Lower tensile strength and elongation level in heat resistant CC steel

[Martinos]

Hi guys,
I have one question for you...

We have heat resistant austenitic stainless steel made by centrifugal casting. It is chromium-manganese steel (20:10 %) with tensile strength about 700 MPa and elongation about 15%, but the tensile strength and elongation level is sometimes suddenly decreased. The chemical composition did not show us any correlation. Process parameters as well.

What is your opinion: What can decrease both, the tensile strength and elongation together in this case?

Thank you!

 

[MagBen]

Two reasons i can think of: coarse grain and grain boundary precipitates.
I donot expect a large elongation with 10% Mn. Do you have a high N content?

 

[metengr]

Please describe what

...but the tensile strength and elongation level is sometimes suddenly decreased

means. Are you referring to new castings supplied and tested for compliance to a material specification or in-service failures or?

 

[arunmrao]

I am not familiar with the alloy and hence pardon me, if my comments are laughable.Some of the causes could be:

a) Sample selection and preparation

b) presence of unsoundness in the sample

c) Any carbides at grain boundaries. ( This is common in Hadfield steel)

d)Is the Nitrogen content varying ?

e) Are these castings heat treated ?

I'm just one step away from being rich, all I need now is money.
( read somewhere on the internet)

 

[EdStainless]

I think that the guys have hit the high points.
The first two that I would check are nitrogen content and grain size.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[Martinos]

Hi guys,
thank you...

to MagBen: Yes, we have nitrogen content about 0,15 % (standard requires 0,1 - 0,3 %)
to metengr: it was typical randomly testing, no in-service failure

I have to write this steel has 0,1 - 0,15 % of sulphur after solidification and the morfology of manganese sulphides is very crazy, MnS size is sometimes too big and looks "slaggy":

Link
Link

Castings are not heat treated. I think some submicroscopic precipitates may play a role in this case, but I can't check it in our conditions.

 

[Martinos]

Another question,

Microstructure of specimen with good properties: Link

Microstructure of specimen with lower properties: Link

How to determine the grain size for this kind of microstructure? According to ASTNM E 112-10 subclause 17.3-5 it is a problem. And as well is a problem to distinguish the grain boundaries for this type of steel castings.

 

[metengr]

Martinos
Castings have variability with regard to mechanical properties. Are the castings with lower properties below the minimum requirement for acceptance or simply lower in value and by how much in comparison to a "good" casting.

 

[Martinos]

Yes,
tensile strength required min. 690, our typical 720, sometimes, like now 677 MPa
elongation required min. 15, our typical 16, sometimes, like now 12 %

 

[MagBen]

The comparison of microstructures clearly tell the story. both casting microstructures are lousy, but the "bad" one showed larger dendrites (not grain structure) and higher tendency for net working!

Possible solutions: 1. increase cooling rate during solidification. 2. decrease casting temperature if possible.

 

[EdStainless]

I would try annealing some samples and see what you get for properties.
Those are poor microstructures.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube