 3.5–3.9 | 2.0–2.1 | 0.3–0.8 | <0.08 | <0.03 |
The quality requirements are not limited to mechanical properties. The casting must be inspected by ultrasonic testing and magnetic particle testing. Defects discovered in the casting cannot be repaired by welding. The flatness, straightness, and parallelism of the guide rail surfaces must be controlled within 5 mm. The casting dimensions must comply with ISO 8062-CT11 level. The maximum blank weight of the casting is 3266 kg. After pouring, the casting must remain in the sand mold until the casting temperature is below 280 °C. Before the casting has cooled to room temperature, no further handling, grinding, or cutting operation is permitted.
The structural configuration of the column is shown in the following illustration.
2. Solidification and Defect Mechanism of Ductile Iron Castings
Ductile iron castings have a well-known “mushy” solidification mode. Unlike grey cast iron, where graphite precipitation occurs throughout solidification and can continuously compensate for solidification shrinkage, ductile iron solidifies with a relatively thick semisolid zone. The eutectic solidification of ductile iron is strongly influenced by graphite nucleation and growth, by the temperature gradient in the mold, and by the rigidity of the mold wall. During eutectic solidification, graphite precipitation causes internal volume expansion. If the mold is rigid enough, this graphitic expansion can be used to feed the casting and to reduce macro-shrinkage defects. However, if the mold wall gives way or if the temperature gradient is not favorable, the expansion will be lost, and shrinkage porosity or shrinkage cavities may form inside the thick sections.
For a large column casting with thick guide rails and A-shaped structural ribs, the main solidification problem is the difference in solidification sequence between the external regions and the center of thick sections. Thick sections solidify slowly, and their central parts may be isolated from liquid feeding. Because ductile iron has a long eutectic solidification range, the last liquid to solidify in the center of a thick section often contains graphite that has already nucleated and grown. If the amount of liquid is insufficient, the center cannot be fed, and shrinkage porosity develops. This problem is particularly serious at the feet or flanges at both ends of the column, where the local wall thickness is large because bolt holes are required for fastening the column to the machine bed.
The modulus concept is useful for describing the solidification behavior of ductile iron castings. The local solidification modulus can be written as:
$$ M = \frac{V}{A} $$
where V is the local volume and A is the cooling surface area. The solidification time is then given by Chvorinov’s rule:
$$ t_s = K \left( \frac{V}{A} \right)^2 $$
where K is a mold-and-material constant. For the 78 mm thick section, the modulus can be approximated as about 39 mm when the section is considered to be a large plate cooled from both sides. This is a very high local modulus. The surrounding thinner walls cool much sooner, so the thick section is unable to obtain liquid feed from the feeder after critical solidus temperature is reached. Therefore, chills must be used to increase the local cooling rate and establish a favorable temperature gradient for directional solidification.
Another important metallurgical consideration in ductile iron castings is the carbon equivalent. A common expression for the carbon equivalent is:
$$ CE = C + \frac{Si}{3} + \frac{P}{3} $$
For the grade QT600-3, the carbon equivalent should be controlled to ensure adequate fluidity while minimizing graphite flotation and shrinkage tendency. In the present work, the chemical composition was controlled strictly within the ranges given above, with particular attention paid to sulfur and phosphorus. Low sulfur is necessary for reliable spheroidization, and low phosphorus is necessary for ductility and for avoiding brittle intergranular segregation.
3. Casting Difficulty Analysis
The production of these ductile iron castings is not a routine foundry job. Several technical difficulties had to be solved in the process design stage.
First, the A-shaped column has substantial metal distribution at both feet. These thick end flanges are necessary for bolting and structural rigidity. Because they are thick and relatively isolated from the guide rail sections, they are prone to shrinkage porosity. There is also a risk of misrun and cold shut if the pouring system is not designed correctly, since liquid iron has to travel a long distance along the column.
Second, the material is ductile iron, and ductile iron solidifies in a pasty manner. The eutectic solidification time is long. The solidification sequence between the outside and the center of thick sections differs considerably. If no special measures are taken, the centerline of the rail and the transition zones will contain micro-shrinkage. The usual solution is to place chills parallel to the thick sections in order to increase the cooling rate and to establish a beneficial temperature gradient. The chills also improve the local soundness of the guide rail surface.
Third, the guide rail surfaces are long and have a strict plane tolerance. The mold must be dimensionally stable, and the pouring system must avoid turbulent flow, slag entrapment, sand erosion, and air aspiration. If the liquid iron enters the mold too slowly, the upper part of the mold may not fill completely. If the liquid iron enters too quickly, the long rails may be eroded and sand inclusions may become trapped on the guide rail surfaces. For this reason, a semi-closed bottom-pouring gating system was selected, with overflow wells and chills used together in a carefully designed feeding concept.
Fourth, the quality standard forbids welding repair. Therefore, any internal defect in the casting is fatal to the casting. The process must be robust enough to consistently produce sound ductile iron castings without rework. This requires a very stable molding process, strict control of molten iron quality, and a robust gating and chilling system.
4. Casting Process Design
4.1 Pouring Position and Parting Surface
After thorough analysis and discussion, the pouring position was selected as horizontal, or more precisely flat casting, in order to obey the principle of directional solidification and to simplify the mold-making process. A two-box molding process was used. The guide rail face is the most critical surface of the casting. It must be free from sand holes, gas holes, slag holes, cracks, shrinkage porosity, and other defects. It also must be dense and uniform so that the hardness remains in the required range. Although the rail face is thick, the best pouring position for such ductile iron castings is to place the guide rail face downward. The parting surface was chosen on a large horizontal plane to simplify pattern construction and core placement.
4.2 Pattern Shrinkage Allowance
The casting shrinkage allowance is affected by many factors, including the alloy composition, the cooling conditions, the resistance to shrinkage caused by the mold and cores, and the geometry of the casting. It is very difficult to predict the exact shrinkage allowance with complete accuracy. In this work, we measured the actual shrinkage of many mass-produced columns by repeated layout inspection. The actual contraction data were recorded and compared statistically. Based on this accumulated experience, the casting shrinkage allowance was finally selected as 1%. This value was applied to all pattern dimensions. The linear shrinkage can be expressed as:
$$ \epsilon_s = \frac{L_{\text{pattern}} – L_{\text{casting}}}{L_{\text{casting}}} \times 100\% $$
where L denotes the corresponding dimension. This empirical approach was reliable because the production equipment, sand system, and pouring conditions remained stable.
4.3 Machining Allowance and Dimensional Tolerances
Machine tool columns require very accurate final dimensions. Therefore, the casting must have a sufficient machining allowance on all machined surfaces. At the same time, excessive allowance increases the casting weight and can make the wall thickness even more difficult to feed. In the present process, the machining allowance on the guide rail surfaces was set to 10 mm. The remaining dimensions were controlled according to the ISO 8062-CT11 class. This dimensional class corresponds to a relatively tight tolerance for a large casting of this size.
4.4 Parting Negative Allowance, Draft, and Core Allowances
Because of the large dimensions, a parting negative allowance of 2 mm was placed on the lower mold. All unspecified gaps were 2 mm. The unspecified draft was 0 to +2 mm. The internal rib cores were designed with a draft of −2 to +3 mm on each side. The end faces near the guide rails were given an additional process compensation allowance of +3 mm. These parameters were necessary to ensure clean mold stripping, accurate core assembly, and final dimensional conformity.
4.5 Summary of Process Parameters
| Parameter | Selected value / instruction |
|---|---|
| Mold method | Two-box molding, furan resin sand |
| Pouring position | Flat pouring, guide rail face downward |
| Casting shrinkage allowance | 1% |
| Machining allowance on guide rail face | 10 mm |
| Other machining allowances | ISO 8062-CT11 |
| Parting negative allowance | 2 mm, located in lower mold |
| Unspecified gap | 2 mm |
| Unspecified draft | 0/+2 mm |
| Internal rib core draft | −2/+3 mm per side |
| End face process compensation | +3 mm |
| Cooling before shakeout | Holding in sand mold at least 72 h; casting temperature below 280 °C before any handling |
5. Gating System Design
5.1 Design Principles
The gating system must ensure that the mold cavity fills quickly, smoothly, and without introducing defects. The casting is large, and the guide rail sections are thick. For the best directional solidification, the ingates should be arranged at the thick guide rail areas and thick wall sections. The gating system was designed as a semi-closed bottom-pouring system. This type of gating system can avoid high-speed falling and splashing at the beginning of pouring, reduce oxidation of liquid iron, and prevent slag from entering the mold cavity. It also creates a favorable temperature gradient because the hottest metal remains in the lower part of the mold, which is the rail side that must be fed longest.
5.2 Calculation of the Minimum Ingate Area
The minimum ingate cross-sectional area was calculated using the flow equation for metal casting. For metal flowing through the gating system, the minimum total ingate area is:
$$ \sum F_{\min} = \frac{G}{\rho t \mu \sqrt{2gH_p}} $$
where:
\( G \) is the total weight of liquid metal flowing through the ingates;
\( \rho \) is the density of the liquid metal;
\( t \) is the mold filling time;
\( \mu \) is the flow loss coefficient of the gating system;
\( g \) is the gravitational acceleration;
\( H_p \) is the average effective static pressure head.
For iron castings, after substituting the liquid iron density and gravitational acceleration, the formula is often simplified to:
$$ \sum F_{\min} = \frac{G}{0.31 t \mu \sqrt{H_p}} $$
Using the calculated pouring weight, the selected filling time, the flow loss coefficient determined from production experience, and the effective static pressure head calculated from the mold geometry, the required minimum ingate area was found to be about 30.2 cm2.
The effective static pressure head for a bottom-gating system can be estimated from:
$$ H_p = H_0 – \frac{p^2}{2C} $$
where \( H_0 \) is the height of the sprue above the ingate, \( p \) is the height of the casting above the ingate, and \( C \) is the total height of the casting cavity. These parameters were carefully measured from the molding layout.
5.3 Gating Layout
Based on the calculation, the production gating system was designed as follows:
- The sprue was made from a ceramic refractory tube with a diameter of ∅70 mm.
- The runner had a cross-section of 120 cm2 and was divided from the center toward both ends of the column.
- Five ingates were made from ∅30 mm ceramic refractory tubes.
The design ratio of sprue area, runner area, and ingate area was intended to be approximately:
$$ \sum F_{\text{sprue}} : \sum F_{\text{runner}} : \sum F_{\text{ingate}} = 1 : 3.1 : 0.78 $$
This is a semi-closed or semi-restricted gating system. The runner provides a large reservoir and reduces the velocity of the liquid iron, while the ingates restrict and control the flow rate. The use of ceramic refractory tubes along the entire gating system minimizes sand erosion. In particular, erosion of the guide rail surface by the incoming metal stream is avoided because the metal enters at the thick lower sections and rises upward in a controlled manner.
5.4 Chills and Overflow System
Because of the thick and uneven wall distribution, chills were placed in the mold at key positions. External chills were laid out along the guide rail area as well as at the thick core positions. The chills promote faster cooling, refine the local microstructure, and create a directional solidification sequence. The chills were cleaned and pre-coated before use so that no foreign material would be introduced into the ductile iron castings.
In addition to chills, an overflow system was designed at the upper part of the mold. The overflow wells allow the first, cooler, or oxidized liquid iron to leave the cavity and enter a sacrificial reservoir. This is particularly useful for preventing oxide films from remaining on the upper surface of the guide rails or at the end faces. The overflow also helps to complete the filling of narrow and distant sections, and it provides a controlled escape path for air and gases. The combination of bottom-pouring, chills, and overflow wells gave a stable filling pattern and a directional solidification pattern throughout the casting.
The relation between the chills and the casting modulus can be understood qualitatively. For a thick section, the chill adds an effective cooling surface. The local solidification time is therefore reduced. The reduced solidification time allows the center of the thick section to remain within the feeding range of the surrounding riser or liquid metal for a shorter period. The result is a fine microstructure and reduced micro-shrinkage. The position and thickness of the chills were decided on the basis of the local wall thickness and the riser feeding distance. Over-chilling was avoided because excessive chill would create carbide and unfavorably hard white-iron spots in the ductile iron castings.
6. Tooling, Molding, and Core Making
6.1 Pattern Making
The pattern was manufactured from dry Korean pine boards and multilayer plywood. This pattern material was selected to ensure that the pattern had adequate overall rigidity and strength. The pattern must not deform during handling, lifting, molding, or stripping. Lifting devices were provided on both sides of the pattern for safe hoisting and mold stripping. The pattern surface finish was controlled to Grade 1, and the pattern was painted with a suitable foundry paint to provide a smooth surface and resistance to sand abrasion. Casting fillets were made directly in the outer pattern and core boxes. The pattern was constructed in two halves, and the parting negative allowance was placed in the lower half.
6.2 Furan Resin Sand Molding
All molds and cores were made from furan resin sand. Furan resin sand provides high strength, good dimensional accuracy, and excellent collapsibility after casting. The high rigidity of the mold is particularly important for ductile iron castings because it enables the graphitic expansion during eutectic solidification to assist internal feeding. If the mold is soft, the expansion is absorbed by the deformation of the mold wall, and shrinkage porosity appears in the center of thick sections.
At the outer pattern guide rail area, chills were placed according to the process layout. Emergency gas vents and riser openings were made at the marked locations in the upper mold. The gating system was assembled using dedicated ceramic refractory tubes. The sprue was placed with a ∅70 mm refractory tube, and the ingate locations were arranged with five ∅30 mm refractory tubes. The core assembly was carefully checked to avoid misalignment.
6.3 Core Making and Assembly
During core making, the hanging bars in the cores were positioned accurately. If the hanging bars are not centered, it is difficult to remove the core from the core box and the core may break during handling. Vent holes were made in the core prints to allow the escape of gas. According to the process specification, chills were placed in the third and fourth core boxes to prevent shrinkage defects in thick sections; without these chills, the thick sections would be very difficult to feed and could cause rejection of the whole casting.
Before assembly, all core prints and outer mold vent holes were checked to ensure that they were clear. Any loose sand or gaps in the mold were repaired carefully. The side cores and the center cores were then placed in sequence. The cores were fixed firmly to each other. The hanging holes were filled with new resin sand and compacted. The repaired areas and all core surfaces were painted with two coats of zirconium silicate or zircon flour coating and then flame-dried. The coating lowers the surface roughness of the ductile iron castings and prevents sand burn-on.
During final assembly stage, the dimensional relation between the cores and the mold was checked against the drawing. The gating system and cavity interior were checked for cleanliness. All core print gaps were sealed completely; otherwise, liquid iron could enter the gaps, causing flash, or the gap could cause “fire” or air aspiration during pouring. The lower mold was then covered with a ring of asbestos rope around the parting line, and a ring of clay was applied. A paper strip test was performed before final clamping to verify that the mold closed correctly. Once the trial closing was accepted, the mold was closed, the joint was sealed with molding sand to prevent runout, and the pouring basin was placed in position.
7. Melting, Spheroidization, Inoculation, and Pouring
7.1 Melting and Spheroidization
The melting equipment used for these ductile iron castings was a 15-ton electric melting furnace. The required charge materials were selected to produce the desired chemical composition with low phosphorus and sulfur. The iron was tapped at about 1400 °C. Nodularization, or spheroidization, was carried out in the pouring ladle using the sandwich method. The ladle and the nodulizer were preheated sufficiently before the liquid iron was tapped, because any loss of temperature during spheroidization would reduce fluidity and increase the risk of misruns.
The nodulizer addition was controlled to 1.8% to 2.0% of the treated liquid iron weight. The amount was selected according to the sulfur content, treatment temperature, nodulizer quality, and the required residual magnesium content. The goal was to obtain complete and consistent spheroidization of graphite throughout the thick sections of the column while avoiding excessive magnesium residual, which can promote carbide and increase shrinkage tendency. The spheroidization result was checked on a quick-test sample before pouring.
7.2 Inoculation
Inoculation is essential for ductile iron castings because it promotes the nucleation of graphite and suppresses the formation of carbides. In the present process, 75% ferrosilicon inoculant was placed in the pouring stream at the tapping spout. Inoculation was performed just before and sometimes during pouring. The combination of spheroidization and late inoculation produced a fine graphite structure and a sound metallic matrix. The final matrix of QT600-3 is mainly pearlitic with some ferrite, giving the high strength and moderate elongation required for machine tool structural ductile iron castings.
7.3 Pouring and Cooling
Before pouring, the liquid iron was slag-removed repeatedly in the furnace and in the ladle. Careful slagging is necessary because slag floating on the surface can easily enter the mold during pouring, causing slag inclusions on the guide rail surfaces. The pouring temperature and pouring time were controlled within the approved process window. The semi-closed bottom gating system promoted calm filling and prevented the velocity from becoming too high.
After pouring, the casting was left in the sand mold for at least 72 hours. This slow cooling is necessary for several reasons. First, it allows the eutectoid transformation to occur in a controlled way so that the desired pearlitic structure is obtained. Second, it prevents internal stress and cracking. Third, it ensures that the casting is not shaken out too early. The specification required that the casting remain in the mold until its temperature was below 280 °C. After this holding period, the mold was shaken out, the casting was separated from the sand, and the gates and risers were removed. The casting was then cleaned by shot blasting to remove adhering sand and oxide scale.
8. Production Results and Quality Inspection
After cleaning, every column was subjected to a complete inspection. Chemical composition and mechanical properties were measured from separately cast test pieces and, in some cases, from the casting body. The microstructure was examined to confirm that the graphite shape was nodular and that the matrix was predominantly pearlitic. Dimensional inspection was carried out by layout measurement, and the guide rail surfaces were checked for flatness, straightness, and parallelism. Ultrasonic testing and magnetic particle testing were performed on the guide rail areas and other critical zones.
The production results are summarized in the table below.
| Inspection item | Result |
|---|---|
| Chemical composition | Within QT600-3 specification |
| Tensile strength | ≥600 MPa |
| Yield strength | ≥370 MPa |
| Elongation | ≥3% |
| Ultrasonic testing | No internal shrinkage cavities or porosity |
| Magnetic particle testing | No surface cracks or linear indications |
| Dimensional tolerance | Conforms to ISO 8062-CT11 |
| Guide rail flatness, straightness, parallelism | Within 5 mm |
| Surface quality | No sand holes, gas holes, slag inclusions, or other casting defects |
The use of the semi-closed bottom gating system, together with chills and overflow wells, successfully solved the difficult problem of internal shrinkage in the thick sections of these large ductile iron castings. The chills increased the cooling rate at the rail surfaces and at the heavy feet, while the overflow system removed the contaminated front part of the metal. The combination created an ideal temperature gradient from the lower rail surface upward, so that the liquid iron continued to feed the solidifying sections both through the spruce and through the internal liquid channels as long as possible.
One important observation from production is that mold rigidity is decisive for the quality of ductile iron castings. The furan resin sand mold with a strongly sealed box allowed the graphitic expansion pressure to be used for internal feeding. In contrast, a soft mold or an improperly sealed mold might absorb the expansion pressure and produce micro-shrinkage in the same casting geometry. Therefore, the molding sand compaction, the box clamping, and the core fixing method were all strictly controlled. Any sign of mold dilation on the test casting was investigated immediately.
The controlled cooling period of 72 hours also contributed to the dimensional stability and metallurgical quality of the ductile iron castings. Rapid cooling immediately after solidification would have caused high internal stress and possible distortion of the long guide rail. The slow cooling cycle allowed the natural stress relaxation to occur while the casting was still in the mold, thereby reducing the risk of distortion during machining.
The results demonstrated that the production process was not only technically feasible but also repeatable. The acceptance rate of the column castings was high, and the castings satisfied both internal and external quality standards. The customer’s inspection approved the chemical composition, mechanical properties, dimensional accuracy, and the absence of surface and internal defects. The guide rail surfaces were clean and dense, and they were ready for subsequent machining and grinding.
9. Conclusions
High-quality machine tool ductile iron castings require a systematic approach. The following conclusions can be drawn from the present work.
First, the casting design and process design must be considered together. The A-shaped column is an excellent structural solution for rigidity, but it creates large thick sections that are difficult to feed. The flat pouring position with the guide rail face downward is the correct choice for such a column because it places the most critical surface in the most favorable solidification position.
Second, the semi-closed bottom gating system is very suitable for large ductile iron castings with long and thick guide rails. It provides calm filling, minimizes oxidation and slag entrainment, and reduces mold erosion. The calculation of the minimum ingate area by the hydraulic formula gives a reliable starting point, which is then adjusted according to actual production experience. The ratio of sprue, runner, and ingate areas must be selected to maintain smooth filling and to avoid aspiration.
Third, chills and overflow wells are necessary for ductile iron castings with thick sections. Chills promote directional solidification and refine the microstructure. Overflow wells remove early contaminated metal and stabilize the filling process. The combination of chills, overflows, and bottom gating provides an effective feeding system that prevents shrinkage cavities and shrinkage porosity in the final product.
Fourth, molten iron treatment is critical. Controlled chemical composition, proper spheroidization, and late inoculation are essential for achieving the graphite morphology and matrix structure required by QT600-3. The addition of 1.8%–2.0% nodulizer, the use of 75% ferrosilicon inoculation, repeated slagging, and tapping at about 1400 °C were the practical measures that ensured stable metallurgical quality in every heat.
Fifth, post-pour cooling cannot be neglected. A minimum holding time of 72 hours in the mold and the requirement that casting temperature be below 280 °C before shakeout are not arbitrary. These conditions reduce residual stress, prevent distortion, and stabilize the pearlitic matrix. This is particularly important for long machine tool columns because the length-to-thickness ratio is high.
Finally, the inspection results proved that the selected process can consistently produce high-quality ductile iron castings for machine tool columns. The casting surface was clean, the dimensions met ISO 8062-CT11, the guide rail surfaces were within the specified flatness and parallelism, and ultrasonic and magnetic particle inspections found no harmful defects. This production experience can be used as a reference for other large machine tool ductile iron castings, including beds, cross beams, headstocks, and tailstocks.
In conclusion, the successful production of these ductile iron castings was achieved by combining careful casting design, modern molding materials, a properly designed gating system, chilling and overflow technology, strict metallurgical treatment, and disciplined process control. The knowledge gained from this work is valuable for the continuous improvement of high-quality ductile iron castings used in advanced CNC machine tools.