Questions about heat treating spring steel AISI 6150
Questions about heat treating spring steel AISI 6150
(OP)
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?
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?





RE: Questions about heat treating spring steel AISI 6150
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?
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
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.
RE: Questions about heat treating spring steel AISI 6150
By the way does the above mentioned procedure achieve 200000 psi TS?
Thanks
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
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.
RE: Questions about heat treating spring steel AISI 6150
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.
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
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
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
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.
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
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.
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
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.
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
You can certainly induction harden some or all surface areas of the 6150 bar if necessary. Can you provide an explanation of why Eleko case hardens their bars?
RE: Questions about heat treating spring steel AISI 6150
Basically, after doing some "research" into how different manufacturers make bars (at least the good ones, Eleiko, Ivanko, Rogue), it is quite evident that the raw bars come heat treated, straightened, ground, polished and turned to the final diameter from the mill itself. Then the bar is knurled and the snap ring grooves are machined. Then the bars undergo some kind of coating process, though not necessarily. It could be chrome plating, zinc plating or black oxide. Following this the sleeves are fit on the bar and the whole setup is tested. The better bars undergo some kind of NDT (mag particle, ultrasonic etc.).
Regarding the knurling and machining, I noticed that the CEO of Eleiko says in one interview, one set of knurling wheels can be used for 6-7 bars at most. I'm guessing this is even with Carbide or Cobalt steel. At the UTS levels of these bars, the hardness likely falls anywhere between 40-45 HRc. Ivanko says the strong steels cut their tooling life to 10% of what they could get with annealed material. But they are quick to point out that though annealed material saves on tooling costs, they would have to re-straighten the bar post heat treatment and then there is a high chance of "squashing" the knurls.
I'm not sure what Alloy Steels these manufacturers use. When they use stainless, it is almost always 17-4 PH age hardened at 900-1000 F. I figure 6150 could be a good starting point for two reasons. One, its good Yield-Tensile strength ratio and two, its decent fatigue resistance.
RE: Questions about heat treating spring steel AISI 6150
6150 has around 0.50% carbon, which is high for a carburizing process; typical steels used for carburizing have 0.10% to 0.20% carbon, or sometimes a plain-carbon steel like 1045 will get carburized (the carbon is higher at about 0.45%, but the only alloying element in 1045 is manganese). While technically possible, it would be very unusual to carburize a steel like 6150 with multiple alloying elements AND medium carbon: the hardness difference between the surface and the core would be relatively small and not worth the extra risk of crack initiation in a high-carbon case. Like tbuelna indicated, induction hardening is the more common way to go with 6150 when a hard surface/soft core combination is desired.
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
I can't imagine 17-4PH CRES being used for this application, since it would be far too costly.
RE: Questions about heat treating spring steel AISI 6150
And, 17-4 PH is used as a bar material. A manufacturer has confirmed this. The bars do get pretty expensive.
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
I agree with tbuelna, nitriding is a nice case operation because is done at (relatively) low temperatures and there is no thermal shock (quenching), so there is no distortion and no need for straightening. Nitriding temperatures are high enough that the core of the bar (below the case) would drop into the mid-30s HRC (~150 ksi tensile), but the surface would be much harder (>325 ksi). I have no idea what the average would work out to over the entire cross-section.
17-4 is probably also used to avoid straightening in the final manufacturing steps. It is soft after quenching (so easy to machine or straighten) and final hardness is gained in a low-temperature step that produces no distortion. Maximum achievable tensile with 17-4 is around 150 ksi. Its a stainless steel, too, which might be nice for avoiding rust due to handling by sweaty hands.
If the surface is under a tensile stress (trying to pull itself away from the center of the bar), it is more likely to crack. If it is under a compressive stress (trying to pull itself further into the bar), that offers some protection from cracking. Shot peening introduces compressive stress to the surface, giving it some protection from cracking. Considering the very high hardness of a nitrided case, I wouldn't expect a light shot peen to damage the knurl.
RE: Questions about heat treating spring steel AISI 6150
Conventional gas nitriding typically involves temperatures of 950-1050degF, so the core of your 4140 bar basically gets tempered at this temperature. Tempering 4140 at 1050degF would give a core hardness of around Rc34 (~160ksi) as Lyrl noted. There are low temp plasma nitriding processes that are performed at around 850degF, which would make it possible to get a core hardness close to Rc43 (~200ksi). In any case you would temper the bar at a temperature suitable for the specific process prior to nitriding.
Here is a vendor website that describes various types of nitriding and benefits such as improved fatigue life from residual surface compressive stress.
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
I don't know what a common commercial spec is for shot peening, but AMS 2430 is used in the US aerospace industry. MIL-S-13165 is an older document that provides some basic guidelines on selection of shot and process parameters for various materials.
Since this is a somewhat cost-sensitive application, you can reduce the processing cost by limiting the shot peening to the surface areas where bending stresses are highest.
Hope that helps.
Terry
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
"....HT 4140 bar to Rc34-38, machine and knurl, polish the knurling to remove sharp edges, gas nitride the bar, shot peen the bar to remove the nitride white layer and pre-stress the bar surface, and then chrome plate the bar....."
If you round the external corners of the raised knurling features enough so that they will not thru harden given the case thickness of the nitriding process used, then there should be no risk of the "knurls chipping off". Typical diamond knurling produces teeth with a 90deg profile and a depth of around .015 to .020 inch. You could either polish the knurled teeth prior to nitriding, or you could mask the knurled teeth.
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
The reason I suggested using shot peening for your bar is because it would allow you to use a bar material with a lower tensile strength, while still achieving an acceptable fatigue life.
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
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.
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
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.
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
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.
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
Induction heating has the same general considerations compared with through hardening, but there are unique specific considerations for each process.
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150
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.
RE: Questions about heat treating spring steel AISI 6150
@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.
RE: Questions about heat treating spring steel AISI 6150
RE: Questions about heat treating spring steel AISI 6150