Welding mild steel to high strength steel

Hi All,

I have an application where I would like to weld mild steel to high strength steel.

Is there anything special I need to consider when welding the two steels together?

Thanks for reading.

MIG/TIG? What steels are being used? There may be a filler rod that is compatible with both materials being fused.

Would you be better off brazing the materials together, rather than welding?

For these sorts of questions, you may be better off directly contacting the suppliers of welding consumables to see what they recommend to be used, as using the incorrect technique/filler can lead to a much weaker weld and embrittlement of the base metals.

Hi Stephen,

I was planning on MIG welding as my TIG skills aren't up to scratch yet:(

The mild steel is a European standard cold rolled steel called DC01 (270-410MPa), which I think is similar to A1008. This is the new steel that I will be welding to the existing high strength steel. I don't have a name for the high strength steel but its tensile strength is 35-45Kgf/mm^2 (343-440MPa).

The filler material I have is ER70S-6.

I'd prefer to avoid using brazing this time around, but it could be a useful option for future projects!

A quick look at the filler materials specifications reads as it would be suitable, but it would be best to confirm, especially if you are welding onto structural components such as roll cages.

Stephen mentioned the heat affected zone, and the danger of the welding affecting the properties of the steel as it will effective be re-tempered at the weld.

As this is for a chassis, and IIRC, a load bearing suspension point, I'd suggest you do some double checking if the load paths are going to be affected, and whether annealing/normalising will be a good idea to reduce the possible brittle temper zone?

Hi lads,

If the risk is that the high strength steel becomes brittle during welding, could this be minimised or even prevented by TIG welding and tightly controlling the heat affected zone? Or is it simply impossible to avoid the welds becoming brittle!?

Hi All,

I found a way to check how susceptible a steel is to hydrogen-induced cold cracking. It involves calculating an "Equivalent Carbon Content". The greater the resulting value (Percentage Carbon Equivalent) the more prone the steel is to cold cracking.

There are a number of different formulae to determine the Equivalent Carbon Content but from the information I found online it seems like the Ito-Bessyo Critical Metal Parameter, Pcm, is the most relevant for steels with a carbon content of less than 0.18%. The steels and filler rod I plan to use all have carbon contents of less than 0.18% so for this reason I used the Pcm formula.

The results were:

DC01 Pcm: 0.15%

HC260LA Pcm: 0.15% *

ER70S-6 Pcm: 0.20%

* I am unsure what steel the car actually uses so I used the details for HC260LA. It is a high strength low alloy steel with similar tensile strengths so this should give me a pretty good indication.

According to the table above this would mean that the steels should have "Excellent" weldability, and therefore very unlikely to develop cold cracking.

So, in theory this all looks good.

If anyone has any feedback on the above I would be grateful for your input.

Thanks.

Hi,

if there is a p problem it will be most likely the chassis, if it's really something like HC260 then you'll have to address that by special rod and only spot welding it, otherwise the HC260 will have a good chance to crack next to the weld. I'm not quite sure what filler and probably wire to use but since Vauxhall/Opel introduced the Vectra C or Astra H models they had to use special mig welding equipment due to the change to high tension. Probably you should visit a Vauxall dealer and ask them what they use for chassis repairs....

On the other hand, if the chassis is something like SJ355 (tension like you imaged in your post) and you would like DC01 (is it really still available in Ireland ?... last time I've seen it in person was like 20 years ago on some stuff eastern Europe made) you can weld it with your normal equipment...

Hi Jan,

Thank you for your feedback on the HC260LA, I will contact my local Opel garage and see if they will share what filling material and welding equipment they use for repairs.

Based on the results of the equivalent carbon content calculations I have done, the HC260LA should not be prone to cracking! Maybe I used the wrong formula :(

The car is a 1980s Mazda and I believe some parts of the chassis use high strength steel, but I don't know what grade. I don't even know if HC260LA was used in cars in the 1980s!

DC01 is the only cold rolled steel available for the general public that I could find in Ireland. I guess we're a bit behind the times :D

Hi All,

Someone has suggested to me that if the chassis uses high strength steel I could preheat the weld area on the chassis and then weld in the mild steel patch panel.

Could this be a viable option?