There is no question about that susceptibility to
cracking increases with hardness. However, it
might not be possible to quantify, since hydrogen levels
and restraint also are important factors.
If you find a nice correlation please post it. I've got problems
with martensite and hydrogen myself. Any input would be
highly appreciated.
/Happy
Hi again KAM,
This is getting interesting. Will IJP be a problem if you try reducing the average heat input? Try lowering the background amperage/voltage parameters (reduce heat) if it will not cause any non-fusion problems. I had only 1 or 2 welds out of thousands with cracks using this process as stated in my earlier post. Both cracks were in the HAZ which propagated into the weld metal. Are the discontinuities your experiencing soley in the HAZ? Hotter is not always better (see the post on the aluminum even though your materials are ferrous).
The problem with my application was IJP/IF discontinuities in the root and groove wall areas. I had wrestled with this for some time before I changed the default parameters on my Auto-M controllers. I obtained instant positive results when this change was made.
When I increased heat input by tweaking the parameters,that is when I first found the cracks. At that point I backed up a little and achieved what I was looking for. Send me an e-mail and let me know how you do on this, or, try contacting Miller, their pretty good with tech-talk.
It is interesting to see things "develop", and I thought that I would give my opinion on the developments. I assume that when you say that you are welding GMAW-P, you mean that that you are using a pulsed spray transfer mode.
The weld size that you mentioned, (1.5mm) is indeed very small. Even with relatively thin base metals, you are liable to have very fast cooling rates. This will necessarily mean more martensite formation. Many people assume that heat input (VxA/Speed) is an absolute indicator of the cooling rates. This is not so, because your cooling rates is related to your temperature gradients. Even if you maintain the same heat input you can vary the temperature gradients.
As an example, if you double the Amps, and double the welding speed, the heat input remains the same. The temperature gradients will however be much bigger. (If you disagree with me on this, let me know, and we can debate it a bit.)
Another factor to consider, is the size of the HAZ. If you are obtaining a very narrow HAZ, it is an indication of high temperature gradients. A small HAZ is advantageous, because the "problem area" is then smaller. Even if a tiny crack develops here, it will be rapidly arrested from propagation by the more ductile weld metal and parent metal. The area of grain growth will also be very small. This area generally leads to a poor weld strength and impact properties.
We therefore have two factors that tend to "balance" each other. Low thermal gradients lead to less martensite, but wider HAZ's. High thermal gradients lead to more martensite, but a narrower HAZ. I believe your problem lies in achieving the "sweet spot" where you have a small enough HAZ, with relatively low martensite present.
Depending on your current welding parameters, it may be advantageous to either increase your temperature gradients, or decrease them. It is impossible for me to tell you which. Obviously, keeping everything else the same, an increase in pre-heat temperature would lead to lower thermal gradients.
Regarding your question of a correlation between hardness and succeptibility to cracking, the answer is yes, there certainly is such a correlation. The correlation is however not simple, because it relates to a number of parameters:
1)Joint design. (The higher the constraint, the lower the hardness to prevent cracking.)
2)Material microstructure.
3)Amount of hydrogen in weld and HAZ. (The less the hydrogen, the higher the hardness before cracking occurs.)
As a rule of thumb, in the industry in which I work, (Pressure vessel manufacture.) a hardness of below 240HB is considdered "safe" for the type of materials that you are talking about. In your industry, it may be different.
I hope this helps.
Niekie Jooste
Sasolburg
South Africa