What is Weldability of Metals? (And what does it depend on?)

Weldability of metals

What is Weldability?
Definition: Weldability of metals is defined as the ability of metals to be welded into permanent strong joints.

In simple words, weldability of metals refers to ease of welding through which two metals can be joined with each other.

Weldability depends on …?
Weldability of metals depends on 4 main parameters.

  • Melting temperature of metal
  • Thermal conductivity of metal
  • Thermal expansion of metal
  • Surface condition of workpiece

The weld quality is highly dependent on the above parameters.

But apart from these factors, the weld quality also depends on the possibility of weld defects, partial loss of constituent elements from the metal due to vaporization, changes in mechanical properties due to extreme heat, etc.

Effect of alloying elements on weldability

The effect of alloying elements on weldability of steel is listed below.

  1. Carbon: The carbon content in the base metal strongly affects the weld quality (or weldability). Especially if the carbon content is more than 0.3 %.
    For such steels, the thermal cycles also play an important role.
    If the metal pieces are welded and then cooled suddenly, then it may result in a brittle zone along the weld.
    In gas welding, the effect of high carbon proportion is seen more as compared to that in arc welding.
  2. Manganese: When the proportion of manganese exceeds 0.6%, it lowers the weldability of the steel.
    In such steel, if the proportion of carbon is also higher, then the weld will result in weld cracking.
    Also if the percentage of manganese is very less, it may result in internal weld porosity.
  3. Silicon: High percentage of silicon along with high carbon content results in excessive cracking of the weld.
  4. Phosphorus: If the proportion of phosphorus is more than 0.04%, then it develops a brittle weld and it results in some unfavorable mechanical properties of the weld.
  5. Sulfur: Generally sulfur is added to steel for providing good machinability to the metal. But high amounts of sulfur results in adverse effects on weldability of metal.
    If sulfur content is more than 0.04%, then it results in cracking.
  6. Nickel, Chromium and Vanadium elements result in hard brittle zones. In order to prevent brittle weld zones, the heat treatment process is required for the steel.

Weldability of different metals

The weldability of different metals are mentioned below.

  • Low carbon steels or mild steel: Because of less proportion of carbon in this type of steel, it can be welded easily. Also there is less tendency to harden except those steels which have upper limit of carbon content.
  • Medium carbon steel: For welding of medium carbon steel, little care is necessary. Hardening effect of higher carbon content can be prevented by preheating the weld area at about 250 °C, and the weld should be allowed to cool slowly.
  • High carbon steel: It is possible to weld high carbon steel, but the preheating of weld area as well as the post weld treatment are necessary. Generally a brazing operation is preferred for high carbon steel, because the brazing temperature does not show more effect on the metallurgical condition of high carbon steel.
  • Low alloy steels: If the carbon content in low carbon steel is below 0.18%, then they have good weldability. But if the carbon content is higher than this percentage, then preheating is necessary along with slow cooling of the final weld.
  • Cast iron: It is difficult to weld cast iron as it contains free graphite and high carbon content. Also due to low ductility of the cast iron, it may develop cracks in the weld. 
    Due to local heating of cast iron, the uneven expansion and contraction takes place. So cast iron can be either hot welded or cold welded.
    • Hot welding: In hot welding, the workpiece is preheated to the temperature of 600 °C to 650 °C and this temperature range is maintained throughout the welding process. 
      A considerable amount of molten metal is formed at the weld to prevent the chilling of metal.
      Hot welding of cast iron can be done with either gas welding flame or electric arc.
    • Cold welding: Cold welding using electric arc is the most used welding method for cast iron as it does not require preheating. This is because the heat is highly concentrated and localized.
  • Stainless steel: In order to weld the stainless steel, proper shielding of the weld area is necessary. This is because the stainless steel contains chromium, and if this chromium reacts with oxygen, it forms extremely dense Cr2O3 film which reduces the corrosion resistant property of the stainless steel.
  • Aluminum alloys: Aluminum and its alloys are weldable but they get oxidized easily and so the film of aluminum oxide is formed on its surface. When aluminum alloys are welded using Oxy acetylene gas welding, or TIG welding or MIG welding, it must be covered with flux to protect the metal against oxidation.
  • Magnesium alloys: Welding of magnesium and its alloys are possible, but they require shielding during a welding process.
    If the magnesium or its alloys are not shielded during the welding process, then it reacts with nitrogen of the atmosphere and forms magnesium nitride which weakens the weld. Also the hydrogen gas dissolves in the weld which leads to porosity in weld.

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