How Hard is Weld?

I work in a mold shop and sometimes we need to weld up a surface because we may have made a mistake or for whatever reason. I would like to know what rockwell the weld is in general. The material I work with is P-20. The problem I am having is that the weld seems to be as hard as hardened tool steel and I am trying to cut it with uncoated carbide 4 flute endmills. Needless to say the cutters are not holding up. Is there any recommendations for a better cutter to use?

The weld deposit hardness will depend on the filler type, the filler vendor, the amount of pre and post-heat and the amount of carbon picked up from the base metal. If I came up with a number here I would just be pulling it out of a dark, wet, place. Some fillers are machinable and some are not. Even a machineable filler can pick up elements from the base metal and become rock hard. Molds are typically large and weld currents low, this creates a quenched weld bead unless pre and post-heat are used. The welder has to know his stuff.

What was the process used: GMAW or TIG? What is the filler used? What size? What was the weld current was used? What bead width was acheived? What was the preheat temperature? What was the postheat temperature and for how long?

There are too many variables in your question, base metal, applied metal, cooling method for starters.
An example- if you are quenching after welding this will harden the weld.

I am not sure what methods the welder uses to weld. I know that it is tig welding but that is it. My question should have been what type of coating do I need to use to cut through the weld? I didn't realize that there were so many variables involved.

Depending on the shape you need for a tool, you can get some ceramic inserted tools. Unfortunately, I think the insert shapes are simple but strong shapes, suitable for light facing type cuts, for example, to level a weld bead with a flat parent surface. Check out Greenleaf for ceramics.

But if you have to mill more intricate surfaces, you might have to resort to using plenty of ballnose endmills, on light fast cuts using a cnc in order to keep control of depth and chipload. Manual machining techniques tend to be a bit too rough and ready for hard milling, IMO.

The OP is asking the wrong question. It doesn't matter what kind of filler metal was used, because what he's seeing is an increase in hardness of the base material in the heat affected zone. P-20 is, I believe, an air hardening steel. I just found a nice article on the Moldmaking Technology web site that explains the problem:

http://www.moldmakingtechnology.com/articles/110401.html

I quote:

"One of the greatest concerns when texturing P20 is welds that might be in the mold. The HAZ (heat-affected zone) around the weld can be 15 or more points of HRC higher than the base metal. With the base material hardness of 28/32 HRC and the HAZ being 15 points higher, the difference in texture depth between the two areas can be as great as 50 to 60 percent. This will cause a halo effect on the textured part and increase the gloss factor, which will increase the amount of handwork required to complete the textured surface, again resulting in higher cost and lost leadtime."

Now of course, if the filler material is a different steel (or stainless) then those parts of the weld may be soft. But any place where the base metal became red hot and then cooled is going to be as hard as diamonds. The only way to avoid this with TIG welding is to send the entire cavity block out for heat treat, to have the temper drawn to a more reasonable hardness.

Either that, or have the repair done with laser welding, which has a tiny HAZ compared to TIG.

Dennis

P-20 is a de-gased 4130. Any welding on this material requires a pre-heat and a Post Weld Heat Treat (PWHT). Interpass temps should be around 400F minimum and the PWHT is right after welding. This material is very prone to embrittlement at the HAZ and will crack if these proceedures are not followed.

After all that, the material needs to be heat treated again. P-20 is an oil hardening steel. Heat to 1550-1600F and then oil quench. A temper at 1000F should give you about a 35 Rc.

Most shops will not weld P-20 unless it's a big or complicated part. Even then, with the warpage due to oil quench, you need to think if it's worth it. I've seen plenty of places try to repair the parting line of P-20 inserts by welding and not doing any heat treat. The area they welded will generally break off without too much use.

JR

Assuming no control over the weld proceedure and needing to power through the cut, then edge geometry and cutter stiffness are more important than cutter material. Use a negative rake insert with a 90 degree edge. Use as much corner radius as your mill can drive without chatter. Use the stiffest cutter you can fit in the cut, either a large diameter insert mill or a face mill.

The average endmill is the exact wrong geometry. An acute cutting edge, positive rake and sharp cutting tips spells chipped cutter.

Pf Changs.