Rate of corrosion of higher carbon steel - does it differ from Mild steel?

[Jackdawg]

I have been playing around with a length of car axle for a month or two. 

Just realised that it doesn't seem to be rusting like a piece of mild steel would, and it has me a little intrigued.

The metal is almost certainly 4140 (of the Australian steel manufacturers, the couple I found that provide info say that is the grade of high tensile steel they supply for use in axle shafts).

The spec for 4140  says it has small amounts of chromium in it, about 1%,  way less than the minimum 10% for a stainless steel.

Firstly I noticed a bit I had beaten into a flat bar and left sitting on the workbench (outside) was still essentially rust free after weeks of exposure, whilst the other bits of metal I left there all had a good patina of rust on them. The other bits had not been forged, so I thought maybe the scale might be acting as a protective barrier.

But now I have ground it in to a basic knife shape. But before I could move on from the 60 grit, the grinder died.  So the cleaned up bit of metal has been sitting on my workbench for a fortnight, (still have not got around to taking the grinder back for a warranty claim). Was in the shed today and I noticed no sign of rust on it at all, humidity has certainly been high enough to cause rust.

So do you think the small amount of chromium in it is the reason it has not as yet shown any sign of rusting? Or do higher quality steels just generally resist corrosion better than my experience with mild steel I am comparing it to.

 

[Steve Sells]

Yes, carbon and other alloy content can effect oxidation rates . it is the reason how we etch a PW billet

Who gave you the 10% number for SS?  the defining  amount of chromium to a stainless steel is 13 % FREE chrome, not total I wont bother to write more here, as I already have a large write up about it, and could be read if interested.

higher quality steel?????

[ThomasPowers]

"The Story of Metals"  ASM, 1950, mentions that low amounts of chromium say up to 1.5 % will increase corrosion resistance as well as hardenability.

[Jackdawg]

cheers for that.