kanthal A1 resistance wire mechanical properties / design considerations (NiCr) - Page 1
Forum section: electronics / projects

author: CopperCone
author_rank: Super Contributor
timestamp: May 04, 2018, 01:22:04 am
content: So this probobly applies for NiCr too, but anyway,I want to know about the thermomechanical properties of heating wire for use with high temperatures (i.e. its 1300C max).I see that heating elements are commonly wound in coil form.It seems easy enough to wind the elements on a bobbin. I want to to insert them into channels cut into insulating bricks.I am aiming to use the highest wire gauge easily workable without a lathe setup into coils that have something like 1/4-3/8 inch overall diameter.My main concern is that once you wind the coils, they will be a bit springy. I can choose the core diameter to get the correct outer dimension, but I am concerned about the lenght of the coils. I see when they are used for heating applications, they are spread out somewhat (makes sense, you don't want shorts), so you can tension sections of runs in a channel to ensure proper spacing.After this wire is fired, i.e. taken to its maximum temperature, does it harden in such a way that makes it mechanically robust?I figured out most aspects of my construction, but I don't know how to do the mechanical mounting for the coils. I was hoping that I could just use temporary iron spacers in a few places to basically get the correct spacing then fire it and hope that it gets kinda brittle and retains its shape.Is this adequate, or do I need some kind of ceramic rods to tension the coils ? Not sure what else to do. Seems like a serious pain in the ass to design some kind of spacer system, considering I want to use tough dense firebricks (i know it will lose more heat but I don't want to use that crumbly shit). I also don't have experience working with ceramics. If I need to make some kind of high temperature tensioning system, can I simply drill holes in firebricks and insert some kind of ceramic rod to act as a 'hook'?Exact shape is unknown, but I expect 3-6kW maximum power with dimensions that are like 12x8x8 or smaller.

author: IanB
author_rank: Super Contributor
timestamp: May 04, 2018, 01:32:59 am
content: I believe that once you heat the wire it will become annealed and will lose much of its springiness. It will also lose some mechanical strength and will become more fragile, so it needs to be supported. From examples of furnace builds I have seen the coiled wire is supported inside channels cut into the firebrick. These channels should provide the mechanical support needed.I am not quite sure of the question. Once the wire is placed inside the channels, what extra support is needed? Maybe a few anchors in key places to hold it in place?

author: CopperCone
author_rank: Super Contributor
timestamp: May 04, 2018, 02:03:25 am
content: I watched a few youtube videos, did not think they were so common.I thought it might like try to retract in on itself with temperature cycling. Is this the case at all?

author: IanB
author_rank: Super Contributor
timestamp: May 04, 2018, 02:36:24 am
content: I haven't built a furnace, but I have purchased some samples of different kinds of heating wire (Kanthal A1, nichrome, Rene 41) to see how it behaves. It doesn't dramatically change in mechanical properties once heated to red hot, but it does develop an oxide coating and it's better not to bend it after heating.What I expect to happen in a furnace is that the wire will expand when hot and contract when cold, which means it will try to "walk" out of its position with repeated cycles. Therefore it will need to be anchored in place at key points to stop this happening. A typical anchor might be a loop or twist of the same wire secured into the brick. You will need to look at some "how to build a furnace" guides to get some ideas.

author: CopperCone
author_rank: Super Contributor
timestamp: May 04, 2018, 03:12:51 am
content: this is what I feared. things walking because of temperature highly annoys me