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Questions about heat treating spring steel AISI 6150
3

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?

RE: Questions about heat treating spring steel AISI 6150

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?

RE: Questions about heat treating spring steel AISI 6150

(OP)
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.

RE: Questions about heat treating spring steel AISI 6150

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.

RE: Questions about heat treating spring steel AISI 6150

(OP)
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

RE: Questions about heat treating spring steel AISI 6150

(OP)
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?

RE: Questions about heat treating spring steel AISI 6150

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.

RE: Questions about heat treating spring steel AISI 6150

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.

RE: Questions about heat treating spring steel AISI 6150

(OP)
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-fa... 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?

RE: Questions about heat treating spring steel AISI 6150

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

RE: Questions about heat treating spring steel AISI 6150

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

RE: Questions about heat treating spring steel AISI 6150

(OP)
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.

RE: Questions about heat treating spring steel AISI 6150

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.

RE: Questions about heat treating spring steel AISI 6150

(OP)
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?

RE: Questions about heat treating spring steel AISI 6150

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.

RE: Questions about heat treating spring steel AISI 6150

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.

RE: Questions about heat treating spring steel AISI 6150

(OP)

Quote (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.

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

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.

RE: Questions about heat treating spring steel AISI 6150

(OP)
Labour is cheap here in India. Even with excesses, the raw material costs me the equivalent of $90. The total deal (and I have a quote) is around $200.

RE: Questions about heat treating spring steel AISI 6150

(OP)
I found some info suggesting that Eleiko case hardens their bars. Can I possibly case harden 6150?

RE: Questions about heat treating spring steel AISI 6150

I've been following this thread and read your descriptions of what this product is used for. Unless I missed something, I don't see why you would want to case harden the bar. It would only be helpful if there is a high level of contact stress or abrasive wear at some location of the bar. Is that an issue with your bar design?

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

(OP)
As Redpicker mentioned above, there might be some benefit to case hardening as it would prevent a surface crack from propagating any further when the crack hits the soft core. I have a question though. It's kind of a tube strength vs rod strength kind of question. Is there any difference at all between the strength of a case hardened bar vs a through hardened bar considering they both have the same surface hardness?

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

Yes, there is a strength difference between a bar that is hard throughout vs hard only on the surface - given the same surface hardness, the bar that is hard throughout will have a higher average tensile strength. The same surface hardness would be an unusual spec, though; typically case hardened parts are around 58 HRC on the surface and 20-40 HRC (depending on steel and section size) in the core.

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

(OP)
I guess with IH, I would require some kind of custom arrangement for the diameter I have in mind. If I straighten the bar post HT, would it require any stress relieving? Finally is there any possibility of fixing the rod while quenching to minimize bending?

RE: Questions about heat treating spring steel AISI 6150

Fractures typically form at a location on the surface that is under high tensile stresses. If there is sufficient tensile stress at a location on the case hardened bar surface to initiate a fracture, the fracture will likely propagate circumferential around the upper surface of the bar before it extends thru the core. If you need a case hardened bar, it would be better to use a steel alloy that can be nitrided. The nitriding will produce a residual compressive stress in the bar surface that will improve fatigue properties of the bar. Nitriding does also not need a quench operation like carburizing does, so there are not quench distortions produced with nitriding. 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.

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

(OP)
Would this achieve the required tensile strength (~200000 psi)? Also would you please explain what pre stressing means? The only thing that pops into my mind when I hear pre stressed steel is suspension bridge cable.

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

(OP)
I missed a couple of questions in my previous post. Wouldn't I require straightening after the HT? Would shot peening destroy the knurl?

RE: Questions about heat treating spring steel AISI 6150

4140 material has similar hardenability to 6150 material.

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

slashragnarok-

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

(OP)
Thanks for all the replies. I think I got it. And regarding shot peening; is there any specification for it? I mean I have to tell the workshop exactly what needs to be done. So is there only one kind of shot peening or various types?

RE: Questions about heat treating spring steel AISI 6150

There are various types of peening: different media (e.g. Conditioned cut wite, glass bead), various intensities (e.g. Almen A), etc. Since you will not use x-Ray diffraction to measure residual stress, I think you will be limited in what you get.

RE: Questions about heat treating spring steel AISI 6150

You need to specify the shot type, shot size, shot hardness, blast intensity, coverage, surfaces areas that will be treated, surface areas that must be masked, surface areas where overspray is acceptable, QA requirements such as use of peen scan or almen strips, etc. Shot peening is a very economical way to significantly improve fatigue capability in metal components. But to get the best results you need to provide detailed process instructions. If you don't take the time to provide specific processing instructions to the vendor then you shouldn't complain about the results.

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

(OP)
tbuelna - Thank you so much. Very helpful. I'll run a FE analysis to determine the areas experiencing the largest stresses.

RE: Questions about heat treating spring steel AISI 6150

(OP)
Would nitriding make the knurls chip off?

RE: Questions about heat treating spring steel AISI 6150

Here's what I said in a previous post:
"....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

(OP)
Ok and what final strength could I expect from the bar?

RE: Questions about heat treating spring steel AISI 6150

If you look back over the posts, you can expect Rc 34-38 from a gas nitrided 4140 steel bar. This equates to a core strength of around 160ksi. You could also get Rc 43 (or ~200ksi) from a plasma nitrided 4140 steel bar, but it would not be practical to diamond knurl the 4140 steel bar at Rc 43 hardness.

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

(OP)
Are the hardness mentioned, core or surface hardness? The final yield strength of the entire bar is very important here. Otherwise the bar would develop a permanent bend over time. What is the overall strength of the bar considering the strength of the nitrided case and the Q&T core?

RE: Questions about heat treating spring steel AISI 6150

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.

RE: Questions about heat treating spring steel AISI 6150

(OP)
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"?

RE: Questions about heat treating spring steel AISI 6150

(OP)
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.

RE: Questions about heat treating spring steel AISI 6150

(OP)
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?

RE: Questions about heat treating spring steel AISI 6150

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.

RE: Questions about heat treating spring steel AISI 6150

(OP)
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-... https://www.againfaster.com/shop/barbells/again-fa... both in the $300-400 range.

RE: Questions about heat treating spring steel AISI 6150

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.

RE: Questions about heat treating spring steel AISI 6150

(OP)
@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?

RE: Questions about heat treating spring steel AISI 6150

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.

RE: Questions about heat treating spring steel AISI 6150

(OP)
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?

RE: Questions about heat treating spring steel AISI 6150

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.

RE: Questions about heat treating spring steel AISI 6150

(OP)
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.

RE: Questions about heat treating spring steel AISI 6150

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?

RE: Questions about heat treating spring steel AISI 6150

(OP)
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?

RE: Questions about heat treating spring steel AISI 6150

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.

RE: Questions about heat treating spring steel AISI 6150

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.

RE: Questions about heat treating spring steel AISI 6150

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.

RE: Questions about heat treating spring steel AISI 6150

(OP)
@ 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.

RE: Questions about heat treating spring steel AISI 6150

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.

RE: Questions about heat treating spring steel AISI 6150

(OP)
I found out that the knurl depth is directly related by the TPI of the knurl which is again dictated by the bar diameter. So on a 28.5mm bar a 20 TPI knurl would be used to prevent double tracking. So the knurling depth is not really something I can control.

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