Dilution Rate in Welding


Dilution rate in welding and dilution calculation in welding​

During fusion welding, each dissimilar base metal and filler metal (if used) are melted together and combined in the weld pool. On solidification, the weld metal will be either a single-phase or a mixture of two or more phases. A phase can be a solid solution, such as that occurring between nickel and copper, or it may be an intermetallic compound (CuAl2), or other compounds such as carbides (Fe3C and TiC).

So, Dilution Rate is The change in the chemical composition of a welding filler metal caused by the admixture of the base metal or previous weld metal in the weld bead. It is measured by the percentage of base metal or previous weld metal in the weld bead. 

weld micro

Factors affecting the Dilution Rate in Welding

The degree of Dilution Rate depends on following factors:

(1) the type of welding joint (groove, fillet or weld overlay) and joint edge preparation (square or V groove);

(2) the welding process (e.g. SMAW, TIG or MIG/MAG), process parameters (including operating current mode, shielding gases), and technique (electrode angle, welding position) used; and

(3) the mismatch between the filler and base metal (Similar or dissimilar metallurgy, overmatch filler). 

Dilution can be defined as the Contribution of Base Material into weld metal (e.g. Base Materials + Filler Metal):

dilution rate formula

Considering the above picture, for autogenous welding techniques (without filler material addition), for the same (Similar) base material, 

Total dilution =100%.

This means, the total contribution of the same similar base metal into weld metal is 100 %, While for Non-autogenous mode (with Filler material addition), dilution % depends on many factors such as:-

  1. Selected Joint design & welding Processes

As can be seen in the figures below, usually with SMAW, GMAW & GTAW processes, 15-20% of the contribution from base metal (i.e total 20 -40 %, from each side of the joint) is expected. While in SAW process, due to its high arc efficiency as much as 80% contribution from base metal is expected.

Groove geometry is also playing an important role as shown in self-explanatory schematics below:-

weld dilution in joints

2. Heat Input (H.I.)

Where; Heat input in (KJ/mm)  = V X I X 60 / S X 1000   

Where, V=voltage, I=Amperage, Travelling speed (S) in mm/min

Higher H.I. can lead to more dilution. If you are joining CS/LAS to SS 304L or 316L using ER309L. You should be more careful to control H.I Since, higher H.I can lead more carbon dilution from CS/LAS side into weld metal resulting in weld solidification cracking due to primary austenite solidification mode only.

However, ER 309L is designed to retain some ferrite in as weld solidified structure to prevent the solidification cracking in the weld joint.

In a fusion Welding process depending on electrode/filler used or not, if used, depending upon matching chemistry of the electrode/filler material with base metal decide the type of solidification-

For example, if no Filler material is used,

Autogenous–No filler material

Homogeneous–Filler metal chemistry similar to the base material

Heterogeneous–Filler metal chemistry is dissimilar to the base material

Example of Dilution rate calculation

Let us understand, the dilution rate calculation using this simple example:

Assume 430 Ferritic SS base metal is joined with 308 filler, with 35% dilution,

 CCrNiMn
Filler composition0.0620.311.31.5
Base metal composition0.0917.30.40.6
Filler X 0.65(1-dilution)0.03913.27.41.2
+ Base X 0.35 (dilution)0.0326.110.10.2
Weld Metal0.07119.37.51.4

Dilution rate calculation Equation

The average percentage of a specific alloying element X in the diluted weld metal can be calculated using the following equation:
Xw = DaXa + DbXb + Xf (1 – Dt) (Reference AWS Handbook Volume 4)

where
Xw = Average % of element X in the weld metal, w;
Xa = % of element X in base metal a;
Xb = % of element X in base metal b;
Xf = % of element X in filler metal f;
Da = % dilution by base metal a, as a decimal;
Db = % dilution by base metal b, as a decimal; and
Dt = % total dilution by base metals a and b, as a decimal.

Dilution rate calculation in groove weld with filler and without filler

Dilution rate calculation in groove weld with filler and without filler

where
a = Base metal
b = Base metal
A = Cross-sectional area of weld in base metal a
B = Cross-sectional area of weld in base metal b
D1 = % total dilution
Da = % dilution in base metal a
Db = % dilution in base metal b
F1 = Cross-sectional area of weld above base metal
F2 = Cross-sectional area of weld below base metal

Dilution rate in different welding processes

Typical values of dilution rates found in different welding processes are shown in the below table. The values are for reference purposes only as different welding variables affect the rate of dilution in welding such as welding current, voltage, welding position, electrode diameter, electrode angle, and shielding gases, etc. to name a few.

Dilution rate in different welding processes

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