Effect of Phosphorus on hardening

[PeterCharles]

Here's the problem. I've got a failed 25mm diameter pin from a clevis joint (it's case hardened). It's 'broken straight through corresponding to the centre of the male end of the clevis.
The quoted material specification is 16MnCr5 (1.7131),
C 0.14-0.19, Mn 1.00-1.30, Si 0.4 max, S 0.035 max, P 0.35 max, Cr 0.80-1.10

Lab report gives,
actual analysis C 0.17, Mn 1.35, Si 0.26, S,0.01, P 0.063, Cr 1.12
Surface hardness 640 Hv, core hardness 350 Hv, grain size 4.5 to ASTM E112, case depth 1.1mm

Lab report says ".. reason for cracking is probably the high P content."

I'm not familiar with the effect of phosphorus on the hardening of steels. Any illumination would be appreciated.

 

[swall]

Basically, the lab did not do an in depth failure analysis to determine the cause of breakage. They found one item out of spec and blamed the failure on that.

 

[PeterCharles]

They did say a bit more which I didn't include
"Microstructure consists of coarse tempered martensite in a core, with retained austenite at the surface. Intergranular cracking was observed at the surface. The structure shows that heat treatment was not done optimally."

Does this help?

 

[swall]

Intergranular cracks could arise from quench cracking during heat treating, grinding of the pin O.D. or they could merely be the result of impact fracture through your (brittle) carburized case. Did the lab determine the initiation point of the fracture? Did they do any electron microscope work? What were the circumstances of the failure?
What is the stress level on the pin? Without addressing these items, it is premature to speculate on the role of an out of spec chemistry.

 

[TVP]

In ferrous physical metallurgy, P is a potent ferrite strengthener, second only to C, I believe. However, it has a profound negative effect on the fracture behavior of quenched and tempered steels, especially after case carburizing, due to its segregation behavior. P segregates to prior austenite grain boundaries, promotes the formation of grain boundary cementite films, all of which embrittles the grain boundaries. The following google search shows a number of articles that discuss the effects of P:

http://www.google.com/search?q=phosphorus+grain+boundary+fracture&hl=en&start=10&sa=N

This one is particular describes the mechanism in case hardened steels, from one of the foremost experts on physical metallurgy of carburized steels:

http://www.springerlink.com/content/0tqhk5v7u56518u7/

Not to discount swall's comments regarding the necessity for a thorough metallurgical failure analysis, but I would be very extremely concerned about a case hardened part that has a P content of ~ 0.060 % by mass. Anything over ~ 0.030 has significant negative effects on grain boundary fracture in case carburized steels.

 

[redpicker]

I'd have to question that high of a phosphorus content. As has been mentioned already, phosphourus can, and will, reduce the fracture toughness and can lead to intergranular cracking. So can retained austenite, particularly under a bearing load. A large grain size (a GS of 4.5 is large) will also reduce toughness.

There are many things you could blame, but the phosphorus content seems really out of range. This really isn't something that a steel mill would make a mistake on; their practices typically don't allow for a phosphorus level even half this. Could the surface have been phosphate coated and contaminated the sample thereby affecting the result?

If you take the phosphorus out of the analysis, I think you still have problems that could lead to a failure, so I think stopping at the phosphorus content in the failure analysis is a little premature.

rp

 

[PeterCharles]

Thanks for the comments. I did not arrange for the lab investigation, just got copies of it.
The batch of pins were manufactured in India.

At least I now know a bit more on the effect of phosphorus, thank you.