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?

 

[slashragnarok]

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.

 

[slashragnarok]

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

 

[tbuelna]

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?

 

[slashragnarok]

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.

 

[Lyrl]

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.

 

[slashragnarok]

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?

 

[tbuelna]

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.

 

[slashragnarok]

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.

 

[slashragnarok]

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

 

[Lyrl]

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.

 

[tbuelna]

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.

 

[slashragnarok]

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?

 

[CoryPad]

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.

 

[tbuelna]

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

 

[slashragnarok]

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

 

[slashragnarok]

Would nitriding make the knurls chip off?

 

[tbuelna]

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.

 

[slashragnarok]

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

 

[tbuelna]

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.

 

[slashragnarok]

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?