Effect of Mn on weldability 1

[Hush]

What effect can I expect on the weldability of 4130 if the following elements are increased:

Mn from .40% to .60% increased to .60% to .80%
Mo from .15% to .25% increased to .25% to .35%
Si from .15% to .35% increased to .20% to .40%

Supplier is offering the modified chemistry to meet 75K RC22 max but I suspect I'll need to requalify my weld procedures. I'd like a little insight before I go talk to a welding engineer so I can grasp what he's telling me.

 

[mcguire]

The increase in alloy content will increase the carbon equivalent by 0.06%, a small but far from insignificant amount making 4130 a more likely candidate for pre-heating and postweld stress relieving.
The formula is CE=%C+%Mn/6+%(Cr+Mo+V)/5+%(Si+Ni+Cu)/15
See the Metals Handbook, desk edition page 30.28 for charts and discussion. The change will correspond to an increase in preheat temperature from 300F to 350F.
The hardness of the martensite is a fuction of the true carbon level and won't be affected. Since your hardenability is increased, you should end up better off if you take the change into account;i.e less chance of less desirable transformation products.

 

[GRoberts]

I would say that requalification would depend on how critical your application is. For instance, in critical offshore work, it is typical to require requalification when the CE or Pcm (index for susceptability to delayed cracking) is increased by .03 above what was qualified with. With the ranges you give, your Pcm would increase by approximately .018 (Pcm = C + Si/30 + Mn/20 + Cu/20 + Cr/20 + Ni/60 + Mo/15 + V/10 + 5B). So it looks like you would most likely not have an increase problem with hydrogen

On the other hand, a material like ASTM A487 Grade 4 has a chemical range of Mn = 1.00 Max, Si = .8 max, & Mo = .15-.3. As you can see, qualifying on a material like this would just about cover both ranges you have listed.

As with most welding, even if the applicable welding code doesn't require requalification, you know how critical your product is, and it is advisable to review you welding to see if it is a good idea to requalify, modify the WPS within the range of essential varaibles allowed, or use the WPS as is.