1. Chemical Composition Tests
Magnet Test (Preliminary Check):
Most stainless steels (e.g., 304, 316) are austenitic and non-magnetic in their annealed state, but some (e.g., 430, 410) are ferritic/martensitic and strongly magnetic.
Inconel alloys (e.g., 600, 718) are consistently non-magnetic due to their high nickel content (typically 50% or more) and alloying elements like chromium and niobium, which stabilize a non-magnetic structure.
Note: This is not definitive for austenitic stainless steels (e.g., 304), which are also non-magnetic, but it can rule out magnetic stainless steels.
2. Corrosion Resistance Tests
Expose a small sample to high heat (e.g., 800–1000°C) in air for 1–2 hours.
Stainless steel (e.g., 304) will form a thin oxide layer but may start to scale or discolor significantly, especially at temperatures above 800°C.
Inconel (e.g., 600, 718) resists oxidation and retains its surface integrity even at these high temperatures due to its high chromium and nickel content, which forms a stable, protective oxide film.
Submerge samples in a strong chloride solution (e.g., 10% NaCl) or dilute acid (e.g., sulfuric acid) for several days.
Stainless steel (e.g., 304) may show pitting, rust, or corrosion, especially in high chloride environments.
Inconel is highly resistant to pitting and general corrosion in such conditions, with minimal surface damage.
3. Mechanical Property Tests
Use a hardness tester (e.g., Rockwell or Vickers):
Stainless steel 304 typically has a hardness of ~70–90 HRB (Rockwell B) or ~150–200 HV (Vickers).
Inconel 718 (heat-treated) has a much higher hardness: ~40–45 HRC (Rockwell C) or ~380–420 HV, making it noticeably harder to scratch or indent.
At elevated temperatures (e.g., 600°C), Inconel retains much higher tensile strength and creep resistance. For example:
Stainless steel 304 loses significant strength above 300°C, with tensile strength dropping to ~300 MPa at 600°C.
Inconel 718 retains tensile strength of ~800–900 MPa at 600°C, making it far more resistant to deformation under heat and stress.
4. Machinability and Spark Test
Inconel is extremely difficult to machine due to its high work-hardening rate, low thermal conductivity (causing tool overheating), and hard intermetallic phases. Cutting Inconel generates minimal chips and quickly dulls tools.
Most stainless steels (e.g., 304) are easier to machine, with better chip formation and less tool wear, though they still work-harden more than carbon steel.
When ground with an abrasive wheel, the spark pattern differs:
Stainless steel produces short, less bright sparks with few branches (due to its high chromium content, which suppresses spark intensity).
Inconel produces even shorter, dimmer sparks (or almost no visible sparks) because of its high nickel content, which further reduces spark formation.
5. Density Check
Density Values:
Stainless steel (e.g., 304) has a density of ~7.9 g/cm³.
Inconel (e.g., 718) is denser, with a density of ~8.19 g/cm³.
Note: This difference is small and requires precise measurement (e.g., using a balance and displacement method) to detect.
6. Certification and Labeling
Stainless steel will list iron as the primary element (60–75%).
Inconel will list nickel as the primary element (50–55% for 718).
