increasing tensile strengthn of 4140 steel shaft 7ft

[jawdayub1]

Hello,

I'm looking to build olympic barbell, i have purchased 4140 grade in annealed form which typical hardness is around 20HRC. I have done knurling on the shaft but i need to increase the tensile strength. the typical tempering and hardness seems to be very expensive to meet the tensile requirements i need it at 205,000PSI. Can i do induction hardness instead? Does induction hardness increase the tensile strength of the bar. We don't have shot peen equipment where i live. How should i meet the UTS with minimum cost?

 

[TugboatEng]

Induction hardening is for mass production items where tight control of shrinkage is required. Induction hardening is a bit of a trial and error process to get right which makes it unsuitable for one off jobs.

What is the reason for requiring such a high UTS?

 

[jawdayub1]

to avoid permanent deformation at heavier weights the bar deforms, barbell manufacturers increase UTS to increase its strength and longevity of the bar.

 

[MagBen]

how big is your shaft? 4140 can be induction hardened, but if you part is too big, you will get a soft core. To get a 205ksi, you will need about 44RC, which is not difficult for 4140.

 

[TugboatEng]

I worry about a brittle failure if it gets too hard as these are already being loaded beyond yield. That could cause serious injury to the lifter.

 

[kingnero]

Scars from a broken barbell would probably mean as much, if not more, to a weight lifter as an olympic medal...

 

[EdStainless]

Quench and temper is the only right way to reach this strength and have usable toughness.


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P.E. Metallurgy, consulting work welcomed

 

[jawdayub1]

That is my first priority, but don’t have large enough furnaces for 7ft also the heat treatment won’t give assurance that it will be 100% straight. Also the charges of full hardness exceed the cost.

 

[mrfailure]

For safety and liability reasons, I wonder whether barbell shaft properties are specified by the governing Olympic or weightlifting organizations.

 

[TugboatEng]

Can you buy the bar in full hard condition and use some thing other than knurling to provide the grip such as an aggressive sand blast?

 

[jawdayub1]

No it's not possible to use other than knurl, knurl is the process of the olympic barbell. Can i do induction hardness or gas nitriding to increase UTS?

 

[EdStainless]

Those will be surface only.
You can Q&T, the mechanically straighten, then temper again for stress relief.

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P.E. Metallurgy, consulting work welcomed

 

[Jughandle]

Instead of heat treatments, any possibility of sleeving up the bar midlength to decrease the bending stresses? Or a shorter bar? That's if you don't mind not having a weightlifting association spec bar.

 

[WKTaylor]

4140 LA Steel seem to be a poor choice for HT205-KSI min.... especially solid bar.

Per MMPDS... a 1.00-inch [2.54-cm] Dia bar is thru hardenable to 200-KSI... this is where the 'equivalent-round' table stops for 4140 aircraft quality steel.

For LA Steel, diameters up-to 1.0-inch equivalent-round require tempering, tempering above 200-KSI, the tempering-temperatures have to be 'lower'... and will enter the ~500-to-900F [285-to-755C] 'blue-brittle' tempering temperature range... which has disastrous effect on toughness due to drastic metallurgical changes specific to being in this 'tempering-temperature' range.

For diameters larger-than 1.0, thru hardening decreases dramatically due to a host of quench/temper issues... again related-to the metallurgy of 4140.

NOTE1. 4330, 4340 LA steels are far better choices for combined strength, toughness, machinability, up ~2.5-inches [6.35-cm] Equivalent round.

NOTE2 Also a serious problem with HT/tempering of any LA steel is preventing oxidation-scaling of the surface. This can be done in ~3-ways.
1. Vacuum or inert atmospheres.
2. Coatings that protect the surfaces... but will-not affect the surfaces... such as copper-plating, ceramic-like coatings, etc].
3. Heat treat unprotected... then simply machine-off all surfaces roughly 0.05-to-0.15-inch [depends]… to physically REMOVE the deep-scale/oxidation, exposing bare/pristine metal.

CAUTION1. High strength LA steels have an annoying way of rusting without protective finishes [inorganic or organic]. This problem is much more serious at higher strength [lower ductility] levels... rusting produces surface defects that are stress concentrations... which can progress to being cracks... rusted = busted.

NOTE2.
A stainless-steel bar, such as PH13-8Mo, precipitation hardened to H1000 would be a very good/close fit with few metallurgical issues to contend with. Although initially more expensive... ultimately it would be far simpler to obtain strength/toughness with fewer issues to worry about... and rusting should NEVER be an issue!



Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[jawdayub1]

Instead of heat treatments, any possibility of sleeving up the bar midlength to decrease the bending stresses? Or a shorter bar? That's if you don't mind not having a weightlifting association spec bar.
/quote]

Not possible the bar length is 7ft. I'm going with 4340 so increasing the depth of induction hardness can increase the tensile strength?

 

[TugboatEng]

Induction hardening is the wrong process for your application!

 

[bobjustbob]

Induction hardening is not the correct heat treatment method for this form. It will require a normalize cycle followed by a quench and temper. To avoid the bar from warping it needs to be properly supported along its length and cooled evenly. For that you will need to go to a commercial heat treater and they may need to develop some special tooling. Even with this care the bar will probably need some straitening.

A sample of the same steel with equivalent dimensions should used as a test coupon. ASTM A1001 covers cast grades of high strength steels and details test bar requirements. I am sure there is a wrought equivalent but I am not familiar. We used this specification for casting lifting devices such as hooks. The tensile and impact properties should be tested using this coupon.

To properly prove out the heat treat process two sacrificial parts would be made first, one sectioned and tested for mechanical properties and the other tested to failure. I am sure there are more factors that I am missing, corrosion resistance and fatigue life for example.

This is why making lifting devices should be left to professionals.

Bob

 

Use A36 bar and leave it fatter in the middle.
Heat treating is expensive and some of the solutions being discussed are overkill.
What about one of the lighter Nitronic SS grades?

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[EdStainless]

Don't bother with Nitronic, the highest strength being regularly produced is 150ksi, and this would be a $500 piece of bar.
This is really an issue of local capabilities.
In southern CA you find many heat treaters that could Q&T this while hanging it vertically.

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P.E. Metallurgy, consulting work welcomed

 

EdS,
Points taken.
But we're flailing because the OP has not stated the full design requirements.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."