Questions about heat treating spring steel AISI 6150 3

[slashragnarok]

Hi,

Currently I am attempting to build myself a piece of weightlifting equipment (a barbell) and have settled on building the main shaft with 2200mm long 29mm dia 6150 spring steel. My dad's friend has offered to heat treat it for me for no charge. But he has asked me to provide him details of the temperatures and time for quenching and subsequent tempering as he has no experience HTing 6150. I tried pulling some data from the ASM Specialty Handbook for Tool Materials. Please help. Can I use that data?

 

[tbuelna]

Those would be core UTS. But you would not use these values for a fatigue analysis of this bar.

The bar would not "develop a permanent bend over time" unless it was subjected to stress beyond its yield point. So what you need to do is make a calculation of the bending stress your bar might experience in service. I think you'll find that the worst case stress is well below what you think it is.

 

[slashragnarok]

Alright. Sounds good enough. I have a query though. If the case strength is 300 ksi and the core strength is 160 ksi what's the overall bar strength assuming a case depth of 0.02"?

 

[slashragnarok]

Also after reading up a bit, I can see that nitriding only gives the bar a hard skin that serves more to increase wear resistance than strength. And I read the previous posts a bit carefully and here's what I think. It is suggested that I HT the bar to 34-38 Rc to facilitate the knurling process. Then I nitride. But I fail to see how even plasma nitriding will increase the core hardness to Rc43 because as far as my knowledge serves me, the only way to re harden the bar would be to quench and temper it again. Tempering can only draw down the hardness not build it back up again. Isn't that true? So my Rc 38 can only go down by tempering it at higher temps than what was already done to achieve Rc38. Any temp lower than that would maintain the hardness.

 

[slashragnarok]

So is there any way I could temper it soft enough for the knurling process and then reharden it to the requisite strength while maintaining straightness?

 

[Lyrl]

It sounds like from your research you found the company you are modeling the process after knurls on the fully hardened bars and just eats the higher tooling costs that result? As far as getting the bar soft enough for more normal tool wear from the knurling and then rehardening, I think it's unlikely to be possible if you need a core hardness of 43 RC minimum. I'm a little skeptical of that 200 ksi tensile requirement because you mentioned some similar products are made of 17-4 material, which is not capable of achieving that hardness/tensile.

I interpreted the mention of plasma nitriding as being capable of being done at lower temperatures, so the core wouldn't get tempered back as far during the nitriding process.

You also mention fatigue resistance - most steels have a very big jump in fatigue resistance when the hardness drops below 40 HRC. If fatigue is a concern, it's in a designer's interest to arrange things so the steel only needs hardness in the high 30s HRC (~175 ksi tensile) in order to minimize fatigue risk.

 

[slashragnarok]

Yes you're right. The top companies knurl the hardened steel and eat the costs. But even smaller companies whose bars are not super expensive are offering tensile strengths of 216000 psi - 264000 psi. I'm flummoxed as to how they can achieve those strengths, knurl the bar and still be economical. Here are some links

http://www.gymandfitness.com.au/force-usa-olympic-lifting-bar-cross-training-use.html

https://www.againfaster.com/shop/barbells/again-faster-klokov-competition-barbells/

both in the $300-400 range.

 

[CoryPad]

S45C and SCM440 steels (listed in the first link) reach 216,000 psi strength range by austenitizing, quenching, and tempering at a low temperature. No expensive costs there. I would expect the knurling to be done prior to heat treatment, similar to socket head cap screws that are around this same strength range.

 

[slashragnarok]

@CoryPad - Even if the bar is knurled prior to HT, surely the quench would cause the bar to bend like a banana. How do you suggest I straighten it back without squashing the knurl?

 

[CoryPad]

The bend can be controlled by stress relieving prior to machining and the hardening process, and by insuring that the part is vertical during quenching.

For straightening, you can use a press and three point loading. I am not aware of your knurl design, but I would not think they would be squashed.

 

[slashragnarok]

As far as I'm aware, stress relieving is done after machining/knurling. What stresses could exist prior to machining? Also does anything change if I'm heat treating by induction heating method?

 

[CoryPad]

The stresses from rolling the bar will be a significant fraction of the yield stress and lead to distortion during quenching.

Induction heating has the same general considerations compared with through hardening, but there are unique specific considerations for each process.

 

[slashragnarok]

Okay, so I could skip stress relieving if I get the stock in the Annealed condition, right? Also given the section size (28mm dia), the bar would probably get through hardened by induction heating.

 

[CoryPad]

You may be able to skip stress relieving after rough machining if you use annealed bar, it will depend on the machining parameters during rough machining and how much stock you leave for final machining after heat treatment. You are able to find a source to do extremely low volume induction hardening? Will this source do vertical scanning? Do you plan to furnace temper or induction temper?

 

[slashragnarok]

Okay so I visited my dad's friend today. Sounds like he can only do HT by conventional oil quench and furnace temper. However since he's basically getting this done for free, he has asked me to give him the complete HT operation cycle and drop the bar at his place. As attached my bar is a long thin piece of steel round with a few knurled sections in the middle and two shallow grooves 5mm from the ends. The final length and dia of the bar are gonna be 2192mm and 28.5mm respectively. Should I directly machine the length and diameter to final spec?

 

[CoryPad]

No, you should leave the diameter larger than final so that you can do light machining after heat treatment to help correct for distortion. You will need to straighten too after heat treatment.

 

[tbuelna]

Regarding knurling after HT with material at >Rc43 hardness, it is possible to cut diamond knurling rather than roll forming it. It requires special tooling, but it can be done. Cutting the diamond pattern of grooves would not produce raised tips above the bar surface like roll forming does. But it is no problem to cut shallow surface grooves in Rc43 steel with a carbide tool.

 

[slashragnarok]

Is this roll formed or cut?

http://i2.wp.com/

http://www.garage-gyms.com/wp-content/uploads/2014/11/klokov-knurling-close.jpg

By the way this is a 264000 psi steel bar.

 

[tbuelna]

From the photo it looks like the knurling was roll formed. The peaks of the diamonds appear to be higher than the adjacent bar surface, and also have corners that appear to be rolled over a bit.

If the claim of this bar having 264ksi UTS is true, that means the bar has a hardness of around Rc52. So the knurling would have been done prior to HT.

I took a look at the bar manufacturer's website, and there was mention of the diamond knurling needing to meet certain requirements.

 

[slashragnarok]

@ CoryPad - Okay but if I knurl prior to HT and leave machining allowance, wouldn't the knurls get destroyed by machining? Straightening by a hydraulic press sounds fine. Also how likely is it that the bar will bend uniformly?

@tbuelna - The only requirements the knurling is supposed to meet are the distances between where each knurled section starts, ends and the distance between the unknurled ring like sections serving to mark the hand placement for Powerlifting and Weightlifting.

 

[CoryPad]

You will need to adjust the knurl depth to accommodate the material removal after heat treatment. You need to use adjustable loading points in the press to make sure you achieve uniform straightening.