Ferrosilicon 75 for Casting: What the Grade Actually Means
Ferrosilicon is quoted by silicon content, and the number in the grade name is the minimum silicon figure. FeSi75 means the alloy contains not less than 75 percent silicon; FeSi72 means not less than 72 percent; FeSi65 means not less than 65 percent. For a foundry the difference is not academic. Silicon added through FeSi raises the silicon content of the iron, controls chill depth in grey iron, and acts as an inoculant when it is added late in the treatment sequence at the right particle size. Because silicon is the element being purchased, the buyer pays per unit of contained silicon, so a lower-grade alloy can be cheaper per tonne and still be more expensive per unit of silicon delivered to the ladle.
Chemistry Bands That Matter in Foundry Use
| Parameter | Typical commercial band | Why it matters |
|---|---|---|
| Silicon | not less than 72, 75 or 65 percent by grade | main purchased element, controls chill and silicon level |
| Aluminium, low-aluminium grades | below 0.1 percent or below 0.5 percent | low aluminium limits pinhole and gas defects in castings |
| Carbon | typically below 0.2 percent | excess carbon affects dissolution in the melt |
| Sulphur and phosphorus | controlled to low residual levels | residual elements carried into the final iron |
| Size range | 10 to 50 mm, 10 to 100 mm, 3 to 10 mm, fines | dissolution rate and inoculation effect |
Both GB/T 2272 for ferrosilicon and ISO 5445 for ferrosilicon specification set out the grade structure and delivery conditions, including the size fractions offered. A purchase order should therefore state the grade, the aluminium class if gas defects are a concern, the size fraction, and the packing, because the same grade supplied as 10 to 50 mm lumps and as fines behaves completely differently in a ladle or in an inoculation stream.
Reference Price Levels for Ferrosilicon
The tables below record reference quotations observed on 14 July. Export levels are USD per tonne FOB Tianjin Port. Domestic levels are yuan per tonne, natural block, ex-works cash price including tax, and apply to low-aluminium material.
| Export grade | Reference price, USD/tonne FOB Tianjin |
|---|---|
| Ferrosilicon 72 | 1,000 to 1,030 |
| Ferrosilicon 75 | 1,080 to 1,100 |
| Domestic product | Specification | Area | Reference price, yuan/tonne |
|---|---|---|---|
| Low aluminium ferrosilicon | Si75, Al below 0.1 percent | Qinghai | 6,700 to 6,750 |
| Low aluminium ferrosilicon | Si75, Al below 0.5 percent | Qinghai | 6,400 to 6,450 |
| Low aluminium ferrosilicon | Si75, Al below 0.1 percent | Inner Mongolia | 6,700 to 6,750 |
| Low aluminium ferrosilicon | Si75, Al below 0.5 percent | Inner Mongolia | 6,400 to 6,450 |
Two observations are useful. The export FeSi75 level sits above FeSi72 by roughly the difference in contained silicon, which confirms that the market prices silicon rather than tonnage. Within the domestic low-aluminium family, tightening aluminium from below 0.5 percent to below 0.1 percent carries a defined premium of about 300 yuan per tonne, so low-aluminium material should only be specified where casting defects justify it.
Why a Very Cheap Offer Usually Is Not Cheap
When a ferrosilicon offer falls well below the range of qualified suppliers, one of four explanations normally applies. The grade may be lower than stated, with silicon below the declared minimum. The aluminium content may be above the low-aluminium class, which shows up later as gas porosity and as higher rejection rates in thin-wall castings. The size fraction may be out of specification, containing excessive fines or oversize lumps that do not dissolve at the intended point of addition. Or the material may be offered on terms that shift freight, insurance and quality risk to the buyer. Each of these can be tested cheaply: check the certificate of analysis for silicon and aluminium, screen a sample for size distribution, and weigh the declared packing against the delivered mass.
Buying Checklist for Foundries and Traders
Fix seven items before ordering: grade as minimum silicon content, aluminium class, size fraction, packing mass and type, ex-works or delivered basis and port, payment terms, and the sampling and analysis method used to settle any dispute. Retain a sealed reference sample of every delivered lot. In foundry use, the practical control is not only the alloy itself but the point of addition: ferrosilicon added as a late inoculant at the right size gives the chill reduction and graphite refinement expected, while the same alloy added early and left to fade delivers much less. Nodularity and graphite form are checked on a cast sample against ISO 945-1 and ASTM A247 reference charts, which keeps the result comparable between shifts and suppliers.
Frequently Asked Questions
Q: Is FeSi72 cheaper to use than FeSi75?
A: Per tonne of alloy, usually yes. Per unit of contained silicon delivered, the difference is much smaller, and lower silicon content means more alloy must be added, which brings more aluminium and carbon into the melt with it.
Q: When is low-aluminium ferrosilicon necessary?
A: Where gas defects, pinholes or surface porosity matter, for example in thin-wall or pressure-tight castings. For heavy sections with generous machining allowance the standard aluminium class is normally adequate.
Q: What size ferrosilicon should be used for inoculation?
A: A finer, closely sized fraction than for bulk silicon addition, so that particles dissolve quickly and distribute evenly. Lumps that are too coarse give uneven inoculation and can leave undissolved residue.
Q: How is silicon content verified on delivery?
A: By sampling the lot and analysing it, with the result compared against the declared minimum for the grade. Keeping a sealed sample of each lot is standard practice for dispute settlement.
Q: Does packing affect the delivered price?
A: Yes. Bulk, one-tonne big bags and 25 kg bags carry different handling costs and different moisture protection, and the declared net mass per package should always be checked against the delivered mass.
Q: What causes chill in grey iron castings?
A: Insufficient silicon at the moment of solidification, an oxidised melt, or an inoculation step that has faded before pouring. Correct grade selection is only part of the answer; timing and dosing matter equally.