Forging vs Rolled Steel 1

[metman]

An AISI 4340 king pin loaded in shear.

1) Will forging produce deeper working of grain structure than hot rolling assuming a simple cylindrical shape i.e. no head?

2) Does a forged king pin have greater shear strength than a cold rolled pin given essentially equal chemistry and final hardness?

3) If statement 2) is true, why?

 

[EngineerTex]

1) Potentially. But I think it would be a function of the individual production lines.

2) I think that it would have the same shear strength.

3) Hardness for most steels is (roughly) directly proportional to yield strength. Assuming an isotropic ductile material, the strength theory of your choice (maximum shear stress or distortion energy) determines shear strength as a function of tensile strength. If the hardness is equal, the shear strength should be equal.

But I have a feeling that this is a trick question.

Engineering is not the science behind building. It is the science behind not building.

 

[CoryPad]

Will these parts be hardened and tempered after forming? If so, I don't think there will be any differences.

If the part had true shape changes, then it is possible for differences between forged and rolled parts.

 

[metengr]

1) Will forging produce deeper working of grain structure than hot rolling assuming a simple cylindrical shape i.e. no head?

This depends on the work reduction ratio for a given process. Assuming similar work reduction - the answer is no.

2) Does a forged king pin have greater shear strength than a cold rolled pin given essentially equal chemistry and final hardness?

No.

 

[MikeHalloran]

Rolled parts can have internal flaws that remain concealed through the usual manufacturing sequence, and reveal themselves in service.

Internal flaws usually reveal themselves to the forging hammer.



Mike Halloran
Pembroke Pines, FL, USA

 

[tigermoth]

Things would be different from a fatigue, fracture toughness and possibly ultimate shear and bending strength point of view if there was a step or other stress raiser - I would think that the forging would perform better. I'm not sure how you would quantify the difference theoretically.

 

[metman]

Ref thread404-258498

The above thread is where these questions came to mind.

No trick EngineerTex.

I was just trying to think why a forging would have greater shear strength than bar stock. Most of your replies confirmed what I thought might be the case except maybe for Mike's comments:

Rolled parts can have internal flaws that remain concealed through the usual manufacturing sequence, and reveal themselves in service.
Internal flaws usually reveal themselves to the forging hammer.

These flaws would be elongated longitudianlly so that a transverse tensile stress will be affected so then is it really isotropic? But this is a moot point because this is a case of essentially pure shear and I think these defects would have minimal infuluence on shear strength but maybe I am wrong.

 

[tigermoth]

I'm pretty sure heavy trailer kingpins experience significant bending and sometimes axial tension also. They are also usually stepped, depending on the type.

 

[metman]

Good point tigermoth. I was idealizing the connection in my head so yes there could be significant bending loads if space between the clevis and yoke but how would axial tension be applied?

As to steps I should think there would only be a head on one end with minimal effect.

 

[dhengr]

Metman:

I’m not a Metallurgist by formal education, but I have rubbed elbows with a bunch of them over the years, when I felt I needed their help. And, we generally spoke the same language and understood each other. My education was in Structural Engineering and Design and Theory of Elasticity, with considerable design experience in Mechanical products too.

Re: your questions: 1) I would agree with EngineerTex, and say there is considerable potentially for grain orientation improvement in the forging process. But, I’d add that it may be more a function of how much forging work is done and how close the forged shape is to the finished shape, so as not to machine the improvement away.

2) I don’t think Zeusfaber (Mil) intended to imply that the shear strength would be greatly better with a forged king pin vs. one made from rolled matr’l.; either hot rolled and treated or cold rolled. He was thinking about the overall performance of these type of products as forgings, see below. I would base the allowable shear stress on the relationship btwn. shear stress and yield or tensile strength. Of course, heat treatment would improve strength and hardness (wear), an important consideration.

3) Given rolled matr’l., the grain improvement caused by the rolling process for a round bar, is essentially machined away before you get to the finished shape. Sufficient forge working causes the near surface grain structure to conform with the near finished shape of the king pin. Thus, you can end up with improved grain orientation at various transition locations, particularly at the radius btwn. the king pin and its upper bolt flange. This particular area is loaded in shear, bending and vertical axial loading and is susceptible to impact loading and fatigue also, thus any grain orientation improvement it to the good. The grain orientation wrt max. stress orientation is the important thing. The axial loading occurs because of relative vertical movement btwn. the tractor and the trailer.

Do you happen to know the thread # of the earlier ‘what’s up with up,’ I thought you were just funnin us, just as I was pullin your leg, and didn’t realize it started someplace else.

 

[tigermoth]

I'm probably thinking of a different style of pin, one with a bolted flange mounting.
Axial loads (though probably not critical in the design)could arise if
1. If the trailer has a high C of G and is undergoing sufficient lateral acceleration during cornering to try to overturn; this is resisted by a download on the kingpin and an upload on one side of the skidplate
2. The tractor and trailer are on planes at an angle to each other eg when turning into a sloped entranceway. Overturning moment is generated by the enforced torsional displacement between tractor and trailer (depending on fifthwheel characteristics).
3. Normal road-going vertical accelerations occur.
A retaining flange or head is usually provided at the bottom end of the pin - this is presumably to prevent vertical movement.
Details are on the websites of the major manufacturers eg Jost, Holland, Ringfeder, Georg Fischer etc

 

[metman]

dhengr,

Yes very good analysis and agrees with all the other input.
My problem is that I oversimplified the design/application but also I was purposely trying not to make the question(s) overcomplicated or difficult to understand.

A bit redundant to reiterate but it does go back to the original post of not providing enough info. Hopefully he will heed the advice given there:
First put the onus on the supplier but also have expert failure analysis done to learn the mode of failure but this also could be improperly oversimplified given all the suggested possible loading combinations available given above. Analysis of one failure might not be typical or the failures might be atypical requiring numerous examples.

Just to satisfy my curiosity I need to look up one or more mfgrs websites provided by tigermoth to get a better idea of the actual geometries involved.

Do you happen to know the thread # of the earlier 'what's up with up,' I thought you were just funnin us, just as I was pullin your leg, and didn't realize it started someplace else.

No I did several searches and could not find it and that is why I started the new thread to see if someone could give me the link to the thread #. All this upitty stuff began plagueing my mind again the other day at work wr2 a technical issue about upsizing some holes. And then I've had a lot of time on my hands this week recuperating from some major dental work. I even got to experince N2O, Nitrous Oxide, Laughing gas -- what a gas in every sense of the phrase.

 

[rmw]

For all the abuse that trailer king pins see, many being mentioned above, the incidence of king pin failure that I have ever heard of (and I do have some unfortunate association with the trucking industry-I can't get rid of the nightmares) is rare.

They are tough little buggers compared to the job they are asked to do.

More often than not, any incident associated with the coupling is due to fifth wheel jaw slack more than any other factor that I have heard of.

And... not that they were very popular, in addition to the flanged mounted king pins, the most common of course, was a drop in version that could be shifted between multiple mounts for changing the front overhang of the trailer. I haven't seen one of those in years, but did 20-30 years ago.

rmw

 

[tigermoth]

rmw - totally agree. Most disconnect incidents I have heard of result from a failure to engage properly in the first place due to the factors we've mentioned. (I've never actually come across a broken kingpin).
The semi is often left behind at the first set of lights.

There are quite a few of the drop-in relocatable kingpins still in service here, mainly on low-bed semis. They would resemble metman's hypothetical pin more closely tho I think they may also be stepped at the point af maximum bending moment.

re the trucking industry - I recognise the symptoms you mention.

 

[macthenife]

Backward extrude your kingpin from cold forgeing wire.
This should be approx 20% larger in diam than the kp.
Heat treatment would destroy the tight grainflow and outer skin work hardening.
A firm with cold headers could eat this job.
Try SPS.

 

[tigermoth]

Is there a quantitative way of predicting that forging is necessary (over machining from stock steel bar)in any given new design? Or is it always a judgement thing, based on superior (but not quantifiable) mechanical properties that can be obtained, plus a history of failures in similar components produced by other means?

 

[MikeHalloran]

It isn't a judgment thing, it's a liability thing.

Specify forging, and most hidden flaws will reveal themselves to the forge shop, not to the ultimate customer.


Mike Halloran
Pembroke Pines, FL, USA