Soft Spots in Carburized Product

[kaufmj42]

thread330-12001

Hello all! I'm currently experiencing issues with soft spots on carburized producted due to surface masking (confirmed by metallographic examination, hardness testing, carbon analysis, etc.) - similar to that described in the referenced thread above.

Some background - the product is machined with a water-based coolant, and is run through a pre-heat before carburization, and is not stacked. The process is well monitored at all stages, and records show that temperatures, gas concentrations, etc. were all spot-on when product in question was run. According to the company producing the coolant, our preheat temperature is sufficient to burn off coolant.

I've been able to replicate the phenomenon by intentionally applying machining coolant to the surface of product prior to carburization. While I'm not ruling out other potential causes of these soft spots, I feel pretty confident that this is something that needs further examination. Which brings me to my question: In the time since the ban of solvent-based coolants, what has your experience been with water-based coolants masking carburized product, and what have you found to be successful in mitigating the phenomenon?

For what it's worth, I've been considering the following:
-Increased pre-heat temperatures/time to burn off any reaction product that may form if coolant sits on the surface for a while
-Chemically cleaning/rinsing product prior to carburizing
-Adding in-line machining coolant monitoring systems to better control the coolant itself

Any and all feedback is appreciated. Thanks!

 

[swall]

I think you nailed it--the coolant is the problem. I went through this once with some coolant that contained chlorine. Don't know if the chlorine was the culprit or something else in the coolant, but switching to a different coolant solved the problem. Running the parts through a caustic washer seemed to help at times, but only the coolant switch made the problem go away.

 

[jsokal]

Make sure there is no silicon anywhere in your process especially in the machining coolant. Silicon will cause soft spots and does not burn off in the furnace, quite the opposite in fact.
If your parts are semi fininshed, a light zirconite blast will usually do the job of removing the conamination off the surface. If you are using any stop-off paints be carefull with the application. If it is applied too thick, a skin forms and the inside remains liquid. When heated in the furnace it bursts spraying the paint on the gear surfaces.

John

Denial ain't just a river in Egypt...Mark Twain

 

[kaufmj42]

Thank you both for the responses.

Swall - do you mind sharing with me what coolant you've had success with? We're currently using a coolant that we're not tremendously satisfied with (giving us poor tool life, expensive, etc.) and I wouldn't mind running trials with new coolants.

jsokal - I'm not certain what the composition of our coolant is, but I'll be talking to the producer here shortly to try and get a handle on it. If I find anything interesting I'll repost and share.

 

[swall]

kaufmj42--I think it was Kleen-Kool, but that was 15 years ago. I also don't remember which type it was. The coolant change was made at a machining vendor of ours based on the sucessful use of the product by another one of our machining vendors. The coolant we used in house was Castrol Syntillo and we never had any spotty case problems with it. For some reason the switch to Kleen-Kool was easier for the machining vendor to accomplish than switching to the Castrol product.

 

[rizzy77boy]

I agree with everything said so far. One other problem can be a contamination of the coolant itself (hydraulic fluids from the turning machines, lubrication oils, etc.) Look closely for leaks surrounding the turning equipment. Finally, we have also found it worthwhile to clean after machining as soon as possible. In other words, it seems that some of these contaminates are very difficult to remove if they are allowed to dry for long times. We try to limit storage to less than a week. Still, we use both pre-carb burn-off AND alkaline washing to fight the uneven carburizing problem.