how to deal with Porocity after heat treating? 1

[ht3jyx]

the job description:
the material is steel 1010, parts are 1 1/4 inch washer. heating 1 hour at 1650F in carbonitride atomsphere. then quench in 90F poplymer. without tempering.

the test on the parts after heat treating shows porosity under the surface.

the question: why porocity happens? and what we can do to make it disappear?

thanks

yush

 

[metengr]

ht3jyx;
What test?

 

[arunmrao]

Are these washers P/M parts? If correct have they not been coined?

Heattreatment does not cause any porosity to occur,but it might expose any preexisting pores.

Perform a DP test on unheattreated washers to detect the presence of pores.

 

[ht3jyx]

answer to metengr:
we send the part to a metallurgic lab. I think they did general material test. but i can verify with the lab again.

questions to arunmrao:

what is P/M parts? what is DP test?

thanks,

yush

 

[ht3jyx]

by the way, can heat treating cure preexisting pores? if so, how?

thanks,

Yush

 

[metengr]

ht3jyx;

P/M is sintered, powdered metal
DP is known as liquid dye penetrant, this is a nondestructive test for detection of surface defects

Are these cast parts?

 

[arunmrao]

ht3jyx,
Heattreatment is used to enhance mechanical properties,improve machinability,stress relieve the component or modify the surface withour affecting the core.

In no way should you progress to heattreatment stage if you have unacceptable parts with preexisting defects.

Can you elaborate on the material used,sizes and the manufacturing process for a better response.

 

[swall]

Porosity in carbonitrided cases comes from nitrogen. Nitrogen is formed by the dissociation of ammonia. Your ammonia flow rate may be too high and you are creating too much nitrogen and the excess nitrogen is causing porosity. High carbon potentials as well as increased processing temperatures can lead to excess nitrogen formation

 

[Yallapragada1022]

I completely agree with Swall. Carbonitriding does create porosity if the surface layer has too much nitrogen concentration. Same is the case with gas nitriding. I am not really sure if you can completely eliminate porosity on plain carbon steels. You can definitely minimize the porosity by lowering the flow rate and processing temperature.

Rao Yallapragada

 

[arunmrao]

swall,I tend to agree with you.

The statement made by OP " The test on the parts after heattreating shows porosity under the surface".

This led to my responses assuming that the case formed was a defect free one.

 

[ht3jyx]

to confirm that the parts are 1-1/4" washer, made out of stamping steel sheets.

here is a strange thing. we run 2 batchs at two identical lines (same carbonitriding atomsphere), the only difference between the two lines is that one using polymer as quench media and another oil. the porocity only happens to polymer quench line.

the photomicrograph shows samples from polymer line porocity at the surface to approximately 0.0015".

I guess i can try to shut off ammonia flow in polymer line to see if we can avoid porocity.

please let me know if you guys have any comments.

thanks.

 

[kenvlach]

For an ASM classic paper on the subect, see
A Practical Study of the Carbonitriding Process,
R. Davies and C.G. Smith, Metal Progress, Vol 114 (No. 4), September 1978, p 40-43, 50-53 © ASM International.

http://www.asminternational.org/pdf/classicpapers/Davies&Smith.pdf

ht3jyx, if you cannot post your micrographs, please compare them to figures in the above paper & tell us which ones match most closely. Please give more information on the gas mixtures & cycles; also, is the problem uniform over surface or greatest near edges? I agree with comments above re ammonia. Doubt that quenching medium is a factor.

A 1993 paper (Effect of NH[3] stepwise increase in gas carbonitriding atmosphere on nitrogen content of low carbon steel) may be useful. Although the journal Nippon Kinzoku Gakkaishi [JOURNAL OF THE JAPAN INSTITUTE OF METALS] is in Japanese, the articles include a synopsis and figure captions in English.

http://cat.inist.fr/?aModele=afficheN&cpsidt=4880612