What is Delong Diagram and its Uses?

Delong Diagram: An Introduction

Delong Diagram clarifies the Schaeffler diagram (Both are constitute diagrams) using the Nitrogen in the calculation as we know Nitrogen is a very powerful austenite stabilizer. While Schaeffler Diagram does not use Nitrogen in its calculations. Another modification in the Delong diagram is the use of Ferrite Number (FN) to represent the ferrite content.The chromium (Cr Eq.) equivalent is  calculated using same equation as in Schaeffler Diagram although the Ni Equivalent (Ni Eq.) is adjusted to below equation: Ni (eq) = Ni + (30 x C) + (0.5 x Mn) + (30 x N).

The Delong diagram, with Cr-Eq and Ni-Eq, is given below. The diagram gives Ferrite level as a percentage along with the ferrite number ‘FN.

As can be seen, the Delong Diagram includes only a limited portion of the alloys recorded in the Schaeffler diagram. It is valid for Nickel equivalents between 10% to 31% and chromium equivalents between 16% to 27%. It can be used to effectively predict the microstructure of metastable austenitic chromium-nickel steels that form at room temperature.

Delong Diagram Calculator

To use DeLong diagram calculation, we need to first calculate the Cr Equivalent. Cr-Equivalent is plotted on the X-axis & is calculated using the below formula:

Chromium Equivalent (Cr Eq)= Cr% + Mo% + 1.5xSi% + 0.5xCb%

Next, we need to find out the Nickel equivalent. Similarly, Nickel Equivalent is plotted on the Y-Axis on the diagram. Ni-Equivalent is calculated using the following formula:

Nickel Equivalent (Ni Eq.) = Ni% + (30 x C%) + (0.5 x Mn%) + (30 x N%)

Both the values are plotted on the DeLong diagram and the equivalent Ferrite Number is identified using the graph.

Uses of Delong Diagram

  1. Predict the 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.
  4. Delong diagram is mainly suitable for metastable austenitic Cr-Ni Steels.

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