The Weld Nugget (Winter 2010) - A Newsletter to inform, educate, and entertain
TRIP Steel - Taking you for a ride
Steel has been the material of choice for structural autobody construction since the early days of the automobile. Its longevity probably stems from a combination of price, strength, and the ease of welding, forming, and machining. As concerns have shifted from performance to focus on better mileage and safety, reducing the weight of the car while improving crash worthiness has become a top priority. Competition from alternative materials such Aluminum and composites/plastics has also forced the steel industry to innovate. TRIP (TRansformation Induced Plasticity) steel is a recent development in a new type of steel that is stronger and hence can use lighter sheet thickness, a.k.a "down gauging", yet has the formability required to produce complex sheet metal shapes. Microstructure of TRIP steel is a mixture of ferrite, bainite, and austenite formed by careful control of carbon content, alloying elements, and heat treatment.
A structural sheet metal is able to bend without breaking if the atoms inside are able to slide past each other. However, after certain amount of sliding, the atoms get all tangled up, the material becomes hard and is no longer able to bend without breaking or developing cracks. TRIP steel is able to overcome these challenges by transforming the most stressed-out part of the austenite phase into martensite; transformation into lower volume martensite results in reduced residual stress and extends the ability of the TRIP steel to endure strain even where other steels would have failed. Ability to phase transform also comes in handy during collisions where the TRIP steel can absorb more of the impact energy, thus protecting the passengers inside the vehicle. By controlling carbon levels, addition of minority elements such as Al, and by careful heat treatment, TRIP steel can be made to be very strong with good formability, and the ability to absorb impact energy; an ideal scenario.
Unfortunately, as it happens in practically every structural alloy development, any improvements in other properties of the steels leads to increased difficulty with welding. Retained austenite with high levels of carbon is quickly transformed to hard martensite during rapid cooling from weld temperature producing a brittle weld and an interface failure where the two sheets separate without transferring material from one sheet to the other. Strategies to improve weld quality include in-process tempering and welding with dissimilar steels with lower carbon and alloy content. Pulsation welding has also shown some benefits. As welding research makes new strides, welding processes will become more robust and TRIP steels will be taking you for a safer and greener ride
Flash welding is a variant of the resistance welding process used to make butt joints between two parts with complex or large sections. This process is used for making autogenous (without adding filler metal) welds for applications such as wheel rims. Wheel rims made of steel start as a flat plate, are cut to length, put through a rolling mill to form a ring shape, and then the free ends are flash welded to form a continuous rim. Of all the welding processes, flash welding of wheel rims is perhaps the most dramatic; sparks rain down on anything within ten feet of the welding station.
In flash welding, the two ends to be welded are clamped securely and are connected to the positive and negative of the secondary transformer of a resistance welding power supply. Voltage is applied but no current flows as long as parts are not in contact. The parts are brought together at controlled rate and as they come in contact initially at the high points, current starts to flow and causes arcing at those high points. As more points come in contact, the arcing spreads across the entire interface and also melts a thin layer of metal on each mating surface; right behind the molten layers is another thin layer of softened metal in plastic state. The parts continue to move towards each other finally closing the gap and pushing out the molten metal which is known as the flash. Ultimately, the weld is formed by forging together the two mating layers in plastic state.
Advantages of flash welding include the ability to butt weld large sections of any shape, removal of surface impurities with the flash, and a very narrow heat affected zone along the weld interface. Disadvantages include fire and safety hazards from the flash, the necessity of a secondary step to remove weld flash, and difficulty in aligning parts with small cross-sections. Flash welding has been successfully used to join steels, aluminum, nickel, and copper alloys; titanium can also be joined with proper shielding gas. Since it is a solid-state process (no fusion across the interface), dissimilar metal welding is also possible. Industrial applications include wheel rims, rails, rings for jet engines, and welding of fittings to oil pipelines.
A Nickel for your thoughts
Nickel (Ni) was discovered by the Swedish chemist, Axel Fredrik Cronstedt in the mineral Niccolite in 1751. One of the larger sources of nickel is the Sudbury region of Canada; it is believed that this deposit of nickel ore is a result of an ancient meteor impact. Nickel is often used in structural alloys as a major alloying element in combination with iron, chromium, and molybdenum to form stainless steels, inconels, and hastelloys. A common industrial application of Ni is plating for corrosion protection, typically on carbon steels. Keep in mind that fusion welding destroys this protective layer and makes the welded area susceptible to corrosion. Pure nickel strips are the material of choice for the battery tab welding. The increasing price of nickel is forcing the industry to look for alternatives but nothing seems to work as well as nickel.
The American 5 cent coin is also called the Nickel, weighs 5 grams but contains only 25% nickel with copper making up the rest. The Dime, which is worth 10 cents, is about half the weight and similar nickel-copper ratio. So technically, based purely on cost of raw materials, a Nickel should be worth two Dimes! Now there is a Nickel for your thoughts.