Sage advice on welding fancy steels for fun and profit without electricity

[Hardware]

I am seeking a sage of the old ways of welding with Oxy acetylene torch. I have a very good handle on the equipment and the safety/responsibilities of handling the gasses, I have been using it as a cutting tool all my life and had a brief introduction to welding with it back in high school (though they skipped bronze brazing for some reason) I have taken on a decently complete setup through inheritance and acquisition as a way around my very under-amped shop space limitations. I am trying to help out my friends with swords who keep breaking them at the welded tangs. I understand the blades to be 5160 and the threaded rod welded to them to be bog standard zinc coated m6 and m8 bolts grade 8.8. I want to get some advice and guidance on how to best go about re-welding these *(fume protection already understood with the zinc) in a way that will not ruin the heat treat of the 5160 and ensure they remain durable with the limitation of an oxy acetylene torch welding set up.

Main things I want to learn is pre-heat and post-heat actions, proper filler rod choice, and considerations for tip size with relation to the material thickness (usually around 3/16ths thick at the point of connection) and of course very important how to protect the temper of the rest of this piece of spring steel while making sure the heat affected zone doesn't cause a brittle spot.


As well as any general advice around oxy acetylene welding beyond the safety stuff that has already been drilled into me all my life by everyone who has ever talked to me about oxy acetylene because yes, I am very aware of it's explosive properties and leak detection complacency concerns. I'm not a welder by trade but I enjoy my industrial work.

 

[KeeponDragon]

pictures may be of help to see what you're working with
in regards to maintaining temper, a possible thought would be to insulate the workpiece by means of wet sogged clothes wrapped around wherever you don't want heat to propagate
(An uncle used this method to do some plumbing repairs, the idea to keep the joint next to the repair intact I think??)
is there material available to you to mimic the sword steel? Or even a broken one to practice on?
better to learn what's needed in regards to order of operations that way
what are they hitting with the sword that is breaking them anyway?

 

[Hardware]

pictures may be of help to see what you're working with
in regards to maintaining temper, a possible thought would be to insulate the workpiece by means of wet sogged clothes wrapped around wherever you don't want heat to propagate
(An uncle used this method to do some plumbing repairs, the idea to keep the joint next to the repair intact I think??)
is there material available to you to mimic the sword steel? Or even a broken one to practice on?
better to learn what's needed in regards to order of operations that way
what are they hitting with the sword that is breaking them anyway?

I can hopefully get some photos later, I have got some coupons made up of the 5160 my friend uses to make swords I was planning to get some practice reps on, I have to first attempt to heat treat them to mimic the sword hardness and temper though.

I've been advised as well that slow cooling in vermiculite is going to be very important as well as a pre heat. I'm acquiring some rg45 rods this afternoon to give this a go tomorrow.

They are hitting other swords, and eachother. I've seen some pretty impressive ways these lads destroy their equipment at this point.

 

[KeeponDragon]

ohh...like the Society for Creative Acronyms...
Yes...some of their sparring events are pretty brutal...a fella can get his bell rung pretty hard, even with a padded helm on his noggin...I've watched a couple YT videos on it.
money's on the welding process not being quite right...the joint's are getting brittle...might be worth trying annealing them after welding?

 

[Hardware]

ohh...like the Society for Creative Acronyms...
Yes...some of their sparring events are pretty brutal...a fella can get his bell rung pretty hard, even with a padded helm on his noggin...I've watched a couple YT videos on it.
money's on the welding process not being quite right...the joint's are getting brittle...might be worth trying annealing them after welding?

This is actually more of a HEMA issue than the SCA, SCA folks are generally gentle with their swords but the heavy fighters break armor and fight with rattan sticks. Usually the broken tangs seem to happen from stress or a drop, blade breaks I can't do anything for as they require a new distal taper and fresh spatulated tips welded on without affecting the heat treat of the rest of the blade again but more significant because the blade is under so much more stress than the tang.

 

[phaxtris]

#2 or #3 tip is where i would start for that thickness, you can always drill it out larger if you find you dont have enough btu's

There will always be a transition zone from the softer weld deposit/bolt to the harder/tougher heat treated blade that will be prone to breakage. A slow cool should help you achieve more toughness than brittleness in that area, i doubt you need to preheat, o/a is so slow and has such a large heat affected zone its likely that will put enough heat into the material to aid in the slow cooling that is needed

How much area you effect the temper will depend on how slow you are and how big that heat affected zone gets, anything that gets above the transition temperature will loose or have the temper affected, how large of an area that is will depend on your technique

 

[Hardware]

#2 or #3 tip is where i would start for that thickness, you can always drill it out larger if you find you dont have enough btu's

There will always be a transition zone from the softer weld deposit/bolt to the harder/tougher heat treated blade that will be prone to breakage. A slow cool should help you achieve more toughness than brittleness in that area, i doubt you need to preheat, o/a is so slow and has such a large heat affected zone its likely that will put enough heat into the material to aid in the slow cooling that is needed

How much area you effect the temper will depend on how slow you are and how big that heat affected zone gets, anything that gets above the transition temperature will loose or have the temper affected, how large of an area that is will depend on your technique

Is there a visual indicator I should watch for regarding the heat affected zone and where the and the transition temperature of 5160 /medium-high carbon steels that can be referred to?

 

[phaxtris]

im not an expert at heat treating, that is a big rabbit hole

the info i can give you, 1400-1500f is roughly the transition temperature for 5160, its a dullish red color if you are looking at a color chart, this only works if the steel is ground shiny/polished, if there is any scale the color will be masked, the curie temperature of steel is also just above 1400f (when steel becomes non-magnetic) so that is an easy one if you are quick with a magnet.

you can guarantee any part was molten will have its temper effected, as well as the areas directly adjacent. The slow cool will help with the crystalline structure.

Using o/a will help with keeping the crystalline structure small because of the large haz and large heat input, and relatively slow cool, so you are winning there

Tempering it after welding would also help with the toughness, if you have an oven large enough for the swords (375-400f for 1-2hrs)

Heat treatment is a science and a bit of a black art, you will probably have to just do the best you can do with the knowledge you can acquire, and then evolve your procedures over time as you learn what works the best for that particular application


EDIT : Absolutely DO NOT grantee that it will not break, tell these customers/friends/clients that there is no guarantees on this type of work, you will do the best you can, but that is all you can do. I am a professional welder, and I would not, nor do i know anyone that would grantee something like this, so take that for what its worth.

 

[Hardware]

im not an expert at heat treating, that is a big rabbit hole

the info i can give you, 1400-1500f is roughly the transition temperature for 5160, its a dullish red color if you are looking at a color chart, this only works if the steel is ground shiny/polished, if there is any scale the color will be masked, the curie temperature of steel is also just above 1400f (when steel becomes non-magnetic) so that is an easy one if you are quick with a magnet.

you can guarantee any part was molten will have its temper effected, as well as the areas directly adjacent. The slow cool will help with the crystalline structure.

Using o/a will help with keeping the crystalline structure small because of the large haz and large heat input, and relatively slow cool, so you are winning there

Tempering it after welding would also help with the toughness, if you have an oven large enough for the swords (375-400f for 1-2hrs)

Heat treatment is a science and a bit of a black art, you will probably have to just do the best you can do with the knowledge you can acquire, and then evolve your procedures over time as you learn what works the best for that particular application


EDIT : Absolutely DO NOT grantee that it will not break, tell these customers/friends/clients that there is no guarantees on this type of work, you will do the best you can, but that is all you can do. I am a professional welder, and I would not, nor do i know anyone that would grantee something like this, so take that for what its worth.

Unfortunately I never guarantee any work, I can warranty things if I felt I did a mechanical rebuild to exact specifications but that's a pretty rare thing to be able to pull off with the lack of documentation and parts available.
Welds like this though are very experimental and are the reason I won't do them anywhere but the tang where a break isn't so much of a safety hazard as it would be up on the actual blade which could break sharp and cause injury. A broken tang is just a missing blade when someone goes to parry and is unfortunate but usually harmless.