Leaded Bronze Porosity,Unequal Lead Dispersion Help! 4

[NHATLEY6]

Hello,

We heat a leaded bronze to 1800 F it cools we then temper it at 725 F. Every once in a while we have a batch that has very bad lead distribution as well as voids. The guys in the shop say it always happens in the spring. Could barometric pressure or humidity play a role in the dispersion of the lead. The other thing is the lead solidifies last and the thermal expansion coefficient is 1.5 times the amount of bronze. So would it be safe to say once the lead cools it shrinks and creates gas pockets? Is there way to increase the lead distribution and or reduce the gas pockets?

Thank you for your time,
Nathan Hatley

 

[arunmrao]

Lead has the lowest freezing point in the alloy,hence it is the last to freeze leaving behind voids.One way to reduce is by cooling fast ,preferably in water cooled mould .

 

[EdStainless]

I would guess that most of the voids are moisture related.
How do you deoxidize?
Do you change deox practice based on the weather?
Do you pre-dry charge metals?

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[racookpe1978]

How big is the porosity?

How "evenly distributed" or just in local spots?

Porosity highest in the thickest sections? The thinnest sections? The first-to-cool? Or the last-to-cool?

 

[mcguire]

Nasty stuff, those alloys. Lead and bronze are not mutually soluble at those temperatures, so any annealing is just going to coalesce the lead, which is molten, into bigger and bigger blobs.

If it began as a casting, it will probably just alter lead precipitate shape, but if it started as powder met, then changes could be dramatic.

Spring=humidity=hydrogen. I doubt heat treating puts enough hydrogen in to cause porosity. In the steel-backed leaded bronze bearing business we didn't ever anneal. I'm wondering why you do.


Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[tbuelna]

What is the composition of the leaded bronze alloy (how much lead)? The lead in these alloys takes the form of discrete particles/globules trapped within a lattice of copper-based materials. The lead is rejected by the copper lattice as it solidifies, and it results in a composite type structure. Even high lead bronze alloys are routinely cast without "gas pockets". Gas porosity within a casting is normally the result of air being entrained within the liquid metal during transfer to the mold, or from outgassing produced by interaction between the hot liquid metal and materials on the mold surfaces.

If you are using a controlled casting process, I don't think you should have a lead segregation problem unless lead content is quite high.

Here are a couple links that might be of interest:
Copper-lead equilibrium phase diagram
Microstructure of continuous cast leaded tin bronze alloy C94100 (20%Pb)

Regarding the tempering step used, does this leaded bronze alloy actually respond to heat treatment?

 

[NHATLEY6]

Thank you for your responses. I am attaching a picture of the porosity. I am also working on the responses to the questions you guys presented.

 

[Maui]

Those "voids" look like areas where lead globules were pulled out of the surface during sample preparation. This could happen during cutting, grinding, or polishing. It would be a good idea to review your sample prep procedures and verify that they actually being followed by your lab technician. Taking shortcuts often create these types of issues.

Maui

http://www.EngineeringMetallurgy.com

 

[NHATLEY6]

it has up to 11 % lead

 

[TVP]

I agree with Maui, you need to review the metallographic preparation of these samples. Here is one source of information on preparation of copper alloys including those alloyed with Pb:

http://www.struers.com/resources/elements/12/101169/Application Notes Copper English.pdf

 

[NHATLEY6]

Ok so we buy the cast bronze with a lead content of up to 11 %. That bronze material then gets melted in a nitrogen rich environment and is bonded to stainless steel. I am starting to believe it is coming from our cooling process. It is happening very randomly on only certain areas of the parts. It is my understanding the longer you take to cool the parts the more time the lead has to segregate. We allow them to cool naturally in a five sided area. It seems like it is possible the parts that are closer to the middle of the batch would take longer to cool ect...

 

[arunmrao]

Enabling a fast cooling rate is definitely beneficial in reducing the incidence of porosity. Based on the geometry of your part, I am sure you could design a suitable chilling mechanism and improve it by having a water cooled jacket.

 

[tbuelna]

Just to be clear, the leaded bronze material is being overcast onto a stainless steel substrate? What process was used to overcast the bronze onto the stainless steel substrate? For example, if this is a sand casting process, the stainless steel substrate is basically a metal "chill" that will promote local heat transfer and cause the bronze material contacting its surface to solidify more quickly than other sections of the casting.

As arunmrao suggests, you might consider modifying your tooling/process to provide a more uniform cooling rate throughout the bronze material. There are commercially available analytical tools for evaluating the solidification of cast metal materials. If this problem is costing your company significant time and money, it might be worth looking into using one of these analytical tools to improve your process.

 

[arunmrao]

NHATLEY6

Have you considered offering the foundry to overlay the lead bronze on stainless steel. This will avoid reprocessing and the foundry will be better equipped to resolve issues regarding segregation and porosity.