Welding S7 to Mild Steel

[Jspool]

I'm replacing my dies on the power hammer with S7.

one set will be mounted to 3/8" mild steel plate

The other set will have 3/8" mild steel tabs welded to them.

I will be MIG welding.

Do I have to preheat the S7 in order to accomplish a good weld?

If so, to what temperature?

Thanks

 

[Charlotte]

This site was posted in the Welding general fab discussions http://www.weldingtipsandtricks.com/  

That guy told me more than I thought there was to know about  technique in mig welding.

 

[BIGGUNDOCTOR]

The weld will need to be annealed after welding, otherwise it will be brittle. 

What about bolting them together?

[Jspool]

I could bolt them together. One of the die sets are 4"x4", the same as the tup.  It's primarily for use with top tools.  The top 4x4 die will slide I to the tup channel, so I can bolt a 1.5" x 3" piece of flat bar, to the tup and die, front and back. 

The bottom 4x4 sits on a 6" round anvil, so a couple of angle taps could be bolted to the die, and the anvil.

i don't understand why the welds would crack as they are just holding the dies in place.

i should add that I do have an Evenheat Oven if that needs to come into play.

[BIGGUNDOCTOR]

When you weld tool steels the welded part becomes hard due to fast cooling of the weld bead. I always welded, then used a torch to anneal the welds to prevent cracking. 

 

S7 is an air hardening tool steel, so I would definitely anneal the welds.

[Jspool]

Thanks for that. I'll definitely do so if I end up welding them.  Should I be heat treating the dies, or leave them around RC20?

[BIGGUNDOCTOR]

Since S7 is a shock resistant steel, I would heat treat them. No sense spending the money on tool steel, then using it in the annealed condition.  Hopefully someone who has more experience with power hammers will chime in on the hardness needed.

[Alan Evans]

When you weld tool steels the welded part becomes hard due to fast cooling of the weld bead. I always welded, then used a torch to anneal the welds to prevent cracking. 

 

S7 is an air hardening tool steel, so I would definitely anneal the welds.

i am bit puzzled by this process. if you take it up to red with a torch to anneal it why does that not cool quickly and harden? How is it different to the weld bead? You are presumably always going to have a heat treated/unheat treated border somewhere...

i have tended to use 312 dissimilar metal rods for welding to pallets. They were not S7 though.

Alan

[BIGGUNDOCTOR]

With the torch you can heat a bigger area than the weld affected, and cool it down slowly. With S7 being an air hardening steel, an oven annealing may be in order, as trying to control the temp with just the torch may be tricky.  With something like 4140 the torch works fine. The main objective is to soften the weld so it is no longer brittle, and non air hardening steels are pretty forgiving in my experience. But yes, you are correct. With an air hardening steel you will always have an area that would probably be hard with the torch method because you are only heating the welds up, not the entire block. It would take quite a bit of time with the torch to bring it down slow enough to not end up with a section that was still hardened. So in this case it would be more of a tempering process than annealing to soften the welds.  All of this being said ,it may be a moot point since he still needs to heat treat the dies anyways. Weld first, then heat treat,and temper. 

To be on the safe side though I would still look into a mechanical method of attachment.

[clinton]

What Biggun said the heat affected zone (HAZ)

[Alan Evans]

With the torch you can heat a bigger area than the weld affected, and cool it down slowly. With S7 being an air hardening steel, an oven annealing may be in order, as trying to control the temp with just the torch may be tricky.  With something like 4140 the torch works fine. The main objective is to soften the weld so it is no longer brittle, and non air hardening steels are pretty forgiving in my experience. But yes, you are correct. With an air hardening steel you will always have an area that would probably be hard with the torch method because you are only heating the welds up, not the entire block. It would take quite a bit of time with the torch to bring it down slow enough to not end up with a section that was still hardened. So in this case it would be more of a tempering process than annealing to soften the welds.  All of this being said ,it may be a moot point since he still needs to heat treat the dies anyways. Weld first, then heat treat,and temper. 

To be on the safe side though I would still look into a mechanical method of attachment.

Yes, it was the welding/annealing with the S7 Air hardening properties I had the difficulty understanding.

I quite agree, mechanical attachment has a lot going for it. It is one of the reasons all my hammer pallets are machined and hardened and then held in place by wedges.

Alan

[patrick]

When you weld steels together, regardless of composition, you have a few things going on. First, the welding procesess melts the two pieces being welded in addition to the filler metal. Therefore the weld region has a composion that is as  mix of all three components. Next you have the heat affected zone (HAZ). The HAZ consists of metal which was so hot it was austenitized and metal which was hot, but not austenitized. The austenitized region can transform to untempered martensite or other structures depending on the grade and cooling rate. When you weld mild steel to mild steel you normally will find the weld is relatively soft because there is not enough carbon and alloying elements to result in a hardened structured on cooling. This can change if the mild steel has a bit of carbon or if the object is so large that it cools the weld very quickly. When you start welding higher carbon and alloy steels you must use pre and post heating techniques otherwise the HAZ will be so brittle that you will get cracks under the weld bead itself. If I were going to weld S7, I would first determine what tempering temperature was needed do get it into a relatively tough state. I'd heat to that temperature, weld, then re-heat to that temperature and cool very slowly. Then I'd fully austentize the component, cool as needed to achieve full hardness, then temper to the desired hardness.

[IronAlchemy]

I have done a few dozen tire hammer dies in S7. I tack weld the S7 to the mild steel plate with the pieces preheated to 550 F. MIG welds should hold fine as long as you do not let the assembly cool below 400 F. You should heat treat the assembly after welding if you want a fully hard die. I like to make them pretty hard, R56-58, so that they do not get dinged by top tools easily. You cannot get them that hard using air cooling. Air cooling only gets you to R38 or so. You need to oil quench and then temper at 450 F. I would be happy to provide a more detailed protocol if you want to go this route.

Doug

[Jspool]

Hi Doug,

Preheating and welding is finished on the top die.  I did not know that I shouldn't have let it cool prior to HT. Fingers crossed.  I am planning to Austenitize at 1750 for three hours (die is 3" thick), and temper at 1000 twice for three hours each as per recommendations I received from another forum.  Is the tempering temp level ultimately what will decide my rc, or the austenitizing temp?  At 58rc, are you getting any chipping?

I have another set to make after this, and then some for the hyd press, and would really appreciate learning more about your protocols.

thanks

Jonathan 

[metalmangeler]

Opperating only from memory I think you are tempering to high, I think for S7 you might want 700, or less. H13 was like 1000. You would do well to look this up rather than trust what you find on the internet forums.

[Jspool]

This is a data sheet I was initially working from.  The forum info that I do trust is from folks that have actually done it.

http://cintool.com/catalog/Shock_resistant/S7.pdf

interesting information

[Jspool]

This is what I ended up doing.

Upper die was preheated and tabs mig welded on.

I machined bolt pockets into the lower die at two opposite corners.

Next step is to dress and heat treat.

 

Edited September 20, 2015 by Jspool

[metalmangeler]

They are recommending the high tempering temps for hot work, and a lower tempering temp for cold work. I of course do not know how you will use your hammer. I hit  a lot of tooling with mine, I want my dies slightly harder than my tools as the tools are cheaper to replace. In the past I left a lot of my tooling in a normalized state on the struck end just for this reason. tooling like this will give out fairly quickly. Now I am making many of my struck tools from 4140 and tempered in to the 600 range. I think these tools would likely mar the dies tempered to 1000 made from S7. Hopefully you made the dies a little proud so you will be able to dress them and keep using them for years. The data sheet that I had been using had a tempering graph showing the rockwell curve at different tempertures. If you can find one of those before you heat treat that might be helpful.

On a side note I have used S7 for tooling but never in larger sizes like you are doing.

[IronAlchemy]

We have not seen any chipping of S7 dies from our 10 machine tire hammer build with them hardened to Rc 56-58. We did a follow on build of power hammer tools also done in S7. I agree with metalmangeler's comments on top tool hardness. We were careful to harden the top tools to more like Rc 50 (temper at 1000 F). I know a lot of people try to leave the striking end of S7 tools annealed but I find that they just do not hold up even to hand hammer work without some hardening. S7 is incredibly tough but I have seen S7 tools chip when they were thin sections air quenched to unknown hardness after getting very hot in use.

The challenge with large sections like dies is to get them hard enough. You may have to work to cool them fast enough. There is a risk that the smaller sections where you machined the mounting holes will cool faster and crack. Personally, I think you will be OK...but I choose to lead life as an optimist. :-)

I have attached the PDF of my welding and heat treatment protocol that I sent by messaging to you so that others can have it as a reference. It has been well tested in my shop but be aware that the section on cryo-treatment has not. I guess I have not yet felt the need to make S7 dies even tougher.

Thanks for sharing your results.

S7 die heat treat protocol.pdf