Tool steel breakage

[idacal]

I built this blade out of s 7 tool steel hardened it with an oil quench and then tempered it to 800 degrees and allowed the kiln to cool down overnight but i only got about 2000 hole punches out of it before it failed and you can see the way it failed
I can change the temper if that would make it last longer. im punching .250 wall pipe from the inside with this would like to get more punches but if thats that, it will have to do

 

[idacal]

There is a small piece of the tip that stayed in the well

 

[sfriedberg]

S7 is generally an air hardening steel. If you quench in oil, the oil needs to be pretty warm, close to 150F, and you should pull the work from the oil as soon as it goes black (around 900F), then let cool in air to 125-150F. Temper immediately after it gets down to that temperature. Cold work tools usually tempered at 400-500F. (HT recipe from Hudson Tool Steel.)

If that piece is as thin as it looks in the photo, I don't think you need an oil quench at all.

 

[idacal]

Its 1/2” thick dont know if that makes a difference had another one in a differrent tool broke today basicly same type of break will try just air hadening the next ones

 

[mhajicek]

If you get it too hot, or keep it hot for too long, before the quench, the grain size can grow and make it brittle even after tempering.

 

[Rickyb]

A good pic of the crack face may help in diagnosis.

 

[White Lightning]

Look st the heating sequence. I believe its recommended to heat in 400° / hour steps. I'm not sure if heating to fast may create a problem. I've been doing this and seem to have good luck.

Sent from my SM-G960U using Tapatalk

 

[idacal]

Okay there is a couple of things i did wrong brought it up to 1200 degrees in an hour then 600 an hour to 1750 held it there for 30 min then quenched. quench was wrong and i guess im heating to fast will a few degrees over hurt anything?
I could get a heat stick at 1725 but it dumps a lot of heat real fast trying to see if anything melted, its a large pottery kiln will try it again and see if it lasts longer. My tempature controll havent been verified to how accurate they are so i went a few over. Didnt think it mattered because forging temp is 2000 or so

 

[Robert R]

I built this blade out of s 7 tool steel hardened it with an oil quench and then tempered it to 800 degrees and allowed the kiln to cool down overnight but i only got about 2000 hole punches out of it before it failed and you can see the way it failed
I can change the temper if that would make it last longer. I am punching .250 wall pipe from the inside with this would like to get more punches but if thats that, it will have to do

Charpy unnotched Impact energy and Rockwell C scale hardness for S7 as a function of tempering temperature:

Single temper at 400 deg F Hc=58, J=180 ft-lb
Single temper at 600 deg F Hc=55, J=228 ft-lb
Single temper at 800 deg F Hc=53, J=179 ft-lb
Single temper at 1000 deg F Hc=51, J=239 ft-lb
Single temper at 1200 deg F Hc=39, J=264 ft-lb

If resistance to cracking from punch impact is a concern, the 800 deg F tempering temperature should be avoided. A 600 deg F temper will give better service for wear resistance and impact strength.

One more comment:
The load on the punch should be transmitted to the press frame through friction in the bolted joint rather than by shear through the two bolts.
Bolted connections that are properly torqued can transmit much higher loads. The crack between the two bolt holes in the punch suggests that the load was carried by the bolts rather than by friction.

 

[gbent]

Didnt think it mattered because forging temp is 2000 or so

The part would be either normalized or fully annealed after forging. The annealing will refine the grain structure and reduce the grain size.

 

[Robert R]

Hudson steel has a quick reference for heat treating S7. The tempering graph shows that the drop in Charpy energy at a 800 deg F temper is also where there is maximum distortion in the part size. There is no change in part size when tempered at 500 deg F.

Tool Steel | S7 Shock Resisting Tool Steel | S7 Steel

This is the same heat treatment recipe that is shown in White Lightning's post (#7)

S7 and the majority of the other air hardening tool steels are on the DO NOT Normalize list . The O, W and L series tool steels can be normalized.

 

[M. Moore]

Looks like you have the answers for the heat treat. I want to know how that oddly shaped punch punches a hole inside a pipe? Can you post a pic of the finished part or the tool set up in the machine?

 

[DDoug]

There is a small piece of the tip that stayed in the well

Looks like you have the answers for the heat treat. I want to know how that oddly shaped punch punches a hole inside a pipe? Can you post a pic of the finished part or the tool set up in the machine?

I think you have your answer up above....

 

[Robert R]

Water wells in sand or clay soils are finished with a string of steel pipe that will include a 20 foot screen section positioned at the water table.The screen is made from slotted tubing with a helical wire wrap to prevent the outer gravel packing from entering the well casing,

It is sometimes necessary to form the screen section in a existing pipe. This might happen if the water table drops below the existing screen.
There are rotary punches made, such as the one shown in the link, that will do the job.

Air Perforators to perforate well casing in the grouund

The punch described by IDACAL in posts 1 &2 above may be part of a larger assembly that punches slots one row at a time.