Heat treatment of soft magnetic materials

[TJMAT]

Does anyone have recommended sources for low volume heat treatment of soft magnetic materials that would include magnetic bias during the thermal process schedule? The objects may be up to 5 feet in length.

 

[EdStainless]

I don't know of anyone that can provide HT in a field on objects that long.


= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[MagBen]

what are the temepratures and the magnetic field strength (i assume it much lower than for permanent magnets)? what kinds of soft magnets? why you need 5' long, is it for a single piece? what are you trying to accomplish? more specific info would let us help you better.

 

[TJMAT]

The materials may include any of the common soft magnetic alloys; Ni-Fe 50, Fe-Co 50, 430FR SS or low carbon Fe (>99.8%). The application will be as a core for long (long is a relative term) cylindrical solenoid core. The heat treatment for Ni-Fe (50/50) will be something like 1180C / 6 hr. anneal followed by cooling at 600C / hr. to 447C where it is held for 24 hr. with 12 Oe bias transverse to the long axis of the core with final cooling at 100C / hr. under bias to ambient conditions. Other materials may have similar heat treatment schedules. Nominal core length will be about 36" but maximum length could be up to 60".

 

[MagBen]

Arnold in Marengo, IL facilities uses solenoids to HT AlNiCo, the field can go up to 1-2 KG, 36'' is for sure, but not quite sure it can go 60''.
The bigger problem is you want to apply a transverse field, i can hardly imagine any one has such huge unit. Are you trying to have a linear B-H curve, so a stable permeability at varying H? Not all soft magnets can be induced magnetic anisotropy by HT at the presence of a magnetic field.

as for Fe-Ni50, you donot want to HT with a field at 447C, which is too high, very close to its curie temperature. The bias field has no use at all at 1180C

 

[EdStainless]

I had thought that this was done by annealing, and then allowing the part to cool in a field.
But then I also thought that this was only done with special alloys.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[TJMAT]

MagBen,

Thank you for your advise. As I review the HT instructions for soft magnetic materials I think I am gaining an understanding of the process. The HT at 1180C (1177C upon further review) is an anneal/stress relief step and applied bias has no effect. The anneal is combined with dry H2 which acts as a reducing agent to prevent surface corrosion/oxidation and remove impurities from the material if the anneal temp is held long enough. This is diffusion process which explains why the extended dwell time is required. Then the temp is lowered to something less than the Curie temp and a magnetic field is applied. Your comment on the dwell at 447C being too close to the Curie temp is well advised. Upon further review the recommended temperature for gain growth is 427C for either Permalloy 50 or 430FR. There are similar but lower numbers for Cobalt alloys. At this stage apparently the grain structure of the alloy grows resulting large grains. Now this will also occur in the absence of an applied field or the ambient Earth's field. I don't think the grain size is a strong function of magnetic bias. So the bias must act to align the domain axis(?). The intent is to linearize the B-H curve (i.e. minimize the internal area of the B-H loop) so the remanence is minimized. I believe there is a facility in FRG that can perform biased HT on medium sized objects however I was hoping a domestic capability was present. It has also been suggested that HT without bias may be enough to linearize the B-H curve. So does it make sense that large grain material would result in lower remanence (with or without magnetic bias during HT)? Please keep I mind that the object is not a thin film and is bar/rod in form. Again, thank you for your advise.

 

[MagBen]

I am not aware a magentic bias affects 430FR

A magnetic bias annealing has nothing to do with grain size ro grian growth. Actually as for Fe-Ni, at 400-500C, there is no grain growth, since the temperature is too low. The bias magnetic field is to help form a preferential alginment of like-atom pairs. due to low temperature, the hold time is long for a slow atom diffusion.

the bigger the grain, the smaller the coercivity, so the smaller the BH area (only apply to traditional materials, not for nanostructured materials). however, the grain size doesnot have much relationship with remanence, the shape of BH does. Low remanence doesnot necessarily mean small BH area.

Annealing in an H2 to Remove impurities (C, S etc) is not effective for bar/rod products, while it is one of main purposes for strip products.