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Re: Stainless steel 17-4PH
Re: Stainless steel 17-4PH -- barrower	Post Reply	Top of thread	Forum

Posted by: crbeebe ® CBeebe 01/24/2005, 10:38:22 Author Profile Mail author Edit

Per MSFC-SPEC-522, 17-4 PH is a "table II" material with only moderate resistance to stress corrosion cracking.  I ran up against this problem before.  Our solution was to use 15-5 PH with a heat treat condition of H1000 or above to replace the problematic 17-4 PH (heat treatment is important here--15-5 has similar SCC problems with heat treatment below H1000).  15-5 PH has very similar properties to 17-4 PH, and can be used in many cases as a direct replacement (depending on heat treatment).

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Re: Stainless steel 17-4PH
Re: Stainless steel 17-4PH -- barrower	Post Reply	Top of thread	Forum

Posted by: workman ® 01/22/2005, 04:55:06

Cracks should not appear if heat treatment cycle is correct. Cracking due to intergranular Corrosion. Improper heat treatment of stainless steels can lead to early failure  under severly corrosive conditions and can greatly reduce service life in many relatively mild enviroments. Unstabilized austentic stainless steels that contain more than .03% C become susceptible to intergranular corrosion because complex chronium carbides percipitate along grain boundries when the steel is exposed to temperatures from about 540C to 845C. As a result , the chronium content near the grain boundries is depleted to the point  where the boundries  have no more corrosion resistance than plain carbon steel . Stainless Steel is susceptible to intergranular corrosion and will not undergo such attack unless a corrosive medium is present . However , many corrosive media , including some considered only mildly corrosive ,will corrode susceptible steels to intergranularly. The possibility of intergranular attack must be ruled out by experiance after solution treatment.Chromium carbide precipitation  in austentic steels can be eliminated by  1.  Heating the steel after final fabrication ,to a temperature high enough to dissolve the carbides 1040 C to 1150C and cooling rapidly enough to avoid reprecipitation.  2- Using a stainless that has been stabilized with niobium or titanium, which combine   with the carbon and thereby prevent harmful precipitation of chromium carbides.  Martensitic precipitation-hardenable stainless steels  heat treatment 17-4PH  .07C(max)1%Mn(max) 1%Si (max) 16.5%Cr (max)4%Ni (max)4%Cu , and .3%Cb is as follows 1- solution heat treat for 1/2 hour at 1035C   2- Cool to room temp approx 72 F and hold for 3 hours   3 - Reheat and age at 480C for one hr per each inch of thickness.   (#1 is at 1/2 hr per each inch of thickness)   The copper rich phase precipatate which contributes to the hardness of 17PH is dissolved by step #1 that develops an austentic structure. Cooling and holding at room temp #2 forms lower carbon martinsite  but does not allow percipitation  of the copper rich phase. At this point the alloy is not particulary strong compared to the properties  developed by reheating to 840C #3 ,whitch causes the copper rich precipitates to form and increases the tensile strength from 1035MPa (150ksi) to about 1345MPa (195ksi)Request charts and Certs from Heat Treater  as well as RC reading. Remember A-286 or 17-7 PH is used where high strength S/S with resistance at elevated temp is required.   17-4 PH or HNM Aplications requiring good corrosion resistance  and scaling resistance at high temp. Stainless Steel is protected from the enviroment with an invisible shield but effective field. If stainless steel ,However, becomes oxygen starved in any particular area by a washer or pile of sand , the passisivity of the stainless steel can break down in these places . The area of stainless steel that is freely exposed  to `the  dissolved oxygen acts as a cathode . Corrosion will take place accordingly under these deposits or crevices where oxygen cannot penetrate and the iron becomes anodic; therfore , when stainless steel is around seawater or other corrosive media, it is important  to avoid crevices where oxygen cannot penatrate . In these cases s/s can corrode just like any other steel. To help you any further  I would need more specifics. Awnser  to Question   Good Stainless Material  to be   to be austenized to the 17-4 PH condition  . A Heat Treater that is respectable  and will certify his work and provide you with charts and hardness RC Verification. 17-4 PH will not have stress cracks  or corrosion if processed correctly . ASM heat Treaters Guide or more info ASM online or ASM Heat treating Volume or ASM correspondance course's Precipitation Hardening Heat Treatments,Heat Treatment of Npnferrous Metals and Alloys,Failure of Metals under Service Conditions.  I hope that this was helpful. ScottWorkman

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Re: Stainless steel 17-4PH
Re: Re: Stainless steel 17-4PH -- workman	Post Reply	Top of thread	Forum

Posted by: workman ® 01/24/2005, 03:17:07

Welding the stainless is a horse of a different color. You defiantly need to be careful. Monel,nickel,and inconel are all readily welded. they can be welded with filler metal containing the same analysis as the base metal; the heat effect of welding does not alter the properties of the base metal to any great extent. In nickel, the Age-hardening alloys of the K-monel,Z- nickel and Hastalloy group are hardenable and may be welded by several methods. Shielded metal arc is usually perfered,but gas welding may be used, although excessive grain growth may take place in the heat effected zone . These alloys are hardenable simply by heating beetween 1100 and 1600F (593 and 871C) and then cooling slowly. There fore a simple annealing proceedure  is all that is required after welding to give the weld and the heat - affected zone uniform hardness and higher corrosion resistance.> The geometry of the part needs to be taken into account , also the racking proceedure prior to heat in furnace. You may need fixture with thin parts or hang. I have used C clamps and 1 inch plate to avoid this distortion. The best way to avoid  distortion if mass production the temp is to be ramped up from 600f  at 150 degree intervals every 30  to 45 min(time ,temp depends on part mass as well as ramp speed can use 5 min with small parts) to set point and held 1 hr per each inch of thickness .Distortion = Time +Temp+ Thickness+ Placement proceedure. Be careful with thick plate it will crack. When I was working with USN Dept of Defense the ramping proceedure on plate for Conditioning  to PH Cond . I used an outlined MIL Spec. I wish that I could remember the specf  but I can't and do not have them in library. I think it is start 600F then ramp 100F each hr and hold 1 hr at 1150F per each inch of thickness then decend 100F each hr  to 600F  open cool to handle. This is pretty close. Bar stock is somewhat different it is machined  90% of time and is usualy purchased in ready annealled state. Machine heat and go. Quick and dirty will work for most applications. I hope this will help you out let me know. Scott Workman

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Re: Stainless steel 17-4PH
Re: Re: Stainless steel 17-4PH -- workman	Post Reply	Top of thread	Forum

Posted by: syed-jaffer ® 01/23/2005, 09:02:32 Author Profile Mail author Edit

Thanks from me too.  Very informative and in detail.  I have a question.  if it is weld fabricated part will there be deformation with this much of temperature 1150 deg C?

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Re: Stainless steel 17-4PH
Re: Re: Stainless steel 17-4PH -- workman	Post Reply	Top of thread	Forum

Posted by: barrower ® 01/22/2005, 10:32:42 Author Profile Mail author Edit

Helpful ? very much so ! Thanks for all these info and in details, it covers the full range. I don't mention that in my first thread but for our customer it is for some applications it is surrounded by seawater. Now the argument seems to be fair, unless S/S specs are very precise and high requirements. Thanks a lot for your help, wonderful!

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