17 4PH stablilization and microstructure 3

[LukaszKukolowicz]

In ITN I foud stabilization after annealing and before aging.

Stabilization
fixture introduction in to the furnace at a temperature not higher than 300 °C, ris e
up to 850 °C, hold for 2 hours , cooling in still air (till T<200°C) and subse quent
cooling in water fo r 2 hours t o guarantee that the temperature on the part is less than 30°C

I can't understand what is the reason for it. Due to Fe-Cu phase diagram at this temperature not all copper is in solution. Also I found A1 near 630^C, and I assume A3 about 1000^C. What is the reason of heating in two phrase region, and maybe more important to me how does it influence on microstructure?
Olso I'am looking for data of microstructure in different temperatures.

 

[remetaper]

Lukas,
Please excuse me but I am unware the acronym ITN and what means. Stabilization is not a heat treatment typical of this grade as well as ITN suggests.
174PH approximately has AC1 = 630°C (1160F) and AC3= 710°C( 1300F). In order the largest amount of Cu could be dissolved in Austenite, it must be treated at 1040°C (1900F. This heat treatment is called " Condition A " or Annealing (ASTM A564).
Lower annealing temperatures cause a variation of mechanical properties in low temperature aging condition.
For instance, in case of condition H900, an annealing at 1700F ( 930°C) causes a reduction of HRC ( from 45 to 39 ), of Rm ( from 195 Ksi to 185 ksi 9, of Rp02 ( from 185 ksi to 170 ksi). This values could have some variation depending on chemical balance of heat number ( mainly C%, N% and Cr/Ni equivalent). In order to understand the behaviour of 174PH and similar grades (soft or Low carbon martensitic stainless steel) ,read annex at page 1 and 2 of annex technical paper (Sevilla, Spain 2005, Stainless Steel Congress). I thing could be useful to clarify.
Really , I am not able to explain the reason why this grade would be treated at 850°C ( 1550F) AFTER annealing 1040°C (1900F).
In other words, an aging after a not complete solution of Cu in Austenite that is the indispensable condition to obtain a good aging.
There is somebody who could find a right metallurgical reason for ITN 850°C heat treatment ?
Let me know if you need more info.
PS: If he right terms were "stress relieving " instead of "stabilization", this is an other story that requires a different explanation! Obviuosly, not at 850° C !!

 

[LukaszKukolowicz]

Thank, you or your answer.
I was told that ITN is number of norm. File I get from client is named itn06710_023. I kow quite well standard heat treatment and behaviour of 17 4PH.
But heat treatment in 850°C is so mysterious for me I had to ask.
It seems you get the right answer. Due to this document characteriastics to obtain ofter aging coreponding to H900 are:
YS 150 ksi
elongation 16%
Impact KV 40 joule
So what they call stabilization is heat tremtment rising elongation & probably ductility. I've missed it at first.

To be shure I need 2 more answers. Do you know what is the temperature at which delta ferrite starts to form? What ductility (KV) should I expect from H900 condition?

 

[remetaper]

Different heats with different chemical balance ( Cr/Nieq ) can contain different % of delta ferrite. By control of chemical balance,the %of Delta Ferrite can be mantained very low until reach zero as 15.5PH. However the amount of ferrite at hot working temperature is influenced by temperature and soaking at temperature. Hot plasticity curve of several 17.4PH give the best range of t° = 1120- 1180°C . To overcome it could be vey dangerous both for ferrite and overheating (burst, decoeasion.. ). A diagram Cr=18 , Ni=4 , C=0,04 gives the field of delta ferrite ( only delta , not gamma +delta ) at 1220°C . Obviously , this t° depends on chemical balance of all elements. Usually , it could be lower.
About Kv at H900 (i.e 1040° C + 480°C x 1 hour ), you can read these values at fig.6 of annex. Please verify how much is important the Cr/Ni eq chemical balance.
Let me know if you need more data .

 

[LukaszKukolowicz]

It was all I wanted to know.
Final conclusion is heat treatment at 850°C rises elongation. Duclility is inchanged or lower. YS & UTS are much lower. Thank you very much indeed.