Welds cracking?

[Sniper]

I have been welding some inserts in a block of hardened H13 the inserts are mild steel, they have to be watertight, each piece has a 6-7x45deg chamfer and there is a gap between the two of about 1-2mm.

I have heated the workpiece up and used a h11 filler rod, I had the welder set to 125amps but after the first run you can see a hairline crack straight down the centre of the weld.

 

[Shox Dr]

How big are the pieces?

What temp have you preheated them to?

Pictures!

 

[TechnicAl]

http://www.weldreality.com/ToolstlsH.htm

There are lots of reasons. Look at this. I havent read it all but the guy usually talks sense (even though hes American)

 

[Sniper]

How big are the pieces?

What temp have you preheated them to?

Pictures!

unfortunatly I dont have any pictures, but the main block is approx 500mm x 500mm x 350 with various cavities in it, and the MS pieces to be welded in are approx 40mm x 80mm x 25mm.

I am not sure about the pre heat temp as I only have a propane gas torch and no way of measuring temp (a guess I would say around 150-200 deg C)

 

[Sniper]

http://www.weldreality.com/ToolstlsH.htm

There are lots of reasons. Look at this. I havent read it all but the guy usually talks sense (even though hes American)

thanks I will have a look through it

 

[Shox Dr]

To be honest I know naff all about welding such material but I do know the likely questions that others would ask. Hence the two questions.

However I do know that's a fair lump of metal there, and your estimated temp you think you achieved, Id say you need far more heat, to get an even temp. What type is the gas torch

 

[Sniper]

it's only a basic one like this but with a bigger nozzle http://www.gasproducts.co.uk/acatalog/Bullfinch_140P_Propane_Gas_Blow_Torch_Kit_.html

 

[Drains]

the main block is approx 500mm x 500mm x 350 with various cavities in it, and the MS pieces to be welded in are approx 40mm x 80mm x 25mm.

I am not sure about the pre heat temp as I only have a propane gas torch and no way of measuring temp (a guess I would say around 150-200 deg C)

You'll be there for hours trying to get sufficient pre-heat on that. As it is you'll just be taking the chill off it. You can go to over 500 degrees C pre-heat according to Mr Craig. And that's what you probably need.

I reckon your mill or two gaps won't help either. You'll need a prep but weld in big gaps will shrink like billy-oh. Fine in M/S but tool steel won't like that at all. You need to consider it a highly restrained weld (that block won't move for you), so as the welds shrink, they can either change shape, or crack.

 

[ChrisJohnsSculpture]

H13 is an air hardening steel so welding it without aerospace grade kit should be considered close to impossible.

Brazing would be more sensible but it sounds like there is a fundamental design issue ~(ie designed by someone who has no clue about matalurgy)

 

[hotrodder]

Aerospace grade kit? It's probably tool and die work i.e. often very different mechanical requirements from the weld to structural stuff/trying to avoid annealing and then rehardening things whenever possible/things that don't make sense until the owner has explained what it does etc

Tempilstiks are your cheapest way of keeping track of temperatures
You don't say how big the torch nozzle is but BTUs versus size are listed here... http://www.bullfinch-gas.co.uk/index.php?option=com_docman&task=doc_view&gid=93

 

[Sniper]

It is tool and die work so the weld for this purpose is not structural it just needs to be water tight so water can pass through to cool when the tool is in use.

The nozzle is approx 25-30mm dia

 

[garethp]

Thats a fair lump to preheat, the torch you've got sounds way too small. A bigger torch and something to keep the heat in are needed - thermolite blocks are a good cheap option, just make sure they're bone dry to stop small explosions as the water turns to steam.