CLASSIFICATION OF FERROUS MATERIALS BASIC INFORMATION AND TUTORIALS

Iron and steel may be classified on the basis of composition, use, shape, method of manufacture, etc. Some of the more important ferrous alloys are described in the sections below.

Ingot iron is commercially pure iron and contains a maximum of 0.15% total impurities. It is very soft and ductile and can undergo severe cold-forming operations. It has a wide variety of applications based on its formability.

Its purity results in good corrosion resistance and electrical properties, and many applications are based on these features. The average tensile properties of Armco ingot iron plates are tensile strength 320 MPa (46,000 lb/in2); yield point 220 MPa (32,000 lb/in2); elongation in 8 in, 30%; Young’s modulus 200 GPa (29 # 106 lb/in2).

Plain carbon steels are alloys of iron and carbon containing small amounts of manganese (up to 1.65%) and silicon (up to 0.50%) in addition to impurities of phosphorus and sulfur. Additions up to 0.30% copper may be made in order to improve corrosion resistance.

The carbon content may range from 0.05% to 2%, although few alloys contain more than 1.0%, and the great bulk of steel tonnage contains from 0.08% to 0.20% and is used for structural applications.

Medium-carbon steels contain around 0.40% carbon and are used for constructional purposes—tools, machine parts, etc. High-carbon steels have 0.75% carbon or more and may be used for wear and abrasion-resistance applications such as tools, dies, and rails.

Strength and hardness increase in proportion to the carbon content while ductility decreases. Phosphorus has a significant hardening effect in low-carbon steels, while the other components have relatively minor effects within the limits they are found.

It is difficult to generalize the properties of steels, however, since they can be greatly modified by cold working or heat treatment.

High-strength low-alloy steels are low-carbon steels (0.10% to 0.15%) to which alloying elements such as phosphorus, nickel, chromium, vanadium, and niobium have been added to obtain higher strength.

This class of steel was developed primarily by the transportation industry to decrease vehicle weight, but the steels are widely used. Since thinner sections are used, corrosion resistance is more important, and copper is added for this purpose.

ELASTIC STRENGTH OF STRUCTURAL MATERIALS BASIC INFORMATION

What is Elastic Strength?

To the user and the designer of machines or structures, one significant value to be determined is a limiting stress below which the permanent distortion of the material is so small that the structural damage is negligible and above which it is not negligible. The amount of plastic distortion which may be regarded as negligible varies widely for different materials and for different structural or machine parts.

In connection with this limiting stress for elastic action, a number of technical terms are in use; some of them are

1. Elastic Limit. The greatest stress which a material is capable of withstanding without a permanent deformation remaining on release of stress. Determination of the elastic limit involves repeated application and release of a series of increasing loads until a set is observed upon release of load.

Since the elastic limit of many materials is fairly close to the proportional limit, the latter is sometimes accepted as equivalent to the elastic limit for certain materials. There is, however, no fundamental relation between elastic limit and proportional limit. Obviously, the value of the elastic limit determined will be affected by the sensitivity of apparatus used.

2. Proportional Limit. The greatest stress which a material is capable of withstanding without a deviation from proportionality of stress to strain. The statement that the stresses are proportional to strains below the proportional limit is known as Hooke’s Law. The numerical values of the proportional limit are influenced by methods and instruments used in testing and the scales used for plotting diagrams.

3. Yield Point. The lowest stress at which marked increase in strain of the material occurs without increase in load. If the stress-strain curve shows no abrupt or sudden yielding of this nature, then there is no yield point. Iron and low-carbon steels have yield points, but most metals do not, including iron and low-carbon steels immediately after they have been plastically deformed at ordinary temperatures.

4. Yield Strength. The stress at which a material exhibits a specified limiting permanent set. Its determination involves the selection of an amount of permanent set that is considered the maximum amount of plastic yielding which the material can exhibit, in the particular service condition for which the material is intended, without appreciable structural damage.

A set of 0.2% has been used for several ductile metals, and values of yield strength for various metals are for 0.2% set unless otherwise stated. The yield strength is generally used to determine the elastic strength for materials whose stress-strain curve in the region pr is a smooth curve of gradual curvature.