Forging vs Rolled Steel
Forging vs Rolled Steel
(OP)
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?





RE: Forging vs Rolled Steel
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.
RE: Forging vs Rolled Steel
If the part had true shape changes, then it is possible for differences between forged and rolled parts.
RE: Forging vs Rolled Steel
This depends on the work reduction ratio for a given process. Assuming similar work reduction - the answer is no.
No.
RE: Forging vs Rolled Steel
Internal flaws usually reveal themselves to the forging hammer.
Mike Halloran
Pembroke Pines, FL, USA
RE: Forging vs Rolled Steel
RE: Forging vs Rolled Steel
Ref thread404-258498: King pin disconnection from trailer
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:
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.
RE: Forging vs Rolled Steel
RE: Forging vs Rolled Steel
As to steps I should think there would only be a head on one end with minimal effect.
RE: Forging vs Rolled Steel
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.
RE: Forging vs Rolled Steel
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
RE: Forging vs Rolled Steel
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.
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.
RE: Forging vs Rolled Steel
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
RE: Forging vs Rolled Steel
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.
RE: Forging vs Rolled Steel
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.
RE: Forging vs Rolled Steel
RE: Forging vs Rolled Steel
Specify forging, and most hidden flaws will reveal themselves to the forge shop, not to the ultimate customer.
Mike Halloran
Pembroke Pines, FL, USA