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Re: Chrome/Moly 4340

To: mdunst@SMTPLINK.COH.ORG
Subject: Re: Chrome/Moly 4340
From: Chasgee@aol.com
Date: Mon, 6 Jan 1997 23:02:56 -0500
Hi Mordy.  Sorry I didn't respond sooner, but its been tough getting on to
AOL lately.  Anyway,

In a message dated 97-01-01 11:59:17 EST, you write:

 <Question: If you had to choose a metal to make a crankshaft which
               would you use.
     
               Moldex uses 4340. Carol Smith's "Engineer to Win" says that 
     it is an excellent metal.  However, they say that there is a 4340
     modified metal that is superior in some respects but possibly not
     in others.  This Modified 4340 has a trace of Vanadium which is 
     supposed to give it more grain streingth.>

4340 is a popular crankshaft material because of its hardenability (uniform
hardness through thick sections) and its good strength and ductility.  It
also doesn't hurt that most machine shops know how to work with it.  The
modified version is usually referred to as 300M and was developed for landing
gear applications.  Essentially, it has better ductility than 4340 at the
same strength levels because of the finer grain formed during heat treatment
due to the presence of the Vanadium.  Interesting to note that the aircraft
insdustry has now moved to a steel called D6AC and in some cases a material
called Aermet 100 (I hear Penske uses this stuff for Indycar axleshafts).
 Both of those materials have even higher strength and ductility than the
300M.  However, since cost is an issue for you, you really don't need to
consider anything other than 4340 for your application.

<Does it matter who makes/processes metal?  i.e. which
               foundry?>

The supplier isn't that important, but the quality level could be.  In other
words, you could order the good stuff or the almost as good stuff.  If it
were me and the cost wasn't that different, I would insist on 4340 to
specification AMS 6414.  Material to this spec is produced to a higher
quality and can give more uniform properties.  If you cheap out, at least
make sure the 4340 they sell you meets AMS 6415.  Still probably O.K. for
your application, but not as good as AMS 6414 stuff.  But remember, you get
what you pay for and your material costs are usually a very small amount of
the overall part cost.  Also, make sure that the material is normalized and
tempered prior to machining and heat treatment.
            
<The book indicates that 4340 should not be hardened to
               Rockwell C of ?43-46- it becomes very brittle.>  

This is true.  The ductility takes a nose dive in that strength range so
avoid it.
               
<After the Crank is made and testing of hardness is done
               can the actual crank be tested or should Coupons treated
               simultaneously with the Crank be tested instead?   Does the 
               hardness testing injure the metal?>

Hardness testing is usually sufficient if the crank is only quenched and
tempered.  If it is nitrided, a sample should be treated with the load to
measure case depth and hardness.  Hardness testing does leave a small
indentation in the part, so it is usually taken in a non-critical area, i.e.,
the end of the shaft.  One other note - get a copy of the heat treat certs to
make sure that the part was at least austenitized, quenched and tempered in
order to develop the properties and not just machined in the as received
condition (unless the material was received in that condition).  This is
important for the fatigue properties. 
     
<Any metallurgists with advice?>

Sorry, don't know any metallurgists that are worth anything :)

Hope this helps....

Chuck Gee
EP Spitfire
Too much materials knowledge 

P.S.  Is Leon's trailer for sale?   Does PIR have hookups we can use during
our event?

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