METALLURGY

DIPLOMA IN FOUNDRY TECHNOLOGY METALLURGY

SESSION 1 :  METAL MELTING

1.1   Introduction

“Metal melting is the process of producing a liquid metal of the required composition at the required rate, and with the required amount of superheat while incurring the minimum cost”.

This is often a difficult operation in which four important factors must be considered:

1.  The compositional range of material to be melted
2.  The fuel or energy used to melt the charge
3.  The degree of refining and control over the process
4.  The type and size of melting unit

All four factors are interconnected and the metal melting operation must be considered as a “total system” rather than viewing each in isolation. For example, there is no point in producing 30 tonnes of liquid metal per day with high compositional control using a virgin metal charge when the specification calls for 5 tonnes of metal per day which could be adequately produced from a scrap charge with limited compositional control. At the same time a melting system must be chosen that allows adequate flexibility for producing the range of composition and quantities required.

The form of the “charge material” has an important influence on the overall efficiency of the operation. This can best be explained by examining the relationship between the surface area and density of the metal charged.

Dense material can be charged into the melt unit at a rapid rate because it has a small surface area; this gives low oxidation losses but also a low melting rate. A compromise must be reached between these points but in general a charge of low surface area and relatively high density, is preferred unless the charge is immediately immersed under a liquid slag or flux.

To this end, baling of light scrap such as off-cut, sheet and swarf is often carried out while virgin metal may be supplied as small ingots or alloying compounds such as briquettes.
Blending of the charge materials is carried out to obtain a metal analysis on melt-out as close as possible to that desired. This is often more important in melting non-ferrous than ferrous alloys.

The solubility of gases in liquid metal is far greater than in the solid state; a fact which can lead to “entrapment” of the gas in the metal with a subsequent reduction in mechanical properties.

The temperature to which the liquid metal is heated above its melting point, the degree of superheat, is important in providing the desired fluidity so that it will fill the mold on pouring. Too low a temperature will lead to surface defects e.g. mis-runs, laps and seams, while too high a temperature will lead to a rough surface finish due to the liquid metal attacking the sand mold.

1.2     Theoretical Aspects of Melting Processes
1.3     Gases in Metals
1.4     Slag and Fluxes
1.5     Melting Units
1.6     Quality Control
1.7     Solidification
1.8     Microstructures

SESSION 2 :  GREY CAST IRON

2.1   Introduction

Iron-carbon alloys with more than about 2 percent of carbon are usually classed as cast iron. Cast iron differs from steel in the form in which the carbon exists. In steel the carbon usually exists as iron-carbide whereas in cast iron carbon can exist as both iron-carbide and free carbon, or graphite.

Apart from carbon, cast iron will also contain other important elements such as silicon, manganese, sulphur and phosphorus which can modify the structure and properties of the iron in a marked manner. Low levels of other elements such as nickel, chromium, molybdenum, copper, or tin may also be added to help develop special properties.

There are many elements which can be present at trace level, either by design of fortuitously, which can have significant effects on structure and properties. Gaseous elements such as nitrogen and hydrogen, can also be present at levels which can affect casting properties and quality.

The simplest method of classifying cast irons is by their structures, as seen under a microscope and, if this is done, it is found that there are three general types of structure:

1.   Those containing flake graphite (grey irons) as shown in Figure 2-1
2.   Those containing no graphite (white irons) Figure 2-2
3.   Those containing nodular or aggregate forms of graphite (ductile (SG) or malleable
(Iron)

In the case of grey cast iron, the graphite flakes will be embedded in a metallic matrix which may comprise of any combination of the five metallurgical constituents: pearlite, ferrite, austenite, iron carbide (cementite) or phosphide-eutectic. Since these micro-constituents have different properties, it will be apparent that if the form and relative amounts of these in the matrix structure can be controlled as desired and likewise the amount and distribution of the graphite can also be controlled, it should be possible to produce a series of cast irons having a wide range of properties.

2.2     Solidification
2.3     Microstructural Constituents
2.4     Inclusions
2.5     Mechanical Property Considerations
2.6     Grey Iron Properties
2.7     The Importance of Cooling Rate and Section Sensitivity
2.8     Section Thickness – Composition and Structure
2.9     Hardness and its Relationship with Tensile Strength
2.10   Alloying & Trace Elements
2.11   Melting – Metallurgy

SESSION 3 :  DUCTILE METALLURGY

3.1   Introduction

Ductile cast irons, sometimes referred to as “nodular” or “spheroidal-graphite” cast iron, constitute a family of cast irons in which the graphite is present in a nodular or spheroidal form as shown in Figure 3-1. The graphite nodules are small and constitute only small areas of weakness in a steel-like matrix. Because of this the mechanical properties of ductile irons relate directly to the strength and ductility of the matrix present – as is the case for steels.

Figure 3 1: Ferritic Ductile Iron (x100)

The graphite occupies about 10 percent to 15 percent of the total material volume and because graphite has negligible tensile strength, the main effect of its presence is to reduce the effective cross-sectional area. This means that ductile-iron has tensile strength, modulus of elasticity and impact strength proportionally lower than that of a carbon-steel of otherwise similar matrix structures.

The matrix of ductile irons can be varied from a soft and ductile ferritic structure, through harder and higher-strength pearlitic structures to a hard, higher-strength and comparatively tough tempered martensitic or bainitic structure. Thus, a wide range of combinations of strength and ductility can be achieved. General engineering grades of ductile-iron commonly have structures which are ferritic/pearlitic or pearlitic.

Ferrite comprises essentially carbon-free iron. The structure of a ferritic-ductile-iron is shown in Figure 3-1. Ferrite is essentially a single-phase solid solution and in cast iron it contains nearly all the silicon. It should be correctly regarded as a silico-ferrite. It is a relatively soft constituent (130HB to 170HB) and is therefore undesirable when high strength and good wear-resistance is required. Ferritic structures are advantageous when ductility and toughness are the important properties.

Pearlite is a constituent produced during transformation of austenite in the solid-state. It occurs during cooling in the temperature range 650°C to 800°C. It consists of alternative lamellar of soft ferrite and hard cementite or iron-carbide. Its structure is shown in Figure 3-2.

Figure 3 2: Lamellar of Soft Ferrite

A pearlitic ductile-iron is shown in Figure 3-3. Pearlite is a desirable constituent when high strength and good wear-resistance are required. Hardness increases with increasing fineness of the laminations in the pearlite.

Typical properties of ductile irons having different types of matrix structures are given in Table 3-1. Reference micro-structure is available which enable the amount of carbide and proportions of ferrite and pearlite to be assessed.

Figure 3 3: Pearlitic Ductile Iron

3.2     Unsatisfactory Graphite Structures
3.3     Control of Matrix Structure
3.4     Carbide in the Structure
3.5     Ductile-Iron Properties
3.6     Effect of Trace Elements
3.7     Effect of the Level of Nucleation
3.8     Suggested Compositions
3.9     Production of Low-Temperature Impact Grades
3.10   Ductile Iron Processes
3.11    Heat-Treat or As-Cast
3.12   Furnace Plant
3.13   Surface Hardening
3.14   Process Control

SESSION 4 :  NON-FERROUS METALLURGY

4.1   Introduction

Biblical reference to the art of the foundrymen is made in First Kings Chapter 7: 15-4 1 where it is recorded that:

4.2   Aluminium Castings Alloys
4.3   Copper-Based Alloys
4.4   Zinc-Based Alloys
4.5   Magnesium-Based Alloys

SESSION 5 :  REFRACTORIES

5.1   Introduction

At some moment in pre-history man baked clay to form a simple, durable, decorative shape. At that moment the interest in refractories began.

From that time and at least as long as metals have been smelted and melted there has been a requirement for suitable materials in which to contain the processes. Initially the simple, natural minerals were suitable, providing the refractory molds for bronze axe-head castings some 50 centuries ago. The clay investment molds developed during early Chinese dynasties were largely still adequate for the bell founders and cannon makers’ art during the Middle Ages.

The industrial revolution in the Western world greatly widened the field of refractories application, introducing new and more intensive hot-working practices additional to the ancient founders and glass makers, incorporating re-heating for working, heat treatment, brick and tile manufacture, ceramic pipes and similar domestic items both utilitarian and decorative.

Then, just over one hundred years ago, Bessemer’s invention for manufacturing cheap steel galvanized hot-metal practices sprang into renewed activity and changed all this. Shortly after this Thomas’s introduction of dolomite to cope with the basic slag of the phosphoric process greatly extended the horizon of knowledge in the field of refractories engineering. This practice has continued to be extended up to the present day where refractory materials play a vital role in the manufacturing industry.

5.2     Test of Refractory Materials
5.3     Classification of Refractory Materials
5.4     Manufacture of Refractories
5.5     Basic Refractories
5.6     Insulating Refractories
5.7     Castables, Mortars and Ramming Mixes
5.8     Refractory Engineering

SESSION 6 :  MELTING AND POURING PRACTICE

The inoculation of cast iron involves the addition of small amounts of a material “inoculant” to molten metal either just before or during pouring. Inoculation increases the number of points available for the precipitation and subsequent growth of graphite. This effect is often described as “increasing the nucleation of the iron”.

The importance of nucleation is shown in Figure 6-1. High nucleation promotes graphitic structures whilst low nucleation can result in the formation of either mottled or white irons and thus “chill” in castings. The need for high nucleation increases as the cooling rate increases and as section size decreases.

Figure 6 1: The Importance of Increasing the Nucleation of the Iron

There are two main methods of inoculation – ladle inoculation and late inoculation:

• Ladle inoculation involves addition of inoculating materials to the metal as it fills the ladle or to the metal in the ladle shortly before pouring
• Late inoculation refers to the addition of an inoculant to the metal stream as it enters the mold, or within the mold itself

The most important point about inoculation is that only small additions of inoculants are made. This is because the effects of inoculants are dependent upon nucleation. The effects do not arise from the change in metal composition caused by the addition of the inoculant.

6.1     Introduction
6.2     Effects of Inoculation
6.3     Inoculating Materials
6.4     Choice of Inoculant
6.5     Fading of Inoculation
6.6     Ladle inoculation Practices
6.7     Amount of Inoculant to Add
6.8     Late Inoculation Techniques
6.9     Problems Associated with Inoculation
6.10   Fundamentals of Running, Gating and Feeding
6.11   Properties of Liquid Metals
6.12   Principles of Flow
6.13   Mold Filling Calculations

SESSION 7 :  METAL CONTROL AND TESTING

7.1   Introduction

Table 7-1, suggests various ways that quality control may be used in iron foundries, especially in the control of raw materials for production.

Table 7 1(a): Control of Raw Materials from Production

7.2     Effect of Some Residual Trace Elements in Cast Iron
7.3     Charge Materials for Electric Induction Furnaces
7.4     Carbon Content of Cast Iron by Thermal Analysis
7.5     Determining Silicon Content of Cast iron by Thermal Analysis
7.6     Sampling for Spectrographic Analysis
7.7     Molten-Metal Temperature Measurement
7.8     Maintaining Quality Control in Iron-Foundries