Schaeffler Diagram Online Calculator: Predict Weld Microstructure for Stainless Steel

The Schaeffler diagram remains the fastest way to estimate whether stainless steel weld metal will solidify as austenite, ferrite, martensite, or a mixture, using nothing more than the filler and base metal chemical composition. This calculator converts your alloy composition into chromium and nickel equivalents, plots the result on an interactive diagram, and flags the phase field and approximate ferrite content you should expect.

What Is the Schaeffler Diagram

The Schaeffler diagram is a constitution diagram published by Anton L. Schaeffler in 1949 to solve a practical welding engineering problem: predicting the room-temperature microstructure of stainless steel weld metal directly from its chemical composition, before a single arc is struck. It grew out of wartime and post-war experience welding austenitic stainless steels, where unpredicted martensite or excessive ferrite in weld deposits was causing cracking and service failures that composition specifications alone could not explain.

The diagram works by collapsing a multi-element alloy composition onto two axes. Elements that stabilise the ferrite (body-centred cubic, alpha) phase — chromium, molybdenum, silicon, niobium — are combined into a single chromium equivalent. Elements that stabilise the austenite (face-centred cubic, gamma) phase — nickel, carbon, manganese — are combined into a nickel equivalent. Every alloy composition becomes a single point on the Cr_eq-Ni_eq plane, and that point falls inside a field labelled austenite, ferrite, martensite, or some combination of the three.

Chromium and Nickel Equivalent Formulas

The two equivalents are empirical weighted sums, calibrated against measured weld metal microstructures rather than derived from first-principles thermodynamics. All composition values are in weight percent.

Cr_eq = %Cr + %Mo + 1.5 x %Si + 0.5 x %Nb

Ni_eq = %Ni + 30 x %C + 0.5 x %Mn

Why Each Coefficient Exists

Chromium contributes to Cr_eq at full weight because it is the primary ferrite stabiliser and defines stainless steel’s corrosion resistance. Molybdenum, added mainly for pitting resistance, is a similarly strong ferrite former and also carries a coefficient of 1.0. Silicon is a comparatively weak ferrite stabiliser but is weighted at 1.5 because small silicon additions (as in high-silicon fluxes or fully deoxidised fillers) disproportionately promote ferrite formation during rapid weld cooling. Niobium, added to some grades for carbide stabilisation, gets a modest 0.5 weighting.

On the nickel equivalent side, carbon receives the largest coefficient of any element in either equation — 30 — because interstitial carbon is an extremely potent austenite stabiliser even in trace amounts; this is also why low-carbon “L-grade” fillers shift noticeably toward higher ferrite predictions than standard grades of otherwise identical composition. Manganese is a moderate austenite stabiliser and is weighted at 0.5, partly because much of its practical role in weld metal is deoxidation and manganese-sulphide formation rather than phase stabilisation.

Reading the Diagram: Phase Regions Explained

Once Cr_eq and Ni_eq are plotted, the resulting point’s location tells you what to expect in the as-welded microstructure.

Austenite (A)

High Ni_eq relative to Cr_eq produces a fully austenitic structure. Fully austenitic weld metal has excellent ductility and toughness but, without any ferrite to disrupt columnar grain boundaries during solidification, is the most susceptible of the three single-phase structures to solidification hot cracking, particularly in highly restrained joints or with elevated sulphur and phosphorus.

Ferrite (F)

High Cr_eq relative to Ni_eq — as in ferritic stainless steels and some high-chromium overlays — produces a fully ferritic structure. Fully ferritic weld metal resists hot cracking well but has limited ductility and toughness, especially after grain coarsening in the heat-affected zone, and is prone to 475°C embrittlement and sigma phase formation on prolonged elevated-temperature exposure.

Martensite (M)

Low Cr_eq and low-to-moderate Ni_eq — typical of the 410/420/440 martensitic grades, and also of carbon steel diluted into a stainless filler at a dissimilar joint — produces martensite on the rapid cooling typical of arc welding. Martensitic weld metal is hard and crack-sensitive; this field is the one to watch most carefully when qualifying dissimilar metal welds, since an unintentionally martensitic fusion zone is a classic cause of delayed hydrogen cracking.

Mixed Fields (A+F, A+M, A+F+M)

Most commercial austenitic stainless steel filler metals — 308, 309, 316 and similar grades — are formulated to land in the A+F field, carrying a controlled few percent of primary ferrite within an austenitic matrix. This is the deliberate “sweet spot” of stainless steel welding: enough ferrite to resist hot cracking, not so much that toughness and corrosion resistance suffer.

Predicting Ferrite Number and Hot Cracking Susceptibility

Within the A+F field, the original Schaeffler diagram carries iso-ferrite contour lines corresponding to approximate volume percent ferrite, typically drawn at 0, 5, 10, 20, 40, 80 and 100 percent. Modern practice has largely moved to Ferrite Number (FN), an arbitrary but standardised scale defined in AWS A4.2 and calibrated against magnetic instrument readings rather than point-counted volume fraction, because magnetic FN measurement is faster, non-destructive, and far more reproducible between laboratories than optical metallography.

A small amount of ferrite in an austenitic weld — commonly targeted in the 4 to 12 FN range for general fabrication — interrupts the columnar austenite grain boundaries that would otherwise run the full length of a solidifying weld bead. Ferrite has a higher solubility for the sulphur and phosphorus that segregate to the solidification front, so it mops up the low-melting-point liquid films that would otherwise wet grain boundaries and tear apart under solidification shrinkage stress. This is why fully austenitic filler metals such as 310 and many nickel-base alloys require tighter control of residual sulphur and phosphorus and more restrictive joint design than a 308L filler carrying a few FN of ferrite.

Too much ferrite is also undesirable. Beyond roughly 10 to 15 volume percent, continuous or near-continuous ferrite networks form, and prolonged service above about 300°C accelerates sigma phase and 475°C embrittlement, both of which sharply reduce impact toughness. Duplex and superduplex stainless steel welds are deliberately engineered around a roughly balanced 40 to 60 percent austenite-ferrite ratio for a different reason: to combine ferrite’s strength and stress corrosion cracking resistance with austenite’s toughness, which is why duplex weld procedures are qualified with tighter heat input control than single-phase austenitic procedures.

Limitations of the Schaeffler Diagram

The diagram’s simplicity is also the source of its limitations, and treating its output as a certified value rather than an engineering estimate is a common mistake.

Nitrogen Is Omitted

Nitrogen is one of the strongest austenite stabilisers available and is a routine, sometimes specified, element in modern stainless fillers and shielding gas blends. Because the original nickel equivalent ignores it entirely, the Schaeffler diagram systematically under-predicts austenite (and over-predicts ferrite) in nitrogen-bearing weld metal, which is exactly the gap the DeLong and WRC-1992 diagrams were built to close.

Fixed Cooling Rate Assumption

The diagram was calibrated against typical shielded metal arc weld cooling rates. Processes or conditions with very different cooling rates — high heat input submerged arc welding, laser welding, or heavy preheat — can shift the actual ferrite content measurably away from the diagram’s prediction for the same nominal composition.

No Direct Dilution Modelling

The diagram itself only accepts a single composition point; dilution from base metal must be calculated separately (typically as a weighted average of filler and base metal compositions based on estimated percent dilution) before the mixed composition is plotted. Skipping this step is the most common error when using the diagram for dissimilar metal welds.

Accuracy Band

Even within its calibrated range, the original Schaeffler diagram carries an accuracy of roughly ±4% ferrite; DeLong tightened this to roughly ±2% FN for the austenitic and A+F region. Neither should substitute for a Welding Procedure Specification qualified with actual mechanical testing and, where required, ferrite measurement per AWS A4.2.

Industrial Applications and Significance

Welding engineers reach for the Schaeffler diagram most often in three situations. First, filler metal selection: matching a filler’s predicted phase balance to the base metal and service requirement, such as choosing a fully austenitic, low-ferrite filler like ER310 for cryogenic service where even trace ferrite would compromise low-temperature toughness. Second, dissimilar metal welding, where the diagram flags whether a proposed filler and dilution combination will land safely in the A+F field or drift toward crack-sensitive martensite at a stainless-to-carbon-steel fusion boundary — a check central to nuclear, petrochemical, and power plant cladding and overlay work. Third, weld overlay and hardfacing design, where the diagram guides the composition needed to achieve a target ferrite level in corrosion-resistant cladding layers deposited over carbon or low-alloy steel vessels.

The diagram also underpins troubleshooting: an unexpected weld cracking failure is often diagnosed by back-calculating the actual deposited composition (accounting for measured dilution) and checking where it falls relative to the crack-sensitive martensite and fully austenitic fields, frequently revealing that dilution pulled the weld metal composition far from what the nominal filler chemistry alone would suggest. For readers building broader weldability judgment, this pairs directly with understanding HAZ microstructure evolution and hydrogen-induced cracking mechanisms, since a martensitic Schaeffler prediction and hydrogen cracking risk very often go hand in hand.

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