Since fusion welding requires or demands that a portion of each base metal or substrate involved in the joint melt along with the filler, heterogeneous fillers inevitably are diluted by the base metal. The degree of dilution depends on
(1) the type of joint and joint edge preparation;
(2) the welding process, process parameters (including operating current mode), and technique used; and
(3) the mismatch between the filler and base metal. In general, dilution (D), expressed as a percentage, is defined as
Weight in this relationship can be taken to be relative areas of a representative cross section of the weld, as shown in figure below
Welding technique can also make a difference in degree of dilution. A series of overlapping, fine stringer beads (made with low heat input) result in less dilution than fewer heavy passes (made with high heat input) in multipass welds. A special technique used to minimize dilution is called buttering. In the case where welding dissimilar base metals could result in metallurgical incompatibilities that could adversely affect weldability (e.g., produce hard spots, general embrittlement, or cracks), buttering is especially useful. The best example is probably welding mild steel to a gray, malleable, or nodular cast iron containing 2-4 wt%C. If either mild steel filler (that matches the mild steel base metal) or an austenitic stainless steel (which is simply compatible with the mild steel base metal) is used, carbon pickup from the cast iron by dilution could render the weld metal susceptible to transformation hardening to form brittle martensite, and would probably result in cracking in the weld.
By buttering the cast iron joint faying surface with fine, side-by-side, overlapping stringer beads (made using very low arc energy and net heat input) of pure nickel (which will not be adversely affected by pickup of carbon), and then filling with either a mild steel or austenitic stainless steel, carbon pickup by the filler can be virtually eliminated, The buttering layer creates a cushion against intermixing between the cast iron and the filler.
Another place where dilution must be taken into account is in weld overlaying, also known as cladding, surfacing, or hardfacing, to provide protection against corrosion or wear. Too much dilution by the base metal can severely degrade the desired properties of the overlay. Multiple layers of overlay (to get past the dilution), or an intermediate, buttering or “cushion coat” (to block dilution) will help.
For multipass welds, Estes and Turner (1964) developed formulas for calculating filler metal dilution and, particularly, composition of the all-important root pass. These formulas are given in figure below
Reference: Principles of welding; Processes, Physics, Chemistry and Metallurgy; Robert W. Messler, Jr.
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