Carbon Steel: Steel Weld-ability or P Number

P Number of steel is an indicator of weld-ability. The standard codes of weld-ability is refer to ASME standard that is approached to qualifying of welding procedures and welders/welding operators. For the purpose of specific toughness testing, each set of P number materials is subdivided into groups. The P Number of number materials are divided into groups 1 through 4, which loosely reflect the strength levels of the materials, as follows:

Group 1: materials with a minimum tensile strength requirement less than 485 Mpa.
Group 2: materials with a minimum tensile strength form 485 Mpa to less than 550 Mpa.
Group 3: materials with a minimum tensile strength from 550 Mpa to less than 620 Mpa.
Group 4: materials with tensile strength properties of more than 620 Mpa.

The purpose of P number is to establish qualification of material group based on weld-ability.

Effect of Chemical Composition on Weld-ability

Chemical composition of steel can affected the ability to be welding. The presence of certain chemical in the final micro-structure can effect to the weldability of steel. Weld-ability generally explain by means of a continuous transformation (CCT) diagram, which is formed by two sets of curved CCT diagram. Chemicals contain like carbon, manganese, chromium and molybdenum suppress the start of austenitic stainless steel, decomposition to lower temperature. Too much carbon contain on steel make more difficult to be welded, but contain metal like chromium, manganese or others make the steel easily to be welded. This is normal like the characteristic of metal generally.

The Quality of Welding

The characteristic of every kind of steel can different on welding apply, the increasing of temperature during welding change the crystal form, there is a limit temperature of each kind of steel to change the Chrystal form, Sometime the formed new crystal can have very bad behavior like easily to crack and even easily to corrode because of high temperature is applied during welding. So welding operator should understand the characteristic of each kind of steel and how they treat welding in order don’t break the crystal structure. Time and temperature of welding electrode will determine the quality of welding, more less a crystal structure change during welding, more better the quality of welding. The quality of welding can be seen under X-Ray, if the quality of welding is bad then the x-ray result showing porous picture. Good quality of welding will showing a solid picture of X-ray result.