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metalht (Automotive) (OP)
21 May 10 7:17
Dear all,

we are doing the gas carburising process in pit type furnace, (average case depth upto 2.8mm). we have observed some different type of microstructure (surface) at certain batch. this structure like a giraffe skin structure. and its coiming only at surface. but we have checked the   hardness on structure(by micro hardness) it's around 57/58 HRC.

as per Our Quality team statement that structure is troosite, but i was not accept. And i dont have any proper document for prove the structure. But i explaind about the troosite structure. enventhrough they are not accepting.

pl guide me how i can explain about the troosite structure.

Remamber: i have to explain to the non-metallugist.
Helpful Member!(4)  redpicker (Materials)
21 May 10 10:02
Troosite is an older term that is usually a mixture of martensite and bainite.  Usually, the bainite etches darker than the martensite, but the bainite will form at the grain boundries, so it could be better described as inverted giraffe skin (the dark is around the edges and the lighter is in the center of the grains).  This condition occurs because either the surface has cooled too low prior to quench, or the quench itself is too slow.  This condiiton normally is of lower hardness than a fully martensitic case.

What I consider "giraffe skin", with light boundries and darker interiors, would occur with a grain-boundry network of carbide (which etches light) with tempered martensite interior (which would etch darker).  This is caused by too high a carbon content on the surface.  With this condition, the hardness is generally unaffected.  This is usually a very undesireable condition since it can crack very easily.

rp
metalht (Automotive) (OP)
22 May 10 0:47
Mr redpicker,
 thanks for your reply, and can you tell me, what is max hardness we can get from troosite structure.

 Regarding giraffe skin =structure we have inspected the carbon content at surface after quenching is 0.65%C (by spectro machine). We have carbides network near the surface. but that also with in range.

pl guide me, if iam wrong,
May be this structure will come, if we have maintained the high carbon content at the time of activation in carburinsing cycle or diffusion time is not sufficient.
or the 850c holding time is very high at low Cp.

 
redpicker (Materials)
24 May 10 12:31
Again, troosite is an older term and I not really that familar with it.  The "old guys" I would talk to in the 1980's would use it, though.  I learned the structure was a mixture of martensite and bainite.  In a carburized case, fully martensitic is what you want, banite is undesireable but better than a carbide network, which should be avoided.

I cannot tell you the max hardness of a troositic structure, I would guess almost as hard as a fully martensitic structure with the same carbon content.

The 0.65% C on the surface seems a bit low for a carburized case, but without knowing the steel chemistry, it may be fine (it would be OK for AISI 9310 steel, but way too low for 1018, for example).  Depending on the steel in question, this could lead to a mixed microstructure in the case, which could be called a troositic structure.  

Whitout knowing exactly how your carburizing operation is set-up, I cannot guess on the problem.  You are correct that a low carbon potential (Cp) with extended holding at 850C can lead to low carbon on the surface.

rp
dbooker630 (Materials)
24 May 10 13:55
I first heard about troostite back in the late 1970's from the old-timers in the bearing business. It was indicative of underquenching and I recall an etching process using picral until straw-colored (good luck getting approval to use that!), followed by a quick nital dip.  

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