Case Depth and Premature Cracking of 1018
Case Depth and Premature Cracking of 1018
(OP)
I have a lot of small rods (o.150" diameter) made of C1018 and C1020 that have been case hardened by heat treatment. The only values that were specified to the supplier were a core of 35 HRC and a case of 50-58 HRC. The QC check for these rods is a 30 degree bend test and it turns out that some are passing the test very well, and some are prematurely cracking at the case around 23-28 degrees. I'm wondering since the case depth has not been specified, if this might have something to do with the issue (i.e. a case that is too deep will be prone to premature cracking). In addition, I'm not sure how they heat treat these small rods but is a case depth of 50-58 possible for 1018 steel with only quench alone, or is some sort of carburization needed to achieve this high hardness value? I'm looking for possible explanations as to why some fail and some don't, and I'm leaning towards difference in case depth as a starting point. Would looking at these samples under the microscope provide any further insight into unraveling this mystery?
Thanks,
Racer
Thanks,
Racer





RE: Case Depth and Premature Cracking of 1018
RE: Case Depth and Premature Cracking of 1018
I am surprised some of the rod don't break.
Carburize surfaces don't like bending.
Mfgenggear
RE: Case Depth and Premature Cracking of 1018
normally a lab test done, Micro hardness, & or superficial.
Then thats it.
Mfgenggear
RE: Case Depth and Premature Cracking of 1018
RE: Case Depth and Premature Cracking of 1018
RE: Case Depth and Premature Cracking of 1018
A nital etch will reveal the case depth on 1018/1020 steels, as will a few other etchants. I would do both a microhardness profile, then etch and compare the measurements. I forget the actual test, there is definitely an ASTM test for measurement of case depth.
Nick
RE: Case Depth and Premature Cracking of 1018
RE: Case Depth and Premature Cracking of 1018
Etching with nital is a tricky procedure, it takes a bit of experience to get it perfect. Then it needs to be measured using a metallograph.
This is generally something that it takes a bit of skill and some experience to get perfect, not really something I would try in the garage.
and yes you cannot get 50-58 HRc in 1018/1020 w/o modifying the chemistry. 0.2 carbon steel as quenched (brine) with a fully martensitic structure barely touches 50 HRc. After tempering you could see as low as 55 HRb.
to through harden to high hardness you'd need something like 4140, that would give you enough carbon to harden, then enough alloy to through harden, and after tempering I think you'd get enough ductility to pass your bend. I dont think you'll get a surface of 58HRc, that would take a lot more carbon, around 0.95 at using a temper at 500F. 0.50 carbon will be around 50HRc after tempering at 500F. *(I may be wrong on the alloy selection, I've been out of practice for a while.)
8pts HRc is a big range for a specification, usually I'd spec 2 (so 56/58 or centering in your band 54/56)
RE: Case Depth and Premature Cracking of 1018
It breaks because it's heat treated & brittle, it has very little percentage of elongation.
the external of the bend is in tension, this is where it will crack.
if you explain what their requirement is then maybe the Material Engineers can help you better.
I have fabricated many projects, so I know from experience.
look at these specs
ASTM E18
ASTM E8
ASTM e384
Mfgenggear
RE: Case Depth and Premature Cracking of 1018
At 50 HRC, I'd suspect you could get a 30 deg bend with high success.
At 58 HRC, I'd be suprised if it didn't crack when bent.
1018 or 1020 will have to be carburized to meet this hardness, unless you want to entertain some other hardening treatment (such as nitriding, or more exotic methods).
If you find that only those 55 HRC and above break, change your hardness to 50-54 HRC. If you heat treater complains about this being too tight of a range, consider droping the lower hardness, ie, 48-54 HRC.
rp
RE: Case Depth and Premature Cracking of 1018
Also, is it common to spec a case depth when heat treating? If I say I want a case between 50 and 54HRC but the case isn't deep enough, I might not meet my requirement for torsional strength along the long axis of the rod. Conversely, if I get a case of 50HRC but it's much deeper than I would want, then it will prematurely crack during the bending yield test...
RE: Case Depth and Premature Cracking of 1018
Case hardening is typically used for wear resistance, not to increase torsional (or tensile, for that matter) strength, but you are correct in that if the parts receive too deep of a case they can be adversely affected. Not knowing the application of your parts, it is difficult to know where this point would be, but with a diameter of only 0.150", you run the risk of through-carburizing with a 0.030" effective case depth, which could also cause the cracking you are observing. In general, deeper case depths take longer times, so the heat treater will generally not produce a deeper case than is specified. However, if they are not experienced in carburizing small diameter rods, they may just not realize they are through-carburizing.
rp