When first humans recognized the metals he also learned how to melt them. He tried to connect them together and learn the simple way of soldering and welding.Bracelets of gold have been found in archeological excavations that date to the period long before and their heads were welded by hammers. In old jewelry, gold particles bonded together by tree's gum and a copper salt and the mixture was burned. Because of burning of gum, copper was restored and small pieces of gold was welded . The ancient Romans used alloys for soldering that are still used in industry.Welding developed in the nineteenth century. In 1887 AD, Russian Brnadvs used coal electrode and arc welding Today, more than a hundreds of welding and cutting and soldering methods are invented and open scientific fields with numerous branches in: welding processes, design, inspection, metallurgy and .... And so many people are working in this industry.

Types of connections
In industry,each Steel Structure is made by casting parts, and machined worked parts and these components are connected in several ways, including: bolts, rivets, thorns, pins, soldering, welding.
These methods can be categorized as follows:
A - Temporary connection: screws, pins, thorns
B - semi-temporary connections: riveted, soldered
C - Constant Connection: Welding

Temporary connection: by separating the binding agent (screws, pins, thorns) base metal is not damaged.
Semi-temporary connection: by separate the binding agent (riveted, soldered) base metal is not damaged but we lost the binding agent.
Permanent link: by removing the binding agent, the binding agent and base metal both are damaged.

The advantage of a permanent connection:
1. Its strength is higher.
2. There is the possibility of sealing.
3. Is faster.
4. Require less preparation.
5. Over the time, the connection will not be loose.
Welding:
Definition: the act of bonding between atoms of two objects is called welding. This bond could be between metallic or non-metallic materials.
Welding can be done with or without heat, with or without pressure, with or without supporting material.Welding could be divided into two general categories from the point of melting:
1.Non-fusion Welding
2. Fusion welding