hardening & tempering HSS, and magnetic effects

B

Bob Blarney

Joined
Nov 19, 2015
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2
Hello,

I'm a visitor from the woodworking universe, and I make my own tool blades from time to time, e.g. smaller plane irons and lathe chisels that can't be found easily or bought reasonably. Right now I'm making a blades for a hand router plane from HSS spade bits. As I was annealing a spade bit for shaping and grinding, I was also thinking about the process of rehardening and tempering. A traditional method is to heat the steel until it no longer attracts a magnet, and then quench it, and temper it. So my questions are:

1. What happens to the hot steel such that it loses its magnetic attraction?

2. What would happen if it was quenched while in a magnetic field? Would it alter the hardness?

I know these might be 'off the wall' questions, but I also know that those who ask questions learn new things, and quicker, than people who just never ask. And when I'm wearing my professor hat, I tell my students that there are many stupid answers but few stupid questions.
 
When steel is heated above a certain temperature (close to 1400F) the atomic crystalline structure of the iron flips from body centered cubic into face centered cubic. This structure is called austenite. It's soft and at that temperature carbon is free to move around and, if quenched fast enough, remain trapped between the iron atoms, straining the bonds and forming a harder structure called martensite. Iron is generally magnetic at room temperature, but the austenitic form is not. So you can tell when it reaches this temperature, the Curie temperature, because it becomes non-magnetic. Incidentally, this is why some non-heat treatable stainless steels are non-magnetic, they're composed almost completely of retained austenite, meaning they remain in the FCC condition at room temperature.

You can not heat treat HSS properly with the "traditional" way you're describing. HSS, because of its alloy content, is normally hardened at about 2,100F. A trip to non-magnetic won't be nearly hot enough. And, unfortunately, HSS does not generally tolerate a second HT without being properly annealed first because the temperature and the soak time needed dissolve the carbides pinning the grain on the second heat caused extreme grain growth. The result is hard, but weak and fragile and coarse grained. It must be brought to a certain temperature (depends on the alloy) and cooled at a certain rate, generally from 10-20 deg per hour before if can be re-hardened. Even a half-way reasonable job of this requires time and temperature control beyond most people. Otherwise, yes, you can get it kind of hard, but it will be a very poor quality structure that won't hold up to use very well.
 
1. A: Curie Point...when the carbon steel is heated to the point the atoms are whizzing around and not lining up to create a magnetic attraction.

2. A: The steel did not reach the point of Transformation where it becomes Martensite (hardened) you have an unknown probably soft or annealed steel not capable of holding an edge.

Common practice for backyard Heat Treating is to do the work in a darkened work area so the proper "Red" color of hot carbon steel can be identified its tricky unless you know what to look for. The Red Hot steel can have a ghost shadow and unless you wait until the metal is thoroughly even color no shadow you can be fooled by a magnet. Once the steel is an even Red Hot test with the magnet then allow another 3-5 minutes soak.
 
Nathan the Machinist said:
When steel is heated above a certain temperature (close to 1400F) the atomic crystalline structure of the iron flips from body centered cubic into face centered cubic. This structure is called austenite. It's soft and at that temperature carbon is free to move around and, if quenched fast enough, remain trapped between the iron atoms, straining the bonds and forming a harder structure called martensite. Iron is generally magnetic at room temperature, but the austenitic form is not. So you can tell when it reaches this temperature, the Curie temperature, because it becomes non-magnetic. Incidentally, this is why some non-heat treatable stainless steels are non-magnetic, they're composed almost completely of retained austenite, meaning they remain in the FCC condition at room temperature.

You can not heat treat HSS properly with the "traditional" way you're describing. HSS, because of its alloy content, is normally hardened at about 2,100F. A trip to non-magnetic won't be nearly hot enough. And, unfortunately, HSS does not generally tolerate a second HT without being properly annealed first because the temperature and the soak time needed dissolve the carbides pinning the grain on the second heat caused extreme grain growth. The result is hard, but weak and fragile and coarse grained. It must be brought to a certain temperature (depends on the alloy) and cooled at a certain rate, generally from 10-20 deg per hour before if can be re-hardened. Even a half-way reasonable job of this requires time and temperature control beyond most people. Otherwise, yes, you can get it kind of hard, but it will be a very poor quality structure that won't hold up to use very well.
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Thank you for the detailed reply. There's much to read and to consider how to use the education that you've provided. Now it's time to make practical use of it.

One matter is about, if the softening treatment that I've done, and the re-hardening and tempering that I will do, will make the blade inferior, useful, or marginal for my purpose. There is a difference between 'bad, good, better, and perfect'. And the enemy of good is often 'perfect'. So I'll give it try and see if the blade is useful for cutting wood and for resharpening.

That said, the second matter is that I have access to another tool in my arsenal: my daughter is a glass artist, and she has a kiln (dad-provided) that is likely perfect for blade treatment. It can achieve very high temperatures, and it has a fully programmable control system.

When I get educated a bit more, I'll ask more questions. Thank you.
 
Bob Blarney said:
Thank you for the detailed reply. There's much to read and to consider how to use the education that you've provided. Now it's time to make practical use of it.

One matter is about, if the softening treatment that I've done, and the re-hardening and tempering that I will do, will make the blade inferior, useful, or marginal for my purpose. There is a difference between 'bad, good, better, and perfect'. And the enemy of good is often 'perfect'. So I'll give it try and see if the blade is useful for cutting wood and for resharpening.

That said, the second matter is that I have access to another tool in my arsenal: my daughter is a glass artist, and she has a kiln (dad-provided) that is likely perfect for blade treatment. It can achieve very high temperatures, and it has a fully programmable control system.

When I get educated a bit more, I'll ask more questions. Thank you.
Click to expand...


I'm not sure how you annealed it, but most likely plenty of carbon is still in solution. I think your best bet for what you're trying to accomplish will be a short moderate heat. A ten minute soak at 2,000F will certainly get it hard again and reduce risk of other problems from a less-than-perfect anneal.

HSS gets a molten salt quench, or an air quench in thin sections. You can use interrupted oil (fire hazard though) but water will wreck it.

The surface has to be protected from decarb because the carbon in the surface of the steel evaporates as CO2 when exposed to oxygen creating a soft skin. Otherwise you need to grind through this skin to get down into hard metal.

HSS is usually tempered to a secondary hardening hump - that's how HSS works. This is three one hour tempers at about 950 degrees. HSS is unlike simple steels that react like you'd think to a "traditional" approach. It takes three tempers and it is a fairly narrow window, 50 degrees can be a couple hardness points.

If you're flying blind as far as the hardness you get you can send them to me and I'll measure it for you.
 
You will need to find out the specific steel (ie T-xx or M-xx) to find the optimal heat treatment.
 
A couple things need to be clarified here. HSS is great for drilling and things that need to hold their hardness at higher temperatures, but a hand plane iron does not need that ability. A plane iron needs edge strength and high sharpenability. While I commend you for thinking about how to re-purpose a spade bit ( which is likely HSS), you aren't really getting what you need.


If you still want to use the spade bits, I would stop and re-think your process. It may be far easier to work the spade bit in the hardened state, and shape and sharpen it with diamond burrs. A set of diamond burrs and wheels is available from Harbor Freight for less than $15. By keeping things cool with dipping in water, you can easily work the hardened spade bit. This will keep most of the original hardness and HT value.

FWIW, I regularly re-shape spade bits to get specific size or shape holes or to make a hole with a bolt head seat. I use my belt grinder or a diamond wheel. I never made a plane iron from a spade bit, but it should be no more difficult.







Now, that said, getting a bar of 1095 or W2 and making plane irons, hand chisels, or lathe tools is a common practice for woodworkers. All shaping is simple because the steel arrives annealed. You can use hack saws and files for all the work, or power tools like wheels and grinders. You will need to leave a little of the edge work to be done after hardening. Once the iron/tool is shaped as you wish, you harden it ... which your daughters kiln will work perfect for.

A 48" bar of .125X1" W2 costs $21. That will make a lot of special plane irons and lathe tools.

The HT is basically;
Pre-heat slowly to 1200F and then ramp to 1500F.
Soak for 10 minutes and quench in brine or oil ( canola oil will work fine).
Temper twice at 425-450F for one hour each time. Cool in water between the temper cycles.

Once hardened, you have to lap the iron, finish grinding the edge and sharpen it, which can be done with wet grinder wheels, flat stones, slip stones, and diamond plates and burrs.






The serious woodworker guys who make their own plane irons often prefer steels like Hitachi white paper and blue paper steel (shirogami and aogami). These steels are not really that much superior to 1095 and W2, but they carry a lot of cache' in the woodworking community. Aldo Bruno, the New jersey Steel Baron, sells Hitachi blue steel that will work very well for your tasks. Woodworkers Heaven in the UK and Dictum in Germany are specialty suppliers to the woodworking crowd, and carry a variety of Hitachi steels.

If you get into special purpose woodworking tools, you can move into the realm of tools steels - D-2, A-2, O-1 and other tool steels are regularly used for this purpose.
 
As Stacy says so well, you can get predictable, excellent results with pretty cheap steels and a low tech heat treat....why use something that's pretty much guaranteed to be troublesome?
Steels such as 10xx and o-1 cost chump change.
That said, thanks for the questions, Nathan's answers are really good, and helped me get a little better perspective. Still wondering about Q. #2
 
Question #2 is specious. All steel, and everything else on earth, is worked in a magnetic field.

Don't think your question was silly - there were are old timers who used to align their anvils on a North-South line so the blade's atoms would all align and make finer grain and stronger steel. This, of course, was nonsense, but it was based on someone teaching it to others and it getting propagated over time by blacksmith "legend and lore". You still hear it every now and then today.


Whether the main magnetic field of the earth, or a distorted pocked caused artificially, the magnetic lines of flux have nothing to do with the arrangement of the iron atoms in relationship to the carbon atoms. As austenite, steel does not react to the lines of flux at all, and as pearlite and martensite, it is always reacting to them. So, doing hardening or any other part of the process in a magnetic field would not alter the results ... but it would make it difficult to handle the steel.





Magnets are used in hardening all the time. In a HT using a forge as the heat source, you bring the blade up to heat and when it gets medium red color ( color is arbitrary, depending on ambient light and the viewers own color perspective), you start checking it on a magnet. As it hits the Curie point around 1420°F, it will suddenly become non-magnetic. The steel is heated about one shade red brighter (about 50-75°F more) and quenched. This is a fairly accurate way of determining how to get the blade to within the 1475-1500°F range. In actuality, most people using this method heat the blade to the 1500-1550°F range, but that is usually OK for the steels that are HTed by this method.
 
You noted that your daughter has a kiln for glass. A friend does glass work also in a kiln and said he would not do anything with knifes in the kiln as the knife (carbons up?) or dirtys the kiln and it adversely affects glass in there afterwards. I would double check this for accuracy, but it could ruin the kiln for glass work.
 
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