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Carbon Analysis in Cast Iron: OES vs Combustion C/S Analyzer
Carbon defines whether your casting is gray iron, ductile iron, white iron or steel. This guide explains how spark optical emission spectrometry (OES) and high-frequency infrared combustion analyzers measure carbon, where each technique wins, and how production foundries use both — with practical mapping to JIEBO Innovate T5, Noble T7, CS995, CS996 and CS2020.
Why carbon matters in cast iron
Carbon is the single most important alloying element in iron casting. The carbon content — together with silicon and the cooling rate — determines whether the casting solidifies as gray iron (carbon as flake graphite, 2.5–4.0 % C), ductile iron (carbon as spheroidal graphite, 3.0–4.0 % C), white iron (carbon as iron carbide, 1.8–3.6 % C), or steel (<2.0 % C). A 0.1 % error in carbon measurement can shift the casting between grades — with direct mechanical and commercial consequences.
Foundries therefore measure carbon at three points: incoming raw material (pig iron, scrap, returns), in-process molten metal (spoon or chill-cup samples before pouring), and finished castings (post-cooling QC). Each point has different speed and accuracy requirements.
Two techniques, both useful
Spark optical emission spectrometry (OES) measures carbon by exciting a solid metal sample with an electrical spark in argon and reading the carbon emission line at 193.09 nm (and others). It returns 20–30 elements in a single 20–30-second burn, making it ideal for in-process composition control where you need carbon plus silicon, manganese, phosphorus and sulfur at once.
High-frequency infrared combustion C/S analyzers measure carbon by burning a weighed metal sample in oxygen inside an induction furnace; the resulting CO₂ is measured by a non-dispersive infrared (NDIR) cell. The technique is more accurate at very low (<0.05 %) and very high (>4 %) carbon levels, but it analyzes only carbon and sulfur — not the other ten elements you also need for grade specification.
Where spark OES wins
Use OES for in-process production control. When the melt is in the holding furnace and you have 60 seconds to decide whether to add carbon, silicon or manganese, an OES burn gives you the full composition in one shot. The Innovate T5 with its Hamamatsu CMOS detector covers the typical foundry range (2.5–4.0 % C in gray iron, sulfur 0.005–0.15 %) at production-grade accuracy.
For research-grade or aerospace foundry work where the carbon must be reported to ±0.005 %, step up to the Noble T7 with constant-temperature optics. Carbon is a non-metallic element with low excitation energy — accurate OES carbon measurement depends on tight calibration, dry argon, and a vacuum or thermostatted optical chamber.
Where combustion C/S wins
Use a dedicated combustion analyzer for certification and the extreme ends of the carbon range. When you must report 0.001 % C in a low-carbon stainless steel, or 4.5 % C in a hyper-eutectic gray iron, the combustion technique is more accurate than OES. The integrated calibration is also simpler — combustion uses a weight basis, not a spectral line ratio, so calibration drift is slower.
JIEBO offers three combustion C/S analyzers spanning entry-level to high-end: the CS995 (bench-type, 0.0005–6 % carbon range, ideal for foundry production), the CS996 (floor-type with expandable cells, 0.0001–10 % range, for mixed-grade plants), and the CS2020 (four-cell auto-cleaning unit, 0.00001–99.99 % range, for high-throughput labs and certification).
How real foundries use both
Most well-run iron foundries run OES on the production floor for in-process control and keep a combustion C/S analyzer in the QC lab for incoming-material certification and finished-casting verification. The two techniques cross-check each other: if the OES carbon reading drifts from the combustion analyzer over a week, the OES needs recalibration. This dual-instrument workflow is the industry standard at every reputable ductile iron foundry we ship to.
The investment cost of running both is lower than people expect. A typical foundry pairing — Innovate T5 plus CS995 — covers the full QC workflow at well under the cost of a single high-end European OES system. We routinely quote both as a package for new foundry customers.
Cast iron grade specifications
Quick reference for the most common iron casting grades and the carbon range each must hit:
| Grade | Carbon (%) | Silicon (%) | Typical use |
|---|---|---|---|
| Gray iron (GG / FC / Class 30) | 2.8 – 3.5 | 1.8 – 2.5 | Engine blocks, machine bases, brake drums |
| Ductile iron (GGG / FCD / 60-40-18) | 3.2 – 3.8 | 1.8 – 2.8 | Crankshafts, pipe fittings, valve bodies |
| White iron | 1.8 – 3.6 | 0.5 – 1.5 | Wear plates, grinding balls, hardened liners |
| Malleable iron | 2.0 – 2.6 | 1.0 – 1.5 | Pipe fittings, agricultural hardware |
| Compacted graphite iron (CGI) | 3.5 – 3.9 | 1.9 – 2.5 | Heavy-duty engine blocks, exhaust manifolds |
| Cast carbon steel (WCB) | 0.20 – 0.30 | 0.60 max | Valve bodies, structural castings |
| Cast low-alloy steel (WC6, WC9) | 0.10 – 0.20 | 0.60 max | High-pressure steam service |
Frequently asked questions
Can I use only an OES and skip the C/S analyzer?
For most cast-iron foundries running standard ductile or gray grades, yes. An OES handles the typical 2.5–4.0 % C range with sufficient accuracy. You need the dedicated C/S analyzer mainly for certification, very low-carbon steels, or very high-carbon hyper-eutectic irons.
Can I use only a C/S analyzer and skip the OES?
No — a combustion analyzer measures only carbon and sulfur. You also need silicon, manganese, phosphorus, copper, chromium, nickel and several others to qualify a casting. OES gives you all of those at once; combustion does not.
How often must each instrument be calibrated?
OES: re-check daily with a control sample, full recalibration every 1–6 months depending on use. Combustion C/S analyzer: re-check every shift with a CRM, full recalibration every 1–3 months. Calibration drift is slower for combustion because it is gravimetric, not spectral.
Which sample preparation does each technique need?
OES needs a flat conductive surface ground or milled to ~20–30 mm diameter, free of paint and oil. C/S analyzer needs a small chip or drilling (0.1–1.0 g) loaded into a graphite or ceramic crucible with tungsten / iron accelerator chips.
Is the answer different for steel vs cast iron?
Yes. For cast iron (carbon 2–4 %) OES is usually accurate enough for production; combustion is needed only for certification. For low-carbon stainless steel (<0.05 %) the combustion analyzer is essential because OES carbon measurement at sub-0.05 % needs careful calibration and dry argon.