Unexpected values of impact strenght ! 1

[stanislasdz]

I have 2 steels with the same composition and the each block of these steels was heat treated in the same conditions QT (according to the heat treatment department !)

The mechanical properties show a low values of impact strenght for the second steel and high values for the first one.

We can see also a homogeneised microstructure for the first one and heterongenous one for the second.

I don't find why we have theses unexpected values of impact strenght in the second steel ?

Stan

 

[arunmrao]

Can you tell some more about the heat treatment cycle for the 2 test bars. Obviously something seems to have gone wrong with the heat treatment of second test bar.

Just a nagging doubt,did someone forget to separate the test coupon from the riser before heat treatment.

Chocolates,men,coffee: are somethings liked better rich!!
(noticed in a coffee shop)

 

[unclesyd]

I concur with arunmrao in that it appears something is rotten in heat treating department or the sample had a miserable first life, thermally.
There is nothing in the chemistry.

The only time I've seen similar microstructure is where the steel was left at the austenizing temperature too long. I would also try to find the prior thermal history of this sample.

 

[arunmrao]

stanislasdz,

If you had not given the micros, I would have suspected test sample preparation and inspection to be a cause for low values.

Chocolates,men,coffee: are somethings liked better rich!!
(noticed in a coffee shop)

 

[stanislasdz]

The test coupon were casting with the part !

Austenization T°C : 880
Tempering : T°C : 610 °C


What is the microstructre showed in the steel N°2 ?

I guess it's tempered martensite with retained austenite (in white) is that true ?

How can i do more investigations ?

 

[SJones]

The magnifications on the micrographs are not given but No2 looks to have a fairly hefty (prior) austenite grain size indicative of some 'overcooking'. First stop should be the heat treatment charts from the furnaces.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdoweb/

 

[stanweld]

stanislasdz:
I tend to agree with Steve. Were T/C's attached to each coupon? Furnace temperatures can vary substantially within the working zone of the furnace, especially when the furnace hasn't been surveyed.

Because the coupons were cast, segregation could also be involved.

 

[redpicker]

As mentioned, it would help to know the magnification used in the micros. The etchant would be helpful, too.

Offhand, I'd seriously wonder if the second sample had been quenched, if heat treated at all. The yeild to tensile ratio of the first sample is 91%, what I would expect for a well-quenched sample. On the second sample, its only 87.9%. With a normalized sample, I'd expect a Y/T ratio of around 75-80%, so this is a little high, but not knowing the section size, it might be higher. Based on the chemistry, this steel has rather high hardenability, so with a small section, I could believe those mechanical test results in the as-cast condition.

I don't know what these microstructures mean without the etchant and magnification. It is very doubtful you are seeing retained austenite with this chemistry. I'd guess that in Steel No. 2, you are looking at either a mixture of bainite and ferrite, or a coarse-grained banitic structure with the different-colored areas being different orientations of the large grains.

Can you get them to re-heat treat steel No 2? If its properties come back to what you have with No. 1, it is obviously a heat treating problem and you really don't have to go any farther than that.

rp