Powder coating changing D2 toolsteels property?
Powder coating changing D2 toolsteels property?
(OP)
We powder coated some D2 Tool steel parts in two batches. The two batches were both heat treated to a hardness of 60-62 HRC. Both batches were double tempered at 400F. However the 2nd batch's fracture force dropped by almost 25% after painting, when the first batch did not drop significantly. We did various tests at the paint shop and believe we have eliminated the iron phosphate rinse as the culprit AND the tests would indicate that the baking WITHOUT the paint is not the cause which leaves me to question the "facts" seemingly demonstrated by our tests. If they are credible, then I would conclude that the cause is a chemical reaction with the paint and subsequent baking (we cannot test painted clips without baking because the paint would not cure).
The powder coating process on the D2 Tool steel consists of the following
- Bath in an iron phosphate solution 120F
- Fresh water rinse
- spray the polyester powder using an electrostatic gun
- Baking at 400F after powder coat (temp. can drift up to 425F) anywhere from 10 to 30 minutes
We had a couple of questions about the problem we've been having
What happens when the temperature if the oven drifts up to 425 F which is higher than clip's tempering temperature?
Could it be the powder chemically reacting with the D2 Tool steel at 400-425F causing the force drop?
What could be causing this inconsistency in fracture force drop?
Do you have any other ideas what might be causing this drop in fracture force?
The powder coating process on the D2 Tool steel consists of the following
- Bath in an iron phosphate solution 120F
- Fresh water rinse
- spray the polyester powder using an electrostatic gun
- Baking at 400F after powder coat (temp. can drift up to 425F) anywhere from 10 to 30 minutes
We had a couple of questions about the problem we've been having
What happens when the temperature if the oven drifts up to 425 F which is higher than clip's tempering temperature?
Could it be the powder chemically reacting with the D2 Tool steel at 400-425F causing the force drop?
What could be causing this inconsistency in fracture force drop?
Do you have any other ideas what might be causing this drop in fracture force?





RE: Powder coating changing D2 toolsteels property?
What is the part configuration?
RE: Powder coating changing D2 toolsteels property?
RE: Powder coating changing D2 toolsteels property?
I would expect this softening from "ovetempered" martensite to produce slightly higher impact results certainly not 25% lower.
TVP's suggestion of possible H2 embrittlement is much more probable.
RE: Powder coating changing D2 toolsteels property?
We tested both lots of clips after heat treat and before the powder coating process. Test results at that point were as expected. It was only after the powder coating process that the fracture force drop was evident and seemingly not on every batch. That is why we undertook testing after the iron phosphate rinse, after rinse and baking (with no powder coating) and the fully processed part. Based on this the conclusion that the powder coating/ baking part of the process is the factor affecting the material property. We had considered hydrogen embrittlement as a possible problem but tended to discount it given our second shop tempering trials on unpainted, heat treated material with no evident change in fracture force.
RE: Powder coating changing D2 toolsteels property?
I can't recall questioning any physical properties of the Heat rated product. We did do a little work with the heat treated directional properties of some shapes and there were a few minor difference in the traverse and longitudinal sections, but not enough to bother us. We also ran some reduced section Chappy Impacts on 2" thick end plates from older pumps that that cycled several hundred time.
You problem could be in the Heat Treat, but D-2 is very forgiving or whether the parts take from traverse or longitudinal section. Check how your parts are stacked for heat treating.
RE: Powder coating changing D2 toolsteels property?
RE: Powder coating changing D2 toolsteels property?
I would be interested to know what going from 2 to 9 tempers does to the physical properties in your view. I would expect pretty much full transformation of any retained austenite which means more consistent behavior under stress. Beyond that?
As far as loading of the parts, we have experienced with heat treat shops all of whom ignore the requirements for the heat treat process. Currently we have just found out that our parts are stacked on edge with the shop's assumption that, that will have no detrimental affect on our part although they had agreed at the outset to hang the parts on moly rods. But because our parts are thin, i suspect this will cause more long lines of distortion rather than uneven heat soak.
Yes, propagation of micro cracking is a possibility but based on previous SEM, we do not believe this to be a case here. We come to the conclusion based on the fact this happens to a complete painted batch whereas micro cracking will be unlikely to be reflected in a consistent manner- that is the influence of micro cracking would cause a inconsistent results within a given batch.
RE: Powder coating changing D2 toolsteels property?
The goal was zero growth during a cyclic operation at 600°F and occasional cleaning with exposure to 1000°F for several hours.
At the time when it came to retained Austenite it was the considered the same as the rabbit and hair problem. You convert 90% the first pass and 90% of the remainder and so on until the material will be stable at temperature. Remember that there was no LN2, cold boxes, and Swiz mountain tops close buy. If my recall is any good 5 tempers would get you by where the clearance's were at 0.0005", but to get to where there we no problems with smaller pumps with 0.0001" clearance ti took 7 or 8 tempers. Yes Zenith could grind this close and we could measure this close using Air Gauges and Micrometers from Curtis Wright.
When LN2 came along we ran numerous tests to verify any new heat treat procedure for these particular parts.
Remember at the time we were limited to 1000x optical microscopy as the SEM wasn't around, There was not any sophisticated electronic or ultrasonic measuring devices either. It was essentially heat and measure. What measurements were accomplished with what is now called a differential screw micrometer and some air gages.
I just checked and what few of the small pumps still in use some have serial number that they were manufactured in 1950s and are still in spec.
Another thing today the metallurgy is better and and the limits are tighter. As we always tempered at secondary hardening peak on the tempering curve we found that the Bethlehem D@ would consistently be 3 points that others.
RE: Powder coating changing D2 toolsteels property?
I agree that H2 should be investigated.
RE: Powder coating changing D2 toolsteels property?
RE: Powder coating changing D2 toolsteels property?
You previously referred to this part as a 'clip', mentioned rectangular 'sheet', and said the parts are 'thin'. A rolled sheet can have mechanical texture: highly directional properties. Are all of your clips oriented in the same direction wrt the sheet from which they are cut? That is one of the points made above by Etarney. If not, maybe the source of your problem is at hand. Perhaps the painted batch had more Ts than Ls. (?)