Heat Treating: Example of Error

[Ray C]

I was going to knock-out some parts today for a pet project and wanted a couple chunks of 4140 to be about Rockwell 38. I got lazy and didn't want to fire-up the electric oven and used a makeshift forging oven and propane.

The heating and quench seemed find. It was tempered by suspending it back in the forging tube and heating with the torch until it hit the 900F needed for that hardness. I used a hand-held IR thermometer but, I must have gotten a bad reading or, the temperature was very uneven inside the tube. Normally, I use an electric oven for tempering because it has a good pyrometer and gives fairly accurate read-outs.

Here's the setup. Quick and dirty and I've used it for lots of simple (unimportant) knock-off parts.


After facing-off the part, you can see exactly what bad heat treating looks like. Look at the area between the 3:00 and 6:00 position on the face of that part; that area is shiny yet, the rest is dull. This is classic uneven tempering. The shiny part is hard and the dull part is not.



Also, I just want to mention that when working with hot-rolled steel, sometimes you get a nice shiny finish once you cut past the scale. If you go a little deeper, most often it starts looking dull again -even if you increase the RPM. This is because the outer surface of the HR steel self-quenched as it cooled and the outer layer is a little harder than the inner core.

....

Yeah, I know... A lot of folks often give heat treating advice that goes like: Heat it till it's medium cherry red then temper it till its' straw colored...

My father was a T&D maker. My uncle was a metallurgist. Even for simple stuff they used an optical pyrometer.

I'll be doing this over.

Ray

 

[bhigdog]

Looks most likely it was uneven temp in the oven maybe coupled with inadequate soak time..........Bob

 

[kev74]

I've got a noob question... Can the part, or any piece of steel for that matter, be re-heat treated, or is it a one time only process?

It seems to me that as long as the surface doesn't degrade beyond what is required, the internal steel should react to the hardening and tempering processes.

 

[Ray C]

I've got a noob question... Can the part, or any piece of steel for that matter, be re-heat treated, or is it a one time only process?

It seems to me that as long as the surface doesn't degrade beyond what is required, the internal steel should react to the hardening and tempering processes.

It can be re-done, no problem. Normal heat treating and quenching does not change the chemical components of the material. For all practical purposes, the process is akin to freezing and thawing water.

The term "Full Annealing" is when metal is re-heated past it's normal heat-treating (Austentizing) temperature then allowed to slowly cool. That basically "resets" the metal and makes it completely ductile.

Ray

 

[Ray C]

Looks most likely it was uneven temp in the oven maybe coupled with inadequate soak time..........Bob

Here's what I think happened -but, I'm not really 100% sure...

The part was fully glowing when removed from the furnace; not just red but, bright glowing red. When it's glowing like that, it's heated all the way through. Also, when it quenched, I could hear the metal "screaming". Metal that's not fully heated does not give-out that characteristic "scream" followed by a long rumble.

When tempering was done, several pieces were processed at the same time and they were stacked horizontally in the tube. I think there's two possibilities.

The piece on the bottom was closest to flame and it got partially annealed or, the heat rose to the top of the tube and the piece at the top got partially annealed.

There were 4 pieces and only 1 of them is messed-up.

Ray

 

[bhigdog]

Be interesting to see what the actual RC is in the different zones. ..................Bob

 

[benmychree]

Looks most likely it was uneven temp in the oven maybe coupled with inadequate soak time..........Bob

And also not long enough time being tempered.

 

[Ray C]

Be interesting to see what the actual RC is in the different zones. ..................Bob

I checked it with the electronic Leeb tester. The mushy part is averaging around 30 and the shiny part is showing only a few points higher. On a properly done piece, the Leeb tester usually gives the same result within 1 or 2 points.

Here's a different piece, done in a different batch earlier in the day. These came out at 40 Rc and give the nice chrome finish.




Ray

 

[Ray C]

And also not long enough time being tempered.

We might never know because I'm not planning to make this mistake again.

For knock-off parts for my own projects, I've used that blast furnace setup many times for the heat treat. For tempering, I always toss it in the oven because I'm usually shooting for a very specific hardness level. My oven is setup for argon purge and the temperature read-out seems very precise. Heat treating is one of my favorite shop tasks and I have books, charts and procedures for every metal under the sun. As a kid, my uncle used to take me to work with him and I was fascinated with it. I have an electronic Leeb tester and a traditional Rockwell unit. When I take my time and do things right the parts are within a couple points of where I wanted them.

Today, I take a shortcut and look were it got me... now I gotta do it over.

Ray

 

[Boswell]

Ray, thanks for sharing the fail and the thought process around what might have happened. I have only worked in Brass and Aluminum so Heat Treating is a mystery to me that someday I will need to figure out.