What is the meaning behind the stipulated range of chemical composition of a steel alloy?

[Metallic007]

Hi,

The range of chemical composition of steel or an alloy has always been stipulated. But, on what basis the RANGE of chemical composition was defined?

I have a Cr-Mn Forged Steel Shaft, where its Chromium (0.9 to 1.5wt%) and Molybdenum (0.15 to 0.3wt%) contents are marginally higher than the stipulated range. What I have got for Cr was 1.6wt% and Mo was 0.6wt%. I know that Cr and Mo are good for corrosion resistance, the more the better. However, will the higher than the stipulated range for both Cr and Mo affect the fatigue strength of the shaft? In other words, can the excessive Cr and Mo cause the shaft to become more susceptible to fatigue?

 

[MJCronin]

Metallica .....

To which material code or standard are you referring ?

ASTM ? ASME ? ISO? EN ?... or something else ?

Supposedly these standards were developed by authorities who put reasonable limits on chemical composition for various reasons.

Limitations on steel chemical composition can be placed for a variety of reasons including long component life, corrosion resistance, creep resistance and to facilitate steel production and quality product. There are several other reasons ...

As a reponsible engineer designing a robust quality product, you are not required to use a steel that meets any international code or standard.

..... but, then again ... you must stand by your decision and explain this to your client

.... and if there is a failure, you cannot blame the quality of manufacture

.... you are responsible

How do others feel about this important issue ???

MJCronin
Sr. Process Engineer

 

[EdStainless]

Provided that your chemistry was done by a lab with certified traceability and standards, and that it was done on a sample of the finished part then you need to look at the analysis specification.
The original material specification sets limits on the original heat (melt) chemistry.
These limits for steels are often based on hardenability and heat treatment response resulting in specific structure and properties.
For a product chemistry there are allowances a little bit outside of these ranges.
More is not better.
The right amounts (and relationships to each other) are critical so that you end up with what was intended.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[arunmrao]

More need not be always good. I had one lot of high chrome iron castings cracking during heat treatment. The lab was convinced that Ni and Mo are friendly elements and could cause no harm in service. The Moly content was about 0.4% higher than specified and Ni about 0.8%. ( Obviously a mixup in charge.). The cracking was due to the increase in the content of Mo and Ni.

Please, request for all elements within tolerance range as given in the standard.

 

[dik]

I'm not sure of the question, but there is a range of allowable chemicals because it's difficult to get an exact percentage of ???. Similarly, when I ask for a contractor to provide sample construction, I spec two samples. It's nearly impossible to have work exactly meet a single sample. Two samples give you a range.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[geesaman.d]

Is there another alloy that has slightly increased quantity of these elements? If there is a good match that is similar in grain structure and properties, that might give an idea of what the properties are going. But behavior of alloying elements is not a simple continuum of mechanical properties.

On the other hand, if early failure is a real risk, don't mess around with out-of-spec material. There are limits because having the right mix of alloying elements maximizes the desired grain structure, but too much can create new phases and interstitials that can be very bad. The test results may also indicate a process that's not sufficiently controlled.

 

[isoca]

Also keep in mind that the limits ensure the homogeneity of the material to be used, which is perhaps as important as the characteristics of the material itself.

 

The product you analyzed did not just miss your specification, it is a completely different grade (and you did not even quote carbon). As previous posters have stated, 'more is not always better'.

Cr and Mo in low alloy steels are there for hardenability; they do not impart much in the way of aqueous corrosion resistance at low or ambient temperature. The benefits of a protective Cr oxide scale (i.e. 'stainless') do not kick in until > 10%.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[Metallic007]

Thank you for the valuable comments from you all.

To Ironic Metallurgist,the specified carbon content is 0.48%max and the chemical analysis result shows that it is within the specification.

 

[mrfailure]

This thread reminds me of one of my pet peeves when I read published literature where authors show chemical composition that does not fall within specification but either ignore it or claim it is close enough. They do not even show if any allowable tolerance is specified that might make the compositions fall in spec. And in some cases, like this one, the alloy is clearly different than the one specified. Sigh.