Effect of heat input in welding
Heat input within the weld pool and the droplet includes heat transfer by conduction, convection and ohmic heating. Heat input for a specific process is dependent upon the arc energy & the welding process efficiency.
Influence of heat input on toughness (CVN)
Toughness of weld metal is influenced by Heat input, chemical composition , microstructure and weld cooling rate. A low heat input results in rapid cooling and can have high toughness microstructure in the weldment while a high Heat Input results in slower cooling but results in very coarse HAZ with poor toughness in the weldment.
Researcher found that Heat input (HI) range of 2 kJ/mm for optimum toughness properties in the weld metal of C-Mn chemistry. Increase in Heat Input resulted in increased bead size and width of the columnar grains with increase in pro-eutectoid ferrite by replacing the acicular ferrite at the expense of toughness properties. A reduction in Tensile Strength resulting is higher Heat Input produced grain growth in reheated regions replacing both coarse and fine grain zones. Increase in Heat Input resulted in larger cellular dendritic cell spacing, decreased acicular ferrite and coarser acicular ferrite laths.
lower heat input values offer better toughness results compare to higher Heat Input in HSLA steel weldment.
High heat input results significant decrease in acicular ferrite with coarse grain structure in the weld metal which resulted in significant decrease in toughness properties of Weld Metal at 0°C. Acicular ferrite in the microstructure significantly affect low temperature toughness properties. An increase in acicular ferrite gives increase to the toughness properties. Hence a lower heat input values favoring growth of acicular ferrite (in the weld metal is beneficial for higher toughness properties as higher Heat Input results in more diffusion-controlled phase during solidification (transformation from austenitic to ferrite) and reduction in the acicular ferrite due to controlled diffusion. A low Heat Input results rapid cooling in the vicinity of the A1 temperature form acicular ferrite which is more equiaxed.
Influence of heat input on tensile strength
Tensile strength (T.S.) and yield strength (Y.S.) of the weld metal reduce with an increment in heat input values. Obviously, high heat input produce slower cooling rate, thus yielding a soft microstructure with reduced yield strength.
Influence of heat input on weld metal microstructure
In ASTM A572 Gr. 50 material which is having similar chemical composition to SA516 Gr. 70 with high heat input SAW welds reported heat input as the main contributing factors for the microstructural properties. High heat input (7.8 kJ/mm-14.3 kJ/mm) produced very low level of acicular ferrite with increase in primary ferrite and aggregates ferrite-carbides. Figure below shows that acicular ferrite & second phase ferrite having steep decrease with increase in heat input values from 4.8kJ/mm to 14.3kJ/mm in SAW weldment. Figure below shows the changes in the microstructure of the weld metal occurred due to increase in heat input. At a heat input values of 4.8kJ/mm, the proportion of acicular ferrite at high level compare to very low levels for high heat input of 14.3kJ/mm. This reduction in acicular ferrite being accompanied by a rise in primary ferrite and ferrite-carbide aggregates.
Figure above, shows the microstructure of a low carbon low alloy steel with predominant presence of acicular ferrite. Right side shows the structure at higher magnification with black spot as the inclusions.
analysed that low heat input with higher weld passes produces ferritic structure vs. graphite structure formation due to high heat input with lower number of weld passes in the weldment zone of SA 516 Gr 70 material using SAW process.
Influence of heat input on weld metal hardness
An increase in HI resulted in decrease in hardness values of weldment. The lowest heat input produced highest hardness and increase in heat input values results in decreased hardness results.