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Ted Mooney, P.E. RET
Pine Beach, NJ

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  for Metal Finishing 1989-2024
  

Testing passivated 440

Hello all. I am an expeditor at an aerospace machine shop. We have parts made out of 440-c that we send out for passivation (ASTM A380 ). According to this spec it must be tested to method 100 or 101 (humidity test or water immersion test). Our processor says if we humidity test these parts they will fail. He could not give me a clear answer as to why they would fail. Does anyone have some insight on this matter?

Due to alloy chemistry variations (C near upper end of its 0.95-1.20 wt% range, Cr near lower end of its 16.0-18.0 range) and improper heat treatments, this alloy can become non-stainless. The Cr content of the metal phase may fall as low as 4 wt% Cr (with most Cr in carbide phases). In such cases, even properly passivated parts may fail QC tests. QQ-P-35C allows exempting 440C from the tests. However, a supplemental chromate treatment can be specified for some improvement. This treatment is given in QQ-P-35C & ASTM A967 as well as in footnote D of Table A2.1 of ASTM A380 .

Search this site for 440C passivation. Letters #19870 (suggests Type VIII per QQ-P-35C), 43504 (covers heat treatment), and 44383 (suitable replacement alloys) are just a few of many.

For further insight, see the C-Cr-Fe phase diagrams in ASM Handbook Volume 3 [taken from G.V. Raynor and V.G. Rivlin, Phase Equilibria in Iron Ternary Alloys, The Institute of Metals, London, (No. 4), 1988] or the heat treatments for martensitic stainless steels in ASM Handbook volume 4.

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