A second heat treatment to dissapear a " Rock candy" defect !

[stanislasdz]

We have a part in steel casting! With 600 mm of thickness !

The final US inspection showed defects witch seems intergranular like Rock Candy.

The aluminium content is 0.031 % but the nitrogen was not measured !

Before the Heat Treatment the part was Ok in MF inspection after Heat Treatment (QT) defects appear in MF and US inspections!

1- Can these defects type rock candy appear during Heat Treatment ?

2 - Can we "save" this part by a second Heat Treatment to dissolve the aluminium nitrides and wich treatement can we apply

Any informations are welcome !

 

[metengr]

stanislasdz;
Have you confirmed that the defects you determined from ultrasonic testing are indeed AlN particles along prior austenitic grain boundaries? In other words, have you confirmed this finding metallographically?

If you are only going by NDT information, this could mislead you into other possible causes of casting defects - porosity, hydrogen cracking, etc.


How were the heats produced - EAF?

 

[redpicker]

I have not seen any relevant ultrasonic indications that can be removed by heat treatment. I am a bit confused how a ultrasonic inspection reveals an intergranular "Rock Candy" type of defect.

What is the chemistry? Are these internal, or do they show up with Magnaflux?

rp

 

[stanislasdz]

Metenger and redpicker thanks

Sorry my thread was not so clear !

By the pas we have a lot problem on many thick parts (see the picture above)

After many investigations we found theses defects were caused by a big amount of Aluminium in first and in less degree by nitrogen (aluminium nitride). We have not observe aluminium nitrides since it's difficult to see Rock Candy fracture under simple light microscopy (we have not a SEM !) (see picture above)

Now, most of these parts since we reduce the alumnium are free of defects!!!

But one kind of these part (remain with defects and we don't now why?

Here some data

Homogneizing @ 1050 °C
Quenching 880°C
Tempering 570°C

C : 0.22
Mn : 1.20
Si : 0.51
S : 0.009
P: 0.013
Ni : 0.15
Cr : 0.24
Mo : 0.05
V : 0.004
AL :0.031

By the past we see that if we have these kind of defects we have an extinction of the signal in Ultrasound detection!

Since that if we see an extinction of US signal we assume that this is due to the Rock-Candy defects !

I also think that this may also due to tempering embrittelement so since it 's a reversible process I think to remove the segregations (Sn, Sb, S, P ) by reheating in a second tempering above 575°C and so we can remove the intergranulaire fracture !

Who can help ?

 

[metengr]

stanislasdz;
This casting should not be used based on the documentation you provided. You cannot reverse the visual indications that you highlighted. Re-heat treatment cannot heal these defects.

 

[stanislasdz]

obviously !!!
It' not the part the picture is just an exemple of an old part

I am looking to reheat a part with non indications in Magnaflux but the US inspection seems à dead signal !

Why i don' t know ?

 

[metengr]

stanislasdz;
What you describe is an attenuation problem with ultrasonic testing. This could be caused by grain size effects or technique or even equipment. Do you have a calibration block for this testing and is the tester qualified for UT? Do you have a procedure for this or did you hire a vendor?

 

[stanislasdz]

Metenger : the device of US inspection work fine and here a summary of the US inspection :

Is it a shrinkage porosity or an embrittlementof grains or a lack of cohesion or only a coarsing of the grain size ?

Ps: un second tempering give the same indications as you said Metenger ! Unfortunately