What is Delong Diagram and its Uses?


Table of Contents

Delong Diagram: An Introduction

Delong Diagram refines the Schaeffler diagram (Both are constitute diagrams) by taking account of the strong austenite stabilizing tendency of nitrogen while Schaeffler Diagram does not use Nitrogen in its calculations. The chromium equivalent is  calculated using same equation as in Schaeffler Diagram although the nickel equivalent is modified to below equation:Ni (eq) = Ni + (30 x C) + (0.5 x Mn) + (30 x N)

The Delong diagram, identifying the phase boundaries is shown in the below figure. Here its represents the ferrite levels in bands, both as percentages, based on metallographic determinations, and as a ferrite number ‘FN‘, based on magnetic determination methods.

Delong Diagram

Uses of Delong Diagram

  1. Predict Ferrite phase in weldment to avoid hot cracking.
  2. Prediction of metallurgical phases based on base metal and filler metal chemistry.
  3. prediction of the constitution of stainless steel weld deposit.

Example of Practical Use of Delong Diagram

Consider, we want to predict the Ferrite Number for a weld of UNS S32205 Duplex Steel welded with a “matching” E2209 (SFA  A5.4/A5.4M) SMAW electrode for duplex stainless.

Let’s assume a 30% dilution rate (Where 2205  base metal contributes a total of 30% of the weld (15 % + 15 % from both sides of the base metal) and the E2209-XX  electrode adds the other 70%). The composition of the resultant weld metal can be predicted as below.

Step: 1 We should have a complete chemical composition of base metal (Mills / material suppliers Certificate / PMI Spectroscopy report)

Chemical Composition of UNS S32205 as per PMI Spectroscopy

CSiMnPSCrNiMoNbCuCoN
0.0230.371.500.0180.00122.375.723.210.110.140.080.177

Step 2

We should have filler/electrode chemical composition (As per ASME SEC IIC, Weld Pad Method/ Electrode manufacturer’s suppliers test report)

The Chemical compositions of the undiluted weld metal as per the ASME SEC IIC weld pad test method

Elements Nominal Values in (Wt%)CMnSiSPCrNiMoCuN
Commercial
E2209-16
0.0311.080.590.0070.02522.389.153.350.0960.18

Step 3

Calculate the Chromium equivalent and Nickel equivalent for base metal and filler/electrode composition:

Cr eq = Cr + Mo +0.7Cb

Ni eq = Ni + 35C+20N+0.25Cu

Step 4 :Plotting the values of Cr eq & Ni eq on the WRC-1992 diagram

The UNS S32205 composition is represented by point B (Cr equivalent 25.6%; Ni equivalent 10.1%) and the E2209-XX   electrode composition  by point A (Cr equivalent 25.7%; Ni equivalent 13.8%). As shown in given at the last. At the same time below Table lists the estimated composition of weld metal/synthetic base metal. Table 1 Calculates Weld metal composition / Synthetic base metal composition from  given base metal and Filler/electrode composition, considering 30:70 dilution (Base metal: Filler Metal).

Delong Diagram

Based on the Creq & Nieq & Plotting their values on the WRC diagram as shown below, you will notice that any output weld metal from this mixture of A and B will be on the line that joins them. As we have presumed 30 percent dilution, point C will give the resultant predicted FN = 42.7 approximately as shown in below figure.  (Notice the calculated Weld metal composition point C which is near thick orange line (FN=40) & between thin blue line (FN=46).

Delong Diagram
FN 42.7 Prediction from WRC 1992 diagram for weld sample prepared by E2209 –XX electrode.

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