[N&W] Re: Steam hard on the track

[nw-mailing-list at nwhs.org]

Gary Rolih writes:

Metallurgical properties of steel rails is a complex topic.  In the 
simplest terms, the hardness and strength of steel is controlled by the 
carbon content and heat treating.  As the carbon content goes up, the 
hardness and strength increase- but simultaneously, the ductility decreases 
and the brittleness increases.  The alloying elements help control the 
changes the steel undergoes in heat treating.  This is "hardenability".

Another simplification. Steel at room temp has a certain tensile strength- 
say 90,000 psi to yield- that is go from elastic deformation to plastic 
deformation.  Most steels can be cold worked to increase the tensile 
strength.  The cold working causes precipitates or grain realignment in the 
steel which causes the increase in strength.  Cold Working is 
DIRECTIONAL!  Roughly, cold working would increase the strength to about 
150,000 psi.

But, cold working is defined as reduction of area.  Imagine a 1/4" thick 
plate; to get a 50% increase in strength, you need to reduce the thickness 
by rolling about 40% or just under 3/16" thickness.

Heat treating with the addition of carbon, increases the yield considerably 
more.  With at least 4 points of carbon, 1040 steel can be heat treated to 
about 200,000psi.  Tool steels with 25 points of carbon and lots of 
alloying elements can get increases up to 350,000 psi to 500,000 psi.  But 
the steel will be hard- Rockwell 60-65, and brittle.

A steam locomotive does have high vertical loads going into the rail from 
the imbalance and the rotary-to-reciprocating motion.  But, they cannot 
cause a reduction of area- the result would be a local hole!  This of 
course would not be desirable.  Obviously, the rail could be "smoothed" by 
the passage of many locomotives.  But, clearly this can be seen as impractical.

I don't' recall what rail has in it as alloys nor exactly how it is 
processed at the mill but it should be something like this:

Low to modest amounts of alloy.  Alloying elements cost money!  Railroads 
buy a lot of steel rail.  Guess what is used- as little alloy as possible. 
The predominant alloy in rail is manganese.  ( A true cold working steel is 
maraging steel.  Think of backhoe teeth.  Maraging steel is high in nickel, 
like 18%, and VERY expensive.)

Some heat treating is done so that the rail head is "through hardened" to 
increase its resistance to abrasive wear and fatigue cracking on the 
surface. Rolling wheels cause local bending causing Hertz contact stresses. 
which cause fatigue, seen as spalling.

The web and flange want to be ductile, bendable and tough. So they probably 
have little or no heat treating.

Some time ago in TRAINS they had a rail article which was quite informative 
about why's and wherefore's.  I would seek it out.