A286 Forgings and Grain Size 2

[metalseng]

We have been buying A286 forgings for high temperature applications. The stock materials are either ASTM A638 Grade 660 or A891. After forging process, they are being solution treated and double aged. Then the mechanical properties are tested from specimens from the forgings and grian structure is also analyzed. The forging process and heat treatment are done by third party venders. Recently we had some issues with grain size and structure. We had to reject these forgings due to their non uniformity in grain size and larger grains. Our requirements call for minimum grain size of ASTM size 4 or finer and most forgings meet this requirement. I am attaching the photomicrographs of the ones with grain size issues. The first three have mixed grain structure. The last one has very large grains. I would like to understand the reason for these unacceptable grain structures.
Any insight or experience on this would be greatly appreciated.

Thanks.

 

[metengr]

metalseng;
Some additional information would help to evaluate what you observed;

1). What was the forging temperature range? This should be available form the 3rd party that is doing this work.

2). What was the solution treatment temperature and time? This information should be available on the MTR from the 3rd party that is performing the heat treatment.

3). What is the approximate size and shape of each forging?

 

[metengr]

Forgot my two last questions...

4) Is any type of forming or straightening operation performed after solution treatment but prior to aging?

5). What is your double aging heat treatment?

 

[metalseng]

Thank you for the interest.
1. The Forging company claims that they stay within specified range. I have no way of checking it.
2. The solution treatment temperature is 1800F. The solution times were 3-4 hours.
3.All Forgings are pancake forgings (thickness ranges 4-5 inch). Diameter ranges 20-22 Inches.
4. None
5. 1400F for 16 hours and 1200F for 16 hours.

Thanks.

 

[metengr]

metalseng;
You absolutely need to track the information in 1) down because the literature I have indicates the forging temperature range for A 286 is narrow, and depends on the amount of work reduction.

I would consider visiting the forging shop and review/audit their procedure for A 286. Also, you should request this information as part of traceability for these forgings. I suspect there could be a problem with a lack of temperature control and subsequent consistency by your forging supplier.

 

[metengr]

If you don't already have this, the web site below provides a good source of reference material that will help you

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19720022813_1972022813.pdf

 

[metalseng]

Thanks for the reply. I agree that the forging temperature is critical. The reference you have provided recommends a forging temperature of 2150F for large amount of deformation. However, I do not understand how how the temperature will stay lower when it is being deformed heavily.

I have another related question. A891 specifies a solution treatment time (at 1800F) of 3-5 hours. Does that apply to even thicker forgings (min thickness of 12 inches)? We cannot drill a hole to insert a thermocouple in the center of the forging to measure the time the center section is at the solution treatment temperature.

Thank you for the link. I was not aware of it.

 

[metengr]

metalseng:

I agree that the forging temperature is critical. The reference you have provided recommends a forging temperature of 2150F for large amount of deformation. However, I do not understand how how the temperature will stay lower when it is being deformed heavily.

I have audited several large forging shops and of the reputable shops I have visited all have some type of process procedure or guide that details forging reduction ratios and temperature hold points. This is why I said you or your organization hire someone to review the forging shop.

Yes. The austenitization time for most ferrous materials occurs much quicker then most believe. The actual soak time at 1800 deg F should start when the part reaches the solution treatment temperature of 1800 deg F, not when you place the forging in the furnace. The hold time allows for the complete dissolution of precipitates that formed during and after forging.

 

[metalseng]

Thanks again. By the way, is there any way to check (right after it has been forged) if the forging has been properly forged before the heat treatment and machining. Sometimes, we find out the grain size is no good after machining and full soltion heat treatment- means all the expense associated with this is wasted. I was thinking of hardness measurements or grain structure. Any guidence would be helpful.

 

[metengr]

I agree, the removal of extra stock for hardness and grain size evaluation from test samples after forging would be beneficial. This requirement would have to be part of the process requirements for the forging vendor to follow. Look at ASTM A788 for guidance.

 

[metalseng]

Thank You Metengr

 

[czarthur]

We are a forging plant in China. In my experience, the critical things for grain structure are material and forging ratio. If we do your forged disks, first we will choose the suitable forging ratio then ultrasoniclly test the billet or ingot. Heat treatment is necessary but has limited effects for grain structure.