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Up Topic Welding Industry / Metallurgy / PWHT CS vs Alloyed steel.
- - By 3.1 Inspector Date 04-18-2008 09:35
Gentlemen and experts,

Why does PWHT for alloyed steel (P5, P9, P91, etc) happend at 720 - 750 degrees celcius?
Is the lower transformation temperature different than for CS - if yes - how do I determine the lower transformation temperature for a given alloy?

Thanks.

Jesper
Parent - - By js55 (*****) Date 04-18-2008 13:46
Optimum PWHT temps for virtually all alloys is generally determined emperically in order to result in the best combination of strength, ductility,and toughness, while of course staying below the lower trqansformation.
As for determining lower transformation I'd suggest that if it is a genuine issue for you that there are many, many materials books and reference volumes available with that info. Try The Metallurgical Society (TMS) or (ASM) American Society of Materials.
You may even be able to google most alloys.
Parent - - By 3.1 Inspector Date 04-18-2008 15:12
Thanks js55,

But am I right in assuming that the lower transformation temperature depends on the alloy(Cr content, etc...) ?

I have done some "googleing" without any CLEAR answer to my question :(

I will try TMS and ASM
Parent - - By js55 (*****) Date 04-18-2008 15:44
It most certainly depends on the alloy content most directly. Thats why transformation temps are most accurately spoken of in terms of temp ranges. Some alloying elements can greatly effect the transformation temp with very small variations in volume percent. For example carbon content in carbon steels, or nitrogen in SS's.
Parent - - By 3.1 Inspector Date 04-18-2008 16:13
Great replys js55....

I will continue my study
Parent - - By 3.1 Inspector Date 04-22-2008 05:24
Thanks Tom,

Exelent sites, I have found the lower temperatures in ASME 31.1

Thanks again....
Parent - - By gwg (*) Date 04-25-2008 00:41
{Why does PWHT for alloyed steel (P5, P9, P91, etc) happend at 720 - 750 degrees celcius?}

First off, PWHT for higher alloy steels is selected or happens to obtain desired mechanical properties; it is not an absolute temperature requirement. In other words, for certain applications, the PWHT can be as low as 300 deg C and as high as 700 deg C. Again, the final selection of PWHT temperature depends on alloy composition, and desired strength and ductility. There is a limit for PWHT, and that temperature limit is the lower critical transformation temperature. If one exceeds this temperature during PWHT, the steel properties will be altered from the desired outcome from PWHT.

 

{Is the lower transformation temperature different than for CS - if yes - how do I determine the lower transformation temperature for a given alloy?}

Yes, it is. The lower transformation temperature can be determined by the chemical composition of the steel. There are actual formula's for establishing the lower critical transformation temperature as a function of C and other alloying elements in the steel.
Parent - By 3.1 Inspector Date 04-25-2008 05:23
thank you gwg, I am all straight about that now - I just needed the last push in the right direction.
Thanks....
Parent - - By js55 (*****) Date 04-25-2008 13:50
Keep in mind the formulas for transformation temps were determined from equlibrium (or near equlibrium) data. Rapid cooling rates will tend to delay and depress transformation. Not to mention the fact that tolerances in chem testing methods can render the formulas somewhat 'fuzzy' as well.
Its really quite similar to the whole history of ferrite estimation with stainless steels.
Chemical testing accruacy, cooling rates, testing methods, etc.
Parent - - By gwg (*) Date 04-26-2008 05:27
js55;
The lower critical transformation temperature does not change as a function of cooling or heating rates. What you describe is the formation of metastable phases  -  bainite, martensite and retained austenite under non-equilibrium cooling conditions. The lower critical transformation temperature is still a defined quantity and does not change with cooling rate, only chemical composition, and grain size. Review a CCT diagram, the lower critical transformation temeprature does not change.
Parent - By js55 (*****) Date 05-01-2008 20:23
OK. I agree. Let me put it differently. And a clarification was in order. With more rapid cooling rates not all grains will transform at the specified transformation temp. With faster cooling rates a higher volume percent of grains transforming are depressed to lower temps, giving us, as you mentioned bainite, martensite, or even just acicular ferrite, and others (since we don't get transformation to ferrite first and then martensite-transformation goes right from austenite to martensite and the others). So even though we say the transformation temp doesn't change, as the cooling rates go up more and more of the grains aren't listening to us.
I'm not disagreeing with you, beause what you are saying is true and I was well corrected, I'm just saying that if an austenite grain is waiting until a lower temperature to transform into martensite instead of ferrite because things is movin too fast, before it gets there it doesn't know anything about an unchanging transformation temp.

Parent - - By Geese_howard (*) Date 07-05-2008 07:22
You are right about the change of critical transformation temperatures depending on alloying content, great book about this is
"the effect of alloying elements in steel", by Mr Edgar Bain, metallurgical legend, an old and genious book indeed. It is somewhere downloable via
pdf, look for it is great piece of metallurgy history.

But, the differences on PWHT temperature, are mainly because of the "resistence to softening", impaired to the steel mainly by carbide forming
elements such as Cr, Mo and V. If you look to some "tempering curves", or diagrams showing decrese in hardness (or UTS or YS), versus
tempering temperatures (after a full quench of course), u will see Cr-Mo steels retains most of their mechanical properties even after high tempering temperatures, but most of the time there is a "inflexion point" in which softening will start more markedly, so a good source of non-code, non-official data to determine the right PWHT
for any alloyed steel, or hardenable stainless, is to check the tempering diagram for that steel, (which is generally readily available in steel producer's brochures), and determine
the temperature at which this marked decrease starts, then for the pwht , 30ºC or 50ºC more should be enough (but not tresspassing lower critical temperature!!)

By the way, this resistence to softening is also the reason why Cr/Mo steels are used in high temperature parts of boilers, and are more resistent to creep

Regards
Parent - - By Geese_howard (*) Date 07-08-2008 05:26
Good, i suggest everyone gives it a look.

By the way, in that site there is also papers and thesis from Teacher Harry Bhadesia, one of the top metallurgists in the field of phase transformations. I have downloaded some but most of them are really out of the reach of my narrow understanding, XD  .... i blame my teachers ... lol
Parent - By js55 (*****) Date 07-08-2008 13:09
Geese,
I hear ya about Bhadesia's work. I've been reading it for years. Much of what I know about carbon steel phases and transformations comes from him. But a mountain of it is way beyond me. But I too would recommend the effort to wade through it.
Parent - - By ssbn727 (*****) Date 08-01-2008 03:28
I posted this thread below with a link to the book list from the University of Cambridge's site back in January...

http://www.aws.org/cgi-bin/mwf/topic_show.pl?tid=14492

Here's the link again, and if you search some more you'll find that I posted a bunch of good info on Ti also. ;)

Respectfully,
Henry
Parent - By Uncle Chuckles (*) Date 08-20-2008 17:54
My Word

That is one heck of a link, It will take me many moons to read a fraction of that book list. But it will be done
Up Topic Welding Industry / Metallurgy / PWHT CS vs Alloyed steel.

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