MikeMech
Mechanical
- Nov 29, 2000
- 92
We have an ongoing job that requires welding 316L base metal with 316 alloy. Magnetic permeability of the entire finished weldment must measure less than 2.0mu on a Severn Gage.
Our customer supplies the base metal, we do the welding. We have had infrequent but recurring problems over the years with seemingly random welds failing the 2.0 permeability test. The welding and testing is being performed by a competent, experienced individual. Why is this happening? How can we best ensure low magnetic permeabilty?
My current theory is as follows: Magnetic permeability is proportional to ferrite percentage/ferrite number. The base metal and filler metal chemistry ranges specified could result in an unacceptable ferrite percentage if the Nickel Equivalent is at the low end of the allowable range and the Chromium Equivalent was at the high end of the acceptable range. A Schaffer-DeLong type diagram indicates this. Statistically, this is unlikely in any particular instance but probable over the long run. My first attempt at a "silver bullet" fix would be to purchase only filler wire with very low, but still within allowable tolerance, ferrite numbers.
I will be performing some experiments with 316 filler metals having ferrite content throughout the allowable standard range. Any advice on weld procedure improvements to try that may help? Annealing after welding is not an option, nor is changing the material.
Comments appreciated!
Thanks,
MikeMech
Our customer supplies the base metal, we do the welding. We have had infrequent but recurring problems over the years with seemingly random welds failing the 2.0 permeability test. The welding and testing is being performed by a competent, experienced individual. Why is this happening? How can we best ensure low magnetic permeabilty?
My current theory is as follows: Magnetic permeability is proportional to ferrite percentage/ferrite number. The base metal and filler metal chemistry ranges specified could result in an unacceptable ferrite percentage if the Nickel Equivalent is at the low end of the allowable range and the Chromium Equivalent was at the high end of the acceptable range. A Schaffer-DeLong type diagram indicates this. Statistically, this is unlikely in any particular instance but probable over the long run. My first attempt at a "silver bullet" fix would be to purchase only filler wire with very low, but still within allowable tolerance, ferrite numbers.
I will be performing some experiments with 316 filler metals having ferrite content throughout the allowable standard range. Any advice on weld procedure improvements to try that may help? Annealing after welding is not an option, nor is changing the material.
Comments appreciated!
Thanks,
MikeMech
