Reverse Engineered

[Alexander]

You are probably not wrong. My basic understanding of metallurgy led me to think that if you get a piece of HSS hot enought that you change the colour that is because you have taken that piece past its critical temperature and the oxygen has left the discoloured look. Doesn't sound like the ideal situation to me. What does everyone else think? Non ferrous metals go brown or blue when the oxygen oxidizes the surface which only happens above the temperature when the metal starts to change. Am I right?

 

[PeterT]

apparentlyI'm no expert either but have been lurking on some heat treating sites as I'm machining O1 tool steel parts that require hardening. My take: assuming proper quenching of typical high carbon tool steels, they have a predictable color change during tempering. Each color occurs at a certain temp during elevation & has a corresponding hardness level when stopped there. So if you were to heat these steel blanks from straw yellow (450F) to blue (575F) you would reduce hardness from about 61 to 55 Rockwell according to the chart. http://www.threeplanes.net/toolsteel.html

But I think modern, premade HSS tool blanks is a different animal with other metallurgy goodies to increase strength, toughness & maintain hardness over elevated temperatures. The graph is kind of generic but shows plain carbon steel to HSS & cobalt alloy for example. Tool steel is probably somewhere in between. But notice that's Temp vs. Hardness. What I've always wondered is if HSS has the same color spectrum as carbon steel but 'displaced' at say 100F higher temp? Haven't seen a chart like that. But I remember being told by Mr. Shop Teacher "don't let your cutter edge turn blue" when grinding, hence the frequent water dip. So whatever temp that equates to means less hard. Now if you are grinding tool steel profile (prior to heat treating) then who cares what color, once profiled you are going to take it up to carrot red anyway for quenching/hardening, but that's a completely different operation.

Another guy told me its confusing because older generation cutters (and therefore text books) were more like modern high oil/water/hardening 'tool' steels. So as the chart shows they would see a more dramatic degradation of hardness with color show. Whereas about the only thing you can do to shape commercial HSS tool blank to best of my knowledge is grind it. Enough theory - lets make something!

 

[Krprice84]

Yea, my understanding (basic as it is) is that modern hss and cobalt alloys are designed for high hot wear resistance. They still will get tempered at high temps, but the color isn't representative of the actual tempering in the same way that it is for other steels.

This is, of course, just my understanding, but that's how it was explained to me a few times, plus on a few YouTube videos.

One guy said that you used to have to quench, but that now it's primarily for comfort holding the tool. You still don't want to get it crazy hot, but he said that you generally don't come near hot enough unless really grinding tons of steel off a tool. Dunno though..... I'll probably quench anyways just to be certain

 

[EricB]

I was going to mention the same ideas as @Rick Runciman I do nothing but threads at work, and I use aluminum shims a lot. The other thing you could look at is making yourself a set of soft-jaws for your chuck, but as Rick said, I would probably just machine the other side first. What kind of jack are you working on, is it an old Walker or something?

 

[Alexander]

It is a hillman toe jack. Fancy and expensive unfortunately they don't sell the part I need so I'll make some.

 

[PeterT]

some additional info I stumbled on re what heat colors imply to grinding HSS tool bits (starts about 13:40)

 

[Johnwa]

Just a comment on grinding tools.

My understanding was that high speed steel doesn't temper or anneal at all below a really quite high temp. I can't say for sure cuz I'm pretty inexperienced, but if it's anything like some other tool steels I've played with, you really need it to glow before you get any annealing or tempering.

I figured the quenching was just to make it so you can hold it.

Am I wrong here?

You're right.

 

[PeterT]

Finally found a link with some good down to earth temperature specs
http://www.gadgetbuilder.com/VerticalShearBit.html

About HSS In "Design and Use of Cutting Tools" Leo St. Clair had some interesting observations on HSS and how to select among the various grades. The basic M1/T1 grade of HSS (without cobalt) is the strongest and toughest material but begins to lose hardness above 750F and should not be used above 1000F. Adding 8% cobalt improves hardness at cutting point temperatures to 1100F but reduces strength and toughness. Adding more cobalt (12%) extends the temperature range to about 1150F but further reduces strength and hardness. The cast alloys (T15, Tantung, Motung, Rex76, Rex95, etc.) handle 1200F or more but are somewhat brittle and their cold hardness is less than some regular HSS.