Mig Welding HSLA Steel - Strength Reduction

[killerbee]

How are HSLA steels affected by MIG welding? Specifically in the gauges used for automotive unitbody construction. Is there a large reduction in Ftu as a result of local heating due to annealing? I was under the impression that annealing did not generate a significant reduction in strength but realized that I have no real information to confirm this.

TIA

 

[metengr]

Killerbee;
The only problem I have seen with MIG welding has been lack of fusion type weld defects due mostly to technique. Watching your preheat and especially interpass temperature is critical for high strength low alloy steel material to avoid over tempering the base material.

 

[killerbee]

What prompted the question was an installation of a sub-frame connector to a unit body vehicle. Stitch welding is recommended (3-4" runs). I'd like to continuously weld it by connecting the beads. What has me worried is that the heating of the original structure will anneal/weaken it.

Based on your response I get the impression that it will have the tendency to increase in hardness? Could you elaborate please?

 

[metengr]

I would be more concerned with the heat input from continuous welds versus recommended stitch welds.

Welding on HSLA steel will result in a narrow band of hardening near the fusion zone of the weld because of the metallurgical changes in the material associated with heat from welding -this is unavoidable. The main concern is to minimize over tempering or softening (reduction in hardness) of the HSLA steel base material adjacent to the weld. This is the reason why preheat is kept to a minimum, and the interpass temperatures are closely monitored during welding to avoid excessive heat input into the base material. I would stay with the stitch weld recommendation.

 

[gdodd]

From my memory the HSLA steels we use (solid solution hardening types) do not alter in properties significantly as evidence from a hardness plot conducted across a sectioned portion of weld bead.

High strength dual phase and TRIP steels though may behave differently.

 

[metengr]

gdodd;
The term HSLA is very broad indeed, and has to be used with care. There are older versions of HSLA steels (like USS T1, HY80, HY100, etc) and you have the newer versions of HSLA steels (microalloy). The more common HSLA steels have been separated out into various classes or forms - weathering steels, microalloyed ferrite/pearlite steels, as-rolled pearlitic steels, low-carbon bainitic steels, dual phase (as you mentioned) steels and inclusion shape controlled steels.

The required strength levels for these steels can be obtained thru specific heat treatments like quench/temper and thru microalloying additions (vanadium,titanium, niobium, nitrogen, copper). The microalloyed HSLA steels during fabrication result in fine grained (enhanced toughness) structures with increasing yield strength obtained thru precipitation strengthening.

As with any welding operation, heat will alter grain size and the distribution/location of phases and preciptates within the HSLA steel microstructure, thus affecting desired properties. Keeping the heat input prior to (preheat temperature) and during welding (interpass temperature) to the minimum recommended by the steel manufacturer is the key to maintaining a localized heat affected zone.