Schaeffler Diagram and its practical uses


What is Schaeffler Diagram?

The Schaeffler diagram is an important tool for predicting the constitution of your stainless steel weld deposit. Depending on the alloying elements it contains, the Schaeffler diagram provides information on the various phases (structures) present.
Ferrite is important in avoiding hot cracking in during cooling from welding of austenitic stainless steels. ‘Constitution diagrams’ are used to predict ferrite levels from the composition by comparing the effects of austenite and ferrite stabilising elements. The Schaeffler diagram is the original methods of predicting the phase balances in austenitic stainless steel welds.

The chromium equivalent (Drawn on the horizontal line) is calculated from the weight percentage of ferrite-forming elements (Cr, Si, Mo, Nb, W) and the nickel equivalent (drawn on the vertical line) is calculated from the weight percentage of austenite-forming elements (C, Ni, Mn, Cu, N). The position in the Schaeffler diagram defined by the Cr- and Ni-equivalents gives the proportions of martensite, austenite, and ferrite in the resulting microstructure.

Read more about Constitution Diagrams:

  1. Delong Diagram & its practical uses.
  2. WRC-1992 Diagram & its practical uses.
Figure 1. Microstructures plotted on Schaeffler Diagram

A Schaeffler diagram can be used to represent the effect of the proportion of two elements (and therefore the composition of the alloy) on the structure obtained after rapid cooling from 1050°C to room temperature.
The figure below shows that chromium is a ferrite stabilizer and nickel is an austenite stabiliser. This diagram shows the limits of the austenitic, ferritic and martensitic phases in relation to the chromium and nickel equivalent, calculated by using these expressions:

• Cr equivalent = (Cr)+2(Si)+1.5(Mo)+5(V)+5.5(Al)+1.75(Nb)+1.5(Ti)+0.75(W)
• Ni equivalent = (Ni)+(Co)+0.5(Mn)+0.3(Cu)+25(N)+30(C)

with all concentrations being expressed in weight percentages.

The Schaeffler diagram is an important tool for predicting the constitution of austenitic Cr-Ni steel welds with carbon contents up to 0.12%. However, it does not allow the determination of the composition and volume of the carbide phase. Furthermore, for a carbon content lower than 0.12%, the agreement of predictions with the actual system is reduced due to the consumption of carbon by the carbide formation process.
The Schaeffler diagram is especially suited to weld metals in order to predict the final deposit structure. Below Schaeffler Diagram shows the positions of various welding consumables with their respective weld deposit microstructure.

Figure 2. Most common welding electrode metallurgy on Schaeffler Diagram

A practical example for Schaeffler Diagram for Dissimilar Welding

As we learned that the position of the material in the Schaeffler diagram is determined from its chromium and nickel equivalents of the base materials, and then a straight line is then drawn between the points obtained by these Creq & Nieq .

Similarly, the position of the proposed filler material is plotted based on its chemistry. When welding symmetrical joints (e.g. single or double V/ U/J groove with equal bevel angles, it can consider that the fusion surfaces of the base material will be more or less equally melted on both levels. Now, A straight line can therefore be drawn between the position of the filler material and the center of the line between the base materials. In general, about 20–40 % of the filler material will be ‘diluted’ by the molten base material, with the result that the structure of the weld metal will be as indicated by a point about 20–40 % of the way along the line from the position of the filler material. If this structure is suitable, then the proposed filler material can be used. If it is not suitable, repeat the above procedure for a filler having a different composition.

The Schaeffler diagram in above Figure represents an example of the above for the case ofa low-alloy steel grade C15 (EN 1.0401), (A), being welded to stainless steel AISI 304 (1.4301), (B), using  ER309LMo (ER 23 12 2 L) filler wire (D). This gives a weld metal structure as shown at point E, i.e. austenite with about 10 % ferrite.

Chemical compositions of the above three materials in the discussion are given below:

Chemical composition % of steel C15 (1.0401): EN 10277-2-2008

CSiMnPS
0.12 – 0.18max   0.40.3 – 0.8max   0.045max   0.045

             Chemical composition % of steel  X5CrNi18-10 (1.4301): EN 10088-2-2005

CSiMnNiPSCrN
max   0.07max   1max   28 – 10.5max   0.045max   0.01517.5 – 19.5max   0.11

Chemical composition % of grade 309LMo ( 309LMo )

CSiMnNiPSCrMoCu
max   0.03max   0.651 – 2.512 – 14max   0.03max   0.0323 – 252 – 3max   0.75

Watch our You Tube video for interactive animation learning on Schaeffler Diagram & its importance.

Figure 3. Distribution of phases on Schaeffler Diagram

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