SAE 4041 Heat Treatment 2

[GTFiji]

What Heat Treatment process(s) would best be used to treat SAE 4041 to an Ultimate tensile strength of around 150,000 psi?

 

[swall]

No such animal as SAE 4041. Do you mean 4140?.But in order to answer your question we will need to know the section size you are interested in hardening.

 

[GTFiji]

Nothing like a critical typo right in the title of the post. Yes, I am interested in 4140. It is a bar section 5' long and 1" x 1" square. The square cross section will be machined to an octagon prior to heat treating.

 

[swall]

A 1" section size in 4140 will not fully harden with an oil quench. You will need to use 4150, 4340 or 5160. You might make it with a water quench, but you risk quench cracking and distortion.

 

[GTFiji]

What tensile could 4140 achieve with an oil quench.

 

[metengr]

The web site below might be of help regarding comparison of mechanical properties for AISI Type 4140 alloy steel, oil quenched and tempered for 1" rounds. Keep in mind you are dealing with variations in hardenability characteristics for the same alloy composition, but for the most part it will give you some idea for comparison.

http://www.matweb.com/search/SpecificMaterial.asp?bassnum=M4140G

 

[swall]

With a 3/4" dia round, austenitized at 1575F, you would quench out (oil quench ) at least Rc 51. This would be for the entire section. Rc 51 is equivalent to a UTS of 264ksi and you would temper at 800F to drop the hardness to around Rc 34 for an UTS of 150 ksi. BUT,with a 1" square section, you will NOT form the desired 90% martensite upon quenching and will have a softer core. With such a variation in structure, you can't predict what UTS you will end up with.

 

[metengr]

swall;
I believe this is one of those situations where you would almost have to try a sample heat treatment using the 1" square bar (versus hardenability projections using a 1" diameter round) and run a tensile test to see how close one could get by using an oil quench/temper heat treatment. The core hardness after oil quench and temper may still be sufficient to reach 150 Ksi. Sounds like fun……

 

[unclesyd]

Bethlehem Steel mass effect data for a 1" Rd gives a tensile strength of 156,000 after oil quenching from 1550°F and tempering at 1000°F.

Instead of all the machining could you use a std 4140 hex bar?
If not standard but close to standard it would be lot easier to machine a hex down that a Rd.

Look at 4140 ann

http://www.frysteel.com/data/MeettheMetals/showmetal.cfm?metal=aircraft

 

[swall]

Well, yeah, you can do an experiment or use some by guess and by gosh method, but why not just stick with sound metallurgical engineering and chose a steel with the necessary hardenability in the first place?

 

[metengr]

Agreed, if GTFiji can upgrade to a higher hardenability steel. My preference would be AISI type 4340. Many times, the horse is let out of the barn before the cowboy (metallurgist) is consulted.

 

[GTFiji]

Yes, the horse is running wild in the hills right now.

My finished machined bar needs to be an octagonal (1" flat to flat) with shallow (0.09" deep) slots machined in 4 of the 8 sides. The 62.5" long bar is for a fitness (gym)application. I need to be able to machine a square (or other) bar to this shape, then heat treat it to my requirements. Requirements are as close to 150,000 UTS as possible & a surface hardness greater than ~Rc 30. I have bearings (Rc 58)riding on the bar flats so the ideal would be a bar as hard as the bearing (but not necassary). The bar will be lifted so it must be strong (safe) and not fail if dropped accidentally (impact). Up to 200 lbs of weight plates may be loaded on.

Any and all material selections are welcomed. Cost as always is an issue. I need the material machined and then hardened (strength/toughness). I would also consider a thin dense chrome (Armaloy) lastly to bump the surface hardness up.

 

[GTFiji]

I meant to also mention, se currenly us Stainless electro polished for cosmetic & corrosion purposes. I think Steel would be OK if I plate/coat it.

 

[swall]

So are you considering switching from stainless because of cost?

 

[unclesyd]

Sorry about the mention of hex bar as it would be hard to get form 6 to 8 sides. I'll have to think square.

Riding into the sunset the sun sometime gets into your eyes.

swall,
Would you please define a "guess and by gosh method" based on sound metallurgical engineering practices?

 

[swall]

unclesyd--you guess that it will work, even though you have forgotten the sound metallurgical engineering practices and by gosh it comes out O.K. Interrupted oil quenching of large D2 tool steel mill rolls comes to mind. See also, "rule of thumb".

 

[GTFiji]

If I choose an AISI 4340 what heat treatment gets me to the 150ksi UTS I am looking for?

I am considering switching from SS to steel for cost savings. I also now realize I may be able to get the appearance of electro polished SS and the corrosion protectionby adding a thin dense chrome coating to steel. I am evaluating both alternatives against one another.

 

[GTFiji]

metengr - what would the oil quench and temper you suggest on MatWeb do to the dimensions of the part?

 

[metengr]

If you use AISI Type 4340 alloy steel, which has noticeably increased hardenability over your original selection (4140), it should respond well to an oil quench for thru-hardening. I believe distortion should be minimal for this material.

 

[swall]

For 4340, austenitize at 1550F, oil quench and temper at 1000F. This will give you an UTS well inexcess of 150ksi.