**1. Introduction**

Steel castings used in high-performance applications, such as cast valve components used in power plants and automotive engines, are subject to stringent quality standards to ensure operational integrity. Such steels may vary from high carbon low alloy steels to low carbon high alloyed steels, the latter designed to withstand highly-corrosive environments and have low susceptibility to brittle fracture. Metal castings of all material and process types are prone to porosity, defined as internal voids or non-metallic inclusions, and classified as internal defects (similar to other internal defects, such as the evolution of unwanted material phases or inhomogeneities in the distribution of alloying elements due to segregation phenomena). Such porosity defects can severely impact the performance of castings, through crack initiation and propagation under fatigue loading, stress corrosion cracking, and

reduced material strength and (hence) fatigue life, among other factors [1–3]. It is therefore common that allowable limits on porosity severity levels are dictated by standards or client specifications. At the same time, it is therefore crucial that foundries develop a clear understanding of porosity defects in (steel) castings and introduce suitable monitoring and improvement protocols in order to remain competitive.

• Secondly, the contraction during solidification in the mushy state, when the liquid metal transforms to the initial solid state microstructure.

*Shrinkage Porosity in Steel Sand Castings: Formation, Classification and Inspection*

*DOI: http://dx.doi.org/10.5772/intechopen.94392*

to compensate for solidification contraction, and (ii) the formation of shrinkage porosity as a result of failure to feed the mushy zone – this is unavoidable due to coherency of the dendritic structure when the mushy zone reaches a particular solid fraction, leading to entrapped interdendritic liquid

*Temperature profile during the solidification stage of a typical low carbon steel.*

**Figure 2.**

**Figure 3.**

**135**

*Mushy zone coherency during solidification shrinkage.*

Contractions for some pure metals at their respective freezing temperatures are given in [4]. In the case of pure Fe, the solidification contraction was found to be 3.16% at the phase transformation temperature of 1536°C. However, as the carbon content increases, it increases to up to 5% for low carbon steels. This is related to the decrease in the solidus temperature as the carbon content increases (see phase diagram in **Figure 2**). The importance of solidification contraction is two-fold: (i) the need for an efficient feeder system

Porosity can generally be categorised as gas porosity (entrapped and segregated gases), inclusions (sand and slag), shrinkage porosity, and hot tears (also known as hot cracks) [4]. Furthermore, these different types of porosity may present both at the micro and macro length scales. These porosity types and their length scales, together with their morphologies, provide clues as to their likely causes, allowing the introduction of mitigation measures by foundries.

This Chapter focuses on shrinkage porosity in steel sand castings. Sandcasting, in particular, is a process that depends on gravitationally pressurised flow, which makes it difficult to feed the solidification contraction of the metal. This makes sandcast components, which is the case for most steel components, prone to shrinkage porosity. In some cases, geometric modifications to part geometry can eliminate shrinkage porosity. However, in other cases, it is unavoidable, but can be reduced through a combination of geometric and process modifications.

During the casting cycle, the steel undergoes three types of contractions that need to be considered, as illustrated in **Figure 1**.

• Firstly, the thermal contraction in the liquid state. In some practices, the liquid melt pouring temperature *Tp* is kept higher to avoid early formation of skin freezing, allowing the metal to solidify directionally inward from the surface. However, it is important to establish whether higher pouring temperatures lead to higher or lower shrinkage porosity. In any case, liquid melt contraction is not an issue in foundries, since its low value (< 1%) is easily compensated through liquid feeding from the feeders. The liquid melt density as a function of temperature *T K*½ � and carbon content C [%wt] can be determined using the empirical relationship [5]:

$$\rho = (7.10 - 0.0732C) - (8.28 - 0.874C) \times 10^{-4} (T - 1823) \tag{1}$$

and the density of solid steel can be determined using Thermo-Calc or similar computational thermodynamic systems.

#### **Figure 1.**

*Contraction of steel during a casting cycle for a low carbon steel (A216 WCB with C = 0.3%, Tliquidus = 1495°C, Tsolidus = 1430°C): liquid contraction = 0.4%; solidification contraction = 3.6%; solid contraction = 6.0%) (Data from Thermo-Calc and [5]).*

*Shrinkage Porosity in Steel Sand Castings: Formation, Classification and Inspection DOI: http://dx.doi.org/10.5772/intechopen.94392*

• Secondly, the contraction during solidification in the mushy state, when the liquid metal transforms to the initial solid state microstructure. Contractions for some pure metals at their respective freezing temperatures are given in [4]. In the case of pure Fe, the solidification contraction was found to be 3.16% at the phase transformation temperature of 1536°C. However, as the carbon content increases, it increases to up to 5% for low carbon steels. This is related to the decrease in the solidus temperature as the carbon content increases (see phase diagram in **Figure 2**). The importance of solidification contraction is two-fold: (i) the need for an efficient feeder system to compensate for solidification contraction, and (ii) the formation of shrinkage porosity as a result of failure to feed the mushy zone – this is unavoidable due to coherency of the dendritic structure when the mushy zone reaches a particular solid fraction, leading to entrapped interdendritic liquid

**Figure 3.** *Mushy zone coherency during solidification shrinkage.*

reduced material strength and (hence) fatigue life, among other factors [1–3]. It is therefore common that allowable limits on porosity severity levels are dictated by standards or client specifications. At the same time, it is therefore crucial that foundries develop a clear understanding of porosity defects in (steel) castings and introduce suitable monitoring and improvement protocols in order to remain

Porosity can generally be categorised as gas porosity (entrapped and segregated gases), inclusions (sand and slag), shrinkage porosity, and hot tears (also known as hot cracks) [4]. Furthermore, these different types of porosity may present both at the micro and macro length scales. These porosity types and their length scales, together with their morphologies, provide clues as to their likely causes, allowing

This Chapter focuses on shrinkage porosity in steel sand castings. Sandcasting, in

During the casting cycle, the steel undergoes three types of contractions that

• Firstly, the thermal contraction in the liquid state. In some practices, the liquid melt pouring temperature *Tp* is kept higher to avoid early formation of skin freezing, allowing the metal to solidify directionally inward from the surface. However, it is important to establish whether higher pouring temperatures lead to higher or lower shrinkage porosity. In any case, liquid melt contraction is not an issue in foundries, since its low value (< 1%) is easily compensated through liquid feeding from the feeders. The liquid melt density as a function of temperature *T K*½ � and carbon content C [%wt] can be determined using the

*<sup>ρ</sup>* <sup>¼</sup> <sup>ð</sup>7*:*<sup>10</sup> � <sup>0</sup>*:*0732*C*Þ � <sup>ð</sup>8*:*<sup>28</sup> � <sup>0</sup>*:*874*C*Þ � <sup>10</sup>�<sup>4</sup>ð Þ *<sup>T</sup>* � <sup>1823</sup> (1)

and the density of solid steel can be determined using Thermo-Calc or similar

*Contraction of steel during a casting cycle for a low carbon steel (A216 WCB with C = 0.3%, Tliquidus = 1495°C, Tsolidus = 1430°C): liquid contraction = 0.4%; solidification contraction = 3.6%; solid contraction = 6.0%) (Data*

particular, is a process that depends on gravitationally pressurised flow, which makes it difficult to feed the solidification contraction of the metal. This makes sandcast components, which is the case for most steel components, prone to shrinkage porosity. In some cases, geometric modifications to part geometry can eliminate shrinkage porosity. However, in other cases, it is unavoidable, but can be

reduced through a combination of geometric and process modifications.

the introduction of mitigation measures by foundries.

*Casting Processes and Modelling of Metallic Materials*

need to be considered, as illustrated in **Figure 1**.

empirical relationship [5]:

computational thermodynamic systems.

**Figure 1.**

**134**

*from Thermo-Calc and [5]).*

competitive.

melt, which results in either pore nucleation (due to volumetric shrinkage) or solid collapse (due to negative pressure), as shown in **Figure 3**. As will be shown later, the freezing range (difference between the liquidus and solidus temperatures) is an important factor in determining the morphology of shrinkage pores.

*eV <sup>f</sup>* ¼ *α Vc* þ *V <sup>f</sup>*

*Shrinkage Porosity in Steel Sand Castings: Formation, Classification and Inspection*

*feeder* ¼) *ts*j

*ts* <sup>¼</sup> *<sup>B</sup> <sup>V</sup> A* � �*<sup>n</sup>*

�K�<sup>1</sup>

4*kρc* � � <sup>1</sup> <sup>þ</sup>

where the modulus *m* has been introduced. Solving Eqs. (1) and (5) simultaneously for a given mould cavity allows the dimensioning of the system of feeders.

*Hotspot formation in the feeder junction regions and the gate area of a steel disc casting for a modulus*

Although Eq. (4) may appear complex, it is relatively straightforward to evaluate given the casting process parameters deployed and the materials used for the sandmould and the casting, the latter being readily available in metal casting databases. It is therefore relatively easy to estimate the solidification time for a specific cast geometry. However, in the design of the feeder system, this is not necessary,

, density *<sup>ρ</sup><sup>m</sup>* in kg�m�<sup>3</sup>

*ts*j

*DOI: http://dx.doi.org/10.5772/intechopen.94392*

*casting* <*ts*j

in terms of the mould geometry, and can be expressed as:

material properties (thermal conductivity *<sup>k</sup>* in W�m�<sup>1</sup>

*<sup>B</sup>* <sup>¼</sup> *<sup>ρ</sup>mL Tm* � *T*<sup>0</sup> � �<sup>2</sup> *π*

since substituting Eq. (3) into Eq. (2) simply requires that:

*V <sup>f</sup> A f*

¼ 1*:*2 *Vc Ac*

and specific heat capacity *<sup>c</sup>* in J�kg�<sup>1</sup>

), given by:

heat of fusion *<sup>L</sup>* in J�kg�<sup>1</sup>

�K�<sup>1</sup>

<sup>J</sup>�kg�<sup>1</sup>

**Figure 5.**

**137**

*ratiom <sup>f</sup> =mc* ¼ 1*:*2 *.*

where the feeder efficiency *e* is generally taken to be in the range of 15–30%. It is necessary to balance the feeder efficiency with feeder size to avoid high waste and energy consumption. Secondly, the solidification time of the feeder/s must be greater than that of the casting, typically by a "safety factor" of 1.2, giving:

The well-known Chvorinov's equation allows the above relation to be expressed

where *V* is the casting volume, *A* is the surface area of the casting through which heat is conducted, *n* is an empirical exponent (equal to 2 for simply shaped castings in silica sand moulds), and B is the moulding constant which is dependent on (a) process parameters (the equilibrium solidification temperature *Tm*, the initial temperature *T*<sup>0</sup> of the mould, and the superheat *ΔTs* ¼ *Tpour* � *Tm*, all in °K), (b) mould

� � (2)

*feeder* ¼ 1*:*2*ts*j*casting* (3)

�K�<sup>1</sup>

*cmΔTs L*

), and (c) casting metal properties (latent

, and specific heat capacity *cm* in

� �<sup>2</sup> " # (5)

¼) *m <sup>f</sup>* ¼ 1*:*2*mc* (6)

, density *<sup>ρ</sup>* in kg�m�<sup>3</sup>

(4)

,

• Thirdly, the solid state contraction of the metal. Even though solid-state solidification values may be used in foundries (especially for pattern design), metals are rarely free to contract due to various microstructural constraints – this leads to internal stresses, and where these stresses overcome resistance, the material can undergo plastic or viscoplastic strain to adapt to the thermal and transformational density changes. Hence, the part may be larger than predicted. These constraints can lead to localised cold cracking (initiated from hot tears during solidification contraction).
