Case Hardening trouble 5

[Kennethhp]

Hello all!
Im an intern at a heat treatment facility as a mechanical engineer. (non english speaking so sorry for typo-o's

I have a task which is causing alot of trouble in our case hardening department.

A costumer is producing different sizes axels in 20MnCr5.

When one certan size axel is treatet they simply dont get any carbon addition in the surface. The other parts in the oven at the same time have perfect result, but this particular axel doesnt.
I have tried to measurre hardness to see if there is any difference after treatment, but no change.

I got my hands on a piece of their raw material before they machined it, and tried to treat that part with great succes. It had the desired case.

So im thinking there is something in the machining of that particular size axel that leaves a protective "shell" so it wont be treated.

Does anyone have any suggestions or experience with similar problems ?

/Kenneth

 

[Kennethhp]

Just to let you all know. The costumer had a meeting with my supervisor and the costumer silently admitted there might be a slight chance in the way they do their machining.
But like we suspected they wont let us in on what difference.

The case is closed until we recieve their next shipment. If we meet the same problem again i will convince my supervisor that we try to blast a single part to see if this will do the trick like Mfgenggear said.

But thank you all for your inputs.

 

[Lyrl]

Kenneth - In addition to the blasting option, also consider treating one part at ~800°F for an hour or so in an air draw before carburizing. As I note in my earlier post, I've had success with this workaround and it has the advantage over blasting of less impact on the part appearance.

 

[redpicker]

A star for Lyrl as this is an excellent suggestion.

In the olden days (prior to Nitrogen/Methanol atmospheres), carburizing was usually done with "endo gas", that is, natural gas converted to as much CO as possible by passing methane and air over a heated catylist in regulated amounts at an elevated temperature. Since the CH4 + O2 --> CO + H2 + H20 reaction absorbs heat, that is is endothermic, the device is known as an endothermic generator, or endo generator and the gas was endo gas. By controlling the temperature the reaction takes place, the amount of hydrogen produced could be maximized and the amount of water minimized. In the furnace, methane was introduced to increase the carbon potential, which also increased the water content. The water content was monitored by checking the dew point of the furnace atmosphere. Endo gas is very reducing and will reduce iron oxide to iron. Endo gas is explosive below 1400F or so and will burn if exposed to oxygen.

Anyway, in the olden days, when carburizing with endo gas, you had to charge the furnace with an air atmosphere, since endo gas was explosive at lower temperatures. Even with such controls, it wasn't uncommon for a carburzing furnace to "blow its doors off" if there was a leak in the furnace. Since you couldn't introduce an atmosphere until the furnace was above 1500F, contamination would burn off and the parts would develop a light scale as the furnace temperature climbed back to its setpoint after charging. This wasn't a problem as once the atmosphere was introduced at 1500F, the scale would be reduced back to iron. This phenomena was noticeable on loads where some problem with the generator prevented introduction of the atmosphere and had to sit at 1500F or so for an extended period. In such cases, the dew point would be way off (high) for a couple of hours as the scale was reduced back to iron

My point (finally getting around to it) is that with the N2/Methanol systems, there is very little oxygen in the furnace when the load is charged (it has been purged with N2), so surface contamination may not burn off. In addition, the parts do not develop the light scale. Lyrl's suggestion would take care of this and might be worth investigating.

 

[mfgenggear]

redpicker good post but there are endo gas generators still out there. again very informative post.

 

[Kennethhp]

@Lyrl. Ah yes ! sounds like the best option. Might have been my "poor" english since i didnt catch that in the first try

@Redpicker, thanks alot for going into details, its nice since im brand new in the branch and as said, "just" an intern.

I will check up on what we are using and make the suggestion next time we get this specific part in house again, if we have the issue.

 

[mfgenggear]

Workpiece material

Typically the materials that are carbonized are low-carbon and alloy steels with initial carbon content ranging from 0.2 to 0.3%. The workpiece surface must be free from contaminants, such as oil oxides, alkaline solutions, which prevent or impede the diffusion of carbon into the workpiece surface.

 

[mfgenggear]

http://www.academia.edu/12252566/The_Effects_of_Rust_on_the_Gas_Carburization_of_AISI_8620_Steel

effects of rust on carburized surfaces on low alloy steels