I have a client who is welding large steel plates. The weld is a mild carbon steel, however the parent plates contain 18% magnesium. Normally magnesium cannot be tested by MT, however with only 18% magnesium I am getting a very good pull from the electromagnetic yoke. The burmah castrol strip pulls up and I am obtaining a field according to the flux meter. A constant temperature of 100°C must be maintained therefore I cannot use PT (unless somebody knows of a penetrant suitable at 100°C?)
Does anybody know if this is still an acceptable means of testing?
Magnesium is a light alloy and cannot be MT inspected.
However, MANGANESE Steels (depending on their other alloying comstituents and concentrations) can be.
The important question is "Does a magnet stick on the surface?"
You are right that there is no steel with 18% Magnesium. Magnesium cannot be alloyed with Fe. As you have pointed out, there are steels with 18% Mn. When the Manganese content exceeds about 12% the steel tends to be Austenitic. As you are very much aware, the Austenitic steels are weekly magnetic and cannot be subjected to MT. But there is a catch.
"THE ORIGINAL AUSTENITIC MANGANESE STEEL, containing about 1.2% C and 12% Mn, was invented by Sir Robert Hadfield in 1882. Hadfield's steel was unique in that it combined high toughness and ductility with high work hardening capacity and, usually, good resistance to wear."
The Hadfield steels are highly amenable to work hardening. When they are subjected to work hardening they undergo a transformation to martensitic phase (near the surface) which is very much ferromagnetic and the part can be subjected to MT. The work hardening is done by impacting or light hammering or during service (the points and frogs in railway tracks). Any part made of this steel, if subjected to even minor work hardening, is bound to develop these magnetic properties to various degrees and certainly the Electromagnetic Yoke sticks.
However the transformation to martensitic phase is very much non-homogenous and to that extent the ferromagnetic field is also non-homogenous and this can be tricky during the MT. The exceptions are when the part is uniformly work hardened like ring forging or cold rolling without further heat treatment, in which case the MT can be happily carried out.
I fully agree with Dent. Since a yoke generates longitudinal magnetic field, the only way we can check appropriate magnetisation is by the weight lift test. Due to the closed nature of magnetic lines of force, the field indicator picks up the direction of magnetic field and NOT the magnetic field in the material being tested. Similarly, since a shim has a known discontinuity , it is picked up by the magnetic field. It no way proves that magnetic lines are travelling in the material being tested.
For one thing, I got to say that I learned something here. It never occured to me of making the 10 lbs and 40 lbs weight in the material that I would be testing, which really make sense, but in this situation, I do have some reservation. P V Sastry stated that the ferromagnetic properties of the material are not homogeneous throughout, so how will one be sure of the inspection being carried out?
Very little details is given about the material except that it is a weld at 100 degrre Celcius. I remember years ago when I was doing my Eddy Current exam, I had to inspect a small plate of Austenetic Stainless Steel and I was having a hard time until I use a differential coil probe. Maybe it is the way to go! Or since it is a weld, why can't good old UT be used? In this case, you may have to use a 0.5 or a 1 MHz transducer and some High Temp couplant.