Necessity of hardened crank pins on 3/4” scale

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jscarmozza
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Re: Necessity of hardened crank pins on 3/4” scale

Post by jscarmozza » Wed Feb 19, 2020 7:29 pm

I'm not all that experienced in this hobby either, but I can say that heat treating in the amateur hobbyist style that most of us use results in scale and a reduction in dimension. Do a test run to see how much of a loss you will get using your heat treating method, then use some Kentucky windage to determine the pre-treatment dimensions that will result in the finished dimensions that you need. Good luck with your project.
John

matthew-s
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Re: Necessity of hardened crank pins on 3/4” scale

Post by matthew-s » Wed Feb 19, 2020 8:41 pm

I just tried this. Much failure ensued.

First shot in the real part: I way over heated it. I don’t think I appreciated how fast it would pass Cherry Red color. Part was pitted and soft afterwards.

Quickly made a part approx the same size and tried again. No pitting. Part shrunk approx 0.0008”. Unfortunately still too soft - easily cut with a file. Maybe I should have let it sit longer, or cool slower?

I’m using 12L14.

daves1459
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Re: Necessity of hardened crank pins on 3/4” scale

Post by daves1459 » Wed Feb 19, 2020 11:18 pm

If I may, 12L14 is un-heat treatable. It is screw machine stock. The miniature nuts and bolts we buy are made from 12L14. The "L" in the designation stands for lead which acts as a chip lubricant during machining. Lead melts around 550 degree F and vaporizes around 1,100 degrees F. With red hot temperatures at 1,300 degrees + the lead is gone and, depending on the actual temperature, can leave pits. However, I don't want to disparage 12L14. It is great stuff for ease of machining, surface finish, holding size, and can be processed a lot like brass. It can be silver soldered, but is not good for brazing and definitely is not weldable.

If your looking for something to run in a bronze bushing just get some " Stress Proof" steel. It is pre heat treated to Rockwell C 28 and machines well with high speed steel tools and gives a good finish with a slight radius on the cutting edge tip of the tool bit.

Case hardening steel is a difficult process for the average modeler. First, the part has to heated past transformation temperature, usually around 1,500 degrees F, then soak it in a hardening media, like Casenit, so the surface can soak up carbon and other trace elements. Then after the part has been "carbonized" it has to be heated again and quenched, usually in water hopefully with soluble oil or dissolved salt in it. Through those two heating's and cooling's the part is very prone to distortion. Through my working career case hardened parts were always made .005" to .010" oversize on the finished surfaces then machine or ground to size after hardening. If you must case harden use S.A.E. 8620. It is design for case hardening so to more readily absorb carbon into it's surface than other alloys.

In stead of fooling around with case hardening use alloys that are designed for hardening as is. One of the best is O-1 (oil hardening tool steel). It is readily available, machines much like cold rolled steel, hardens easily, quench temperature is lower than other alloys and those with a carbon case, and very dimensional stable. For the hobbyist it can be quenched in non-detergent engine oil.

All of the steel alloys mention are available through internet suppliers such as onlinemetals.com and speedymetals.com

Dave

Harold_V
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Re: Necessity of hardened crank pins on 3/4” scale

Post by Harold_V » Thu Feb 20, 2020 4:16 am

It should be noted that any carbon steel material with an ending number less than 30 won't be heat treatable by normal processes, although it can be pack hardened, or a hardening agent used. It's the carbon content that makes the material hard, with anything less than .30% (the last two numbers in the identifying number, such as 1045, which would have .45% carbon) not responding to normal heat treat methods. Adding carbon by pack hardening is a slow process, as it takes about an hour per ten thou penetration. A good option for those who have the necessary equipment, including grinding capabilities, so the parts can be properly sized after hardening.

I heartily endorse the use of Stressproof (1144), although I'd like to comment that it is not pre-heat-treated. It has particular qualities that are attributed to the cold working of the material, a patented process by LaSalle. It is recommended for use where hardened materials may be desired, but they aren't subjected to heat treat. It has exceptional workability, and is easily sized, unlike most carbon steels. Of interest, it can be heat treated (note that the last digits are 44, or .44% carbon).

H
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matthew-s
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Re: Necessity of hardened crank pins on 3/4” scale

Post by matthew-s » Thu Feb 20, 2020 7:46 am

I have some stressproof on hand. And have machined it with success. That would be the easiest approach for me (by far).

Harold_V
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Re: Necessity of hardened crank pins on 3/4” scale

Post by Harold_V » Thu Feb 20, 2020 5:04 pm

An even better choice (in regards to performance) would be prehardened 4140, as it's much tougher, but it doesn't respond to finish cuts the way Stressproof does. It likes to skip and tear much the way mild steel does, just not as much. In order for size to be achieved, typical approach is to machine a thou oversized, then polish (progressively) to size. Stressproof is a much nicer experience and will serve far better than plain old mild steel.

Want to guild the lily? Use 17-4 PH stainless, hardened to H900. It offers the same tensile strength as 4140, with corrosion resistance as a bonus.

Being precipitation hardening, it doesn't scale, as it's heated to just 900° F and held for a couple hours for heat treat. It can be polished to size before heat treat, keeping in mind it has a shrink coefficient of about .0006"/inch after heat treat. It can also be sized by polishing after heat treat. Makes little difference.

17-4 machines quite nicely, rarely tearing. A dull tool will raise razor sharp spines, however. Keep the tool sharp and it cuts beautifully.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

daves1459
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Re: Necessity of hardened crank pins on 3/4” scale

Post by daves1459 » Thu Feb 20, 2020 10:03 pm

I have never worked with pre-hardened 4140 so can not comment. Although I have found annealed 4140 not the most pleasant metal to machine. Annealed 41L40 is readily available and with the lead in it machines very well. It heat treats about the same a standard 4140.

LaSalle has another version of it's pre-processed 1144 steel called Fatigue Proof. It has a minimum hardness of Rockwell C 30. It has much the same properties as Stress Proof with higher hardness and is still machinable with high speed steel tools, just no quite as easily. Both Stress Proof and Fatigue Proof are available from McMaster-Carr. As I recall Stress Proof is designated with 100,000 psi yield and Fatigue Proof with 125,000 psi yield.

My apologies for my misleading description of the processing of Stress Proof. But, Stress Proof is heat treated as an important part of the manufacturing process. The 1144 steel is indeed cold worked to give it it's mechanical properties. After cold working it is furnace stress relieved of residual stresses from cold working. The end product has good strength, enough hardness for good wear, is stable, and machines well.

Dave

matthew-s
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Re: Necessity of hardened crank pins on 3/4” scale

Post by matthew-s » Thu Feb 20, 2020 10:25 pm

Fatigue proof sounds promising. I’ve got a lot to read up on.

As an aside. What do people like to work with when it comes to flat steel stock such as the side rods themselves? I picked up cold rolled 1018, as Kozo provided just “steel” as the specification.

I now wonder how well that will machine, and whether it will turn into a pretzel from residual stress?

Any wisdom to share here?

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NP317
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Re: Necessity of hardened crank pins on 3/4” scale

Post by NP317 » Thu Feb 20, 2020 10:45 pm

Cold rolled steel will deform with machining, unless heat relieved.
Hot rolled still will not deform/twist, since it is stress relieved when rolled hot.
Machinable stainless steel makes dandy rods.
RussN
Rods1small.jpg

Harold_V
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Re: Necessity of hardened crank pins on 3/4” scale

Post by Harold_V » Fri Feb 21, 2020 2:59 am

Yep. What RussN said. Cold rolled has an incredible amount of internal stress, so it's pretty much a guarantee that it will move radically when machined. There are procedures that will allow its use, however. It's just a matter of how much discipline one has, and if they are willing to stick to tried and proven methods of addressing the issue. I harp on it all the time. It's called roughing before finishing.

Regardless of your material choice, It helps if you can machine the sides progressively, as that relieves the sides gradually, so the amount of movement that occurs is not as drastic, and the cut on the opposite face tends to move the material back to the original condition. Still, if in the finished part you remove a large percentage of material from either side, you can expect some movement, even with hot rolled materials. Unless they are roughed, stress relieved or process annealed, then finish machined, they'll still move some. Just a matter of how much, and how much you can live with.

I've read on more than one occasion where a builder claims no movement with hot rolled. Quite frankly, I don't believe it. I have too many years of watching metal deform when machined to think that a novice has some magical power that overrules what nature does. What I suspect is that they don't recognize what's happening, and they are happy with the end result. Some after the fact inspection may reveal things are not as they think, however. Pretty much guaranteed.

In regards to stainless, if a guy is willing to spring for the fun stuff, 303 S machines quite nicely and polishes up beautifully. It still suffers from the same problems (internal stresses), but, as I said, that can be addressed by procedure choice. If one has deep pockets, it's a nice choice.

H
Wise people talk because they have something to say. Fools talk because they have to say something.

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cbrew
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Re: Necessity of hardened crank pins on 3/4” scale

Post by cbrew » Fri Feb 21, 2020 8:38 am

for that it is worth, I use 1144 (stress proof) for all my axles and pins.
I do not harden pins. so far this has not been any issues with wear. but i also run propane fired locomotives so i do not have any issues with coal dust etc.
If it is not live steam. its not worth it.

matthew-s
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Re: Necessity of hardened crank pins on 3/4” scale

Post by matthew-s » Sat Feb 22, 2020 7:43 pm

Ok. So here is where I think I landed:

1144 for the crank pins.
I found some annealed 303 for the side rods. Yeah, it was speedy, but for a model of this size, and the slow speed at which I work, the $80 will be survivable.

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