Welding carburized 8620 bushing

[rudylyon57]

We have are welding an 8620 bushing (masked on all surfaces except bore during carburizing) into an A36 plate. The bushing is about 7.5" OD with a 1.25" wall and we are applying a fillet weld on the OD corner. That is, we are applying weld heat about 1" to 1.25" from the carburized surface. The problem is that we are cracking the parts. The bushing is being preheated to 400-500F before welding and the assembly is air-cooled. Any suggestions are greatly appreciated.

 

[metengr]

Locally remove the carburized surface on the bushing and then weld with elevated preheat - 500 to 600 deg F. Otherwise, the high carbon in the carburized case will result in extremely hard heat affaected zones in the bushing base material that will crack during or afer welding.

 

[rudylyon57]

Thanks for your insight but we did mask the surfaces to be welded during carburizing using stop-off. Therefore they should have the carbon content of 8620. I can also add that we are using 71A85 ultrcore wire.

 

[metengr]

Ok. Where exactly are the cracks located relative to the fillet weld? Can you describe or show what they look like - orientation relative to the fillet weld? How close is the carburized layer to the edge of the fillet weld?

 

[rudylyon57]

At worst case, we are probably welding within 1/2" of the carbon-rich (hardened) areas. The spec for the part is to temper the case to 55-60 Rc. I can probably live with 50-55 and wonder if the higher temper and lower hardness of the case would reduce the probability for cracking. I will attach a few pictures in another reply.

 

[Guest102023]

This is not a job that you should even start to try and make work. By the time you have made the bushing 'weldable', it won't be similar to the original requirements for the part.

 

[rudylyon57]

Hi Brimstoner, would you elaborate a little more? Why do you say that by making the bushing weldable we will lose the case characteristics. Are you suggesting the case will be tempered out by the weld heat? Thanks.

 

[unclesyd]

Two little points to make. I don't think they will completely solve

your problem but here goes.
If you used copper as stopoff it may be initiating the cracking in the bushing. In some alloys Cu will initiate cracking when
incorporated into the weld metal. This coupled with having an
extremely brittle base metal could b part of the cause.

Normally when we weld an alloy steel where there is quite a
different in the base metals we use a high preheat as
recommended in the post by meteng. We always use a
higher alloy electrode such as Allstate 275 (ESAB. This is similar to 312 SS. The last part of our procedure is we immediately cover the weldment with insulation to achieve a slow cool.

Have you looked a alternate designs like mild steel with a tension bushing or,bore welding, or shrinking in a hardened bushing?

You may also call someone like Weld Mold Company and get their recommendations for an electrode and procedure.

http://www.weldmold.com/index.php?option=com_content&view=article&id=131&Itemid=13

 

[Duwe6]

In addition to metengr and unclesyd'srecommendations, weld at least two passes. The first pass is partially PWHT'd by the second one -- look up 'Temper Bead'.

A good cooling set-up [insulation to slow cooling rate] is a metal pan of vermeculite deep enough to cover the part by at lease 2" or so, with a hardwood board at the bottom of the pan to keep the part off the bottom of the pan.

Also use a process that uses a flux as it will help with the problems caused by the oxide caoting. SMAW 'stick' or FCAW-G 'gas-shielded flux-core' come to mind.