PressEquip
Civil/Environmental
- Oct 26, 2009
- 35
Thank you in advance for your time and potential responses
At the shop they repair equipment used at the facility. In this case it is a fork lift. There is pivot point (pin with a bushing), that eventually develops a flat spot (especially on the ones from the factory which are mild steel). The pin is welded to an axle that has rubber seals. In the past the shop has upgraded these weld-on pins to 4340 on several of the fork lifts. Hardness measurement was taken of one of the replacement 4340 pins: 297 HB (~31 HRC). This hardness leads me to thinking the material is quenched and tempered (such as A434 4340 BC or A434 4340 BD). They had a sample of the machined upgraded part which they showed me. On the sample, the contact surface with the bushing was discolored due to what we suspect was localized heat treatment to create a case hardened region to prolong the life of the pin. This localized hardening was probably either induction or direct flame hardening. A hardness measurement was taken on the dis-colored surface: 380 HB (~ 41 HRC). Unfortunately there isn't any documentation on record that I'm aware of such as the MTR or details on the case hardening heat treatment.
The replacement part is ~4.125" OD at the shoulder, then reduces to ~3"OD at the bushing mating surface, over-all length ~3.625".
I talked to the shop welder who usually does the welding on the fork-lifts. Since the pin is welded to an axle that has rubber seals, he only does a min. 250°F pre-heat so as not to damage the seals. The shoulder allows for up to ~0.75" leg fillet weld. The factory weld is much smaller but the mechanics manager told the welder to put on as large of weld as possible, so it is now a much larger fillet weld. They have done this repair on several fork lift and it has held up fine for years, no cracks or issues that I was told about.
I'm concerned that if the case hardening is direct flame that the surface of the part could become decarburized, induction hardening would be preferred in my opinion.
The bushing is a soft material such as bronze and there is grease channels in bushing and a grease nipple to supply grease to the bushing.
Ideally the welding procedure should be changed to have a higher pre-heat (min. 600-800°F) and post heat (min. 800°F) but that would require the rubber seals to be removed and then re-installed which is a significant amount of work.
Please provide feedback on the following:
1) Does it even make sense for the replacement pin to be case hardened in the contact region with the bushing?
2) Are there economical ways to improve this repair process?
3) Since the shop has been successful with previous repairs using the above method, is that proof enough or should certain aspects of the repair process be changed? I'm specifically thinking of the welding procedure.
If you have any questions or want clarification please request that information in the responses
Thanks
A Fan of EngTips, share the knowledge
At the shop they repair equipment used at the facility. In this case it is a fork lift. There is pivot point (pin with a bushing), that eventually develops a flat spot (especially on the ones from the factory which are mild steel). The pin is welded to an axle that has rubber seals. In the past the shop has upgraded these weld-on pins to 4340 on several of the fork lifts. Hardness measurement was taken of one of the replacement 4340 pins: 297 HB (~31 HRC). This hardness leads me to thinking the material is quenched and tempered (such as A434 4340 BC or A434 4340 BD). They had a sample of the machined upgraded part which they showed me. On the sample, the contact surface with the bushing was discolored due to what we suspect was localized heat treatment to create a case hardened region to prolong the life of the pin. This localized hardening was probably either induction or direct flame hardening. A hardness measurement was taken on the dis-colored surface: 380 HB (~ 41 HRC). Unfortunately there isn't any documentation on record that I'm aware of such as the MTR or details on the case hardening heat treatment.
The replacement part is ~4.125" OD at the shoulder, then reduces to ~3"OD at the bushing mating surface, over-all length ~3.625".
I talked to the shop welder who usually does the welding on the fork-lifts. Since the pin is welded to an axle that has rubber seals, he only does a min. 250°F pre-heat so as not to damage the seals. The shoulder allows for up to ~0.75" leg fillet weld. The factory weld is much smaller but the mechanics manager told the welder to put on as large of weld as possible, so it is now a much larger fillet weld. They have done this repair on several fork lift and it has held up fine for years, no cracks or issues that I was told about.
I'm concerned that if the case hardening is direct flame that the surface of the part could become decarburized, induction hardening would be preferred in my opinion.
The bushing is a soft material such as bronze and there is grease channels in bushing and a grease nipple to supply grease to the bushing.
Ideally the welding procedure should be changed to have a higher pre-heat (min. 600-800°F) and post heat (min. 800°F) but that would require the rubber seals to be removed and then re-installed which is a significant amount of work.
Please provide feedback on the following:
1) Does it even make sense for the replacement pin to be case hardened in the contact region with the bushing?
2) Are there economical ways to improve this repair process?
3) Since the shop has been successful with previous repairs using the above method, is that proof enough or should certain aspects of the repair process be changed? I'm specifically thinking of the welding procedure.
If you have any questions or want clarification please request that information in the responses
Thanks
A Fan of EngTips, share the knowledge
