Case Depth and Premature Cracking of 1018

[mxracer17788]

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

 

[mxracer17788]

In addition, what is the best method for measuring the case depth on a small cylindrical part like this? I would assume the best way would be to cut a transverse section from the rod and use a microhardness tester to measure the hardness at different locations moving toward the center axis. When the hardness of the case matches the core, this is the total case depth. Is there a standard for this test method other than SAE J423?

 

[mfgenggear]

MxRacer

I am surprised some of the rod don't break.
Carburize surfaces don't like bending.

Mfgenggear

 

[mfgenggear]

Who decided that a bend test was required?
normally a lab test done, Micro hardness, & or superficial.
Then thats it.
Mfgenggear

 

[mxracer17788]

So in order for 1018 steel to be surface hardened to 50-58 HRC it must be carburized? This isnt possible from just austenizing then quenching and tempering? (dumb question I know, go easy on me I'm a civil engineer).

 

[mxracer17788]

The bending requirement is necessary for their application. At this point, I'm just looking into ways to figure out why some break and some don't. My first guess is differences in case depth since some rods are capable of bending up to 60 degrees without visible cracking.

 

[NickE]

You are right about taking a metallurgical section of the rod, you could even make some very detailed longitudinal profiles too...

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

 

[mxracer17788]

How long do I nital etch? Is there a procedure for this test method? After etching, how will I be able to visualize the case depth?

 

[NickE]

You need to do a full metallurgical mount and polish, down to 0.03um Al2O3. this is required for both the microhardness profile and the etched structure evaluation.

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)

 

[mfgenggear]

MXracer

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

 

[redpicker]

Check the surface hardness of the rods prior to bending.

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

 

[mxracer17788]

Awesome, thanks so much for the help guys.

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...

 

[redpicker]

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...

Yes, it is common to specify a depth; it is usually specified as a depth to a specific hardness (say, minimum effective case depth of 0.030" to 50 HRC) or, not nearly as common, as total case depth; that is, the total depth the carbon content has been increased. The first method is easily verified by a microhardness traverse on a cross section of either an actual part of a coupon while the second method is much more involved since it includes making chemical analysis tests at various depths.

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