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“The higher the Mg content, the better the nodularization” — that sentence has cost foundries money and destabilised their processes.
A more accurate statement is this: Mg content is not a multiple-choice question, it is an arithmetic one. The percentage printed on the bag is what you pay for. What actually determines whether graphite grows as nodules is the 0.03%–0.05% of magnesium still sitting in the iron once treatment is finished. Between those two numbers sit three separate losses and one recovery rate.
Which is why the same FeSiMg8RE3 runs at a 1.4% addition rate in Foundry A and needs 1.9% in Foundry B. The material did not change; the process did. The reverse holds too: if improving your process lifts recovery, moving to a lower Mg grade may well cost you less overall.
What follows takes that calculation apart, far enough that you can apply it directly.
Once the nodularizer goes into the melt, only part of its magnesium stays behind to do any work. There are three destinations:
① Desulphurization. Magnesium reacts with sulphur to form MgS, which floats off into the slag. This is unavoidable cost — the higher the sulphur in the base iron, the more magnesium this channel swallows.
② Oxidation and vapourization losses. This is the one that gets underestimated. Magnesium boils at 1091°C while nodularizing treatment runs at roughly 1380–1520°C. Magnesium starts escaping towards its vapour phase the moment it enters the iron, taking with it whatever is consumed by atmospheric oxygen and surface oxides.
③ Residual magnesium. The portion that actually nodularizes. Ductile iron is typically held at 0.03%–0.05% residual Mg: below that, nodularity suffers; above roughly 0.06%, chill and cementite become a concern — more on that in section 3.
|
Destination |
How controllable |
Governed by |
|
Desulphurization |
Partly (pre-treatment possible) |
Base iron sulphur, target final sulphur |
|
Oxidation / vapourization |
Compressible through practice |
Treatment process, temperature, cover conditions |
|
Residual magnesium |
This is the target |
Section thickness, pouring rhythm |
The judgement worth drawing out: of the three channels, only the second can be compressed substantially through process choices. And it is a physical fact — magnesium vapourizes above 1091°C — that sets how far it can be compressed.
That gives us the backbone of the calculation:
Total Mg required = (target residual Mg + desulphurization consumption) ÷ Mg recovery rate
Nodularizer addition = total Mg required ÷ Mg content of the grade
The recovery rate already carries the second channel’s losses inside it. So in practice only one thing remains to pin down: under your process, what is your Mg recovery?
Answer these four before choosing a Mg level. They do not carry equal weight, so take them in order.
Magnesium utilisation varies widely between nodularizing methods. This is the first thing to settle.
|
Treatment process |
Typical Mg recovery |
Mg band it suits |
Notes |
|
Pour-over, open ladle / plunge |
30%–45% |
Mg 7%–9% |
Low capital cost; violent reaction, heavy fume |
|
Covered ladle / sandwich |
40%–55% |
Mg 6%–8% |
Cover or cover material suppresses turbulence; steadier than open pour-over |
|
Cored-wire injection |
45%–60% |
Mg 5%–8% |
Real value depends on injection depth and speed; needs a wire feeder |
|
Transfer ladle (GF converter / rocking ladle) |
50%–65% |
Mg 5%–7% |
Gradual release inside the ladle; among the higher recovery options |
|
In-mould nodularization |
60%–80% |
Mg 4%–6% |
Reaction chamber design decides success or failure; suits high-volume repeat castings |
|
Wire injection + transfer ladle combined |
55%–70% |
Mg 5%–7% |
Balances stability against scheduling flexibility |
Figures represent typical performance for each process family and shift significantly with ladle geometry, cover practice and batch size. Use your own measured figures.
One thing in that table deserves a pause: the more advanced the process, the lower the Mg band it suits. That is no coincidence — since the point of choosing such a process is to raise recovery, you do not then need a high Mg grade to pile enough residual magnesium into the iron.
Magnesium boils at 1091°C; the iron is treated at 1380–1520°C. The wider that gap, the heavier the vapourization loss.
The working rule: each 50°C or so of added treatment temperature costs a measurable drop in Mg recovery, and the drop steepens at the top of the range. If a foundry is struggling with unstable nodularization and high magnesium consumption, temperature belongs high on the list of things to check.
There is a knock-on effect worth noting. Overly hot tapping tends to do two kinds of damage at once: magnesium losses rise, and the iron’s oxidation tendency increases, which blunts inoculation as well. So the instinct to “take it a bit hotter to be safe” runs in the wrong direction at this step of the process.
This is the one channel that can be calculated exactly. The reaction stoichiometry fixes the mass ratio:
Mg consumed by desulphurization = (sulphur before treatment − target final sulphur) × 0.76
The 0.76 coefficient is the ratio of atomic weights (Mg 24.3 ÷ S 32.06 ≈ 0.76).
Example: with base iron at 0.025% S and a 0.010% S target, desulphurization consumes
(0.025 − 0.010) × 0.76 = 0.0114% Mg per tonne of iron, or 0.114 kg.
Set that beside the target residual magnesium and the scale becomes obvious: with a 0.045% target residual, desulphurization accounts for roughly a quarter as much again. If base iron sulphur doubles, so does this line item.
That points to a conclusion people often miss: if your base iron runs persistently high in sulphur, external desulphurization usually beats switching to a higher Mg grade. Desulphurizing agents cost far less than magnesium.
|
Melting route |
Typical base iron sulphur |
What it means for grade selection |
|
Electric furnace, good scrap/carburiser balance |
0.010%–0.025% |
Little desulphurization load; mid to low Mg grades open up |
|
Electric furnace with variable scrap quality |
0.020%–0.040% |
Leave headroom for desulphurization |
|
Cupola |
0.040%–0.080% |
Pre-treatment strongly advisable, otherwise Mg consumption runs very high |
|
Induction furnace with desulphurization pre-treatment |
≤0.015% |
Low Mg grades with high-recovery processes work well |
Residual magnesium does not stay put once it is in. It fades with time. The thicker the section and the longer the interval between pouring and solidification, the higher the starting residual needs to be:
|
Casting type |
Target residual Mg |
Additional requirement |
|
Thin sections (<15 mm), fast pour |
0.030%–0.040% |
Hold the upper limit down; excess brings chill |
|
Medium sections (15–50 mm) |
0.035%–0.045% |
Normal selection window |
|
Heavy sections (>80 mm) |
0.045%–0.055% |
Needs anti-fading elements (RE or yttrium-based) |
|
Very heavy castings / slow pours |
≥0.050% |
Heavy rare earth nodularizer plus in-mould inoculation |
The difficulty with heavy sections is not how much magnesium to add, it is whether any is still there when solidification arrives. Raising the grade’s Mg content helps only marginally here; the effective answer is heavy rare earth, which section 5 covers.
Back to that opening line. High Mg grades genuinely have their place, but treating them as a universal safeguard fails in three ways:
One: more magnesium simply burns off. A higher Mg grade reacts more vigorously per unit mass, the bath churns harder, and a larger share of the magnesium leaves as fume. Magnesium you paid a premium for goes out the extraction duct instead of into the residual account.
Two: violent reaction brings slag inclusions. Churning drags air in and generates more oxide inclusions. Residual magnesium may come in on target while castings fail on slag holes and inclusions anyway — which trades a composition problem for a cleanliness problem.
Three: excess residual magnesium has side effects. As noted earlier, above roughly 0.06% residual Mg thin sections pick up chill and cementite risk. Excess magnesium also raises the tendency to shrinkage porosity. So “adding a bit extra for safety” carries a cost on thin-wall work.
Taken together, the sequence for choosing a grade should run the other way:
✕ Usual approach: pick the grade first (“we have used FeSiMg8RE3 for years”) → set the addition rate by experience → add more when problems appear
✓ Better order: fix the process and measure recovery → calculate the total Mg required → then work out which Mg band is most economical
The grade is the output of that last step, not the starting assumption.
Now to run the arithmetic from section 1 across two options treating the same iron.
Given conditions (one line, one batch of castings):
Option A: FeSiMg8RE3, pour-over method, recovery taken as 40%
Mg for desulphurization = (0.025% − 0.010%) × 0.76 = 0.0114%
Target residual = 0.0450%
Effective Mg needed, subtotal = 0.0564%
Total Mg to charge = 0.0564% ÷ 0.40 (recovery) = 0.1410%
Nodularizer addition = 0.1410% ÷ 8% (grade Mg) = 1.76%
→ 17.6 kg per tonne of iron
Option B: FeSiMg6RE2, switched to a transfer ladle, recovery taken as 55%
Total Mg to charge = 0.0564% ÷ 0.55 (recovery) = 0.1025%
Nodularizer addition = 0.1025% ÷ 6% (grade Mg) = 1.71%
→ 17.1 kg per tonne of iron
The addition rates come out almost identical (17.6 vs 17.1 kg per tonne).
The real difference sits in unit price. Magnesium is the costliest constituent of the nodularizer, so FeSiMg6RE2 normally carries a lower tonne price than FeSiMg8RE3. Hence:
|
|
Option A · FeSiMg8RE3, pour-over |
Option B · FeSiMg6RE2, transfer ladle |
|
Addition per tonne of iron |
17.6 kg |
17.1 kg |
|
Delivered price of nodularizer |
【填入：FeSiMg8RE3 单价】 |
【填入：FeSiMg6RE2 单价】 |
|
Alloying cost per tonne of iron |
addition × unit price |
addition × unit price |
That bottom line is what to compare: not the price per tonne of nodularizer but the cost per tonne of iron. Option A may look dearer or cheaper on unit price, but what lands on the accounts is the last row.
Worth adding that this kind of calculation is worth doing regardless of whether it saves money. Once the “addition ÷ recovery” model exists, every process change becomes quantifiable — a new ladle, a different temperature, a change in sulphur — instead of a matter for trial and opinion.
The job rare earths do inside a nodularizer is neutralizing interfering elements. Titanium, lead, bismuth, antimony and arsenic in the base iron degrade graphite nodule roundness; rare earths bind with them preferentially and so protect the magnesium’s nodularizing action.
That protection has a downside — excess rare earth increases chill and cementite risk, particularly in thin sections. RE content should therefore follow the cleanliness of your base iron:
|
Operating condition |
Recommended RE band |
Reason |
|
Electric furnace, good-quality scrap |
RE 1%–2% (low) |
Clean charge, few interfering elements |
|
Electric furnace, mixed scrap sources |
RE 2%–4% (medium) |
Leave capacity to neutralize |
|
Cupola iron |
RE 3%–5% (medium-high) |
Higher in both interfering elements and sulphur |
|
Heavy sections / slow pours |
Heavy RE (yttrium-based) or higher RE |
The requirement is resistance to fading |
|
Thin-wall castings |
Take the lower end of RE |
Avoid chill tendency |
How to tell where you stand: if raising RE does not improve nodularity but does increase chill, the constraint almost certainly lies elsewhere — go back and examine sulphur or temperature.
Compressed into something you can run with:
Step 1 · Measure your own Mg recovery. Do not look it up — measure it. Same batch of iron, same practice, record the addition and the final residual Mg, then back-calculate actual recovery. Three heats is enough for a usable average, and it beats any published table.
Step 2 · Stabilise base iron sulphur. Sulphur swings invalidate any carefully calculated addition rate. If the variation exceeds ±0.01%, fix that before anything else.
Step 3 · Work out total Mg requirement using the formula in section 1: target residual plus desulphurization consumption, divided by recovery.
Step 4 · Derive the Mg band from that requirement, rather than carrying on with the grade you have used up to now.
Step 5 · Specify RE against the table in section 5, and watch that you do not overshoot.
Step 6 · Compare suppliers on alloying cost per tonne of iron, not price per tonne of nodularizer.
Questions to put to your supplier:
Henan Longchuang Metallurgical Material Co., Ltd. supplies steel mills and foundries with RE-MgFeSi nodularizers, inoculants, alloy cored wire, ferrosilicon, manganese and chromium series alloys and some 20 further products, produced to specified composition windows and particle size requirements.
On this question specifically:
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