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Functions of Carbon & Sulfur Analyzers

1. Significance of Carbon & Sulfur in Ferrous Materials

Carbon and sulfur are critical elements that determine the grade and quality of steel products.

Materials with carbon content above 1.70% are defined as cast iron; those below 1.70% are steel.
High-carbon steel: carbon content > 0.60%
Medium-carbon steel: carbon content 0.25% ~ 0.60%
Low-carbon steel: carbon content < 0.25%
Industrial pure iron: carbon content < 0.04%

Carbon exerts a decisive influence on steel properties:

As carbon content rises, steel hardness and tensile strength increase, while toughness and ductility decline. Conversely, lower carbon content reduces hardness and strength but improves ductility and toughness.

Carbon & sulfur analyzers are standard metering instruments for physical and chemical labs. They quantitatively detect carbon and sulfur in metallic and non-metallic materials, widely adopted in metallurgy, foundry, machinery, automotive, pump & valve, ore, environmental protection, quality inspection and other sectors. They support rapid testing for raw material incoming inspection, on-furnace analysis and finished product verification.

Sulfur impairs steel quality and deteriorates mechanical performance, corrosion resistance and weldability. When sulfur exists as iron sulfide (FeS) inside steel, its low melting point (~1000℃) causes hot shortness (red-shortness). Hot working at high temperatures will generate cracks, damaging product quality and service life. Therefore, sulfur content in steel should be kept as low as possible:

Ordinary steel: S ＜ 0.050%
Tool steel: S ＜ 0.045%
High-quality steel: S ＜ 0.020%

Given the vital impact of carbon and sulfur on steel quality and performance, carbon-sulfur analysis is of great practical importance.

2. Core Testing Principle

Carbon and sulfur in steel/stainless steel oxidize into CO₂ and SO₂ when combusted with oxygen at high temperatures (1200℃ ~ 1400℃), which forms the theoretical foundation of combustion-based carbon-sulfur testing.

Working principle overview:

Samples are fully combusted with oxygen in high-temperature furnaces (resistance tube furnace, electric arc furnace, high-frequency induction furnace, etc.), generating CO₂ and SO₂ gas. This separates carbon and sulfur from base metals and their compounds. The concentration of CO₂ and SO₂ is measured to calculate the mass percentage of carbon and sulfur in the specimen.

3. Main Analytical Methods

3.1 Infrared Absorption Method

Samples are heated to high temperature under oxygen-rich atmosphere to oxidize carbon and sulfur into CO₂ and SO₂. Purified gas flows into dedicated absorption cells to absorb characteristic infrared radiation. Detectors convert light signals into electrical signals, and a computer calculates test results.

Advantages: High accuracy, fast response, excellent sensitivity, suitable for both high and low C/S ranges.

Infrared carbon-sulfur analyzers feature high automation but relatively high cost, ideal for applications requiring strict analytical precision.

3.2 Volumetric Method

Common techniques: gas volumetry & non-aqueous titration for carbon; iodometry & acid-base titration for sulfur.

Gas volumetric carbon measurement combined with iodometric sulfur testing is the most widely adopted combined C/S detection method in China, balancing speed and precision.

Instrument performance: lower detection limit 0.050% for carbon, 0.005% for sulfur, meeting most industrial testing demands.

3.3 Gravimetric Method

Carbon test: Absorb CO₂ with asbestos alkali and calculate carbon content via mass gain.

Sulfur test: Wet chemical digestion oxidizes samples into sulfate; barium chloride is added under hydrochloric acid medium to precipitate barium sulfate. The precipitate is filtered, washed, ignited and weighed to calculate sulfur concentration.

Disadvantages: Slow testing speed, unsuitable for on-site industrial inspection.

Advantages: Superior accuracy; still recognized as standard reference method worldwide for standard laboratories and research institutes.

3.4 Conductometric Method

Features: Accurate, fast and sensitive; limitations include narrow measurement range and high reagent consumption, mostly used for low-carbon and low-sulfur samples.

3.5 Other Testing Technologies for Carbon & Sulfur in Metals

ICP spectrometry, optical emission spectrometry (OES), X-ray fluorescence (XRF), mass spectrometry, chromatography, activation analysis, each with unique merits and applicable scenarios.

4. Main Types of Carbon & Sulfur Analyzers Widely Used in China

Infrared Carbon & Sulfur Analyzer
High-Speed Carbon & Sulfur Analyzer
Non-Aqueous Titration Carbon & Sulfur Analyzer

Users may select models according to actual testing demands and the pros & cons of each instrument type.

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