FC200 (JIS G5501) is Japan’s most common gray cast iron grade — the material in machine tool beds, engine blocks, brake rotors, and pump housings that has been enabling industrial production for over a century. Its graphite is in flake (lamellar) form: a microstructure that makes FC200 inherently brittle in tension but uniquely good at damping mechanical vibration, resisting thermal shock, and machining cleanly. The “200” designates the minimum tensile strength in MPa, which is the weakest direction — compressive strength (600–700 MPa) is 3–4× higher, and it is compressive loading that most machine bases actually experience. Understanding this asymmetry between tensile weakness and compressive strength is the key to using FC200 correctly: it is not a poor-quality steel substitute; it is a specifically engineered material for applications where vibration damping and compressive loading dominate.
- JIS FC Grade Family & International Equivalents
- Chemical Composition
- Mechanical Properties
- Vibration Damping: The Key Property Advantage
- Machinability
- Heat Treatment Options
- Common Mistakes
- When to Choose FC vs FCD
- FAQ
1. JIS FC Grade Family & International Equivalents
| JIS Grade | ASTM A48 | EN (GJL) | Min Tensile (MPa) | Typical Application |
|---|---|---|---|---|
| FC100 | Class 20 | GJL-100 | ≥ 100 | Non-structural castings, decorative |
| FC150 | Class 25 | GJL-150 | ≥ 150 | Light housings, covers |
| FC200 | Class 30 | GJL-200 | ≥ 200 | Machine tool beds, engine blocks, pump housings |
| FC250 | Class 35 | GJL-250 | ≥ 250 | Heavy machine bases, cylinder blocks |
| FC300 | Class 45 | GJL-300 | ≥ 300 | High-pressure valves, gears |
| FC350 | Class 50 | GJL-350 | ≥ 350 | Heavy-duty engine components |
Note: JIS G5501 specifies grades by minimum tensile strength. ASTM A48 classes and EN GJL grades do not map exactly to JIS grades — Class 30 (ASTM) is closest to FC200 in tensile requirement, but composition ranges differ by standard. Always verify against the actual specification for critical applications.
2. Chemical Composition
JIS G5501 specifies FC grades by tensile strength only — it does not mandate specific composition ranges. The following represents typical foundry practice for FC200:
| Element | Typical FC200 | FC250 (comparison) | Role |
|---|---|---|---|
| C (total) | 3.0–3.6% | 2.9–3.3% | Forms graphite; higher C → more flake graphite → better damping, lower tensile strength |
| Si | 1.4–2.5% | 1.2–2.0% | Promotes graphite formation (graphitizer); controls carbon equivalent |
| Mn | 0.5–0.9% | 0.6–1.0% | Promotes pearlite matrix; partially offsets Si graphitization |
| P | ≤ 0.25% | ≤ 0.15% | Improves fluidity but reduces toughness — minimized in structural grades |
| S | ≤ 0.12% | ≤ 0.10% | Promotes flake graphite; excessive S reduces machinability |
Carbon Equivalent (CE) is the key composition parameter for gray iron: CE = C + (Si + P)/3. FC200 typically has CE = 3.7–4.0. Above CE ~4.26 (eutectic), the iron is hypereutectic — more graphite, better damping, lower tensile strength. Below ~3.8, iron tends toward harder, stronger, but more wear-resistant microstructure.
3. Mechanical Properties
| Property | FC200 | FC250 | Notes |
|---|---|---|---|
| Tensile strength (min) | 200 MPa (29 ksi) | 250 MPa (36 ksi) | Weakest direction — tension opens flakes |
| Compressive strength | 600–700 MPa (87–102 ksi) | 700–800 MPa | 3–4× tensile strength — the governing property for machine bases |
| Bending strength | ~350–450 MPa | ~400–500 MPa | Used for beam loading calculations |
| Elongation | < 0.5% | < 0.5% | Brittle — no plastic deformation before fracture |
| Hardness | 170–230HBW | 180–250HBW | Section-dependent; varies with cooling rate |
| Elastic modulus | ~100–145 GPa (14,500–21,000 ksi) | ~100–145 GPa | ~50% of steel — stiffer designs needed |
| Thermal conductivity | ~50–55 W/(m·K) | ~50 W/(m·K) | Higher than steel; good for heat dissipation |
4. Vibration Damping: The Key Property Advantage
Gray iron’s vibration damping capacity — the ability to absorb vibrational energy and convert it to heat — is 10–20× higher than structural steel. This property determines why machine tool beds, lathes, milling machines, and grinding machine bases are almost universally made from gray cast iron rather than welded steel even when steel would be structurally adequate:
- Graphite flakes as damping mechanism: Under vibration, the graphite flakes flex and their interfaces with the iron matrix dissipate energy through friction. The specific damping capacity (SDC) of gray iron is 20–30%; steel is 1–2%. This means gray iron absorbs 10–15× more vibrational energy per cycle
- Effect on machined surface quality: A lathe bed that damps vibration produces less chatter — the periodic variation in cutting force that creates surface waviness. The same cutting operation on a steel frame bed with equivalent stiffness produces measurably worse surface finish. This is quantifiable: gray iron beds reduce surface roughness Ra by 30–50% compared to equivalent steel at identical cutting conditions
- Thermal stability: Gray iron’s lower elastic modulus (100–145 GPa vs steel’s 200 GPa) and high thermal mass make it more dimensionally stable during thermal cycling in machine tool operation — important for maintaining geometrical accuracy during warm-up
5. Machinability
FC200 is one of the most easily machined engineering materials. Machinability index ~110–130 (vs AISI 1212 free-cutting steel = 100). The reasons:
- Graphite as solid lubricant: Graphite flakes exposed at the machined surface act as dry lubricants, reducing tool-workpiece friction
- Chip fragmentation: Flake graphite acts as internal notches — chips break into short powder-like fragments rather than continuous ribbons. No chip entanglement, no coolant needed for chip control
- Dry machining is standard: Gray iron is typically machined dry or with minimal air cooling. Coolant can cause thermal shock cracking in thin sections
Carbide cutting tools are standard. Coated carbide inserts at cutting speeds of 150–250 m/min for roughing; 200–300 m/min for finishing. Tool life is excellent — gray iron is significantly less abrasive than high-silicon aluminum alloys or reinforced polymers.
6. Heat Treatment Options
Gray iron heat treatment is limited compared to steel — there is no through-hardening via martensite transformation (the high C content creates brittle white iron, not martensite, if quenched rapidly).
| Treatment | Temperature | Purpose | Effect |
|---|---|---|---|
| Stress relief anneal | 500–600°C (932–1112°F), slow cool | Relieve casting residual stresses before precision machining | Reduces distortion during machining; no hardness change |
| Flame / induction hardening | Surface to 900–950°C, water quench | Surface hardening of wear surfaces (ways, guides) | Surface 50–60HRC; core remains soft; depth 1–3 mm |
| Graphitizing anneal | 700–760°C, slow cool | Convert hard white iron zones (chilled areas) to gray iron | Reduces hard spots; improves machinability of problem areas |
| Normalizing | 850–900°C, air cool | Increase pearlite content; raise hardness and strength | +20–30 MPa tensile, +10–20HBW; reduced ductility |
Machine tool beds and slides are routinely stress-relief annealed at 550°C before precision surface grinding — eliminating the dimensional change that would occur if residual casting stresses were released during machining. This step is critical for maintaining slideways to ±2 μm flatness tolerances.
7. Common Mistakes
8. When to Choose FC vs FCD
Vibration damping is the primary performance requirement: machine tool beds, lathes, mills, grinding machines, press frames. When the dominant loading is compressive rather than tensile or cyclic-bending: machine bases, housings, covers under static compressive load. When excellent machinability is required without coolant: automotive cylinder blocks, engine blocks, brake rotors. When cost is critical and mechanical strength requirements are moderate.
Tensile loading, cyclic bending, or impact loading is present: crankshafts, connecting rods, gears, axle housings, agricultural equipment. When welding or secondary heat treatment is required. When elongation above 1% is needed. FCD’s 10–22% elongation versus FC’s <0.5% is the critical distinction — in any application where the component must deform plastically before fracture, FCD is mandatory.
Full weldability is required (cast iron welds are difficult and unreliable for structural repairs). High tensile strength (> 350 MPa in tension) combined with good ductility. Components that will be heat treated after casting for enhanced mechanical properties. Critical structural castings where sudden fracture cannot be tolerated and plastic deformation before fracture is required by design code.
9. FAQ
Q: Why are machine tool beds made from cast iron rather than welded steel?
Both would be structurally adequate, but gray cast iron provides 10–20× better vibration damping capacity than structural steel. This translates directly into better surface finish on machined parts (less chatter), lower tool wear from vibration, and better geometric accuracy during thermal cycling. The cost difference (cast iron beds are more expensive to produce than welded steel frames) is accepted because the performance difference is measurable and significant.
Q: Can FC200 be welded?
With difficulty and precautions. The high carbon content produces a hard, brittle heat-affected zone and promotes white iron formation during rapid solidification. Welding requires: preheat to 300–400°C, nickel-alloy filler rod (ENi-CI or ENiFe-CI), controlled interpass temperature, and post-weld stress relief at 550°C. For structural repairs, welding is feasible. For new designs requiring weldability, cast steel (SC grade) should be specified instead.
Q: What is the difference between FC200 and FC250?
Composition and section size. FC250 requires lower Carbon Equivalent (thinner graphite flakes, finer pearlite matrix) to achieve 250 MPa tensile versus FC200’s 200 MPa. In practice, the same iron poured into different section sizes will meet different grade requirements — a thin section produces harder, stronger iron (faster cooling = less graphite precipitation) while a thick section produces softer, more graphitic iron. The grade distinction reflects both composition control and section thickness in the actual casting.
Summary
- FC200 (JIS G5501) = ASTM A48 Class 30 = EN GJL-200: flake graphite gray iron, ≥ 200 MPa tensile, ≥ 600 MPa compressive
- Vibration damping 10–20× better than steel — the defining property for machine tool beds, not an incidental benefit
- Brittle in tension (< 0.5% elongation) — design must keep tensile stresses below 200 MPa; use for compressively loaded structures
- Excellent machinability: graphite flakes provide chip breaking and lubrication; dry machining is standard
- Stress-relief anneal at 550°C before finish machining is standard practice for precision castings
- For tensile/fatigue-loaded parts: FCD (ductile iron); for weldable castings: SC (cast steel)
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