300M - 4340M AMS 6419 Hardness All over

[MechyMarco]

Hi all,
Have some parts made from 300M. Been doing standard processing per AMS 6419. Tensile specimens come back with good ductility and yield. Issue is recent brittle failures at low stress levels. Potentially impact loaded, but nothing that's very different from normal use.

I've been playing with temper temperature to dial down hardness in favour of toughness. I'm using my old Aerospace Structural Metals Handbook (1963). Data shows double peak behavior. Optimum at 575F to 600F. Then Valley up to about 1000F where all properties actually get worse.

Still waiting for all data to come in from labs, but my gut says 575F is done for good reasons. Bloody Heat Treaters Guide even says don't temper outside 550F to 600F.

My question relates to the hardness. On large parts tempered at 575 and 600F the surface hardness is about 2-3 HRC higher than smaller 1" rounds I've been playing with. The parts are only about 2" thick max so don't get why such a difference. Leads me to think I should be chasing a hardness spec instead of going with default 575F. Anyone else see 54-55 HRC on parts of 300M tempered at 575F? Maybe I soak at austenitizing temp longer and temper longer. Currently doing 2 hour soak and 2+2 double temper. The 1" rounds and parts are heat treated at the same time. So really confused why the surface hardness would be so different.

 

[ShataCastings]

Different thickness will have different effects on heat treatment.
Compare the hardness, depth of hardness between part and specimens, maybe helpful to know the reason

 

[MechyMarco]

Yeah case depth survey probably a good idea on some used parts. Compare them all and see if there's a pattern.

 

[mfgenggear]

Op
Follow the heat treat spec AMS 2759, AMS2759/3. , done thousands of Aerospace parts. With no issues. Timkens hand book for heat treat for Metallurgical is a good guide for max diameter or max depth.
I always run test for actual results

 

[MechyMarco]

Yeah I got that too. Just noticing that hardness is not following temper temps reliably. 52 HRC is the sweet spot for balance between strength and toughness/ductility.

What hardness ranges you getting on your tensiles? I'm in the 53-54 HRC with great mechanicals. So maybe my brittle failures are just how the cookie is crumbling due to impacts and potentially SCC.

 

[mfgenggear]

Op
With material over 40 HRc and post processing with chemicals it is important to follow the AMS 2759/9 and qq-p-416 spec for post baking.

 

[mfgenggear]

Op
Far as hardness goes, I going by memory which has been a while, I believe 50-55 HRc

 

[MechyMarco]

I'll post back with some case depth survey data. Not coating right now. Sometimes electrolysis nickel with full 24hr 375F bake.

 

[jessieleo99]

The larger parts, even though only 2" thick, have more thermal mass than the 1" rounds. This can lead to slightly slower cooling rates after austenitizing, which may affect the final microstructure and result in a slightly harder surface. You might try adjusting the cooling process or pre-heating the larger parts before tempering to see if it makes a difference.

 

[mfgenggear]

Op
Look up timkens practical data for metalurgist hand book. It is free and has been very accurate.

 

[EdStainless]

Have you checked as quenched hardness?
You may need to tweak the austenitizing temperature slightly.

 

[mfgenggear]

Op
There is many variables. Quenching in oil is common, but there is fast or slow oil types. The thickness or variable sizes with effect which section cool faster or slower in quench. Some parts I have done have mar tempered or salt or oil quench at an elevated temperature to prevent distortion. But time the quench us critical to have complete or almost complete change from austenitic to martensetic transformation, then cold stabilize for complete transformation. I apologize for my mis spelling if I have done so. But what is important is the procedure.
I have done the same with vacuum gas quench. It's all in the recipe.