Acceptable content for an unspecified element ladle analisys

[Cello]

Hi all,
I'm dealing with a standard CrMoV steel where the Nb maximum acceptable ladle value is not specified. I think in this case I should consider it as residual element, tough I don't have any clue of where is the limit between residual and intentionally added. There might be an international standard such as ASTM or ASME, which clearly defines this limit for this steel family, but I couldn't find anything in this regard. Does any of you know standards that could answer my doubt?

Thanks a lot for your help.

 

[Maui]

What exactly is the grade of steel that you're working with? If the Nb content does not appear in your chemical specifications for this grade, then the Nb is by definition a residual element. The company which manufactures this steel may have an internal maximum aim for the Nb content. By contacting them you should be able to find out if this is the case. If they don't have any internal aims, then this element probably appears in such low concentrations that it does significantly impact the properties of this alloy. For example, M8 high speed steel used a 1% Nb addition. The claim was that this addition minimized the tendency of this steel to decarburize. Since no one was ever able to substantiate this claim, the grade fell by the wayside. To the best of my knowledge, Nb does little if anything to enhance the properties of either tool steels or high speed steels.


Maui

 

[Maui]

I should read my responses with more care before posting them[purple]. My sentence in the above response should read,"If they don't have any internal aims, then this element probably appears in such low concentrations that it does NOT significantly impact the properties of this alloy."


Maui

 

[mcguire]

The niobium will be present as NbC below about 1000C. If your alloy can stand the loss of carbon ( about 1/8th the amount of Nb by wieght ) then the Nb is insignificant. It is not the type of element that should be present as a residual impurity so its presence would be curious and worthy of a question to the producer.
Nb is a key constituent of some high strength/low alloy steels. It could be useful or detrimental depending on your alloy and the use to which you intend to put it.

 

[Cello]

Thankyou guys for the clarifying replies. To go deeper, this is the average steel analisys:
C=0,30%
Cr=1%
Mo=1%
V=0.30%
On the heats the Nb content which is not specified by applicable spec. scatters from low values of 0.002 up to values of 0.02. While I feel that the low values could well be considered as residual impurity from scrap, I do not have the same concern on the higher ones. Therefore I was trying to figure out whether there would be international codes and/or standards that limit the unspecified elements to typical residual maximum contents. Regardless the possible harmful effect that such a Nb content would have on the steel properties, which I don’t think is an issue for this steel grade, setting such a limit by means of a code/standard would then help me a lot through the worthy discussion with the steel mill.

Have a good day

 

[JSMAT]

Mb can be used in a similar way to V to improve structure, but at 0.02% it would have little effect. The variations could be explained by variable scrap sources or carryover from previous heats.

 

[stanweld]

It would appear that Nb is not being intentionally added by the manufacturer based on the 0.002% to .02% range assuming the steel is from the same manufacturer. It may simply have been contained in the vanadium ferroalloys. If the higher Nb content is from heats made by another manufacturer, it most probably was intentionally added.

Nb has been used as a microalloy for decades in carbon and low alloy steels as a grain refiner and strengthener in additions as low as 0.015% (aim) and is often used together with Vanadium. It combines with carbon and nitrogen to form fine niobium carbo-nitrides when used in your high temperature alloy and will provide real increases in creep properties at the 0.02% level.

Nb is also used in low temp. steels. It essentially permits the lowering of carbon content yet maintaining high strength.