re-heat treating 17-4ph 3

[engamopar]

Given: Raw Material 17-4ph (AMS 5643- Solution Annealed).
Parts fabricated & then aged to H900 for 1hr. (Result Rc 42.3 - As expected. Only problem: Was supposed to be H1075 for 4 HRs!!! Can we just re-run thru [CORRECT]
aging process as is?

 

[monkeydog]

I'll let the metals guys answer the grain structure type of questions, but your dimensionals my move a little, by re-annealing and re-heat treating.

 

[israelkk]

Yes.

 

[metalonis]

I agree with israelkk. Just run the aging procedure again. I had a similar situation myself.

 

[EdStainless]

Unless you have paticular fatigue properties to meet I would just re-age. Often material responds well and sometimes you even get better properties doing this.

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[RPstress]

We re-aged Custom 455 from H950 to H1025 with some success some years back. Discoloration was an issue, but static properties and toughness (KIc) were fine. We had trouble showing it was was ok from a crack-growth point of view.

 

[unclesyd]

We have never encountered any problems in re-aging of the PH alloys, mainly 455 and 17/4. We generally use H1125 for our components. As we purposely force an oxide on the components is no problem.
The process where we use these materials causes an increase in the hardness with resulting lost of toughness over time. To restore the physicals we routinely use an over-aging treatment. We are able to restore 80% to 90% of the original physical values.

If one has to remove the bluest heat treat tint on 17/4 I strongly recommend that you use the alkaline permanganate bath prior to the HNO3/HF. By doing this you hardly affect the surface finish of the part.

 

[israelkk]

unclesyd

Our experience with chemically removal of heat treat tint on 17-4, 15-5, custom 455 is unpredictable and uncontrollable from the dimensional changes of the parts point of view. It may remove 0.05mm (0.002") thick layer from the part surfaces. For parts manufactured to 0.001" dimensional tolerance this may be disastrous.

The more severe issue is hydrogen embrittlement for parts heat treated to H1000, H950, H900 and maybe even H1025. Chemical cleaning for high strength heat treatments is forbidden by ASTM passivating specs. Mechanical cleaning process such as vapor blast etc. is recommended. Any process should be considered not to remove too much material. The best is to final machine the precise dimension after heat treatment and heat tint cleaning. For aerospace and military use passivation of precipitation hardening stainless still is a must and mandatory. Therefore, the issues of heat tint removal and passivation has to be carefully thought and planned.

 

[israelkk]

Sorry for the typo error should be steel not still

 

[unclesyd]

I understand the concern and requirements for H2 embrittlement problems.
Our PH parts operate 585F with very near to the yield. I have cleaned numerous parts, for identification purposes, with this method in this service with no problems.
This isn't a particularly bad environment for problems with H2 embrittlement we have have trouble with plated 4140 components.

The problem I found is that most people tend to try to get the pickling solution to do all the work by increasing the HF and lowering the HNO3 concentration.

I used the AP// HNO3/HF to clean PH spinnerets with up to 1260 0.0090" holes without the loss of surface and hole edge integrity. I use 1 hr in 15% NaOH + 3% KMO4 at 200F or better. Then dip in the 17.5% HNO3 + 2% HF at ambient, 70F for less than 10 seconds. Pressure or steam rinse.

 

[kenvlach]

The ASTM passivation procedures A380 & A967 do not prohibit the use of acid or chemical cleaning on precipitation-hardening SS, although perils and precautions are given, and mechanical descaling is recommended. See below.
The 0.002" removal mentioned by israelkk seems excessive to me, although maybe moot, as dimensional changes during hardening make finish machining necessary for tight tolerances.

The alkaline permanganate scale conditioner given by unclesyd is an oxidizing solution; no danger of hydrogen embrittlement. ASM Handbook vol. 5 Surface Engineering, p. 74 (1994) even mentions that immersion times longer than 8 hours can be used if necessary for 400 series stainless. This info is incorporated by reference in ASTM A380 Paragraph 5.2.4, "...Chemical descaling methods, factors in their selection, and precautions in their use are described in the Metals Handbook." Salt baths, anodic cleaning and electropolishing are also generally safe w.r.t. hydrogen embrittlement.

For the HNO₃/HF bath, a general rule is to keep the HNO₃-to-HF ratio > 10 for materials subject to hydrogen embrittlement.

ASTM A380-99 (2005):
Re ACID DESCALING (PICKLING):
"A1.6 Hardenable 400 Series alloys, maraging alloys, and precipitation-hardening alloys in the hardened condition are subject to hydrogen embrittlement or intergranular attack by acids. Descaling by mechanical methods is recommended where possible. If acid pickling is unavoidable, parts should be heated at 250 to 300°F (121 to 149°C) for 24 h immediately following acid treatment to drive off the hydrogen and reduce the susceptibility to embrittlement."

Re ACID CLEANING:
"A2.4 Hardenable 400 Series, maraging, and precipitation- hardening alloys in the hardened condition are subject to hydrogen embrittlement or intergranular attack when exposed to acids. Cleaning by mechanical methods or other chemical methods is recommended. If acid treatment is unavoidable..."

 

[israelkk]

unclesyd and kenvlach

Thank you for your insight and putting things in perspective and I agree the the ASTM doesn't forbid chemical cleaning but recommend mechanical cleaning.

I am no metallurgist or chemist and my experience is from 30 years of working in close relationship with heat treaters, platers, metallurgists and chemists.

At that time many hundred of thousands of US dollars went to the drain due to repeated excessive material removal or hydrogen embrittlement even when used by the most respected plating houses, including the in house facilities in my own previous company which is well known worldwide. The outsourcing and even the in house plating workers has no time to take a deep look in the part drawing and the purchasing guy sometime even do not know how to look at it.

I agree that there are very professional heat treating and plating houses who can do the job right. I just came to the conclusion that its not worth taking the risk.

Today there is a rush to get products out to the market on the expense of careful design and testing. Add to this the tendency of managers and purchasing departments to seek for the cheaper plating and heat treating houses. And the worst of all is that most of the time the designer has no control who will do the job and in what country for the next 20 ot 30 years of production.

More than that sometimes when you sell a system to foreign countries you also allow them to manufacture part or all of the system and then things get real messy.