Heat Treatment of UNS S15500

[obriend1]

Metallurgists,

Our pressure vessels use S15500 stainless steel aged to H1025. The material size is a 10" bar with a 2" bore hole drilled through the center. Here is the heat treatment and aging process:

- Material originally in Condition A
- Heat treat to 1725°F for 4 hours
- Remove from oven and quench in agitated oil bath set at 120°F for 1 hour
- Wash oil off of part with hot water set at 150°F
- Cool to room temp
- Age to 1025 for 4 hours and air cool.

The problem we are having is inconsistent toughness (charpy) values. We require 20 ft-lbs average for 3 full size samples (taken transversely to the rolling axis) with no single value less than 20 ft-lbs. From past test results the charpy values have been very inconsistent - anywhere from 15 ft-lbs to 40 ft-lbs.

The reasoning behind the pseudo-sol’n annealing temperature of 1725°F, is the fear that a large diameter bar will not be fully sol'n annealed (at the mill) throughout the entire thickness. Therefore, heat treating the part to 1725F will still allow some diffusion of precipitates and quenching from 1725 will not be as severe as quenching from 1900F so there is less chance that the part will crack due to rapid quenching.

I have some questions:

- How important is the cooling rate when quenched from 1900F? Some literature I have reviewed stated that the part should be cooled to 90F within 1 hour - Is this important? Will the cooling rate affect toughness?

- How important is the melting process when the material is initially made? Our spec indicates that the material must be VAR (Vacuum Arc Remelt) on the second melting process, but we don't specify any conditions for the initial melting process. Usually the initial melting process is VIM or ARC. Can the melting process affect final toughness?

- How can we get more consistent toughness results from our heat treating?

- Where can I find a phase diagram for S15500? I have tried the net, but was unsuccessful.

Thank-you

 

[unclesyd]

One question, why are you doing a solution treatment to the bar.

We have numerous pressure containing parts made from 15/5 PH and the we heat treat similar to 17/4. I don't remember ever doing a solution treatment from condition A.

 

[obriend1]

unclesyd,

That is the question other metallurgists are asking as well. From ASTM, normally we would heat treat the condition A material to 1025F for 4 hours and air cool to RT.

The spec was written in '96 buy a metallurgical engineer at our local test lab. From my conversation with the author of the spec, the reasoning behind this pseudo-soln annealing temperature is explained in my first thread. Apparently, according to him, this step also allows the material to have better toughness (14% elongation, 20 ft-lbs charpy minimum) and adequate strength (155 ksi tensile, 150 ksi yield) than aging to a standard H1025. However, I am skeptical since our charpy impact results have been all over the map. The elongation and strengths are always within spec.

Is this heat treatment to 1725F causing lower toughness than a standard heat treatment? Can I expect better or more consistent results if heat treated to a standard H1025 condition?

Thanks

 

[EdStainless]

The uniformity of the structure depends on the forging/breakdown process than the melting. You might see diff in fatigue due to melting practice, but not impact.

If you are going to anneal, then anneal. I am afraid that you are messing the structure up.
Have you tried direct aging from the original condition? The material that I would worry about the most is the center of the bar, and that stuff is gone.

Are you making 15% elongations? Are you aging long enough? with 4" wall I would look into that also.

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[obriend1]

Ed,

The elongation is consistently 14% to 17% - it never falls below 14%. As for the aging time, I believe we are aging our part long enough because the heat treaters guide specifies 4 hours minimum and other heat treat specifications from mills such as Carpenter Steels specify a half hour per inch of material thickness. We try to age our parts 1 hour per inch of material thickness as a precaution to ensure the entire part is aged to 1025F.

Some specs indicate that the part must be rapidly quenched from 1900F to 90F within one hour. After the 1725F step, our part is quenched in agitated oil set at 120F for one hour, rinsed with hot water set at 150F, and cooled to RT. Other than our pseudo-soln annealing step, would toughness be also affected because the part was not cooled fast enough to 90F?

Thanks

 

[mcguire]

The literature I have seen shows this material in the H1025 condition having a average charpy toughness of 35 ft-lbs with a standard deviation of about 7 ft-lbs. So an occasional value below 20 is what the material is designed to produce. The variations in toughness are most likely due to amount and morphology of delta ferrite,which has been the Achilles Heel of martensitic PH alloys.
AK's data sheet on 15-5 recommends at least 1875F solution treatment and cooling directly to 90F. Slack quenching these alloys, which don't have enough carbon to have a significant quench crack danger, coarsens grain size and stabilizes austenite which can hurt the mechanical properties you are seeking to optimize. I would aggree with Ed. If you're going to solution anneal, then don't screw around at 1725. And quench less timidly.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[EdStainless]

McG, I haven't worked much with 15-5, but what about the possibility of incompleate transformation in this section thickness? Would chilling the parts help?

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Rust never sleeps
Neither should your protection

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[mcguire]

Chilling could help because of the stabilizing of the austenite from slow cooling in the interior, if retained austenite is the problem. I'm betting on ferrite. If not that or what we recommended before, he'll have to increase the precipitation temperature to buy toughness.The chemistry balance on these alloys is so delicate that it's hard to think that there is a single heat treat path which will always produce low variability. The variability is fairly built in.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[unclesyd]

My question is still why re-solution heat treat.

As stated before we have thousands of pressure containing parts made from various PH alloys. A majority of these parts are hogged from quite thick sections, 6" to 8", of the starting materials after heat treating to Condition H1150 especially for 15/5 and 17/4. We have never called out a Solution Heat Treatment for any of our components even after we see an increase in hardness and loss of ductility after numerous thermal cycles due to our process requirements. We use a overaging heat treatment to recover a large percentage of the physicals.

I personally don't think that you can work with the idea of particular maximum of any of the physical properties of any of the common PH Alloys. The closest we come is using the minimum of the TS and YS published values and even there you still have some scatter.

 

[mcguire]

Concur with Unclesyd. Nothing good is coming from an extra heat tretment and it may be harming.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[obriend1]

Thanks guys, I really appreciate your input. You have provided some very sound advice on how we can hopefully improve the toughness properties of 15-5. I guess the first thing we have to do is get rid of the 1725F stage and then ensure that the part is rapidly quenched to 90F or colder.

Increasing the aging temperature is not an option because we need the high strength of H1025.

 

[mcguire]

Consider 13-8 which would allow you to increase toughness and strength.

Michael McGuire

http://stainlesssteelforengineers.blogspot.com/

 

[obriend1]

Gentlemen,

As an addition to Ed's question to Mike regarding chilling of the part. Do you mean chill the part from 1875F (we have to re-solution anneal the part and our local heat treater can only go to 1875F) or 1025F or both?

What is the recommended cooling rate and minimum cooling temperature? The heat treaters guide says that after aging to 1025F for 4 hours the part must be air cooled to RT or lower. Can toughness increase if the part is rapidly quenched from the aging temperature?

Our local heat treaters can go down to sub-zero temperatures, but they can't reach the recommended 1900F soln annealing temperature. The part will be first quenched in oil set at around 120F-150F and then chilled to below RT. Any suggestions.

I would like to maximize toughness while retaining tensile properties around 155 ksi tensile and 150 ksi yield.

Thanks

 

[EdStainless]

My thought was to sub-zero chill after the solution. But, I have to agree with others. Unless you can show that the original solution treatment was incompleate don't re-anneal. Minor chemistry differences between heats of these grades changes the aging response, you may need higher aging temp on some lots of material.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm