Effect of Multiple heat treatment on CA15 / 410SS 2

[mahadhatu]

I am having difficulty hardening a CA15 casting. The chemistry is within the ASTM spec including the trace elements. The heat treatment I have been running has worked on different lots with similar composition. 1850F - 3h Fan cool usually results in close to 400BHN hardness. On this lot the hardness the first time around was 280BHN, after running the same cycle it dropped to 250 BHN and after running the same cycle third time it dropped down to 200BHN.

I am trying to understand the reason for the hardness to drop after consecutive normalize cycles.

Also is there any way I can regain the 400BHN hardness on this casting?

Please comment.
Thank you

Mahadhatu

 

[arunmrao]

Can you tell what is the weight of each casting? How many castings do you load into the furnace per batch? Is there sufficient air circulation and also how do you ensure that all the castings get equal amounts of air. Do you see some still red while a few turned dark.

Have you attempted oil quenching. perhaps this will be a better option for you to get 400 BHN uniformly and conaistently in your castings.

 

[Yallapragada1022]

How do you know that the chemistry is within the specification limits? Are you going by the material certification or have you performed a chemical analysis?

Are you using a portable (Equotip) teseter to check the hardness? Based on the hardness results that you got, it appears to me that the cooling was not fast enough to get 400 HBW. Have you looked at the microstructure?

Rao Yallapragada

 

[mahadhatu]

The batch has 5 rings 4"ID x 7"OD x 1"thick. All 5 of these castings were loaded in the furnace at the same time. The heat treatment furnace is a gas fired furnace box (3ft x 3ft x 3ft). The rings were removed from the furnace and placed in front of a 6ft diameter fan to air quench on a cold table. The rings cooled down to room temperature wichin 5 minutes.

Oil quenching is an option but I would consider it as a last resort.

What I am trying to understand is the reason for the decrease in hardness after multiple normalizing cycles.

Thank you

 

[mahadhatu]

Rao,

The chemistry was measured using a SpectroMAX Spectrometer when the I poured the heat. The hardness was measured using a KING Brinnel hardness tester.

I have not yet looked at the microstructure.

Thank you for your input.

 

[arunmrao]

Has the scaling on the casting prevented complete reaustenitization? What if you take the soaking temperature from your current 1010 to 1025 C?

 

[mahadhatu]

I use a protective coating on 410/CA15 and other scale prone alloys. This coating completely eliminates any scaling and peels off after the austenitizing cycle. It is benificial in maintaing wall thicknesses after heat treatment.

Today, I am going to try increasing the austenitizing temperature to 1025C as u suggested and oil quench to see if I can prevent any retained austenite or ferrite.

Thank you for your suggestions and comments. I will post my findings.

 

[GRoberts]

Are you grinding down through the de-carburized zone of the casting (which will be softer) in order to take your hardness reading. I'm not completely sure if this is accurate, so someone can correct me if I'm wrong, but he de-carburized zone will grow in thickness with each austenitizing heat treatment, so that could explain the lowering hardness readings.

 

[metengr]

mahadhatu;
Perhaps this might be a rather simple question, but did you check your thermocouples on the gas fired furnace or did you verify the actual austenitizing temperature of the batch before removing from the furnace?

 

[mahadhatu]

Here's the Vertict!!!!

GRooberts,
Thanks for the valuable tip. I ground a spot on the castings about 1/8" deep on from the surface and checked the harndess and wallah !! The hardness was 350BHN consistant on all the castings. Multiple austenitizing heat treatments had de carburized the surface to almost 1/8" deep. I am glad we had enough machining stock on all surfaces. That saved a valuable recast and a lot of time!!!!

I appreciate the input from all the Gurus out here. It sure helps when we look at the problem from 10 different point of views as opposed to one.

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There is always some thing better out there...
Mahadhatu