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Material selection for machinery pins 1

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gibsoni

gibsoni

Agricultural
Aug 23, 2010
25
AU
Hi All,

I am new and posted previously but not sure if it was clearly posted.

I am looking for some advice on material and treatment to make 25mm pins out of that are loaded in shear on a tractor driven mulcher. Below is some pictures of previously failed pins.

I have a standard machine shop and furnace.

4140? 1045? flame hardened by hand with oxy-acet and water jet?

They need to be very hard on the surface as the existing ones show signs of wear and were not touched by a centre punch.

Thanks in advance for your advice





 
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Hi gibsoni

There are no links to the five attachments,

desertfox
 
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Hi gibsoni

How long have the pins that failed been in service?
I think I would go with the 1045 steel, however if the pins have not lasted very long I would try to find out why they have failed, your cross section picture shows a brittle and quite rapid failure but its hard to see any surface detail.

desertfox
 
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Try 4140 heat treated (quenched and tempered), with a tempering temp. of 1,000 F. Use spray-on dry film lube that has moly disulfide.

You need full cross-sectional strength for that pin, and 4140 should give it to you. Also consider having the pins nitrided after the previous HT.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein
 
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Regarding the failed pins, what do you know about them? Where were they made?

If 4140 or 4340 won't work, there is a problem with the design. Heck, like desertfox said, 1045 should be plenty strong for a typical application. It's not a spaceship, go with 1045 and if that breaks, you better look at the overall design. Stronger pins like 4140 might just cause it to fail somewhere else.

I suspect a problem with the original pins.
 
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Thanks Guys,

This machine works very hard in a rough environment and hits rocks and all other things that should be avoided.

Indeed, pin failure is desirable as the point of failure before anything else.

The original pins were made by the mulcher manufacturer and appear to be forged, with the head as integral and turned later then presumably case hardened.

The pin case was very hard and showed signs of wear. Given this, I would like to have the 4140 pins a bit harder than they presently are (case). What would you recommend?

Or is another steel better to achieve this but still maintain the same (suitable) shear strength.

Is 4140 supplied in the H/T state needing re-hardening and tempering?

Thanks for your help guys, I really appreciate it.

Regards, Iain.

 
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additionally, if using 1045, what heat treatment would be desirable?

Hardened at what temp?

Tempered at what temp?


 
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Based on the failure mode and the stated environment, I agree with the suggestion from Metalguy: 4140 quenched and tempered, with a subsequent nitriding treatment should perform well in terms of strength, wear resistance, and corrosion resistance. Heat treatment of 4140 is something like austenitize at 845 C, oil quench, temper at 500-550 C. This will produce hardness of ~ 30 HRC and ultimate tensile strength ~ 950-1000 MPa. Nitriding is usually performed in the range of 500-600 C, with higher temperatures lowering the core strength/hardness, which would be undesirable. I recommend a specific type of nitriding known as ferritic nitrocarburizing, also known as salt bath nitriding. Nitromet has a good website with information on the subject. The nitriding will confer greater wear and corrosion resistance.
 
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Who is the mulcher manufacturer? Well, maybe we don't want to attract any lawyers, so I'll just say it like this: if the pins were made in China (especially), India, or Eastern Europe, you might save yourself all this trouble by just replacing them with German or Japanese or British or American, etc. pins.

Assuming that this pin is intended to shear to prevent a much more expensive failure elsewhere, I don't like the looks of that break. Like desertfox said, it looks more like brittle failure than a shear overload. That leads me to believe that the pin itself was faulty, or was designed too hard/brittle to begin with.
 
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Just remember the issues of using super hard/strong pins in a clevis hole made of mild steel. Any slop in the hole-pin fit and the clevis holes will get wallowed out by the pin hammering on them. This will allow the pin to move and chatter then subsequently fail due to fatigue. Hard pins work well if they are placed in tight holes but usually mounting pins require a fit loose enough to put them in and take them out by hand or with a small mallet. A pin with a high hardness subjected to fatigue loading is pretty easy to get a crack in the surface since it it more brittle than lesser grade steel pins.

In short, be wary of high strength steels in fatigue applications with a large stress range. High strength steel works well in high load applications as long as long as the range of stress due to fluctuating loads is small. When the K value of max/min stress gets low (less than 0.4 or so) or goes into the negative range of reversing loads then high strength steel is no better than plain A36 and can actually perform worse due to the reduced ductility not allowing the pin to locally deform to equalize the stresses.

Have you investigated some sort of bushings between the pin and the mounting clevis? Using multiple pins? Some sort of damper between the attachment mounting arm and the pin, similar to a suspension?

You may have already though of all this but I have seen problems before where the bigger/better/harder/stronger mindset makes people forget about what they are trading off for the increased hardness/strength.

 
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>"In short, be wary of high strength steels in fatigue applications with a large stress range. High strength steel works well in high load applications as long as long as the range of stress due to fluctuating loads is small. When the K value of max/min stress gets low (less than 0.4 or so) or goes into the negative range of reversing loads then high strength steel is no better than plain A36 and can actually perform worse due to the reduced ductility not allowing the pin to locally deform to equalize the stresses."<

The subject pin doesn't have any stress raisers in the loaded area, so the fatigue strength will rise with the ultimate strength, at the levels of strength involved here. The fatigue strength should be ~50% of the ultimate.

Note that nitrided Nitralloy 135 Mod. is/was frequently used for highly stressed gears in helicopters, and gear teeth have significant bending stresses. 1045 can't be nitrided.





"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein
 
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The point is that the surface finish is not going to remain at the "polished" level required to use the rule of thumb that endurance limit ~ 45-50% of UTS. Attachment pins like this are going to get small nicks that will act as a stress raiser just like a notch, will get corrosion from oxidation or galvanic from its mating parts that can be just as bad. If the pin was not going to be out in the open and subjected to whatever nature throws at it I'd agree. Gears are protected inside a sealed and well lubricated enclosure so they don't tend to develop all the minor surface defects that pins like this will.

Refer to the attached chart for an example of how "notches" or corrosion can de-rate the endurance limit for even high strength steel.

 
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Hi gibsoni

You might well be confused now with the different advice however it seems to me that either material for the pin would be suitable with the correct heat treatment.
The 4145 would certainly be stronger and with nitriding after heat treatment give better wear, but in one of your earlier posts you indicated that the pin should fail in preference to anything else, in that case you might well be better off talking to the OEM and find out what material they used.
What you will find if you do a google search is that hitch pins in 1045 steel are common.

 
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Thankyou all so much for you help.

I have asked my supplier what temperature my current stock of 4140 was tempered at and for some reason he cant track this lot but says it would definately fall in the range of

570-650 degrees C.

Is this ok?

Regards, Iain.

 
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