Effect of excessive austenitizing time 6

[armourian78]

Hi all,

Our heat treatment supplier, has heat treated some parts and they have had an excessively long austentizing time, the austenitizing temp was correct. We are looking to convert the microstructure to martensite, with a fraction of retained austenite.

What are the potential knock on effects of the long austenitizing time?

I am aware of grain growth, and the potential mechanical properties changes.
Could the retained austenite be affected? I cannot see how, as the austenitizing temp was correct and retained austenite level is dependant on quench rate and subsequent tempering.

Anything else?

Thanks
Ian (Metallurgist)
Plymouth

 

[arunmrao]

Scaling, depth of decarb layer and any movement in dimensions are some of the other critical factors to be considered.

 

[Maui]

What grade of steel (if it is steel) are these parts made from? What are the details of the heat treating process? What was your aim hardness? What type of furnace was used? Vacuum? Atmosphere? A hand torch and a bucket of quench oil? Without the necesary details it is very difficult for anyone here to provide a meaningful answer to your question.

Maui

http://www.EngineeringMetallurgy.com

 

[redpicker]

arunmrao and Maui both have excellent points.

In particular, the specifics are the key. If you are talking about some 4130 forgings that were austenitzed at 1650F for 6 hours instead of 1.5 hours and the remainder of the processing, including testing and finish machining, are as expected, I don't think there would be much to worry about, even in an open atmosphere furnace, since the scale and decarb from the extended heat treatment is insignificant compared to that resulting from the forging operations.

On the other hand, if you are dealing with a high carbon tool steel that was finish mahcined that was austenitized at 1750F for 10 hours instead of 30 minutes, well, you could see a lot of problems caused by the extended austenitizing time. Perhaps the decarb, scaling, amount of carbon going into solution, are all such that the process returns acceptable product when the shorter time is used, but with the longer time, unacceptable product is the result.

rp

 

[armourian78]

Hi all,

I work in the bearing industry. The steel is 52100. The parts were heat treated in a seal quench furnace, with the carbon potential matched to the alloy content to prevent carb or de-carb. The hardening time was 100 minutes, which we would class as excessive as the parts are very small (>25 mm OD). The parts are not finished machined, they require grinding to bring to size, therefore any extra heat treatment distortion will be deal with.

The target hardness was 62-65 HRC after tempering, the parts achieved 63.5-64.5 HRC.

In order to accept the parts, I have asked for grain size analysis, microstructure analysis of the surface layer (for any carb or de-carb) and a core hardness check. We will perform the retained austenite check, as the supplier cannot accurately measure this.

Thanks
Ian (Metallurgist)

 

[Maui]

The only things that I [italic]might[/italic] be concerned about would be the grain size and the level of intergranular oxidation. But grain size is not likely to be an issue for a soak time of only 100 minutes, since the effects of soak time are secondary to temperature in this respect. And the temperature was apparently correct. Since you will be grinding these parts as part of your finishing operations, any intergranular oxidation that is present should be removed (provided the grinding removal is sufficient). As long as this isn't a life-critical application, based upon the information that you've provided I wouldn't be too concerned.

Maui

http://www.EngineeringMetallurgy.com

 

[TVP]

I concur with Maui. 100 minutes is unlikely to produce any meaningful problems so long as the temperature was correct (part temperature not furnace temperature).

 

[armourian78]

The investigation of the parts has revealed, that the grain size is ok (surprisingly is was finer than expected anyway), microstructure revealed no de-carb or carb present, and that the core hardness was within spec and consistent with the surface hardness. Just awaiting retained austenite results, but I do not believe the level of retianed austenite will have been affected.