A Martensite Question & An Austenite Question. 6

[tc7]

Hello,
Martensite Question:
I am looking for data on Martensite Start and Martensite Finish temperatures for carbon steels and its alloys. I have looked in the "Atlas of I-T & C-T Diagrams" available at the nearest (not very near) University but this book only show M-sub-S and M-sub-90 temperatures. An online resource would be ideal or any other recommended reference appreciated. I have not found any.

Austenite Question:
In the Q&T of carbon steels, such as my favorite 4130, I understand that the it is best to quench such that you have arrived at a full martensite structure and then the martensite is tempered to the desired hardness. BUT what is the result if we do not arrive at the full martensite condition? How will the remaining austenite effect acheiveable strength, hardness, ductility and toughness?

Thankyou

 

[NickE]

For Ms and Mf temps based on alloy content you can probably calculate it with some degree of certainty. I dont have the reference here, I'm almost certain I remember that from school. (Maui, got any hints?)

Retained Austenite will eventually fall down the hill and end up as ferrite+pearlite, the dimensional change can be trouble. More likely an interruppted quench will produce a multi-phase (IIRC) structure containing Tempered martensite, Bainite, ferrite+pearlite, and possibly some odd X-formation products.

I personally have performed some Marquenching, or Martempering, or austempering. (I didn't get to do any metallography, and dont have a salt bath.)

O-1 tool steel was austenitized (1500F), then quenched into a tempering furnace at 525F, held for 4hrs, and cooled in breezy air at ~72F. (in front of the air inlet for the compressor room)

I ended up with an HRc 30-35, a bit lower than I had predicted, likely the quench was not truly fast enough (atmosphere furnace). However as it was a random experiment on a part for an inspection microscope, the part works great. (1010 would have worked too....)

Nick
I love materials science!

 

[CoryPad]

You already have reviewed the standard reference - The ASM Atlas. What would M_F give you that M₉₀ doesn't? Cryogenic considerations?

To answer NickE's question, there is prior work on calculation:

G. Buza, H.P. Hougardy, and M. Gergely, Calculation of the Isothermal Transformation Diagram from Measurements with Continuous Cooling, Steel Res., Vol 57 (No. 12), 1986, p 650-653

K. Andrews, Empirical Formulae for the Calculation of Some Transformation Temperatures, J. Iron Steel Inst., Vol 203, 1965, p 721–72

G. Krauss, Steels: Heat Treatment and Processing Principles, ASM INTERNATIONAL, 1989

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[tc7]

Thanks NickE & Corypad-
My interest is in regard to a welding issue that I am having problems with and I was told that for the post weld treatment it was important to drop the temperature down to or below the M-sub-F temperature before raising it to the stress relief temperature. So when I found the ASM ATLAS and saw the M-sub-90 temperature is ~510 deg F and my weld preheat and holding temp is already much lower than that (400 deg F), I reasoned that M-sub-S has to be much lower than that, maybe in the 250 deg range? And so I am curious as to what happens in the weld and HAZ if I don't reach the M-sub-F temp before I attempt to temper.

 

[NickE]

Whoa.....

Mf is always lower than Ms....

(Unless I've got my concepts backwards...)

 

[tc7]

NickE-
oops, my mis-print. Corrected as follows:

My interest is in regard to a welding issue that I am having problems with and I was told that for the post weld treatment it was important to drop the temperature down to or below the M-sub-F temperature before raising it to the stress relief temperature. So when I found the ASM ATLAS and saw the M-sub-90 temperature is ~510 deg F and my weld preheat and holding temp is already much lower than that (400 deg F), I reasoned that M-sub-F has to be much lower, maybe in the 250 deg range? And so I am curious as to what happens in the weld and HAZ if I don't reach the M-sub-F temp before I attempt to temper.

 

[Robertmet]

4130 - Ms 710 F, Mf 550 F. If you don't complete the martensite transformation ,depending on alloy times and temperatures, you will get any number of possibilities -pearlite ,bainite, or mixes.You should be looking for a uniform structure that will give you good properties that will not change in time .

 

[metengr]

And so I am curious as to what happens in the weld and HAZ if I don't reach the M-sub-F temp before I attempt to temper.

The austenite will transform AFTER tempering to possibly fresh martensite or a mixture of microstructures as mentioned above.

 

[tc7]

So I guess it makes sense when welding high carbon steels, that we permit some slow cooling to occur prior to PWHT so that we are assured that all the martensite that can be formed, will be formed.

This is counter to a common practice we commonly use with higher carbon steels of rushing the weldment right into post weld heat treat without any chance of cooling.

 

[NickE]

"This is counter to a common practice we commonly use with higher carbon steels of rushing the weldment right into post weld heat treat without any chance of cooling."

Do you cross the Ms temp? if not you might be getting a bainitic structure that is relatively tough and strong...and might work just fine.

 

[tc7]

NickE
Even if we rush the weldment into PWHT, it probably reaches down to 400-350 deg F before the PWHT rampup can begin. And this would be our practice with Cr-Mo steels that we use from time to time. So I don't know how to answer the question of crossing the Ms temp. Thankyou for your interest.

 

[NickE]

From your description, it sounds to me like you are getting mostly martensite in the HAZ/Weld... Thus giving you tempered martensite after PWHT.

I dont think you wnat to cool much below 350F, Cracking could be a serious concern.

 

[metengr]

tc7;
I just want to clarify some of the information regarding welding and PWHT of carbon and low alloy steels. If the carbon equivalent (CE) is at or below 0.45, you can indeed go right into PWHT (if specified)or you can cool to RT followed by PWHT, because the ability to retain austenite below 300 deg F is nil for this CE.

As you increase in CE, preheat becomes essential to first reduce the cooling rate from 800 deg C to 500 deg C, to assure complete austenite transformation, and to reduce thermal gradients. It is inevitable that you will form martensite and other mixed microstructures upon cooling for CE values above 0.45. The objective is to keep martensite formation to a minimum amount, and more importantly not to have any retained austenite prior to PWHT.

Now, for even higher alloy steels where the Cr content exceeds 5%, when you weld, preheat is an essential requirement along with the interpass temperature so that one remains in a region between martensite/austenite. It is an absolute requirement to cool these higher alloy steels to below 200 deg F to allow for complete transformation of remaining austenite to martensite, prior to PWHT.

Keep in mind that most pressure vessel (C-Mn,C-Mn-Si) steels and of course the Cr-Mo low alloy steels contain less than 0.35% C by design, and are considered very weldable. If the Cr is less than 2.25%, you can go right from welding to PWHT or even slow cool down to RT before PWHT with typically no adverse effects related to cracking.

 

[tc7]

That is an extremely concise summary of all possible conditions.

When you say, "...It is an absolute requirement to cool these higher alloy steels to below 200 deg F..." I understand it is because of the martensite implications, but does "absolute requirement" come from a Code paragraph??

 

[metengr]

No, it's my specific requirement, as derived from technical literature, in my company WPS's.

 

[tc7]

Metengr-
As I study your 2 Oct recommendation(to the point of memorizing it word-for-word!) I notice a few points that are difficult to interpret. Please advise:
1.
What is meant by, " As you increase in CE, preheat becomes essential to first reduce the cooling rate from 800 deg C to 500 deg C...", more specifically the phrase...cooling rate from 800 deg C to 500 deg C... is not understood; are you recommending preheats be increased so that we achieve an optimal cooling rate of 500 deg C per hour? If that was your intent, it still seems a pretty fast cooling rate and I don't know that production welding lends itself to controlling cooling rates to any degree of accuracy.

2.
The next point of confusion is, "...more importantly not to have any retained austenite prior to PWHT...". I was pretty sure that untempered martensite was the bain of our existence but why is retained austenite so hazardous to weld integrity prior to PWHT?

Thanks again Met, your advice is always welcomed.

 

[metengr]

tc7;
In lieu of me attempting to explain the 800/500 cooling rate equation for welding preheat requirements, let me recommend you visit the web site below (this is one of the best I have seen for actual preheat requirement caculations and other calcs;

http://homepage3.nifty.com/yurioka/exp.html

 

[metengr]

tc7;
Regarding your second question;

2.
The next point of confusion is, "...more importantly not to have any retained austenite prior to PWHT...". I was pretty sure that untempered martensite was the bain of our existence but why is retained austenite so hazardous to weld integrity prior to PWHT?

Very simply, you don't want any retained austenite prior to PWHT because upon cooling from PWHT, the retained austenite would most likely transform to fresh marteniste.

 

[0707]

http://www.georgesbasement.com/Microstructures/LowAlloySteels/Lesson-2/Specimen03.htm

http://www.matter.org.uk/steelmatter/metallurgy/7_1_2.html

luis marques

 

[tc7]

For the case of 4130 (CE=.695) is it correct to say that really sloooow cooling through the 800 to 500 deg C range may produce bainite, or pearlite or combinations and these will NOT subsequently convert to martensite upon further cooling to room temperature? Or will martensite ALWAYS form at and below the M-sub-S temperature no matter what?


My take away on all of this is that slow cooling is necessary for two reasons:
1. by raising preheat temperature we will reduce quench rates through the HAZ that could possibly cause retained austenite (a bad thing) in the martensite.
2. when welding is complete, a slow cooling mode can result in austenite transforming into pearlite, bainite and things that are good (i.e., not hard) , as long as retained austenite does not result.

If this idea is correct, then the practical problem is how do you specify “slow cooling” in a way that is meaningful to the welder ?