P6 Mold Steel Carburizing Prob - Low Case Hardness 1

[mewhg]

I sent some samples of a P6 Mold steel to our heat treater last week for an R&D run before we will heat treat the main run of parts.

On these particular small parts (about 1/4" thk x 1.5" long) we wanted to get a .030" thick case with a hardness of 61 HRC (91 R15-N). I have a data sheet for this steel from Cartech that gives specific heat treating instructions that unfortunately the heat treater did not follow.

Specifically there are 3 different heat treatments listed in the alloy data sheet:

Treatment A:
Carburize at 1600/1650F, pack cool or air cool. for hardening, heat the parts to 1500F, quench in oil. Reheat parts to 1425F, quench in oil. Temper at 200/300F. This will produce parts showing the best combination of case properties and core strength; however the multiple heat treatment will subject the parts to greater distortion and size change.

Treatment B:
To secure less distortion in hardening, the following alternate treatment may be used. Carburize at 1550/1600F, air cool or pack cool. Heat the parts at 1450F, quench in oil. Temper at 200/300F.

TreatmentC:
For highest core properties with some sacrifice of case properties, parts may be carburized at 1550/1600F and quenched directly in oil. This should be followed by tempering at 200/300F.

For a .030" case my heat treater said it would take about 8 hours of carburizing at 1550F. He said he can't tie up a furnace that long so he upped the carburizing temperature to 1750F, carburized for 1 hour then direct quenched.

The case hardness is only about 56HRC (88 R15-N) after quench and about 55 HRC after a 1 hour temper.

Here are my questions:

1. Why are we seeing lower hardness at the higher carburizing temperature? (for reference the normalizing temp for this alloy is 1650/1750F)

2. Why are there 3 different heat treatment methods? What does each one do for the alloy?

3. What does it mean to have the "best combination of case and core properties"?

4. This is a note on the data sheet saying higher carburizing temps up to 1650F may be used if followed by a double hardening operation as described under Treatment A. How are the higher carburizing temp and heat treatment related?

5. There is also a note "During carburizing, it is generally desirable to produce a surface carbon content between 0.80% and 1.0%. This accounts for the relatively low carburizing temperature". How is surface carbon and carburizing temperature related?

I know there are lot of questions and I appreciate any and all help. I am pretty much stuck with this alloy for now but could change in the future. I can deal with a thinner case (.015) but I really need the RC61 case hardness.

For reference the trade name of P6 mold steel is 'Carpenter158' and its chemical composition is C=.1%, Si=.3%, Ni=3.5%, Mn=.5%, Cr=1.5%

Thanks again,
Bill

 

[Carburize]

The low hardness is due to retained austenite - in a steel of the composition quoted when carburized to a surface carbon level of 0.8% the martensite finish temperature will be around -150 F so unless the part is sub-zero treated it will have large amounts of soft retained austenite.
Also at 1750 for 1 hour I don't believe you will be close to a 0.030-inch effective case. I don't have any direct experience with the P6 material but I would estimate a case of no more than 0.020-inch
The rate of carbon diffusion and hence the case depth produced is time and temperature dependent higher temperatures reduce the cycle time but are associated with increase grain growth within the core, increase potential for intergranular oxidation, increased distortion and on a practical level reduced furnace life for racks, grids, refractories etc. Typically commercial practice uses 1725F as the upper end of the carburizing temperature range - I emphasise typically a few operations do use higher temperatures but not many.
The various heat treatment options are intended balance case and core properties , attempt to minimise distortion etc. Cool down to room temperature and re-heat and quench is intended to do a degree of grain refinement and improve core properties but there are those who claim that this produces the greatest shape change. Direct quench from carburize is supposed to give the least distortion the best case but perhaps a slightly less tough core.
Many of these observations are material and facility specific so particular heat treatment shops often have to develope procedures specific to their materials.

 

[metman]

Following was copied from ASM desk edition on-line:

P20 T51620 0.28-0.40 0.60-1.00 0.20-0.80
AISI UNS C Mn Si

0.40-2.00 . . . 0.30-0.55

Cr Ni Mo

. . . . . . . . .

W V Co

mewhg,

Either you have listed the wrong composition for Carpenter 158 or you don't have P20. P20 has almost exactly the same comp as AISI 4130 but a cleaner steel to minimize inclusions that could blemish the surface and leave unsightly marks on molded plastic parts.

I will guess that Carburized P20 (4130) does not require cold treatment but will have to check unless someone would like to beat me to it. Otherwize what Carburize says is true and the comp you listed is possibly what you have and all you need is some dry ice.

 

[metman]

ASM Hdbk 8th Ed Vol 1 and 2 indicate nothing about cold treatment for carburized P20. Usual working range for carburized P20 is 58-64. You should have no problem achieving HRC61 surface with the small cross section you are heat treating if it were P20. I re-read the Cartech ht instructions in your post which compare with ASM hdbk for carburized P20.
ASM also recommends max carburizing temperature of 1650F to maintain polishability (because of grain growth I suspect).


Carburize ,
I am also interested in his question, "How is surface carbon and carburizing temperature related?" I have forgotten.

 

[mewhg]

Metman, the alloy is P6, not P20. This alloy has the low carbon (.1%), 3.5% Nickel and no Moly. Thanks for the tip that P20 is basically 4130...that is good to know.

Is there a way to post attachements on this forum? I have the datasheet in PDF format.

The grain growth at the higher carburizing temperature makes sense.

Bill

 

[TVP]

Until Carburize responds again, here are some useful links for more information on carburizing:

http://www.keytosteel.com/Articles/Art114.htm

http://www.industrialheating.com/

Industrial Heating frequently has articles on carburizing including those by Daniel Herring (The Heat Treat Doctor). Perform a keyword search to find specific articles.

 

[Carburize]

The carburizing process depends on complex gas-metal reactions and diffusion under condtions which never become stable. The surface carbon is a influenced by, amongst other things, the partial pressure of CO at the surface which determines the forward - carburizing reaction and the partial pressures of hydrogen and water vapour which determine the backward - decarburizing reaction. With no other atmosphere controls in place ( such as oxygen probes) as temperature changes it is possible for the CO/CO2 concentataion to vary and this will influence the surface equilibrium carbon level.

There was an excellent review of the gas metal reaction rates, carbon diffusion rates etc published after an ASM conference on carburizing around 1989 - if I can find the reference I will post it here.

 

[Carburize]

CARBURIZING - Processing and Performance
Edited by George Krauss
ASM International 1989
ISBN 0-87170-360-2

 

[mewhg]

This a follow up to my first post.

I modified the heat treatment procedure to have a cryogenic freeze at -120F for 2 hours after quench and then a standard temper cycle.

Surface hardness is now at 92 R-15N (64RC). The part gained 8 points on the C scale.

The cryo treatment worked well in this case.

 

[Carburize]

Yep - the low temperature treatment completed the transformation and significantly reduced the level of retained austenite.