Spot welding stainless

[Glyn]

I've just got a aquired a spot welder as payment for some work. Had a play and it's fine on mild steel but when I use it on stainless (no idea of grade but magnetic) the welds seem brittle and can be pulled apart, not easy but easier than the mild steel. The weld it's self is strong, it tears a hole in one side of the metal but seems quite weak compared to the mild steel. Tell me why please someone.

 

[Archie]

If I remember my materials lectures then magnetic stainless is ferritic and is the poorest quality stainless. If you're saying the weld does not break but the material around it tears then I'd hazard a guess that it's just rubbish metal.

 

[Glyn]

Now, if I hadn't done the same test using mig, I'd agree but I've used this metal for all sorts of little projets and in the past tried to snap the stuff at the welds and it's been fine! I've just stuck a piece of mild to a piece of stainless and the mild stuff tore. Not a fair test as the stainless is half a mill thicker than the mild. Would stainless not tend to be more brittle when welded? I wouldnt like to think my next project of a stainless, last a lifetime wheelbarrow is going to fall apart at the spot welds. The metal came from a printing press, I'm told. It's covered in scratched in patterns.

 

[Hitch]

Not all types of stainless can be resistance welded well, the 300 grades are okay, 304,316,321,347 etc.... ferretic types are okay aswell i think.
Its the MArtenistic ones that dont spot weld very well, 410 i think is one type.
Something to do with the hardness i belive.

Difficult to guess at anything else without knowing what stainless it is.

 

[zxr400]

Could it be that you need to preheat? Or possibly stress relieve by annealing post welding?

Cheers,

Mark

 

[pidgeon chit welder]

I have spot welded thousands & thousands of drinker brackets and cage 400 x 600 tops , all of the same quality stainless steel sheet 0.5 mm thick using one of the hand operated 240 volt spotwelder machines from Machine Mart. ( set up on a bracket in a blacksmiths vice )

Every now & then I had to reset the faces of the electrodes ,occasionally refiling them round at the business end with a flat file.

I cannot recall if the welds were brittle around a " heat affected zone ( HAZE) as it was nine years ago , I've been to sleep since then .

As soon as I noticed the size of the weldspot getting bigger or getting blued I automatically redid the tip .

As the tip melts you get a bigger surface heating things up so draw more current to do the job this leads to excessive heating methinks & would be likely to produce a HAZE.

So I would service the copper electrodes and reset them correctly to have the ball end of each electrode touching the other at the smallest point of contact then see how things are .

 

[Glyn]

Cheers for the answers chaps, I have re-shaped the electrodes and it does seem slightly better. ZXR400, I'm never going to get that involved, as I say, I only use the stainless for silly little things such as the wheelbarrow I'm going to make. What I have noticed though is, different timer adjustments make a big difference. Just messing about with it, I noticed the longer the timer setting, the more brittle the weld seams to be. So I guess pidgon chit welder was right. Thanks again.

 

[pidgeon chit welder]

Glyn ,

I've just re read the thread.... as you use the spot welder for longer periods the resistance changes as it heats up and this will also affect the length of timing you use.