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Tempering

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Derek Lane30/12/2018 00:25:44
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206 forum posts
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I am in the process of hardening and tempering some hacksaw blades after I have worked them. I have heard that tempering can be done in an oven is this correct and if so what is the best way to go about it.

The parts are smallish so if this is possible may be easier that heating with a flame to straw colour

Hopper30/12/2018 02:29:24
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3709 forum posts
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Light straw color indicates about 200C/400F so if you set your oven to that temp, let it get up to temp then bung the parts in the oven for long enough for them to reach ambient temp in there, it would be the same as heating them with a torch to that temperature.

Just make sure your hacksaw blades are the carbon steel type with HSS teeth. I don't think you will have much luck heat treating the solid HSS type without precision temperature control. But I have never tried it myself.

Derek Lane30/12/2018 12:55:02
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206 forum posts
49 photos

Thank you Hopper. Unfortunately the blade is unknown type as it is a cheap one from a saw I got second hand.

I did make one some time ago but did not treat it in any way as i ground it so the cutting edge was from right next to the teeth. I say ground as it was a very slow process as I did not want to change the structure of the already treated tooth area so far it seems to hold a very fine sharp edge as it is for wood carvingdscf4408.jpg

Nick Hulme31/12/2018 01:10:21
703 forum posts
37 photos

Tempering temperature/oxide colour charts are all over the internet.
Google search, pick a couple that agree and you have your temperatures.
I used to post links but you got here so you can get there.......

Chris Trice31/12/2018 05:11:16
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1362 forum posts
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Yes, you can temper in your kitchen oven. Most go up to 250C if electric. I'd recommend a separate high temperature oven thermometer (available from Lakeland) visible through the door as the cooker markings are not always reliable indicators of the true temperature. I found doing it the old fashioned way by flame heating and trying to judge colour changes in the metal very hit and miss.

Derek Lane31/12/2018 10:12:34
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206 forum posts
49 photos

Thank you guys. I will be trying it in the new year.

ega31/12/2018 10:26:17
1265 forum posts
108 photos

I use the deep fat fryer.

SillyOldDuffer31/12/2018 10:58:44
4711 forum posts
1010 photos
Posted by Derek Lane 2 on 30/12/2018 00:25:44:

I am in the process of hardening and tempering some hacksaw blades after I have worked them. ...

In the event of disappointing results it may not be your fault. Hacksaw blades, particularly modern ones, are often made from a thin strip of hardenable steel welded to a much larger mild-steel backing. Only the teeth are set in the hard steel. It's done to make the blade tougher whilst allowing the teeth to be harder (and more brittle) than would be possible in a homogenous blade. This layer construction might be exactly what's wanted in a small wood-working knife but cheap hacksaw blades might be a better bet if they happen to be made from Carbon Steel throughout.

There's a real problem today re-using anything manufactured because modern* engineering is highly tuned to achieve 'value for money'. Even simple articles may be composites of different alloys and other materials recruited from the awkward squad. Not knowing what you're dealing with is a first class beginner booby-trap, which is why I prefer to cough up for known metals.

My advice is to experiment with scrap before putting a lot of work into it.

* By "modern" I mean mainly after WW2, but the trend started much earlier.

Dave

Martin Cargill31/12/2018 11:35:14
107 forum posts

I've tempered knife blades in a domestic gas oven before. It worked quite easily and with reliable results, as the items could be held at quite a precise temperature (rather than using a flame where temperature control is difficult) the only problem was that the item (a larger knife blade) was laid on the grille in the oven and where it touched the bars of the grill it reduced the temperature thus creating a series of temper variations along the blade.

Martin

Chris Trice31/12/2018 11:45:36
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1362 forum posts
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Posted by Martin Cargill on 31/12/2018 11:35:14:

I've tempered knife blades in a domestic gas oven before. It worked quite easily and with reliable results, as the items could be held at quite a precise temperature (rather than using a flame where temperature control is difficult) the only problem was that the item (a larger knife blade) was laid on the grille in the oven and where it touched the bars of the grill it reduced the temperature thus creating a series of temper variations along the blade.

Martin

Vermiculite boards are our friend.

speelwerk31/12/2018 12:16:43
331 forum posts
1 photos

For me the easiest way to controlled temper is the flame of a spirit burner with just enough heat to the reach temperture and the object inbedded in a tray with brass filings. Niko.

Derek Lane31/12/2018 13:39:33
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206 forum posts
49 photos
Posted by Martin Cargill on 31/12/2018 11:35:14:

I've tempered knife blades in a domestic gas oven before. It worked quite easily and with reliable results, as the items could be held at quite a precise temperature (rather than using a flame where temperature control is difficult) the only problem was that the item (a larger knife blade) was laid on the grille in the oven and where it touched the bars of the grill it reduced the temperature thus creating a series of temper variations along the blade.

Martin

Would a piece of steel plate help reduce this problem

Chris Trice31/12/2018 17:07:27
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1362 forum posts
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Can you slide it inside a copper tube?

Tim Stevens31/12/2018 17:36:47
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1085 forum posts

A traditional way to even out the heat was to use a tray of sand. However you do it, the trick is to avoid any chance of overheating the thin, sharpened edge compared with the rest of the blade.

Wassail

Tim

Michael Gilligan31/12/2018 17:40:43
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14011 forum posts
608 photos
Posted by Tim Stevens on 31/12/2018 17:36:47:

A traditional way to even out the heat was to use a tray of sand.

.

... and Clockmakers tend to use a tray of Brass swarf, for the same purpose.

[ probably something to do with 'available material' ]

MichaelG.

Clive B01/01/2019 11:09:53
24 forum posts
20 photos

+1 for the deep fat fryer. I have a cheap one from Asda which, on full power, cycles over 180-190 degrees C (measured with a decent thermometer). This temperature is fine for silver steel and gauge plate, not sure about unknown types of steel though....

Clive

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