Tempering question

R

Ryan J

Joined
Jul 14, 2014
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Hello again everyone. I recently posted a thread about making my first liner lock. I found out at the end that I'd botched the heat treat by over heating the 1095. It was suggested that I try tempering it back further. So I put it in the oven in my house for an hour at 450. The blades still fubar but I got a result I don't understand.
The colors indicate it wasn't evenly heated at all and got much higher than 450 degrees in spots
What caused this? Is the home oven unacceptable for the task? I'm asking because many use a toaster oven. I've decreed to send off for heat treat until I have a proper oven. However, I'd like to better understand what's happening here. Here are some pics




It seems counter intuitive to me that the thickest part seems to of gotten the hottest. That is, along the spine and the plunge line. Meanwhile the center of the blade is gold/straw like I wanted. Does it heat from the edges in? I'd assume it would heat evenly when it's in an oven. Nothing lost because this knife was made from scrap stuff around the shop as a learning experience in folder making. Any insight I to what happening here is appreciated.
 
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i hope to help.
An house oven is generally a better choice than a toaster oven, but either will overshoot the temperature, their thermostat is not reliable and the heat is not the same inside in different spots.
The best bet is placing some firebricks and a sand pot inside to give more heat mass inside, letting the temperature equalize (1 hour minimum) and then placing the blade inside, shielded from the direct exposition to the radiating elements, buried into the sand.
Get a multimeter with its flexible cheap termocouple and place it in the sand...relay on its reading instead of just assuming the oven is at the preset temperature. This method is quite reliable.

The colors on the blade are just a hint...the blade should be fresh grinded and totally degreased; also the steel type could diverge from your color chart, but 1095 should develope the correct colors
 
The colours are a very rough guide if nothing else is available.

The colours are interference colours caused by light reflected from the front of the Oxide layer interfering with the light reflected from the back of the Oxide layer. The colour depends on the thickness of the Oxide layer. It is exactly the same effect as you see with colours from a film of oil on water.

The thickness of the Oxide layer depends on many things including temperature, time, surface contamination, steel composition and the composition of the atmosphere in contact with the steel.

The old-school method is to Austenitizing and quench, sand to bare steel then heat the steel well away from the working edge and watch the colours run. When the edge reaches the desired colour, quench.

This method removes almost all the variables except temperature. Clean to bare steel removes surface contamination. Heating away from the edge means the atmosphere producing the colours is normal air. The whole process is done quickly enough that time can be neglected. With plain Carbon steels, steel composition can be considered constant.

In your process, you have probably got surface contamination. Time is probably an hour or more. Atmosphere may be different if you have a gas oven. Steel composition might be different too.

It all adds up to your oven controls will give you a much more accurate measurement of temperature than the colours will.
 
Color tempering is used in flame tempering and other "hot tempers". These are when a much hotter object ( flame, red hot steel, etc.) are placed on the spine of the blade or the shank of a chisel/graver, and the colors are observed as they run down the metal toward the cutting edge. Once the desired color reached the edge, the piece was quickly cooled off in water and it was considered "tempered". Similarly, springs were placed in a pan and the pan heated to watch the colors turn. When the correct "blue" was seen, the springs were removed from the pan.
This is not the same, nor as good as a proper oven temper.


On your blade:

If the blade was wiped clean after the quench and then oven tempered - the colors mean nothing.

If the blade went into the oven directly from quench and a little cleanup - the rainbow of colors on a blade after an oven temper mean very little.

It the blade was sanded to bright metal and carefully cleaned - it should be a straw-bronze color ... but could still have other rainbow colors in spots due to contamination.

A good thermometer or pyrometer ad a clock are the tools for getting a good temper ... not colors.
 
Stacy E. Apelt - Bladesmith said:
Color tempering is used in flame tempering and other "hot tempers". These are when a much hotter object ( flame, red hot steel, etc.) are placed on the spine of the blade or the shank of a chisel/graver, and the colors are observed as they run down the metal toward the cutting edge. Once the desired color reached the edge, the piece was quickly cooled off in water and it was considered "tempered". Similarly, springs were placed in a pan and the pan heated to watch the colors turn. When the correct "blue" was seen, the springs were removed from the pan.
This is not the same, nor as good as a proper oven temper.


On your blade:

If the blade was wiped clean after the quench and then oven tempered - the colors mean nothing.

If the blade went into the oven directly from quench and a little cleanup - the rainbow of colors on a blade after an oven temper mean very little.

It the blade was sanded to bright metal and carefully cleaned - it should be a straw-bronze color ... but could still have other rainbow colors in spots due to contamination.

A good thermometer or pyrometer ad a clock are the tools for getting a good temper ... not colors.
Click to expand...

This is what confused me. I'm not saying that I want to know the color to temper by. I'm not planning to rely on that. But this blade was buffed to a mirror and scrubbed with dawn before I put it in to the oven. After I seen the colors after the first temper I took these steps to avoid contamination before the second temper because I wanted to keep the colors and didn't want fingerprints on itit. So I think contamination shouldn't of been an issue. Like you said it should of been straw colored. I'm asking about this because I had the blade on the bottom rack about 5 inches from the oven burner. The oven is electric as well so I don't think it's an atmospheric issue. I'm wondering if having the blade that close to the burner caused it to get too hot. I take them directly from quench to oven usually but this one had its final finish because I was trying to temper it back further to reduce the chipping. Like I say I scrubbed it with Dawn and hit water for quite a while. I wonder if maybe the colors were caused by residual buffing compound?
 
You do not want the blade close to or in direct sight of any burners.

You want to oven up to temp and give 15-30 minutes to settle down before putting the blade in.

I get blue & purple colors on W2 at 450f for one hour and still have 61-62 Rc hardness.
 
"...... . But this blade was buffed to a mirror and scrubbed with dawn before I put it in to the oven. ....."


That is why I said the colors mean virtually nothing. You need a surface with no oxides on it.

If you buffed it you polished all sorts of contaminants into the pores and surface that will affect the colors. Cghrome oxide, iron oxide, tin oxide, etc. These are all oxides.

Dawn will leave its own residue.

The surface for checking temper by color is a smooth freshly ground surface at 120-220 grit. As said before, you need steel surface with no oxides on it.
 
I see. Thanks for the replies everyone. So much to learn. All of you freely giving your time to help is appreciated.
 
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