Metallurgy and Technology of Steel Castings

The history of steel castings is associated with previously developed technologies for obtaining castings of bronze, copper and cast iron. These formed the basis for the improvement and development of casting technology of metal with a higher melting temperature and higher mechanical properties, and then - better utility properties (corrosion resistance, impact toughness at low temperatures and finally - smelting in a vacuum). The development of steel smelting technology and methods of obtaining steel castings have been presented against the background of extremely extensive literature on bronze casting in ancient times.

The primary technology of steel making in the foundry is a basic step to obtain a good quality of molten steel: with proper deoxidation, with low gases quantity and the control of non-metallic inclusions. These processes are discussed in detail through oxidation of elements and then during deoxidation in reduction period and in the ladle.

Slags play an important role in melting processes (dephosphorization, desulfurization and deoxidation) and during tapping of steel in the ladle. During melting of steel in basic EAF, the slags change the chemical composition in each melt. These aspects govern the quality of molten steel for castings and in consequence – the properties of each grade of cast steel. The structure of multicomponent systems, the structure of slags according to molecular or ionic theories, composition of slags during oxidation and reduction periods of melting and calculation of their basicity are shown.

Gases constitute a number of threats to the good quality of steel castings. Therefore, changes in the solubility of oxygen, nitrogen and hydrogen during solidification and defects, caused by rapid reduction of solubility in the liquidus temperature: microshrinkage, microporosity and non-metallic inclusions, have been presented. The types of inclusions and their dependences on the chemical composition of cast steel have been defined. The examples of adverse effect of inclusions on mechanical properties of cast steel have been given. To reduce their impact on the quality of castings, examples of modifications of inclusions and elements leading to changes in the morphology of inclusions, have been identified.

Deoxidation of steel intended for castings is particularly important. Because rimmed steel, or even half-killed steel, cannot be used for castings, the oxygen content must be as low as possible. Low oxygen content is also associated with non-metallic inclusions nascent during solidification. Their morphology (number, location and size) decides on the mechanical properties in service conditions (e.g. sub-zero temperatures and in heat resistance conditions). Elimination of oxygen from steel must be combined with the removal of inclusions and their impact on the quality of cast steel.

Improvement in the quality of cast steel is based on the application of secondary metallurgy methods. The differences in the techniques used partly result from the capacities of melting furnaces and ladles. In small casting ladles, the temperature of the molten steel falls much faster, which requires attention to timescales with respect to argon blowing or standing in vacuum, or else additional heating of the metal in the ladle. For these reasons, typical techniques for foundries have been presented, such as argon stirring, AOD, VOD, VD, LF and DETEM. For each technique, the benefits and effects of its use in practice have been given.

Low-alloy cast steels are used in many branches of industry for various mechanical parts with very different wall thicknesses (40-400mm). The mechanical properties in such sections must be equal throughout. For these reasons, there are several grades of cast steels, which fulfil the requirements for different uses. This chapter characterises the parameters of low-alloy cast steels, intended for several service conditions.

In engineering structure castings play important role, spatially in minus temperature. These conditions arise with additional stresses cracks and defects in steel castings. Since different properties of materials show different crack propagation, it is necessary to find the limiting parameters, which ensure the proper work of machines. For this reason, this chapter describes the fracture toughness, transition temperature, brittle and ductile fractures, chemical composition of cast steels and effect of heat treatment.

Cast steels are chemically unstable in air-saturated water and other solutions at ambient temperatures. Their commercial applications therefore depend on protective mechanisms: passivation and formation of thin oxide films, prevention of passivity breakdown, cathodic protection, coatings, control of chemical composition and microstructure, along with their service conditions (stress, fatigue, voids and erosioncorrosion). The corrosion of steel castings is usually considered as a problem involving chemical (carbon content, alloying), microstructure (austenite, ferrite-austenite) and quality (non-metallic inclusions, gases, segregation, grain size) parameters. In this chapter, some topics concerning the manufacturing of corrosion-resistant steel castings are considered.

The high temperature oxidation of cast alloys represents an important problem in several industries: petrochemicals, chemical processing, the arms industry, gas turbines, aircraft engines and cement mill equipment. Many alloyed cast steels are used in these areas. The present chapter describes the alloys’ resistance to corrosion at elevated temperatures, the stability of their microstructures and three group of cast steels: Fe-Cr; Fe-Cr-Ni and Fe-Ni-Cr.

Steel castings are chosen for several wear conditions as elements of machines working in mining, transport and military industries. They work mainly in contact with ceramic materials (quartz sands, different ores, water-solid particles). In general, castings work with abrasion condition with high stresses. This chapter shows the types of wear, effect of hardness, load, and temperatures on weight losses. In the present work, the microstructure of used grades, their heat treatment and mechanical properties are shown.

The quality of steel castings depends upon how the liquid metal enters into the mould and solidifies. The technology of pouring molten steel to the ladle and then to the mould must be controlled if no defect is to be obtained. These factors must be accounted for in designing the gating system, the feeding and risering of castings. Gating system ought to control a directional solidification of each part of the casting in such a way, that no wall thickness is isolated from the feeding during the freezing time. Position and size of the riser ensure elimination of shrinkage which takes place when a molten steel changes from the liquid to solid state.

This chapter highlights the fact that every steel casting should be heat treated. The structure of cast steel in as-cast conditions requires changes – this applies to virtually all types of cast steel grades. As a result of heating and cooling (according to various technological options), beneficial changes in mechanical and also other properties (e.g. hardness, corrosion resistance and wear resistance) are obtained. This chapter includes the types of heat treatment of cast steel, possible practical effects after each treatment and the structural changes caused during its course. There are examples of the microstructure in as-cast conditions, after quenching and after tempering at various temperatures.