RANorrisPE
Mechanical
Hi all,
Back in March I sent out for a few sets of prototype bevel gears to be made. For cost, capability (I've spent the last decade doing spurs and none of my local shops wanted anything to do with 2 sets of 88 deg bevels) and timing reasons, I went with a shop I had never worked with before in Canada. I had originally specified the material to be 4140, but they said all they had available was 8620, so I revised the print to case carburize 8620 to 50 HRC. I've never worked with 8620, but the analysis said the parts should be sufficient for the prototype life and stresses.
The 3 weeks promise time got to around 8 weeks and so I asked for one of the sets to be sent to us without heat treat so I could do an assembly fit up, I would then swap in the hardened gears when they came in.
Despite calling and emailing ever since, and believing the shop had ghosted me and my last gear set I finally got an email last week that the parts had been lost at the heat treaters but were finally back from heat treat and were on their way.
When I received them they were covered in brown, black and yellow scale and the surface finish as compared to the non-heat treated gears was abysmal. I have spec'd case carburizing before and have never seen this type scaling.
That got me wondering if this is unique to 8620? Or perhaps since they were lost they were treated while rusting or were mishandled? Or have I been lucky with the shops I've worked at and they either pickle or post HT grind? I want to update my drawings and specs going forward to avoid this, but I will admit my knowledge on the heat treat process is still a work in progress and I would prefer to not leave it to a note saying to remove scale unless that is best practice. (I did have a note on the drawing for final surface finish requirements that was clearly ignored). My note reads: "MATERIAL: 8620 STEEL CASE CARBURIZED 48-50 HRC 0.05"-0.06" DEPTH." With a finish call-out on the drawing of 32 microinches. Which has been a typical kind of note that I've seen and made at both industrial and aerospace companies in the past.
Thanks for your help folks! I've been reading as much as I can get my hands on on gear HT processing, just not able to find this case quite yet.
Back in March I sent out for a few sets of prototype bevel gears to be made. For cost, capability (I've spent the last decade doing spurs and none of my local shops wanted anything to do with 2 sets of 88 deg bevels) and timing reasons, I went with a shop I had never worked with before in Canada. I had originally specified the material to be 4140, but they said all they had available was 8620, so I revised the print to case carburize 8620 to 50 HRC. I've never worked with 8620, but the analysis said the parts should be sufficient for the prototype life and stresses.
The 3 weeks promise time got to around 8 weeks and so I asked for one of the sets to be sent to us without heat treat so I could do an assembly fit up, I would then swap in the hardened gears when they came in.
Despite calling and emailing ever since, and believing the shop had ghosted me and my last gear set I finally got an email last week that the parts had been lost at the heat treaters but were finally back from heat treat and were on their way.
When I received them they were covered in brown, black and yellow scale and the surface finish as compared to the non-heat treated gears was abysmal. I have spec'd case carburizing before and have never seen this type scaling.
That got me wondering if this is unique to 8620? Or perhaps since they were lost they were treated while rusting or were mishandled? Or have I been lucky with the shops I've worked at and they either pickle or post HT grind? I want to update my drawings and specs going forward to avoid this, but I will admit my knowledge on the heat treat process is still a work in progress and I would prefer to not leave it to a note saying to remove scale unless that is best practice. (I did have a note on the drawing for final surface finish requirements that was clearly ignored). My note reads: "MATERIAL: 8620 STEEL CASE CARBURIZED 48-50 HRC 0.05"-0.06" DEPTH." With a finish call-out on the drawing of 32 microinches. Which has been a typical kind of note that I've seen and made at both industrial and aerospace companies in the past.
Thanks for your help folks! I've been reading as much as I can get my hands on on gear HT processing, just not able to find this case quite yet.
to buy material, mfg, heat, not. Testing and inspection. A proper heat for post finishing would have bead blast the gears clean then fi is grind all the critical dimensions.