hydrogen embrittlement relief for PH 15-5 H 925 2

[maya04]

Hi All
our drawing specified hydrogen embrittlement relief for PH 15-5 H 925 after passivation process.
the processor forgot this stage.
someone told me that for ph 15-5 after passivation in nitric acid you dont have to perform hydrogen embrittlement relief .
is that true? and why?

 

[israelkk]

When you heat treat PH 15-5 to H 925 there will always some colored heat tint on the surface and even if you do the heat treating in a high vaccum oven there still will be a thin transparent layer that needed to be removed before passivation. If the heat tint is removed by a chemical process before passivation then hydrogen embrittlement relief is required because the cleaning process produces hydrogen. If the surface were cleaned by a mechanical process such as ultra fine mesh aluminum oxide blast then the passivation process doesn't produce hydrogen and the relief is not required.

Alternative process is to machine the whole part after heat treatment thereby resulting in a clean part ready for passivation with no need for hydrogen relief. This is also prevent the dimension change of precision parts due to the shrinkage of the part as the result of the heat treatment.

 

[TVP]

You should review the requirements specified in SAE AMS 2759/9B Hydrogen Embrittlement Relief (Baking) of Steel Parts. Table 1 shows the Minimum Baking Time (Hours) at Standard Temperature of 375 °F (191 °C) and has two categories related to Chemical Treatments. One is called "Chemical Treatments Chem (3)" and one is called "Chemical Treatments Etch (4)" with the following descriptions:

(3) Chemical Processing: Etching, Stripping, Pickling, Chemical Milling

(4) Etch Inspection, without surfactant addition to etchant, to detect abusive grinding/machining

All PH stainless steels require a minimum of 3 hours baking for Chemical Processing like pickling or passivation. Baking is considered Not Applicable for all stainless steels after Etch Inspection.

 

[maya04]

dear israelkk and TVP
thank you for your answers
TVP,
Im familiar with the spec you have mentioned but Ive also heard that some aerospace companies follow what israelkk said.
indeed we cleaned the scale with alumina and then performed passivation.
israelkk
do you have any spec that you can address me to conforming what you said?

 

[israelkk]

maya04

I am not aware of a spec that addresses the processes I recommended. I came to this process as a result of 30 years of bad experience with endless false and failed promisses from passivators and heat treaters claiming they can do the job without harming the parts.

One more important issue is that the heat tint chemical removal is not a reliable controlled process that can remove a significant amount of base metal resulting in severe dimension loss no matter if the heat treatment was done in a vacuum oven or air oven.

I have encountered expensive precision parts with a dimension tolerance of 0.01 mm (0.0004") that lost 0.05 mm (0.002") after passivation because there was a chemical cleaning of the heat tint before passivation. Every effort to control the cleaning process failed. The end result was to machine the whole part after heat treatment.

A compromise process is to machine the part to final dimensions except precise dimensions and functional surfaces, heat treatment, mechanical cleaning (fine blasting, etc.), final machining/gringing of precise dimensions and finally passivation.

 

[unclesyd]

israelkk,
There is a method to remove the tint from PH SS parts without getting to the surface.
This is a slight modification of the standard alkaline permanganate (AP)+ Nitric/HF treatment given in most literature on PH steels.

Give the parts 45 minute soak in a 15% NaOH + 3% KMnO4 @ 200F. Thoroughly rinse and blow dry. Immerse in a 10% HNO3 + 2% HF bath at RT for no more than 30 seconds. Immediately power rinse with water or steam as the parts will appear unchanged because the oxide will still show. It requires a little force to remove.

The parts I clean using this procedure are 17/4 H1125 spinnerets with 1280 0.0090 holes. The holes are to have no surface breaking edge @ 100X.

There are several other process that take special equipment that will do the job.

 

[israelkk]

unclesyd

Are you claiming that using this cleaning method doesn't produce hydrogen and/or doesn't remove any base metal? What is the required tolerance on the 0.0090 hole? Can you guaranty that a part machined to a diameter of 1.000 mm, heat treated in air oven resulted with a heat tint on its surface will stay within 5.995 to 6.000 after removing the heat tint and passivating?

 

[unclesyd]

One point I forgot to mention is that you don't let the part dry down while rinsing the oxide off.

Don't know about the adsorption of H2 but on the other aspects there is no measurable effect on the base metal if all the oxide is heat tint. If the oxide is generated by other means there will be base metal loss based on how the scale was formed and the amount of base metal it took to form it.
I will guarantee this procedure will remove heat tint and there will be far less loss than your tolerance of 0.005.

We worked in the range +0.0 - 0.00005" and we measure to 0.000002" on a broach we ground with a diameter of 0.0090" +0.00000" - 0.00001". Our broach had a way better tolerance than we needed but our process produced these tolerances. I can't recall the exact tolerances on the hole diameter but it was formed by the above broach. The holes were then plugged and lapped to achieve the required edge condition. The plug was ground on the same machines and it's tolerances were the same or in most cases better as it was straight and the broach was a tear drop shape.

We never passivated any 17/4 and only cleaned these components and a few others. On most components going into a service with a possibility of any corrosion we would leave the heat tint and force a heavier oxide by steam/air oxidation prior to exposure.