The principal methods of hot working are extrusion, drawing, forging and rolling. Rolling is the most extensive employed forming process, though some limitations may apply to the process. Rolling mainly consists of three major sub-categories.. flat rolling, shape rolling (with specifically designed roll grooves) and pipe rolling (including piercing). Forging may be sub-categorized as hamming, pressing, etc.
Forging may be performed under hammers, in mechanical presses and upsetters or by a method known as roll forging. Pressing generally includes the manufacture of forged articles in hydraulic presses. Extrusion usually is performed in hydraulic presses which force the hot steel through a die. Rolling is performed in rolling mills of a variety of types.
The two principal reasons for perform metal forming at elevated temperatures (hot working) are to reduce the forming loads through the reduction of the resistance of the steel to deformation, and to develop preferred metallurgical structures for strength and ductility of the finish products.
The most appropriate manufacturing method of a product will be decided with consideration of its material, sizes, shape, use, standards and other properties.
There are numerous processes for manufacturing butt weld fittings, several examples listed as follows.
One of the most common manufacturing methods for manufacturing Elbows from pipes. After heating the raw material, it is pushed over a die called "mandrel" which allows the pipe to expand and bend simultaneously. Applicable to a wide size range.
Elbows of steel pipe joints are used in the industrial plants and are mainly manufactured by the hot mandrel bending from raw material of straight steel pipe. Elbows are generally manufactured at elevated temperature by means of pushing, expanding and bending of pipes simultaneously, using the inner tool of mandrel. Characteristics of mandrel bending strongly depend on the integrated shape and dimensions of the mandrel.
Elbows manufactured by using hot mandrel bending have advantages of small thickness deviation and shorter bending radius than those of any other bending method type.
All the steps mentioned above give a brief explanation (the basic principle) about Hot forming, extrusion method. Many more steps are needed, but hopefully the pictures give a good description.
Benkan Japan KK
G. Dieter.. Mechanical Metallurgy, 2nd Ed.
Making, Shaping and Treating of Steel.
Kurt Lange.. Hand Book of Metal Forming. SME.