Structure change due to overheating 2

[deco0404]

Hi guys,

just got asked whether accidental local overheating and quick cooling of pipe can change the structure to martensite, I said probably not, but my question to you guys is...

What are the conditions ( if any ) that could possibly change a structure ( lets just assume A106 gr.B) from pearlitic to bainitic or martensitic. I am familiar with T-T-T diagrams, but not in reverse. I would imagine the material still needs to go above 800C before any change is likely....

Declan

 

[TemperedMartensite]

Here is the quick checklist-

Alloy
Time at temperature
temperature
heat extraction rate

I worked at a company that made thin walled tubes (.018") out of 4140. Out process was to drop them into a furnace at 1600F and quench them 5 minutes later - fully martensitic.

 

[metengr]

The bottom line is this, what was the maximum temperature and time of exposure? If the temperature was below either the lower critical transformation temperature or original tempering temperature (if applicable), nothing would happen.

If the maximum temperature exceeded either of the two critical temperatures above, you have an altered material.

 

[cloa]

The question is probably has the pipe really changed to martensite or just embrittled possibly hydrogen, heavy metal or carbon contamination. Are the operators just guessing?

 

[deco0404]

Cloa,

that is correct, they just want to be sure its not martensitic, and I'm fairly sure its not. I would be very surprised if the temperature reached the A3 temp.

When the material is originally formed, if it is quenched from austenite fast enough to avoid the pearlite transformation, giving you a martensitic structure (ok so I am simplfying this slightly). Would there be no change if you reheat the material(as pearlite) to just below the A3, and then quench it, would some martensite form, or would you just get larger, harder pearlite grains?

Declan

 

[davefitz]

In the case of overheating a formed part, the analysis gets more complicated. For a short term overheat of a thick part, the degree of overheat depends on positon thru the wall thickness, and type of new crystal formed is depenednet on the rate of cool down, which also varies as a function of position thru the wall thickness.

Also, in addition to crystal structure, there are also strenght enhancing strace metlas that are added to the original melt- the hi temp creep strength is dependent on an equal distribution of these trace metals thru the material. A short term overheat could allow some of these trace metals to come out of solution and agglomerate- so even if teh corect crystal structure is formed, the lack of evenly distributed trace metals will lower the long term creep srength.

 

[deco0404]

Thanks Dave,

just to qualify this better. The pipe is std wall X55 gas pipe, , creep resistance is not a consideration, it is not in any kind of dynamic environment, except for pressure drops and increases, max operating pressure of approx 45Bar.


Declan

 

[metengr]

If you did not exceed the lower critical transformation temperature there would be no martensite formation. If the material was originally Q&T, you will further temper the material if you exceed the original tempering temperature.

 

[stanweld]

deco0408,
How did the overheating occur? Did someone try to heat straighten the pipe; with a rosebud or better yet with oxy-acetylene torch? If so, you can almost be guaranteed that the lower critical temperature was exceeded. Did they water quench on one side after heating?

 

[unclesyd]

A test that is somewhat less than a proper microstructural analysis is the old fashioned hammer test. Even though various methods exist to examine an actual sample of the affected material it is still very common to test any steel that had been overheated with this test. I've tested pot still distillation columns, heating fluid lines, boiler tubes using this test.
One thing this type testing enables is the ability to cover wide areas of affected material. A case in point is some heating media vapor lines exposed to flames due to firemen knocking off the asbestos insulation so they could keep the lines cool we were able to safely bring the lines backup very quickly to mitigate further loss.
This type physical testing is not for every case but can be assistance in some cases but is not a substitute for a formal metallurgical analysis.

Please don't hammer test anything pressurized as I have found some very brittle material using this method.

 

[dik]

unklesyd:

What is the hammer test and if it is just impacting the pipe what are you expecting as a result?

Dik

 

[unclesyd]

The nomenclature isn't exactly correct but you will hear the term burnt metal used with hammer testing. If there is been any transformation of the metal it have very low ductility and in some cases will not deform and actually chip out when struck. The test isn't absolute unless the material breaks when struck. I was taught to especially check around welds or any place where there was any signs of the metal yielding.
I haven't use the test on process materials since 1989 but have used it when flame straightening structural steel where it would take more than one heating cycle to straighten the section.
A little history on hammer testing. When I first started work it was quite common to hammer test a new fabrication when hydrotesting. Again it was mostly in the area of any welding or the weld proper. You always wanted to have a little deformation of the area struck.
I should have stated that with the portable equipment available the hammer test has been retired, but you have to remember that it was used for years in the day of Acetylene Welding, Twin Arc Weld and sometimes material with a shaky pedigree.

 

[rob768]

If it is a vital part, you can have the structure tested through a replica, or perhaps ultrasonic hardness testing will tell you what you need to know.

 

[BrianE22]

Can I throw a related question in here? When a steel shaft heats up enough to turn blue can you assume that something has happened to the steel? The steel in this case is Stressproof.

 

[metengr]

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