Solidification and Methods Evaluation in Steel Castings

Synopsis: Steel casting process is an operation which is sensitive to a number of factors like mold thickness, steel’s thermal conductivity and superheat temperature of the molten steel, changing mold thickness also has some effect of solidification. The heat transfer equations governing the process solidification assume significance in production of sound casings.

The results show that the most important factor in solidified process is the thickness of steel in the mold, the magnitude of molten steel and its superheat temperature prior to entry in the mold.

In a major oil transportation project several pumps were ordered, as the first batch of two pump casings were delivered to the pump manufacturer and machining began the defects stared to show up in these castings. The initial assessment of casting was made with X-ray of the defective casings and excavations of those areas were done to expose and remove defects. Figure 1 to 3 depicts those steps.

Both pump casings were cast out of ASTM A 352 LCB, each pump weighed 3,000 kg and were both discovered to have defects that caused a review of the casting methodology and practices followed by the foundry. The defects were a cause of concern for both pump manufacturers as well as to the end user of the pump were analyzed. The defects were identified to be caused by shrinkage during the solidification of the casing.

This gave an opportunity to learn, and identify the location of defects and understand what are shrinkage porosities and how they manifest themselves and what can be done to control them.

Figure 1: Excavated shrinkage defect.

Subsequent to the study of the defective castings, an analysis was carried out to evaluate casting- methods simulation had considered all aspects of shrinkage and temperature dynamics in the casting design process. The following is a brief description and the end result of the review process. This article describes the principles and concept of the shrinkage formation in castings and data that are required as input for simulation of shrinkage forming in castings. The result of such simulation can be effectively used to improve the quality of casting.

Figure 2: Overview of the Pump casing and the location one of the Shrinkage defects.

Castings if not properly designed and the process not properly executed can turn into a huge disappointment. Castings can have defects ranging from gas porosities to shrinkages and even cracks.

Defects in castings can be avoided or controlled by;

• Proper design of casting itself,
• The pattern design,
• Selection of type of mold or design of the mold,
• Mold materials.

The process design can also contribute towards the control of casting defects. Some of the design practices that can help control shrinkage defects would include the following;

• Material composition,
• Temperature of the mold,
• Melting temperature of the material,
• Pouring temperature of the material,
• Rate of pouring,
• Cooling and solidification control,
• Feeding of fluid material into the fast solidifying cavities of the mold,
• Escaping of the gases.

Figure 3: The defect location in the second casting.

Since the pump casing castings had shrinkage related porosities, it is imperative to know and discuss about what they are and how they occur and then the methods available to mitigate these defects from castings or at least control these defects to a level which can be acceptable in a sound casting. Going by the specifications there can be various levels of acceptable defects in castings including the level of shrinkage in castings. The level of acceptability can also vary on the basis of castings ultimate application and the location, size and dispersion levels of the shrinkage related porosities.

Classification of Shrinkage Porosity

Shrinkage related defects in intricately shaped castings are a major cause of rejections and rework in the casting industry. In 2001, Lee et. al. classified the shrinkage porosity in castings by the size and cause.

The shrinkage porosities by size of pore are:

• Macro-porosity
•        and
• Micro-porosity;
• And by the cause for the pores forming are:
• Shrinkage porosity
•         and
• Gas porosity.

In 2002 Sabau et. al. considered porosity as usually to be either;

1. Hydrogen porosity
2.         or
3. Shrinkage porosity.

Hydrogen porosity is the term given to porosity that is generally rounded, isolated, and well distributed. They are deemed less damaging to the integrity of casting.

Porosity that are interconnected or clustered and have an irregular shape corresponding to the shape of the interdendritic region are linked to shrinkage and often termed as shrinkage porosity.

In general, the occurrence of micro-porosity in alloys is due to the combined effects of solidification shrinkage and gas precipitation.

In 2008 study A. Reis et. al. classified important defects that arise from shrinkage solidification as;

• External defects: pipe shrinkage and caved surfaces;
• Internal defects: macro-porosity and micro-porosity.

It may be noted that generally short freezing alloys are more prone to internal defects, whereas long freezing alloys are more prone to surface depressions.

 

Formation of shrinkage porosity

For any student of foundry and metallurgy the formation of porosity is a complex and interesting subject. There are several variables that have very complex relationship to the production of a defect free casting. These variables can be summarized as the following.

1. The heat transfer and thermal properties, like latent heat of fusion and thermal conductivity of the alloy being cast.
2. Casting’s chemical composition which determines the alloys freezing range and thus the dissolved gas content in it,
3. The mold type and its properties,
4. The geometry of the casting.

All the above are important factors that determine the quality of the final cast product.

The relative effect of these variables is complex and complicated. These effects and their impact on casting’s quality have been studied in detail for years, yet there is no clear identification of specific mechanism that controls the formation of porosity. In the absence of a clear scientific understanding, foundry methods engineer often use empirical rules to design castings. The absence of clear identification of specific mechanism highlights the complexities of variables, and their interactions. Though there are number of variables, shrinkage in castings is empirically defined as;

• Shrinkage is the obstruction of fluid flow in the mold, and difference in the specific volume of liquid and solid metal.

This definition can be explained as; as the casting solidifies, the fluid metal would try to flow in the mold to compensate for the liquid/solid volume change; however, that free flow may be hindered by the solidified sections of the casting. If a poorly fed region is large and completely cut off from a source of liquid metal, then a large void is formed. The resulting void is termed `macroporosity’. The gas solubility differences may also contribute to macro pore formation. The sections of casting where macro pores are formed solidify after the surrounding sections termed as a `hot spot’ with reference to the islands of hot metal completely surrounded by colder material.

Pellini’s observations in 1953 to the theoretical thermal analysis are of some importance in this regards. Where in the length of riser is considered as an important factor to feed in the required material during the solidification and resulting shrinkage process.

Volumetric shrinkage refers to the density difference between the solid and liquid alloy phases. This is explained as, as solidification proceeds, the volume diminishes and surrounding liquid flows in to fill in the now empty spaces. Depending on the amount and distribution of solid, the fluid flow may be impeded or even completely blocked. When sufficient liquid is not present to flow in to fill the cavities, voids (pores) form. This shrinkage porosity can either be many small distributed pores or one large void.

D.R. Gunasegarama et al.in 2009 believed that shrinkage porosity defects occurring in castings are strongly influenced by the time-varying temperature profiles inside the solidifying casting. This is explained as the temperature gradient within the casting would determine if a region that is just solidifying has access to sufficient amounts of feed metal at a higher temperature. Shrinkage pores will emerge in regions experiencing volume reduction due to phase change with no access to feed metal.

1. Campbell in 1991 provided good idea about the initiation of the shrinkage porosity with the help of pictorial view of solidification steps occurred during cooling of casting.

Other researchers have studied the formation of shrinkage porosity by offering theoretical models or empirical prediction criteria.

Factors Affecting Shrinkage Porosity

Campbell in 1991 and Gunasgaram in 2009 referenced above have established that Heat transfer rates at the casting/mold interface (CMI) play a significant role in determining the temperature gradients in the solidifying casting in molds.

That is because CMI is the rate controlling factor due to the fact that it offers the largest resistance to heat flowing out of the casting and into the mold. Heat flux Q (W/m²) across CMI is the product of the heat transfer coefficient h (Wm⁻² K⁻¹), which quantifies the degree of thermal contact between the casting and the mold, and T (K),which is the temperature difference between the surface of the casting at the CMI and that of the mold at the same CMI. The thermal resistance h of the CMI is attributed to the mold coat until an air gap forms between the expanding mold and the contracting casting. This is the insulating gap, the gap once formed acts as the major contributor to the resistance to heat flow out of the casting.

Temperature gradients are also a function of the geometry of the casting, size and location of sprue, location of gates and lengths of runners. Since thinner sections would solidify sooner than thicker areas, a temperature gradient exists from thinner to thicker sections.

Melt flow patterns inside the casting cavity and filling durations are also functions of the temperature gradients within the casting. Depending on the flow length after the runners have delivered sufficiently hot fluid metal to the casting, of the melt inside the cavity between the instant it enters the cavity and the moment it comes to rest, the amount of heat lost also will vary. Melt flowing longer distances would be colder after losing greater amounts of heat to the mold.

Need for Defect Prediction

The complexities of the melt flow and temperature gradient that determines the quality of the casting produced demands that there be some method to predict to a level of accuracy and confidence that will be acceptable to industry.

The task of a methods engineer in a foundry is to coordinate with the pattern makers, mold designer and foundry engineer to make an optimized geometric casting design and choose proper process parameters that eliminate or minimize porosity development. But porosity formation is a complex phenomenon where the final sizes and the distribution of porosity voids are determined by several strongly interacting process and alloy variables. As a result, it is usually difficult to eliminate the porosity completely from metal castings, while reducing it or moving it to an unimportant area can be a choice.

This highlights the need for some prediction technique which would predict the location and size of the porosity in the casting.

 

Prediction of Casting Shrinkage

The pump casing casting method has been analyzed using solidification simulation for assessment and prediction of the location of shrinkage porosity. Simulation result reasonably compares with available knowledge of the defects located by radiography and excavation of the casting.

As discussed in previous paragraphs, porosities in a casting due to shrinkage during the solidification process are one of the most common defects in castings. Various existing techniques of shrinkage porosity prediction like modulus and equi-solidification time and criterion function including Niyama criterion, a dimensionless criterion, Lee et al. criterion and Franco criterion for prediction of shrinkage porosity shrinkages and even cracks are used by foundry engineers and designers.

 

Modeling of Shrinkage Porosity

Once porosity forms, the pores will grow until they have reached equilibrium between all the forces acting on them including pressure and interfacial energy. Hence, to model pore nucleation and growth, the following physics based on Lee et al., 2001is analyzed and simulated.

(i) Analysis of the thermal field;

(ii) Analysis of the flow field (for pressure, heat and mass transport);

(iii) Analysis of the fraction solid (nucleation and growth of the solid grains and their   interaction with the thermal and solute concentration fields);

(iii) Analysis of the impingement of pores upon growing grains (altering both the interfacial energy and imposing curvature restrictions on the bubbles).

An ideal model would include this phenomenon. However, due to the complexity of the problem, most of the commercially available models appear to only consider a few of these phenomena and assumptions are made about rest as negligible. The validity of the model assumptions is dependent upon the alloy, process and particular casting design.

However if a model did account for all of the phenomena discussed above it may not be industrially viable, being so computationally intense that it would not be cost effective. Additionally, such a model would be so complex that the necessary boundary conditions and material properties might be difficult to obtain with required accuracy. The methods that have been proposed to model pore formation are categorized below into four different groups; each with its own benefits and drawbacks as far as industrial application is concerned (Lee et al., 2001).

• Analytical solutions.
• Criterion function models, based on empirical functions.
• Numerical solutions of Stokes flow (Darcy’s law), coupled with energy and mass conservation, and continuity equations.
• Models using a stochastic approach to nucleation of pores and grains in combination with continuum solutions for diffusion, taking into consideration the pore and microstructure interactions.

Casting Solidification Simulation

The aim of casting simulation is to;

• Predict the pattern of solidification, indicating where shrinkage cavities and associated defects may arise.
• Simulate solidification with the casting in various positions, so that the optimum position may be selected.
• Calculate the volumes and weights of all the different materials in the solid model.
• Provide a choice of quality levels, allowing, for example, the highlighting or ignoring of micro-porosity.
• Perform over a range of metals. The methods engineer should be able to demonstrate the key calculations that affect the quality of casting and are key tools to control the shrinkage porosities in castings.

Sprue Area Calculation Derivation of Formula: Sprue Area = M.T/t.d(2.g.H)^0.5.Cd

Where,

M = cast weight, casting and feeders, kg;

T = bottom pour correction factor: average filling rate/initial filling rate at start of pour;

t = measured or required filling time of casting including feeders, seconds;

d = liquid metal density, kg/m³;

g = gravity constant; 9.81ms²

H = metal head: in the case of the sprue exit this will be the total head above it including the pouring basin; for the sprue entrance this will be the head in the pouring basin, in mm;

Cd = discharge coefficient to allow for variations in mass distribution in the mold and friction losses; when T is based on experimental measurement this is equal to 1;

Solidification Modelling, asset of figure 4 shows the temperature gradients at various stages of material flow in the casing. The analysis resulted in changes in Sprue location and diameter and use of chills.

Figure 4A: Before simulation indicating heat and flow of molten metal in the mold

Figure 4B: After simulation indicating heat and flow of molten metal in the mold

The simulation program should be able to accept the following variables described below, as inputs to calculate the required output.

 

Figure 4C: Final result of heat and flow simulation

 

Casting Weight: The total weight of metal to be cast, including running system (estimated at the design stage) and feeders.

Estimated Fill Time: The total time between start and end of pouring including filling of feeders. The time should be chosen to ensure the metal fills as slowly as possible conducive with filling all of the casting and not introducing casting defects such as cold shuts. It is often useful to time the filling of similar successful castings to obtain data on which to base a judgment. Very complicated methods of calculating filling time are available but there is usually quite wide latitude within which acceptable castings can be made. A useful indication is given by the relationship: Fill Time (secs) = k x (Cast Weight, kgs)^0.5

Metal Density: The density of molten metal is used in the calculation. If this is known it can be entered, otherwise the density of the solid can be used. A correction factor (x 0.9) is applied to give an approximate liquid density. Densities of some alloys are given in the following table the density of alloy steel and carbon steel is highlighted yellow.

Material (Solid) Density Table (Density is in lb/in³)

Lb/in³ = 27679.9 kg/m³

1 kg/m³ = 0.0624 lb/ft³ = 0.000036127 lb/in³

MATERIAL	DENSITY	MATERIAL	DENSITY
Aluminum 1100	0.098	Magnesium, Cast, AZ91A	0.066
Aluminum 3003	0.099	Manganese	0.267
Aluminum 5052	0.097	Molybdenum	0.369
Aluminum 6061	0.098	Nickel, Pure	0.309
Aluminum 7075	0.101	Phosphor Bronze	0.32
Aluminum, Cast, 356	0.097	Platinum, (99.9)	0.775
Aluminum, Cast, 384	0.102	Silver, Fine	0.379
Beryllium	0.0675	Stainless Steel 301	0.285
Beryllium Copper	0.298	Stainless Steel 302	0.284
Brass, Cartridge	0.308	Stainless Steel 316	0.284
Brass, Free Cutting	0.307	Stainless Steel 420	0.278
Brass, Naval	0.304	Stainless Steel 430	0.283
Brass, Red	0.316	Stainless Steel 440	0.279
Cast Iron, Grey, CL35	0.252	Steel, Mild	0.284
Chromium	0.26	Steel, Carbon Alloy	0.284
Copper OFHC	0.323	Tin	0.264
Gold, (99.95)	0.682	Titanium	0.163
Iron, Pure	0.284	Tungsten	0.697
Kovar	0.302	Zinc	0.258
Lead	0.41	Zinc, Cast	0.24
Magnesium	0.063	Zirconium	0.23

Metal Head: The total height the metal falls from the level in the pouring basin to the bottom of the sprue.

Basin Depth: The head of metal above the sprue entrance diameter. This, with the entrance diameter, determines how much metal enters the sprue. During filling it is very important that the pouring basin is kept full to prevent air and surface films being drawn into the metal stream and into the casting. Ideally the sprue entrance is blocked with a stopper while the basin fills; the stopper is withdrawn immediately after the basin is full.

Bottom Pour Correction Factor: The correction factor allows for the slowing down in the filling rate of bottom poured castings as the mold fills. This slowing down is due to two factors:

1. the difference between the metal levels in the pouring basin and the mold (and hence the metallostatic head) which reduces during filling;
2. friction losses in the system e.g. at changes of direction in the metal flow.

This field can be left blank when a value of 1.5 is used which is suitable for most casting shapes. The factor of 1.5 was obtained by comparing measured filling times with theoretical predictions for many castings. It therefore includes the effects of friction in the system which is often allowed for by incorporating a coefficient of discharge; thus the coefficient of discharge is unity under these conditions.

The difference between measured and theoretical filling times is also affected by the distribution of masses of the casting in the mold, e.g. a casting with most of its mass in the bottom of the mold will take less time to fill than the same casting turned upside down with most of its mass at the top of the mold.

The factor of 1.5 is suitable for castings with a fairly even variation in mass distribution in different parts of castings. In practice once feeders have been added to castings the mass distribution is often fairly uniform. If castings are of completely uniform cross-sectional area in the vertical direction e.g. an unfed plate, the theoretical factor is 2 if friction losses are ignored.

Metal Speed in Ingates: This is to minimize the enfoldment of surface films into the body of the metal which can result in defects in castings.

Number of Ingates: The total number of entry points for the metal into the casting. It is used to calculate the dimensions of each ingate, assuming they are of equal cross-sectional area, which will give the required metal speed in the ingate.

Ingate Thickness: By setting one dimension of the ingate the other dimension of a rectangular cross-section can be calculated from the cross-sectional area. These dimensions will maintain the metal speed provided flow is uniform, the ingate is full and there are no momentum effects: such conditions rarely obtained in practice.

Real-time radiography during filling has shown that momentum effects in the metal stream have an important influence on turbulence and metal speed in individual metal streams, particularly when filters are not used. Thus these ingate and runner dimensions do not guarantee that filling will be non-turbulent; however they are a good starting point.

Metal Speed in Runners: As for Metal Speed in Ingate above, except applied to runners.

Number of runner bars: As for Metal Speed in Ingate above, except applied to runners.

Runner Width: As for Ingate Thickness above except applied to runners.

When all the above inputs are entered in a simulation model the following data can be produced as an Output.

Sprue Entrance Diameter or Rectangular Dimensions

This diameter is solely determined by the head of metal in the pouring basin and controls the amount of metal entering the sprue. The dimensions of a rectangle of equivalent area are given when one of the dimensions is set. The area of the entrance has been increased above the theoretical minimum by a factor for several reasons: to provide a safety margin to ensure that metal flow is always controlled by the sprue exit diameter; real-time radiography of filling indicates that the sprue fills completely more quickly. Two versions of the calculator are available with different safety factors. Either may be used, the factor is not overly critical providing that it is more than 1.

1. Factor based on diameter: Sprue entrance diameter is calculated to be 1.2 times greater than the minimum. This gives the largest diameter. Ref: ^{HMSO Ship Department Publication SDP18 for sand castings by M.Cox and D.W.Townsend published by HMSO;}  
2. Factor based on area: Sprue entrance cross-sectional area is calculated to be 1.2 times greater than the minimum. ^{Ref. Prof. John Campbell in “Castings” (Pub.: Butterworth Press).}

Fillet Radius on Sprue Entrance Diameter: A generous fillet radius minimizes separation of the metal stream from the mold walls.

Sprue Length between Runner and Basin: This dimension is provided for pattern making purposes.

Sprue Length between Fillet Radii: This dimension is provided for pattern making purposes.

Sprue Exit Diameter: This diameter, together with the total metal head, determines the amount of metal leaving the sprue and is often called the choke because it is the smallest cross-sectional area through which the metal flows into the casting. When the Bottom Pour Correction factor discussed above is set to 1, it can be used to dimension the size of hole in a nozzle at the bottom of a ladle or the size of hole required to control the fill of a top poured casting when the total metal head, is set appropriately.

 

Fillet Radius on Sprue Exit Diameter: A generous fillet radius minimizes separation of the metal stream from the mold walls.

Result and the Corrective action

As a result of the analysis, for the pump casing the sprue diameter was changed and heat transfer was corrected with placing of chill in the critical area of the casting shown in figures 5 and 6 as before and after conditions of the casing method.

Figure 5A: Sprue diameter and location before the analysis causing shrinkage defects.

Figure 5B: Sprue diameter and location before the analysis causing shrinkage defects.

Figure 6A: Sprue diameter and location after the analysis causing shrinkage defects.

Figure 6B: Sprue diameter and location after the analysis causing shrinkage defects.

Based on the above a comprehensive quality control check can be established to review the effectiveness of simulations and methods used at the foundry to control shrinkage pores in a casting. The following check list would establish the input and output data validity of the simulation program used by the methods engineer.

Check-list for the Methods department to validate simulation program.

Points Number	Description of Points to Check	Unit	Remarks
1.	Total cast weight	kg
2.	Estimated fill time	Seconds
3.	Steel Density (solid)	g/mm3 x 1000
4.	Steel density (Liquid)	g/mm3 x 1000
5.	Metal head (Cope depth)	mm
6.	Basin Depth	mm
7.	Bottom Pore correction
8.	Number of Ingates
9.	Metal speed in Ingates	mm/ Sec
10.	Metal speed in runners	mm/Sec
11.	Sprue location and size
12.	Number of runners
13.	Size of runner bars
14.	Fill Time (secs) = k x (Cast Weight, kgs)0.5

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