processes can be used. Cooling slowly in the furnace is called annealing and is not commonly used, except as an intermedi- ate treatment to allow some grades to be machined. Cooling in still air is called normalizing and is the most common treatment, providing good strength and ductility. Rapidly cooling in water or oil is known as quenching. Although steel must be reheated or tempered after quenching to improve ductility, quenching and tempering give the high- est strength available from any grade. A wide range of final strength levels with quenched and tempered grades can be achieved by varying tempering time and temperature.
Strain Strain is the amount of stretching in
a loaded component. Steel can be bent, twisted or stretched without breaking, and this ability to absorb strain without fracturing is critical to safety and reli- ability. Strain is measured by determin- ing the amount of permanent stretch (plastic deformation) in the tensile bar test. The increase in length is elongation, and the change in area at the point of fracture of the bar is reduction of area. The ability to stretch without cracking is called ductility. Although many designers think in
terms of the material’s strength, most steel production is in the lower strength grades, which have good ductility. Carbon content and heat treatment also influence strength and ductility, as shown in Fig. 1. Carbon contents typically are kept well below 0.3% to avoid problems with cracks in heat treating or welding. The ratio of stress to strain is the elastic
modulus, and the data is derived from a tensile test. The modulus of elasticity is based solely on the material; heat treat- ment does not affect modulus. Steels have a modulus of approximately 30 x 106
psi,
which is the highest modulus of elasticity of commonly used cast or wrought mate- rials. The larger the modulus, the smaller the deflection of a part. Therefore, steel provides good stiffness.
Fatigue Fatigue is the failure of a component
when it is repeatedly loaded, even at levels well below the yield strength of the steel. It is measured by repeated loading of several bars at different stress levels and determining the number of cycles until failure. A typical result of the stress versus cycles is shown in Fig. 2. Low-cycle fatigue is below 100,000 loadings where ductility is needed. High-cycle fatigue is normally above
22 Metal Casting Design & PurChasing 2010 Casting sourCe DireCtory
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