For the Metallurgists/Structural Engineers

Trains Magazine General Discussion
1

One of the advantages of stainless steel passengercar construction (I have read) is that cars so built were lighter weight than conventional steel cars. But I haven’t been able to find the answer as to exactly why…

  • Is stainless steel stronger than conventional steel-so the structural members are actually smaller/thinner?

  • Is stainless steel simply lighter-so the structural members are the same size but don’t weigh as much?

  • Is it a combination? 50-50? 60-40? 70-30?

I have gathered that welding saves weight over riveting (and that some experiments were made with welding heavyweight equipment to save weight) but does that favor stainless over conventional construction?

2

Stainless steel as used in passenger car (and freight car) construction is stronger than mild steel and thinner sections can be used. However, to prevent this thinner material buckling, it was common to use corrugated sheet for roof areas. The curved “fluting” below the windows of the classic trains like the 1937 Super Chief was decorative, but was clipped in place between structural ribs that stiffened the side of the car. Another feature of the original Budd cars was the electric “shot” welding, a high power spot welding process that joined the stainless without heating the rest of the structure and reducing its higher strength.

Budd actually developed spot welding as used in automobile construction and had a patented design of pressed steel automobile wheel which were Budd’s profitable products and the stainless steel trains were a sideline, but very profitable in the late 1940s.

M636C

3

Light weight construction is not limited to stainless steel passenger cars. A lot of smooth-side lightweights were built as well. Usual spotting features are the lower roof-line, 4 wheel trucks, & welding.

Budd stainless steel cars were built using stainless steel, and fluted sides/roof were shot-welded to the truss frame - they were not decorative panels(they were part of the structure). P-S cars with fluting did have ‘decorative’ panels and there were a lot of rust related issued due to to water seeping between the fluting and the smooth side panels. Budd also built passenger cars with a sort of ‘seamed’ side panels(GN/NP) that matched their smooth-side P-S/ACF cars, and were painted to match.

Jim

4

The weight savings is due to not having to factor in corrosion. Stainless is actually a little heavier than carbon steel. Also a corrugated panel is heavier than a flat panel for the same area. So the lack of a corrosion factor results in a substantial weight savings in spite of these drawbacks. The stainless was affixed to carbon steel framework very much like the heavyweight cars but lighter gauge steel.

5

As the comments above indicate, reducing weight by reducing metal thickness can be a ‘2-edged sword’, or another demonstration of 'a ‘Law of Unintended Consequences’ - make it too thin, and it has less resistance to compression buckling under load, and so then metal has to be added back anyway in the form of corrugations or other stiffening - which negates the intended benefit of weight reduction !

For a little more insight and facts, here’s some info from a data sheet on 304 and 304L stainless steels by AK Steel, at -

http://www.aksteel.com/pdf/markets_products/stainless/austenitic/304_304L_Data_Sheet.pdf

Density = 0.29 lbs./ cu. in. = 8.03 g/ cm^^3

Modulus of Elasticity, KSI = 28.0 X 10^^3 in tension = 193 X 10

6

I also did a search of John H. White, Jr.'s seminal work - The American Railroad Passenger Car, Johns Hopkins University Press, 1978 (hardcover), 1985 (softcover as 2 vols.), on Google Books for ‘‘stainless’’, as well as ‘Shotweld’. I didn’t find anything on this question worth quoting or mentioning here in the few little snippets that are available ‘on-line’, but there is an internal reference to the whole subject being discussed at more length in Chapter Two - The Day Coach: Era of the Metal Car (pp. 116 - 201). So that’s a reference that is likely worth looking into and ‘checking out’.

  • Paul North.
7

Couple of items not mentioned so far.

Shot welding is the preferred method for cosmetic welding stainless.

You can flame weld it, but it takes a really experienced welder.

Arc or electric welding is the preferred method for structural welding.

You use stainless for the exterior for a few simple but dollar saving reasons.

First, you don’t have to paint it.

Yup, paint can add a lot of weight, just look at the Space Shuttle external fuel tank.

Remember when it was painted white?

Guess what, the whiz kids at NASA realized they could save money, and about 6000lbs lift weight by leaving it primer red.

Yup, 6000lbs, three tons of weight they don’t have to lift.

On a train or rail car, you don’t have to do much more than use a high pressure spray to clean the car, instead of a schedule of strip and paint.

And it is “stainless” after all.

Other reasons, the fluting is a structural design, not just a cosmetic one.

Any panel with a raised portion or design is stronger than a plain flat panel.

Note the waffle design of the sides of a lot of boxcars and thing like the body panels of your automobile.

Rarely do you find a truly flat panel on a auto, because any crease or bend increases the stiffness of that panel.

Flat panels suffer from air drag, they can distort and flex, but a ribbed panel stays “stiff”.

Try this…take a piece of printer paper, hold it a ¼" off the surface of your desk, and blow across the face, (not the edge) of the paper it will lift and flutter and move around.

Now fold it a few time, make a few creases in it, then flatten it out leaving a few “ridges” in the paper, and blow across it again, note how much stiffer it is.

Stainless allows you to use thinner parts, not necessarily lighter parts, that require less maintenance.

Thinner exterior panels mean shorter rivets, wider interior spaces, or more room for engineering space like duct wor

8

White’s book is supposed to be ‘shelved’ in a couple of our local libraries. I may take a look at it sometime in the next few weeks . . . if I do, I’ll post here if it has any insight on this question.

In the meantime, consider this: Loof at a typical ‘broadside’ view of one of the old ‘heavyweight’ cars - preferably one without a lot of battery boxes, air conditioning ice chests or equipment, water tanks, air reservoirs, etc. to get in the way and clutter up the view. Notice that big heavy riveted steel ‘fishbelly’ center sill/ frame ? How much do you think that weighs ?

Now find a similar view of one of the modern ‘lightweight’ stainless steel or even standard steel cars. Likely you’ll not see that center sill . . . it’s been replaced by the tubular shape of the carbody, kind of the railroad’s version of ‘unibody’ automobile construction.

In another thread I did a ‘quick and dirty’ computation about like this - to withstand 2 million lbs. = 2,000 kips or KIloPound

9

See this photo of the underside of an old MKT heavyweight coach (talk about me getting lucky in finding that one !):