Cracks in Low C steel due to decarburization or grain growth? 1

[gearman1234]

We have 8620 Hot Rolled annealed material 8.5 inch dia. We finish turn it to 8.448 Gear OD in soft turning. In the recent supply we found that atleast 15% parts showed cracks on the OD after soft turning. I know that we have to use a bigger bar size to leave more allowance for finish turning. But my question is why we see cracks on the OD? Is it because of the decarb layer or because of the high soaking time in bar making leading to excessive grain growth and resulting in cracks.

I have earlier never heard of cracing due to decarb layer.Does the decarb layer cause cracking ein bar making? What else could be the reason for5 vracks.

 

[Carburize]

The minimum stock removal for this size of bar should be 0.4-inch.
it is difficult to know exactly what you are finding, they may in fact be cracks but it is also possible they are seams or other surface imperfections from the rolling process which you are not removing because of the lack of stock removal.

 

[TVP]

Just to expand on what Carburize said, you are likely finding "seams" or "laps", not "cracks". Seams and laps are two types of surface defects that have different origins. Seams originate as porosity in the solidified billet, usually spherical in shape (also called pinholes or gas bubbles). This porosity is then elongated by the rolling process, and forms seam defects. Laps are surface defects that are produced by the rolling process, in which small misalignment between the bar and the rolls causes a small section of metal to be rolled over onto another piece of the bar, but the two pieces are not completely forged together. Neither of these defects is caused by decarburization. Seams, laps, decarburization, and other surface defects (cracks, pits, etc.) are usually removed by ensuring sufficient stock removal, which Carburize provided. Keep in mind that some producers can routinely produce bars with fewer surface defects and hence less stock removal is required.

 

[gearman1234]

Thank you for the explanation provided by all.

Do the seams or laps have a potential to get transformed into crack during further processing?
If suppose we process the gears with the seams /laps and heat treat it to 58/62 HRc then do the seams or cracks develop further or they remain the same as they were?

On the present lot I observed the "cracks" that is the seams/laps more closely. We took out a cut to machine the recommeded allowance. The discontinuities got removed. But the gear OD becomes undersize. We could not get the proper dia stock material in time that is why this bar was used. Although we have about 70-75 % parts without this problem.

 

[Carburize]

If you try to carburize, quench and temper the parts with any kind of sharp flaws in the surface whether they are crack, seams, laps or whatever the likelyhood is that the part will develop deep surface cracks during processing.

 

[SMF1964]

If you are going to be using this as a gear application, then I would expect any surface laps/seams will initiate cracks in service.

The trick is to make sure the cracks don't become an issue until after the warrantee has expired. ;-)

 

[gearman1234]

Thanks guys. I appreciate your responses.