Ⅰ.The role of carbon element on the performance of stainless steel mechanism 

   1.1Double effect on austenite stability

When the carbon content exceeds 0.08% (the upper limit of the 304L standard), carbon atoms form Cr23C6-type carbides with chromium. This precipitation phase causes the chromium content at the grain boundaries to drop below 12%, destroying the continuity of the passivation film and decreasing the corrosion resistance by 40%-60%. However, a moderate amount of carbon content (0.03%-0.08%) can improve the strength of the material, for example, 316L steel grade carbon content increased by 0.01%, tensile strength increased by about 15MPa.

    1.2Enhanced phase transition temperature sensitivity

Carbon content exceeds the standard will make the critical temperature interval of solid solution tratment narrow, when the smelting process temperature fluctuations of more than ± 15 ℃, prone to δ ferrite segregation, resulting in the impact toughness reduced to the standard value of 50% -70%.

A petrochemical project has been due to the carbon content of 0.12% of 317L steel pipe at -20 ℃ environment of brittle fracture, direct economic losses of more than 3 million yuan!

Ⅱ.Empirical Effects of Excess Carbon Content on Physical Properties

   1.Mechanical properties parameter change

Target	Standard Values（C≤0.08%）	Exceed Standard Values（C≤0.12%）	Performance Decay Rate
Strength of Extension(MPa)	520-670	580-720	+8%
Tensile Elongation(%)	≥40	28-32	-30%
Hardness(HRB)	≤95	102-108	+12%
Fatigue Life	1×10⁷	3×10⁶	-70%

          2.Typical failure case analysis 

• Intergranular corrosion accident: an offshore platform used 2205 duplex steel pipe with 0.10% carbon content, penetrating cracks appeared after 18 months of service in Cl-containing media, and the repair cost was 5 times of the cost of the pipe.

• Cracking defects in cold bending: cross-border e-commerce customer feedback from the return case shows that the carbon content of 0.09% of the 304H pipe in the bending radius of 3D processing cracking probability of 3 times more than the standard products

Ⅲ.Key nodes for carbon control in the production process

      1.Raw material melting stage

Adopting AOD refining technology to control the carbon content in the range of ±0.005%.

Spectral analysis (PMI test) is performed in each furnace, and a melting number traceability system is established.

 

      2.Hot processing control points

Keep the rolling temperature at 1050-1150℃ to avoid abnormal precipitation of carbide.

Cold bed cooling rate control at 15-25 ℃ / s, to prevent secondary carbonization.

Test Item	Detection Method	Control Standards	Test Frequency
Carbon Content	Direct-Reading Spectrometer	≤0.08%	Per Heat
Carbide Distribution	Metallographic Microscope	Carbide Size ≤5μm	Sampling 10% per lot
Intergranular Corrosion	ASTM A262 Practice	Flexure Without Crack	Monthly Complete Inspection

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