Ferritic heat resistant casting failed in tensile testing test

[Martinos]

Dear all.
I have just received the results from external lab with tensile testing results for heat resistant centrifugal casting (as is specified below).
Customer need us to reach: (+ results)
tensile strength at least 450 MPa / 65 KSI ........ we reached only 290 MPa, 2 specimens, third was ruptured at radius
0.2% yield strength at least 400 MPa / 58 KSI ........ not measured, UTS was reached before Rp0.2%.
elongation at least 5 % ........ we reached only 0.5 %

Chemical composition of centrifugal casting is 0.4%C; 2.3% Si; 0.9% Mn; 17.6% Cr; 0.9% Ni; 1.6 % Nb, 1.7% W. S = 0.02%. P = 0.018% = conforming to customer standard.

Hardness measured = 244 HBW 5/750 - conforming to standard
Microstructure = ferritic matrix with two types of carbides embedded - conforming (please chceck the direct download link) Link

Have you got any idea why our results are so poor please? Thank you!

 

[metengr]

What was the casting quality? Did you evaluate the failed tensile specimens to check for possible defects?

 

[EdStainless]

What heat treatment did you use?
Have you ever made this alloy before?
Were Nb and W added shortly before casting?

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P.E. Metallurgy, Plymouth Tube

 

[Martinos]

Quality was evaluated as good.

Tensile specimens are in the lab for archivation. I do not have access to specimens now.

I suspect the laboratory (possibly not good manufacturing of specimens) or impurities. But metallographiic examination of the same tube did not show us any impurities, inclusions or quality problems.

 

[Martinos]

I have re-checked the chemical composition analysis. Antimony content of 0.1% was measured by internal spectrometer. This is very interesting.

 

[Martinos]

Ed Stainless,
no heat treeatment, heat treatment is not allowed from the customer,

this is the first time we made this alloy

FeW and FeNb were added not shortly before casting, but sooner.

W and Nb content in tube is conforming to standard and the same is from the melt analysis.

 

[arunmrao]

Coarse grains due to superheating of the metal can be a reason.Any grain refiners added. Also, any inclusions observed on the fracture surface.


"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

[EdStainless]

What orientation are the micros?
If this is centri-cast it should have a columbner grain texture in the radial (through wall) direction.
The grain size looks way too large.
Did you de-ox before adding Nb and W? Looks like some dirt in there also.

Who set the standard? Are you trying to reverse engineer someones alloy?

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P.E. Metallurgy, Plymouth Tube

 

[Martinos]

Ed, orientation of micros is radial. Casted tube have outer diameter 94 mm, inner diameter 28 mm and is 710 mm long.

Our melting technology is "dirt" based on induction remelting of scrap + adding feroalloys (100 kg furnaces). De-ox was not implemented in this case. Our common de-ox is only by aluminium in the ladle, but in this case was not implemented. It was only our first specimen-pouring. Now we know we cant guarantee properties from customer standard.
It is standard adjusted for investment casting. Customer need us to made this steel according this standard, but by centrifugal. This alloy should replace AISI 309 forged pipes.

arun, our melting temperature is high because we are pouring very small inner diameters. Temperature of melt in furnace before pouring into the ladle was 1660 deg. C. But we use similar parameters for all of our 10 heat resistant alloys and we do not have problems like these there. Tensile strength 290 MPa is too poor even if you have larger grains. Inclusions at fracture? Yes, I think this is good point of view from your side. Thank you guys. If you have some other notes, please, do not hesitate to ask me. Thanks a lot.

 

[arunmrao]

@Martinos,

I remember processing for a well known valve manufacturer sand casting in Inconel 600. The casting was machined and tested ok.

But the mechanical test failed due to poor elongation values. This was not in my scope of supply but the customer shifted the goal post.

I failed convincing him the difference between sand cast test bar and the reported standard values mechanically worked product.

Hope, you are not in a similar situation.


"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

[metengr]

This material looks similar to ASTM A 743 CB30 plus the addition of several spices. The tensile properties without an anneal heat treatment are 65 Ksi UTS, 30 Ksi YS and 241 HBW hardness

The anneal by specification at 1450 deg F and air cool is

95 Ksi UTS
60 Ksi YS
15% Elong
195 HBW