A little more about Steel...

The material Steel has been used throughout this blog. I think it's about time I clarified what Steel exactly is!

Steel is not just a natural material. It is in fact, an alloy. The term Alloy, is used to describe a material that has been made up of various other materials. Steel consists mainly of Iron. Iron is not that strong as a single substance, by adding other metals to this iron, we produce steel. This steel can be up to a thousand times strong than the original steel!

Above, a photo of pure Iron.



I mentioned the Iron is bonded with other materials, Carbon essentially. Steel typically will have a carbon content ranging from 0.2% to 2.1% pending on weight and quality. Carbon acts and a hardening element, this prevents dislocations in the atomic structure.

Other materials added into the Steel alloy, usually contain the following, Manganese, Chromium, Vanadium and Tungsten. Each are usued to offer different properties to the steel. These can vary from

• Hardness - Ability to stop deforming when a force is applied.

• Ductility - Ability to deform elastically without fracture

• Tensile strength - Amount of stress the material will take without rupture.

With that in mind, different percentages of each material, is used to create the ideal steel for the job.



















A sword, for example, must be lightweight, have a high tensile strength, but, ideally, not bend! (Where as a spring, should be very ductile!)

Stress and Strain Behaviours

My colleague has published a post making a general comparison of Steel, against, Aluminium. I will be looking to develop on this matter furthermore, so, as a group, we can be sure we're making the right choice.
As mentioned, mechanical properties vary from low strength alloys, to medium and high strength alloys. Regardless, there is no such aluminium alloy that compares to the strength of steel. But, as mentioned beforehand, strength is not alloys a deciding factor when selecting materials.

Below, there is a graph showing the stress strain curve (for, temperatures from -30c to 80c) you can see how each alloy, behaves very, very differently. Because of this, we shall have to select materials carefully.

Heat, does have a big factor also. Heat can have an impact from the weilding taking place in the manufacturing process, but also from the outdoor temperature. Aluminium's suffer more damage than Steel's, especially heat treated aluminium alloys. This results in lower actual strengths than metals in a stable environment.
(NB. If using mechanical fasteners, instead of weildings, there will be no heat damage)

As you can see from the above diagram, an important factor, steel is linearly elastic, up the 0.2% limit, however, aluminium, the proportional limit fp, is lower then fo.

Corrosion

Corrosion Theory
Humans have most likely been trying to understand and control corrosion for as long as they have been using metal objects. The most important periods of prerecorded history are named for the metals that were used for tools and weapons (Iron Age, Bronze Age). With a few exceptions, metals are unstable in ordinary aqueous environments. Metals are usually extracted from ores through the application of a considerable amount of energy.
Certain environments offer opportunities for these metals to combine chemically with elements to form compounds and return to their lower energy levels. A modern and comprehensive document on the subject is the second edition of the classic CORROSION BASICS textbook. Some excerpts of that document are used here.
Corrosion is the primary means by which metals deteriorate. Most metals corrode on contact with water (and moisture in the air), acids, bases, salts, oils, aggressive metal polishes, and other solid and liquid chemicals. Metals will also corrode when exposed to gaseous materials like acid vapors, formaldehyde gas, ammonia gas, and sulfur containing gases. Corrosion specifically refers to any process involving the deterioration or degradation of metal components. The best known case is that of the rusting of steel. Corrosion processes are usually electrochemical in nature, having the essential features of a battery.
When metal atoms are exposed to an environment containing water molecules they can give up electrons, becoming themselves positively charged ions, provided an electrical circuit can be completed. This effect can be concentrated locally to form a pit or, sometimes a crack, or it can extend across a wide area to produce general wastage. Localized corrosion that leads to pitting may provide sites for fatigue initiation and, additionally, corrosive agents like seawater may lead to greatly enhanced growth of the fatigue crack. Pitting corrosion also occurs much faster in areas where microstructural changes have occurred due to welding operations.
The corrosion process (anodic reaction) of the metal dissolving as ions generates some electrons, as shown in the simple model on the left, that are consumed by a secondary process (cathodic reaction). These two processes have to balance their charges. The sites hosting these two processes can be located close to each other on the metal's surface, or far apart depending on the circumstances. This simple observation has a major impact in many aspects of corrosion prevention and control, for designing new corrosion monitoring techniques to avoiding the most insidious or localized forms of corrosion.

Steel Rope.

As there is a winch system within our crane, we shall have to use a strong type of steel rope, to hoist our load. We have previously stated we shall be using stainless steel. As it is a 'rope' we cannot just buy steel in a rope form. Instead, to achieve these desired results, a special blend of steel is produced, giving it strength, and ductility. When this is made, several of these rolls, or strands, are woven together, around a typically steel core, to produce one large 'rope'. By doing this, it enables the user to have extra support.

Ropes are favoured now due to chains. This is due to the fact they have a higher reliabililty compared to metal chains. Flaws in chains can lead to disaster, i.e. a dropping of the load. With wires however, the flaws do not make much of an impact.
The flexibililty of the rope is critical in our crane. The rope can then be wound around a cylindrical barrel, to lift a load, and unwound to put down a load.






















Above, we can see the wires, which are bonded together to form a strand, which is wound together with other strands to form the rope.