Properties of metals.

WELDING FOR BEGINNERS

Applications of welding are very useful to our day to lives in this modern society. Welding is a permanent joint between two weldments (the parts to be joined). It is reliable and one of the oldest methods of joining materials. And also, most of the times it is very practical than mechanical bonding. Welders always work with materials; most of the time they work with metals. Then they need to know the properties of metals. There are three types of properties and they are,

1.       Chemical properties

2.       Physical properties

3.       Mechanical properties

    1) Chemical properties of metals are one of the key factors which are important to know as a welder.     We discuss a few of them on purpose as a beginning and when we go ahead with our lessons, we’ll discuss this topic in depth. Well, let’s begin.

·            Metals have the capability to mix with other metals and non-metals. For example ferrous mix with carbon (as the main composition) and make Mild steel, Hard steel, Cast iron, High carbon steel, Wrought iron. They getting different from each other according to the carbon percentage of the composition.

·            Metals have the capability to resist corrosion. Maybe you already know that the Stainless steel and White steel resistance to corrosion. And also, there are alloys like Monel steel.

·            Metals have the capability to resist to eco-climate change.

·            Metals have the capability to react with chemicals or resist chemical reactions.

·            During the welding process steel occurs oxidation. Then the weld seam should protect from the atmospheric contamination. Before start the welding, the weldment is better to grind and it removes the oxides or the passive protection against the corrosion.

·            Atomic weight

·            Atomic number:- It’s the number of protons in the nucleus of an atom, which is characteristic of chemical elements and determines its place in the periodic table.

·            Molecular weight:- the mass of a given molecular.

·            Valency:- The measure of its combining power with other atoms when it from chemical compounds.

·            Acidity:- The measure of the amount of free hydrogen ion in a solution.

·            Alkalinity:- The chemical measurement of ability to neutralize acids.

·            Chemical composition:- The arrangement, ratio, and type of atoms in molecules of chemical substances.  

     2)    Physical properties of metals can describe the physical conduct of metals. It helps to control and protect the useful properties of a metal. There are types of physical properties. The first one is the thermal properties. The second one is electrical and magnetic properties and the third one is the visible properties.

                                           I.   Thermal properties,

·         Thermal conductivity:- Rate of heat transfer through a unit the thickness of the material per unit area per temperature difference.

·         Thermal convection:- Heat transfer by mass motion of a fluid such as air or water when the heated fluid is caused to move away from the source of heat-carrying energy with it.

·         Melting point:- The temperature at which a given metal getting into molten metal.

·         Thermal radiation:- Electro-magnetic radiation emitted from a material that is due to the heat of the material, the characteristic of which depends on its temperature.

·         Coefficient of linear expansion:- The change in length of a specimen one unit ling when its temperature is changed by one degree Celsius.

·         Specific heat capacity:- The heat required to raise the temperature of the unit mass of a given substance by a given amount (usually one degree).

                                        II.      Electrical and magnetic properties,

·         Electrical conductivity:- Electrical conductivity is the result of the movement of electrically charged particles. The atoms of metal elements are characterized by the presence of valence electrons, which are electrons in the outer shell of an atom that are free to move about. It is these “free electrons” that allow metals to conduct an electric current. Because valence electrons are free to move, they can travel through the lattice that forms the physical structure of a metal.

·         Magnetization:- The density of magnetic dipole moment that is induced in a magnetic material when it is placed near a magnet (Also known as magnetic polarization). And there are two types of magnets, the first type is short-lived magnets. They are made of Silicon alloys, low carbon steel, Ferrous + Nickel alloys, or high carbon steel. And the second type is permanent magnets. Those are made of Cobalt alloys.

                                     III.                        Visible properties,

·         When talking about visible properties it could be the surface or cross-section of the material. There are five types of visible properties.

                                                                                                                                      i.      Color

                                                                                                                                   ii.      Specific gravity

                                                                                                                                 iii.      Density

                                                                                                                                  iv.      Porosity

                                                                                                                                    v.      Structure

·         Color:- Most metals have a characteristic color. For example, Copper has a reddish-brown color, Aluminium has a silvery color, Bronze has a metallic brown color and brass has a yellow color.

·         Specific gravity:- Every material has a unique specific gravity. Specific gravity means the ratio between the density of an object, and a reference substance (usually water).

·         Density:- It is the mass of a unit volume of a material substance.

·         Porosity:- Open spaces that contain in a material.

·         Structure:- The arrangement of and relations between the parts or elements of something complex.

3)     The third property of the metal is the mechanical properties. After studying this section we can understand how it can be used for our work and what material is suitable for our necessity.

·         Strength properties,

v  Tensile strength:- The resistance of a material to break under tension.

v  Compressive strength:- The resistance of a material to break under compression.

v  Shear strength:- The strength of a material or component against the type of yield or structural failure where the material or component fails in shear.

·         Ductility:- Measurement of a material’s ability to undergo significant plastic deformation before rupture or breaking, which may be expressed as percent elongation or percent area reduction from a tensile test.

·         Elasticity:- The property where a slid material changes it is shape and size under the action of opposing forces but recovers its original configuration when the forces are removed.

·         Malleability:- The property of a metal that permits mechanical deformation by extrusion, forging, rolling without fracturing. ( And there are a lot of mechanical deformation methods )

·         Plasticity:- The quality of being easily shaped or molded.

·         Rigidity:- The property of a material which resists a change in its physical shape

·         Toughness:- The ability of a structural material to resist shock or impact or ability to absorb energy before fracture.

·         Hardness:- Resistance of a material to deformation, indentation, or penetration by means such as impact, scratch, drill, abrasion measured by hardness tests such as Hard Rockwell, Brinell, Vickers.

·         Brittleness:- That unique of material that is manifested by sudden r abrupt failure without appreciable prior ductile or plastic deformation.      

·         Resilience:- The ability of a material that has been subjected to an external force to recover its size or shape, following deformation.

·         Machinability:- this is a unique property of a material, such as a metal, that makes it easy to shape, cut, grind or drill, and other machinability work. Good machinability relatively saves power and cost.

·         Weldability:- According to AWS (American Welding Society), Weldability is the capacity of a metal to be welded under the fabrication conditions imposed into a specific, suitably designed structure and to perform satisfactorily in the intended services.

·         Creep:- When a material is subjected to a constant load (below its yield point), at high temperature, for a prolonged period, it undergoes permanent deformation. 

That is enough for now. Let’s meet with another lesson.