Rockwell (macrohardness) topic [split]
07-19-2007, 05:44 PM,
#1
Rockwell (macrohardness) topic [split]
Gordon,

Further the discussion of the Rockwell (macrohardness) topic, I will like to ask, if my metallorgical evaluation jhas passed all the micro structure, like unmelt, oxide level, proroxiy...etc.. but fail on rockwell hardness, lower than the requirement by ~10%, What should I do to increase it assuming adjusting the plasma gasses flow rate as my last option to consider?

This is because the rest of the attributes or coating characteristic are meeting the specs, I do not want to take the risk of increasing the flow rate and that may change the coating outcome and invite defects on other area.

1. How could I increase the particle velocity and increase dentify of coating for plasma spray? (Chrome Carbide Coating).

2. How significant the coating thickness contribute to the rockwell hardness failure? If I have already spray to min 0.015" of thickness? in this case spraying t to 0.020 or more, would it help?

3. How would oxide level affect the rockwell hardness as the indentation is done on the coating surface? Should I back-off the stand-off distance to increase oxide level which should be within the allowable limits to increase hardness reading?

4. If I have to adjust the plasma jgasses flow rate, how to balance the primary & 2nd gas ratio? or I should try only increase primary gas only?

Pls advice.

Thanks

Regards,
Alexangel1226
Reply
07-24-2007, 02:53 PM,
#2
RE: Rockwell Hardness-15Y
Hi Alex

Very difficult to answer your questions in a generic manner as not all coatings respond in the same way.

Generally, your microstructure, particularly density should be reflected in your hardness readings (well most of the time Happy0193)

Quote:1. How could I increase the particle velocity and increase dentify of coating for plasma spray? (Chrome Carbide Coating).
This is a bit of a balancing act between the many variable parameter settings. One way would be to change to a narrower bore nozzle and use so called high energy plasma HEP parameters (not really significantly higher thermal energy, but kinetic). Without hardware changes, and assuming it is even possible to optimise the coating density or perhaps hardness, I would start by increasing primary plasma gas flow only, by small amount and evaluate effect. Then possibly this change would need to be balanced with increases in secondary gas to increase heating to offset reduced particle dwell time. As I'm sure you know it is very complicated, as you will probably also need to consider re-optimising things like powder injection and spray distance etc..

Quote:2. How significant the coating thickness contribute to the rockwell hardness failure? If I have already spray to min 0.015" of thickness? in this case spraying t to 0.020 or more, would it help?
If you are well above the recommended minimum thickness for the particular hardness test, then the actual thickness should have little effect on your readings. With coatings that are too thin, the substrate then influences the hardness reading.

Quote:3. How would oxide level affect the rockwell hardness as the indentation is done on the coating surface? Should I back-off the stand-off distance to increase oxide level which should be within the allowable limits to increase hardness reading?
Oxide levels can effect hardness, but it does depend on the coating material and nature of the oxides formed. Pure metals generally show increasing hardness as oxide content rises, but not necessarily with coatings containing carbon and other easily oxidised hardening agents. Also sometimes oxides can weaken coating cohesion and reduce hardness (note wear resistance and hardness do not necessarily equate in these conditions).
You could try changing stand-off distance from one extreme to the other within your allowable limits. Not sure whether you will see any benefit. Oxide content is not the only factor, increasing spray distance could in fact reduce hardness further, via less density and poorer coating cohesion counteracting oxide effects.

Quote:4. If I have to adjust the plasma jgasses flow rate, how to balance the primary & 2nd gas ratio? or I should try only increase primary gas only?
My advice would be to only change one thing at a time until know what effect you are getting. Usually increasing primary gas will give minor increase in power level, but overall effect seems to be cooling of particles (increased velocity, lower dwell time), may need to be balanced with higher secondary gas, which probably also increases velocity, but has a more pronounced effect on particle heating. Don't forget other parameters may also need to be balanced with these changes. Taguchi and other experimental design study statistical methods for achieving optimised coatings Happy0193 well I'll leaves that for another discussion.
Reply
07-24-2007, 03:05 PM,
#3
RE: Rockwell (macrohardness) topic [split]
Hi Alex

Hope you don't mind me splitting the thread you originally posted to, but I thought this deserved a thread of it's own.

Split from http://www.gordonengland.co.uk/sef/rockw...t-209.html
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07-31-2007, 11:18 AM,
#4
Big Grin  RE: Rockwell (macrohardness) topic [split]
Gordon- Thanks. The spliting is great! and thanks for the advices. I am trying it and see if it works. Thanks again.
Reply
08-16-2007, 04:11 PM, (This post was last modified: 08-24-2007, 11:22 AM by Alexangel1226.)
#5
RE: Rockwell (macrohardness) topic [split]
Gordon,

this is a tough ones.. tried many times, dun work still.. I am using the extension gun S.Metco 7MT gun with standard nozzle etoc 718 nozzle To spray an internal ID of ~5.5". I am spraying T-400 coating (triballoy) at min 0.015" to 0.020". I have met the rest of the characteristic.

But too bad my hardness is alwys @ ave 73~78 vs the requirement of Rockwell (R15N scale) of ave 80min. Pretty close but not there yet.

Primary gas had been tried from 65lpm 75, to 80lpm, H2 from 3.0 to 5.0lpm..idstance @ 3.5"(perpendicular to substrate). I find that the &MT gun spraying angle is weird @~65deg.., Current @ 375A..and Volage manage to get 70~80V.

Not sure what is the prob now.

1. I would like to ask, if the hardness is performed with the bond coat of Metco 443 @ 0.005". would it affect the hordness reading? The specimen is a bond and top coating, of 443 & T400, but the hardness req of 80 min is actually only for T-400 deposited onto a substrate? I have not yet verify this. Will try out by spraying only T400 and see if it will pass.

2. theoritically, would the spraying angle of 65deg compared to 90deg, give a significant difference on hardness? any smaller bore nozzle for this extension gun of 7MT to recommend?

thanks. Pls comment.

regards,
alexangel1226
Reply
08-16-2007, 06:48 PM,
#6
RE: Rockwell (macrohardness) topic [split]
Hi Alex

First, before you start banging your head against a brick wall with parameter development, check your hardness testing procedure. To get an accurate HR15N reading at around the hardness levels you are looking at will require a minimum of around 0.015" coating thickness of T400 (that's after surface prep and does not include thickness of bond coat). If you are including the thickness of your 443 bond coat in your 0.015-0.20" thickness, then it is possible that the softer 443 is reducing the hardness reading. As you have indicated, the best way to verify this is to spray a thicker T400 on a test piece and test it.

Are the specifications/standards that you are working to specifically for internal diameters/ plasma extension guns/specific component or a generic T400 coating that you might spray normally through a standard 9MB gun? To some extent you should expect some deterioration in coating properties using angled spraying with extension guns in combination with internal spraying, compared to normal. 80 HR15N sounds about right for a normally sprayed coating.

I take it that you are using nitrogen primary gas. I believe the 718 is the only nozzle available for nitrogen. 713 and 714 are Argon nozzles. Its been nearly 20 years since I played with a 7MT, so don't remember too much on the nozzle differences.

What are the conditions like for spraying? such as controlling dust/over-spray material and temperature control.
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08-16-2007, 07:11 PM,
#7
RE: Rockwell (macrohardness) topic [split]
Hi Alex and all

Seems like from this forum anyway, that people have big problems meeting specifications for Tribaloy T400 and T800 coatings. Is this due to setting specification targets too high or just poor parameter recommendations/standards?
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08-24-2007, 11:22 AM, (This post was last modified: 08-24-2007, 11:22 AM by Alexangel1226.)
#8
RE: Rockwell (macrohardness) topic [split]
Gordon,

Got it right this time on the same issue. The change was still on adjustment on 1st & 2nd gas. I find the ratio of 1sr/2nd gas does have significant impact to it. e.g, secondary gas ratio is optimum at ~ 10% of the primary gas. Not so sure if it is the same for the other coating type, what should be the recommended one for normal plasma (9MB) and extention gun 7MT.

I believe the incremental 2nd gas increases the velocity of the jet and particle besides primary gas. As the velocity of particle increases, the impact is stronger, resulting in splatter laminar structure which is denser and more compact, also result in voids & porosity reduction. and increase hardness. As the 2nd gas (H2) is adding more heating capacity flame/plasma jet to the compensate the dwell time reduction as a result of velocity increase.

2nd thing is thickness build-up to 0.0025" to obtain ready confidently above the 80 marks. but do you think it is too much for the test though the min thickness shd be 0.015"?

Is it advisable to polish the surface to certain flatness? would the high and low rough profile (as bond coat is rough) affect Rockwell hardness?

Regards.
Alexangel1226
Reply
08-24-2007, 01:34 PM,
#9
RE: Rockwell (macrohardness) topic [split]
Hi Alex

Good to know your making progress with sorting your problems Big Grin

Quote:Got it right this time on the same issue. The change was still on adjustment on 1st & 2nd gas. I find the ratio of 1sr/2nd gas does have significant impact to it. e.g, secondary gas ratio is optimum at ~ 10% of the primary gas. Not so sure if it is the same for the other coating type, what should be the recommended one for normal plasma (9MB) and extention gun 7MT.

The info I have is old (Metco 66F-NS tech bulletin ~1980 for 7MB), so it may not be current recommendation for recent 9MB.

Set 1 - Ar/H2, GH, PP#2, Ar 150 FMR@100psi, H2 10 FMR@50psi, 500A, 70-75V, 37 CGF, 4" SD, 6 lb/hr SR,

Set 2 - Ar Only, 704, PP#3, Ar 50 FMR@55psi, 1000A, 32-37V, 32 CGF, 4" SD, 6 lb/hr SR

Old 9MB manual (7MC era) only gives the Ar only parameters.

Just occurred to me that you are probably using N2/H2 on your 7MT angled extension gun, so its probably pointless comparing the above parameters Ashamed0002 No info at all for N2. Where did you get your starting point parameters from?

Quote:2nd thing is thickness build-up to 0.0025" to obtain ready confidently above the 80 marks. but do you think it is too much for the test though the min thickness shd be 0.015"?

No, I think that's OK for the hardness test piece, best to be safely above minimum thickness for hardness test. I agree that it is best to test as close to the actual conditions of the job in question, but it is pointless if that makes the test invalid.

Quote:Is it advisable to polish the surface to certain flatness? would the high and low rough profile (as bond coat is rough) affect Rockwell hardness?

Yes, perhaps not necessarily to the extreme that you would for Vickers microhardness test, but certainly to a fine ground finish. I usually cut a smaller piece from the test piece (makes it much easier), grind flat (~ 600 grit) removing all previous coating texture and also grinding the back side to ensure good anvil support (use smallest flat support anvil Wink).
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09-06-2007, 11:27 AM, (This post was last modified: 09-06-2007, 11:28 AM by Alexangel1226.)
#10
RE: Rockwell (macrohardness) topic [split]
Gordon- Thanks for the sharing. Will continue to monitor and try your recommendation. And to add on to better and more effectively problem solve the process issue, esp when time is not at your favor, do engage the hardware or material supplier for the problem solving, it is of great help. After all it gives you the synergy result when you work closely with them. Thanks.
Reply
09-07-2007, 12:24 PM,
#11
RE: Rockwell (macrohardness) topic [split]
Cheers Alex
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