Passivation of 440C 2

[cooblacrouse]

I have currently run into a problem with passivation of 440C. We have a customer who requires this material and passivation per QQ-P-35,Type II. In the spec there are warnings about possible etching when passivating 440C. We had one lot of parts processed without a problem. The next lot came back etched. The vendor says that there is no way to guarantee 440C in this process. The mix of the bath in both lots were in the middle of their tolerance, according to the spec.

Does anyone know if there is a way to control this process better to avoid the etching problem?

Thank you,
Bob

 

[kenvlach]

Yes, better quality control of the heat treatment.

440C (16.0-18.0 wt% Cr, 0.95-1.20 C) contains enough carbon to tie-up all of the chromium as Cr₂₃C₆, leaving a non-stainless metal matrix. This metal will etch during passivation, leaving the dark carbide on the surface. Hence, it is essential to very rapidly quench 440C from the austenizing (solutionizing) temperature in order to maintain enough Cr in the metal phase. See thread330-50491 for essentially the same issue.

Also, an improper atmosphere during heat treatment can ruin the material. MIL-H-6875H requires a neutral, inert or vacuum furnace atmosphere (neither oxidizing nor reducing). Oxidation is frequently a problem, depleting the metal surface of Cr.

Passivation, like many other processes, obeys the following maxim: Garbage in, garbage out.

 

[cooblacrouse]

Kenvlach

Thank you for your response.

We currently do the following process to the material.

1. MATERIAL: 440C ANNEALED SST PER AMS 5630
2. ROUGH MACHINE
3. STRESS RELIEVE FOR 4 HOURS ±10 MIN AT 1100°±10°F
4. FINISH MACHINE

The furnace is a circulating air type.
We do not need the material for it's hardenability but purely for similar material as the bearing it is pressed onto. I thought the warning for the austenizing was in the 900 deg range. You are saying that if I adjust my heat treatment to include a rapid quench and an inert environment I could avoid etching?

Bob

 

[kenvlach]

The 440C is etching because the anneal allowed lots of Cr₂₃C₆ formation in the grain boundaries. This alloy has its best corrosion resistance in the hardened condition. Solutionizing at high temperature (1850-1950°F) will dissolve the carbides and allow diffusion throughout the grains, then rapid quenching will pretty much preserve the compositional distribution. Depending upon the part size & geometry, oil quenching may suffice. Tempering/stress relief at 300-800°F is advised, depending upon the hardness desired (ASM Metals Reference Book, 3rd Edn., pp. 375-6); sensitization can occur at 800-1100°F.
See Thread330-50491 for more heat treatment details from metrat and maui.