Weight reduction or “lightweighting” is a hot topic and it’s driving interest in high strength alloys. These “super alloy” steels allow reduced material thickness. That means a fabrication can be lighter yet as strong or stronger than one made from regular grades of steel. Inevitably though, there’s a downside. These alloys have a different composition to other fabrication steels, and that makes them difficult to weld.
At Wiley Metal we make it our business to stay on top of developments like these. That way, if or when you ask us about welding high strength steel we’ll be ready. Here’s a summary of what we know.
What are super alloys?
You’ll hear two terms: High Strength Steel (HSLA), and Advanced High Strength Steels(AHSS). AHSS in particular is something of a favorite in the auto industry, and is often used in a galvanized condition.
HSLA and AHHS aren’t one or two specific types but rather, span a family of steel alloys. They consist of low carbon steel alloyed with elements like niobium, titanium and vanadium. Their superior strength stems from a smaller grain structure, which increases notch hardness and tensile strength. Indeed, tensile strengths of super alloy steels range from around 600 Megapascal (MPa) to as high as 1,000 Mpa. (For comparison, SAE A36 grade steel, which is a general steel used in many structures, is around 250Mpa.)
The welding challenge
Reduced carbon content, plus these alloying elements, results in steels that are hard to weld. This stems from the propensity of the weld to crack. (We discussed avoiding weld cracks back in May 2016 and the problem of welding aluminum in November 2015.) Other problems result from the reduced material thicknesses and the presence of galvanized surfaces. (Galv tends to increase spatter and porosity.) In addition, stress relieving these materials can be challenging as some of their strength comes from a martensitic structure, which heat treatment can affect adversely.
How are super alloys welded?
The simple answer is, it depends on the precise nature of the steel. In general the approach is to preheat to 316°C (600°F). Then, after welding some form of annealing is usually required to bring back some ductility. During the actual welding process it’s important to control the amount of heat put in to the metal, which is why pulsed MIG and what are called “synergistic systems” are used.
In pulsed MIG the welder rapidly switches back and forth between two arc strengths. That lets a drop of metal spray hit the weld, but then allows a moment of cooling before sending the next drop. This puts less heat into the weld. “Synergistic systems” could be thought of as a “smart” pulsed MIG welder in that they automatically compensate for changes in wire feed speed.
Speaking of wire, these alloys are very particular about the type of welding consumables used. In most cases they must be selected to suit the specific grades of HSLA or AHHS being welded.
Other welding options
There are many more welding technologies than just MIG, and several show promise for these super alloys. Plasma arc, capacitor discharge and even friction stir welding are all interesting, although the one that makes us smile is laser welding.
And why would we smile? Well laser cutting is one of our preferred ways of producing steel shapes for fabrication. There’s a certain irony in using a laser to put those blanks back together.
Are super alloys in your future?
HSLA and AHHS let you reduce the weight of rigid steel fabricated structures. The problem is that welding these materials takes expert knowledge. We’ve made a point of being informed about how super alloys are changing the welding process and we’re ready to help.