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]

What makes you feel you need heat treated 6150 steel for your 29mm bar? Have you looked at the stress levels your 29mm bar will experience in service?

For an application like a weightlifting bar, it seems like you would want a ductile material that will bend and fail gracefully rather than a brittle material that will fracture at failure. Just how much weight do you plan to lift with this bar?

 

[slashragnarok]

Firstly please pardon my ignorance. The criteria I am looking at is approximately 200000 psi Tensile Strength. The idea is to select a shaft of 29mm that will not develop permanent bends over time. The TS requirement is a standard industry practice. I had also looked at heat treated 4340 and 4140.

 

[dbooker630]

There are two ways to do this, quench & temper, or austemper. Appears the barbell needs properties similar to an automobile anti-sway bar.

Quench & temper: heat to 1600F for 90 minutes, oil quench, temper at 850-900F.
Austemper: heat to 1600F, quench in 625F salt.

The austempered bar may have less distortion than the Q&T bar, I would avoid straightening. But you may need to find out more about the capabilities of your dad's friend with regard to heat treat equipment.

 

[slashragnarok]

The heat treater has basic hardening and tempering facilities along with induction hardening and stress relieving equipment. What baffles me though is bars that are designed for weightlifting having over 260000 psi TS. How do you machine such a bar economically? Is there a special steel for that?

By the way does the above mentioned procedure achieve 200000 psi TS?

Thanks

 

[slashragnarok]

Also I believe that unless I go for destructive testing, I would have to gauge final strength after temper by the surface hardness. Is this correct? Do I need to worry about brittle failure at these strength levels (200000 psi)? Would the material lose some strength if the surface is machined to reduce the diameter after heat treatment?

 

[Lyrl]

Most steels (including 6150) have a very consistent relationship between hardness and tensile strength. You can be confident that the part's strength is appropriate if the hardness is in the range you select: 43 HRC minimum for the 200ksi minimum, and I'd suggest putting a 47 HRC maximum on there to minimize the risk of brittle fracture. The risk of brittle fracture at these hardness levels applies to impact loading (like if the fully weighted bar is dropped onto a hard surface from a good height); if the overload condition is applied gradually the bar will bend before breaking.

6150 material is through-hardening in the diameter you mention (29mm) and will not lose hardness even if it is machined after heat treatment.

The suggested process is good. The company I'm at (a commercial heat treater) doesn't have a lot of history with 6150, so I can't speak with confidence on the tempering temperature; if we received a part of 6150 with that hardness requirement I'd start with a 750°F temper just to be conservative (it's a lot easier to retemper at a higher temperature to soften a part compared to doing the whole process over again if the part becomes too soft).

Austemper would give you better properties, but the equipment required is less common than quench & temper equipment. It sounds like your contact doesn't have austemper capability, so you'll pass on the quench & temper process information to him.

 

[CoryPad]

Machining steel at the strengths you mentioned is straightforward, you just have to use harder tools (e.g. WC/Co instead of HSS).

To get steel at 260 ksi would likely mean using a tool steel like H11. This would be extremely sensitive to environment-assisted cracking (hydrogen embrittlement, stress corrosion cracking, etc.). Another steel option would be maraging steel. I cannot imagine using steel at 260 ksi for an application that involves human contact or operation near humans due to the cracking risk. A non-steel material like MP35N is an option at this strength level, and it is essentially immune to environment-assisted cracking.

 

[slashragnarok]

Thank you. This answers most of my questions. Here's a link to the barbell I was talking about.

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

What baffles me further is that even at this high strength and hardness, the bar has been knurled. How do you knurl such a bar?

 

[slashragnarok]

Also the bar needs to be pretty straight for my application. How do I ensure that?

 

[redpicker]

My guess is that they machine the bar prior to hardening. I'd also guess that the bars are heat treated by induction hardening. I don't know if it would be through-hardened, or just case-hardened, say 1/4" to 3/16", which would give some protection to brittle failure, not only by providing a tough core, but by also limiting the depth a brittle crack could grow. My guess tempering after hardening would be in a batch furnace, where time and temperature cycles can be better controlled.

Straightness is going to be a big issue. With a l/d ratio of 75 or better, immersion quenching is not going to be very successful. With induction hardening, with a proper equipment set-up including properly sized and aligned rolls, a uniform spray quench, and controlled heating and cooling, straightening after HT would still be needed, but it could be kept to a minimum. Non-Destructive Examination (by Mag Particle would be my choice) after HT is highly recommended.

rp

 

[slashragnarok]

Thanks. Does induction hardening offer a straighter end result than austempering? Tempering AFAIK could also be done by induction heating. Does tempering affect straightness? Also I believe that since 6150 is a spring steel, it will be through hardened. Regarding machining; this bar (2.2 m long) will undergo knurling to improve grip and there would also be two grooves machined on either end of the shaft 5 mm from the ends. These grooves would accommodate snap rings. So the grooves would require pretty tight tolerances. Wouldn't heat treatment hinder that? Knurling on the other hand would be much easier on the pre HT shaft.

 

[slashragnarok]

I came across this thread which suggests IH has issues with 6150. thread330-172036

 

[tbuelna]

You probably don't want to diamond knurl steel that is harder than about Rc38, since it will be very hard on the knurling die. If you diamond knurl the middle surface section of the bar prior to HT, you might want to be careful of quench cracks occurring at the small radius root fillets of the knurling. After HT your bar material will be very hard through its thickness, which will make it very difficult (and costly) to finish machine.

As noted quenching will present a problem with this long, slender bar. Have you fully considered what it will cost for the custom tooling needed to IH this bar? Mechanically straightening the bar after HT will be costly since it is largely a manual operation. And machining the (Rc56) bar after HT will be even more expensive since the material removal rate will be very limited. There is also no guarantee the bar will retain its shape after mechanical straightening or finish machining. The residual stresses in the bar from HT may slowly relieve over time as it is subjected to repeated bending load cycles in service, which might result in the bar taking a permanent sag.

Since there is a IWF diameter requirement for this bar, if you knurl the bar prior to HT, and you intend to finish machine the bar ends after HT to correct any distortions, have you considered just how much machine stock you will need to add and where to add it? If this 7ft long bar has say 1/16" of bow from the center to the ends, you will need to add 1/8" to the diameter of the bar for finish machine stock. You will also have to machine the center section of the bar that gets knurled to size prior to HT.

Lastly, since this bar is stressed in bending, a cheap and easy way to improve its fatigue capability would be to shot peen it. In terms of fatigue properties, shot peening gives great bang-for-the-buck.

 

[slashragnarok]

Thanks. That's a lot of things to consider. I would temper the bar to about Rc43. That's what I need for the strength requirement. Does that change things? Could I knurl and machine it reasonably at Rc43? Could you suggest what else I could do to make the shaft?

 

[slashragnarok]

Basically this is what I want to achieve. (Attached)

 

[tbuelna]

Finish machining at Rc43 is not too bad, but diamond knurling at that hardness is not a good idea. As I said, a cold forming operation like knurling is pushing things a bit even at Rc38. In theory, you can knurl steel at Rc43, but the knurling dies will take a beating. It will cost quite a bit of money to knurl your bar at Rc43 given the amount of surface being knurled and how often the dies will need to be replaced. You must remember that the knurling dies have a hardness of around Rc55-60, but they also have very tiny features that quickly become burnished from the contact pressures created by cold forming hard surfaces. The knurling process requires creating enough surface pressure on the material to plastically deform it.

Figure out what the acceptable straightness limit is for your barbell. From looking at the similar product shown in your link, it appears that there will be a 18" or so bearing mounted hub at each end of the bar that the weights are mounted to. So this might reduce the overall straightness required in your bar. Your most pressing issues seems to be finding a vendor that can knurl and trim your bar prior to HT, and then finding a vendor that has furnace and quench equipment to handle a 7ft long part and that can straighten the bar after HT to the tolerances you require. Just remember that heat treaters will provide no guarantee regarding the outcome of your parts. So you should run a sample part to verify the process is satisfactory prior to processing a full batch of parts.

 

[metengr]

Looks to me that I could buy this far cheaper, and at least have something that is reliable and safe for use rather then trying to reverse engineer.

 

[slashragnarok]

Looks to me that I could buy this far cheaper, and at least have something that is reliable and safe for use rather then trying to reverse engineer.

Probably if I was in the US but I'm not. In fact I don't even have a sample to reverse engineer. This is a prototype (2 actually) I'm building for myself and a friend. The ones available here are simply unacceptable. Athletes use Eleikos which I cannot afford as a hobbyist weightlifter.

Regarding straightness, I would like it to be no more than 0.015" per foot out of straightness. That's a 2.73mm deviation over the length of the barbell. I had thought of using 17-4 PH stainless steel which when aged at 900 F achieves the same strength sans the warping and with the added benefit of corrosion resistance. But the cost is prohibitive at the moment. I'm looking for some other solutions. So to reiterate, I basically need a 200000 psi TS steel, no more than 0.015" per foot out of straightness which will undergo a fair bit of diamond knurling and machining.

 

[tbuelna]

Appreciate the fact that you are looking at making your own equipment to save some money. But unfortunately, once you consider the cost of raw material, rough machining operations, heat treatment, straightening, and finish machining, it will likely cost more than purchasing the very nice $1000 Eleikos Olympic bar. The raw material for the bar and end fittings will cost around $150, the heat treating and straightening will cost around $200, the rough machining will cost around $300, the knurling will cost around $100, the plating will cost around $100, and the bearings/hardware will cost another $100. The processing prices for heat treating or plating are lot charges and you can reduce them by processing several parts at a time. But even so, unless you have a desire to make your own Olympic bars, it would be better to buy one rather than going through the hassle and cost of making one.