"It depends." As noted above, Strasburg successfully machined main rods for a large 2-8-0 out of steel billet material and they've apparently been working fine for several years. I suspect the Bluebell Railway is fine with what they're doing as well.

OTOH- forging rods from high strength steel to minimize weight and achieve other desirable characteristics may be a necessity for building things like reproduction Pennsy T-1's.

.

An example of the final CNC machining for a set of rods;

http://www.northernsteam.com/Motion.htm

Rods, for an SAR Cape gauge (3'+6") 25NC, modeled on 7+1/4" gauge,

If these would have been full scale they surely would have been waterjet cut to get them closer, it all depends on the cutting time used to determine which method will end up the least expensive. This was done on a Hass (not known for great rigidity or a deep cut).

The customer only provided the shop with the set of (accurate) detailed drawings. The shop provided the materials and all machine work and delivered the finnished rods. The cost was very reasonable, but I'm not sure I'm supposed to give that out freely. This is just under 1/4 size/scale, so a lot less chips were made on this starting from billet, than would be made from a full scale rod project, even if they were water-jet cut initially and finnished on the CNC machine.

Hello,

Very interesting initial post along with all the follow-up dialogue.

Even with near net shape forging, even closed die forging, any post machining is going to breach the grain pattern. In my younger days I recall the hot rodders always speaking glowingly of billet crankshafts as the ultimate for high performance car/ race engines. Likewise H beam pattern versus I beam patterns for connecting rods were preferred to blend into the big end bolted joint....these may have been billet also.

Most routine applications are not pushing stresses to the limit. Our heavy duty diesel and natural gas engines designed for full load 24-7 operation appear to have forged conn rods but minimal machining. Do I recall such parts being shot peened to add a skin of material under compression? Of course aggressive shot peening could distort a part.

Kelly's comments on today's high quality materials was interesting. The initial post sure described a complex process which had many opportunities to slip up and result in a defective part.

Is there really any analytical evaluation which can be related to forging, especially grain flow?

It's certainly much simpler to machine from billet. Especially in low production runs.

Great information!

Thank you all for posting.

Cheers!

Randy Musselman

>Do I recall such parts being shot peened to add a skin of material under compression?

There are two distinct 'schools of thought' about this; I think they are both valid but require somewhat different methodologies:

The 'classic' shot peening approaches are to relieve stress raisers introduced by surface machining or residual surface characteristics. This is the purpose that used to be served by polishing high-stress members like IC connecting rods in the '30s. (Superfinishing, both the Chrysler and the modern chemical methods, are something different, although there is some overlap)

Some of the current peening methods do appear to introduce some directional 'worked' structure into the underlying metal in a complex part. I've seen some discussion of this in performance-automobile magazines. However, I am old-fashioned in thinking that you'd need to work the part at elevated temperature to get the effect of 'forging' -- most of the existing "peening" techniques are documented as if they are a form of cold working...

Found a slightly different recipe for the N&W J in its drawings:

N&W J Class
Timken High Dynamic Steel (4339 modified)
Carbon: .40-.43
Nickel: 1.7-1.8
Chromium: .7-.8
Molybdenum: .28-.33
Manganese: .70-.80

Yield: 110,000 psi min
Elongation: 18% min
Reduction: 55% min
Brinell: 269 min
120D: 50 min

As I recall, it was later found that 'nickel steel' fatigued quickly under use. It was a new alloy in the late 1930s, and had fallen out by 1950.
With modern alloys, you don't need forging. A 4140 or 4340 casting (or billet) will oftentimes do the job better than the antiquated 1018 forging ever thought about!

CD