8620 vs 4320

[davidt85]

I am looking to possibly replace parts using 4320 steel wih 8620 steel. I am interested in what changes I can expect in regards to impact strength, fracture chacteristics, and compressive strength if I switch from 4320 steel to 8620 steel. Will there be any negative consequences If I switch alloys? The parts are carburized to 60 Rc. These parts are under constant repeating impact so fracture and fatigue resistance will need to be good.

Are there any resources(books, websites, etc) out there that will help me in my decision?

Any help guys?

 

[DAVIDSTECKER]

You might want to look into "Hadfield" manganese steel, ASTM A128.
Stulz-Sickles Steel is where I have purchased plate and bar from.
It will work harden to Rc60 so if you need to do much machining you'll need to look into a great grade of tooling.
ASTM A128 is a cast steel specification so there are foundries that can cast a part to reduce any machining.
It used for crushers and railroad switch gear normally.
I am currently making some parts from this stuff and it is tuff.
Hope this helps, David

 

[salmon2]

8620 is quite popular for carburization, so I don't see your problem with afterwards carburization process.

But 8620 typicall has 0.5% Ni vs. 2.0% of 43XX. Ni will give you excellent impact toughness. So maybe you won't have enough toughness, unless currently you have plenty to cut down.

 

[swall]

Why are you looking to make the change?

 

[hydtools]

See the table 7.5 in the link:

http://books.google.com/books?id=bA...&resnum=9&ved=0CEwQ6AEwCA#v=onepage&q&f=false

The charpy impact numbers are similar. The yield strength of 8620 is higher. Looks like a fair substitution.

Ted

 

[davidt85]

Swall, I am looking for changes due to cost.

Hydrools I've been comparing the two materials and come to the conclusion that charpy values matter the most. Your post makes me more confident that I am looking at the right values to compare.

I am primarily concerned with impact and fracture resistance due to our parts experiencing cyclic impact. Charpy values is what I should be concerned with right? What about crack propgation? I know charpy values measure toughness and how resisant the material is to initial cracks but what happens when cracks and start propgrating? Would 4340 be more resistant to cracks propgrating?

 

[hydtools]

Once cracks start in the carburized case, it is probably a crap shoot which alloy is better. That depends on the structure of the core which depends on the heattreatment.

Ted

 

[redpicker]

You also need to evaluate the effect of the hardenability of these steels and the section size of the part you are making. 8260 has lower hardenability, so, for other than the smallest parts (< 0.75" cross section), you will have a softer core and for larger parts, you may not get the case to develop full harndess. 4320, on the other hand, has much more nickel, so you have to more take precautions against retained austenite in the case. For a wide range or parts, though, you can make this substitute.

rp

 

[salmon2]

Impact toughness comparison at RT is one thing, low temperature or high strain rate is another thing.