Welding Chemistry is a relationship chemically between the base metal, metal fillers, and other chemicals present in welding process. The capabilities of the parent metal and filler metal to fuse without causing an effect bad chemistry is important in conjunction with weld abilities. There are 8 factors need to be considered in welding chemistry.
When welding takes place, the weld metal in a liquid state must be protected from unwanted elements, namely carbon, oxygen, hydrogen and nitrogen. These elements are present in air, flames, and surface contaminants. In SMAW the protection comes from evaporation of the electrode shield (flux), in tungsten arc welding (GTAW), the shield from the gas through the welding handlebar. In SAW welding the shield comes from a powder flux.
Weld metal composition
The composition of the weld metal consists of various sources, namely the base metal, welding metal, flux, and protection. There are welds that do not use filler metal, such as point welding, plate welding, meaning that the weld metal comes from the parent metal itself.
Welding chemistry of the specific parent metal
The composition of the base metal and the weld metal must be the same to get the best welding result. Usually the electrodes are designed to be able to weld all of the parent metals. As the carbon percentage increases the weldability drops. If the carbon content is between 0.15% – 0.30%, it is generally easy to weld. If more than 0.3% becomes difficult to weld, it must be preheated.
Calculation of Carbon Equivalent
The weldability of steel depends on the chemical composition of the parent metal. The formula for determining the weldability of a steel is based on the amount of Carbon Equivalent (Ceq). The carbon equivalent is calculated as follows:
Ceq = % C + 1/6 (% Mn +% Si) + 1/5 (% Cr +% Mo) + 1/15 (% Ni +% Cu).
- If Ceq is up to 0.4, it is recommended that the parent metal be heated at a temperature of 200 – 400 F.
- If the Ceq value is 0.4 – 0.6 preheating at a temperature of 400 – 700 F.
- If Ceq is above 0.4, low hydrogen is recommended, and heating at the end of welding process is recommended.
Welding chemistry of stainless steel
In highly corrosive environments stainless steels corrode at high rates. Stainless steel has a Chromium content of at least 12%. There are four types of stainless steels, namely ferritic, martensitic, austenitic, and precipitation hardening. The three types in advance have a stable phase at room temperature, while the fourth requires precipitation hardening, which is the opposite of quenching and tempering. Ferritic stainless steels are easy to weld with almost any type of electrode, whereas martensitic stainless steels require preheating and finishing.
Welding chemistry of aluminum alloy.
Aluminum alloys have an aluminum oxide layer on their surface which makes aluminum difficult to weld. To be welded, the aluminum oxide layer must be destroyed first. To weld it must be used electric welding with alternating current (AC).
Welding Chemistry of Copper Alloys.
Unlike steel, pure copper and its alloys cannot be hardened by quenching and tempering. The aluminum alloy is cold worked.
Welding Chemistry of Reactive Metals.
There are three groups of reactive metals: titanium, zirconium and tantalum. These three metals are very reactive to other elements, especially if they experience heat in welding. When they react with other elements e.g. oxygen, hydrogen, nitrogen, metals – these metals became too hard and brittle.