Fracturing of Seam Welded G60 IF Steel 1

[dbb]

When a seam-welded 5-gallon body is flanged (to accept top and bottom), the nugget on the flange fractures sometimes. The sample I have has one flange cracked and the other OK. The steel is .005 C, .13 Mn, .008 S, .043 Al, .058 Ti, .0039 N. Base metal ASTM grain is 9-10. Zinc coating weight is high at 1.2 oz/ft.sq. The properties of the base steel blank are uniform edge to edge, good DDS. The grain in the HAZ is blown to ASTM 2 to 4 and no failure. The grain in the fusion zone is ASTM 7 to 8 around the contact region with surface grain ASTM 4 to 5. In the failed nugget, the grain boundaries of surface grains have separated and there is mainly transgranular cracking through the nugget. No obvious collections of zinc in the joint. The fracture is brittle, not ductile.
Seems like some sort of embrittlement? What is the cause? Is this a form of CW embrittlement? Has increasing the welding current to accommodate the heavy zinc coating caused problems?

 

[Metalguy]

off the top, I think you may have a Zn embrittlement problem.

 

[dbb]

Metallurgy: Are you suggesting that some detrimental iron-zinc alloys now exist in the nugget or zinc is existing in the structure? Can you suggest a way to confim your suspicions since I see no interruption of the etching of the nugget when using Nital (or Picral). However, I am confounded by the fact that the grain is highly well-defined around the fusion zone---normally I see lousy grain definition in Ti-stabilized steel (as in the base steel and in the HAZ herein). Thanks

 

[Metalguy]

The best way to determine if you have a Zn-related liquid-metal-embrittlement (LME) problem is to have a lab. look for Zn all over the fracture surfaces in a SEM/EDAX. LME *usually* causes intergranular fracture, so I may be wrong with my opinion. However, there are cases of Zn-LME of steel, usually higher strength stuff like 4140, 4340, etc.

I don't think you'll find the Zn via etching.

 

[dbb]

Metalguy: Perhaps the separation of the vertical grain boundaries at those larger surface grains could be due LME, particularly because the zinc was right there prior to being pressed away. I hear you about another mechanism for the brittle fracturing of the internal grains. I'll try the SEM to see if it will tell me something (after I demount my one and only sample!!!). Thanks for your input.

 

[alexalexander]

I have dobts about britellness in IF espceially with Ti alloyed one, if you want please send me your picture I will put my comments on it. grain what you find in weld metal zone are not fully ferritic,therfore I do not think that it is good idea to put grain size comparion between HAZ, BM and weld,
ASTM is based on single polygonal ferrrite grain, in weld you have high speed coled ferrite grains which has various shape and dimension,
If -Ti has an higher graingrowth tendecy in HAz but not necessarly in FZ, IF-Boron,has no grain graowth tendency in HAZ as well as at weld junction.

I do not think that zinc can couse those probelm may be dicontiounty but not transgranuler crack which is more for high stregth hardenabil steel;

good luck
alex